diff --git a/README.md b/README.md
index 40eb3c0..872e4fa 100644
--- a/README.md
+++ b/README.md
@@ -208,6 +208,12 @@ jobs:
           token: ${{ secrets.GH_TOKEN_FOR_UPDATES }}
 ```
 
+## Sign-off commit
+
+The bot can produce commits with a sign-off. This is useful for projects that require a sign-off to be present in the commit message.
+To enable this, set the `signoff` input to `true`.
+
+
 ## With GPG commit signing
 
 It's possible for the bot to produce GPG-signed commits.
diff --git a/action.yml b/action.yml
index 2ed0d92..c216d3e 100644
--- a/action.yml
+++ b/action.yml
@@ -34,7 +34,7 @@ inputs:
       Automated changes by the [update-flake-lock](https://github.com/DeterminateSystems/update-flake-lock) GitHub Action.
 
       ```
-      {{ env.GIT_COMMIT_MESSAGE }}
+      {{ env.FLAKE_UPDATE_OUTPUT }}
       ```
 
       ### Running GitHub Actions on this PR
@@ -42,7 +42,6 @@ inputs:
       GitHub Actions will not run workflows on pull requests which are opened by a GitHub Action.
 
       **To run GitHub Actions workflows on this PR, close and re-open this pull request.**
-
   pr-labels:
     description: "A comma or newline separated list of labels to set on the Pull Request to be created"
     required: false
@@ -75,6 +74,10 @@ inputs:
     description: "Set to true if the action should sign the commit with GPG"
     required: false
     default: "false"
+  signoff:
+    description: "Set to true if the action should add a Signed-off-by line to the commit message"
+    required: false
+    default: "false"
   gpg-private-key:
     description: "GPG Private Key with which to sign the commits in the PR to be created"
     required: false
@@ -175,15 +178,6 @@ runs:
         path: pr_body.template
         contents: ${{ inputs.pr-body }}
       env: {}
-    - name: Set additional env variables (GIT_COMMIT_MESSAGE)
-      shell: bash
-      run: |
-        DELIMITER=$(dd if=/dev/urandom bs=15 count=1 status=none | base64)
-        COMMIT_MESSAGE="$(git log --format=%b -n 1)"
-        echo "GIT_COMMIT_MESSAGE<<$DELIMITER" >> $GITHUB_ENV
-        echo "$COMMIT_MESSAGE" >> $GITHUB_ENV
-        echo "$DELIMITER" >> $GITHUB_ENV
-        echo "GIT_COMMIT_MESSAGE is: ${COMMIT_MESSAGE}"
     - name: Interpolate PR Body
       uses: pedrolamas/handlebars-action@2995d7eadacbc8f2f6ab8431a01d84a5fa3b8bb4 # v2.4.0
       with:
@@ -207,6 +201,10 @@ runs:
         base: ${{ inputs.base }}
         branch: ${{ inputs.branch }}
         delete-branch: true
+        commit-message: |
+          ${{ inputs.commit-msg }}
+
+          ${{ env.FLAKE_UPDATE_OUTPUT }}
         committer: ${{ env.GIT_COMMITTER_NAME }} ${{ env.GIT_COMMITTER_EMAIL }}
         author: ${{ env.GIT_AUTHOR_NAME }} ${{ env.GIT_AUTHOR_EMAIL }}
         title: ${{ inputs.pr-title }}
@@ -215,3 +213,4 @@ runs:
         labels: ${{ inputs.pr-labels }}
         reviewers: ${{ inputs.pr-reviewers }}
         body: ${{ steps.pr_body.outputs.content }}
+        signoff: ${{ inputs.signoff }}
diff --git a/dist/index.js b/dist/index.js
index e04b0a3..9b38b39 100644
--- a/dist/index.js
+++ b/dist/index.js
@@ -95249,27 +95249,26 @@ function makeOptionsConfident(actionOptions) {
  * Copyright (c) 2018-2020 [Samuel Carreira]
  */
 //# sourceMappingURL=index.js.map
+// EXTERNAL MODULE: external "stream"
+var external_stream_ = __nccwpck_require__(2203);
 ;// CONCATENATED MODULE: ./dist/index.js
 // src/nix.ts
-function makeNixCommandArgs(nixOptions, flakeInputs, commitMessage) {
+function makeNixCommandArgs(nixOptions, flakeInputs) {
   const flakeInputFlags = flakeInputs.flatMap((input) => [
     "--update-input",
     input
   ]);
-  const lockfileSummaryFlags = [
-    "--option",
-    "commit-lockfile-summary",
-    commitMessage
-  ];
   const updateLockMechanism = flakeInputFlags.length === 0 ? "update" : "lock";
-  return nixOptions.concat(["flake", updateLockMechanism]).concat(flakeInputFlags).concat(["--commit-lock-file"]).concat(lockfileSummaryFlags);
+  return nixOptions.concat(["flake", updateLockMechanism]).concat(flakeInputFlags);
 }
 
 // src/index.ts
 
 
 
+
 var EVENT_EXECUTION_FAILURE = "execution_failure";
+var COMMIT_MESSAGE_MAX_LENGTH = 65536;
 var UpdateFlakeLockAction = class extends DetSysAction {
   constructor() {
     super({
@@ -95277,7 +95276,6 @@ var UpdateFlakeLockAction = class extends DetSysAction {
       fetchStyle: "universal",
       requireNix: "fail"
     });
-    this.commitMessage = inputs_exports.getString("commit-msg");
     this.flakeInputs = inputs_exports.getArrayOfStrings("inputs", "space");
     this.nixOptions = inputs_exports.getArrayOfStrings("nix-options", "space");
     this.pathToFlakeDir = inputs_exports.getStringOrNull("path-to-flake-dir");
@@ -95291,20 +95289,25 @@ var UpdateFlakeLockAction = class extends DetSysAction {
   async update() {
     const nixCommandArgs = makeNixCommandArgs(
       this.nixOptions,
-      this.flakeInputs,
-      this.commitMessage
+      this.flakeInputs
     );
     core.debug(
       JSON.stringify({
         options: this.nixOptions,
         inputs: this.flakeInputs,
-        message: this.commitMessage,
         args: nixCommandArgs
       })
     );
+    let output = "";
     const execOptions = {
       cwd: this.pathToFlakeDir !== null ? this.pathToFlakeDir : void 0,
-      ignoreReturnCode: true
+      ignoreReturnCode: true,
+      outStream: new external_stream_.Writable({
+        write: (chunk, _, callback) => {
+          output += chunk.toString();
+          callback();
+        }
+      })
     };
     const exitCode = await exec.exec("nix", nixCommandArgs, execOptions);
     if (exitCode !== 0) {
@@ -95314,6 +95317,15 @@ var UpdateFlakeLockAction = class extends DetSysAction {
       core.setFailed(`non-zero exit code of ${exitCode} detected`);
     } else {
       core.info(`flake.lock file was successfully updated`);
+      if (output.length > COMMIT_MESSAGE_MAX_LENGTH) {
+        core.warning(
+          `commit message is too long, truncating to ${COMMIT_MESSAGE_MAX_LENGTH} characters`
+        );
+      }
+      core.exportVariable(
+        "FLAKE_UPDATE_OUTPUT",
+        output.trim().slice(0, COMMIT_MESSAGE_MAX_LENGTH)
+      );
     }
   }
 };
diff --git a/dist/index.js.map b/dist/index.js.map
index 2e2062e..794c0ba 100644
--- a/dist/index.js.map
+++ b/dist/index.js.map
@@ -1 +1 @@
-{"version":3,"sources":["../src/nix.ts","../src/index.ts"],"sourcesContent":["// Build the Nix args out of inputs from the Actions environment\nexport function makeNixCommandArgs(\n  nixOptions: string[],\n  flakeInputs: string[],\n  commitMessage: string,\n): string[] {\n  const flakeInputFlags = flakeInputs.flatMap((input) => [\n    \"--update-input\",\n    input,\n  ]);\n\n  // NOTE(cole-h): In Nix versions 2.23.0 and later, `commit-lockfile-summary` became an alias to\n  // the setting `commit-lock-file-summary` (https://github.com/NixOS/nix/pull/10691), and Nix does\n  // not treat aliases the same as their \"real\" setting by requiring setting aliases to be\n  // configured via `--option <alias name> <option value>`\n  // (https://github.com/NixOS/nix/issues/10989).\n  // So, we go the long way so that we can support versions both before and after Nix 2.23.0.\n  const lockfileSummaryFlags = [\n    \"--option\",\n    \"commit-lockfile-summary\",\n    commitMessage,\n  ];\n\n  const updateLockMechanism = flakeInputFlags.length === 0 ? \"update\" : \"lock\";\n\n  return nixOptions\n    .concat([\"flake\", updateLockMechanism])\n    .concat(flakeInputFlags)\n    .concat([\"--commit-lock-file\"])\n    .concat(lockfileSummaryFlags);\n}\n","import { makeNixCommandArgs } from \"./nix.js\";\nimport * as actionsCore from \"@actions/core\";\nimport * as actionsExec from \"@actions/exec\";\nimport { DetSysAction, inputs } from \"detsys-ts\";\n\nconst EVENT_EXECUTION_FAILURE = \"execution_failure\";\n\nclass UpdateFlakeLockAction extends DetSysAction {\n  private commitMessage: string;\n  private nixOptions: string[];\n  private flakeInputs: string[];\n  private pathToFlakeDir: string | null;\n\n  constructor() {\n    super({\n      name: \"update-flake-lock\",\n      fetchStyle: \"universal\",\n      requireNix: \"fail\",\n    });\n\n    this.commitMessage = inputs.getString(\"commit-msg\");\n    this.flakeInputs = inputs.getArrayOfStrings(\"inputs\", \"space\");\n    this.nixOptions = inputs.getArrayOfStrings(\"nix-options\", \"space\");\n    this.pathToFlakeDir = inputs.getStringOrNull(\"path-to-flake-dir\");\n  }\n\n  async main(): Promise<void> {\n    await this.update();\n  }\n\n  // No post phase\n  async post(): Promise<void> {}\n\n  async update(): Promise<void> {\n    // Nix command of this form:\n    // nix ${maybe nix options} flake ${\"update\" or \"lock\"} ${maybe --update-input flags} --commit-lock-file --commit-lockfile-summary ${commit message}\n    // Example commands:\n    // nix --extra-substituters https://example.com flake lock --update-input nixpkgs --commit-lock-file --commit-lockfile-summary \"updated flake.lock\"\n    // nix flake update --commit-lock-file --commit-lockfile-summary \"updated flake.lock\"\n    const nixCommandArgs: string[] = makeNixCommandArgs(\n      this.nixOptions,\n      this.flakeInputs,\n      this.commitMessage,\n    );\n\n    actionsCore.debug(\n      JSON.stringify({\n        options: this.nixOptions,\n        inputs: this.flakeInputs,\n        message: this.commitMessage,\n        args: nixCommandArgs,\n      }),\n    );\n\n    const execOptions: actionsExec.ExecOptions = {\n      cwd: this.pathToFlakeDir !== null ? this.pathToFlakeDir : undefined,\n      ignoreReturnCode: true,\n    };\n\n    const exitCode = await actionsExec.exec(\"nix\", nixCommandArgs, execOptions);\n\n    if (exitCode !== 0) {\n      this.recordEvent(EVENT_EXECUTION_FAILURE, {\n        exitCode,\n      });\n      actionsCore.setFailed(`non-zero exit code of ${exitCode} detected`);\n    } else {\n      actionsCore.info(`flake.lock file was successfully updated`);\n    }\n  }\n}\n\nfunction main(): void {\n  new UpdateFlakeLockAction().execute();\n}\n\nmain();\n"],"mappings":";AACO,SAAS,mBACd,YACA,aACA,eACU;AACV,QAAM,kBAAkB,YAAY,QAAQ,CAAC,UAAU;AAAA,IACrD;AAAA,IACA;AAAA,EACF,CAAC;AAQD,QAAM,uBAAuB;AAAA,IAC3B;AAAA,IACA;AAAA,IACA;AAAA,EACF;AAEA,QAAM,sBAAsB,gBAAgB,WAAW,IAAI,WAAW;AAEtE,SAAO,WACJ,OAAO,CAAC,SAAS,mBAAmB,CAAC,EACrC,OAAO,eAAe,EACtB,OAAO,CAAC,oBAAoB,CAAC,EAC7B,OAAO,oBAAoB;AAChC;;;AC7BA,YAAY,iBAAiB;AAC7B,YAAY,iBAAiB;AAC7B,SAAS,cAAc,cAAc;AAErC,IAAM,0BAA0B;AAEhC,IAAM,wBAAN,cAAoC,aAAa;AAAA,EAM/C,cAAc;AACZ,UAAM;AAAA,MACJ,MAAM;AAAA,MACN,YAAY;AAAA,MACZ,YAAY;AAAA,IACd,CAAC;AAED,SAAK,gBAAgB,OAAO,UAAU,YAAY;AAClD,SAAK,cAAc,OAAO,kBAAkB,UAAU,OAAO;AAC7D,SAAK,aAAa,OAAO,kBAAkB,eAAe,OAAO;AACjE,SAAK,iBAAiB,OAAO,gBAAgB,mBAAmB;AAAA,EAClE;AAAA,EAEA,MAAM,OAAsB;AAC1B,UAAM,KAAK,OAAO;AAAA,EACpB;AAAA;AAAA,EAGA,MAAM,OAAsB;AAAA,EAAC;AAAA,EAE7B,MAAM,SAAwB;AAM5B,UAAM,iBAA2B;AAAA,MAC/B,KAAK;AAAA,MACL,KAAK;AAAA,MACL,KAAK;AAAA,IACP;AAEA,IAAY;AAAA,MACV,KAAK,UAAU;AAAA,QACb,SAAS,KAAK;AAAA,QACd,QAAQ,KAAK;AAAA,QACb,SAAS,KAAK;AAAA,QACd,MAAM;AAAA,MACR,CAAC;AAAA,IACH;AAEA,UAAM,cAAuC;AAAA,MAC3C,KAAK,KAAK,mBAAmB,OAAO,KAAK,iBAAiB;AAAA,MAC1D,kBAAkB;AAAA,IACpB;AAEA,UAAM,WAAW,MAAkB,iBAAK,OAAO,gBAAgB,WAAW;AAE1E,QAAI,aAAa,GAAG;AAClB,WAAK,YAAY,yBAAyB;AAAA,QACxC;AAAA,MACF,CAAC;AACD,MAAY,sBAAU,yBAAyB,QAAQ,WAAW;AAAA,IACpE,OAAO;AACL,MAAY,iBAAK,0CAA0C;AAAA,IAC7D;AAAA,EACF;AACF;AAEA,SAAS,OAAa;AACpB,MAAI,sBAAsB,EAAE,QAAQ;AACtC;AAEA,KAAK;","names":[]}
\ No newline at end of file
+{"version":3,"sources":["../src/nix.ts","../src/index.ts"],"sourcesContent":["// Build the Nix args out of inputs from the Actions environment\nexport function makeNixCommandArgs(\n  nixOptions: string[],\n  flakeInputs: string[],\n): string[] {\n  const flakeInputFlags = flakeInputs.flatMap((input) => [\n    \"--update-input\",\n    input,\n  ]);\n\n  const updateLockMechanism = flakeInputFlags.length === 0 ? \"update\" : \"lock\";\n\n  return nixOptions\n    .concat([\"flake\", updateLockMechanism])\n    .concat(flakeInputFlags);\n}\n","import { makeNixCommandArgs } from \"./nix.js\";\nimport * as actionsCore from \"@actions/core\";\nimport * as actionsExec from \"@actions/exec\";\nimport { DetSysAction, inputs } from \"detsys-ts\";\nimport { Writable } from \"stream\";\n\nconst EVENT_EXECUTION_FAILURE = \"execution_failure\";\nconst COMMIT_MESSAGE_MAX_LENGTH = 65536;\n\nclass UpdateFlakeLockAction extends DetSysAction {\n  private nixOptions: string[];\n  private flakeInputs: string[];\n  private pathToFlakeDir: string | null;\n\n  constructor() {\n    super({\n      name: \"update-flake-lock\",\n      fetchStyle: \"universal\",\n      requireNix: \"fail\",\n    });\n\n    this.flakeInputs = inputs.getArrayOfStrings(\"inputs\", \"space\");\n    this.nixOptions = inputs.getArrayOfStrings(\"nix-options\", \"space\");\n    this.pathToFlakeDir = inputs.getStringOrNull(\"path-to-flake-dir\");\n  }\n\n  async main(): Promise<void> {\n    await this.update();\n  }\n\n  // No post phase\n  async post(): Promise<void> {}\n\n  async update(): Promise<void> {\n    // Nix command of this form:\n    // nix ${maybe nix options} flake ${\"update\" or \"lock\"} ${maybe --update-input flags}\n    // Example commands:\n    // nix --extra-substituters https://example.com flake lock --update-input nixpkgs\n    // nix flake update\n    const nixCommandArgs: string[] = makeNixCommandArgs(\n      this.nixOptions,\n      this.flakeInputs,\n    );\n\n    actionsCore.debug(\n      JSON.stringify({\n        options: this.nixOptions,\n        inputs: this.flakeInputs,\n        args: nixCommandArgs,\n      }),\n    );\n\n    let output = \"\";\n\n    const execOptions: actionsExec.ExecOptions = {\n      cwd: this.pathToFlakeDir !== null ? this.pathToFlakeDir : undefined,\n      ignoreReturnCode: true,\n      outStream: new Writable({\n        write: (chunk, _, callback) => {\n          output += chunk.toString();\n          callback();\n        },\n      }),\n    };\n\n    const exitCode = await actionsExec.exec(\"nix\", nixCommandArgs, execOptions);\n\n    if (exitCode !== 0) {\n      this.recordEvent(EVENT_EXECUTION_FAILURE, {\n        exitCode,\n      });\n      actionsCore.setFailed(`non-zero exit code of ${exitCode} detected`);\n    } else {\n      actionsCore.info(`flake.lock file was successfully updated`);\n      if (output.length > COMMIT_MESSAGE_MAX_LENGTH) {\n        actionsCore.warning(\n          `commit message is too long, truncating to ${COMMIT_MESSAGE_MAX_LENGTH} characters`,\n        );\n      }\n      actionsCore.exportVariable(\n        \"FLAKE_UPDATE_OUTPUT\",\n        output.trim().slice(0, COMMIT_MESSAGE_MAX_LENGTH),\n      );\n    }\n  }\n}\n\nfunction main(): void {\n  new UpdateFlakeLockAction().execute();\n}\n\nmain();\n"],"mappings":";AACO,SAAS,mBACd,YACA,aACU;AACV,QAAM,kBAAkB,YAAY,QAAQ,CAAC,UAAU;AAAA,IACrD;AAAA,IACA;AAAA,EACF,CAAC;AAED,QAAM,sBAAsB,gBAAgB,WAAW,IAAI,WAAW;AAEtE,SAAO,WACJ,OAAO,CAAC,SAAS,mBAAmB,CAAC,EACrC,OAAO,eAAe;AAC3B;;;ACdA,YAAY,iBAAiB;AAC7B,YAAY,iBAAiB;AAC7B,SAAS,cAAc,cAAc;AACrC,SAAS,gBAAgB;AAEzB,IAAM,0BAA0B;AAChC,IAAM,4BAA4B;AAElC,IAAM,wBAAN,cAAoC,aAAa;AAAA,EAK/C,cAAc;AACZ,UAAM;AAAA,MACJ,MAAM;AAAA,MACN,YAAY;AAAA,MACZ,YAAY;AAAA,IACd,CAAC;AAED,SAAK,cAAc,OAAO,kBAAkB,UAAU,OAAO;AAC7D,SAAK,aAAa,OAAO,kBAAkB,eAAe,OAAO;AACjE,SAAK,iBAAiB,OAAO,gBAAgB,mBAAmB;AAAA,EAClE;AAAA,EAEA,MAAM,OAAsB;AAC1B,UAAM,KAAK,OAAO;AAAA,EACpB;AAAA;AAAA,EAGA,MAAM,OAAsB;AAAA,EAAC;AAAA,EAE7B,MAAM,SAAwB;AAM5B,UAAM,iBAA2B;AAAA,MAC/B,KAAK;AAAA,MACL,KAAK;AAAA,IACP;AAEA,IAAY;AAAA,MACV,KAAK,UAAU;AAAA,QACb,SAAS,KAAK;AAAA,QACd,QAAQ,KAAK;AAAA,QACb,MAAM;AAAA,MACR,CAAC;AAAA,IACH;AAEA,QAAI,SAAS;AAEb,UAAM,cAAuC;AAAA,MAC3C,KAAK,KAAK,mBAAmB,OAAO,KAAK,iBAAiB;AAAA,MAC1D,kBAAkB;AAAA,MAClB,WAAW,IAAI,SAAS;AAAA,QACtB,OAAO,CAAC,OAAO,GAAG,aAAa;AAC7B,oBAAU,MAAM,SAAS;AACzB,mBAAS;AAAA,QACX;AAAA,MACF,CAAC;AAAA,IACH;AAEA,UAAM,WAAW,MAAkB,iBAAK,OAAO,gBAAgB,WAAW;AAE1E,QAAI,aAAa,GAAG;AAClB,WAAK,YAAY,yBAAyB;AAAA,QACxC;AAAA,MACF,CAAC;AACD,MAAY,sBAAU,yBAAyB,QAAQ,WAAW;AAAA,IACpE,OAAO;AACL,MAAY,iBAAK,0CAA0C;AAC3D,UAAI,OAAO,SAAS,2BAA2B;AAC7C,QAAY;AAAA,UACV,6CAA6C,yBAAyB;AAAA,QACxE;AAAA,MACF;AACA,MAAY;AAAA,QACV;AAAA,QACA,OAAO,KAAK,EAAE,MAAM,GAAG,yBAAyB;AAAA,MAClD;AAAA,IACF;AAAA,EACF;AACF;AAEA,SAAS,OAAa;AACpB,MAAI,sBAAsB,EAAE,QAAQ;AACtC;AAEA,KAAK;","names":[]}
\ No newline at end of file
diff --git a/flake.lock b/flake.lock
index f42a293..e15eaaf 100644
--- a/flake.lock
+++ b/flake.lock
@@ -2,12 +2,12 @@
   "nodes": {
     "nixpkgs": {
       "locked": {
-        "lastModified": 1753939845,
-        "narHash": "sha256-K2ViRJfdVGE8tpJejs8Qpvvejks1+A4GQej/lBk5y7I=",
-        "rev": "94def634a20494ee057c76998843c015909d6311",
-        "revCount": 837094,
+        "lastModified": 1754725699,
+        "narHash": "sha256-iAcj9T/Y+3DBy2J0N+yF9XQQQ8IEb5swLFzs23CdP88=",
+        "rev": "85dbfc7aaf52ecb755f87e577ddbe6dbbdbc1054",
+        "revCount": 841808,
         "type": "tarball",
-        "url": "https://api.flakehub.com/f/pinned/NixOS/nixpkgs/0.1.837094%2Brev-94def634a20494ee057c76998843c015909d6311/019866ba-4140-7e21-9b2b-75f13bfb023e/source.tar.gz"
+        "url": "https://api.flakehub.com/f/pinned/NixOS/nixpkgs/0.1.841808%2Brev-85dbfc7aaf52ecb755f87e577ddbe6dbbdbc1054/01989280-4b63-70f9-95b3-49c511cb4d92/source.tar.gz"
       },
       "original": {
         "type": "tarball",
diff --git a/src/index.ts b/src/index.ts
index fd80134..2504837 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -2,11 +2,12 @@ import { makeNixCommandArgs } from "./nix.js";
 import * as actionsCore from "@actions/core";
 import * as actionsExec from "@actions/exec";
 import { DetSysAction, inputs } from "detsys-ts";
+import { Writable } from "stream";
 
 const EVENT_EXECUTION_FAILURE = "execution_failure";
+const COMMIT_MESSAGE_MAX_LENGTH = 65536;
 
 class UpdateFlakeLockAction extends DetSysAction {
-  private commitMessage: string;
   private nixOptions: string[];
   private flakeInputs: string[];
   private pathToFlakeDir: string | null;
@@ -18,7 +19,6 @@ class UpdateFlakeLockAction extends DetSysAction {
       requireNix: "fail",
     });
 
-    this.commitMessage = inputs.getString("commit-msg");
     this.flakeInputs = inputs.getArrayOfStrings("inputs", "space");
     this.nixOptions = inputs.getArrayOfStrings("nix-options", "space");
     this.pathToFlakeDir = inputs.getStringOrNull("path-to-flake-dir");
@@ -33,28 +33,34 @@ class UpdateFlakeLockAction extends DetSysAction {
 
   async update(): Promise<void> {
     // Nix command of this form:
-    // nix ${maybe nix options} flake ${"update" or "lock"} ${maybe --update-input flags} --commit-lock-file --commit-lockfile-summary ${commit message}
+    // nix ${maybe nix options} flake ${"update" or "lock"} ${maybe --update-input flags}
     // Example commands:
-    // nix --extra-substituters https://example.com flake lock --update-input nixpkgs --commit-lock-file --commit-lockfile-summary "updated flake.lock"
-    // nix flake update --commit-lock-file --commit-lockfile-summary "updated flake.lock"
+    // nix --extra-substituters https://example.com flake lock --update-input nixpkgs
+    // nix flake update
     const nixCommandArgs: string[] = makeNixCommandArgs(
       this.nixOptions,
       this.flakeInputs,
-      this.commitMessage,
     );
 
     actionsCore.debug(
       JSON.stringify({
         options: this.nixOptions,
         inputs: this.flakeInputs,
-        message: this.commitMessage,
         args: nixCommandArgs,
       }),
     );
 
+    let output = "";
+
     const execOptions: actionsExec.ExecOptions = {
       cwd: this.pathToFlakeDir !== null ? this.pathToFlakeDir : undefined,
       ignoreReturnCode: true,
+      outStream: new Writable({
+        write: (chunk, _, callback) => {
+          output += chunk.toString();
+          callback();
+        },
+      }),
     };
 
     const exitCode = await actionsExec.exec("nix", nixCommandArgs, execOptions);
@@ -66,6 +72,15 @@ class UpdateFlakeLockAction extends DetSysAction {
       actionsCore.setFailed(`non-zero exit code of ${exitCode} detected`);
     } else {
       actionsCore.info(`flake.lock file was successfully updated`);
+      if (output.length > COMMIT_MESSAGE_MAX_LENGTH) {
+        actionsCore.warning(
+          `commit message is too long, truncating to ${COMMIT_MESSAGE_MAX_LENGTH} characters`,
+        );
+      }
+      actionsCore.exportVariable(
+        "FLAKE_UPDATE_OUTPUT",
+        output.trim().slice(0, COMMIT_MESSAGE_MAX_LENGTH),
+      );
     }
   }
 }
diff --git a/src/nix.ts b/src/nix.ts
index e3d829a..0cf75a0 100644
--- a/src/nix.ts
+++ b/src/nix.ts
@@ -2,30 +2,15 @@
 export function makeNixCommandArgs(
   nixOptions: string[],
   flakeInputs: string[],
-  commitMessage: string,
 ): string[] {
   const flakeInputFlags = flakeInputs.flatMap((input) => [
     "--update-input",
     input,
   ]);
 
-  // NOTE(cole-h): In Nix versions 2.23.0 and later, `commit-lockfile-summary` became an alias to
-  // the setting `commit-lock-file-summary` (https://github.com/NixOS/nix/pull/10691), and Nix does
-  // not treat aliases the same as their "real" setting by requiring setting aliases to be
-  // configured via `--option <alias name> <option value>`
-  // (https://github.com/NixOS/nix/issues/10989).
-  // So, we go the long way so that we can support versions both before and after Nix 2.23.0.
-  const lockfileSummaryFlags = [
-    "--option",
-    "commit-lockfile-summary",
-    commitMessage,
-  ];
-
   const updateLockMechanism = flakeInputFlags.length === 0 ? "update" : "lock";
 
   return nixOptions
     .concat(["flake", updateLockMechanism])
-    .concat(flakeInputFlags)
-    .concat(["--commit-lock-file"])
-    .concat(lockfileSummaryFlags);
+    .concat(flakeInputFlags);
 }