diff --git a/go/ai/embedder.go b/go/ai/embedder.go
index bec1ff492..fca80fa43 100644
--- a/go/ai/embedder.go
+++ b/go/ai/embedder.go
@@ -18,6 +18,7 @@ import (
 	"context"
 
 	"github.com/firebase/genkit/go/core"
+	"github.com/firebase/genkit/go/internal/atype"
 )
 
 // EmbedderAction is used to convert a document to a
@@ -34,13 +35,13 @@ type EmbedRequest struct {
 // DefineEmbedder registers the given embed function as an action, and returns an
 // [EmbedderAction] that runs it.
 func DefineEmbedder(provider, name string, embed func(context.Context, *EmbedRequest) ([]float32, error)) *EmbedderAction {
-	return core.DefineAction(provider, name, core.ActionTypeEmbedder, nil, embed)
+	return core.DefineAction(provider, name, atype.Embedder, nil, embed)
 }
 
 // LookupEmbedder looks up an [EmbedderAction] registered by [DefineEmbedder].
 // It returns nil if the embedder was not defined.
 func LookupEmbedder(provider, name string) *EmbedderAction {
-	action := core.LookupActionFor[*EmbedRequest, []float32, struct{}](core.ActionTypeEmbedder, provider, name)
+	action := core.LookupActionFor[*EmbedRequest, []float32, struct{}](atype.Embedder, provider, name)
 	if action == nil {
 		return nil
 	}
diff --git a/go/ai/generate.go b/go/ai/generate.go
index ddf0822da..b27657bae 100644
--- a/go/ai/generate.go
+++ b/go/ai/generate.go
@@ -25,6 +25,7 @@ import (
 
 	"github.com/firebase/genkit/go/core"
 	"github.com/firebase/genkit/go/core/logger"
+	"github.com/firebase/genkit/go/internal/atype"
 )
 
 // A ModelAction is used to generate content from an AI model.
@@ -63,7 +64,7 @@ func DefineModel(provider, name string, metadata *ModelMetadata, generate func(c
 		}
 		metadataMap["supports"] = supports
 	}
-	return core.DefineStreamingAction(provider, name, core.ActionTypeModel, map[string]any{
+	return core.DefineStreamingAction(provider, name, atype.Model, map[string]any{
 		"model": metadataMap,
 	}, generate)
 }
@@ -71,7 +72,7 @@ func DefineModel(provider, name string, metadata *ModelMetadata, generate func(c
 // LookupModel looks up a [ModelAction] registered by [DefineModel].
 // It returns nil if the model was not defined.
 func LookupModel(provider, name string) *ModelAction {
-	return core.LookupActionFor[*GenerateRequest, *GenerateResponse, *Candidate](core.ActionTypeModel, provider, name)
+	return core.LookupActionFor[*GenerateRequest, *GenerateResponse, *Candidate](atype.Model, provider, name)
 }
 
 // Generate applies a [ModelAction] to some input, handling tool requests.
diff --git a/go/ai/prompt.go b/go/ai/prompt.go
index 43d88c65a..a3fca0ab4 100644
--- a/go/ai/prompt.go
+++ b/go/ai/prompt.go
@@ -18,6 +18,7 @@ import (
 	"context"
 
 	"github.com/firebase/genkit/go/core"
+	"github.com/firebase/genkit/go/internal/atype"
 )
 
 // PromptRequest is a request to execute a prompt template and
@@ -50,5 +51,5 @@ func RegisterPrompt(provider, name string, prompt Prompt) {
 		"prompt": prompt,
 	}
 	core.RegisterAction(provider,
-		core.NewStreamingAction(name, core.ActionTypePrompt, metadata, prompt.Generate))
+		core.NewStreamingAction(name, atype.Prompt, metadata, prompt.Generate))
 }
diff --git a/go/ai/retriever.go b/go/ai/retriever.go
index 0579eadec..50b405ffd 100644
--- a/go/ai/retriever.go
+++ b/go/ai/retriever.go
@@ -18,6 +18,7 @@ import (
 	"context"
 
 	"github.com/firebase/genkit/go/core"
+	"github.com/firebase/genkit/go/internal/atype"
 )
 
 type (
@@ -52,25 +53,25 @@ func DefineIndexer(provider, name string, index func(context.Context, *IndexerRe
 	f := func(ctx context.Context, req *IndexerRequest) (struct{}, error) {
 		return struct{}{}, index(ctx, req)
 	}
-	return core.DefineAction(provider, name, core.ActionTypeIndexer, nil, f)
+	return core.DefineAction(provider, name, atype.Indexer, nil, f)
 }
 
 // LookupIndexer looks up a [IndexerAction] registered by [DefineIndexer].
 // It returns nil if the model was not defined.
 func LookupIndexer(provider, name string) *IndexerAction {
-	return core.LookupActionFor[*IndexerRequest, struct{}, struct{}](core.ActionTypeIndexer, provider, name)
+	return core.LookupActionFor[*IndexerRequest, struct{}, struct{}](atype.Indexer, provider, name)
 }
 
 // DefineRetriever registers the given retrieve function as an action, and returns a
 // [RetrieverAction] that runs it.
 func DefineRetriever(provider, name string, ret func(context.Context, *RetrieverRequest) (*RetrieverResponse, error)) *RetrieverAction {
-	return core.DefineAction(provider, name, core.ActionTypeRetriever, nil, ret)
+	return core.DefineAction(provider, name, atype.Retriever, nil, ret)
 }
 
 // LookupRetriever looks up a [RetrieverAction] registered by [DefineRetriever].
 // It returns nil if the model was not defined.
 func LookupRetriever(provider, name string) *RetrieverAction {
-	return core.LookupActionFor[*RetrieverRequest, *RetrieverResponse, struct{}](core.ActionTypeRetriever, provider, name)
+	return core.LookupActionFor[*RetrieverRequest, *RetrieverResponse, struct{}](atype.Retriever, provider, name)
 }
 
 // Index runs the given [IndexerAction].
diff --git a/go/ai/tools.go b/go/ai/tools.go
index 53b48ccf6..e3ef6735a 100644
--- a/go/ai/tools.go
+++ b/go/ai/tools.go
@@ -20,6 +20,7 @@ import (
 	"maps"
 
 	"github.com/firebase/genkit/go/core"
+	"github.com/firebase/genkit/go/internal/atype"
 )
 
 // A Tool is an implementation of a single tool.
@@ -41,7 +42,7 @@ func RegisterTool(definition *ToolDefinition, metadata map[string]any, fn func(c
 	}
 	metadata["type"] = "tool"
 
-	core.DefineAction("local", definition.Name, core.ActionTypeTool, metadata, fn)
+	core.DefineAction("local", definition.Name, atype.Tool, metadata, fn)
 }
 
 // toolActionType is the instantiated core.Action type registered
@@ -51,7 +52,7 @@ type toolActionType = core.Action[map[string]any, map[string]any, struct{}]
 // RunTool looks up a tool registered by [RegisterTool],
 // runs it with the given input, and returns the result.
 func RunTool(ctx context.Context, name string, input map[string]any) (map[string]any, error) {
-	action := core.LookupAction(core.ActionTypeTool, "local", name)
+	action := core.LookupAction(atype.Tool, "local", name)
 	if action == nil {
 		return nil, fmt.Errorf("no tool named %q", name)
 	}
diff --git a/go/core/action.go b/go/core/action.go
index 8b8cf4f37..8df4c083b 100644
--- a/go/core/action.go
+++ b/go/core/action.go
@@ -24,6 +24,7 @@ import (
 	"github.com/firebase/genkit/go/core/logger"
 	"github.com/firebase/genkit/go/core/tracing"
 	"github.com/firebase/genkit/go/internal"
+	"github.com/firebase/genkit/go/internal/atype"
 	"github.com/invopop/jsonschema"
 )
 
@@ -54,7 +55,7 @@ type streamingCallback[Stream any] func(context.Context, Stream) error
 // Each time an Action is run, it results in a new trace span.
 type Action[In, Out, Stream any] struct {
 	name         string
-	atype        ActionType
+	atype        atype.ActionType
 	fn           Func[In, Out, Stream]
 	tstate       *tracing.State
 	inputSchema  *jsonschema.Schema
@@ -67,35 +68,39 @@ type Action[In, Out, Stream any] struct {
 // See js/core/src/action.ts
 
 // DefineAction creates a new Action and registers it.
-func DefineAction[In, Out any](provider, name string, atype ActionType, metadata map[string]any, fn func(context.Context, In) (Out, error)) *Action[In, Out, struct{}] {
+func DefineAction[In, Out any](provider, name string, atype atype.ActionType, metadata map[string]any, fn func(context.Context, In) (Out, error)) *Action[In, Out, struct{}] {
 	return defineAction(globalRegistry, provider, name, atype, metadata, fn)
 }
 
-func defineAction[In, Out any](r *registry, provider, name string, atype ActionType, metadata map[string]any, fn func(context.Context, In) (Out, error)) *Action[In, Out, struct{}] {
+func defineAction[In, Out any](r *registry, provider, name string, atype atype.ActionType, metadata map[string]any, fn func(context.Context, In) (Out, error)) *Action[In, Out, struct{}] {
 	a := NewAction(name, atype, metadata, fn)
 	r.registerAction(provider, a)
 	return a
 }
 
-func DefineStreamingAction[In, Out, Stream any](provider, name string, atype ActionType, metadata map[string]any, fn Func[In, Out, Stream]) *Action[In, Out, Stream] {
+func DefineStreamingAction[In, Out, Stream any](provider, name string, atype atype.ActionType, metadata map[string]any, fn Func[In, Out, Stream]) *Action[In, Out, Stream] {
 	return defineStreamingAction(globalRegistry, provider, name, atype, metadata, fn)
 }
 
-func defineStreamingAction[In, Out, Stream any](r *registry, provider, name string, atype ActionType, metadata map[string]any, fn Func[In, Out, Stream]) *Action[In, Out, Stream] {
+func defineStreamingAction[In, Out, Stream any](r *registry, provider, name string, atype atype.ActionType, metadata map[string]any, fn Func[In, Out, Stream]) *Action[In, Out, Stream] {
 	a := NewStreamingAction(name, atype, metadata, fn)
 	r.registerAction(provider, a)
 	return a
 }
 
+func DefineCustomAction[In, Out, Stream any](provider, name string, metadata map[string]any, fn Func[In, Out, Stream]) *Action[In, Out, Stream] {
+	return DefineStreamingAction(provider, name, atype.Custom, metadata, fn)
+}
+
 // NewAction creates a new Action with the given name and non-streaming function.
-func NewAction[In, Out any](name string, atype ActionType, metadata map[string]any, fn func(context.Context, In) (Out, error)) *Action[In, Out, struct{}] {
+func NewAction[In, Out any](name string, atype atype.ActionType, metadata map[string]any, fn func(context.Context, In) (Out, error)) *Action[In, Out, struct{}] {
 	return NewStreamingAction(name, atype, metadata, func(ctx context.Context, in In, cb NoStream) (Out, error) {
 		return fn(ctx, in)
 	})
 }
 
 // NewStreamingAction creates a new Action with the given name and streaming function.
-func NewStreamingAction[In, Out, Stream any](name string, atype ActionType, metadata map[string]any, fn Func[In, Out, Stream]) *Action[In, Out, Stream] {
+func NewStreamingAction[In, Out, Stream any](name string, atype atype.ActionType, metadata map[string]any, fn Func[In, Out, Stream]) *Action[In, Out, Stream] {
 	var i In
 	var o Out
 	return &Action[In, Out, Stream]{
@@ -114,7 +119,7 @@ func NewStreamingAction[In, Out, Stream any](name string, atype ActionType, meta
 // Name returns the Action's name.
 func (a *Action[In, Out, Stream]) Name() string { return a.name }
 
-func (a *Action[In, Out, Stream]) actionType() ActionType { return a.atype }
+func (a *Action[In, Out, Stream]) actionType() atype.ActionType { return a.atype }
 
 // setTracingState sets the action's tracing.State.
 func (a *Action[In, Out, Stream]) setTracingState(tstate *tracing.State) { a.tstate = tstate }
@@ -194,7 +199,7 @@ func (a *Action[In, Out, Stream]) runJSON(ctx context.Context, input json.RawMes
 // action is the type that all Action[I, O, S] have in common.
 type action interface {
 	Name() string
-	actionType() ActionType
+	actionType() atype.ActionType
 
 	// runJSON uses encoding/json to unmarshal the input,
 	// calls Action.Run, then returns the marshaled result.
diff --git a/go/core/action_test.go b/go/core/action_test.go
index 5cc1ee9d0..59ac581be 100644
--- a/go/core/action_test.go
+++ b/go/core/action_test.go
@@ -19,6 +19,8 @@ import (
 	"context"
 	"slices"
 	"testing"
+
+	"github.com/firebase/genkit/go/internal/atype"
 )
 
 func inc(_ context.Context, x int) (int, error) {
@@ -26,7 +28,7 @@ func inc(_ context.Context, x int) (int, error) {
 }
 
 func TestActionRun(t *testing.T) {
-	a := NewAction("inc", ActionTypeCustom, nil, inc)
+	a := NewAction("inc", atype.Custom, nil, inc)
 	got, err := a.Run(context.Background(), 3, nil)
 	if err != nil {
 		t.Fatal(err)
@@ -37,7 +39,7 @@ func TestActionRun(t *testing.T) {
 }
 
 func TestActionRunJSON(t *testing.T) {
-	a := NewAction("inc", ActionTypeCustom, nil, inc)
+	a := NewAction("inc", atype.Custom, nil, inc)
 	input := []byte("3")
 	want := []byte("4")
 	got, err := a.runJSON(context.Background(), input, nil)
@@ -51,7 +53,7 @@ func TestActionRunJSON(t *testing.T) {
 
 func TestNewAction(t *testing.T) {
 	// Verify that struct{} can occur in the function signature.
-	_ = NewAction("f", ActionTypeCustom, nil, func(context.Context, int) (struct{}, error) { return struct{}{}, nil })
+	_ = NewAction("f", atype.Custom, nil, func(context.Context, int) (struct{}, error) { return struct{}{}, nil })
 }
 
 // count streams the numbers from 0 to n-1, then returns n.
@@ -68,7 +70,7 @@ func count(ctx context.Context, n int, cb func(context.Context, int) error) (int
 
 func TestActionStreaming(t *testing.T) {
 	ctx := context.Background()
-	a := NewStreamingAction("count", ActionTypeCustom, nil, count)
+	a := NewStreamingAction("count", atype.Custom, nil, count)
 	const n = 3
 
 	// Non-streaming.
@@ -101,7 +103,7 @@ func TestActionStreaming(t *testing.T) {
 func TestActionTracing(t *testing.T) {
 	ctx := context.Background()
 	const actionName = "TestTracing-inc"
-	a := NewAction(actionName, ActionTypeCustom, nil, inc)
+	a := NewAction(actionName, atype.Custom, nil, inc)
 	if _, err := a.Run(context.Background(), 3, nil); err != nil {
 		t.Fatal(err)
 	}
diff --git a/go/core/flow.go b/go/core/flow.go
index 25a38b0dc..6201fe3d8 100644
--- a/go/core/flow.go
+++ b/go/core/flow.go
@@ -27,6 +27,7 @@ import (
 	"github.com/firebase/genkit/go/core/logger"
 	"github.com/firebase/genkit/go/core/tracing"
 	"github.com/firebase/genkit/go/internal"
+	"github.com/firebase/genkit/go/internal/atype"
 	"github.com/google/uuid"
 	"github.com/invopop/jsonschema"
 	otrace "go.opentelemetry.io/otel/trace"
@@ -258,7 +259,7 @@ func (f *Flow[In, Out, Stream]) action() *Action[*flowInstruction[In], *flowStat
 		tracing.SetCustomMetadataAttr(ctx, "flow:wrapperAction", "true")
 		return f.runInstruction(ctx, inst, streamingCallback[Stream](cb))
 	}
-	return NewStreamingAction(f.name, ActionTypeFlow, metadata, cback)
+	return NewStreamingAction(f.name, atype.Flow, metadata, cback)
 }
 
 // runInstruction performs one of several actions on a flow, as determined by msg.
diff --git a/go/core/registry.go b/go/core/registry.go
index 5e5a18dd4..a41fba888 100644
--- a/go/core/registry.go
+++ b/go/core/registry.go
@@ -26,6 +26,7 @@ import (
 	"sync"
 
 	"github.com/firebase/genkit/go/core/tracing"
+	"github.com/firebase/genkit/go/internal/atype"
 	sdktrace "go.opentelemetry.io/otel/sdk/trace"
 	"golang.org/x/exp/maps"
 )
@@ -90,23 +91,6 @@ const (
 	EnvironmentProd Environment = "prod" // production: user data, SLOs, etc.
 )
 
-// An ActionType is the kind of an action.
-type ActionType string
-
-const (
-	ActionTypeChatLLM   ActionType = "chat-llm"
-	ActionTypeTextLLM   ActionType = "text-llm"
-	ActionTypeRetriever ActionType = "retriever"
-	ActionTypeIndexer   ActionType = "indexer"
-	ActionTypeEmbedder  ActionType = "embedder"
-	ActionTypeEvaluator ActionType = "evaluator"
-	ActionTypeFlow      ActionType = "flow"
-	ActionTypeModel     ActionType = "model"
-	ActionTypePrompt    ActionType = "prompt"
-	ActionTypeTool      ActionType = "tool"
-	ActionTypeCustom    ActionType = "custom"
-)
-
 // RegisterAction records the action in the global registry.
 // It panics if an action with the same type, provider and name is already
 // registered.
@@ -138,7 +122,7 @@ func (r *registry) lookupAction(key string) action {
 
 // LookupAction returns the action for the given key in the global registry,
 // or nil if there is none.
-func LookupAction(typ ActionType, provider, name string) action {
+func LookupAction(typ atype.ActionType, provider, name string) action {
 	key := fmt.Sprintf("/%s/%s/%s", typ, provider, name)
 	return globalRegistry.lookupAction(key)
 }
@@ -146,7 +130,7 @@ func LookupAction(typ ActionType, provider, name string) action {
 // LookupActionFor returns the action for the given key in the global registry,
 // or nil if there is none.
 // It panics if the action is of the wrong type.
-func LookupActionFor[In, Out, Stream any](typ ActionType, provider, name string) *Action[In, Out, Stream] {
+func LookupActionFor[In, Out, Stream any](typ atype.ActionType, provider, name string) *Action[In, Out, Stream] {
 	a := LookupAction(typ, provider, name)
 	if a == nil {
 		return nil
diff --git a/go/core/servers_test.go b/go/core/servers_test.go
index 2ba017019..a14249ab2 100644
--- a/go/core/servers_test.go
+++ b/go/core/servers_test.go
@@ -24,6 +24,7 @@ import (
 	"testing"
 
 	"github.com/firebase/genkit/go/core/tracing"
+	"github.com/firebase/genkit/go/internal/atype"
 	"github.com/google/go-cmp/cmp"
 	"github.com/google/go-cmp/cmp/cmpopts"
 	"github.com/invopop/jsonschema"
@@ -38,10 +39,10 @@ func TestDevServer(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	r.registerAction("devServer", NewAction("inc", ActionTypeCustom, map[string]any{
+	r.registerAction("devServer", NewAction("inc", atype.Custom, map[string]any{
 		"foo": "bar",
 	}, inc))
-	r.registerAction("devServer", NewAction("dec", ActionTypeCustom, map[string]any{
+	r.registerAction("devServer", NewAction("dec", atype.Custom, map[string]any{
 		"bar": "baz",
 	}, dec))
 	srv := httptest.NewServer(newDevServeMux(r))
diff --git a/go/internal/atype/atype.go b/go/internal/atype/atype.go
new file mode 100644
index 000000000..f5b48c207
--- /dev/null
+++ b/go/internal/atype/atype.go
@@ -0,0 +1,33 @@
+// Copyright 2024 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package atype provides types for Genkit actions.
+package atype
+
+// An ActionType is the kind of an action.
+type ActionType string
+
+const (
+	ChatLLM   ActionType = "chat-llm"
+	TextLLM   ActionType = "text-llm"
+	Retriever ActionType = "retriever"
+	Indexer   ActionType = "indexer"
+	Embedder  ActionType = "embedder"
+	Evaluator ActionType = "evaluator"
+	Flow      ActionType = "flow"
+	Model     ActionType = "model"
+	Prompt    ActionType = "prompt"
+	Tool      ActionType = "tool"
+	Custom    ActionType = "custom"
+)