Agent Evaluation

Agent Evaluation

Quick nav: Why Evaluate? · Metrics to Track · Implementation · Example · Tools

Why Evaluate Agents?

When you create custom agents in .claude/agents/, you're encoding specialized expertise into reusable workflows. But how do you know if your agents are actually effective?

Without evaluation, you're building blind:

• ❌ No way to measure if agent responses are improving or degrading over time
• ❌ Can't compare different agent configurations objectively
• ❌ Difficult to identify which aspects of agent context/instructions need refinement
• ❌ No data to justify investment in agent development

With evaluation, you iterate with confidence:

• ✅ Quantify agent quality through metrics (response time, accuracy, tool usage)
• ✅ A/B test different agent configurations with measurable outcomes
• ✅ Identify patterns in successful vs failed interactions
• ✅ Build feedback loops for continuous improvement

Core principle: Agents are code. Like all code, they need tests, metrics, and observability.

Metrics to Track

1. Response Quality Metrics

What to measure:

• Task completion rate: Did the agent accomplish the stated goal?
• Correctness: Were the agent's outputs factually accurate?
• Relevance: Did the response stay on-topic and address the actual question?
• Hallucination rate: How often did the agent invent information?

How to track:

# Post-response hook: .claude/hooks/log-response-quality.sh
# Triggered after each agent response

# Log structure:
{
  "timestamp": "2026-02-10T14:32:00Z",
  "agent_id": "backend-architect",
  "task_completed": true,
  "correctness_score": 4.5,  # User rating 1-5
  "hallucinations": 0,
  "response_tokens": 1250
}

Implementation tip: Use user feedback prompts (thumbs up/down) or automated checks (test suite passing after agent code generation).

2. Tool Usage Metrics

What to measure:

• Tool call success rate: Percentage of tool calls that executed without errors
• Tool selection accuracy: Did agent choose the right tool for the task?
• Tool call efficiency: Minimum calls to achieve goal (avoid unnecessary reads/searches)
• Error recovery: Did agent handle tool failures gracefully?

How to track:

# Post-tool-use hook: .claude/hooks/log-tool-usage.sh
# Triggered after each tool call

# Log structure:
{
  "timestamp": "2026-02-10T14:32:05Z",
  "agent_id": "backend-architect",
  "tool_name": "Read",
  "tool_success": true,
  "tool_parameters": {"file_path": "src/auth.ts"},
  "execution_time_ms": 45
}

Implementation tip: Use Claude Code hooks system (see examples/hooks/) to automatically log tool calls.

3. Performance Metrics

What to measure:

• Response time: Total time from user prompt to complete response
• Token efficiency: Input/output tokens used per task
• Context utilization: How much of context window was used?
• Cost per task: API cost for the full interaction

How to track:

# Session-end hook: .claude/hooks/log-performance.sh
# Triggered at end of session

# Log structure:
{
  "timestamp": "2026-02-10T14:35:00Z",
  "agent_id": "backend-architect",
  "session_duration_s": 180,
  "input_tokens": 3500,
  "output_tokens": 2800,
  "total_cost_usd": 0.15,
  "context_utilization": 0.42
}

Implementation tip: Parse Claude Code session logs or use MCP observability tools.

4. User Satisfaction Metrics

What to measure:

• Explicit feedback: User ratings, comments, bug reports
• Implicit signals: Did user accept agent's suggestions? Did they retry the prompt?
• Adoption rate: How often is this agent used vs alternatives?
• Retention: Do users return to this agent for similar tasks?

How to track:

# Manual feedback collection
# After agent completes task, prompt user:
"Rate this agent's performance (1-5): _"

# Log:
{
  "timestamp": "2026-02-10T14:35:10Z",
  "agent_id": "backend-architect",
  "user_rating": 5,
  "user_comment": "Perfect analysis of auth flow",
  "would_use_again": true
}

Implementation tip: Add feedback prompts to agent templates or use post-session surveys.

Implementation Patterns

Pattern 1: Logging Hook System

Use Case: Automatically track all agent interactions without manual intervention

Setup:

# .claude/hooks/post-tool-use.sh
#!/bin/bash
# Triggered after every tool call

AGENT_ID=$(echo "$CLAUDE_AGENT_ID" | jq -r)
TOOL_NAME=$(echo "$CLAUDE_TOOL_NAME" | jq -r)
TOOL_SUCCESS=$(echo "$CLAUDE_TOOL_SUCCESS" | jq -r)

# Append to metrics log
echo "{\"timestamp\":\"$(date -Iseconds)\",\"agent\":\"$AGENT_ID\",\"tool\":\"$TOOL_NAME\",\"success\":$TOOL_SUCCESS}" \
  >> .claude/logs/agent-metrics.jsonl

Pros: Zero manual overhead, complete coverage, time-series data Cons: Requires parsing Claude Code environment variables (may change across versions)

Pattern 2: Agent Unit Tests

Use Case: Regression testing to ensure agent improvements don't break existing capabilities

Setup:

# tests/agents/backend-architect.test.sh
#!/bin/bash

# Test 1: Agent correctly identifies hexagonal architecture layers
echo "Test: Hexagonal architecture analysis"
RESULT=$(claude agent backend-architect "Analyze src/auth.ts for layer violations")
if echo "$RESULT" | grep -q "domain layer"; then
  echo "✅ PASS: Identified layers"
else
  echo "❌ FAIL: Did not identify layers"
  exit 1
fi

# Test 2: Agent recommends correct patterns
echo "Test: Pattern recommendations"
RESULT=$(claude agent backend-architect "Improve error handling in src/api.ts")
if echo "$RESULT" | grep -q "Result<T, E>"; then
  echo "✅ PASS: Recommended Result pattern"
else
  echo "❌ FAIL: Incorrect pattern"
  exit 1
fi

Pros: Automated, catches regressions, CI/CD integration Cons: Requires maintenance, may have false positives/negatives

Pattern 3: A/B Testing Configurations

Use Case: Compare two versions of agent to determine which performs better

Setup:

# .claude/agents/backend-architect-v1.md (control)
name: backend-architect
version: 1.0
instructions: |
  You are a backend architect specializing in...
  [original instructions]

# .claude/agents/backend-architect-v2.md (experiment)
name: backend-architect-v2
version: 2.0
instructions: |
  You are a backend architect specializing in...
  [modified instructions with new pattern emphasis]

Evaluation:

# Run same task with both agents, compare metrics
# Task: "Analyze src/auth.ts for security issues"

# Version 1 metrics:
# - Response time: 45s
# - Issues found: 3
# - User rating: 4/5

# Version 2 metrics:
# - Response time: 38s
# - Issues found: 5 (2 additional critical issues)
# - User rating: 5/5

# Conclusion: Version 2 is more thorough and faster → promote to production

Pros: Data-driven decisions, quantifiable improvements Cons: Requires discipline to run controlled experiments

Pattern 4: Feedback Loop Integration

Use Case: Continuously improve agent based on real-world usage data

Setup:

# After agent completes task
echo "How would you rate this response? (1-5, or 'skip'): "
read RATING

if [ "$RATING" != "skip" ]; then
  echo "Any specific feedback?: "
  read COMMENT

  # Log feedback
  echo "{\"timestamp\":\"$(date -Iseconds)\",\"agent\":\"$AGENT_ID\",\"rating\":$RATING,\"comment\":\"$COMMENT\"}" \
    >> .claude/logs/agent-feedback.jsonl
fi

# Weekly: Review feedback.jsonl, identify patterns
# Monthly: Update agent instructions based on aggregated feedback

Pros: Aligns agent with actual user needs, identifies edge cases Cons: Requires manual review and action on feedback

Example: Agent with Evaluation

Full template available: examples/agents/analytics-with-eval/ includes complete agent definition, hooks, analysis scripts, and report template.

Setup: Analytics Agent with Built-in Metrics

# .claude/agents/analytics-agent.md
---
name: analytics-agent
description: SQL query generator with evaluation hooks
version: 1.0
tools:
  - Read
  - Write
  - Bash
hooks:
  post_response: .claude/hooks/log-analytics-metrics.sh
---

# Analytics Agent

You are an expert SQL analyst helping users query databases.

## Evaluation Criteria

After each query:
1. **Correctness**: Does query produce expected results?
2. **Performance**: Query execution time < 5s?
3. **Safety**: No destructive operations (DELETE, DROP, TRUNCATE)?
4. **Best practices**: Uses proper JOINs, indexes, parameterized queries?

## Instructions

[... agent instructions ...]

Metrics Hook

# .claude/hooks/log-analytics-metrics.sh
#!/bin/bash
# Triggered after analytics-agent response

# Extract query from response (naive grep, improve with jq)
QUERY=$(echo "$CLAUDE_RESPONSE" | grep -oP 'SELECT.*?;')

if [ -n "$QUERY" ]; then
  # Test query (requires database connection)
  EXEC_TIME=$( (time psql -U user -d db -c "$QUERY") 2>&1 | grep real | awk '{print $2}')

  # Check for destructive operations
  if echo "$QUERY" | grep -iE 'DELETE|DROP|TRUNCATE'; then
    SAFETY="FAIL"
  else
    SAFETY="PASS"
  fi

  # Log metrics
  echo "{\"timestamp\":\"$(date -Iseconds)\",\"query\":\"$QUERY\",\"exec_time\":\"$EXEC_TIME\",\"safety\":\"$SAFETY\"}" \
    >> .claude/logs/analytics-metrics.jsonl
fi

Analysis

# Monthly review: Analyze metrics
jq -s 'group_by(.safety) | map({safety: .[0].safety, count: length})' \
  .claude/logs/analytics-metrics.jsonl

# Output:
# [
#   {"safety": "PASS", "count": 127},
#   {"safety": "FAIL", "count": 3}
# ]

# Action: Review 3 failed queries, update agent instructions to prevent future violations

Tools & References

Open-Source Evaluation Frameworks

nao (Analytics Agents)

URL: github.com/getnao/nao

What it provides:

• Built-in evaluation framework for analytics agents
• Unit testing capabilities for agent responses
• Metrics collection (response quality, tool usage, performance)
• Feedback loop integration

How to adapt for Claude Code:

• Context builder pattern: Apply nao's structured context approach to .claude/agents/ config
• Evaluation hooks: Translate nao's evaluation framework to Claude Code hooks system
• Metrics schema: Use nao's metrics schema as template for your logs

Status: Production-ready, actively maintained, TypeScript + Python

Claude Code Native Patterns

Hooks system: .claude/hooks/ for automated logging (see examples/hooks/README.md)

Agents directory: .claude/agents/ for custom agent definitions (see guide/ultimate-guide.md Section 4)

MCP observability: Use MCP servers for advanced logging and metrics aggregation

Best Practices

Start Simple

Week 1: Add basic logging hook (tool calls only) Week 2: Add user feedback prompt (manual ratings) Week 3: Build dashboard to visualize metrics Week 4: Run first A/B test on agent configuration

Focus on Actionable Metrics

Don't track metrics you won't act on. Prioritize:

1. Task completion rate → Refine agent instructions
2. Tool call errors → Improve context or add examples
3. User ratings → Identify confusing or unhelpful responses

Automate Where Possible

Manual evaluation doesn't scale. Use:

• Hooks for automatic logging
• CI/CD integration for agent unit tests
• Scripts for periodic metric aggregation

Build Feedback Loops

Metrics are useless without action:

• Weekly: Review metrics, identify patterns
• Monthly: Update agent instructions based on data
• Quarterly: Major agent refactoring if needed

Related Sections

• Agents: Creating custom agents
• Hooks: Automation with event hooks
• Observability: Logging and monitoring strategies
• AI Ecosystem: External frameworks like nao

Next steps:

1. Add logging hook to your most-used agent
2. Collect 1 week of metrics
3. Analyze and refine agent based on data

Template: See examples/agents/analytics-with-eval/ for complete implementation with hooks, scripts, and report template