fix: translate tool_choice object format between Responses and Chat APIs

open-sse/translator/request/openai-responses.ts

@@ -244,6 +244,20 @@ export function openaiResponsesToOpenAIRequest(
     return true;
   });
 
+  // Translate tool_choice object format: Responses {type,name} → Chat {type,function:{name}}
+  if (result.tool_choice && typeof result.tool_choice === "object" && !Array.isArray(result.tool_choice)) {
+    const tc = toRecord(result.tool_choice);
+    const tcType = toString(tc.type);
+    if (tcType === "function" && tc.name !== undefined && !tc.function) {
+      result.tool_choice = { type: "function", function: { name: tc.name } };
+    } else if (tcType && tcType !== "function" && tcType !== "allowed_tools") {
+      // Built-in tool types (web_search_preview, file_search, etc.) have no Chat equivalent
+      throw unsupportedFeature(
+        `Unsupported Responses API feature: tool_choice type '${tcType}' is not supported by omniroute`
+      );
+    }
+  }
+
   // Cleanup Responses API specific fields
   // Note: prompt_cache_key is intentionally preserved — it is used by Codex and other
   // providers as a cache-affinity signal. Stripping it breaks prompt caching (#517).
@@ -440,6 +454,23 @@ export function openaiToOpenAIResponsesRequest(
     });
   }
 
+  // Translate tool_choice: Chat {type,function:{name}} → Responses {type,name}
+  if (root.tool_choice !== undefined) {
+    if (typeof root.tool_choice === "string") {
+      result.tool_choice = root.tool_choice;
+    } else if (typeof root.tool_choice === "object" && !Array.isArray(root.tool_choice)) {
+      const tc = toRecord(root.tool_choice);
+      if (tc.type === "function" && tc.function) {
+        const fn = toRecord(tc.function);
+        result.tool_choice = { type: "function", name: fn.name };
+      } else {
+        result.tool_choice = root.tool_choice;
+      }
+    } else {
+      result.tool_choice = root.tool_choice;
+    }
+  }
+
   // Pass through relevant fields
   if (root.service_tier !== undefined) result.service_tier = root.service_tier;
   if (root.temperature !== undefined) result.temperature = root.temperature;

tests/unit/responses-translation-fixes.test.mjs

@@ -158,3 +158,59 @@ test("Chat→Responses: file content part converted to input_file", () => {
   assert.equal(filePart.file_id, "file-abc");
   assert.equal(filePart.filename, "data.csv");
 });
+
+test("Responses→Chat: tool_choice {type:'function', name} wrapped to {type:'function', function:{name}}", () => {
+  const body = {
+    model: "gpt-4",
+    input: "hello",
+    tool_choice: { type: "function", name: "get_weather" },
+    tools: [{ type: "function", name: "get_weather", parameters: {} }],
+  };
+  const result = openaiResponsesToOpenAIRequest(null, body, null, null);
+  assert.deepEqual(result.tool_choice, {
+    type: "function",
+    function: { name: "get_weather" },
+  });
+});
+
+test("Chat→Responses: tool_choice {type:'function', function:{name}} unwrapped to {type:'function', name}", () => {
+  const body = {
+    model: "gpt-4",
+    messages: [{ role: "user", content: "hello" }],
+    tool_choice: { type: "function", function: { name: "get_weather" } },
+    tools: [{ type: "function", function: { name: "get_weather", parameters: {} } }],
+  };
+  const result = openaiToOpenAIResponsesRequest("gpt-4", body, true, null);
+  assert.deepEqual(result.tool_choice, {
+    type: "function",
+    name: "get_weather",
+  });
+});
+
+test("Responses→Chat: string tool_choice passes through unchanged", () => {
+  const body = { model: "gpt-4", input: "hello", tool_choice: "auto" };
+  const result = openaiResponsesToOpenAIRequest(null, body, null, null);
+  assert.equal(result.tool_choice, "auto");
+});
+
+test("Chat→Responses: string tool_choice passes through unchanged", () => {
+  const body = {
+    model: "gpt-4",
+    messages: [{ role: "user", content: "hello" }],
+    tool_choice: "required",
+  };
+  const result = openaiToOpenAIResponsesRequest("gpt-4", body, true, null);
+  assert.equal(result.tool_choice, "required");
+});
+
+test("Responses→Chat: built-in tool_choice type throws unsupported error", () => {
+  const body = {
+    model: "gpt-4",
+    input: "hello",
+    tool_choice: { type: "web_search_preview" },
+  };
+  assert.throws(
+    () => openaiResponsesToOpenAIRequest(null, body, null, null),
+    (err) => err.message.includes("web_search_preview")
+  );
+});