Adds a plugin seam at the top of agent/error_classifier.classify_api_error()
(step 0, before the built-in pipeline) so model-provider plugins can classify
their provider's error quirks without patching core:
- New "classify_api_error" entry in VALID_HOOKS. Callbacks receive the parsed
error context (provider, model, status_code, error_type, error_code,
error_message, error_body, error, approx_tokens, context_length,
num_messages), self-scope on `provider`, and return None to pass or a dict
{"reason": "<FailoverReason name>", ...optional recovery-hint overrides}.
- get_plugin_error_classification() helper mirrors
get_pre_tool_call_block_message(): first valid result wins, invalid dicts
and unknown reasons are skipped, callback exceptions are isolated — a
broken plugin can never break classification. Zero behavior change when no
plugin claims the error (all 179 existing classifier tests pass untouched).
- Bundled reference plugin `openrouter-tool-use-404` (opt-in, like all
bundled standalone plugins) re-implements PR #58451: OpenRouter's
"No endpoints found that support tool use" 404 carries no
_MODEL_NOT_FOUND_PATTERNS signal, so it classifies as unknown/retryable
and the retry loop burns 3-5 attempts on a deterministic rejection.
The plugin classifies it as model_not_found (retryable=False,
should_fallback=True) so the fast-fallback path fires immediately —
demonstrating a waiting core PR converted to a publishable plugin.
Motivation: ~10 open PRs are single-provider error-classification patches
(#58451, #58355, #58502, #58474, #58366, ...). This hook turns that whole
class of contribution into plugin territory.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FWMcB7RPSYUpsXDfBgwjzM
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