Tyler Yust 0deb8b0da1 fix: recover from context overflow caused by oversized tool results (#11579) ...

* fix: gracefully handle oversized tool results causing context overflow

When a subagent reads a very large file or gets a huge tool result (e.g.,
gh pr diff on a massive PR), it can exceed the model's context window in
a single prompt. Auto-compaction can't help because there's no older
history to compact — just one giant tool result.

This adds two layers of defense:

1. Pre-emptive: Hard cap on tool result size (400K chars ≈ 100K tokens)
   applied in the session tool result guard before persistence. This
   prevents extremely large tool results from being stored in full,
   regardless of model context window size.

2. Recovery: When context overflow is detected and compaction fails,
   scan session messages for oversized tool results relative to the
   model's actual context window (30% max share). If found, truncate
   them in the session via branching (creating a new branch with
   truncated content) and retry the prompt.

The truncation preserves the beginning of the content (most useful for
understanding what was read) and appends a notice explaining the
truncation and suggesting offset/limit parameters for targeted reads.

Includes comprehensive tests for:
- Text truncation with newline-boundary awareness
- Context-window-proportional size calculation
- In-memory message truncation
- Oversized detection heuristics
- Guard-level size capping during persistence

* fix: prep fixes for tool result truncation PR (#11579) (thanks @tyler6204)

2026-02-07 17:40:51 -08:00

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