Use this skill when load testing services, benchmarking API performance, planning capacity, or identifying bottlenecks under stress. Triggers on k6, Artillery, JMeter, load testing, stress testing, soak testing, spike testing, performance benchmarks, throughput testing, and any task requiring load or performance testing.
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Use this skill when load testing services, benchmarking API performance, planning capacity, or identifying bottlenecks under stress. Triggers on k6, Artillery, JMeter, load testing, stress testing, soak testing, spike testing, performance benchmarks, throughput testing, and any task requiring load or performance testing.
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Load Testing
A practitioner's guide to load testing production services. This skill covers test
design, k6 implementation, CI integration, results analysis, and capacity planning
with an emphasis on when each test type is appropriate and what to measure.
Designed for engineers who need to validate performance before and after launches.
When to use this skill
Trigger this skill when the user:
Writes a k6, Artillery, JMeter, or Gatling test script
Plans a load, stress, soak, or spike test campaign
Benchmarks API throughput or latency
Defines performance SLOs or pass/fail thresholds
Integrates load tests into CI/CD pipelines
Analyzes load test results to find bottlenecks
Capacity plans for an upcoming traffic event (launch, sale, campaign)
Do NOT trigger this skill for:
Unit or integration tests that don't involve concurrent load (use a testing skill)
Frontend performance (Lighthouse, Core Web Vitals - use a frontend performance skill)
Key principles
Test in production-like environments - A load test against a single-instance
staging box with seeded data tells you nothing about your production fleet. Match
CPU/memory ratios, replica counts, and dataset sizes. Synthetic data that doesn't
reflect production cardinality produces misleading results.
Define pass/fail criteria before testing - Decide what "passing" means before
you run the first request. "P95 latency < 300ms, error rate < 0.1%, RPS >= 500"
is a pass/fail criterion. "It felt fast" is not. Set thresholds in code so tests
fail automatically in CI.
Ramp up gradually - Never go from 0 to peak load instantly. A sudden spike
obscures whether failure was caused by the ramp itself or sustained load. Use stages:
warm up, ramp to target, hold steady, ramp down. A gradual ramp mirrors real traffic
and gives infrastructure time to autoscale.
Test with realistic data and scenarios - A test that hits a single cached
endpoint with the same user ID is not a load test; it is a cache benchmark. Use
parameterized data (real user IDs, varied payloads), model the full user journey,
and include think time between requests to simulate realistic concurrency.
Automate load tests in CI - Load tests only provide value if they run
consistently. Gate every deployment with a smoke-level load test. Run full stress
and soak tests on a schedule (nightly or pre-release). Fail the build on threshold
violations. Trends over time catch regressions earlier than one-off runs.
Core concepts
Test types
Type
Goal
Duration
VU shape
Smoke
Verify the test script works; baseline sanity
1-2 min
1-5 VUs, constant
Load
Validate behavior at expected production traffic
15-30 min
Ramp to target, hold
Stress
Find the breaking point; measure degradation curve
30-60 min
Ramp beyond expected until failure
Soak
Detect memory leaks, connection pool exhaustion, drift
Choose the test type based on what question you're trying to answer - not habit.
Most teams only run load tests and miss soak and spike scenarios where real incidents
happen.
Key metrics
Metric
What it measures
Typical target
RPS / throughput
Requests per second the system handles
Depends on expected traffic
P50 / P95 / P99 latency
Response time distribution
P99 < 2x your SLO
Error rate
% of requests returning 4xx/5xx
< 0.1% under load
Time to first byte (TTFB)
Server processing latency
Proxy for backend work
Checks passed %
Business logic assertions in the test
100% expected
Always track percentiles (p95, p99), not averages. An average of 100ms with a p99
of 5000ms means 1 in 100 users waits 5 seconds - that is a bad service.
Think time
Think time (or "sleep") is the pause between requests a virtual user makes to simulate
a real user reading a page or filling a form. Without think time, virtual users fire
requests as fast as possible, which does not reflect real traffic patterns and saturates
the system unrealistically. Use sleep(randomBetween(1, 3)) to add variance.
Virtual users vs RPS
Virtual users (VUs) model concurrent users - each VU executes the full scenario
loop. RPS is a result of VU count, think time, and iteration duration.
Open vs closed workload models:
Closed (VU-based): Fixed pool of VUs, each completes a request before starting
the next. System naturally caps throughput. Best for session-based applications.
Open (arrival rate): New requests arrive at a fixed rate regardless of system
state. Queues build under saturation. Best for stateless APIs and microservices.
k6 supports both: vus/duration for closed, constantArrivalRate/ramping ArrivalRate executors for open.
Common tasks
Write a basic load test
// k6 basic load test - smoke then loadimport http from'k6/http';
import { sleep, check } from'k6';
exportconst options = {
stages: [
{ duration: '30s', target: 10 }, // ramp up
{ duration: '1m', target: 10 }, // hold
{ duration: '15s', target: 0 }, // ramp down
],
thresholds: {
http_req_duration: ['p(95)<300'], // 95% of requests under 300mshttp_req_failed: ['rate<0.01'], // less than 1% errors
},
};
exportdefaultfunction () {
const res = http.get('https://api.example.com/health');
check(res, {
'status is 200': (r) => r.status === 200,
'response time < 500ms': (r) => r.timings.duration < 500,
});
sleep(1);
}
Run with: k6 run script.js. Add --out json=results.json to export raw data.
Use SharedArray to avoid loading large data files per-VU. Model real think time
between steps - a user takes seconds between actions, not milliseconds.
Use abortOnFail: false during stress tests - you want to observe the degradation
curve, not abort at the first threshold breach. The breaking point is the RPS where
error rate exceeds tolerance or latency becomes unusable.
Gate PRs on smoke tests (1-5 VUs, 2 min). Run full load tests on merge to main.
Run soak tests nightly. Keep load tests in tests/load/ and treat them like
production code - review them, version them, maintain them.
Analyze results and identify bottlenecks
After a k6 run, the summary output shows key metrics. Here is how to read it:
scenarios: (100.00%) 1 scenario, 50 max VUs, 6m30s max duration
default: 50 looping VUs for 6m0s (gracefulStop: 30s)
checks.........................: 99.34% 12841 out of 12921
data_received..................: 48 MB 130 kB/s
data_sent......................: 2.4 MB 6.6 kB/s
http_req_blocked...............: avg=1.2ms p(95)=2.1ms p(99)=250ms
http_req_duration..............: avg=142ms p(95)=389ms p(99)=1204ms
http_req_failed................: 0.52% 67 out of 12921
http_reqs......................: 12921 35.89/s
Read the results in this order:
Error rate - http_req_failed above 0.1% needs investigation first
P99 vs p95 gap - a large gap (e.g., p95=389ms, p99=1204ms) signals high tail
latency, often from slow DB queries, GC pauses, or lock contention
http_req_blocked - high p99 here means connection pool exhaustion or
DNS issues, not application latency
Checks passed % - below 100% means business logic failures under load
Throughput (req/s) - compare to your expected traffic to confirm headroom
Bottleneck identification checklist:
Symptom
Likely cause
Next step
Error rate climbs at X VUs
Thread/connection saturation
Profile CPU and connection pool
P99 diverges from p95 at scale
GC pauses or lock contention
Heap profiling, slow query logs
http_req_blocked spikes
Connection pool exhausted
Increase pool size or reduce VUs
Latency grows linearly with VUs
No caching on hot path
Add caching, check indexes
Error rate recovers after ramp-down
Temporary saturation, no leak
System is resilient, note max VUs
Anti-patterns
Anti-pattern
Why it's wrong
What to do instead
Testing against production with no traffic shielding
Unexpected degradation hits real users
Test in a production-like staging environment or use a dark traffic approach
Using averages to judge performance
Average hides the worst 5-10% of requests that real users experience
Always track and gate on p95 and p99
No think time between steps
Generates unrealistically high RPS; stresses network, not application logic
Add sleep(randomBetween(1, 3)) between logical steps
Single hardcoded test data record
Hits the same cache key every time; measures cache, not system
Parameterize with a pool of realistic IDs and payloads
Treating load tests as one-off checks
Regressions silently reintroduce themselves after each deploy
Automate in CI with defined thresholds; fail the build on violations
Running load tests with no resource monitoring
Test results show latency but not why - you cannot fix what you cannot see
Correlate k6 results with CPU, memory, DB slow logs, and APM traces
Gotchas
k6 VU-based (closed) model produces misleadingly low RPS at high think times - If your scenario has 5 seconds of think time and you run 50 VUs, your max throughput is 50/5 = 10 RPS. This feels like the system is underloaded when it is actually VU-constrained. Use the ramping-arrival-rate executor to control RPS directly when benchmarking throughput capacity.
http_req_blocked spikes are invisible in aggregate dashboards - Aggregate p95 latency can look healthy while p99 http_req_blocked (connection pool wait time) is 2-3 seconds, indicating connection exhaustion. Always check http_req_blocked and http_req_connecting separately from http_req_duration before declaring a test passing.
Shared test data loaded with open() per-VU causes OOM on large datasets - Loading a large JSON file with open('./data/users.json') at the top level of the default function runs once per VU, not once per run. Use SharedArray to load data once and share it across all VUs without duplicating memory.
Threshold failures abort the test before you see the full breakdown curve - During stress tests, setting abortOnFail: true on latency thresholds stops the test the moment it crosses the boundary, preventing you from seeing how the system degrades at higher load. Use abortOnFail: false for stress and spike tests; reserve abort behavior for smoke tests in CI.
Load testing authenticated endpoints requires token refresh logic - Tokens generated in setup() expire during long soak tests (2-24 hours). VUs that use an expired token receive 401s that inflate error rates without revealing the real cause. Implement token refresh in the VU loop or generate tokens with a lifetime longer than the test duration.
References
For detailed comparisons and implementation patterns, read the relevant file from
the references/ folder:
references/tool-comparison.md - k6 vs Artillery vs JMeter vs Gatling: when to
use each, scripting model, CI integration, and ecosystem
Only load a references file if the current task requires it - they will consume context.
Companion check
On first activation of this skill in a conversation: check which companion skills are installed by running ls ~/.claude/skills/ ~/.agent/skills/ ~/.agents/skills/ .claude/skills/ .agent/skills/ .agents/skills/ 2>/dev/null. Compare the results against the recommended_skills field in this file's frontmatter. For any that are missing, mention them once and offer to install:
