fix(ai): keep tools when Anthropic tool_choice is none (#38553)

packages/ai/src/protocols/anthropic-messages.ts

@@ -159,7 +159,7 @@ const AnthropicTool = Schema.Struct({
 type AnthropicTool = Schema.Schema.Type<typeof AnthropicTool>
 
 const AnthropicToolChoice = Schema.Union([
-  Schema.Struct({ type: Schema.Literals(["auto", "any"]) }),
+  Schema.Struct({ type: Schema.Literals(["auto", "any", "none"]) }),
   Schema.Struct({ type: Schema.tag("tool"), name: Schema.String }),
 ])
 
@@ -297,7 +297,7 @@ const lowerTool = (breakpoints: Cache.Breakpoints, tool: ToolDefinition, inputSc
 const lowerToolChoice = (toolChoice: NonNullable<LLMRequest["toolChoice"]>) =>
   ProviderShared.matchToolChoice("Anthropic Messages", toolChoice, {
     auto: () => ({ type: "auto" as const }),
-    none: () => undefined,
+    none: () => ({ type: "none" as const }),
     required: () => ({ type: "any" as const }),
     tool: (name) => ({ type: "tool" as const, name }),
   })
@@ -542,7 +542,6 @@ const outputConfig = (request: LLMRequest) => {
 }
 
 const fromRequest = Effect.fn("AnthropicMessages.fromRequest")(function* (request: LLMRequest) {
-  const toolChoice = request.toolChoice ? yield* lowerToolChoice(request.toolChoice) : undefined
   const generation = request.generation
   const toolSchemaCompatibility = request.model.compatibility?.toolSchema
   const outputLimit = request.model.defaults?.limits?.output ?? request.model.route.defaults.limits?.output ?? 4096
@@ -551,7 +550,7 @@ const fromRequest = Effect.fn("AnthropicMessages.fromRequest")(function* (reques
   // over-mark we keep their tool hints and shed the message-tail ones first.
   const breakpoints = Cache.newBreakpoints(ANTHROPIC_BREAKPOINT_CAP)
   const tools =
-    request.tools.length === 0 || request.toolChoice?.type === "none"
+    request.tools.length === 0
       ? undefined
       : request.tools.map((tool) =>
           lowerTool(
@@ -560,6 +559,9 @@ const fromRequest = Effect.fn("AnthropicMessages.fromRequest")(function* (reques
             ToolSchemaProjection.modelCompatibility(tool.inputSchema, toolSchemaCompatibility),
           ),
         )
+  // Anthropic rejects tool_choice when tools are absent; "none" is only meaningful with tools present.
+  const toolChoice =
+    tools === undefined || !request.toolChoice ? undefined : yield* lowerToolChoice(request.toolChoice)
   const system =
     request.system.length === 0
       ? undefined

packages/ai/test/provider/anthropic-messages.test.ts

@@ -235,6 +235,34 @@ describe("Anthropic Messages route", () => {
     }),
   )
 
+  it.effect("keeps tools and sends tool_choice none", () =>
+    Effect.gen(function* () {
+      const prepared = yield* LLMClient.prepare<AnthropicMessages.AnthropicMessagesBody>(
+        LLM.request({
+          id: "req_tool_choice_none",
+          model,
+          tools: [{ name: "lookup", description: "Look things up", inputSchema: { type: "object", properties: {} } }],
+          messages: [
+            Message.user("What is the weather?"),
+            Message.assistant([ToolCallPart.make({ id: "call_1", name: "lookup", input: { query: "weather" } })]),
+            Message.tool({ id: "call_1", name: "lookup", result: { forecast: "sunny" } }),
+          ],
+          toolChoice: "none",
+          cache: "none",
+        }),
+      )
+
+      expect(prepared.body.tools).toEqual([
+        {
+          name: "lookup",
+          description: "Look things up",
+          input_schema: { type: "object", properties: {} },
+        },
+      ])
+      expect(prepared.body.tool_choice).toEqual({ type: "none" })
+    }),
+  )
+
   // Regression: read tool results must stay structured so base64 media data is
   // not JSON-stringified into `tool_result.content`.
   it.effect("lowers media tool-result content as structured blocks", () =>