fix(openai-compatible): clamp negative outputTokens.text from inconsistent provider usage

[shaper]

Problem

Providers can return internally inconsistent usage where completion_tokens_details.reasoning_tokens exceeds completion_tokens. Observed with Baseten serving moonshotai/Kimi-K3: the model spent its whole output budget on reasoning and hit the length stop before emitting any text, and the response reported:

"usage": {
  "prompt_tokens": 951,
  "completion_tokens": 6000,
  "total_tokens": 6952,
  "prompt_tokens_details": { "cached_tokens": 60 },
  "completion_tokens_details": { "reasoning_tokens": 6001 }
}

(total_tokens = 951 + 6001, so completion_tokens is the field undercounting the actual generation here.)

convertOpenAICompatibleChatUsage computes text = completion_tokens - reasoning_tokens, which yields outputTokens.text: -1 for this response. A negative token count is nonsensical and breaks downstream consumers that validate usage values (e.g. schemas requiring non-negative counts).

Fix

Clamp the text share at 0 — the text portion of completion tokens can never be negative. total and reasoning remain exactly as reported by the provider, so consumers that aggregate text + reasoning stay aligned with the actual generation (6001 reasoning tokens here).

Testing

• Added convert-openai-compatible-chat-usage.test.ts with a regression case replaying the incident payload, plus null/normal cases.
• Updated 5 snapshots that had baked in negative text values (-318, -229, -338, -201) from recorded xAI fixtures — the same class of provider-side usage inconsistency, previously recorded as expected output.
• Reproduced end-to-end via a mock fetch replaying the incident payload through the Baseten provider on both the generateText and streamText paths: textTokens: -1 pre-fix, 0 post-fix.
• Full @ai-sdk/openai-compatible suite (node + edge, 238 tests) and tsc --build pass.

[github-actions]

✅ Backport PR created: #18620

[github-actions]

🚀 Published in:

Package	Version
@ai-sdk/baseten	2.1.6 github npm
@ai-sdk/cerebras	3.0.28 github npm
@ai-sdk/deepinfra	3.0.28 github npm
@ai-sdk/fireworks	3.0.30 github npm
@ai-sdk/google-vertex	5.0.48 github npm
@ai-sdk/huggingface	2.0.28 github npm
@ai-sdk/moonshotai	3.0.31 github npm
@ai-sdk/openai-compatible	3.0.28 github npm
@ai-sdk/togetherai	3.0.29 github npm
@ai-sdk/vercel	3.0.28 github npm