feat: Compress MCP tool output via PostToolUse hook

[jmeiracorbal]

About

Adds a PostToolUse hook that compresses MCP tool responses before they reach Claude's context. Unlike Bash commands (intercepted via PreToolUse), MCP tool outputs were previously untouched by rtk.

Claude Code supports updatedMCPToolOutput in PostToolUse hooks which allows replacing a tool's response before the agent sees it. This is the only deterministic interception point available for MCP tools (it doesn't depend on agent cooperation like Read actions).

How it works

1. rtk init --global installs rtk-post-tool-use.sh alongside the existing rtk-rewrite.sh and registers it under PostToolUse in ~/.claude/settings.json with matcher mcp__.*.
2. On every MCP tool call, the hook pipes the PostToolUse JSON to rtk filter-mcp-output.
3. The subcommand deduplicates repeated lines and truncates responses over 3000 chars, returning updatedMCPToolOutput only when there is actual savings — pass-through when the response is already short.
4. Errors are logged to ~/.local/share/rtk/logs/mcp-filter.log and the hook always exits 0 so it never blocks Claude.

Changes

• src/mcp_filter.rs: Compression logic (deduplicate + truncate).
• hooks/rtk-post-tool-use.sh: The same behavior that ~/.claude/hooks/rtk-rewrite.sh.
• src/main.rs: filter-mcp-output subcommand.
• src/init.rs: Can install/uninstall PostToolUse hook alongside existing PreToolUse hook.

Test plan

• cargo test: 615 passed, 0 failed.
• Manually verified hook fires for mcp__.* tools in Claude Code.
• Verified compression triggers on responses > 3000 chars.
• Verified no output (pass-through) when response is under threshold.
• rtk init -g --auto-patch idempotent: Can detects hook already present.
• Errors logged to file, hook never blocks Claude on failure.

[CLAassistant]

All committers have signed the CLA.

[pszymkowiak]

Hi! Two things needed before we can review:

1. Retarget to develop — this PR targets master, but all PRs should target develop. You can change the base branch in the PR settings (right sidebar).
2. Sign the CLA — if not already done, please sign at https://cla-assistant.io/rtk-ai/rtk

Thanks!

[aeppling]

Hey

We are cleaning up the codebase and improving the project structure for better onboarding. As part of this effort, PR #826 reorganizes src/ from a flat layout into subfolders.

No logic changes — only file moves and import path updates.

What you need to do

Rebase your branch on develop when receiving this comment:

git fetch origin && git rebase origin/develop

Git detects renames automatically. If you get import conflicts, update the paths:

use crate::git;        // now: use crate::cmds::git::git;
use crate::tracking;   // now: use crate::core::tracking;
use crate::config;     // now: use crate::core::config;
use crate::init;       // now: use crate::hooks::init;
use crate::gain;       // now: use crate::analytics::gain;

Need help rebasing? Tag @aeppling