Your Kimi K3.1 Pro Responses Truncate Silently

Connecting to the Kimi K3.1 Pro service via an OpenAI-compatible client proves straightforward initially. A generation request completes successfully, but the response is unexpectedly truncated, cutting off the answer mid-sentence. The client receives a successful response, the stream closes cleanly, and the user only receives part of the answer.

Detecting truncation involves comparing the request size to the documented context limit and choosing between the Pro and Highspeed variants, based on evidence rather than assumptions. Every completion comes with a finish_reason, which reveals whether the model finished naturally or hit the max_tokens limit. The failure occurs in streaming calls when the connection closes before inspecting the final chunk containing the finish_reason == "length".

Most hand-written stream loops only read delta.content, leading to truncated answers being stored as complete. The max_tokens parameter exacerbates this issue, as leaving it unset defaults to a conservative value, while setting it too high causes failures when the prompt exceeds the context limit. Detecting truncation requires checking finish_reason in the stream.

The loop reads the text and records the termination reason from the last chunk. If finish_reason equals "length", a RuntimeError is raised, revealing that the generation hit the token cap mid-output. This check runs after the loop, not inside it, as the final chunk often carries finish_reason with empty content. A typo in the model ID returns a 404 error, which can be mistaken for an authentication issue.

Choosing the right variant is crucial, as Kimi provides two versions of K3.1 Pro with distinct tradeoffs. K3.1 Pro Correctness prioritizes accuracy but is slower per request, while K3.1 Pro Highspeed focuses on latency and throughput, potentially altering the output. Testing the parity between variants with real prompts before migrating is essential.

Use a constant model string to avoid accidental changes in output due to variant switches. Budget tokens before sending the request to avoid truncation or request failure. Calculate remaining tokens by subtracting prompt tokens from the context limit, ensuring the prompt does not exceed the limit. This helps prevent truncation caused by exceeding the context limit, while ensuring max_tokens is sufficient to capture the complete answer. Implement a retry loop that handles rate-limit errors without duplicating side effects.