Answer delivered entirely in reasoning tokens instead of content #426
Description
Bug: Final answer delivered entirely in reasoning tokens instead of content
Summary
When using moonshotai/kimi-k2.5:nitro with reasoning enabled ({ max_tokens: 8000, exclude: false }), the model frequently puts the final answer entirely into reasoning tokens with zero content tokens.
This happens in ~19.5% of responses across 190 messages in our production dataset.
Expected behavior
The model should put its internal thinking into reasoning deltas and the user-facing answer into content deltas. When finish_reason: stop, there should be meaningful content.
Actual behavior
The final step streams only reasoning deltas with empty content. The entire user-facing answer (with markdown formatting, bold text, etc.) arrives as reasoning tokens.
Reproduction
Request config:
- Model:
moonshotai/kimi-k2.5:nitro - Reasoning:
{ max_tokens: 8000, exclude: false } - Multi-step tool-calling agent (AI SDK v6 ToolLoopAgent)
- After tool results are returned, the model's final "answer" step puts everything into reasoning
Raw response chunks (all reasoning, no content):
// 67 chunks, ALL in the reasoning field — zero content deltas
{"choices":[{"delta":{"reasoning":" **Resultaat**\n\n..."}}]}
{"choices":[{"delta":{"reasoning":"Gisteren (5 maart 2026) waren er **36 unieke bezoekers** vanuit Amsterdam."}}]}
// ...
{"choices":[{"finish_reason":"stop"}]}
{"usage":{"prompt_tokens":5456,"completion_tokens":68,"completion_tokens_details":{"reasoning_tokens":56}}}Impact
- Downstream consumers (AI SDK, chat UIs) expect
contentfor the answer - With
exclude: true, the answer is lost entirely - With
exclude: false, the answer renders as "thinking" instead of the response
Stats from production (190 assistant messages)
| Pattern | Count | % |
|---|---|---|
| Normal (reasoning + text content) | 141 | 74.2% |
| Answer in reasoning only | 37 | 19.5% |
| Reasoning + tool (incomplete) | 9 | 4.7% |
| Other | 3 | 1.6% |
Environment
- Provider: OpenRouter → Parasail
- Resolved model:
moonshotai/kimi-k2.5-0127 @openrouter/ai-sdk-provider- AI SDK v6 with
ToolLoopAgent