fix: gate Claude WebSearch native mapping by target format

Only translate Claude Code web_search_YYYYMMDD server tools to native Responses web_search when the final target is OpenAI Responses. Keep the Chat Completions target on function-tool shape and cover the full translateRequest path.

open-sse/translator/index.ts

@@ -183,12 +183,15 @@ export function translateRequest(
         const toOpenAI = getRequestTranslator(sourceFormat, FORMATS.OPENAI);
         if (toOpenAI) {
           // Forward Copilot UA marker to source→openai translators only.
-          const step1Credentials = options?.copilotClient
-            ? {
-                ...(credentials && typeof credentials === "object" ? credentials : {}),
-                _copilotClient: true,
-              }
-            : credentials;
+          const hasTargetHint = targetFormat != null;
+          const step1Credentials =
+            options?.copilotClient || hasTargetHint
+              ? {
+                  ...(credentials && typeof credentials === "object" ? credentials : {}),
+                  ...(options?.copilotClient ? { _copilotClient: true } : {}),
+                  ...(hasTargetHint ? { _targetFormat: targetFormat } : {}),
+                }
+              : credentials;
           result = toOpenAI(model, result, stream, step1Credentials);
           // Log OpenAI intermediate format
           reqLogger?.logOpenAIRequest?.(result);

open-sse/translator/request/claude-to-openai.ts

@@ -65,8 +65,17 @@ function hasClaudeServerWebSearchTool(tools: unknown): boolean {
   return Array.isArray(tools) && tools.some((tool) => isClaudeServerWebSearchTool(tool));
 }
 
+function shouldUseNativeResponsesWebSearch(credentials: unknown): boolean {
+  return (
+    credentials !== null &&
+    typeof credentials === "object" &&
+    !Array.isArray(credentials) &&
+    (credentials as JsonRecord)._targetFormat === FORMATS.OPENAI_RESPONSES
+  );
+}
+
 // Convert Claude request to OpenAI format
-export function claudeToOpenAIRequest(model, body, stream) {
+export function claudeToOpenAIRequest(model, body, stream, credentials: unknown = null) {
   const result: {
     model: string;
     messages: JsonRecord[];
@@ -127,11 +136,13 @@ export function claudeToOpenAIRequest(model, body, stream) {
   // Fix missing tool responses - OpenAI requires every tool_call to have a response
   fixMissingToolResponses(result.messages);
 
+  const useNativeResponsesWebSearch = shouldUseNativeResponsesWebSearch(credentials);
+
   // Tools
   if (body.tools && Array.isArray(body.tools)) {
     const normalizedTools = body.tools
       .map((tool) => {
-        if (isClaudeServerWebSearchTool(tool)) {
+        if (useNativeResponsesWebSearch && isClaudeServerWebSearchTool(tool)) {
           return convertClaudeServerWebSearchTool(tool);
         }
 
@@ -160,7 +171,7 @@ export function claudeToOpenAIRequest(model, body, stream) {
   if (body.tool_choice) {
     result.tool_choice = convertToolChoice(
       body.tool_choice,
-      hasClaudeServerWebSearchTool(body.tools)
+      useNativeResponsesWebSearch && hasClaudeServerWebSearchTool(body.tools)
     );
   }
 

tests/unit/translator-claude-to-openai.test.ts

@@ -3,6 +3,8 @@ import assert from "node:assert/strict";
 
 const { claudeToOpenAIRequest } =
   await import("../../open-sse/translator/request/claude-to-openai.ts");
+const { translateRequest } = await import("../../open-sse/translator/index.ts");
+const { FORMATS } = await import("../../open-sse/translator/formats.ts");
 
 test("Claude -> OpenAI maps system blocks, parameters, tool declarations and tool choice", () => {
   const result = claudeToOpenAIRequest(
@@ -71,7 +73,8 @@ test("Claude -> OpenAI maps Claude server WebSearch to native Responses web_sear
       ],
       tool_choice: { type: "tool", name: "web_search" },
     },
-    true
+    true,
+    { _targetFormat: FORMATS.OPENAI_RESPONSES }
   );
 
   assert.deepEqual(result.tools, [
@@ -87,6 +90,46 @@ test("Claude -> OpenAI maps Claude server WebSearch to native Responses web_sear
   assert.deepEqual(result.tool_choice, { type: "web_search" });
 });
 
+test("translateRequest maps Claude server WebSearch natively only for Responses targets", () => {
+  const body = {
+    messages: [{ role: "user", content: "Search docs" }],
+    tools: [{ type: "web_search_20250305", name: "web_search" }],
+    tool_choice: { type: "tool", name: "web_search" },
+  };
+
+  const responses = translateRequest(
+    FORMATS.CLAUDE,
+    FORMATS.OPENAI_RESPONSES,
+    "gpt-5.5",
+    structuredClone(body),
+    true
+  );
+  assert.deepEqual(responses.tools, [{ type: "web_search" }]);
+  assert.deepEqual(responses.tool_choice, { type: "web_search" });
+
+  const chat = translateRequest(
+    FORMATS.CLAUDE,
+    FORMATS.OPENAI,
+    "gpt-4o",
+    structuredClone(body),
+    true
+  );
+  assert.deepEqual(chat.tools, [
+    {
+      type: "function",
+      function: {
+        name: "web_search",
+        description: "",
+        parameters: { type: "object", properties: {} },
+      },
+    },
+  ]);
+  assert.deepEqual(chat.tool_choice, {
+    type: "function",
+    function: { name: "web_search" },
+  });
+});
+
 test("Claude -> OpenAI skips invalid tool payloads without crashing", () => {
   const result = claudeToOpenAIRequest(
     "gpt-4o",