fix(agent-core): cap compaction output at 128k by default

[wbxl2000]

Related Issue

Follow-up to #1129 and #1131.

Problem

After #1129, compaction reads maxOutputSize from the model config. But when maxOutputSize is not configured, compaction still falls back to the full context window size (max_context_tokens, often 1M+), which can exceed the max_tokens ceiling enforced by OpenAI-compatible providers (e.g. [1, 393216]), triggering 400 Invalid max_tokens errors.

#1131 added a 128k ceiling to the OpenAI Legacy provider, but the Kimi provider has no such ceiling, and compaction was intentionally left uncapped so it can always produce a summary. This leaves Kimi and any future chat-completions provider without a safe default.

What changed

• When maxOutputSize is not set and the model's context window is known, compaction now caps its output at min(max_context_tokens, 128k) instead of the full context window. This keeps compaction uncapped enough to produce a summary while staying below common provider ceilings.
• When the context window is unknown (max_context_tokens === 0), the existing conservative fallback (32k or reservedContextSize) is preserved, so the unknown-context behavior is unchanged.
• When maxOutputSize is explicitly configured, it still takes precedence — this only changes the default.
• Added a test verifying the default 128k cap is applied when maxOutputSize is not configured.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update.

[changeset-bot]

🦋 Changeset detected

Latest commit: aa0bd5d

The changes in this PR will be included in the next version bump.

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Name	Type
@moonshot-ai/kimi-code	Patch

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