diff --git a/open-sse/executors/chatgpt-session/errors.ts b/open-sse/executors/chatgpt-session/errors.ts
new file mode 100644
index 0000000000..20d874f40a
--- /dev/null
+++ b/open-sse/executors/chatgpt-session/errors.ts
@@ -0,0 +1,85 @@
+/**
+ * Maps every failure this provider can produce onto the HTTP contract the router expects.
+ *
+ * The distinction that matters: a 503 with `connection_cooldown` lets combo routing skip this
+ * connection without opening the provider circuit breaker, while a 400 is terminal and must not
+ * be retried (a changed ChatGPT DOM will not fix itself on a retry).
+ */
+
+import { ChatGptSessionInputError } from "./messages.ts";
+
+export interface ChatGptSessionErrorClass {
+  status: number;
+  code: string;
+  fallbackHint?: "connection_cooldown";
+}
+
+interface ErrorLike {
+  message: string;
+  name?: string;
+  status?: number;
+  code?: string;
+}
+
+function asErrorLike(error: unknown): ErrorLike {
+  if (error instanceof Error) {
+    const typed = error as Error & { status?: unknown; code?: unknown };
+    return {
+      message: error.message,
+      name: error.name,
+      ...(typeof typed.status === "number" ? { status: typed.status } : {}),
+      ...(typeof typed.code === "string" ? { code: typed.code } : {}),
+    };
+  }
+  if (error && typeof error === "object") {
+    const typed = error as Record<string, unknown>;
+    return {
+      message: typeof typed.message === "string" ? typed.message : String(error),
+      ...(typeof typed.name === "string" ? { name: typed.name } : {}),
+      ...(typeof typed.status === "number" ? { status: typed.status } : {}),
+      ...(typeof typed.code === "string" ? { code: typed.code } : {}),
+    };
+  }
+  return { message: String(error ?? "") };
+}
+
+const BROWSER_UNAVAILABLE =
+  /No supported Chrome|browserType\.launch|Executable doesn't exist|chromium.*not installed/i;
+const MISSING_CREDENTIALS =
+  /credentials are missing|Cookie or verified browser storage state is required|Cookie header is missing/i;
+const SESSION_EXPIRED = /not authenticated|storage state is invalid|sign ?in|log ?in|logged out/i;
+const RATE_LIMITED = /rate limit|usage limit|too many requests|message limit/i;
+const ROUTE_UNAVAILABLE =
+  /not available for this|not available while the account|is not supported/i;
+const UI_TIMEOUT = /waitForSelector|Timeout \d+ms exceeded|actionability|interception/i;
+
+export function classifyChatGptSessionError(error: unknown): ChatGptSessionErrorClass {
+  if (error instanceof ChatGptSessionInputError) {
+    return { status: 400, code: error.code };
+  }
+
+  const like = asErrorLike(error);
+
+  if (BROWSER_UNAVAILABLE.test(like.message)) {
+    return { status: 503, code: "browser_unavailable", fallbackHint: "connection_cooldown" };
+  }
+  if (MISSING_CREDENTIALS.test(like.message)) {
+    return { status: 401, code: "missing_credentials" };
+  }
+  if (SESSION_EXPIRED.test(like.message)) {
+    return { status: 401, code: "session_expired" };
+  }
+  if (RATE_LIMITED.test(like.message)) {
+    return { status: 429, code: "rate_limited" };
+  }
+  if (ROUTE_UNAVAILABLE.test(like.message)) {
+    return { status: 400, code: "route_unavailable" };
+  }
+  if (like.name === "TimeoutError" || UI_TIMEOUT.test(like.message)) {
+    return { status: 400, code: "browser_ui_timeout" };
+  }
+  if (typeof like.status === "number" && like.status >= 400) {
+    return { status: like.status, code: like.code ?? "turn_failed" };
+  }
+  return { status: 502, code: "turn_failed" };
+}
diff --git a/tests/unit/chatgpt-session-errors.test.ts b/tests/unit/chatgpt-session-errors.test.ts
new file mode 100644
index 0000000000..cb83c747f6
--- /dev/null
+++ b/tests/unit/chatgpt-session-errors.test.ts
@@ -0,0 +1,77 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+
+import { classifyChatGptSessionError } from "../../open-sse/executors/chatgpt-session/errors.ts";
+import { ChatGptSessionInputError } from "../../open-sse/executors/chatgpt-session/messages.ts";
+
+test("a missing browser is a cooldown-hinted 503", () => {
+  const result = classifyChatGptSessionError(
+    new Error("No supported Chrome or Chromium executable was found")
+  );
+  assert.equal(result.status, 503);
+  assert.equal(result.code, "browser_unavailable");
+  assert.equal(result.fallbackHint, "connection_cooldown");
+});
+
+test("a playwright launch failure is also a cooldown-hinted 503", () => {
+  const result = classifyChatGptSessionError(
+    new Error("browserType.launch: Executable doesn't exist at /root/.cache/ms-playwright/x")
+  );
+  assert.equal(result.status, 503);
+  assert.equal(result.fallbackHint, "connection_cooldown");
+});
+
+test("missing credentials are a 401", () => {
+  assert.equal(
+    classifyChatGptSessionError(new Error("ChatGPT browser credentials are missing")).status,
+    401
+  );
+});
+
+test("an expired session is a 401 session_expired", () => {
+  const result = classifyChatGptSessionError(new Error("ChatGPT page is not authenticated"));
+  assert.equal(result.status, 401);
+  assert.equal(result.code, "session_expired");
+});
+
+test("a rate-limit dialog is a 429", () => {
+  const result = classifyChatGptSessionError(new Error("ChatGPT reported a usage limit"));
+  assert.equal(result.status, 429);
+  assert.equal(result.code, "rate_limited");
+});
+
+test("an account-capability mismatch is a terminal 400", () => {
+  const result = classifyChatGptSessionError(
+    new Error("pro is not available for this non-Pro connection")
+  );
+  assert.equal(result.status, 400);
+  assert.equal(result.code, "route_unavailable");
+  assert.equal(result.fallbackHint, undefined);
+});
+
+test("a DOM timeout is a terminal 400, not a retryable 5xx", () => {
+  const timeout = new Error("locator.waitForSelector: Timeout 30000ms exceeded");
+  timeout.name = "TimeoutError";
+  const result = classifyChatGptSessionError(timeout);
+  assert.equal(result.status, 400);
+  assert.equal(result.code, "browser_ui_timeout");
+});
+
+test("input errors map to their own 400 codes", () => {
+  const result = classifyChatGptSessionError(
+    new ChatGptSessionInputError("vision_unsupported", "no images")
+  );
+  assert.equal(result.status, 400);
+  assert.equal(result.code, "vision_unsupported");
+});
+
+test("an explicit upstream status wins over message matching", () => {
+  const result = classifyChatGptSessionError({ message: "anything", status: 502, code: "x" });
+  assert.equal(result.status, 502);
+});
+
+test("an unrecognised failure is a retryable 502", () => {
+  const result = classifyChatGptSessionError(new Error("something odd happened"));
+  assert.equal(result.status, 502);
+  assert.equal(result.code, "turn_failed");
+});