diff --git a/go/plugins/googleai/googleai.go b/go/plugins/googleai/googleai.go
index 6bd4b3eaef..8b192542a4 100644
--- a/go/plugins/googleai/googleai.go
+++ b/go/plugins/googleai/googleai.go
@@ -91,12 +91,10 @@ func Init(ctx context.Context, apiKey string) (err error) {
 	state.client = client
 	state.initted = true
 	for model, caps := range knownCaps {
-		if _, err := DefineModel(model, &caps); err != nil {
-			return fmt.Errorf("googleai.Init: failed to define known model %q: %w", model, err)
-		}
+		defineModel(model, caps)
 	}
 	for _, e := range knownEmbedders {
-		DefineEmbedder(e)
+		defineEmbedder(e)
 	}
 	return nil
 }
@@ -111,8 +109,8 @@ func IsKnownModel(name string) bool {
 // The second argument describes the capability of the model.
 // Use [IsKnownModel] to determine if a model is known.
 func DefineModel(name string, caps *ai.ModelCapabilities) (*ai.Model, error) {
-	// state.mu.Lock()
-	// defer state.mu.Unlock()
+	state.mu.Lock()
+	defer state.mu.Unlock()
 	if !state.initted {
 		panic("googleai.Init not called")
 	}
@@ -152,8 +150,8 @@ func defineModel(name string, caps ai.ModelCapabilities) *ai.Model {
 
 // DefineEmbedder defines an embedder with a given name.
 func DefineEmbedder(name string) *ai.Embedder {
-	// state.mu.Lock()
-	// defer state.mu.Unlock()
+	state.mu.Lock()
+	defer state.mu.Unlock()
 	if !state.initted {
 		panic("googleai.Init not called")
 	}
diff --git a/go/plugins/vertexai/vertexai.go b/go/plugins/vertexai/vertexai.go
index 2b5b6f6058..a822189a59 100644
--- a/go/plugins/vertexai/vertexai.go
+++ b/go/plugins/vertexai/vertexai.go
@@ -114,12 +114,10 @@ func Init(ctx context.Context, projectID, location string) error {
 	}
 	state.initted = true
 	for model, caps := range knownCaps {
-		if _, err := DefineModel(model, &caps); err != nil {
-			return fmt.Errorf("vertexai.Init: failed to define known model %q: %w", model, err)
-		}
+		defineModel(model, caps)
 	}
 	for _, e := range knownEmbedders {
-		DefineEmbedder(e)
+		defineEmbedder(e)
 	}
 	return nil
 }
@@ -128,8 +126,8 @@ func Init(ctx context.Context, projectID, location string) error {
 // The second argument describes the capability of the model.
 // Use [IsKnownModel] to determine if a model is known.
 func DefineModel(name string, caps *ai.ModelCapabilities) (*ai.Model, error) {
-	// state.mu.Lock()
-	// defer state.mu.Unlock()
+	state.mu.Lock()
+	defer state.mu.Unlock()
 	if !state.initted {
 		panic("vertexai.Init not called")
 	}
@@ -143,13 +141,17 @@ func DefineModel(name string, caps *ai.ModelCapabilities) (*ai.Model, error) {
 	} else {
 		mc = *caps
 	}
+	return defineModel(name, mc), nil
+}
 
+// requires state.mu
+func defineModel(name string, mc ai.ModelCapabilities) *ai.Model {
 	meta := &ai.ModelMetadata{
 		Label:    "Vertex AI - " + name,
 		Supports: mc,
 	}
 	g := &generator{model: name, client: state.gclient}
-	return ai.DefineModel(provider, name, meta, g.generate), nil
+	return ai.DefineModel(provider, name, meta, g.generate)
 }
 
 // IsKnownModel reports whether a model is known to this plugin.
@@ -169,13 +171,18 @@ func KnownModels() []string {
 	return keys
 }
 
-// DefineModel defines an embedder with the given name.
+// DefineEmbedder defines an embedder with the given name.
 func DefineEmbedder(name string) *ai.Embedder {
-	// state.mu.Lock()
-	// defer state.mu.Unlock()
+	state.mu.Lock()
+	defer state.mu.Unlock()
 	if !state.initted {
 		panic("vertexai.Init not called")
 	}
+	return defineEmbedder(name)
+}
+
+// requires state.mu
+func defineEmbedder(name string) *ai.Embedder {
 	fullName := fmt.Sprintf("projects/%s/locations/%s/publishers/google/models/%s", state.projectID, state.location, name)
 	return ai.DefineEmbedder(provider, name, func(ctx context.Context, req *ai.EmbedRequest) (*ai.EmbedResponse, error) {
 		return embed(ctx, fullName, state.pclient, req)