fix(ai): omit OpenAI tool choice when tools are empty (#48857)

packages/ai/src/protocols/openai-chat.ts

@@ -766,7 +766,7 @@ export const fromRequest = Effect.fn("OpenAIChat.fromRequest")(function* (
               supportsStrictMode,
             ),
           ),
-    tool_choice: request.toolChoice ? yield* lowerToolChoice(request.toolChoice) : undefined,
+    tool_choice: hasActiveTools && request.toolChoice ? yield* lowerToolChoice(request.toolChoice) : undefined,
     stream: true as const,
     ...(supportsUsageInStreaming ? { stream_options: { include_usage: true } } : {}),
     ...(zaiToolStream && hasActiveTools ? { tool_stream: true } : {}),

packages/ai/src/protocols/openai-responses.ts

@@ -199,8 +199,10 @@ const fromRequest = Effect.fn("OpenAIResponses.fromRequest")(function* (request:
         ? undefined
         : yield* Effect.forEach(request.tools, (tool) => lowerToolEntry(tool, toolSchemaCompatibility)),
     tool_choice:
-      OpenResponses.allowedToolChoice(request) ??
-      (request.toolChoice ? yield* lowerToolChoice(request.toolChoice, request.tools) : undefined),
+      request.tools.length === 0
+        ? undefined
+        : (OpenResponses.allowedToolChoice(request) ??
+          (request.toolChoice ? yield* lowerToolChoice(request.toolChoice, request.tools) : undefined)),
   })
 })
 

packages/ai/test/provider/empty-tool-choice.test.ts

@@ -0,0 +1,80 @@
+import { expect } from "bun:test"
+import { Effect } from "effect"
+import { LLM, Message, ToolCallPart } from "../../src/index.js"
+import { OpenAIChat } from "../../src/protocols/openai-chat.js"
+import { OpenAIResponses } from "../../src/protocols/openai-responses.js"
+import { Auth } from "../../src/route/auth.js"
+import { compileRequest } from "../../src/route/client.js"
+import { it } from "../lib/effect.js"
+
+const tool = { name: "lookup", description: "Look up a value", inputSchema: { type: "object", properties: {} } }
+
+for (const route of [OpenAIChat.route, OpenAIResponses.route]) {
+  const model = route
+    .with({ endpoint: { baseURL: "https://api.openai.test/v1" }, auth: Auth.bearer("test") })
+    .model({ id: "gpt-4.1-mini" })
+
+  for (const choice of [
+    { type: "auto" },
+    { type: "none" },
+    { type: "required" },
+    { type: "tool", name: "lookup" },
+  ] as const) {
+    it.effect(`${route.id} omits ${choice.type} tool choice without active tools`, () =>
+      Effect.gen(function* () {
+        for (const messages of [
+          [Message.user("Say OK.")],
+          [
+            Message.user("Look up the value."),
+            Message.assistant([ToolCallPart.make({ id: "call_1", name: "lookup", input: {} })]),
+            Message.tool({ id: "call_1", name: "lookup", result: "OK", resultType: "text" }),
+          ],
+        ]) {
+          const prepared = yield* compileRequest(
+            LLM.request({ model, messages, tools: [], toolChoice: choice, cache: "none" }),
+          )
+          expect(prepared.body.tool_choice).toBeUndefined()
+        }
+      }),
+    )
+
+    it.effect(`${route.id} preserves ${choice.type} tool choice with active tools`, () =>
+      Effect.gen(function* () {
+        const prepared = yield* compileRequest(
+          LLM.request({ model, prompt: "Look up the value.", tools: [tool], toolChoice: choice }),
+        )
+        expect(prepared.body.tool_choice).toEqual(
+          choice.type !== "tool"
+            ? choice.type
+            : route.id === "openai-chat"
+              ? { type: "function", function: { name: "lookup" } }
+              : { type: "function", name: "lookup" },
+        )
+      }),
+    )
+  }
+}
+
+it.effect("OpenAI Responses omits allowed tool choice without tool definitions", () =>
+  Effect.gen(function* () {
+    const model = OpenAIResponses.route
+      .with({ endpoint: { baseURL: "https://api.openai.test/v1" }, auth: Auth.bearer("test") })
+      .model({ id: "gpt-4.1-mini" })
+    for (const tools of [[], [tool]]) {
+      const prepared = yield* compileRequest(
+        LLM.request({
+          model,
+          prompt: "Look up the value.",
+          tools,
+          toolChoice: "required",
+          providerOptions: { allowedTools: { mode: "required", toolNames: ["lookup"] } },
+        }),
+      )
+      expect(prepared.body.tool_choice).toEqual(
+        tools.length === 0
+          ? undefined
+          : { type: "allowed_tools", mode: "required", tools: [{ type: "function", name: "lookup" }] },
+      )
+    }
+  }),
+)