[Bug] Classify provider error 1261 as context overflow #52961

Description

In our OpenCode Sandbox integration, a long-running session can fail when the accumulated prompt becomes too large for the Zhipu Coding Plan GLM model. The provider returns HTTP 400 with this response:

{"error":{"code":"1261","message":"Prompt exceeds max length"}}

OpenCode receives the message Prompt exceeds max length, but the latest provider error classifier still has no matching rule for it. The response is therefore treated as a generic provider error instead of a context-overflow error, and the existing compaction/recovery path is not entered.

I checked the latest public release, v1.18.34: packages/llm/src/provider-error.ts still has no Prompt exceeds max length match.

Plugins

None.

OpenCode version

v1.18.34 (source commit aec0b9a6d8898f68f923aaf08b7306d931fd9d76)

Steps to reproduce

1. Start a long-lived OpenCode session using glm-5.3-flash through the Zhipu Coding Plan provider.
2. Continue the same session through multiple tool calls, file reads, and edits so the assembled chat request grows large.
3. Send another prompt.
4. The provider responds with HTTP 400, error code 1261, and message Prompt exceeds max length.
5. OpenCode reports the provider error without entering its context-overflow compaction/recovery path.

Expected behavior

An HTTP 400 provider error with message Prompt exceeds max length (or Prompt exceeds maximum length) should be classified as a context-overflow failure so OpenCode can use its existing compaction/recovery behavior, or report a specific context-overflow error when recovery is not possible.

Proposed scope

This appears to be a focused change to the shared provider error classifier, with regression coverage for the HTTP 400 invalid_request_error shape and negative cases such as max_tokens exceeds max length, ordinary bad requests, and HTTP 429 rate limits. I can prepare a small PR for this classifier change if the direction is in scope.

Operating System

Linux x64 Sandbox image

Terminal

Sandbox API integration