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name: build-eval description: "Write rigorous evals for LLM agents, multi-agent systems, skills, MCP servers, and prompts. Use when: building test suites, measuring agent effectiveness, evaluating coordination, or choosing eval frameworks. Covers: DeepEval, Braintrust, RAGAS, precision/recall, F1, task completion, pass@k, iterative metrics, multi-agent coordination."

Eval-1337

Write evals that measure what matters. Not vanity metrics.

The key to success is measuring performance and iterating (Anthropic 2026).

The Core Problem

# This tells you NOTHING
activation_rate = 100%  # Activates on every prompt = useless

Single metrics lie. You need to measure BOTH failure modes.

Three Grader Types

Type	Examples	Use When
Code-based	String match, regex, test suites, outcome verification	Deterministic checks (speed, objectivity)
Model-based	LLM rubric scoring, pairwise comparison, multi-judge	Open-ended tasks (flexibility, nuance)
Human	Expert review, crowdsourced judgment, spot-check	Gold standard (expensive, slow)

Code-based first: Prefer deterministic graders; use model-based for flexibility; apply partial credit for multi-component tasks. "Grade what the agent produced, not the path it took" (Anthropic 2026).

Match Eval to Agent Type

Agent Type	Primary Grader	Key Metrics	Benchmark
Coding	Code (test suites)	Tests pass, no regressions	SWE-bench Verified
Conversational	Multi (state + transcript + rubric)	Resolution, turn limits, tone	τ2-Bench
Research	Model (groundedness, coverage)	Claim support, source quality	Custom
Computer Use	Code (screenshot, state inspection)	GUI state, file system	WebArena, OSWorld
Skills	Code (activation) + Model (methodology)	F1 + adherence rubric	Custom
Multi-Agent	Multi (milestones + coordination)	Task score, handoff success	MultiAgentBench
Pipeline	Per-stage + handoffs + end-to-end	Stage success, bottleneck	Custom

Non-Determinism Metrics

LLMs are stochastic. Run 5+ trials per task.

Metric	Formula	Use When
pass@k	P(≥1 success in k trials)	One success is enough
pass^k	P(all k trials succeed)	Reliability-critical

Example: 75% per-trial success → pass@3 ≈ 98%, pass^3 ≈ 42%.

Use pass@k for exploration; pass^k for production reliability.

Iterative Metrics (Ralph Pattern)

Traditional pass@k treats trials as independent. Iterative eval uses failures as feedback:

Metric	Formula	Question
pass@k (iterative)	Success within k retries with feedback	Can it recover?
iterations_to_pass	Retries until success	Learning speed
recovery_rate	(pass@k - pass@1) / (1 - pass@1)	% failures that recover
feedback_sensitivity	Δscore per iteration	Does guidance help?

Use case: Agent has 60% pass@1. Is that its ceiling, or can it do better with feedback?

Iterative eval result:
├── pass@1: 60%         (baseline)
├── pass@3: 91%         (with retry + feedback)
└── recovery_rate: 78%  → Deploy with retry loop, not better prompts

Multi-Agent Metrics

Single-agent metrics miss coordination failures:

Metric	Formula	Measures
Task Score	Σ(milestone × weight)	Goal achievement
Handoff Success	Completed / expected	Task transfers work?
Comm Efficiency	Useful messages / total	Signal vs noise
Role Adherence	On-role actions / total	Staying specialized?
ToM Score	Passed scenarios / total	Theory of mind

Match Metric to Target

Target	What to Measure	Metric	Framework
Agents	Task completion	pass@k / accuracy	DeepEval
Agents	Tool usage	ToolCorrectnessMetric	DeepEval
Skills	Activation (L1)	Precision/Recall/F1	Custom
Skills	Methodology (L2)	LLM rubric	DeepEval GEval
MCP Servers	Tool calls	ToolCallAccuracy	RAGAS
MCP Servers	Reliability	MCPGauge 4-dim	Custom
Prompts	Output quality	LLM-as-judge	Braintrust
Any	Traces	Span analysis	Phoenix (local)
Any	Behavioral	Bloom scenarios	Anthropic Bloom

Building Evals: The Roadmap

From Anthropic's agent evaluation guide (2026):

Step 0: Start early with 20-50 tasks from actual failures
Step 1: Write unambiguous tasks (pass expert test)
Step 2: Build balanced problem sets (positive AND negative)
Step 3: Robust harness with clean environments per trial
Step 4: Thoughtful grader design (deterministic preferred)
Step 5: Read transcripts (verify graders, understand failures)
Step 6: Monitor saturation (100% → only tracks regressions)
Step 7: Maintain as living artifact (dedicated ownership)

Common Pitfalls

Trap	Fix
Single test run	5+ runs - stochastic outputs
One-sided test set	Balance positive/negative - prevents overfitting
Measuring recall only	Add precision - high recall + low precision = noise
"Forced eval" inflation	Realistic conditions - forced mode inflates scores
No ground truth	Label expectations - must_trigger, should_not
Grader too rigid	Accept valid variations - grade outcome, not path
Shared state between runs	Isolate environments - leftover files cause correlation
Bypass vulnerabilities	Design to require solving - agents exploit loopholes
Eval saturation	Expand difficulty - high pass rates mask improvements

Classification Metrics (F1)

Use when you have TWO failure modes:

                      ACTUAL
                      Yes         No
                  +-----------+-----------+
EXPECTED    Yes   |    TP     |    FN     |
                  |  Correct  |  Missed   |
                  +-----------+-----------+
            No    |    FP     |    TN     |
                  |   Noise   |  Correct  |
                  +-----------+-----------+

Precision = TP / (TP + FP)   "when it fires, is it right?"
Recall    = TP / (TP + FN)   "when it should fire, does it?"
F1        = 2×(P×R)/(P+R)    "balanced score"

Labeled Test Cases

{"input": "What crate for CLI args?", "expectation": "must_trigger"}
{"input": "Write a haiku", "expectation": "should_not_trigger"}
{"input": "Explain ownership", "expectation": "acceptable"}

Label	Meaning	Measures
must_trigger	Should definitely fire	Recall (misses)
should_not_trigger	Must not fire	Precision (noise)
acceptable	Either outcome fine	Excluded

Defense in Depth (Swiss Cheese)

No single eval method catches everything. Layer them:

Method	Speed	Coverage	Best For
Automated evals	Fast	Narrow	Regression prevention
Production monitoring	Real-time	Broad	Real behavior
A/B testing	Days/weeks	Statistical	Outcome measurement
Manual transcript review	Slow	Deep	Building intuition
Human studies	Very slow	Gold-standard	Subjective quality

Use multiple methods; each layer catches what others miss.

Framework Decision

Situation	Use	Why
Python agent evals	DeepEval	TaskCompletionMetric, ToolCorrectness
TypeScript/Node	Braintrust	Identical Python/TS API
RAG pipelines	RAGAS	ToolCallF1, context metrics
Skill activation	Custom	Precision/recall with labeled expectations
Behavioral evals	Bloom	Automated scenario generation
Infrastructure	Harbor, Promptfoo	Containerized, YAML-based

Quick Reference

AGENTS
  Coding: Test suites (SWE-bench pattern)
  Conversational: Multi-grader (state + transcript + rubric)
  Research: LLM groundedness + coverage
  Metrics: pass@k (exploration), pass^k (reliability)

MULTI-AGENT
  Task: Milestone-weighted task score
  Coordination: Handoff success, comm efficiency
  Roles: Role adherence, work duplication
  Advanced: Theory of Mind (ToM) scenarios

PIPELINE (Sequential A → B → C)
  Level 1: Single-agent metrics per stage
  Level 2: Handoff quality between stages
  Level 3: End-to-end pipeline metrics
  Key: Find bottleneck stage, error propagation

ITERATIVE (Ralph Pattern)
  When: Deciding retry loop vs better prompts
  Metrics: iterations_to_pass, recovery_rate, feedback_sensitivity
  Key insight: pass@1 ≠ capability ceiling

SKILLS
  Level 1 (Activation): F1 with labeled expectations
  Level 2 (Methodology): GEval rubric (evidence, WHY, verification)
  Observable: skill_check, skill_match spans

MCP SERVERS
  RAGAS: ToolCallAccuracy, ToolCallF1
  MCPGauge: proactivity, compliance, effectiveness, overhead

BEHAVIORAL
  Bloom: Automated scenario generation for alignment properties
  Targets: sycophancy, self-preservation, sabotage

OBSERVABILITY
  ALL extensions should have OTel spans
  Skills: skill_check, skill_match
  Agents: agent_run, llm_call, tool_call
  MCP: mcp_server, mcp_call

Domain Routing

Detected	Load
agent, task completion, pass@k	agents.md
multi-agent, coordination, handoff	multi-agent.md
pipeline, sequential, stage, chain	multi-agent.md
iterative, retry, recovery, ralph	iterative.md
skill, activation, trigger	skills.md
methodology, behavioral, adherence	methodology.md
MCP, tool call, server	mcp.md
prompt, quality, judge	prompts.md
trace, debug, analyze spans	observability.md
security, red team, adversarial	security.md
benchmark, SWE-bench, WebArena	benchmarks.md
dataset, labeling	datasets.md
DeepEval, Braintrust, RAGAS	frameworks.md
Full citations	sources.md