Agent Diff Viewer — Real-Time Code Review Tool

A localhost tool for reviewing and steering agent-generated code diffs between turns. Cursor-style diff panes driven by Claude Code hooks, with a clipboard-b…

Agent Diff Viewer — Real-Time Code Review Tool

Partially superseded. This page describes the original design (May 2026). DiffViewer has since grown to include an OpenCode adapter, Pi extension (mid-turn blocking review), mobile companion MVP-0 (Tailscale PWA + Commandr bus approval tokens), and CodeBoarding architecture tab. The bus-watching re-homing (Phase 2) is in progress. See DiffViewer for current state and Desktop AI Agent Control Plane — Architecture Synthesis for the Tauri roadmap.

A localhost tool for reviewing and steering agent-generated code diffs between turns. Cursor-style diff panes driven by Claude Code hooks, with a clipboard-based steer injection loop.

Problem

Claude Code generates file changes across multiple tool calls per turn. There’s no native UI to review the full set of changes before continuing — you either catch them in terminal scroll or check git diff after the fact. Cursor solves this with diff panes; Claude Code has no equivalent.

Core Design Decisions

Injection model: between-turn, not mid-turn

Steering happens between agent iterations (when Claude is waiting for user input), not mid-generation. True mid-turn interruption would require a bidirectional pause protocol — fragile, no native primitive. Between-turn steering is just a follow-up prompt, which maps cleanly to the Claude Code interaction loop.

Turn boundary via Stop hook

Claude Code’s stop hook (PostResponse) fires when Claude finishes its full response. This is the turn boundary signal — no polling, no timeouts. Events accumulate per turn; Stop hook triggers the flush to the browser.

Transport: PostToolUse hook → HTTP POST

The PostToolUse hook fires after every Write/Edit/MultiEdit and exposes CLAUDE_TOOL_INPUT_JSON with file path + content. A two-line curl in the hook body POSTs to the local server. Zero polling, sub-second browser update.

Steer injection: clipboard bridge → tmux (staged)

v1: user types steer in browser UI, clicks Send, app writes to clipboard. User ⌘+Tab to terminal, paste, Enter.
v2: app calls tmux send-keys -t <pane> "$(cat ~/.claude/pending-steer.md)" Enter — fully automated.

Architecture

Claude Code
  PostToolUse hook → curl POST :3333/event    (per file change)
  Stop hook        → curl POST :3333/turn-end  (turn boundary)

~/.claude/tools/diff-viewer/server.js (Hono, always-running daemon)
  POST /event      → buffer into current turn
  POST /turn-end   → flush turn to SSE stream
  POST /steer      → copy to clipboard (v1) / tmux send-keys (v2)
  GET  /stream     → SSE to browser

browser (diff2html + vanilla JS + SSE)
  SSE listener → render per-turn grouped diffs → steer input box

Placement

~/dotfiles/.claude/tools/diff-viewer/ stowed to ~/.claude/tools/diff-viewer/. Co-located with Claude Code settings and skills. Path is machine-agnostic across all machines that run the dotfiles bootstrap. node_modules/ excluded from git; npm install in machine bootstrap.

Daemon started in a named tmux window: tmux new-window -n diff-viewer 'node ~/.claude/tools/diff-viewer/server.js'

Hooks registered globally in ~/.claude/settings.json — active in every project, zero per-project config.

Staged Roadmap

v1 — File diff review + clipboard steer

• Hono server, SSE, diff2html rendering
• PostToolUse + Stop hooks wired
• Per-turn grouped diff cards in browser
• Steer box → clipboard on Send

v2 — Full turn summary + tmux injection

• Extend hook capture to Bash commands
• Collapsible turn groups with file tree summary
• tmux send-keys injection replaces clipboard

Out of scope

• Git history traversal (use lazygit/Fugitive)
• Mid-turn interruption
• Multi-project diff history (v3 candidate: append to ~/.claude/diff-history.jsonl)

Related

• Agent Harness — hook system, PostToolUse event shape
• Tool Design for Agents — error messages as recovery instructions
• Agent Skills — skill invocation patterns