diff --git a/packages/ai/src/route/client.ts b/packages/ai/src/route/client.ts
index bbaf46515da..6b92bd53d7f 100644
--- a/packages/ai/src/route/client.ts
+++ b/packages/ai/src/route/client.ts
@@ -60,7 +60,8 @@ export interface Route<
   readonly body: RouteBody<Body>
   readonly supportsEffortUpdates?: (request: LLMRequest) => boolean
   readonly sanitizer?: LanguageModelSanitizerCompatibility
-  readonly namespaces: NamespaceStyle
+  /** How protocol bodies receive tool namespaces; defaults to `"flat"`. */
+  readonly namespaces?: NamespaceStyle
   readonly with: {
     <Next extends CompactionOperations | undefined>(
       patch: RoutePatch<Body, Prepared> & { readonly compact: Next },
@@ -393,7 +394,7 @@ function makeFromTransport<Body, Prepared, Frame, Event, State>(
       body: protocol.body,
       supportsEffortUpdates: protocol.supportsEffortUpdates,
       sanitizer: protocol.sanitizer,
-      namespaces: protocol.namespaces ?? "flat",
+      namespaces: protocol.namespaces,
       with: (patch: RoutePatch<Body, Prepared>) => {
         const { compact, id, provider, providerMetadataKey, auth, transport, endpoint, ...defaults } = patch
         return build({
@@ -576,7 +577,8 @@ const prepareRequest = (request: LLMRequest) => {
 }
 
 // Protocols receive wire tool names; callers keep declared names in both directions.
-const lowerRequest = (request: LLMRequest) => ToolNames.lower(prepareRequest(request), request.model.route.namespaces)
+const lowerRequest = (request: LLMRequest) =>
+  ToolNames.lower(prepareRequest(request), request.model.route.namespaces ?? "flat")
 
 const compile = Effect.fn("LLM.compile")(function* (request: LLMRequest, options?: StreamOptions) {
   const lowered = yield* lowerRequest(request)
diff --git a/packages/core/src/aisdk.ts b/packages/core/src/aisdk.ts
index 36339b7b96e..f27ad01f375 100644
--- a/packages/core/src/aisdk.ts
+++ b/packages/core/src/aisdk.ts
@@ -368,7 +368,6 @@ function modelFromLanguage(info: RuntimeInfo, language: LanguageModelV3) {
     protocol: "ai-sdk",
     // AI SDK providers convert tool schemas themselves, so model-family sanitizers stay off here.
     sanitizer: "none",
-    namespaces: "flat",
     endpoint: Endpoint.path("/", { baseURL: "https://ai-sdk.local" }),
     auth: Auth.none,
     transport: {