diff --git a/open-sse/transformer/responsesTransformer.ts b/open-sse/transformer/responsesTransformer.ts
index 1ef35e4aa6..f22f38f9ed 100644
--- a/open-sse/transformer/responsesTransformer.ts
+++ b/open-sse/transformer/responsesTransformer.ts
@@ -7,6 +7,15 @@ import {
 } from "../utils/reasoningPlaceholder.ts";
 import * as fs from "fs";
 import * as path from "path";
+
+// #10223: threshold for detecting corrupted request_id fields. Normal
+// request IDs are <100 chars. DeepSeek's SSE encoder bug produces 200+
+// char values with response-ID fragments. The 100-char gap between normal
+// (<100) and threshold (200) provides safety margin for providers that
+// use moderately longer IDs. The transformer never reads request_id, so
+// stripping it has no functional impact on the output.
+const CORRUPTED_REQUEST_ID_THRESHOLD = 200;
+
 /**
  * Responses API Transformer
  * Converts OpenAI Chat Completions SSE to Codex Responses API SSE format
@@ -605,6 +614,21 @@ export function createResponsesApiTransformStream(
             continue;
           }
 
+          // #10223: strip request_id when it looks corrupted (suspiciously
+          // long — normal request IDs are <100 chars). Some providers
+          // (DeepSeek) have SSE encoder bugs that leak response-ID fragments
+          // into this field, producing 200+ char values. Well-behaved
+          // providers' request_id is preserved.
+          if (
+            typeof parsed.request_id === "string" &&
+            parsed.request_id.length >= CORRUPTED_REQUEST_ID_THRESHOLD
+          ) {
+            logger?.logInput(
+              `[ResponsesTransformer] stripped corrupted request_id (${parsed.request_id.length} chars)`
+            );
+            delete parsed.request_id;
+          }
+
           if (parsed.usage) {
             state.usage = normalizeResponsesUsage(state.usage, parsed.usage);
           }
diff --git a/tests/unit/responses-transformer-corrupted-request-id.test.ts b/tests/unit/responses-transformer-corrupted-request-id.test.ts
new file mode 100644
index 0000000000..c952527b6d
--- /dev/null
+++ b/tests/unit/responses-transformer-corrupted-request-id.test.ts
@@ -0,0 +1,83 @@
+// Regression guard for #10223: DeepSeek's SSE encoder bug leaks response-ID
+// fragments into `request_id`, producing suspiciously long (200+ char) values.
+// The transformer never reads `request_id` for its own output, but it must
+// strip a corrupted one (logging that it did) and must NOT touch a normal,
+// well-behaved provider's request_id.
+import test from "node:test";
+import assert from "node:assert/strict";
+
+const { createResponsesApiTransformStream } = await import(
+  "../../open-sse/transformer/responsesTransformer.ts"
+);
+
+const encoder = new TextEncoder();
+const decoder = new TextDecoder();
+
+async function runTransformStream(chunks, logger = null) {
+  const stream = createResponsesApiTransformStream(logger, 3000, {});
+  const writer = stream.writable.getWriter();
+  const reader = stream.readable.getReader();
+
+  const output = [];
+  const readerTask = (async () => {
+    while (true) {
+      const { value, done } = await reader.read();
+      if (done) break;
+      output.push(decoder.decode(value));
+    }
+  })();
+
+  for (const chunk of chunks) {
+    await writer.write(encoder.encode(chunk));
+  }
+  await writer.close();
+  await readerTask;
+
+  return output.join("");
+}
+
+function makeMockLogger() {
+  const inputs = [];
+  return {
+    inputs,
+    logInput: (event) => inputs.push(event),
+    logOutput: () => {},
+    flush: () => {},
+  };
+}
+
+test("BUG #10223: a corrupted (>=200 char) request_id is stripped and logged", async () => {
+  const corruptedId = "r".repeat(250);
+  const logger = makeMockLogger();
+
+  await runTransformStream(
+    [
+      `data: {"id":"chatcmpl_1","request_id":"${corruptedId}","choices":[{"index":0,"delta":{"content":"Hi"}}]}\n\n`,
+      'data: {"choices":[{"index":0,"delta":{},"finish_reason":"stop"}]}\n\n',
+    ],
+    logger
+  );
+
+  const stripLog = logger.inputs.find(
+    (entry) => typeof entry === "string" && entry.includes("stripped corrupted request_id")
+  );
+  assert.ok(stripLog, "logger.logInput should be called noting the corrupted request_id was stripped");
+  assert.match(stripLog, /\(250 chars\)/);
+});
+
+test("a normal (<200 char) request_id is left untouched — no strip logged", async () => {
+  const logger = makeMockLogger();
+
+  await runTransformStream(
+    [
+      'data: {"id":"chatcmpl_1","request_id":"req_normal_12345","choices":[{"index":0,"delta":{"content":"Hi"}}]}\n\n',
+      'data: {"choices":[{"index":0,"delta":{},"finish_reason":"stop"}]}\n\n',
+    ],
+    logger
+  );
+
+  const stripLog = logger.inputs.find(
+    (entry) => typeof entry === "string" && entry.includes("stripped corrupted request_id")
+  );
+  assert.equal(stripLog, undefined, "a normal-length request_id must never be stripped");
+});