Claude Code's Stop hook blocks the agent from finishing until verification passes. Combine it with PostToolUse feedback injection to build a 3-layer verification loop (syntax, intent, regression) in 20 minutes. The result: the agent can't say "done" until it actually is.

Two hook setups. Same Claude Code session. Different outcomes:

# What most devs have: a formatting hook
# PostToolUse: runs prettier after file edits

# What this post builds: a verification loop
# PostToolUse: checks syntax on every file change
# Stop: blocks completion until tests pass + intent verified
# Result: agent can't say "done" until it actually is

The first catches formatting. The second catches logic errors, missed requirements, and broken tests before the agent claims it's finished.

LangChain's PreCompletionChecklistMiddleware is the most documented example of this pattern. It contributed to a 13.7-point benchmark gain using harness changes alone. This post builds the Claude Code equivalent using hooks.

What does "verification" actually mean for an AI coding agent?

Verification means checking that the agent's output matches the task's intent, not just that the code compiles. Only 3% of developers report high trust in AI-generated code (Qodo, State of AI Code Quality, 2025). Most developers stop at syntax checks (lint, format, type-check). Production verification needs two more layers.

Three verification layers, each catching a different class of failure:

"Run the tests" only covers Layer 3. Tests verify what you wrote tests for, not what you asked the agent to do. If you asked Claude to add pagination and it added sorting instead, every test still passes. Layer 2 catches that.

Spotify's Honk system demonstrates this at scale: 1,500+ PRs merged through verification loops, handling roughly 50% of all PRs automatically (Spotify Engineering, Dec 2025). Their key design choice: the agent doesn't know how verification works. It just gets pass/fail feedback. That separation keeps the agent focused on the task, not on gaming the verifier.

How does Claude Code's Stop hook work?

The Stop hook fires every time Claude finishes responding. Exit code 2 blocks Claude from stopping and forces it to continue working. This single mechanism prevents the agent from saying "done" when it isn't.

Here's the critical part most tutorials skip: the stop_hook_active field.

#!/bin/bash
# .claude/hooks/verify-before-stop.sh
INPUT=$(cat)

# CRITICAL: prevent infinite verification loops
# When true, Claude is already in a forced-continuation state
if [ "$(echo "$INPUT" | jq -r '.stop_hook_active')" = "true" ]; then
    exit 0  # Let Claude stop — don't loop forever
fi

# Run tests — block stop if they fail
npm test 2>&1 || {
    echo "Tests failing. Fix before completing." >&2
    exit 2
}

exit 0

Without checking stop_hook_active, the hook blocks every stop attempt. Claude fixes the tests, tries to stop, gets blocked again, fixes more, tries to stop, gets blocked again. Infinite loop. Always check this field.

Two ways to send feedback back to the model:

• Exit 2 + stderr: The stderr message appears as feedback. Claude reads it, acts on it, then tries to stop again.
• Exit 0 + JSON with additionalContext: Inject context into the agent's next turn without blocking. Good for warnings that don't require immediate action.

Feedback via additionalContext is capped at 10,000 characters. If your test output is longer, filter it. HumanLayer learned this the hard way: 4,000 lines of passing tests flooded the context window and the agent lost track of the task. Surface failures only.

How do you build a 3-layer verification loop?

Compose three hooks across two events: a PostToolUse command hook for syntax (Layer 1), a Stop command hook for regression (Layer 3), and a Stop prompt hook for intent (Layer 2). Each runs automatically. The agent gets feedback and self-corrects.

Layer 1: Syntax verification (PostToolUse)

Runs after every Write or Edit tool call. Checks lint and type errors on the changed file. Fast, deterministic, zero tokens.

#!/bin/bash
# .claude/hooks/verify-syntax.sh
INPUT=$(cat)
FILE_PATH=$(echo "$INPUT" | jq -r '.tool_input.file_path // empty')

# Skip non-JS/TS files
if [[ ! "$FILE_PATH" =~ \.(ts|tsx|js|jsx)$ ]]; then
    exit 0
fi

# Run ESLint on the changed file, surface errors only
LINT_OUTPUT=$(npx eslint "$FILE_PATH" --quiet 2>&1)
LINT_EXIT=$?

if [ $LINT_EXIT -ne 0 ]; then
    echo "{\"additionalContext\": \"Lint errors in $FILE_PATH:\n$LINT_OUTPUT\"}"
    exit 0
fi

exit 0

The key detail: this hook returns exit 0, not exit 2. PostToolUse hooks can't undo the file write. Instead, the additionalContext field injects the lint errors into Claude's next turn. Claude sees the errors and fixes them on its own.

Layer 2: Intent verification (Stop prompt hook)

Runs when Claude tries to stop. Asks an LLM to check whether the original request was actually addressed. This is the Claude Code equivalent of LangChain's PreCompletionChecklistMiddleware.

{
  "type": "prompt",
  "prompt": "Review what was accomplished in this session. Check if all requirements from the user's original request were addressed. If anything is incomplete or missing, respond with {\"decision\": \"block\", \"reason\": \"Incomplete: <what remains>\"}. If everything looks complete, respond with {\"decision\": \"allow\"}."
}

For complex tasks, swap the prompt hook for an agent hook. Agent hooks spawn a subagent that can Read files, Grep the codebase, and run Bash commands. More thorough, but adds 2-10 seconds.

Layer 3: Regression verification (Stop command hook)

Runs when Claude tries to stop. Deterministic check: do the tests pass? Does the build succeed?

#!/bin/bash
# .claude/hooks/verify-regression.sh
INPUT=$(cat)

# Anti-loop protection, MANDATORY
if [ "$(echo "$INPUT" | jq -r '.stop_hook_active')" = "true" ]; then
    exit 0
fi

# Run tests
TEST_OUTPUT=$(npm test 2>&1)
if [ $? -ne 0 ]; then
    TRIMMED=$(echo "$TEST_OUTPUT" | tail -50)
    echo "Tests failing. Fix before completing:\n$TRIMMED" >&2
    exit 2
fi

# Run build
BUILD_OUTPUT=$(npm run build 2>&1)
if [ $? -ne 0 ]; then
    TRIMMED=$(echo "$BUILD_OUTPUT" | tail -30)
    echo "Build failing:\n$TRIMMED" >&2
    exit 2
fi

exit 0

The complete configuration

All three layers in one .claude/settings.json:

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Write|Edit",
        "hooks": [
          { "type": "command", "command": "bash .claude/hooks/verify-syntax.sh" }
        ]
      }
    ],
    "Stop": [
      {
        "hooks": [
          { "type": "command", "command": "bash .claude/hooks/verify-regression.sh" },
          {
            "type": "prompt",
            "prompt": "Review what was accomplished. Check if all requirements from the user's original request were addressed. If incomplete, respond with {\"decision\": \"block\", \"reason\": \"<what remains>\"}. If complete, respond with {\"decision\": \"allow\"}."
          }
        ]
      }
    ]
  }
}

Stop hooks run in definition order. Put the fast command hook (Layer 3) first. If tests fail, there's no point running the slower prompt hook (Layer 2).

Boris Cherny, creator of Claude Code, reports that verification feedback loops improve quality significantly: "Give Claude a way to verify its work. If Claude has that feedback loop, it will 2-3x the quality of the final result" (X thread, 2026).

What's the cost of running verification hooks?

Verification hooks add roughly 10-20% token overhead per session, primarily from the prompt/agent Stop hooks. Command hooks cost zero tokens and under 5 seconds of wall time. But skipping verification costs significantly more: teams lose an average of 7 hours per week per engineer to AI-related inefficiency, and AI code rework rates hit 20-30% when AI-generated code exceeds 40% of the codebase (Exceeds AI, 2026).

You don't need all 3 layers at once. Layer 3 alone (the test-runner Stop hook) is the highest-ROI single addition. It's 15 lines of bash, costs zero tokens, and catches the most common failure: the agent says "done" while tests are broken.

When should you use each verification layer?

Use Layer 1 (syntax) always. It's free, catches the obvious, and runs in under 2 seconds. Use Layer 3 (regression) when your project has a test suite. It's the highest-ROI single hook. Use Layer 2 (intent) for complex or multi-step tasks where the agent might solve the wrong problem entirely.

How to adopt gradually:

• Week 1: Add the Layer 3 Stop hook (test runner). Copy the verify-regression.sh script above. This single hook catches the most common failure mode.
• Week 2: Add the Layer 1 PostToolUse hook (syntax). Copy verify-syntax.sh. Now lint errors get fixed automatically instead of piling up.
• When you hit an intent failure: Add the Layer 2 prompt hook. You'll know you need it when Claude completes a task that passes all tests but doesn't match what you asked for.

This follows the failure-first method: add constraints after real failures, not before imagined ones.

FAQ

What is a self-verification loop in Claude Code?

A self-verification loop is a system of hooks that automatically checks Claude Code's output at multiple levels (syntax, intent, regression) before allowing the agent to finish. It uses PostToolUse hooks for per-file checks and Stop hooks for task-completion verification. The agent receives feedback and self-corrects without manual review.

Does verification slow down Claude Code?

Command hooks add under 5ms. Prompt hooks add 300-2000ms per Stop event. Agent hooks add 2-10 seconds. These fire once when Claude tries to stop, not on every tool call. The overhead is minimal compared to the 7 hours per week teams lose to AI-related rework.

What is the Stop hook in Claude Code?

The Stop hook fires every time Claude finishes responding. Exit code 2 blocks Claude from stopping and forces it to continue with feedback from stderr. The stop_hook_active field prevents infinite loops by signaling when Claude is already in a forced-continuation state.

How do I prevent infinite loops in verification hooks?

Always check the stop_hook_active field in your Stop hook. When the value is true, Claude is already in a forced-continuation state from a previous block. Return exit 0 to let it stop. Without this check, the hook blocks every stop attempt indefinitely, creating an infinite loop that burns tokens until the session times out.

What is harness engineering?

Harness engineering is the discipline of building constraints, tools, feedback loops, and observability around an AI agent to make it reliable in production. The formula: Agent = Model + Harness. Self-verification loops are one harness engineering example. For the full framework, see Harness Engineering: The System Around AI Matters More Than AI.

Try it now: Copy verify-regression.sh into .claude/hooks/, add the Stop hook config to .claude/settings.json, make it executable with chmod +x, and ask Claude to make a code change. Watch the Stop hook fire when tests fail. Confirm the agent fixes the issue before completing.

What layer would you add first — syntax, intent, or regression? Drop it in the comments.

Originally published on ShipWithAI. I write about Claude Code workflows, AI-assisted development, and shipping software faster with structured AI.