diff --git a/README.md b/README.md
index a8d9bf96..8f709207 100644
--- a/README.md
+++ b/README.md
@@ -1246,8 +1246,8 @@ when an activity class is referenced in a workflow before it has been explicitly
 > Temporalio::Workflow::Unsafe.illegal_call_tracing_disabled block.
 
 This comes from `bootsnap` via `zeitwork` because it is lazily loading a class/module at workflow runtime. It is not
-good to lazily load code durnig a workflow run because it can be side effecting. Workflows and the classes they
-reference should not be eagerly loaded.
+good to lazily load code during a workflow run because it can be side effecting. Workflows and the classes they
+reference should be eagerly loaded.
 
 To resolve this, either always eagerly load (e.g. `config.eager_load = true`) or explicitly `require` what is used by a
 workflow at the top of the file.
diff --git a/temporalio/Cargo.lock b/temporalio/Cargo.lock
index fe87d94b..436db1af 100644
--- a/temporalio/Cargo.lock
+++ b/temporalio/Cargo.lock
@@ -312,9 +312,9 @@ dependencies = [
 
 [[package]]
 name = "bitflags"
-version = "2.9.4"
+version = "2.9.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "2261d10cca569e4643e526d8dc2e62e433cc8aba21ab764233731f8d369bf394"
+checksum = "6a65b545ab31d687cff52899d4890855fec459eb6afe0da6417b8a18da87aa29"
 
 [[package]]
 name = "bumpalo"
@@ -351,11 +351,10 @@ checksum = "37b2a672a2cb129a2e41c10b1224bb368f9f37a2b16b612598138befd7b37eb5"
 
 [[package]]
 name = "cc"
-version = "1.2.37"
+version = "1.2.33"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "65193589c6404eb80b450d618eaf9a2cafaaafd57ecce47370519ef674a7bd44"
+checksum = "3ee0f8803222ba5a7e2777dd72ca451868909b1ac410621b676adf07280e9b5f"
 dependencies = [
- "find-msvc-tools",
  "jobserver",
  "libc",
  "shlex",
@@ -384,9 +383,9 @@ checksum = "613afe47fcd5fac7ccf1db93babcb082c5994d996f20b8b159f2ad1658eb5724"
 
 [[package]]
 name = "chrono"
-version = "0.4.42"
+version = "0.4.41"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "145052bdd345b87320e369255277e3fb5152762ad123a901ef5c262dd38fe8d2"
+checksum = "c469d952047f47f91b68d1cba3f10d63c11d73e4636f24f08daf0278abf01c4d"
 dependencies = [
  "num-traits",
  "serde",
@@ -432,9 +431,9 @@ dependencies = [
 
 [[package]]
 name = "clap"
-version = "4.5.47"
+version = "4.5.45"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "7eac00902d9d136acd712710d71823fb8ac8004ca445a89e73a41d45aa712931"
+checksum = "1fc0e74a703892159f5ae7d3aac52c8e6c392f5ae5f359c70b5881d60aaac318"
 dependencies = [
  "clap_builder",
  "clap_derive",
@@ -442,9 +441,9 @@ dependencies = [
 
 [[package]]
 name = "clap_builder"
-version = "4.5.47"
+version = "4.5.44"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "2ad9bbf750e73b5884fb8a211a9424a1906c1e156724260fdae972f31d70e1d6"
+checksum = "b3e7f4214277f3c7aa526a59dd3fbe306a370daee1f8b7b8c987069cd8e888a8"
 dependencies = [
  "anstream",
  "anstyle",
@@ -454,9 +453,9 @@ dependencies = [
 
 [[package]]
 name = "clap_derive"
-version = "4.5.47"
+version = "4.5.45"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "bbfd7eae0b0f1a6e63d4b13c9c478de77c2eb546fba158ad50b4203dc24b9f9c"
+checksum = "14cb31bb0a7d536caef2639baa7fad459e15c3144efefa6dbd1c84562c4739f6"
 dependencies = [
  "heck",
  "proc-macro2",
@@ -753,7 +752,7 @@ dependencies = [
  "libc",
  "option-ext",
  "redox_users",
- "windows-sys 0.61.0",
+ "windows-sys 0.60.2",
 ]
 
 [[package]]
@@ -787,18 +786,18 @@ checksum = "48c757948c5ede0e46177b7add2e67155f70e33c07fea8284df6576da70b3719"
 
 [[package]]
 name = "enum-iterator"
-version = "2.3.0"
+version = "2.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a4549325971814bda7a44061bf3fe7e487d447cba01e4220a4b454d630d7a016"
+checksum = "c280b9e6b3ae19e152d8e31cf47f18389781e119d4013a2a2bb0180e5facc635"
 dependencies = [
  "enum-iterator-derive",
 ]
 
 [[package]]
 name = "enum-iterator-derive"
-version = "1.5.0"
+version = "1.4.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "685adfa4d6f3d765a26bc5dbc936577de9abf756c1feeb3089b01dd395034842"
+checksum = "a1ab991c1362ac86c61ab6f556cff143daa22e5a15e4e189df818b2fd19fe65b"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -825,23 +824,22 @@ checksum = "877a4ace8713b0bcf2a4e7eec82529c029f1d0619886d18145fea96c3ffe5c0f"
 
 [[package]]
 name = "erased-serde"
-version = "0.4.8"
+version = "0.4.6"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "259d404d09818dec19332e31d94558aeb442fea04c817006456c24b5460bbd4b"
+checksum = "e004d887f51fcb9fef17317a2f3525c887d8aa3f4f50fed920816a688284a5b7"
 dependencies = [
  "serde",
- "serde_core",
  "typeid",
 ]
 
 [[package]]
 name = "errno"
-version = "0.3.14"
+version = "0.3.13"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "39cab71617ae0d63f51a36d69f866391735b51691dbda63cf6f96d042b63efeb"
+checksum = "778e2ac28f6c47af28e4907f13ffd1e1ddbd400980a9abd7c8df189bf578a5ad"
 dependencies = [
  "libc",
- "windows-sys 0.61.0",
+ "windows-sys 0.60.2",
 ]
 
 [[package]]
@@ -862,12 +860,6 @@ dependencies = [
  "windows-sys 0.60.2",
 ]
 
-[[package]]
-name = "find-msvc-tools"
-version = "0.1.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "7fd99930f64d146689264c637b5af2f0233a933bef0d8570e2526bf9e083192d"
-
 [[package]]
 name = "fixedbitset"
 version = "0.5.7"
@@ -899,9 +891,9 @@ checksum = "d9c4f5dac5e15c24eb999c26181a6ca40b39fe946cbe4c263c7209467bc83af2"
 
 [[package]]
 name = "form_urlencoded"
-version = "1.2.2"
+version = "1.2.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "cb4cb245038516f5f85277875cdaa4f7d2c9a0fa0468de06ed190163b1581fcf"
+checksum = "e13624c2627564efccf4934284bdd98cbaa14e79b0b5a141218e507b3a823456"
 dependencies = [
  "percent-encoding",
 ]
@@ -1051,7 +1043,7 @@ dependencies = [
  "js-sys",
  "libc",
  "r-efi",
- "wasi 0.14.7+wasi-0.2.4",
+ "wasi 0.14.2+wasi-0.2.4",
  "wasm-bindgen",
 ]
 
@@ -1102,7 +1094,7 @@ dependencies = [
  "futures-core",
  "futures-sink",
  "http",
- "indexmap 2.11.3",
+ "indexmap 2.10.0",
  "slab",
  "tokio",
  "tokio-util",
@@ -1209,9 +1201,9 @@ checksum = "df3b46402a9d5adb4c86a0cf463f42e19994e3ee891101b1841f30a545cb49a9"
 
 [[package]]
 name = "humantime"
-version = "2.3.0"
+version = "2.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "135b12329e5e3ce057a9f972339ea52bc954fe1e9358ef27f95e89716fbc5424"
+checksum = "9b112acc8b3adf4b107a8ec20977da0273a8c386765a3ec0229bd500a1443f9f"
 
 [[package]]
 name = "hyper"
@@ -1268,9 +1260,9 @@ dependencies = [
 
 [[package]]
 name = "hyper-util"
-version = "0.1.17"
+version = "0.1.16"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3c6995591a8f1380fcb4ba966a252a4b29188d51d2b89e3a252f5305be65aea8"
+checksum = "8d9b05277c7e8da2c93a568989bb6207bef0112e8d17df7a6eda4a3cf143bc5e"
 dependencies = [
  "base64 0.22.1",
  "bytes",
@@ -1384,9 +1376,9 @@ checksum = "b9e0384b61958566e926dc50660321d12159025e767c18e043daf26b70104c39"
 
 [[package]]
 name = "idna"
-version = "1.1.0"
+version = "1.0.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3b0875f23caa03898994f6ddc501886a45c7d3d62d04d2d90788d47be1b1e4de"
+checksum = "686f825264d630750a544639377bae737628043f20d38bbc029e8f29ea968a7e"
 dependencies = [
  "idna_adapter",
  "smallvec",
@@ -1415,9 +1407,9 @@ dependencies = [
 
 [[package]]
 name = "indexmap"
-version = "2.11.3"
+version = "2.10.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "92119844f513ffa41556430369ab02c295a3578af21cf945caa3e9e0c2481ac3"
+checksum = "fe4cd85333e22411419a0bcae1297d25e58c9443848b11dc6a86fefe8c78a661"
 dependencies = [
  "equivalent",
  "hashbrown 0.15.5",
@@ -1443,9 +1435,9 @@ dependencies = [
 
 [[package]]
 name = "io-uring"
-version = "0.7.10"
+version = "0.7.9"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "046fa2d4d00aea763528b4950358d0ead425372445dc8ff86312b3c69ff7727b"
+checksum = "d93587f37623a1a17d94ef2bc9ada592f5465fe7732084ab7beefabe5c77c0c4"
 dependencies = [
  "bitflags",
  "cfg-if",
@@ -1509,9 +1501,9 @@ checksum = "4a5f13b858c8d314ee3e8f639011f7ccefe71f97f96e50151fb991f267928e2c"
 
 [[package]]
 name = "jobserver"
-version = "0.1.34"
+version = "0.1.33"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "9afb3de4395d6b3e67a780b6de64b51c978ecf11cb9a462c66be7d4ca9039d33"
+checksum = "38f262f097c174adebe41eb73d66ae9c06b2844fb0da69969647bbddd9b0538a"
 dependencies = [
  "getrandom 0.3.3",
  "libc",
@@ -1519,9 +1511,9 @@ dependencies = [
 
 [[package]]
 name = "js-sys"
-version = "0.3.80"
+version = "0.3.77"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "852f13bec5eba4ba9afbeb93fd7c13fe56147f055939ae21c43a29a0ecb2702e"
+checksum = "1cfaf33c695fc6e08064efbc1f72ec937429614f25eef83af942d0e227c3a28f"
 dependencies = [
  "once_cell",
  "wasm-bindgen",
@@ -1563,9 +1555,9 @@ dependencies = [
 
 [[package]]
 name = "libredox"
-version = "0.1.10"
+version = "0.1.9"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "416f7e718bdb06000964960ffa43b4335ad4012ae8b99060261aa4a8088d5ccb"
+checksum = "391290121bad3d37fbddad76d8f5d1c1c314cfc646d143d7e07a3086ddff0ce3"
 dependencies = [
  "bitflags",
  "libc",
@@ -1583,9 +1575,9 @@ dependencies = [
 
 [[package]]
 name = "linux-raw-sys"
-version = "0.11.0"
+version = "0.9.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "df1d3c3b53da64cf5760482273a98e575c651a67eec7f77df96b5b642de8f039"
+checksum = "cd945864f07fe9f5371a27ad7b52a172b4b499999f1d97574c9fa68373937e12"
 
 [[package]]
 name = "litemap"
@@ -1605,15 +1597,15 @@ dependencies = [
 
 [[package]]
 name = "log"
-version = "0.4.28"
+version = "0.4.27"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "34080505efa8e45a4b816c349525ebe327ceaa8559756f0356cba97ef3bf7432"
+checksum = "13dc2df351e3202783a1fe0d44375f7295ffb4049267b0f3018346dc122a1d94"
 
 [[package]]
 name = "lru"
-version = "0.16.1"
+version = "0.16.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "bfe949189f46fabb938b3a9a0be30fdd93fd8a09260da863399a8cf3db756ec8"
+checksum = "86ea4e65087ff52f3862caff188d489f1fab49a0cb09e01b2e3f1a617b10aaed"
 dependencies = [
  "hashbrown 0.15.5",
 ]
@@ -1940,9 +1932,9 @@ dependencies = [
 
 [[package]]
 name = "percent-encoding"
-version = "2.3.2"
+version = "2.3.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "9b4f627cb1b25917193a259e49bdad08f671f8d9708acfd5fe0a8c1455d87220"
+checksum = "e3148f5046208a5d56bcfc03053e3ca6334e51da8dfb19b6cdc8b306fae3283e"
 
 [[package]]
 name = "petgraph"
@@ -1951,7 +1943,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "3672b37090dbd86368a4145bc067582552b29c27377cad4e0a306c97f9bd7772"
 dependencies = [
  "fixedbitset",
- "indexmap 2.11.3",
+ "indexmap 2.10.0",
 ]
 
 [[package]]
@@ -2046,9 +2038,9 @@ dependencies = [
 
 [[package]]
 name = "potential_utf"
-version = "0.1.3"
+version = "0.1.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "84df19adbe5b5a0782edcab45899906947ab039ccf4573713735ee7de1e6b08a"
+checksum = "e5a7c30837279ca13e7c867e9e40053bc68740f988cb07f7ca6df43cc734b585"
 dependencies = [
  "zerovec",
 ]
@@ -2100,9 +2092,9 @@ dependencies = [
 
 [[package]]
 name = "proc-macro-crate"
-version = "3.4.0"
+version = "3.3.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "219cb19e96be00ab2e37d6e299658a0cfa83e52429179969b0f0121b4ac46983"
+checksum = "edce586971a4dfaa28950c6f18ed55e0406c1ab88bbce2c6f6293a7aaba73d35"
 dependencies = [
  "toml_edit",
 ]
@@ -2266,9 +2258,9 @@ dependencies = [
 
 [[package]]
 name = "quinn"
-version = "0.11.9"
+version = "0.11.8"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b9e20a958963c291dc322d98411f541009df2ced7b5a4f2bd52337638cfccf20"
+checksum = "626214629cda6781b6dc1d316ba307189c85ba657213ce642d9c77670f8202c8"
 dependencies = [
  "bytes",
  "cfg_aliases",
@@ -2277,7 +2269,7 @@ dependencies = [
  "quinn-udp",
  "rustc-hash 2.1.1",
  "rustls",
- "socket2 0.6.0",
+ "socket2 0.5.10",
  "thiserror 2.0.16",
  "tokio",
  "tracing",
@@ -2286,9 +2278,9 @@ dependencies = [
 
 [[package]]
 name = "quinn-proto"
-version = "0.11.13"
+version = "0.11.12"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f1906b49b0c3bc04b5fe5d86a77925ae6524a19b816ae38ce1e426255f1d8a31"
+checksum = "49df843a9161c85bb8aae55f101bc0bac8bcafd637a620d9122fd7e0b2f7422e"
 dependencies = [
  "bytes",
  "getrandom 0.3.3",
@@ -2307,16 +2299,16 @@ dependencies = [
 
 [[package]]
 name = "quinn-udp"
-version = "0.5.14"
+version = "0.5.13"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "addec6a0dcad8a8d96a771f815f0eaf55f9d1805756410b39f5fa81332574cbd"
+checksum = "fcebb1209ee276352ef14ff8732e24cc2b02bbac986cd74a4c81bcb2f9881970"
 dependencies = [
  "cfg_aliases",
  "libc",
  "once_cell",
- "socket2 0.6.0",
+ "socket2 0.5.10",
  "tracing",
- "windows-sys 0.60.2",
+ "windows-sys 0.59.0",
 ]
 
 [[package]]
@@ -2395,9 +2387,9 @@ dependencies = [
 
 [[package]]
 name = "raw-cpuid"
-version = "11.6.0"
+version = "11.5.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "498cd0dc59d73224351ee52a95fee0f1a617a2eae0e7d9d720cc622c73a54186"
+checksum = "c6df7ab838ed27997ba19a4664507e6f82b41fe6e20be42929332156e5e85146"
 dependencies = [
  "bitflags",
 ]
@@ -2474,9 +2466,9 @@ dependencies = [
 
 [[package]]
 name = "regex"
-version = "1.11.2"
+version = "1.11.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "23d7fd106d8c02486a8d64e778353d1cffe08ce79ac2e82f540c86d0facf6912"
+checksum = "b544ef1b4eac5dc2db33ea63606ae9ffcfac26c1416a2806ae0bf5f56b201191"
 dependencies = [
  "aho-corasick",
  "memchr",
@@ -2486,9 +2478,9 @@ dependencies = [
 
 [[package]]
 name = "regex-automata"
-version = "0.4.10"
+version = "0.4.9"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "6b9458fa0bfeeac22b5ca447c63aaf45f28439a709ccd244698632f9aa6394d6"
+checksum = "809e8dc61f6de73b46c85f4c96486310fe304c434cfa43669d7b40f711150908"
 dependencies = [
  "aho-corasick",
  "memchr",
@@ -2497,9 +2489,9 @@ dependencies = [
 
 [[package]]
 name = "regex-syntax"
-version = "0.8.6"
+version = "0.8.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "caf4aa5b0f434c91fe5c7f1ecb6a5ece2130b02ad2a590589dda5146df959001"
+checksum = "2b15c43186be67a4fd63bee50d0303afffcef381492ebe2c5d87f324e1b8815c"
 
 [[package]]
 name = "relative-path"
@@ -2656,15 +2648,15 @@ version = "0.1.0"
 
 [[package]]
 name = "rustix"
-version = "1.1.2"
+version = "1.0.8"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "cd15f8a2c5551a84d56efdc1cd049089e409ac19a3072d5037a17fd70719ff3e"
+checksum = "11181fbabf243db407ef8df94a6ce0b2f9a733bd8be4ad02b4eda9602296cac8"
 dependencies = [
  "bitflags",
  "errno",
  "libc",
  "linux-raw-sys",
- "windows-sys 0.61.0",
+ "windows-sys 0.60.2",
 ]
 
 [[package]]
@@ -2706,9 +2698,9 @@ dependencies = [
 
 [[package]]
 name = "rustls-webpki"
-version = "0.103.6"
+version = "0.103.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8572f3c2cb9934231157b45499fc41e1f58c589fdfb81a844ba873265e80f8eb"
+checksum = "0a17884ae0c1b773f1ccd2bd4a8c72f16da897310a98b0e84bf349ad5ead92fc"
 dependencies = [
  "ring",
  "rustls-pki-types",
@@ -2738,11 +2730,11 @@ dependencies = [
 
 [[package]]
 name = "schannel"
-version = "0.1.28"
+version = "0.1.27"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "891d81b926048e76efe18581bf793546b4c0eaf8448d72be8de2bbee5fd166e1"
+checksum = "1f29ebaa345f945cec9fbbc532eb307f0fdad8161f281b6369539c8d84876b3d"
 dependencies = [
- "windows-sys 0.61.0",
+ "windows-sys 0.59.0",
 ]
 
 [[package]]
@@ -2753,9 +2745,9 @@ checksum = "94143f37725109f92c262ed2cf5e59bce7498c01bcc1502d7b9afe439a4e9f49"
 
 [[package]]
 name = "security-framework"
-version = "3.4.0"
+version = "3.3.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "60b369d18893388b345804dc0007963c99b7d665ae71d275812d828c6f089640"
+checksum = "80fb1d92c5028aa318b4b8bd7302a5bfcf48be96a37fc6fc790f806b0004ee0c"
 dependencies = [
  "bitflags",
  "core-foundation",
@@ -2766,9 +2758,9 @@ dependencies = [
 
 [[package]]
 name = "security-framework-sys"
-version = "2.15.0"
+version = "2.14.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "cc1f0cbffaac4852523ce30d8bd3c5cdc873501d96ff467ca09b6767bb8cd5c0"
+checksum = "49db231d56a190491cb4aeda9527f1ad45345af50b0851622a7adb8c03b01c32"
 dependencies = [
  "core-foundation-sys",
  "libc",
@@ -2776,9 +2768,9 @@ dependencies = [
 
 [[package]]
 name = "semver"
-version = "1.0.27"
+version = "1.0.26"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d767eb0aabc880b29956c35734170f26ed551a859dbd361d140cdbeca61ab1e2"
+checksum = "56e6fa9c48d24d85fb3de5ad847117517440f6beceb7798af16b4a87d616b8d0"
 
 [[package]]
 name = "seq-macro"
@@ -2788,28 +2780,18 @@ checksum = "1bc711410fbe7399f390ca1c3b60ad0f53f80e95c5eb935e52268a0e2cd49acc"
 
 [[package]]
 name = "serde"
-version = "1.0.225"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "fd6c24dee235d0da097043389623fb913daddf92c76e9f5a1db88607a0bcbd1d"
-dependencies = [
- "serde_core",
- "serde_derive",
-]
-
-[[package]]
-name = "serde_core"
-version = "1.0.225"
+version = "1.0.219"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "659356f9a0cb1e529b24c01e43ad2bdf520ec4ceaf83047b83ddcc2251f96383"
+checksum = "5f0e2c6ed6606019b4e29e69dbaba95b11854410e5347d525002456dbbb786b6"
 dependencies = [
  "serde_derive",
 ]
 
 [[package]]
 name = "serde_derive"
-version = "1.0.225"
+version = "1.0.219"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "0ea936adf78b1f766949a4977b91d2f5595825bd6ec079aa9543ad2685fc4516"
+checksum = "5b0276cf7f2c73365f7157c8123c21cd9a50fbbd844757af28ca1f5925fc2a00"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -2818,24 +2800,23 @@ dependencies = [
 
 [[package]]
 name = "serde_json"
-version = "1.0.145"
+version = "1.0.143"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "402a6f66d8c709116cf22f558eab210f5a50187f702eb4d7e5ef38d9a7f1c79c"
+checksum = "d401abef1d108fbd9cbaebc3e46611f4b1021f714a0597a71f41ee463f5f4a5a"
 dependencies = [
  "itoa",
  "memchr",
  "ryu",
  "serde",
- "serde_core",
 ]
 
 [[package]]
 name = "serde_spanned"
-version = "1.0.1"
+version = "1.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "2789234a13a53fc4be1b51ea1bab45a3c338bdb884862a257d10e5a74ae009e6"
+checksum = "40734c41988f7306bb04f0ecf60ec0f3f1caa34290e4e8ea471dcd3346483b83"
 dependencies = [
- "serde_core",
+ "serde",
 ]
 
 [[package]]
@@ -3024,15 +3005,15 @@ checksum = "1ac9aa371f599d22256307c24a9d748c041e548cbf599f35d890f9d365361790"
 
 [[package]]
 name = "tempfile"
-version = "3.22.0"
+version = "3.21.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "84fa4d11fadde498443cca10fd3ac23c951f0dc59e080e9f4b93d4df4e4eea53"
+checksum = "15b61f8f20e3a6f7e0649d825294eaf317edce30f82cf6026e7e4cb9222a7d1e"
 dependencies = [
  "fastrand",
  "getrandom 0.3.3",
  "once_cell",
  "rustix",
- "windows-sys 0.61.0",
+ "windows-sys 0.60.2",
 ]
 
 [[package]]
@@ -3205,7 +3186,9 @@ dependencies = [
 name = "temporalio_bridge"
 version = "0.1.0"
 dependencies = [
+ "async-trait",
  "futures",
+ "log",
  "magnus",
  "parking_lot",
  "prost",
@@ -3355,9 +3338,9 @@ dependencies = [
 
 [[package]]
 name = "tokio-rustls"
-version = "0.26.3"
+version = "0.26.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "05f63835928ca123f1bef57abbcd23bb2ba0ac9ae1235f1e65bda0d06e7786bd"
+checksum = "8e727b36a1a0e8b74c376ac2211e40c2c8af09fb4013c60d910495810f008e9b"
 dependencies = [
  "rustls",
  "tokio",
@@ -3389,14 +3372,14 @@ dependencies = [
 
 [[package]]
 name = "toml"
-version = "0.9.6"
+version = "0.9.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ae2a4cf385da23d1d53bc15cdfa5c2109e93d8d362393c801e87da2f72f0e201"
+checksum = "75129e1dc5000bfbaa9fee9d1b21f974f9fbad9daec557a521ee6e080825f6e8"
 dependencies = [
- "indexmap 2.11.3",
- "serde_core",
+ "indexmap 2.10.0",
+ "serde",
  "serde_spanned",
- "toml_datetime",
+ "toml_datetime 0.7.0",
  "toml_parser",
  "toml_writer",
  "winnow",
@@ -3404,22 +3387,27 @@ dependencies = [
 
 [[package]]
 name = "toml_datetime"
-version = "0.7.1"
+version = "0.6.11"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a197c0ec7d131bfc6f7e82c8442ba1595aeab35da7adbf05b6b73cd06a16b6be"
+checksum = "22cddaf88f4fbc13c51aebbf5f8eceb5c7c5a9da2ac40a13519eb5b0a0e8f11c"
+
+[[package]]
+name = "toml_datetime"
+version = "0.7.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bade1c3e902f58d73d3f294cd7f20391c1cb2fbcb643b73566bc773971df91e3"
 dependencies = [
- "serde_core",
+ "serde",
 ]
 
 [[package]]
 name = "toml_edit"
-version = "0.23.5"
+version = "0.22.27"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c2ad0b7ae9cfeef5605163839cb9221f453399f15cfb5c10be9885fcf56611f9"
+checksum = "41fe8c660ae4257887cf66394862d21dbca4a6ddd26f04a3560410406a2f819a"
 dependencies = [
- "indexmap 2.11.3",
- "toml_datetime",
- "toml_parser",
+ "indexmap 2.10.0",
+ "toml_datetime 0.6.11",
  "winnow",
 ]
 
@@ -3541,7 +3529,7 @@ checksum = "d039ad9159c98b70ecfd540b2573b97f7f52c3e8d9f8ad57a24b916a536975f9"
 dependencies = [
  "futures-core",
  "futures-util",
- "indexmap 2.11.3",
+ "indexmap 2.10.0",
  "pin-project-lite",
  "slab",
  "sync_wrapper",
@@ -3639,9 +3627,9 @@ checksum = "e421abadd41a4225275504ea4d6566923418b7f05506fbc9c0fe86ba7396114b"
 
 [[package]]
 name = "trybuild"
-version = "1.0.111"
+version = "1.0.110"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "0ded9fdb81f30a5708920310bfcd9ea7482ff9cba5f54601f7a19a877d5c2392"
+checksum = "32e257d7246e7a9fd015fb0b28b330a8d4142151a33f03e6a497754f4b1f6a8e"
 dependencies = [
  "dissimilar",
  "glob",
@@ -3685,9 +3673,9 @@ dependencies = [
 
 [[package]]
 name = "unicode-ident"
-version = "1.0.19"
+version = "1.0.18"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f63a545481291138910575129486daeaf8ac54aee4387fe7906919f7830c7d9d"
+checksum = "5a5f39404a5da50712a4c1eecf25e90dd62b613502b7e925fd4e4d19b5c96512"
 
 [[package]]
 name = "unicode-xid"
@@ -3703,14 +3691,13 @@ checksum = "8ecb6da28b8a351d773b68d5825ac39017e680750f980f3a1a85cd8dd28a47c1"
 
 [[package]]
 name = "url"
-version = "2.5.7"
+version = "2.5.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "08bc136a29a3d1758e07a9cca267be308aeebf5cfd5a10f3f67ab2097683ef5b"
+checksum = "32f8b686cadd1473f4bd0117a5d28d36b1ade384ea9b5069a1c40aefed7fda60"
 dependencies = [
  "form_urlencoded",
  "idna",
  "percent-encoding",
- "serde",
 ]
 
 [[package]]
@@ -3727,9 +3714,9 @@ checksum = "06abde3611657adf66d383f00b093d7faecc7fa57071cce2578660c9f1010821"
 
 [[package]]
 name = "uuid"
-version = "1.18.1"
+version = "1.18.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "2f87b8aa10b915a06587d0dec516c282ff295b475d94abf425d62b57710070a2"
+checksum = "f33196643e165781c20a5ead5582283a7dacbb87855d867fbc2df3f81eddc1be"
 dependencies = [
  "getrandom 0.3.3",
  "js-sys",
@@ -3775,40 +3762,30 @@ checksum = "ccf3ec651a847eb01de73ccad15eb7d99f80485de043efb2f370cd654f4ea44b"
 
 [[package]]
 name = "wasi"
-version = "0.14.7+wasi-0.2.4"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "883478de20367e224c0090af9cf5f9fa85bed63a95c1abf3afc5c083ebc06e8c"
-dependencies = [
- "wasip2",
-]
-
-[[package]]
-name = "wasip2"
-version = "1.0.1+wasi-0.2.4"
+version = "0.14.2+wasi-0.2.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "0562428422c63773dad2c345a1882263bbf4d65cf3f42e90921f787ef5ad58e7"
+checksum = "9683f9a5a998d873c0d21fcbe3c083009670149a8fab228644b8bd36b2c48cb3"
 dependencies = [
- "wit-bindgen",
+ "wit-bindgen-rt",
 ]
 
 [[package]]
 name = "wasm-bindgen"
-version = "0.2.103"
+version = "0.2.100"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ab10a69fbd0a177f5f649ad4d8d3305499c42bab9aef2f7ff592d0ec8f833819"
+checksum = "1edc8929d7499fc4e8f0be2262a241556cfc54a0bea223790e71446f2aab1ef5"
 dependencies = [
  "cfg-if",
  "once_cell",
  "rustversion",
  "wasm-bindgen-macro",
- "wasm-bindgen-shared",
 ]
 
 [[package]]
 name = "wasm-bindgen-backend"
-version = "0.2.103"
+version = "0.2.100"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "0bb702423545a6007bbc368fde243ba47ca275e549c8a28617f56f6ba53b1d1c"
+checksum = "2f0a0651a5c2bc21487bde11ee802ccaf4c51935d0d3d42a6101f98161700bc6"
 dependencies = [
  "bumpalo",
  "log",
@@ -3820,9 +3797,9 @@ dependencies = [
 
 [[package]]
 name = "wasm-bindgen-futures"
-version = "0.4.53"
+version = "0.4.50"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a0b221ff421256839509adbb55998214a70d829d3a28c69b4a6672e9d2a42f67"
+checksum = "555d470ec0bc3bb57890405e5d4322cc9ea83cebb085523ced7be4144dac1e61"
 dependencies = [
  "cfg-if",
  "js-sys",
@@ -3833,9 +3810,9 @@ dependencies = [
 
 [[package]]
 name = "wasm-bindgen-macro"
-version = "0.2.103"
+version = "0.2.100"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "fc65f4f411d91494355917b605e1480033152658d71f722a90647f56a70c88a0"
+checksum = "7fe63fc6d09ed3792bd0897b314f53de8e16568c2b3f7982f468c0bf9bd0b407"
 dependencies = [
  "quote",
  "wasm-bindgen-macro-support",
@@ -3843,9 +3820,9 @@ dependencies = [
 
 [[package]]
 name = "wasm-bindgen-macro-support"
-version = "0.2.103"
+version = "0.2.100"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ffc003a991398a8ee604a401e194b6b3a39677b3173d6e74495eb51b82e99a32"
+checksum = "8ae87ea40c9f689fc23f209965b6fb8a99ad69aeeb0231408be24920604395de"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -3856,9 +3833,9 @@ dependencies = [
 
 [[package]]
 name = "wasm-bindgen-shared"
-version = "0.2.103"
+version = "0.2.100"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "293c37f4efa430ca14db3721dfbe48d8c33308096bd44d80ebaa775ab71ba1cf"
+checksum = "1a05d73b933a847d6cccdda8f838a22ff101ad9bf93e33684f39c1f5f0eece3d"
 dependencies = [
  "unicode-ident",
 ]
@@ -3878,9 +3855,9 @@ dependencies = [
 
 [[package]]
 name = "web-sys"
-version = "0.3.80"
+version = "0.3.77"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "fbe734895e869dc429d78c4b433f8d17d95f8d05317440b4fad5ab2d33e596dc"
+checksum = "33b6dd2ef9186f1f2072e409e99cd22a975331a6b3591b12c764e0e55c60d5d2"
 dependencies = [
  "js-sys",
  "wasm-bindgen",
@@ -3914,11 +3891,11 @@ checksum = "ac3b87c63620426dd9b991e5ce0329eff545bccbbb34f3be09ff6fb6ab51b7b6"
 
 [[package]]
 name = "winapi-util"
-version = "0.1.11"
+version = "0.1.10"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c2a7b1c03c876122aa43f3020e6c3c3ee5c05081c9a00739faf7503aeba10d22"
+checksum = "0978bf7171b3d90bac376700cb56d606feb40f251a475a5d6634613564460b22"
 dependencies = [
- "windows-sys 0.61.0",
+ "windows-sys 0.60.2",
 ]
 
 [[package]]
@@ -3936,7 +3913,7 @@ dependencies = [
  "windows-collections",
  "windows-core",
  "windows-future",
- "windows-link 0.1.3",
+ "windows-link",
  "windows-numerics",
 ]
 
@@ -3957,7 +3934,7 @@ checksum = "c0fdd3ddb90610c7638aa2b3a3ab2904fb9e5cdbecc643ddb3647212781c4ae3"
 dependencies = [
  "windows-implement",
  "windows-interface",
- "windows-link 0.1.3",
+ "windows-link",
  "windows-result",
  "windows-strings",
 ]
@@ -3969,7 +3946,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "fc6a41e98427b19fe4b73c550f060b59fa592d7d686537eebf9385621bfbad8e"
 dependencies = [
  "windows-core",
- "windows-link 0.1.3",
+ "windows-link",
  "windows-threading",
 ]
 
@@ -4001,12 +3978,6 @@ version = "0.1.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "5e6ad25900d524eaabdbbb96d20b4311e1e7ae1699af4fb28c17ae66c80d798a"
 
-[[package]]
-name = "windows-link"
-version = "0.2.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "45e46c0661abb7180e7b9c281db115305d49ca1709ab8242adf09666d2173c65"
-
 [[package]]
 name = "windows-numerics"
 version = "0.2.0"
@@ -4014,7 +3985,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9150af68066c4c5c07ddc0ce30421554771e528bde427614c61038bc2c92c2b1"
 dependencies = [
  "windows-core",
- "windows-link 0.1.3",
+ "windows-link",
 ]
 
 [[package]]
@@ -4023,7 +3994,7 @@ version = "0.3.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "56f42bd332cc6c8eac5af113fc0c1fd6a8fd2aa08a0119358686e5160d0586c6"
 dependencies = [
- "windows-link 0.1.3",
+ "windows-link",
 ]
 
 [[package]]
@@ -4032,7 +4003,7 @@ version = "0.4.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "56e6c93f3a0c3b36176cb1327a4958a0353d5d166c2a35cb268ace15e91d3b57"
 dependencies = [
- "windows-link 0.1.3",
+ "windows-link",
 ]
 
 [[package]]
@@ -4062,15 +4033,6 @@ dependencies = [
  "windows-targets 0.53.3",
 ]
 
-[[package]]
-name = "windows-sys"
-version = "0.61.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e201184e40b2ede64bc2ea34968b28e33622acdbbf37104f0e4a33f7abe657aa"
-dependencies = [
- "windows-link 0.2.0",
-]
-
 [[package]]
 name = "windows-targets"
 version = "0.52.6"
@@ -4093,7 +4055,7 @@ version = "0.53.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d5fe6031c4041849d7c496a8ded650796e7b6ecc19df1a431c1a363342e5dc91"
 dependencies = [
- "windows-link 0.1.3",
+ "windows-link",
  "windows_aarch64_gnullvm 0.53.0",
  "windows_aarch64_msvc 0.53.0",
  "windows_i686_gnu 0.53.0",
@@ -4110,7 +4072,7 @@ version = "0.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b66463ad2e0ea3bbf808b7f1d371311c80e115c0b71d60efc142cafbcfb057a6"
 dependencies = [
- "windows-link 0.1.3",
+ "windows-link",
 ]
 
 [[package]]
@@ -4211,18 +4173,21 @@ checksum = "271414315aff87387382ec3d271b52d7ae78726f5d44ac98b4f4030c91880486"
 
 [[package]]
 name = "winnow"
-version = "0.7.13"
+version = "0.7.12"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "21a0236b59786fed61e2a80582dd500fe61f18b5dca67a4a067d0bc9039339cf"
+checksum = "f3edebf492c8125044983378ecb5766203ad3b4c2f7a922bd7dd207f6d443e95"
 dependencies = [
  "memchr",
 ]
 
 [[package]]
-name = "wit-bindgen"
-version = "0.46.0"
+name = "wit-bindgen-rt"
+version = "0.39.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f17a85883d4e6d00e8a97c586de764dabcc06133f7f1d55dce5cdc070ad7fe59"
+checksum = "6f42320e61fe2cfd34354ecb597f86f413484a798ba44a8ca1165c58d42da6c1"
+dependencies = [
+ "bitflags",
+]
 
 [[package]]
 name = "writeable"
@@ -4266,18 +4231,18 @@ dependencies = [
 
 [[package]]
 name = "zerocopy"
-version = "0.8.27"
+version = "0.8.26"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "0894878a5fa3edfd6da3f88c4805f4c8558e2b996227a3d864f47fe11e38282c"
+checksum = "1039dd0d3c310cf05de012d8a39ff557cb0d23087fd44cad61df08fc31907a2f"
 dependencies = [
  "zerocopy-derive",
 ]
 
 [[package]]
 name = "zerocopy-derive"
-version = "0.8.27"
+version = "0.8.26"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "88d2b8d9c68ad2b9e4340d7832716a4d21a22a1154777ad56ea55c51a9cf3831"
+checksum = "9ecf5b4cc5364572d7f4c329661bcc82724222973f2cab6f050a4e5c22f75181"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -4354,7 +4319,7 @@ dependencies = [
  "bzip2",
  "crc32fast",
  "flate2",
- "indexmap 2.11.3",
+ "indexmap 2.10.0",
  "memchr",
  "zopfli",
  "zstd",
@@ -4398,9 +4363,9 @@ dependencies = [
 
 [[package]]
 name = "zstd-sys"
-version = "2.0.16+zstd.1.5.7"
+version = "2.0.15+zstd.1.5.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "91e19ebc2adc8f83e43039e79776e3fda8ca919132d68a1fed6a5faca2683748"
+checksum = "eb81183ddd97d0c74cedf1d50d85c8d08c1b8b68ee863bdee9e706eedba1a237"
 dependencies = [
  "cc",
  "pkg-config",
diff --git a/temporalio/ext/Cargo.toml b/temporalio/ext/Cargo.toml
index 73eb3475..285bc4e8 100644
--- a/temporalio/ext/Cargo.toml
+++ b/temporalio/ext/Cargo.toml
@@ -10,7 +10,9 @@ publish = false
 crate-type = ["cdylib"]
 
 [dependencies]
+async-trait = "0.1"
 futures = "0.3"
+log = "0.4"
 magnus = "0.7"
 parking_lot = "0.12"
 prost = "0.13"
diff --git a/temporalio/ext/sdk-core b/temporalio/ext/sdk-core
index b3acbe2e..d21a257e 160000
--- a/temporalio/ext/sdk-core
+++ b/temporalio/ext/sdk-core
@@ -1 +1 @@
-Subproject commit b3acbe2e83c2a2b798aeb3d0261d75a721988667
+Subproject commit d21a257e7bdd1e039e386f561a3c87b4efc317ce
diff --git a/temporalio/ext/src/lib.rs b/temporalio/ext/src/lib.rs
index 9979acc1..623c7c7f 100644
--- a/temporalio/ext/src/lib.rs
+++ b/temporalio/ext/src/lib.rs
@@ -46,6 +46,14 @@ macro_rules! id {
     }};
 }
 
+#[macro_export]
+macro_rules! lazy_id {
+    ($str:expr) => {{
+        static VAL: magnus::value::LazyId = magnus::value::LazyId::new($str);
+        &VAL
+    }};
+}
+
 #[magnus::init]
 fn init(ruby: &Ruby) -> Result<(), Error> {
     Lazy::force(&ROOT_ERR, ruby);
diff --git a/temporalio/ext/src/runtime.rs b/temporalio/ext/src/runtime.rs
index a2f01cb4..88efaee1 100644
--- a/temporalio/ext/src/runtime.rs
+++ b/temporalio/ext/src/runtime.rs
@@ -294,6 +294,29 @@ impl RuntimeHandle {
         });
     }
 
+    /// Same as spawn, but does not spawn in a background Tokio task and does not provide a
+    /// non-GVL awaitable. This simply submits the callback to the Ruby thread inline and
+    /// returns.
+    pub(crate) fn spawn_sync_inline<F>(&self, with_gvl: F)
+    where
+        F: FnOnce(Ruby) -> Result<(), Error> + Send + 'static,
+    {
+        // Ignore fail to send in rare case that the runtime/handle is
+        // dropped before this Tokio future runs
+        let _ = self
+            .async_command_tx
+            .clone()
+            .send(AsyncCommand::RunCallback(Box::new(
+                move || match Ruby::get() {
+                    Ok(ruby) => with_gvl(ruby),
+                    Err(err) => {
+                        log_error!("Unable to get Ruby instance in async callback: {}", err);
+                        Ok(())
+                    }
+                },
+            )));
+    }
+
     pub(crate) fn fork_check(&self, action: &'static str) -> Result<(), Error> {
         if self.pid != std::process::id() {
             Err(error!(
diff --git a/temporalio/ext/src/worker.rs b/temporalio/ext/src/worker.rs
index 2de1b489..3fba95eb 100644
--- a/temporalio/ext/src/worker.rs
+++ b/temporalio/ext/src/worker.rs
@@ -1,6 +1,7 @@
 use std::{
     cell::RefCell,
     collections::{HashMap, HashSet},
+    marker::PhantomData,
     sync::Arc,
     time::Duration,
 };
@@ -8,15 +9,20 @@ use std::{
 use crate::{
     ROOT_MOD,
     client::Client,
-    enter_sync, error, id, new_error,
+    enter_sync, error, id, lazy_id, new_error,
     runtime::{AsyncCommand, Runtime, RuntimeHandle},
-    util::{AsyncCallback, Struct},
+    util::{AsyncCallback, SendSyncBoxValue, Struct},
 };
 use futures::{StreamExt, stream::BoxStream};
 use futures::{future, stream};
 use magnus::{
-    DataTypeFunctions, Error, IntoValue, RArray, RString, RTypedData, Ruby, TypedData, Value,
-    class, function, method, prelude::*, typed_data,
+    DataTypeFunctions, Error, Exception, IntoValue, RArray, RClass, RModule, RString, RTypedData,
+    Ruby, TypedData, Value,
+    block::Proc,
+    class, function, method,
+    prelude::*,
+    typed_data,
+    value::{Lazy, LazyId},
 };
 use prost::Message;
 use temporal_sdk_core::{
@@ -27,14 +33,18 @@ use temporal_sdk_core::{
 use temporal_sdk_core_api::{
     errors::{PollError, WorkflowErrorType},
     worker::{
-        PollerBehavior, SlotKind, WorkerDeploymentOptions, WorkerDeploymentVersion,
-        WorkerVersioningStrategy,
+        PollerBehavior, SlotInfo, SlotInfoTrait, SlotKind, SlotKindType, SlotMarkUsedContext,
+        SlotReleaseContext, SlotReservationContext, SlotSupplier, SlotSupplierPermit,
+        WorkerDeploymentOptions, WorkerDeploymentVersion, WorkerVersioningStrategy,
     },
 };
 use temporal_sdk_core_protos::coresdk::workflow_completion::WorkflowActivationCompletion;
-use temporal_sdk_core_protos::coresdk::{ActivityHeartbeat, ActivityTaskCompletion};
+use temporal_sdk_core_protos::coresdk::{
+    ActivityHeartbeat, ActivitySlotInfo, ActivityTaskCompletion, LocalActivitySlotInfo,
+    NexusSlotInfo, WorkflowSlotInfo,
+};
 use temporal_sdk_core_protos::temporal::api::history::v1::History;
-use tokio::sync::mpsc::{Sender, channel};
+use tokio::sync::mpsc::{Sender, UnboundedSender, channel, unbounded_channel};
 use tokio_stream::wrappers::ReceiverStream;
 
 pub fn init(ruby: &Ruby) -> Result<(), Error> {
@@ -105,7 +115,7 @@ impl Worker {
 
         let worker = temporal_sdk_core::init_worker(
             &client.runtime_handle.core,
-            build_config(options)?,
+            build_config(options, &client.runtime_handle)?,
             client.core.clone().into_inner(),
         )
         .map_err(|err| error!("Failed creating worker: {}", err))?;
@@ -419,7 +429,7 @@ impl WorkflowReplayer {
         let (tx, rx) = channel(1);
 
         let core_worker = temporal_sdk_core::init_replay_worker(ReplayWorkerInput::new(
-            build_config(options)?,
+            build_config(options, &runtime.handle)?,
             ReceiverStream::new(rx),
         ))
         .map_err(|err| error!("Failed creating worker: {}", err))?;
@@ -450,7 +460,7 @@ impl WorkflowReplayer {
     }
 }
 
-fn build_config(options: Struct) -> Result<WorkerConfig, Error> {
+fn build_config(options: Struct, runtime_handle: &RuntimeHandle) -> Result<WorkerConfig, Error> {
     WorkerConfigBuilder::default()
         .namespace(options.member::<String>(id!("namespace"))?)
         .task_queue(options.member::<String>(id!("task_queue"))?)
@@ -522,6 +532,7 @@ fn build_config(options: Struct) -> Result<WorkerConfig, Error> {
             options
                 .child(id!("tuner"))?
                 .ok_or_else(|| error!("Missing tuner"))?,
+            runtime_handle,
         )?))
         .workflow_failure_errors(
             if options.member::<bool>(id!("nondeterminism_as_workflow_fail"))? {
@@ -541,24 +552,39 @@ fn build_config(options: Struct) -> Result<WorkerConfig, Error> {
         .map_err(|err| error!("Invalid worker options: {}", err))
 }
 
-fn build_tuner(options: Struct) -> Result<TunerHolder, Error> {
+fn extract_poller_behavior(poller_behavior: Struct) -> Result<PollerBehavior, Error> {
+    Ok(if poller_behavior.member::<usize>(id!("initial")).is_ok() {
+        PollerBehavior::Autoscaling {
+            minimum: poller_behavior.member::<usize>(id!("minimum"))?,
+            maximum: poller_behavior.member::<usize>(id!("maximum"))?,
+            initial: poller_behavior.member::<usize>(id!("initial"))?,
+        }
+    } else {
+        PollerBehavior::SimpleMaximum(poller_behavior.member::<usize>(id!("simple_maximum"))?)
+    })
+}
+
+fn build_tuner(options: Struct, runtime_handle: &RuntimeHandle) -> Result<TunerHolder, Error> {
     let (workflow_slot_options, resource_slot_options) = build_tuner_slot_options(
         options
             .child(id!("workflow_slot_supplier"))?
             .ok_or_else(|| error!("Missing workflow slot options"))?,
         None,
+        runtime_handle,
     )?;
     let (activity_slot_options, resource_slot_options) = build_tuner_slot_options(
         options
             .child(id!("activity_slot_supplier"))?
             .ok_or_else(|| error!("Missing activity slot options"))?,
         resource_slot_options,
+        runtime_handle,
     )?;
     let (local_activity_slot_options, resource_slot_options) = build_tuner_slot_options(
         options
             .child(id!("local_activity_slot_supplier"))?
             .ok_or_else(|| error!("Missing local activity slot options"))?,
         resource_slot_options,
+        runtime_handle,
     )?;
 
     let mut opts_build = TunerHolderOptionsBuilder::default();
@@ -575,16 +601,21 @@ fn build_tuner(options: Struct) -> Result<TunerHolder, Error> {
         .map_err(|err| error!("Failed building tuner options: {}", err))
 }
 
-fn build_tuner_slot_options<SK: SlotKind>(
+fn build_tuner_slot_options<SK: SlotKind + Send + Sync + 'static>(
     options: Struct,
     prev_slots_options: Option<ResourceBasedSlotsOptions>,
+    runtime_handle: &RuntimeHandle,
 ) -> Result<(SlotSupplierOptions<SK>, Option<ResourceBasedSlotsOptions>), Error> {
     if let Some(slots) = options.member::<Option<usize>>(id!("fixed_size"))? {
         Ok((SlotSupplierOptions::FixedSize { slots }, prev_slots_options))
     } else if let Some(resource) = options.child(id!("resource_based"))? {
         build_tuner_resource_options(resource, prev_slots_options)
+    } else if let Some(custom) = options.member::<Option<Value>>(id!("custom"))? {
+        build_custom_slot_supplier(custom, runtime_handle).map(|v| (v, None))
     } else {
-        Err(error!("Slot supplier must be fixed size or resource based"))
+        Err(error!(
+            "Slot supplier must be fixed size, resource based, or custom"
+        ))
     }
 }
 
@@ -615,14 +646,557 @@ fn build_tuner_resource_options<SK: SlotKind>(
     ))
 }
 
-fn extract_poller_behavior(poller_behavior: Struct) -> Result<PollerBehavior, Error> {
-    Ok(if poller_behavior.member::<usize>(id!("initial")).is_ok() {
-        PollerBehavior::Autoscaling {
-            minimum: poller_behavior.member::<usize>(id!("minimum"))?,
-            maximum: poller_behavior.member::<usize>(id!("maximum"))?,
-            initial: poller_behavior.member::<usize>(id!("initial"))?,
+fn build_custom_slot_supplier<SK: SlotKind + Send + Sync + 'static>(
+    supplier: Value,
+    runtime_handle: &RuntimeHandle,
+) -> Result<SlotSupplierOptions<SK>, Error> {
+    Ok(SlotSupplierOptions::Custom(Arc::new(CustomSlotSupplier::<
+        SK,
+    > {
+        supplier: Arc::new(SendSyncBoxValue::new(supplier)),
+        runtime_handle: runtime_handle.clone(),
+        _phantom: PhantomData,
+    })))
+}
+
+struct CustomSlotSupplier<SK: SlotKind> {
+    supplier: Arc<SendSyncBoxValue<Value>>,
+    runtime_handle: RuntimeHandle,
+    _phantom: PhantomData<SK>,
+}
+
+struct CancelGuard<F: FnOnce()> {
+    f: Option<F>,
+}
+
+impl<F: FnOnce()> CancelGuard<F> {
+    fn new(f: F) -> Self {
+        Self { f: Some(f) }
+    }
+
+    fn defuse(mut self) {
+        self.f.take();
+    }
+}
+
+impl<F: FnOnce()> Drop for CancelGuard<F> {
+    fn drop(&mut self) {
+        if let Some(f) = self.f.take() {
+            f();
+        }
+    }
+}
+
+#[async_trait::async_trait]
+impl<SK: SlotKind + Send + Sync> SlotSupplier for CustomSlotSupplier<SK> {
+    type SlotKind = SK;
+
+    async fn reserve_slot(&self, ctx: &dyn SlotReservationContext) -> SlotSupplierPermit {
+        // Do this in an infinite loop until we get success (log and sleep on error)
+        let ctx = SlotReserveContextHolder::new(SK::kind(), ctx);
+        loop {
+            match self.attempt_reserve_slot(ctx.clone()).await {
+                Ok(permit) => {
+                    return SlotSupplierPermit::with_user_data(Arc::new(permit));
+                }
+                Err(err) => {
+                    log::error!("Slot reserve failed: {err}");
+                    tokio::time::sleep(std::time::Duration::from_secs(1)).await
+                }
+            }
         }
+    }
+
+    fn try_reserve_slot(&self, ctx: &dyn SlotReservationContext) -> Option<SlotSupplierPermit> {
+        let res = self.call_supplier_sync(
+            lazy_id!("try_reserve_slot"),
+            SlotReserveContextHolder::new(SK::kind(), ctx),
+            |v| {
+                if v.is_nil() {
+                    None
+                } else {
+                    Some(SendSyncBoxValue::new(v))
+                }
+            },
+        );
+        match res {
+            Ok(v) => v.map(|v| SlotSupplierPermit::with_user_data(Arc::new(v))),
+            Err(err) => {
+                log::error!("Slot try-reserve failed: {err}");
+                None
+            }
+        }
+    }
+
+    fn mark_slot_used(&self, ctx: &dyn SlotMarkUsedContext<SlotKind = Self::SlotKind>) {
+        let res = self.call_supplier_sync(
+            lazy_id!("mark_slot_used"),
+            SlotMarkUsedContextHolder::new(ctx),
+            |_| (),
+        );
+        if let Err(err) = res {
+            log::error!("Mark slot used failed: {err}");
+        }
+    }
+
+    fn release_slot(&self, ctx: &dyn SlotReleaseContext<SlotKind = Self::SlotKind>) {
+        let res = self.call_supplier_sync(
+            lazy_id!("release_slot"),
+            SlotReleaseContextHolder::new(ctx),
+            |_| (),
+        );
+        if let Err(err) = res {
+            log::error!("Mark slot used failed: {err}");
+        }
+    }
+}
+
+impl<SK: SlotKind> CustomSlotSupplier<SK> {
+    async fn attempt_reserve_slot(
+        &self,
+        ctx: SlotReserveContextHolder,
+    ) -> Result<SendSyncBoxValue<Value>, String> {
+        // Make a call with channels getting info
+        let (result_tx, mut result_rx) = unbounded_channel();
+        let (cancel_proc_ready_tx, mut cancel_proc_ready_rx) = unbounded_channel();
+        let result_err_tx = result_tx.clone();
+        self.start_supplier_call(
+            lazy_id!("reserve_slot"),
+            ctx,
+            move |_, val| {
+                let _ = result_tx.send(Ok(SendSyncBoxValue::new(val)));
+            },
+            move |err| {
+                let _ = result_err_tx.send(Err(err));
+            },
+            Some(cancel_proc_ready_tx),
+        );
+
+        // Wait for cancel proc ready or result. Note, we may not get a cancel proc in the case
+        // that its tx is dropped before the result tx is recorded, so we just ignore and move on.
+        let (cancel_proc, result) = tokio::select! {
+            cancel_proc = cancel_proc_ready_rx.recv() => (cancel_proc, None),
+            result = result_rx.recv() => (None, Some(result.ok_or_else(|| "Unable to get result".to_string()))),
+        };
+        // If we got a result, return it
+        if let Some(result) = result {
+            return result.flatten();
+        }
+
+        // Now that we have a cancel proc, we can setup the guard
+        let runtime_handle = self.runtime_handle.clone();
+        let cancel_guard = cancel_proc.map(move |cancel_proc| {
+            CancelGuard::new(move || {
+                runtime_handle.spawn(async {}, move |ruby, _| {
+                    // TODO(cretz): Should do this in a Ruby thread to not block main Ruby
+                    // reactor loop?
+                    let cancel_proc = cancel_proc.value(&ruby);
+                    if let Err(err) = cancel_proc.call::<_, Value>(()) {
+                        log::error!("Failed canceling: {err}");
+                    }
+                    Ok(())
+                });
+            })
+        });
+
+        // Wait for result
+        let result: Result<SendSyncBoxValue<Value>, String> = result_rx
+            .recv()
+            .await
+            .ok_or_else(|| "Unable to get result".to_string())
+            .flatten();
+
+        // Defuse guard (we don't want it to run cancel proc), and return
+        if let Some(cancel_guard) = cancel_guard {
+            cancel_guard.defuse();
+        }
+        result
+    }
+
+    fn call_supplier_sync<R: Send + 'static>(
+        &self,
+        method: &'static LazyId,
+        ctx: impl TryIntoValue + Send + 'static,
+        map_result: impl Fn(Value) -> R + Send + 'static,
+    ) -> Result<R, String> {
+        // Make call
+        let (result_tx, result_rx) = std::sync::mpsc::channel();
+        let result_err_tx = result_tx.clone();
+        self.start_supplier_call(
+            method,
+            ctx,
+            move |_, val| {
+                let _ = result_tx.send(Ok(map_result(val)));
+            },
+            move |err| {
+                let _ = result_err_tx.send(Err(err));
+            },
+            None,
+        );
+
+        // Wait for result. We recognize this blocks, but we expect users not to block. We cannot
+        // use Tokio's blocking_recv or block_on because we're already in a Tokio runtime and so
+        // they panic. We also can't use tokio::task::block_in_place because this is sometimes
+        // started on a non-Tokio-started thread from a workflow context, and block_in_place fails
+        // even in legitimate Tokio contexts if the thread wasn't started by Tokio. So, we just wait
+        // and block this thread, but we have told users they should not block on these sync calls.
+        result_rx
+            .recv()
+            .map_err(|err| format!("Call canceled: {err}"))
+            .flatten()
+    }
+
+    fn start_supplier_call(
+        &self,
+        method: &'static LazyId,
+        ctx: impl TryIntoValue + Send + 'static,
+        on_success: impl Fn(&Ruby, Value) + Send + 'static,
+        on_error: impl Fn(String) + Clone + Send + 'static, // May be called multiple times
+        cancel_proc_ready_tx: Option<UnboundedSender<SendSyncBoxValue<Proc>>>,
+    ) {
+        // Run in Ruby. We have to use spawn_sync_inline instead of spawn here because spawn starts
+        // a new Tokio task and we cannot always start a new Tokio task because we're not always in
+        // a multi-threaded situation where it can be waited on.
+        let supplier = self.supplier.clone();
+        self.runtime_handle.spawn_sync_inline(move |ruby| {
+            if let Err(err) = start_supplier_call_in_ruby(
+                ruby,
+                supplier,
+                method,
+                ctx,
+                on_success,
+                on_error.clone(),
+                cancel_proc_ready_tx,
+            ) {
+                // May have already sent a result here, but it's harmless to try
+                on_error(format!("Raised exception: {err}"));
+            }
+            Ok(())
+        });
+    }
+}
+
+fn start_supplier_call_in_ruby(
+    ruby: Ruby,
+    supplier: Arc<SendSyncBoxValue<Value>>,
+    method: &'static LazyId,
+    ctx: impl TryIntoValue,
+    on_success: impl Fn(&Ruby, Value) + Send + 'static,
+    on_error: impl Fn(String) + Send + 'static,
+    cancel_proc_ready_tx: Option<UnboundedSender<SendSyncBoxValue<Proc>>>,
+) -> Result<(), Error> {
+    // Context
+    let method = LazyId::get_inner_with(method, &ruby);
+    let ctx = ctx.try_into_value(&ruby)?;
+
+    // Put functions in options so we can take it just once in closure
+    let mut on_success = Some(on_success);
+    let mut on_error = Some(on_error);
+
+    // Create proc for callback
+    let proc = ruby.proc_from_fn(move |ruby, args, _block| {
+        let result = args
+            .first()
+            .copied()
+            .unwrap_or_else(|| ruby.qnil().as_value());
+        // If it's an exception, that's an error, otherwise it's a success
+        if let Some(err) = Exception::from_value(result) {
+            if let Some(on_error) = on_error.take() {
+                on_error(format!("Raised exception: {err}"));
+            }
+        } else if let Some(on_success) = on_success.take() {
+            on_success(ruby, result);
+        } else {
+            log::error!("Custom slot supplier callback unexpectedly invoked multiple times");
+        }
+    });
+
+    // Build arg set. If there is a cancel_proc_ready, we create a cancellation and provide it to
+    // the sender.
+    let mut maybe_cancellation = None;
+    if let Some(cancel_proc_ready_tx) = cancel_proc_ready_tx {
+        // Create cancellation and provide cancellation as second arg and send back cancel proc
+        let cancellation = ruby
+            .get_inner(&CANCELLATION_CLASS)
+            .funcall(id!("new"), ())?;
+        let cancel_proc = RArray::to_ary(cancellation)?.entry::<Value>(1)?;
+        let _ = cancel_proc_ready_tx.send(SendSyncBoxValue::new(
+            Proc::from_value(cancel_proc).expect("Expecting proc"),
+        ));
+        maybe_cancellation = Some(cancellation)
+    };
+
+    // Call custom supplier with the block, it can be a two-arg form with context or one-arg w/out
+    let supplier = supplier.value(&ruby);
+    if let Some(cancellation) = maybe_cancellation {
+        supplier.funcall_with_block::<_, _, Value>(method, (ctx, cancellation), proc)?;
     } else {
-        PollerBehavior::SimpleMaximum(poller_behavior.member::<usize>(id!("simple_maximum"))?)
-    })
+        supplier.funcall_with_block::<_, _, Value>(method, (ctx,), proc)?;
+    }
+    Ok(())
+}
+
+pub static CANCELLATION_CLASS: Lazy<RClass> = Lazy::new(|ruby| {
+    ruby.class_object()
+        .const_get::<_, RModule>("Temporalio")
+        .unwrap()
+        .const_get::<_, RClass>("Cancellation")
+        .unwrap()
+});
+
+pub static CUSTOM_CLASS: Lazy<RClass> = Lazy::new(|ruby| {
+    ruby.class_object()
+        .const_get::<_, RModule>("Temporalio")
+        .unwrap()
+        .const_get::<_, RClass>("Worker")
+        .unwrap()
+        .const_get::<_, RClass>("Tuner")
+        .unwrap()
+        .const_get::<_, RClass>("SlotSupplier")
+        .unwrap()
+        .const_get::<_, RClass>("Custom")
+        .unwrap()
+});
+
+pub static RESERVE_CONTEXT_CLASS: Lazy<RClass> = Lazy::new(|ruby| {
+    ruby.get_inner(&CUSTOM_CLASS)
+        .const_get::<_, RClass>("ReserveContext")
+        .unwrap()
+});
+
+pub static MARK_USED_CONTEXT_CLASS: Lazy<RClass> = Lazy::new(|ruby| {
+    ruby.get_inner(&CUSTOM_CLASS)
+        .const_get::<_, RClass>("MarkUsedContext")
+        .unwrap()
+});
+
+pub static RELEASE_CONTEXT_CLASS: Lazy<RClass> = Lazy::new(|ruby| {
+    ruby.get_inner(&CUSTOM_CLASS)
+        .const_get::<_, RClass>("ReleaseContext")
+        .unwrap()
+});
+
+pub static SLOT_INFO_MODULE: Lazy<RModule> = Lazy::new(|ruby| {
+    ruby.get_inner(&CUSTOM_CLASS)
+        .const_get::<_, RModule>("SlotInfo")
+        .unwrap()
+});
+
+pub static SLOT_INFO_WORKFLOW_CLASS: Lazy<RClass> = Lazy::new(|ruby| {
+    ruby.get_inner(&SLOT_INFO_MODULE)
+        .const_get::<_, RClass>("Workflow")
+        .unwrap()
+});
+
+pub static SLOT_INFO_ACTIVITY_CLASS: Lazy<RClass> = Lazy::new(|ruby| {
+    ruby.get_inner(&SLOT_INFO_MODULE)
+        .const_get::<_, RClass>("Activity")
+        .unwrap()
+});
+
+pub static SLOT_INFO_LOCAL_ACTIVITY_CLASS: Lazy<RClass> = Lazy::new(|ruby| {
+    ruby.get_inner(&SLOT_INFO_MODULE)
+        .const_get::<_, RClass>("LocalActivity")
+        .unwrap()
+});
+
+pub static SLOT_INFO_NEXUS_CLASS: Lazy<RClass> = Lazy::new(|ruby| {
+    ruby.get_inner(&SLOT_INFO_MODULE)
+        .const_get::<_, RClass>("Nexus")
+        .unwrap()
+});
+
+trait TryIntoValue {
+    fn try_into_value(self, ruby: &Ruby) -> Result<Value, Error>;
+}
+
+#[derive(Clone)]
+struct SlotReserveContextHolder {
+    slot_kind: SlotKindType,
+    task_queue: String,
+    worker_identity: String,
+    worker_deployment_name: String,
+    worker_build_id: String,
+    is_sticky: bool,
+}
+
+impl SlotReserveContextHolder {
+    fn new(slot_kind: SlotKindType, ctx: &dyn SlotReservationContext) -> Self {
+        Self {
+            slot_kind,
+            task_queue: ctx.task_queue().to_string(),
+            worker_identity: ctx.worker_identity().to_string(),
+            worker_deployment_name: ctx
+                .worker_deployment_version()
+                .clone()
+                .map(|v| v.deployment_name)
+                .unwrap_or_default(),
+            worker_build_id: ctx
+                .worker_deployment_version()
+                .clone()
+                .map(|v| v.build_id)
+                .unwrap_or_default(),
+            is_sticky: ctx.is_sticky(),
+        }
+    }
+}
+
+impl TryIntoValue for SlotReserveContextHolder {
+    fn try_into_value(self, ruby: &Ruby) -> Result<Value, Error> {
+        ruby.get_inner(&RESERVE_CONTEXT_CLASS).funcall(
+            id!("new"),
+            (
+                // Slot type
+                match self.slot_kind {
+                    SlotKindType::Workflow => id!("workflow"),
+                    SlotKindType::Activity => id!("activity"),
+                    SlotKindType::LocalActivity => id!("local_activity"),
+                    SlotKindType::Nexus => id!("nexus"),
+                },
+                // Task queue
+                self.task_queue,
+                // Worker identity
+                self.worker_identity,
+                // Worker deployment name
+                self.worker_deployment_name,
+                // Worker build ID
+                self.worker_build_id,
+                // Sticky
+                self.is_sticky,
+            ),
+        )
+    }
+}
+
+struct SlotMarkUsedContextHolder {
+    slot_info: SlotInfoHolder,
+    permit: Option<Arc<SendSyncBoxValue<Value>>>,
+}
+
+impl SlotMarkUsedContextHolder {
+    fn new<SK: SlotKind>(ctx: &dyn SlotMarkUsedContext<SlotKind = SK>) -> Self {
+        Self {
+            slot_info: SlotInfoHolder::new(ctx.info().downcast()),
+            permit: ctx
+                .permit()
+                .user_data::<Arc<SendSyncBoxValue<Value>>>()
+                .cloned(),
+        }
+    }
+}
+
+impl TryIntoValue for SlotMarkUsedContextHolder {
+    fn try_into_value(self, ruby: &Ruby) -> Result<Value, Error> {
+        ruby.get_inner(&MARK_USED_CONTEXT_CLASS).funcall(
+            id!("new"),
+            (
+                // Slot info
+                self.slot_info.try_into_value(ruby)?,
+                // Permit
+                self.permit
+                    .map(|v| v.value(ruby))
+                    .unwrap_or_else(|| ruby.qnil().as_value()),
+            ),
+        )
+    }
+}
+
+struct SlotReleaseContextHolder {
+    slot_info: Option<SlotInfoHolder>,
+    permit: Option<Arc<SendSyncBoxValue<Value>>>,
+}
+
+impl SlotReleaseContextHolder {
+    fn new<SK: SlotKind>(ctx: &dyn SlotReleaseContext<SlotKind = SK>) -> Self {
+        Self {
+            slot_info: ctx.info().map(|i| SlotInfoHolder::new(i.downcast())),
+            permit: ctx
+                .permit()
+                .user_data::<Arc<SendSyncBoxValue<Value>>>()
+                .cloned(),
+        }
+    }
+}
+
+impl TryIntoValue for SlotReleaseContextHolder {
+    fn try_into_value(self, ruby: &Ruby) -> Result<Value, Error> {
+        ruby.get_inner(&RELEASE_CONTEXT_CLASS).funcall(
+            id!("new"),
+            (
+                // Slot info
+                if let Some(slot_info) = self.slot_info {
+                    Some(slot_info.try_into_value(ruby)?)
+                } else {
+                    None
+                },
+                // Permit
+                self.permit
+                    .map(|v| v.value(ruby))
+                    .unwrap_or_else(|| ruby.qnil().as_value()),
+            ),
+        )
+    }
+}
+
+enum SlotInfoHolder {
+    Workflow(WorkflowSlotInfo),
+    Activity(ActivitySlotInfo),
+    LocalActivity(LocalActivitySlotInfo),
+    Nexus(NexusSlotInfo),
+}
+
+impl SlotInfoHolder {
+    fn new(slot_info: SlotInfo) -> Self {
+        match slot_info {
+            SlotInfo::Workflow(v) => SlotInfoHolder::Workflow(v.clone()),
+            SlotInfo::Activity(v) => SlotInfoHolder::Activity(v.clone()),
+            SlotInfo::LocalActivity(v) => SlotInfoHolder::LocalActivity(v.clone()),
+            SlotInfo::Nexus(v) => SlotInfoHolder::Nexus(v.clone()),
+        }
+    }
+}
+
+impl TryIntoValue for SlotInfoHolder {
+    fn try_into_value(self, ruby: &Ruby) -> Result<Value, Error> {
+        match self {
+            SlotInfoHolder::Workflow(v) => {
+                ruby.get_inner(&SLOT_INFO_WORKFLOW_CLASS).funcall(
+                    id!("new"),
+                    (
+                        // Workflow type
+                        v.workflow_type,
+                        // Sticky
+                        v.is_sticky,
+                    ),
+                )
+            }
+            SlotInfoHolder::Activity(v) => {
+                ruby.get_inner(&SLOT_INFO_ACTIVITY_CLASS).funcall(
+                    id!("new"),
+                    (
+                        // Activity type
+                        v.activity_type,
+                    ),
+                )
+            }
+            SlotInfoHolder::LocalActivity(v) => {
+                ruby.get_inner(&SLOT_INFO_LOCAL_ACTIVITY_CLASS).funcall(
+                    id!("new"),
+                    (
+                        // Activity type
+                        v.activity_type,
+                    ),
+                )
+            }
+            SlotInfoHolder::Nexus(v) => ruby.get_inner(&SLOT_INFO_NEXUS_CLASS).funcall(
+                id!("new"),
+                (
+                    // Service
+                    v.service,
+                    // Operation
+                    v.operation,
+                ),
+            ),
+        }
+    }
 }
diff --git a/temporalio/lib/temporalio/internal/bridge/worker.rb b/temporalio/lib/temporalio/internal/bridge/worker.rb
index 1677edd0..a890033e 100644
--- a/temporalio/lib/temporalio/internal/bridge/worker.rb
+++ b/temporalio/lib/temporalio/internal/bridge/worker.rb
@@ -40,6 +40,7 @@ class Worker
         TunerSlotSupplierOptions = Struct.new(
           :fixed_size,
           :resource_based,
+          :custom,
           keyword_init: true
         )
 
@@ -103,6 +104,55 @@ def complete_activity_task_in_background(proto)
           # TODO(cretz): Log error on this somehow?
           async_complete_activity_task(proto.to_proto, queue)
         end
+
+        class CustomSlotSupplier
+          def initialize(slot_supplier:, thread_pool:)
+            @slot_supplier = slot_supplier
+            @thread_pool = thread_pool
+          end
+
+          def reserve_slot(context, cancellation, &block)
+            run_user_code do
+              @slot_supplier.reserve_slot(context, cancellation) { |v| block.call(v) }
+            rescue Exception => e # rubocop:disable Lint/RescueException
+              block.call(e)
+            end
+          end
+
+          def try_reserve_slot(context, &block)
+            run_user_code do
+              block.call(@slot_supplier.try_reserve_slot(context))
+            rescue Exception => e # rubocop:disable Lint/RescueException
+              block.call(e)
+            end
+          end
+
+          def mark_slot_used(context, &block)
+            run_user_code do
+              block.call(@slot_supplier.mark_slot_used(context))
+            rescue Exception => e # rubocop:disable Lint/RescueException
+              block.call(e)
+            end
+          end
+
+          def release_slot(context, &block)
+            run_user_code do
+              block.call(@slot_supplier.release_slot(context))
+            rescue Exception => e # rubocop:disable Lint/RescueException
+              block.call(e)
+            end
+          end
+
+          private
+
+          def run_user_code(&)
+            if @thread_pool
+              @thread_pool.execute(&)
+            else
+              yield
+            end
+          end
+        end
       end
     end
   end
diff --git a/temporalio/lib/temporalio/worker/tuner.rb b/temporalio/lib/temporalio/worker/tuner.rb
index 60f8916b..42f208aa 100644
--- a/temporalio/lib/temporalio/worker/tuner.rb
+++ b/temporalio/lib/temporalio/worker/tuner.rb
@@ -22,10 +22,11 @@ def initialize(slots) # rubocop:disable Lint/MissingSuper
           end
 
           # @!visibility private
-          def _to_bridge_options
+          def _to_bridge_options(_tuner)
             Internal::Bridge::Worker::TunerSlotSupplierOptions.new(
               fixed_size: slots,
-              resource_based: nil
+              resource_based: nil,
+              custom: nil
             )
           end
         end
@@ -36,7 +37,7 @@ def _to_bridge_options
         class ResourceBased < SlotSupplier
           attr_reader :tuner_options, :slot_options
 
-          # Create a reosurce-based slot supplier.
+          # Create a resource-based slot supplier.
           #
           # @param tuner_options [ResourceBasedTunerOptions] General tuner options.
           # @param slot_options [ResourceBasedSlotOptions] Slot-supplier-specific tuner options.
@@ -46,7 +47,7 @@ def initialize(tuner_options:, slot_options:) # rubocop:disable Lint/MissingSupe
           end
 
           # @!visibility private
-          def _to_bridge_options
+          def _to_bridge_options(_tuner)
             Internal::Bridge::Worker::TunerSlotSupplierOptions.new(
               fixed_size: nil,
               resource_based: Internal::Bridge::Worker::TunerResourceBasedSlotSupplierOptions.new(
@@ -55,14 +56,175 @@ def _to_bridge_options
                 min_slots: slot_options.min_slots,
                 max_slots: slot_options.max_slots,
                 ramp_throttle: slot_options.ramp_throttle
+              ),
+              custom: nil
+            )
+          end
+        end
+
+        # A slot supplier that has callbacks invoked to handle slot supplying.
+        #
+        # Users should be cautious when implementing this and make sure it is heavily tested and the documentation for
+        # every method is well understood.
+        #
+        # @note WARNING: This API is experimental.
+        class Custom < SlotSupplier
+          # Context provided for slot reservation on custom slot supplier.
+          #
+          # @!attribute slot_type
+          #   @return [:workflow, :activity, :local_activity, :nexus] Slot type.
+          # @!attribute task_queue
+          #   @return [String] Task queue.
+          # @!attribute worker_identity
+          #   @return [String] Worker identity.
+          # @!attribute worker_deployment_name
+          #   @return [String] Worker deployment name or empty string if not applicable.
+          # @!attribute worker_build_id
+          #   @return [String] Worker build ID or empty string if not applicable.
+          # @!attribute sticky?
+          #   @return [Boolean] True if this reservation is for a sticky workflow task.
+          ReserveContext = Data.define(
+            :slot_type,
+            :task_queue,
+            :worker_identity,
+            :worker_deployment_name,
+            :worker_build_id,
+            :sticky?
+          )
+
+          # Context provided for marking a slot used.
+          #
+          # @!attribute slot_info
+          #   @return [SlotInfo::Workflow, SlotInfo::Activity, SlotInfo::LocalActivity, SlotInfo::Nexus] Information
+          #     about the slot. This is never nil.
+          # @!attribute permit
+          #   @return [Object] Object that was provided as the permit on reserve.
+          MarkUsedContext = Data.define(
+            :slot_info,
+            :permit
+          )
+
+          # Context provided for releasing a slot.
+          #
+          # @!attribute slot_info
+          #   @return [SlotInfo::Workflow, SlotInfo::Activity, SlotInfo::LocalActivity, SlotInfo::Nexus, nil]
+          #     Information about the slot. This may be nil if the slot was never used.
+          # @!attribute permit
+          #   @return [Object] Object that was provided as the permit on reserve.
+          ReleaseContext = Data.define(
+            :slot_info,
+            :permit
+          )
+
+          # Reserve a slot.
+          #
+          # This can/should block and must provide the permit to the (code) block. The permit is any object (including
+          # nil) that will be given on the context for mark_slot_used and release_slot.
+          #
+          # Just returning from this call is not enough to reserve the slot, a permit must be provided to the block
+          # (e.g. via yield or block.call). If the call completes, the system will still wait on the block (so this can
+          # be backgrounded by passing the block to something else). Reservations may be canceled via the given
+          # cancellation. Users can do things like add_cancel_callback, but it is very important that the code in the
+          # callback is fast as it is run on the same reactor thread as many other Temporal Ruby worker calls.
+          #
+          # @note WARNING: This call should never raise an exception. Any exception raised is ignored and this is called
+          #   again after 1 second.
+          #
+          # @param context [ReserveContext] Contextual information about this reserve call.
+          # @param cancellation [Cancellation] Cancellation that is canceled when the reservation is no longer being
+          #   asked for.
+          # @yield [Object] Confirm reservation and provide a permit.
+          def reserve_slot(context, cancellation, &)
+            raise NotImplementedError
+          end
+
+          # Try to reserve a slot.
+          #
+          # @note WARNING: This should never block, this should return immediately with a permit, or nil if the
+          #   reservation could not occur.
+          #
+          # @note WARNING: This call should never raise an exception. Any exception raised is ignored and the slot
+          #   reservation attempt fails (i.e. same as if this method returned nil).
+          #
+          # @param context [ReserveContext] Contextual information about this reserve call.
+          # @return [Object, nil] A non-nil object to perform the reservation successfully, a nil to fail the
+          #   reservation.
+          def try_reserve_slot(context)
+            raise NotImplementedError
+          end
+
+          # Mark a slot as used.
+          #
+          # Due to the nature of Temporal polling, slots are reserved before they are used and may never get used. This
+          # call is made as just a notification when a slot is actually used.
+          #
+          # @note WARNING: This should never block, this should return immediately.
+          #
+          # @note WARNING: This call should never raise an exception. Any exception raised is ignored.
+          #
+          # @param context [MarkUsedContext] Contextual information about this reserve call.
+          def mark_slot_used(context)
+            raise NotImplementedError
+          end
+
+          # Release a previously reserved slot.
+          #
+          # @note WARNING: This should never block, this should return immediately.
+          #
+          # @note WARNING: This call should never raise an exception. Any exception raised is ignored.
+          #
+          # @param context [ReleaseContext] Contextual information about this reserve call.
+          def release_slot(context)
+            raise NotImplementedError
+          end
+
+          # @!visibility private
+          def _to_bridge_options(tuner)
+            Internal::Bridge::Worker::TunerSlotSupplierOptions.new(
+              fixed_size: nil,
+              resource_based: nil,
+              custom: Internal::Bridge::Worker::CustomSlotSupplier.new(
+                slot_supplier: self,
+                thread_pool: tuner.custom_slot_supplier_thread_pool
               )
             )
           end
+
+          # Slot information.
+          module SlotInfo
+            # Information about a workflow slot.
+            #
+            # @!attribute workflow_type
+            #   @return [String] Workflow type.
+            # @!attribute sticky?
+            #   @return [Boolean] Whether the slot was for a sticky task.
+            Workflow = Data.define(:workflow_type, :sticky?)
+
+            # Information about an activity slot.
+            #
+            # @!attribute activity_type
+            #   @return [String] Activity type.
+            Activity = Data.define(:activity_type)
+
+            # Information about a local activity slot.
+            #
+            # @!attribute activity_type
+            #   @return [String] Activity type.
+            LocalActivity = Data.define(:activity_type)
+
+            # Information about a Nexus slot.
+            #
+            # @!attribute service
+            #   @return [String] Nexus service.
+            # @!attribute operation
+            #   @return [String] Nexus operation.
+            Nexus = Data.define(:service, :operation)
+          end
         end
 
         # @!visibility private
-        def _to_bridge_options
-          raise ArgumentError, 'Tuner slot suppliers must be instances of Fixed or ResourceBased'
+        def _to_bridge_options(_tuner)
+          raise ArgumentError, 'Tuner slot suppliers must be instances of Fixed, ResourceBased, or Custom'
         end
       end
 
@@ -75,10 +237,9 @@ def _to_bridge_options
       # @!attribute target_cpu_usage
       #   @return [Float] A value between 0 and 1 that represents the target (system) CPU usage. This can be set to 1.0
       #     if desired, but it's recommended to leave some headroom for other processes.
-      ResourceBasedTunerOptions = Struct.new(
+      ResourceBasedTunerOptions = Data.define(
         :target_memory_usage,
-        :target_cpu_usage,
-        keyword_init: true
+        :target_cpu_usage
       )
 
       # Options for a specific slot type being used with {SlotSupplier::ResourceBased}.
@@ -94,11 +255,10 @@ def _to_bridge_options
       #
       #     This value matters because how many resources a task will use cannot be determined ahead of time, and thus
       #     the system should wait to see how much resources are used before issuing more slots.
-      ResourceBasedSlotOptions = Struct.new(
+      ResourceBasedSlotOptions = Data.define(
         :min_slots,
         :max_slots,
-        :ramp_throttle,
-        keyword_init: true
+        :ramp_throttle
       )
 
       # Create a fixed-size tuner with the provided number of slots.
@@ -161,27 +321,36 @@ def self.create_resource_based(
       # @return [SlotSupplier] Slot supplier for local activities.
       attr_reader :local_activity_slot_supplier
 
+      # @return [ThreadPool, nil] Thread pool for custom slot suppliers.
+      attr_reader :custom_slot_supplier_thread_pool
+
       # Create a tuner from 3 slot suppliers.
       #
       # @param workflow_slot_supplier [SlotSupplier] Slot supplier for workflows.
       # @param activity_slot_supplier [SlotSupplier] Slot supplier for activities.
       # @param local_activity_slot_supplier [SlotSupplier] Slot supplier for local activities.
+      # @param custom_slot_supplier_thread_pool [ThreadPool, nil] Thread pool to make all custom slot supplier calls on.
+      #   If there are no custom slot suppliers, this parameter is ignored. Technically users may set this to nil which
+      #   will not use a thread pool to make slot supplier calls, but that is dangerous and not advised because even the
+      #   slightest blocking call can slow down the system.
       def initialize(
         workflow_slot_supplier:,
         activity_slot_supplier:,
-        local_activity_slot_supplier:
+        local_activity_slot_supplier:,
+        custom_slot_supplier_thread_pool: ThreadPool.default
       )
         @workflow_slot_supplier = workflow_slot_supplier
         @activity_slot_supplier = activity_slot_supplier
         @local_activity_slot_supplier = local_activity_slot_supplier
+        @custom_slot_supplier_thread_pool = custom_slot_supplier_thread_pool
       end
 
       # @!visibility private
       def _to_bridge_options
         Internal::Bridge::Worker::TunerOptions.new(
-          workflow_slot_supplier: workflow_slot_supplier._to_bridge_options,
-          activity_slot_supplier: activity_slot_supplier._to_bridge_options,
-          local_activity_slot_supplier: local_activity_slot_supplier._to_bridge_options
+          workflow_slot_supplier: workflow_slot_supplier._to_bridge_options(self),
+          activity_slot_supplier: activity_slot_supplier._to_bridge_options(self),
+          local_activity_slot_supplier: local_activity_slot_supplier._to_bridge_options(self)
         )
       end
     end
diff --git a/temporalio/sig/temporalio/internal/bridge/worker.rbs b/temporalio/sig/temporalio/internal/bridge/worker.rbs
index 747b0f00..3381e642 100644
--- a/temporalio/sig/temporalio/internal/bridge/worker.rbs
+++ b/temporalio/sig/temporalio/internal/bridge/worker.rbs
@@ -63,10 +63,12 @@ module Temporalio
         class TunerSlotSupplierOptions
           attr_accessor fixed_size: Integer?
           attr_accessor resource_based: TunerResourceBasedSlotSupplierOptions?
+          attr_accessor custom: CustomSlotSupplier?
 
           def initialize: (
             fixed_size: Integer?,
-            resource_based: TunerResourceBasedSlotSupplierOptions?
+            resource_based: TunerResourceBasedSlotSupplierOptions?,
+            custom: CustomSlotSupplier?
           ) -> void
         end
 
@@ -94,6 +96,32 @@ module Temporalio
 
         def complete_activity_task_in_background: (untyped proto) -> void
 
+        class CustomSlotSupplier
+          def initialize: (
+            slot_supplier: Temporalio::Worker::Tuner::SlotSupplier::Custom,
+            thread_pool: Temporalio::Worker::ThreadPool?
+          ) -> void
+
+          def reserve_slot: (
+            Temporalio::Worker::Tuner::SlotSupplier::Custom::ReserveContext context,
+            Temporalio::Cancellation cancellation
+          ) { (untyped) -> void } -> void
+
+          def try_reserve_slot: (
+            Temporalio::Worker::Tuner::SlotSupplier::Custom::ReserveContext context
+          ) { (untyped) -> void } -> void
+
+          def mark_slot_used: (
+            Temporalio::Worker::Tuner::SlotSupplier::Custom::MarkUsedContext context
+          ) { (untyped) -> void } -> void
+
+          def release_slot: (
+            Temporalio::Worker::Tuner::SlotSupplier::Custom::ReleaseContext context
+          ) { (untyped) -> void } -> void
+
+          def run_user_code: [T] { -> T } -> T
+        end
+
         # Defined in Rust
 
         def self.new: (Client client, Options options) -> Worker
diff --git a/temporalio/sig/temporalio/worker/tuner.rbs b/temporalio/sig/temporalio/worker/tuner.rbs
index 23c08a63..f46b9073 100644
--- a/temporalio/sig/temporalio/worker/tuner.rbs
+++ b/temporalio/sig/temporalio/worker/tuner.rbs
@@ -18,12 +18,58 @@ module Temporalio
           ) -> void
         end
 
-        def _to_bridge_options: -> Internal::Bridge::Worker::TunerSlotSupplierOptions
+        class Custom < SlotSupplier
+          def reserve_slot: (ReserveContext context, Cancellation cancellation) { (untyped) -> void } -> void
+
+          def try_reserve_slot: (ReserveContext context) -> untyped
+
+          def mark_slot_used: (MarkUsedContext context) -> void
+
+          def release_slot: (ReleaseContext context) -> void
+
+          class ReserveContext
+            attr_reader slot_type: :workflow | :activity | :local_activity | :nexus
+            attr_reader task_queue: String
+            attr_reader worker_identity: String
+            attr_reader worker_deployment_name: String
+            attr_reader worker_build_id: String
+            attr_reader sticky?: bool
+          end
+
+          class MarkUsedContext
+            attr_reader slot_info: SlotInfo::Workflow | SlotInfo::Activity | SlotInfo::LocalActivity | SlotInfo::Nexus
+            attr_reader permit: untyped
+          end
+
+          class ReleaseContext
+            attr_reader slot_info: SlotInfo::Workflow | SlotInfo::Activity | SlotInfo::LocalActivity | SlotInfo::Nexus | nil
+            attr_reader permit: untyped
+          end
+
+          module SlotInfo
+            class Workflow
+              attr_reader workflow_type: String
+              attr_reader sticky?: bool
+            end
+            class Activity
+              attr_reader activity_type: String
+            end
+            class LocalActivity
+              attr_reader activity_type: String
+            end
+            class Nexus
+              attr_reader service: String
+              attr_reader operation: String
+            end
+          end
+        end
+
+        def _to_bridge_options: (Tuner tuner) -> Internal::Bridge::Worker::TunerSlotSupplierOptions
       end
 
       class ResourceBasedTunerOptions
-        attr_accessor target_memory_usage: Float
-        attr_accessor target_cpu_usage: Float
+        attr_reader target_memory_usage: Float
+        attr_reader target_cpu_usage: Float
 
         def initialize: (
           target_memory_usage: Float,
@@ -32,9 +78,9 @@ module Temporalio
       end
 
       class ResourceBasedSlotOptions
-        attr_accessor min_slots: Integer?
-        attr_accessor max_slots: Integer?
-        attr_accessor ramp_throttle: Float?
+        attr_reader min_slots: Integer?
+        attr_reader max_slots: Integer?
+        attr_reader ramp_throttle: Float?
 
         def initialize: (
           min_slots: Integer?,
@@ -60,11 +106,13 @@ module Temporalio
       attr_reader workflow_slot_supplier: SlotSupplier
       attr_reader activity_slot_supplier: SlotSupplier
       attr_reader local_activity_slot_supplier: SlotSupplier
+      attr_reader custom_slot_supplier_thread_pool: ThreadPool?
 
       def initialize: (
         workflow_slot_supplier: SlotSupplier,
         activity_slot_supplier: SlotSupplier,
-        local_activity_slot_supplier: SlotSupplier
+        local_activity_slot_supplier: SlotSupplier,
+        ?custom_slot_supplier_thread_pool: ThreadPool?
       ) -> void
 
       def _to_bridge_options: -> Internal::Bridge::Worker::TunerOptions
diff --git a/temporalio/test/sig/worker_test.rbs b/temporalio/test/sig/worker_test.rbs
new file mode 100644
index 00000000..bf86e00b
--- /dev/null
+++ b/temporalio/test/sig/worker_test.rbs
@@ -0,0 +1,7 @@
+class WorkerTest < Test
+  class TrackingSlotSupplier < Temporalio::Worker::Tuner::SlotSupplier::Custom
+    attr_reader events: Array[[Symbol, untyped]]
+
+    def add_event: (Symbol method, untyped context) -> void
+  end
+end
\ No newline at end of file
diff --git a/temporalio/test/sig/workflow_utils.rbs b/temporalio/test/sig/workflow_utils.rbs
index 3a8d896d..817d09e3 100644
--- a/temporalio/test/sig/workflow_utils.rbs
+++ b/temporalio/test/sig/workflow_utils.rbs
@@ -19,8 +19,9 @@ module WorkflowUtils
     ?task_timeout: duration?,
     ?on_worker_run: Proc?,
     ?unsafe_workflow_io_enabled: bool,
+    ?priority: Temporalio::Priority,
     ?start_workflow_client: Temporalio::Client,
-    ?priority: Temporalio::Priority
+    ?tuner: Temporalio::Worker::Tuner
   ) -> Object? |
   [T] (
     singleton(Temporalio::Workflow::Definition) workflow,
@@ -42,8 +43,9 @@ module WorkflowUtils
     ?task_timeout: duration?,
     ?on_worker_run: Proc?,
     ?unsafe_workflow_io_enabled: bool,
+    ?priority: Temporalio::Priority,
     ?start_workflow_client: Temporalio::Client,
-    ?priority: Temporalio::Priority
+    ?tuner: Temporalio::Worker::Tuner
   ) { (Temporalio::Client::WorkflowHandle, Temporalio::Worker) -> T } -> T
 
   def assert_eventually_task_fail: (
diff --git a/temporalio/test/worker_test.rb b/temporalio/test/worker_test.rb
index 1207ae36..a8deeca3 100644
--- a/temporalio/test/worker_test.rb
+++ b/temporalio/test/worker_test.rb
@@ -239,4 +239,224 @@ def test_can_run_with_autoscaling_poller_behavior
       end
     end
   end
+
+  class BasicPermit < Object; end
+
+  class TrackingSlotSupplier < Temporalio::Worker::Tuner::SlotSupplier::Custom
+    attr_reader :events
+
+    def reserve_slot(context, _cancellation, &)
+      add_event(:reserve_slot, context)
+      yield BasicPermit.new
+    end
+
+    def try_reserve_slot(context)
+      add_event(:try_reserve_slot, context)
+      BasicPermit.new
+    end
+
+    def mark_slot_used(context)
+      add_event(:mark_slot_used, context)
+    end
+
+    def release_slot(context)
+      add_event(:release_slot, context)
+    end
+
+    private
+
+    def add_event(method, context)
+      (@events ||= []) << [method, context]
+    end
+  end
+
+  class SlotSupplierActivity < Temporalio::Activity::Definition
+    def execute; end
+  end
+
+  class SlotSupplierWorkflow < Temporalio::Workflow::Definition
+    def execute
+      Temporalio::Workflow.execute_activity(SlotSupplierActivity, start_to_close_timeout: 10)
+      Temporalio::Workflow.execute_local_activity(SlotSupplierActivity, start_to_close_timeout: 10)
+    end
+  end
+
+  def test_custom_slot_supplier_simple
+    supplier = TrackingSlotSupplier.new
+    execute_workflow(
+      SlotSupplierWorkflow,
+      activities: [SlotSupplierActivity],
+      tuner: Temporalio::Worker::Tuner.new(
+        workflow_slot_supplier: supplier,
+        activity_slot_supplier: supplier,
+        local_activity_slot_supplier: supplier
+      )
+    )
+    # Assert the events are as expected...
+    events = supplier.events
+
+    # Had to get reserved for workflow (sticky and non), activity, and local activity
+    assert(events.any? { |(m, e)| m == :reserve_slot && e.slot_type == :workflow && !e.sticky? })
+    assert(events.any? { |(m, e)| m == :reserve_slot && e.slot_type == :workflow && e.sticky? })
+    assert(events.any? { |(m, e)| m == :reserve_slot && e.slot_type == :activity })
+    assert(events.any? { |(m, e)| m == :reserve_slot && e.slot_type == :local_activity })
+
+    # Since the activity was eager, had to get a try reserve for it
+    assert(events.any? { |(m, e)| m == :try_reserve_slot && e.slot_type == :activity })
+
+    # # Had to get mark used for workflow (sticky and non), activity, and local activity
+    assert(events.any? do |(m, e)|
+      m == :mark_slot_used && e.slot_info.is_a?(Temporalio::Worker::Tuner::SlotSupplier::Custom::SlotInfo::Workflow) &&
+        e.slot_info.workflow_type == 'SlotSupplierWorkflow' && !e.slot_info.sticky? && e.permit.is_a?(BasicPermit)
+    end)
+    assert(events.any? do |(m, e)|
+      m == :mark_slot_used && e.slot_info.is_a?(Temporalio::Worker::Tuner::SlotSupplier::Custom::SlotInfo::Workflow) &&
+        e.slot_info.workflow_type == 'SlotSupplierWorkflow' && e.slot_info.sticky? && e.permit.is_a?(BasicPermit)
+    end)
+    assert(events.any? do |(m, e)|
+      m == :mark_slot_used && e.slot_info.is_a?(Temporalio::Worker::Tuner::SlotSupplier::Custom::SlotInfo::Activity) &&
+        e.slot_info.activity_type == 'SlotSupplierActivity' && e.permit.is_a?(BasicPermit)
+    end)
+    assert(events.any? do |(m, e)|
+      m == :mark_slot_used &&
+        e.slot_info.is_a?(Temporalio::Worker::Tuner::SlotSupplier::Custom::SlotInfo::LocalActivity) &&
+        e.slot_info.activity_type == 'SlotSupplierActivity' && e.permit.is_a?(BasicPermit)
+    end)
+
+    # Must be a release for every reserve
+    assert_equal(
+      events.count { |(m, _)| m == :reserve_slot || m == :try_reserve_slot },
+      events.count { |(m, _)| m == :release_slot }
+    )
+  end
+
+  class BlockingSlotSupplier < Temporalio::Worker::Tuner::SlotSupplier::Custom
+    attr_reader :canceled_contexts
+
+    def reserve_slot(context, cancellation, &)
+      # We'll block on every reserve, waiting for queue to get resolved
+      queue = Queue.new
+      (@waiting ||= []) << [context, queue]
+      cancellation.add_cancel_callback do
+        (@canceled_contexts ||= []) << context
+        queue.push(StandardError.new('Canceled'))
+      end
+      yield queue.pop
+    end
+
+    def try_reserve_slot(_context)
+      # No try-reserve
+      None
+    end
+
+    def mark_slot_used(_context)
+      # Do nothing
+    end
+
+    def release_slot(_context)
+      # Do nothing
+    end
+
+    def resolve_a_reserve(slot_type)
+      waiting_idx = @waiting&.index { |(context, _)| context.slot_type == slot_type }
+      raise 'Not found' unless waiting_idx
+
+      _, queue = @waiting.delete_at(waiting_idx)
+      queue << BasicPermit.new
+    end
+
+    def waiting_contexts
+      @waiting.map(&:first)
+    end
+  end
+
+  class BlockingSlotSupplierWorkflow < Temporalio::Workflow::Definition
+    def execute
+      Temporalio::Workflow.execute_activity(SlotSupplierActivity, start_to_close_timeout: 10)
+    end
+  end
+
+  def test_custom_slot_supplier_blocking
+    supplier = BlockingSlotSupplier.new
+    waiting_contexts = execute_workflow(
+      BlockingSlotSupplierWorkflow,
+      activities: [SlotSupplierActivity],
+      tuner: Temporalio::Worker::Tuner.new(
+        workflow_slot_supplier: supplier,
+        activity_slot_supplier: supplier,
+        local_activity_slot_supplier: supplier
+      ),
+      # Core does not progress if you don't let sticky be reserved, and we want to manually control every slot
+      max_cached_workflows: 0
+    ) do |handle|
+      # Make sure history has not started a task
+      assert handle.fetch_history_events.none?(&:workflow_task_started_event_attributes)
+      # Some slower test runners don't start asking for reserve until after we've reached here, so we will wait until
+      # we see reservation requests for workflow and activity
+      assert_eventually do
+        assert(supplier.waiting_contexts.any? { |ctx| ctx.slot_type == :workflow })
+        assert(supplier.waiting_contexts.any? { |ctx| ctx.slot_type == :activity })
+      end
+      # Let one reserve call for workflow task through
+      supplier.resolve_a_reserve(:workflow)
+      # Make sure history has a task completed
+      assert_eventually { assert handle.fetch_history_events.any?(&:workflow_task_completed_event_attributes) }
+
+      # Now resolve one activity and wait for history to show activity completion
+      supplier.resolve_a_reserve(:activity)
+      assert_eventually { assert handle.fetch_history_events.any?(&:activity_task_completed_event_attributes) }
+
+      # Now resolve one workflow task to let that progress
+      supplier.resolve_a_reserve(:workflow)
+
+      # Confirm the workflow completes successfully
+      handle.result
+
+      # Collect the waiting contexts
+      supplier.waiting_contexts
+    end
+
+    # Confirm we canceled all waiting reservations
+    assert_equal waiting_contexts.size, supplier.canceled_contexts.size
+    waiting_contexts.each { |w| assert_includes supplier.canceled_contexts, w }
+  end
+
+  class RaisingSlotSupplier < Temporalio::Worker::Tuner::SlotSupplier::Custom
+    def reserve_slot(context, _cancellation, &)
+      # We'll only raise on non-workflows
+      raise 'Intentional error' unless context.slot_type == :workflow
+
+      yield BasicPermit.new
+    end
+
+    def try_reserve_slot(_context)
+      raise 'Intentional error'
+    end
+
+    def mark_slot_used(_context)
+      raise 'Intentional error'
+    end
+
+    def release_slot(_context)
+      raise 'Intentional error'
+    end
+  end
+
+  class SlotSupplierRaisingWorkflow < Temporalio::Workflow::Definition
+    def execute
+      'done'
+    end
+  end
+
+  def test_custom_slot_supplier_raising
+    supplier = RaisingSlotSupplier.new
+    assert_equal 'done', execute_workflow(
+      SlotSupplierRaisingWorkflow,
+      tuner: Temporalio::Worker::Tuner.new(
+        workflow_slot_supplier: supplier,
+        activity_slot_supplier: supplier,
+        local_activity_slot_supplier: supplier
+      )
+    )
+  end
 end
diff --git a/temporalio/test/workflow_utils.rb b/temporalio/test/workflow_utils.rb
index f642685a..539c013d 100644
--- a/temporalio/test/workflow_utils.rb
+++ b/temporalio/test/workflow_utils.rb
@@ -32,7 +32,8 @@ def execute_workflow(
     on_worker_run: nil,
     unsafe_workflow_io_enabled: false,
     priority: Temporalio::Priority.default,
-    start_workflow_client: client
+    start_workflow_client: client,
+    tuner: Temporalio::Worker::Tuner.create_fixed
   )
     worker = Temporalio::Worker.new(
       client:,
@@ -45,7 +46,8 @@ def execute_workflow(
       workflow_payload_codec_thread_pool:,
       max_heartbeat_throttle_interval:,
       interceptors:,
-      unsafe_workflow_io_enabled:
+      unsafe_workflow_io_enabled:,
+      tuner:
     )
     worker.run do
       on_worker_run&.call