hooks/hermes: add transform_tool_result output filter for high-token tools

[cfournel]

Problem

The rtk-rewrite plugin currently only reduces tokens on the input side — rewriting terminal commands before execution via pre_tool_call. Tool outputs (what gets injected back into the conversation context) are untouched.

Analysis of 388 real Hermes sessions (~2,900 tool calls) shows four tools dominate output token consumption:

Tool	Calls	Est. output tokens	Avg tok/call
browser_navigate	281	330,175	1,175
execute_code	410	344,030	839
read_file	130	132,701	1,020
terminal	—	—	~235 (observed)

terminal was identified as the largest single contributor in a real kanban session (55% of total context tokens — 23,502 chars across 25 calls), yet was previously unfiltered. Its output is a JSON wrapper {"output": "..."} containing raw shell output with blank lines, repeated lines, and long listings.

The browser_navigate output is a JSON blob containing an accessibility tree snapshot heavily padded with LayoutTable*, LayoutTableRow, LayoutTableCell, LayoutInline*, LayoutBlock* structural nodes that carry no semantic value for the model.

Solution

Add a transform_tool_result hook (supported by Hermes since model_tools.py:1116) that filters these tools before their output enters the context:

• browser_navigate — parse JSON, strip Layout* accessibility-tree noise lines, deduplicate, drop stealth_warning/stealth_features metadata fields, truncate snapshot to 120 lines
• terminal — parse JSON wrapper, apply line dedup + blank-line strip + truncate to 200 lines on the output field, repack JSON
• read_file / execute_code — strip blank lines, deduplicate repeated lines, truncate to 200 lines

All filters fail open: returning None leaves the original result untouched. No existing behaviour is changed.

Benchmark

Tested live on https://news.ycombinator.com:

rtk: filtered browser_navigate: 8712 -> 6826 chars (21% reduction)

Observed on a real kanban session (t_c094df87, 42 messages, 1h32m):

Tool	Calls	Chars (unfiltered)	Now filtered
terminal	25	23,502	✅
execute_code	3	18,773	✅
kanban_show	1	53	—

Changes

• plugin.yaml — declare transform_tool_result hook so Hermes loads the plugin for it
• __init__.py — add _transform_tool_result, _filter_browser, _filter_terminal, _filter_text; pre_tool_call path is untouched

🤖 Generated with Claude Code

[cfournel]

This allows also to compact tools output in Hermes kanban so all cards are optimized too .