Agents: Claude Code, Cursor, Codex

Claude Code

.claude/settings.json

{
  "hooks": {
    "SessionStart": [
      {
        "matcher": "startup|resume|compact|clear",
        "hooks": [
          {
            "type": "command",
            "command": "node \"$CLAUDE_PROJECT_DIR/node_modules/@usefragments/cli/dist/loop.js\" hook --agent claude --event SessionStart"
          }
        ]
      }
    ],
    "PostToolUse": [
      {
        "matcher": "Write|Edit|MultiEdit",
        "hooks": [
          {
            "type": "command",
            "command": "node \"$CLAUDE_PROJECT_DIR/node_modules/@usefragments/cli/dist/loop.js\" hook --agent claude --event PostToolUse"
          }
        ]
      }
    ],
    "Stop": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "node \"$CLAUDE_PROJECT_DIR/node_modules/@usefragments/cli/dist/loop.js\" hook --agent claude --event Stop"
          }
        ]
      }
    ]
  }
}

Claude Code hook events
Event	Loop step	What the agent receives
SessionStart	card	the card as additional context
PostToolUse on a write	check	findings for that file as additional context, phrased as replace <raw> with <Fragment>
Stop	prove	nothing when the diff is clean; a block with the findings and the repair command when it is not

In (Claude Code, Write tool):

{"hook_event_name":"PostToolUse","cwd":"<repo>","tool_name":"Write",
 "tool_input":{"file_path":"<repo>/apps/cloud/src/prod-smoke/ProdSmokeGovernance.tsx"}}

Out (what the agent reads next):

{"hookSpecificOutput":{"hookEventName":"PostToolUse","additionalContext":
 "apps/cloud/src/prod-smoke/ProdSmokeGovernance.tsx:9 FUI1004 Bespoke <button> has a library equivalent. Swap to <Button> from @usefragments/ui. → Replace <button> with <Button>\napps/cloud/src/prod-smoke/ProdSmokeGovernance.tsx:12 FUI2005 Raw color #ff00aa on inline `color`. Use a CSS variable instead. → no fix"}}

Recorded 2026-09-02 on the Fragments repository; the Cursor postToolUse payload carries the same text under additional_context.

Cursor

.cursor/hooks.json

{
  "version": 1,
  "hooks": {
    "sessionStart": [
      { "command": "node ./node_modules/@usefragments/cli/dist/loop.js hook --agent cursor --event sessionStart" }
    ],
    "postToolUse": [
      { "command": "node ./node_modules/@usefragments/cli/dist/loop.js hook --agent cursor --event postToolUse" }
    ],
    "stop": [{ "command": "node ./node_modules/@usefragments/cli/dist/loop.js hook --agent cursor --event stop" }]
  }
}

Cursor hook events
Event	Loop step	What the agent receives
sessionStart	card	additional_context
postToolUse	check	additional_context with findings for the edited file
stop	prove	followup_message with the findings, at most once per session

Codex

npx @usefragments/cli hook install --agent codex writes .codex/hooks.json with the same three events. SessionStart prints the card to stdout; PostToolUse and Stop use the Claude Code protocol. Codex loads project hooks only after you trust them: run /hooks in Codex and approve the file.

{
  "hooks": {
    "SessionStart": [
      {
        "matcher": "startup|resume|compact|clear",
        "hooks": [
          {
            "type": "command",
            "command": "node ./node_modules/@usefragments/cli/dist/loop.js hook --agent codex --event SessionStart"
          }
        ]
      }
    ],
    "PostToolUse": [
      {
        "matcher": "apply_patch|Write|Edit",
        "hooks": [
          {
            "type": "command",
            "command": "node ./node_modules/@usefragments/cli/dist/loop.js hook --agent codex --event PostToolUse"
          }
        ]
      }
    ],
    "Stop": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "node ./node_modules/@usefragments/cli/dist/loop.js hook --agent codex --event Stop"
          }
        ]
      }
    ]
  }
}

Budgets

The loop is on the write path, so it is fast or it is off.

Loop step budgets
Step	Budget	Measured
card	≤ 100 ms	64 ms mean (hyperfine, 20 runs)
check on save	≤ 300 ms p95	38 ms mean warm (daemon), 568 ms cold in-process
prove on stop	≤ 1 s	230 ms mean warm on this repository (git diff dominates); 622 ms in-process on a 10-file fixture
network calls	0	asserted by test

A resident daemon holds the compiled manifest; the hook is a thin client. If the daemon is not running, the hook runs in process and starts it.

Proving the chain

npx @usefragments/cli doctor

doctor fires each hook with a real payload and prints what the agent would receive. Green means every step above works on this machine.

Next steps

The loopFive minutes from init to a first correction.MCPLocal search, get, check, tokens — and the hosted endpoint.CLIGenerated command reference.