Synchronous hook that lets plugins inspect and optionally block messages
before they are written to the session JSONL file. Primary use case is
private mode... when enabled, the plugin returns { block: true } and the
message never gets persisted.
The hook runs on the hot path (synchronous, like tool_result_persist).
Handlers execute sequentially in priority order. If any handler returns
{ block: true }, the write is skipped immediately. Handlers can also
return a modified message to write instead of the original.
Changes:
- src/plugins/types.ts: add hook name, event/result types, handler map entry
- src/plugins/hooks.ts: add runBeforeMessageWrite() following tool_result_persist pattern
- src/agents/session-tool-result-guard.ts: invoke hook before every originalAppend() call
- src/agents/session-tool-result-guard-wrapper.ts: wire hook runner to the guard
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
* 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)
