DeepSeek's new Responses endpoint (deepseek-v4-flash) intermittently rejects valid request bodies with serde deserialization errors such as 'input: invalid type: string ..., expected a sequence'. These were not classified as compatibility errors, so affected conversations died instead of falling back to Chat Completions.
The wire format is correct (input serializes as a list), so this is a server-side Responses compatibility issue; Chat Completions is strictly more permissive, making fallback safe. Extend the fallback classifier to recognize serde body-parsing markers. Repeated failures still trip the existing circuit breaker.
When the Responses API fails repeatedly (3 consecutive compatibility
errors), skip it and fall back directly to Chat Completions. Unlike a
permanent disable, the circuit re-probes after 5 minutes so recovery
is automatic when the API comes back. Success resets the counter.
Keyed per (model, reasoning_effort) so a failure with one model does
not affect others.
arcdrake22
chengyongru
chengyongru
outlook84
Xubin Ren
Mohamed Elkholy