Measured against the production pod: a reasoning model asked for 64 tokens
spent all 64 inside its thought, returned NO content, and the response said
finish_reason "stop".

    max_tokens=64    finish=stop  content=0 chars ''             reasoning=262
    max_tokens=1500  finish=stop  content=13 chars 'XMU-5635-FB' reasoning=448

Two layers were each sure the other knew better. The proxy hardcoded
finish_reason = "stop" and never read choices[0].finish_reason from the remote;
OpenAIFormatter then hardcoded it again, so even a caller that worked the
reason out had it overwritten on the way to the client. vLLM had been saying
"length" all along.

It matters because "length" is the one reason a client can act on. An answer
cut off at the limit is half a sentence, and told it stopped normally the client
has nothing to retry on and no sign anything is missing. On a reasoning model
the thought is generated FIRST and spends the same budget, so a tight limit
returns an EMPTY answer that claims to have finished of its own accord -- which
is how this surfaced, as a retrieval test scoring zero for reasons that had
nothing to do with what it was measuring.

The remote's word is now carried through the per-call sink beside reasoning,
tool_calls and logprobs (get_last_finish_reason, buffered and streamed), the
formatter accepts the caller's reason and keeps "stop" only as the default, and
a tool call still wins -- that describes the answer's shape, and a truncation
after the call was formed must not relabel it. Fixed at all five response
exits, not just the one it was found at: streamed chat (both terminal-chunk
paths), buffered chat, and streamed and buffered /v1/completions.

A local backend is never told a reason, so the budget decides: generation that
used every token it was allowed was cut off, not finished. Drawn only when a
limit was actually set and reached -- never from a token count alone, or every
long completion would be mislabelled.

Tests verified against the reverted files: assert None == 'length' for the
backends, TypeError: unexpected keyword 'finish_reason' for the formatter.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S8LAP3ueNzQJtwzhgVbq66