When a proxy (Ollama, OpenRouter) rejects the MODEL's own unparseable
tool-call JSON with `400 invalid tool call arguments`, the classifier returned
the generic format_error verdict (`should_fallback=True`) and the non-retryable
client-error path cascaded through every fallback provider: 4-5 sequential
calls, 20-60s per occurrence, ending on a model that produced the same broken
JSON (#12770).
- error_classifier: explicit `_MALFORMED_TOOL_ARGS_PATTERNS` checked before the
request-validation and overflow heuristics, returning format_error with
`retryable=False, should_fallback=False`.
- turn_api_error: the client-error settlement honours `should_fallback`; the
verdicts that legitimately reach that branch (policy block, TLS chain, MoA
shape/preset errors) now state `should_fallback=True` explicitly, so the gate
changes behaviour only for the new verdict. Local validation errors keep
their historical fallback.
Fixes#12770. Pattern list and gating approach from #16022 by @cuyua9 (stale
base); tests trimmed to two invariants.
Co-authored-by: cuyua9 <2114364329@qq.com>
dfd4aa4a94