Port from google-gemini/gemini-cli#24440.
Browser snapshots (accessibility trees from browser_snapshot) are the
largest single tool outputs — each one can be 8,000+ characters. Only
the most recent snapshot reflects the current page state; older ones
waste context-window tokens.
New pre-pass runs before every API call (zero LLM cost):
- Scans messages for browser_snapshot and browser_vision tool results
- Replaces all but the most recent with a compact placeholder
- Skips short outputs (<200 chars, likely error messages)
- Idempotent — already-superseded snapshots are not re-processed
This complements the existing generic tool output pruning in
ContextCompressor._prune_old_tool_results(), which only triggers
during full compression. The new function runs proactively every turn,
specifically targeting the highest-token-cost tool outputs.
Includes 11 tests covering all edge cases.
Teknium
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