fix(ai): detect Z.AI CN endpoint context overflow errors

The CN endpoint returns code 1261 with "Prompt exceeds max length"
instead of "Prompt too long", which isContextOverflow() did not match.

fixes #10208

packages/ai/CHANGELOG.md

@@ -6,6 +6,10 @@
 
 - Added the lightweight `@earendil-works/pi-ai/models` entry point for model collections and provider construction without loading TypeBox, built-in catalogs, or provider SDKs.
 
+### Fixed
+
+- Fixed context overflow detection for Z.AI CN endpoint `Prompt exceeds max length` errors ([#10208](https://github.com/earendil-works/pi/issues/10208))
+
 ## [0.99.1] - 2026-09-29
 
 ### Added

packages/ai/src/utils/overflow.ts

@@ -27,7 +27,7 @@ import type { AssistantMessage } from "../types.ts";
  * - DS4: "Prompt has X tokens, but the configured context size is Y tokens"
  * - Cerebras: "400/413 status code (no body)"
  * - Mistral: "Prompt contains X tokens ... too large for model with Y maximum context length"
- * - z.ai: `{"code":"1261","message":"Prompt too long"}` or silent overflow via usage.input > contextWindow
+ * - z.ai: `{"code":"1261","message":"Prompt too long"}`, `{"code":"1261","message":"Prompt exceeds max length"}` (CN endpoint), or silent overflow via usage.input > contextWindow
  * - Xiaomi MiMo: Truncates input to fill contextWindow exactly, then returns finish_reason "length"
  *   with output=0 (no room left to generate). Detected via stopReason "length" + zero output +
  *   input filling the context window.
@@ -36,6 +36,7 @@ import type { AssistantMessage } from "../types.ts";
  */
 const OVERFLOW_PATTERNS = [
 	/prompt (?:is )?too long/i, // Anthropic and z.ai token overflow
+	/prompt exceeds max length/i, // z.ai CN endpoint token overflow
 	/request_too_large/i, // Anthropic request byte-size overflow (HTTP 413)
 	/input is too long for requested model/i, // Amazon Bedrock
 	/exceeds the context window/i, // OpenAI (Completions & Responses API)

packages/ai/test/overflow.test.ts

@@ -41,6 +41,12 @@ describe("isContextOverflow", () => {
 		expect(isContextOverflow(message, 1048576)).toBe(true);
 	});
 
+	it("detects z.ai CN endpoint prompt-exceeds-max-length errors", () => {
+		// Regression for #10208.
+		const message = createErrorMessage('400 {"code":"1261","message":"Prompt exceeds max length"}', "zai");
+		expect(isContextOverflow(message, 1048576)).toBe(true);
+	});
+
 	it("detects Together AI context length errors", () => {
 		const message = createErrorMessage(
 			"400 The input (516368 tokens) is longer than the model's context length (262144 tokens).",