* includes: prompt overhead in compaction safeguard calculation.
Subtracts SUMMARIZATION_OVERHEAD_TOKENS from maxChunkTokens in both the main summarization path and the dropped-messages summarization path.
This ensures the chunk budget leaves room for the prompt overhead that generateSummary wraps around each chunk.
* adds: budget for overhead tokens to use an effectiveMax instead of maxTokens naïvely.
- Added `SUMMARIZATION_OVERHEAD_TOKENS = 4096` — a budget for the tokens that `generateSummary` adds on top of the serialized conversation (system prompt, `<conversation>` tags, summarization instructions, `<previous-summary>` block, and reasoning: "high" thinking budget).
- `chunkMessagesByMaxTokens` now divides `maxTokens` by `SAFETY_MARGIN` (1.2) before comparing against estimated token counts. Previously, the safety margin was only used in `computeAdaptiveChunkRatio` and `isOversizedForSummary` but not in the actual chunking loop — so chunks could be built that fit the estimated budget but exceeded the real budget once the API tokenized them properly.
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