diff --git a/open-sse/translator/helpers/openaiHelper.ts b/open-sse/translator/helpers/openaiHelper.ts
index b53bbb64e7..502d309fe3 100644
--- a/open-sse/translator/helpers/openaiHelper.ts
+++ b/open-sse/translator/helpers/openaiHelper.ts
@@ -148,6 +148,9 @@ export function filterToOpenAIFormat(body) {
   // Strip Claude-specific fields that OpenAI-compatible providers reject
   delete body.metadata;
   delete body.anthropic_version;
+  // Codex clients send a top-level `client_metadata` object; OpenAI rejects it
+  // with 400 "Unknown parameter: 'client_metadata'" (9router#1157).
+  delete body.client_metadata;
 
   // Map max_output_tokens (from Vercel AI SDK) to max_tokens logic
   if (body.max_output_tokens !== undefined) {
diff --git a/tests/unit/translator-strip-client-metadata-1157.test.ts b/tests/unit/translator-strip-client-metadata-1157.test.ts
new file mode 100644
index 0000000000..243b1407a3
--- /dev/null
+++ b/tests/unit/translator-strip-client-metadata-1157.test.ts
@@ -0,0 +1,26 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+const openaiHelper = await import("../../open-sse/translator/helpers/openaiHelper.ts");
+
+// Regression for upstream issue 9router#1157: Codex-origin requests carry a
+// top-level `client_metadata` object. On the OpenAI->OpenAI chat-completions
+// passthrough, `filterToOpenAIFormat` is the only sanitizer, and it forwarded
+// `client_metadata` to api.openai.com, which rejects it with
+// 400 "Unknown parameter: 'client_metadata'". It must be stripped alongside
+// the other Claude/Codex-specific fields.
+test("filterToOpenAIFormat strips top-level client_metadata (9router#1157)", () => {
+  const body = {
+    messages: [{ role: "user", content: "hi" }],
+    client_metadata: { user_id: "abc", session_id: "xyz" },
+    metadata: { remove: true },
+    anthropic_version: "2023-06-01",
+  };
+
+  const result = openaiHelper.filterToOpenAIFormat(body);
+
+  assert.equal("client_metadata" in result, false);
+  assert.equal("metadata" in result, false);
+  assert.equal("anthropic_version" in result, false);
+  assert.equal(result.messages.length, 1);
+});