diff --git a/pkg/bass/cache_path.go b/pkg/bass/cache_path.go
index 124e5a46..c7dada3a 100644
--- a/pkg/bass/cache_path.go
+++ b/pkg/bass/cache_path.go
@@ -12,13 +12,22 @@ import (
 
 // CachePath is a Path within an ephemeral directory managed by the runtime.
 type CachePath struct {
-	ID   string
-	Path FileOrDirPath
+	ID              string
+	ConcurrencyMode ConcurrencyMode
+	Path            FileOrDirPath
 }
 
+type ConcurrencyMode int
+
+const (
+	ConcurrencyModeShared = iota
+	ConcurrencyModePrivate
+	ConcurrencyModeLocked
+)
+
 var _ Value = CachePath{}
 
-func NewCacheDir(id string) CachePath {
+func NewCacheDir(id string, mode ...ConcurrencyMode) CachePath {
 	return NewCachePath(id, ParseFileOrDirPath("."))
 }
 
@@ -86,6 +95,7 @@ func (path *CachePath) UnmarshalProto(msg proto.Message) error {
 	}
 
 	path.ID = p.Id
+	path.ConcurrencyMode = ConcurrencyMode(p.Concurrency)
 
 	return path.Path.UnmarshalProto(p.Path)
 }
diff --git a/pkg/bass/ground.go b/pkg/bass/ground.go
index d625fa7f..a39bb703 100644
--- a/pkg/bass/ground.go
+++ b/pkg/bass/ground.go
@@ -801,9 +801,31 @@ func init() {
 	)
 
 	Ground.Set("cache-dir",
-		Func("cache-dir", "[id]", NewCacheDir),
-		`returns a cache directory corresponding to the string identifier`,
-		`Cache directories may be mounted to thunks. Their content persists across thunk runs.`)
+		Func("cache-dir", "[id & mode]", func(id string, mode ...Symbol) (CachePath, error) {
+			cache := NewCacheDir(id)
+			if len(mode) > 0 {
+				switch mode[0] {
+				case "locked":
+					cache.ConcurrencyMode = ConcurrencyModeLocked
+				case "private":
+					cache.ConcurrencyMode = ConcurrencyModePrivate
+				case "shared":
+					cache.ConcurrencyMode = ConcurrencyModeShared
+				default:
+					return CachePath{}, fmt.Errorf("invalid cache mode: %s", mode[0])
+				}
+			}
+
+			return cache, nil
+		}),
+		`returns a cache directory with the given identifier and concurrency mode`,
+		`Cache directories may be mounted to thunks. Their content persists across thunk runs.`,
+		`The :shared concurrency mode allows multiple thunks to run concurrently with the same cache.`,
+		`The :locked mode allows only one thunk to run at a time with the same cache.`,
+		`The :private mode creates a new mount for concurrent thunks.`,
+		`The default mode is :shared.`,
+		`=> (cache-dir "foo")`,
+		`=> (cache-dir "foo" :locked)`)
 
 	Ground.Set("binds?",
 		Func("binds?", "[scope sym]", (*Scope).Binds),
diff --git a/pkg/bass/proto.go b/pkg/bass/proto.go
index 533dfecd..c5a8b7d6 100644
--- a/pkg/bass/proto.go
+++ b/pkg/bass/proto.go
@@ -77,8 +77,9 @@ func FromProto(val *proto.Value) (Value, error) {
 		}, nil
 	case *proto.Value_CachePath:
 		return CachePath{
-			ID:   x.CachePath.Id,
-			Path: fod(x.CachePath.Path),
+			ID:              x.CachePath.Id,
+			Path:            fod(x.CachePath.Path),
+			ConcurrencyMode: ConcurrencyMode(x.CachePath.Concurrency),
 		}, nil
 	case *proto.Value_LogicalPath:
 		fsp := &FSPath{}
@@ -308,7 +309,8 @@ func (value *FSPath) MarshalProto() (proto.Message, error) {
 
 func (value CachePath) MarshalProto() (proto.Message, error) {
 	pv := &proto.CachePath{
-		Id: value.ID,
+		Id:          value.ID,
+		Concurrency: proto.ConcurrencyMode(value.ConcurrencyMode),
 	}
 
 	pathp, err := value.Path.MarshalProto()
diff --git a/pkg/proto/bass.pb.go b/pkg/proto/bass.pb.go
index e75091b6..e590648c 100644
--- a/pkg/proto/bass.pb.go
+++ b/pkg/proto/bass.pb.go
@@ -20,6 +20,55 @@ const (
 	_ = protoimpl.EnforceVersion(protoimpl.MaxVersion - 20)
 )
 
+type ConcurrencyMode int32
+
+const (
+	ConcurrencyMode_CONCURRENCY_MODE_SHARED  ConcurrencyMode = 0
+	ConcurrencyMode_CONCURRENCY_MODE_PRIVATE ConcurrencyMode = 1
+	ConcurrencyMode_CONCURRENCY_MODE_LOCKED  ConcurrencyMode = 2
+)
+
+// Enum value maps for ConcurrencyMode.
+var (
+	ConcurrencyMode_name = map[int32]string{
+		0: "CONCURRENCY_MODE_SHARED",
+		1: "CONCURRENCY_MODE_PRIVATE",
+		2: "CONCURRENCY_MODE_LOCKED",
+	}
+	ConcurrencyMode_value = map[string]int32{
+		"CONCURRENCY_MODE_SHARED":  0,
+		"CONCURRENCY_MODE_PRIVATE": 1,
+		"CONCURRENCY_MODE_LOCKED":  2,
+	}
+)
+
+func (x ConcurrencyMode) Enum() *ConcurrencyMode {
+	p := new(ConcurrencyMode)
+	*p = x
+	return p
+}
+
+func (x ConcurrencyMode) String() string {
+	return protoimpl.X.EnumStringOf(x.Descriptor(), protoreflect.EnumNumber(x))
+}
+
+func (ConcurrencyMode) Descriptor() protoreflect.EnumDescriptor {
+	return file_bass_proto_enumTypes[0].Descriptor()
+}
+
+func (ConcurrencyMode) Type() protoreflect.EnumType {
+	return &file_bass_proto_enumTypes[0]
+}
+
+func (x ConcurrencyMode) Number() protoreflect.EnumNumber {
+	return protoreflect.EnumNumber(x)
+}
+
+// Deprecated: Use ConcurrencyMode.Descriptor instead.
+func (ConcurrencyMode) EnumDescriptor() ([]byte, []int) {
+	return file_bass_proto_rawDescGZIP(), []int{0}
+}
+
 type Value struct {
 	state         protoimpl.MessageState
 	sizeCache     protoimpl.SizeCache
@@ -1812,8 +1861,9 @@ type CachePath struct {
 	sizeCache     protoimpl.SizeCache
 	unknownFields protoimpl.UnknownFields
 
-	Id   string          `protobuf:"bytes,1,opt,name=id,proto3" json:"id,omitempty"`
-	Path *FilesystemPath `protobuf:"bytes,2,opt,name=path,proto3" json:"path,omitempty"`
+	Id          string          `protobuf:"bytes,1,opt,name=id,proto3" json:"id,omitempty"`
+	Path        *FilesystemPath `protobuf:"bytes,2,opt,name=path,proto3" json:"path,omitempty"`
+	Concurrency ConcurrencyMode `protobuf:"varint,3,opt,name=concurrency,proto3,enum=bass.ConcurrencyMode" json:"concurrency,omitempty"`
 }
 
 func (x *CachePath) Reset() {
@@ -1862,6 +1912,13 @@ func (x *CachePath) GetPath() *FilesystemPath {
 	return nil
 }
 
+func (x *CachePath) GetConcurrency() ConcurrencyMode {
+	if x != nil {
+		return x.Concurrency
+	}
+	return ConcurrencyMode_CONCURRENCY_MODE_SHARED
+}
+
 type Secret struct {
 	state         protoimpl.MessageState
 	sizeCache     protoimpl.SizeCache
@@ -2644,53 +2701,63 @@ var file_bass_proto_rawDesc = []byte{
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+	0x4c, 0x6f, 0x67, 0x69, 0x63, 0x61, 0x6c, 0x50, 0x61, 0x74, 0x68, 0x12, 0x2c, 0x0a, 0x04, 0x66,
+	0x69, 0x6c, 0x65, 0x18, 0x01, 0x20, 0x01, 0x28, 0x0b, 0x32, 0x16, 0x2e, 0x62, 0x61, 0x73, 0x73,
+	0x2e, 0x4c, 0x6f, 0x67, 0x69, 0x63, 0x61, 0x6c, 0x50, 0x61, 0x74, 0x68, 0x2e, 0x46, 0x69, 0x6c,
+	0x65, 0x48, 0x00, 0x52, 0x04, 0x66, 0x69, 0x6c, 0x65, 0x12, 0x29, 0x0a, 0x03, 0x64, 0x69, 0x72,
+	0x18, 0x02, 0x20, 0x01, 0x28, 0x0b, 0x32, 0x15, 0x2e, 0x62, 0x61, 0x73, 0x73, 0x2e, 0x4c, 0x6f,
+	0x67, 0x69, 0x63, 0x61, 0x6c, 0x50, 0x61, 0x74, 0x68, 0x2e, 0x44, 0x69, 0x72, 0x48, 0x00, 0x52,
+	0x03, 0x64, 0x69, 0x72, 0x1a, 0x34, 0x0a, 0x04, 0x46, 0x69, 0x6c, 0x65, 0x12, 0x12, 0x0a, 0x04,
+	0x6e, 0x61, 0x6d, 0x65, 0x18, 0x01, 0x20, 0x01, 0x28, 0x09, 0x52, 0x04, 0x6e, 0x61, 0x6d, 0x65,
+	0x12, 0x18, 0x0a, 0x07, 0x63, 0x6f, 0x6e, 0x74, 0x65, 0x6e, 0x74, 0x18, 0x02, 0x20, 0x01, 0x28,
+	0x0c, 0x52, 0x07, 0x63, 0x6f, 0x6e, 0x74, 0x65, 0x6e, 0x74, 0x1a, 0x46, 0x0a, 0x03, 0x44, 0x69,
+	0x72, 0x12, 0x12, 0x0a, 0x04, 0x6e, 0x61, 0x6d, 0x65, 0x18, 0x01, 0x20, 0x01, 0x28, 0x09, 0x52,
+	0x04, 0x6e, 0x61, 0x6d, 0x65, 0x12, 0x2b, 0x0a, 0x07, 0x65, 0x6e, 0x74, 0x72, 0x69, 0x65, 0x73,
+	0x18, 0x02, 0x20, 0x03, 0x28, 0x0b, 0x32, 0x11, 0x2e, 0x62, 0x61, 0x73, 0x73, 0x2e, 0x4c, 0x6f,
+	0x67, 0x69, 0x63, 0x61, 0x6c, 0x50, 0x61, 0x74, 0x68, 0x52, 0x07, 0x65, 0x6e, 0x74, 0x72, 0x69,
+	0x65, 0x73, 0x42, 0x06, 0x0a, 0x04, 0x70, 0x61, 0x74, 0x68, 0x2a, 0x69, 0x0a, 0x0f, 0x43, 0x6f,
+	0x6e, 0x63, 0x75, 0x72, 0x72, 0x65, 0x6e, 0x63, 0x79, 0x4d, 0x6f, 0x64, 0x65, 0x12, 0x1b, 0x0a,
+	0x17, 0x43, 0x4f, 0x4e, 0x43, 0x55, 0x52, 0x52, 0x45, 0x4e, 0x43, 0x59, 0x5f, 0x4d, 0x4f, 0x44,
+	0x45, 0x5f, 0x53, 0x48, 0x41, 0x52, 0x45, 0x44, 0x10, 0x00, 0x12, 0x1c, 0x0a, 0x18, 0x43, 0x4f,
+	0x4e, 0x43, 0x55, 0x52, 0x52, 0x45, 0x4e, 0x43, 0x59, 0x5f, 0x4d, 0x4f, 0x44, 0x45, 0x5f, 0x50,
+	0x52, 0x49, 0x56, 0x41, 0x54, 0x45, 0x10, 0x01, 0x12, 0x1b, 0x0a, 0x17, 0x43, 0x4f, 0x4e, 0x43,
+	0x55, 0x52, 0x52, 0x45, 0x4e, 0x43, 0x59, 0x5f, 0x4d, 0x4f, 0x44, 0x45, 0x5f, 0x4c, 0x4f, 0x43,
+	0x4b, 0x45, 0x44, 0x10, 0x02, 0x42, 0x0b, 0x5a, 0x09, 0x70, 0x6b, 0x67, 0x2f, 0x70, 0x72, 0x6f,
+	0x74, 0x6f, 0x62, 0x06, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x33,
 }
 
 var (
@@ -2705,113 +2772,116 @@ func file_bass_proto_rawDescGZIP() []byte {
 	return file_bass_proto_rawDescData
 }
 
+var file_bass_proto_enumTypes = make([]protoimpl.EnumInfo, 1)
 var file_bass_proto_msgTypes = make([]protoimpl.MessageInfo, 33)
 var file_bass_proto_goTypes = []interface{}{
-	(*Value)(nil),            // 0: bass.Value
-	(*Thunk)(nil),            // 1: bass.Thunk
-	(*ThunkAddr)(nil),        // 2: bass.ThunkAddr
-	(*ThunkPort)(nil),        // 3: bass.ThunkPort
-	(*ThunkTLS)(nil),         // 4: bass.ThunkTLS
-	(*ThunkImage)(nil),       // 5: bass.ThunkImage
-	(*ImageRef)(nil),         // 6: bass.ImageRef
-	(*ImageArchive)(nil),     // 7: bass.ImageArchive
-	(*ImageDockerBuild)(nil), // 8: bass.ImageDockerBuild
-	(*ImageBuildInput)(nil),  // 9: bass.ImageBuildInput
-	(*BuildArg)(nil),         // 10: bass.BuildArg
-	(*Platform)(nil),         // 11: bass.Platform
-	(*ThunkDir)(nil),         // 12: bass.ThunkDir
-	(*ThunkMountSource)(nil), // 13: bass.ThunkMountSource
-	(*ThunkMount)(nil),       // 14: bass.ThunkMount
-	(*Array)(nil),            // 15: bass.Array
-	(*Object)(nil),           // 16: bass.Object
-	(*Binding)(nil),          // 17: bass.Binding
-	(*Null)(nil),             // 18: bass.Null
-	(*Bool)(nil),             // 19: bass.Bool
-	(*Int)(nil),              // 20: bass.Int
-	(*String)(nil),           // 21: bass.String
-	(*CachePath)(nil),        // 22: bass.CachePath
-	(*Secret)(nil),           // 23: bass.Secret
-	(*CommandPath)(nil),      // 24: bass.CommandPath
-	(*FilePath)(nil),         // 25: bass.FilePath
-	(*DirPath)(nil),          // 26: bass.DirPath
-	(*FilesystemPath)(nil),   // 27: bass.FilesystemPath
-	(*ThunkPath)(nil),        // 28: bass.ThunkPath
-	(*HostPath)(nil),         // 29: bass.HostPath
-	(*LogicalPath)(nil),      // 30: bass.LogicalPath
-	(*LogicalPath_File)(nil), // 31: bass.LogicalPath.File
-	(*LogicalPath_Dir)(nil),  // 32: bass.LogicalPath.Dir
+	(ConcurrencyMode)(0),     // 0: bass.ConcurrencyMode
+	(*Value)(nil),            // 1: bass.Value
+	(*Thunk)(nil),            // 2: bass.Thunk
+	(*ThunkAddr)(nil),        // 3: bass.ThunkAddr
+	(*ThunkPort)(nil),        // 4: bass.ThunkPort
+	(*ThunkTLS)(nil),         // 5: bass.ThunkTLS
+	(*ThunkImage)(nil),       // 6: bass.ThunkImage
+	(*ImageRef)(nil),         // 7: bass.ImageRef
+	(*ImageArchive)(nil),     // 8: bass.ImageArchive
+	(*ImageDockerBuild)(nil), // 9: bass.ImageDockerBuild
+	(*ImageBuildInput)(nil),  // 10: bass.ImageBuildInput
+	(*BuildArg)(nil),         // 11: bass.BuildArg
+	(*Platform)(nil),         // 12: bass.Platform
+	(*ThunkDir)(nil),         // 13: bass.ThunkDir
+	(*ThunkMountSource)(nil), // 14: bass.ThunkMountSource
+	(*ThunkMount)(nil),       // 15: bass.ThunkMount
+	(*Array)(nil),            // 16: bass.Array
+	(*Object)(nil),           // 17: bass.Object
+	(*Binding)(nil),          // 18: bass.Binding
+	(*Null)(nil),             // 19: bass.Null
+	(*Bool)(nil),             // 20: bass.Bool
+	(*Int)(nil),              // 21: bass.Int
+	(*String)(nil),           // 22: bass.String
+	(*CachePath)(nil),        // 23: bass.CachePath
+	(*Secret)(nil),           // 24: bass.Secret
+	(*CommandPath)(nil),      // 25: bass.CommandPath
+	(*FilePath)(nil),         // 26: bass.FilePath
+	(*DirPath)(nil),          // 27: bass.DirPath
+	(*FilesystemPath)(nil),   // 28: bass.FilesystemPath
+	(*ThunkPath)(nil),        // 29: bass.ThunkPath
+	(*HostPath)(nil),         // 30: bass.HostPath
+	(*LogicalPath)(nil),      // 31: bass.LogicalPath
+	(*LogicalPath_File)(nil), // 32: bass.LogicalPath.File
+	(*LogicalPath_Dir)(nil),  // 33: bass.LogicalPath.Dir
 }
 var file_bass_proto_depIdxs = []int32{
-	18, // 0: bass.Value.null:type_name -> bass.Null
-	19, // 1: bass.Value.bool:type_name -> bass.Bool
-	20, // 2: bass.Value.int:type_name -> bass.Int
-	21, // 3: bass.Value.string:type_name -> bass.String
-	23, // 4: bass.Value.secret:type_name -> bass.Secret
-	15, // 5: bass.Value.array:type_name -> bass.Array
-	16, // 6: bass.Value.object:type_name -> bass.Object
-	1,  // 7: bass.Value.thunk:type_name -> bass.Thunk
-	24, // 8: bass.Value.command_path:type_name -> bass.CommandPath
-	25, // 9: bass.Value.file_path:type_name -> bass.FilePath
-	26, // 10: bass.Value.dir_path:type_name -> bass.DirPath
-	29, // 11: bass.Value.host_path:type_name -> bass.HostPath
-	28, // 12: bass.Value.thunk_path:type_name -> bass.ThunkPath
-	30, // 13: bass.Value.logical_path:type_name -> bass.LogicalPath
-	2,  // 14: bass.Value.thunk_addr:type_name -> bass.ThunkAddr
-	22, // 15: bass.Value.cache_path:type_name -> bass.CachePath
-	5,  // 16: bass.Thunk.image:type_name -> bass.ThunkImage
-	0,  // 17: bass.Thunk.args:type_name -> bass.Value
-	0,  // 18: bass.Thunk.stdin:type_name -> bass.Value
-	17, // 19: bass.Thunk.env:type_name -> bass.Binding
-	12, // 20: bass.Thunk.dir:type_name -> bass.ThunkDir
-	14, // 21: bass.Thunk.mounts:type_name -> bass.ThunkMount
-	17, // 22: bass.Thunk.labels:type_name -> bass.Binding
-	3,  // 23: bass.Thunk.ports:type_name -> bass.ThunkPort
-	4,  // 24: bass.Thunk.tls:type_name -> bass.ThunkTLS
-	1,  // 25: bass.ThunkAddr.thunk:type_name -> bass.Thunk
-	25, // 26: bass.ThunkTLS.cert:type_name -> bass.FilePath
-	25, // 27: bass.ThunkTLS.key:type_name -> bass.FilePath
-	6,  // 28: bass.ThunkImage.ref:type_name -> bass.ImageRef
-	1,  // 29: bass.ThunkImage.thunk:type_name -> bass.Thunk
-	7,  // 30: bass.ThunkImage.archive:type_name -> bass.ImageArchive
-	8,  // 31: bass.ThunkImage.docker_build:type_name -> bass.ImageDockerBuild
-	11, // 32: bass.ImageRef.platform:type_name -> bass.Platform
-	28, // 33: bass.ImageRef.file:type_name -> bass.ThunkPath
-	2,  // 34: bass.ImageRef.addr:type_name -> bass.ThunkAddr
-	11, // 35: bass.ImageArchive.platform:type_name -> bass.Platform
-	28, // 36: bass.ImageArchive.file:type_name -> bass.ThunkPath
-	11, // 37: bass.ImageDockerBuild.platform:type_name -> bass.Platform
-	9,  // 38: bass.ImageDockerBuild.context:type_name -> bass.ImageBuildInput
-	10, // 39: bass.ImageDockerBuild.args:type_name -> bass.BuildArg
-	28, // 40: bass.ImageBuildInput.thunk:type_name -> bass.ThunkPath
-	29, // 41: bass.ImageBuildInput.host:type_name -> bass.HostPath
-	30, // 42: bass.ImageBuildInput.logical:type_name -> bass.LogicalPath
-	26, // 43: bass.ThunkDir.local:type_name -> bass.DirPath
-	28, // 44: bass.ThunkDir.thunk:type_name -> bass.ThunkPath
-	29, // 45: bass.ThunkDir.host:type_name -> bass.HostPath
-	28, // 46: bass.ThunkMountSource.thunk:type_name -> bass.ThunkPath
-	29, // 47: bass.ThunkMountSource.host:type_name -> bass.HostPath
-	30, // 48: bass.ThunkMountSource.logical:type_name -> bass.LogicalPath
-	22, // 49: bass.ThunkMountSource.cache:type_name -> bass.CachePath
-	23, // 50: bass.ThunkMountSource.secret:type_name -> bass.Secret
-	13, // 51: bass.ThunkMount.source:type_name -> bass.ThunkMountSource
-	27, // 52: bass.ThunkMount.target:type_name -> bass.FilesystemPath
-	0,  // 53: bass.Array.values:type_name -> bass.Value
-	17, // 54: bass.Object.bindings:type_name -> bass.Binding
-	0,  // 55: bass.Binding.value:type_name -> bass.Value
-	27, // 56: bass.CachePath.path:type_name -> bass.FilesystemPath
-	25, // 57: bass.FilesystemPath.file:type_name -> bass.FilePath
-	26, // 58: bass.FilesystemPath.dir:type_name -> bass.DirPath
-	1,  // 59: bass.ThunkPath.thunk:type_name -> bass.Thunk
-	27, // 60: bass.ThunkPath.path:type_name -> bass.FilesystemPath
-	27, // 61: bass.HostPath.path:type_name -> bass.FilesystemPath
-	31, // 62: bass.LogicalPath.file:type_name -> bass.LogicalPath.File
-	32, // 63: bass.LogicalPath.dir:type_name -> bass.LogicalPath.Dir
-	30, // 64: bass.LogicalPath.Dir.entries:type_name -> bass.LogicalPath
-	65, // [65:65] is the sub-list for method output_type
-	65, // [65:65] is the sub-list for method input_type
-	65, // [65:65] is the sub-list for extension type_name
-	65, // [65:65] is the sub-list for extension extendee
-	0,  // [0:65] is the sub-list for field type_name
+	19, // 0: bass.Value.null:type_name -> bass.Null
+	20, // 1: bass.Value.bool:type_name -> bass.Bool
+	21, // 2: bass.Value.int:type_name -> bass.Int
+	22, // 3: bass.Value.string:type_name -> bass.String
+	24, // 4: bass.Value.secret:type_name -> bass.Secret
+	16, // 5: bass.Value.array:type_name -> bass.Array
+	17, // 6: bass.Value.object:type_name -> bass.Object
+	2,  // 7: bass.Value.thunk:type_name -> bass.Thunk
+	25, // 8: bass.Value.command_path:type_name -> bass.CommandPath
+	26, // 9: bass.Value.file_path:type_name -> bass.FilePath
+	27, // 10: bass.Value.dir_path:type_name -> bass.DirPath
+	30, // 11: bass.Value.host_path:type_name -> bass.HostPath
+	29, // 12: bass.Value.thunk_path:type_name -> bass.ThunkPath
+	31, // 13: bass.Value.logical_path:type_name -> bass.LogicalPath
+	3,  // 14: bass.Value.thunk_addr:type_name -> bass.ThunkAddr
+	23, // 15: bass.Value.cache_path:type_name -> bass.CachePath
+	6,  // 16: bass.Thunk.image:type_name -> bass.ThunkImage
+	1,  // 17: bass.Thunk.args:type_name -> bass.Value
+	1,  // 18: bass.Thunk.stdin:type_name -> bass.Value
+	18, // 19: bass.Thunk.env:type_name -> bass.Binding
+	13, // 20: bass.Thunk.dir:type_name -> bass.ThunkDir
+	15, // 21: bass.Thunk.mounts:type_name -> bass.ThunkMount
+	18, // 22: bass.Thunk.labels:type_name -> bass.Binding
+	4,  // 23: bass.Thunk.ports:type_name -> bass.ThunkPort
+	5,  // 24: bass.Thunk.tls:type_name -> bass.ThunkTLS
+	2,  // 25: bass.ThunkAddr.thunk:type_name -> bass.Thunk
+	26, // 26: bass.ThunkTLS.cert:type_name -> bass.FilePath
+	26, // 27: bass.ThunkTLS.key:type_name -> bass.FilePath
+	7,  // 28: bass.ThunkImage.ref:type_name -> bass.ImageRef
+	2,  // 29: bass.ThunkImage.thunk:type_name -> bass.Thunk
+	8,  // 30: bass.ThunkImage.archive:type_name -> bass.ImageArchive
+	9,  // 31: bass.ThunkImage.docker_build:type_name -> bass.ImageDockerBuild
+	12, // 32: bass.ImageRef.platform:type_name -> bass.Platform
+	29, // 33: bass.ImageRef.file:type_name -> bass.ThunkPath
+	3,  // 34: bass.ImageRef.addr:type_name -> bass.ThunkAddr
+	12, // 35: bass.ImageArchive.platform:type_name -> bass.Platform
+	29, // 36: bass.ImageArchive.file:type_name -> bass.ThunkPath
+	12, // 37: bass.ImageDockerBuild.platform:type_name -> bass.Platform
+	10, // 38: bass.ImageDockerBuild.context:type_name -> bass.ImageBuildInput
+	11, // 39: bass.ImageDockerBuild.args:type_name -> bass.BuildArg
+	29, // 40: bass.ImageBuildInput.thunk:type_name -> bass.ThunkPath
+	30, // 41: bass.ImageBuildInput.host:type_name -> bass.HostPath
+	31, // 42: bass.ImageBuildInput.logical:type_name -> bass.LogicalPath
+	27, // 43: bass.ThunkDir.local:type_name -> bass.DirPath
+	29, // 44: bass.ThunkDir.thunk:type_name -> bass.ThunkPath
+	30, // 45: bass.ThunkDir.host:type_name -> bass.HostPath
+	29, // 46: bass.ThunkMountSource.thunk:type_name -> bass.ThunkPath
+	30, // 47: bass.ThunkMountSource.host:type_name -> bass.HostPath
+	31, // 48: bass.ThunkMountSource.logical:type_name -> bass.LogicalPath
+	23, // 49: bass.ThunkMountSource.cache:type_name -> bass.CachePath
+	24, // 50: bass.ThunkMountSource.secret:type_name -> bass.Secret
+	14, // 51: bass.ThunkMount.source:type_name -> bass.ThunkMountSource
+	28, // 52: bass.ThunkMount.target:type_name -> bass.FilesystemPath
+	1,  // 53: bass.Array.values:type_name -> bass.Value
+	18, // 54: bass.Object.bindings:type_name -> bass.Binding
+	1,  // 55: bass.Binding.value:type_name -> bass.Value
+	28, // 56: bass.CachePath.path:type_name -> bass.FilesystemPath
+	0,  // 57: bass.CachePath.concurrency:type_name -> bass.ConcurrencyMode
+	26, // 58: bass.FilesystemPath.file:type_name -> bass.FilePath
+	27, // 59: bass.FilesystemPath.dir:type_name -> bass.DirPath
+	2,  // 60: bass.ThunkPath.thunk:type_name -> bass.Thunk
+	28, // 61: bass.ThunkPath.path:type_name -> bass.FilesystemPath
+	28, // 62: bass.HostPath.path:type_name -> bass.FilesystemPath
+	32, // 63: bass.LogicalPath.file:type_name -> bass.LogicalPath.File
+	33, // 64: bass.LogicalPath.dir:type_name -> bass.LogicalPath.Dir
+	31, // 65: bass.LogicalPath.Dir.entries:type_name -> bass.LogicalPath
+	66, // [66:66] is the sub-list for method output_type
+	66, // [66:66] is the sub-list for method input_type
+	66, // [66:66] is the sub-list for extension type_name
+	66, // [66:66] is the sub-list for extension extendee
+	0,  // [0:66] is the sub-list for field type_name
 }
 
 func init() { file_bass_proto_init() }
@@ -3278,13 +3348,14 @@ func file_bass_proto_init() {
 		File: protoimpl.DescBuilder{
 			GoPackagePath: reflect.TypeOf(x{}).PkgPath(),
 			RawDescriptor: file_bass_proto_rawDesc,
-			NumEnums:      0,
+			NumEnums:      1,
 			NumMessages:   33,
 			NumExtensions: 0,
 			NumServices:   0,
 		},
 		GoTypes:           file_bass_proto_goTypes,
 		DependencyIndexes: file_bass_proto_depIdxs,
+		EnumInfos:         file_bass_proto_enumTypes,
 		MessageInfos:      file_bass_proto_msgTypes,
 	}.Build()
 	File_bass_proto = out.File
diff --git a/pkg/runtimes/buildkit.go b/pkg/runtimes/buildkit.go
index 5755c4ef..ed50712c 100644
--- a/pkg/runtimes/buildkit.go
+++ b/pkg/runtimes/buildkit.go
@@ -1312,10 +1312,20 @@ func (b *buildkitBuilder) initializeMount(ctx context.Context, source bass.Thunk
 		return llb.AddMount(targetPath, st, llb.SourcePath(sp)), sp, false, nil
 
 	case source.Cache != nil:
+		var mode llb.CacheMountSharingMode
+		switch source.Cache.ConcurrencyMode {
+		case bass.ConcurrencyModeShared:
+			mode = llb.CacheMountShared
+		case bass.ConcurrencyModePrivate:
+			mode = llb.CacheMountPrivate
+		case bass.ConcurrencyModeLocked:
+			mode = llb.CacheMountLocked
+		}
+
 		return llb.AddMount(
 			targetPath,
 			llb.Scratch(),
-			llb.AsPersistentCacheDir(source.Cache.ID, llb.CacheMountLocked),
+			llb.AsPersistentCacheDir(source.Cache.ID, mode),
 			llb.SourcePath(source.Cache.Path.FilesystemPath().FromSlash()),
 		), "", false, nil
 
diff --git a/pkg/runtimes/dagger.go b/pkg/runtimes/dagger.go
index 03c29186..d9105e5b 100644
--- a/pkg/runtimes/dagger.go
+++ b/pkg/runtimes/dagger.go
@@ -413,11 +413,21 @@ func (runtime *Dagger) mount(ctx context.Context, client *dagger.Client, ctr *da
 			return nil, fmt.Errorf("mounting subpaths of cache not implemented yet: %s", fsp.Slash())
 		}
 
+		var mode dagger.CacheSharingMode
+		switch src.Cache.ConcurrencyMode {
+		case bass.ConcurrencyModeShared:
+			mode = dagger.Shared
+		case bass.ConcurrencyModePrivate:
+			mode = dagger.Private
+		case bass.ConcurrencyModeLocked:
+			mode = dagger.Locked
+		}
+
 		return ctr.WithMountedCache(
 			target,
 			client.CacheVolume(src.Cache.ID),
 			dagger.ContainerWithMountedCacheOpts{
-				Sharing: dagger.Locked,
+				Sharing: mode,
 			},
 		), nil
 	case src.FSPath != nil:
diff --git a/pkg/runtimes/suite.go b/pkg/runtimes/suite.go
index 355a684a..5a15986b 100644
--- a/pkg/runtimes/suite.go
+++ b/pkg/runtimes/suite.go
@@ -169,18 +169,6 @@ func Suite(t *testing.T, runtimeConfig bass.RuntimeConfig, opts ...SuiteOpt) {
 		},
 		{
 			File: "cache-sync.bass",
-			Result: bass.NewList(
-				bass.NewList(bass.String("1")),
-				bass.NewList(bass.String("2")),
-				bass.NewList(bass.String("3")),
-				bass.NewList(bass.String("4")),
-				bass.NewList(bass.String("5")),
-				bass.NewList(bass.String("6")),
-				bass.NewList(bass.String("7")),
-				bass.NewList(bass.String("8")),
-				bass.NewList(bass.String("9")),
-				bass.NewList(bass.String("10")),
-			),
 		},
 		{
 			File:   "cache-cmd.bass",
diff --git a/pkg/runtimes/testdata/cache-sync.bass b/pkg/runtimes/testdata/cache-sync.bass
index 0e0805d5..21cefdac 100644
--- a/pkg/runtimes/testdata/cache-sync.bass
+++ b/pkg/runtimes/testdata/cache-sync.bass
@@ -1,5 +1,5 @@
 (def test-cache-path
-  (cache-dir (str "test-cache-sync-" (now 0))))
+  (cache-dir (str "test-cache-sync-" (now 0)) :locked))
 
 (defn counter [tag]
   (subpath
@@ -18,7 +18,8 @@
   (let [files (map counter tags)]
     (-> ($ sh -c "cat $@ | sort -n" ignored & $files)
         (with-image (linux/alpine))
-        (read :unix-table)
+        (read :lines)
         all)))
 
-(counts "a" "b" "c" "d" "e" "f" "g" "h" "i" "j")
+(assert = ["1" "2" "3" "4" "5" "6" "7" "8" "9" "10"]
+  (counts "a" "b" "c" "d" "e" "f" "g" "h" "i" "j"))
diff --git a/proto/bass.proto b/proto/bass.proto
index eb6d580d..dc86a675 100644
--- a/proto/bass.proto
+++ b/proto/bass.proto
@@ -163,6 +163,13 @@ message String {
 message CachePath {
   string id = 1;
   FilesystemPath path = 2;
+  ConcurrencyMode concurrency = 3;
+};
+
+enum ConcurrencyMode {
+  CONCURRENCY_MODE_SHARED = 0;
+  CONCURRENCY_MODE_PRIVATE = 1;
+  CONCURRENCY_MODE_LOCKED = 2;
 };
 
 message Secret {