diff --git a/open-sse/translator/index.ts b/open-sse/translator/index.ts
index 123221eb84..65341d5393 100644
--- a/open-sse/translator/index.ts
+++ b/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);
diff --git a/open-sse/translator/request/claude-to-openai.ts b/open-sse/translator/request/claude-to-openai.ts
index fdf6687512..f426c99857 100644
--- a/open-sse/translator/request/claude-to-openai.ts
+++ b/open-sse/translator/request/claude-to-openai.ts
@@ -27,8 +27,55 @@ function normalizeOpenAIReasoningEffort(effort: unknown): string | undefined {
   return normalized || undefined;
 }
 
+function isClaudeServerWebSearchTool(tool: unknown): tool is JsonRecord {
+  if (!tool || typeof tool !== "object" || Array.isArray(tool)) return false;
+  const record = tool as JsonRecord;
+  return (
+    record.name === "web_search" &&
+    typeof record.type === "string" &&
+    /^web_search_\d{8}$/.test(record.type)
+  );
+}
+
+function toStringArray(value: unknown): string[] {
+  if (!Array.isArray(value)) return [];
+  return value
+    .filter((entry): entry is string => typeof entry === "string")
+    .map((entry) => entry.trim())
+    .filter(Boolean);
+}
+
+function convertClaudeServerWebSearchTool(tool: JsonRecord): JsonRecord {
+  const allowedDomains = toStringArray(tool.allowed_domains);
+  const blockedDomains = toStringArray(tool.blocked_domains);
+  const filters: JsonRecord = {};
+  if (allowedDomains.length > 0) filters.allowed_domains = allowedDomains;
+  if (blockedDomains.length > 0) filters.blocked_domains = blockedDomains;
+
+  return {
+    type: "web_search",
+    ...(Object.keys(filters).length > 0 ? { filters } : {}),
+    ...(tool.user_location && typeof tool.user_location === "object" && !Array.isArray(tool.user_location)
+      ? { user_location: tool.user_location }
+      : {}),
+  };
+}
+
+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[];
@@ -89,30 +136,31 @@ 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) => {
-        const name = typeof tool.name === "string" ? tool.name.trim() : "";
+        if (useNativeResponsesWebSearch && isClaudeServerWebSearchTool(tool)) {
+          return convertClaudeServerWebSearchTool(tool);
+        }
+
+        if (!tool || typeof tool !== "object" || Array.isArray(tool)) return null;
+        const record = tool as JsonRecord;
+        const name = typeof record.name === "string" ? record.name.trim() : "";
         if (!name) return null; // skip tools with empty/invalid name
 
         return {
           type: "function",
           function: {
             name,
-            description: typeof tool.description === "string" ? tool.description : "", // fix: never null (#276)
-            parameters: normalizeToolSchema(tool.input_schema),
+            description: typeof record.description === "string" ? record.description : "", // fix: never null (#276)
+            parameters: normalizeToolSchema(record.input_schema),
           },
         };
       })
-      .filter(
-        (
-          tool
-        ): tool is {
-          type: "function";
-          function: { name: string; description: string; parameters: unknown };
-        } => Boolean(tool)
-      );
+      .filter((tool): tool is JsonRecord => Boolean(tool));
 
     if (normalizedTools.length > 0) {
       result.tools = normalizedTools;
@@ -121,7 +169,10 @@ export function claudeToOpenAIRequest(model, body, stream) {
 
   // Tool choice
   if (body.tool_choice) {
-    result.tool_choice = convertToolChoice(body.tool_choice);
+    result.tool_choice = convertToolChoice(
+      body.tool_choice,
+      useNativeResponsesWebSearch && hasClaudeServerWebSearchTool(body.tools)
+    );
   }
 
   // Reasoning effort: map Claude-side thinking controls to OpenAI reasoning_effort.
@@ -320,7 +371,7 @@ function convertClaudeMessage(msg) {
 }
 
 // Convert tool choice
-function convertToolChoice(choice) {
+function convertToolChoice(choice, hasServerWebSearch = false) {
   if (!choice) return "auto";
   if (typeof choice === "string") return choice;
 
@@ -330,6 +381,9 @@ function convertToolChoice(choice) {
     case TOOL_CHOICE_ANY:
       return "required";
     case "tool":
+      if (hasServerWebSearch && choice.name === "web_search") {
+        return { type: "web_search" };
+      }
       return { type: "function", function: { name: choice.name } };
     default:
       return "auto";
diff --git a/tests/unit/translator-claude-to-openai.test.ts b/tests/unit/translator-claude-to-openai.test.ts
index 9c082f1c93..c5a01a9db1 100644
--- a/tests/unit/translator-claude-to-openai.test.ts
+++ b/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(
@@ -52,6 +54,136 @@ test("Claude -> OpenAI maps system blocks, parameters, tool declarations and too
   });
 });
 
+
+
+test("Claude -> OpenAI maps Claude server WebSearch to native Responses web_search", () => {
+  const result = claudeToOpenAIRequest(
+    "gpt-5.5",
+    {
+      messages: [{ role: "user", content: "Search docs" }],
+      tools: [
+        {
+          type: "web_search_20250305",
+          name: "web_search",
+          allowed_domains: ["docs.anthropic.com", "", 123, { domain: "bad.example" }],
+          blocked_domains: ["spam.example", false],
+          max_uses: 8,
+          user_location: { type: "approximate", country: "US" },
+        },
+      ],
+      tool_choice: { type: "tool", name: "web_search" },
+    },
+    true,
+    { _targetFormat: FORMATS.OPENAI_RESPONSES }
+  );
+
+  assert.deepEqual(result.tools, [
+    {
+      type: "web_search",
+      filters: {
+        allowed_domains: ["docs.anthropic.com"],
+        blocked_domains: ["spam.example"],
+      },
+      user_location: { type: "approximate", country: "US" },
+    },
+  ]);
+  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",
+    {
+      messages: [{ role: "user", content: "hi" }],
+      tools: [null, "bad", 42, [], { name: "", input_schema: { type: "object" } }, { name: "ok" }],
+    },
+    false
+  );
+
+  assert.deepEqual(result.tools, [
+    {
+      type: "function",
+      function: {
+        name: "ok",
+        description: "",
+        parameters: { type: "object", properties: {} },
+      },
+    },
+  ]);
+});
+
+test("Claude -> OpenAI leaves ordinary web_search function tools as functions", () => {
+  const result = claudeToOpenAIRequest(
+    "gpt-4o",
+    {
+      messages: [{ role: "user", content: "Search docs" }],
+      tools: [
+        {
+          type: "custom",
+          name: "web_search",
+          description: "User-defined search function",
+          input_schema: { type: "object", properties: { query: { type: "string" } } },
+        },
+      ],
+      tool_choice: { type: "tool", name: "web_search" },
+    },
+    false
+  );
+
+  assert.deepEqual(result.tools[0], {
+    type: "function",
+    function: {
+      name: "web_search",
+      description: "User-defined search function",
+      parameters: { type: "object", properties: { query: { type: "string" } } },
+    },
+  });
+  assert.deepEqual(result.tool_choice, {
+    type: "function",
+    function: { name: "web_search" },
+  });
+});
+
 test("Claude -> OpenAI converts assistant text and both base64 and URL images", () => {
   const result = claudeToOpenAIRequest(
     "gpt-4o",