diff --git a/open-sse/executors/codex.ts b/open-sse/executors/codex.ts
index fcbcbab1cd..294c4e6ba5 100644
--- a/open-sse/executors/codex.ts
+++ b/open-sse/executors/codex.ts
@@ -336,6 +336,20 @@ function normalizeCodexTools(body: Record<string, unknown>): void {
     }
 
     const tool = toolValue as Record<string, unknown>;
+
+    // Preserve namespace tools (MCP tool groups used by Codex/OpenAI Responses API).
+    // Codex API supports them natively; register sub-tool names for tool_choice validation.
+    if (tool.type === "namespace" && Array.isArray(tool.tools)) {
+      for (const st of tool.tools as unknown[]) {
+        if (st && typeof st === "object" && !Array.isArray(st)) {
+          const subTool = st as Record<string, unknown>;
+          const name = typeof subTool.name === "string" ? subTool.name.trim() : "";
+          if (name) validToolNames.add(name);
+        }
+      }
+      return true;
+    }
+
     if (tool.type !== "function") {
       return false;
     }
diff --git a/open-sse/translator/request/openai-responses.ts b/open-sse/translator/request/openai-responses.ts
index e4f3a4b4c8..5cd5da520d 100644
--- a/open-sse/translator/request/openai-responses.ts
+++ b/open-sse/translator/request/openai-responses.ts
@@ -55,12 +55,14 @@ export function openaiResponsesToOpenAIRequest(
     for (const toolValue of tools) {
       const tool = toRecord(toolValue);
       const toolType = toString(tool.type);
-      // Allow: function tools, tools already in Chat format (have .function property), and CLI subagent tools
+      // Allow: function tools, tools already in Chat format (have .function property), CLI subagent tools,
+      // and namespace tools (MCP tool groups used by Codex/OpenAI Responses API).
       if (
         toolType &&
         toolType !== "function" &&
         toolType !== "custom" &&
         toolType !== "command" &&
+        toolType !== "namespace" &&
         !tool.function
       ) {
         throw unsupportedFeature(