Gemini CLI Hook Support (PR #131 Part 3)

[ahundt]

PR 131 Part 3: Gemini CLI Hook Support

Branch: feat/gemini-support-v2 | Base: master (stacks on Part 1)
Tests: 560 pass | Parallel to: Part 2 (zero file conflicts)
Split from: PR #131
New deps: None
PR: #158

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Context

Addresses item 1 from FlorianBruniaux's split request:

Gemini CLI support (rtk init --gemini) -- standalone, no deps on the rest

FlorianBruniaux said "standalone" but code analysis shows gemini_hook.rs imports 6 items from hook.rs (types and functions). This PR depends on PR 1 for the shared hook infrastructure, but is parallel to PR 2 (data safety).

Coordination with PR #141: FlorianBruniaux noted overlap with #141's JS-based hook. This PR achieves the same Windows support goal via compiled Rust binary -- no bash, node, or bun. Both PRs wanted cross-platform support; this approach has zero external runtime dependencies.

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Merge Sequence

After PR 1 merges: retarget feat/gemini-support-v2 -> master, then merge. Can merge before or after PR 2.

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Summary

Adds Gemini CLI support via rtk hook gemini and rtk init --gemini. Shares 65% of hook logic with Claude Code -- only the JSON wire format differs.

Impact: First multi-platform support. Future platforms (Cursor, Windsurf, etc.) only need a new protocol handler -- 35% code per platform, 65% shared.

rtk hook gemini -- BeforeTool handler

Different wire format from Claude:

• Input: hook_event_name (must be "BeforeTool") + tool_name (must be "run_shell_command")
• Output: decision/reason/updated_input (not permissionDecision/updatedInput)
• Exit 0 with decision: "deny" works (unlike Claude bug #4669)

rtk init --gemini -- Setup

Patches ~/.gemini/settings.json, creates ~/.gemini/GEMINI.md with @RTK.md reference.

rtk init --gemini             # Gemini only
rtk init --claude             # Claude only
rtk init                      # Both platforms (default)
rtk init --gemini --uninstall # Remove Gemini hooks

Manual setup:

// ~/.gemini/settings.json
{"hooks":{"BeforeTool":[{"matcher":"run_shell_command","hooks":[{"type":"command","command":"rtk hook gemini"}]}]}}

Multi-platform init logic

Command	Claude	Gemini
rtk init	Yes	Yes
rtk init --claude	Yes	No
rtk init --gemini	No	Yes
rtk init --claude --gemini	Yes	Yes

Shared infrastructure extracted from init.rs:

• patch_settings_shared() -- JSON patching for both Claude and Gemini
• patch_instruction_file() -- Adds @RTK.md reference to CLAUDE.md or GEMINI.md
• remove_hook_from_settings_file() -- Uninstall hook entry

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Changes

6 files changed (+1037, -6)

New: src/cmd/gemini_hook.rs (490 lines, 19 tests)

Modified: src/cmd/mod.rs (+gemini_hook), src/main.rs (+HookCommands::Gemini, +--gemini/--claude init flags), src/init.rs (+run_gemini(), +uninstall_gemini(), +patch_gemini_settings(), extracted shared functions), INSTALL.md (+Gemini setup), README.md (+Gemini integration section)

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Review Guide

Focus areas:

1. src/cmd/gemini_hook.rs -- Gemini protocol compliance, guard checks
2. src/init.rs -- Multi-platform initialization, shared function extraction

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Not Included (Future)

Additional protocol handlers for other LLM CLIs (Cursor, Windsurf, Aider) will follow the same 35% code pattern established here.

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Test Plan

• cargo test -- 560 tests pass (gemini_hook: 19 new)
• echo '{"hook_event_name":"BeforeTool","tool_name":"run_shell_command","tool_input":{"command":"git status"}}' | cargo run -- hook gemini -- allow with rewrite
• echo '{"hook_event_name":"BeforeTool","tool_name":"write_file","tool_input":{"path":"test.txt"}}' | cargo run -- hook gemini -- NoOpinion (exit 0, no output)
• cargo run -- init --gemini --auto-patch -- settings patched
• cargo run -- init -- both platforms configured

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Related PRs (Split from PR #131)

Part	PR	Description
1	#156	Hook Engine + Chained Commands
2	#157	Data Safety Rules
3	#158	Gemini CLI Support (this PR)

Merge order: Part 1 first → retarget Parts 2 & 3 to master → merge in any order