k6 vs promptfoo

k6 embeds Sobek (a Go-based JavaScript runtime forked from Goja) to execute test scripts written in JavaScript, enabling developers to write load tests as version-controlled code files that can be modularized and integrated into CI/CD pipelines. The Sobek runtime compiles JavaScript to bytecode and executes it within Go's performance envelope, allowing a single k6 process to simulate thousands of concurrent virtual users without the overhead of spawning separate processes or VMs.

Unique: Uses Sobek (Go-based JavaScript VM) instead of Node.js or V8, enabling k6 to run thousands of VUs in a single process with minimal memory overhead while maintaining JavaScript syntax familiarity. The Sobek compiler optimizes bytecode execution for load testing workloads.

vs alternatives: Faster VU scaling than Node.js-based tools (JMeter, Locust) because Sobek's bytecode compilation and Go's concurrency primitives avoid interpreter overhead; more familiar than Go-based tools (Vegeta) for JavaScript developers.

k6 provides native modules for HTTP, WebSocket, gRPC, and browser automation (via Chromium), each implementing protocol-specific request/response handling, connection pooling, and metrics collection. The module architecture uses a registry pattern where each protocol module (http, ws, grpc, browser) exposes a standardized interface that integrates with k6's VU context and metrics engine, allowing developers to mix protocols within a single test script.

Unique: Implements protocol modules as pluggable Go packages that expose JavaScript APIs through Sobek bindings, enabling native performance for each protocol while maintaining a unified scripting interface. The HTTP module uses Go's net/http with custom dialer for TLS control; gRPC module uses grpc-go; browser module wraps Chromium via CDP.

vs alternatives: Supports more protocols natively than Locust (Python) or JMeter (Java) without requiring separate plugins; faster than Selenium-based tools for browser testing because it uses Chromium directly rather than WebDriver protocol.

k6's HTML module (k6/html) provides jQuery-like selectors for parsing HTML responses and extracting data (form fields, links, etc.). The module uses Go's html package for parsing and supports CSS selectors via the goquery library. Developers can extract form fields, CSRF tokens, and other HTML elements from responses, then use extracted values in subsequent requests. This enables realistic workflows like login forms with CSRF protection or multi-step processes requiring data extraction.

Unique: Implements HTML parsing via Go's html package with jQuery-like selector API (goquery), enabling efficient parsing without JavaScript execution overhead. The module integrates with k6's response handling, allowing extracted values to be used in subsequent requests within the same VU iteration.

vs alternatives: More efficient than Selenium-based tools because it parses HTML without rendering; more intuitive than JMeter's XPath extractors because it uses familiar CSS selectors.

k6 provides detailed request/response tracing via the --http-debug flag, which logs all HTTP requests and responses (headers, body, timing) to stdout. The http.Client constructor also accepts a debug parameter for per-VU tracing. Tracing output includes request/response headers, body content (truncated for large payloads), and timing breakdowns (DNS, TLS, connection, request, wait, receive). This enables developers to diagnose issues like unexpected status codes, malformed requests, or slow response phases.

Unique: Implements request/response tracing via Go's http.Client tracing hooks (httptrace package), capturing detailed timing information for each request phase. Output is formatted as human-readable text with color-coded headers and body content.

vs alternatives: More detailed timing breakdowns than Postman's request inspector; easier to use than JMeter's View Results Tree plugin because it's built-in and requires no UI interaction.

k6 organizes tests into scenarios, where each scenario defines a specific test flow (executor, duration, VU count, script function). Multiple scenarios can run concurrently within a single test, enabling complex test compositions like 'ramp-up scenario' + 'sustained load scenario' + 'spike scenario' running in parallel. Scenarios are defined in options.scenarios and can target different script functions (via exec parameter), allowing different VU cohorts to execute different test logic simultaneously.

Unique: Implements scenarios as independent execution contexts that run concurrently within a single k6 process, each with its own executor and VU pool. Scenarios can target different script functions via the exec parameter, enabling diverse user behavior simulation without separate test runs.

vs alternatives: More flexible than JMeter's thread groups because scenarios can run concurrently with different executors; more intuitive than Locust's task sets because scenarios are declaratively configured rather than programmatically defined.

k6 provides the group() function to organize test logic into named groups, which aggregate metrics (response time, error rate) separately from the global metrics. Groups can be nested, creating a hierarchy of test flows. Metrics for each group are tracked independently and reported separately, enabling developers to analyze performance of specific test sections (e.g., login flow, checkout flow) without manual metric tagging.

Unique: Implements groups as a metrics aggregation mechanism that automatically tracks metrics separately for each group without requiring manual metric instrumentation. Groups can be nested, creating a hierarchical metrics structure that mirrors test logic.

vs alternatives: More automatic than JMeter's transaction controllers (which require manual metric configuration); more intuitive than Locust's custom metrics because groups are built-in and require no setup.

k6 supports parameterizing test scripts via environment variables and command-line flags, enabling test configuration without modifying script code. Environment variables are accessed via __ENV object in test scripts; command-line parameters are passed via --env flag (e.g., --env BASE_URL=https://api.example.com). This enables CI/CD integration where test parameters (API endpoint, load profile, credentials) are injected at runtime without script changes.

Unique: Implements environment variable injection via the __ENV global object, which is populated from OS environment variables and --env CLI flags. This enables simple parameterization without requiring external configuration files or script modification.

vs alternatives: Simpler than JMeter's property files because it uses standard environment variables; more flexible than Locust's command-line arguments because it supports both environment variables and CLI flags.

k6 manages a pool of Virtual Users (VUs) where each VU is a lightweight goroutine executing the test script's default function in a loop. The VU lifecycle includes setup (executed once per VU before the test loop), the main test loop (executed repeatedly per scenario), and teardown (executed once per VU after the loop completes). Each VU maintains isolated state through a context object that persists across iterations, enabling stateful test scenarios like login-then-request patterns.

Unique: Implements VUs as goroutines (not threads or processes), enabling k6 to spawn 10,000+ VUs on modest hardware. Each VU has isolated context but shares the same JavaScript runtime instance, reducing memory overhead compared to process-per-VU tools like JMeter.

vs alternatives: More memory-efficient VU scaling than JMeter (which uses threads) or Locust (which uses greenlets with Python GIL contention); setup/teardown hooks are more intuitive than JMeter's thread group initialization.

Executes structured test suites defined in YAML/JSON config files against LLM prompts, agents, and RAG systems. The evaluator engine (src/evaluator.ts) parses test configurations containing prompts, variables, assertions, and expected outputs, then orchestrates parallel execution across multiple test cases with result aggregation and reporting. Supports dynamic variable substitution, conditional assertions, and multi-step test chains.

Unique: Uses a monorepo architecture with a dedicated evaluator engine (src/evaluator.ts) that decouples test configuration from execution logic, enabling both CLI and programmatic Node.js library usage without code duplication. Supports provider-agnostic test definitions that can be executed against any registered provider without config changes.

vs alternatives: Simpler than hand-written test scripts because test logic is declarative config rather than code, and faster than manual testing because all test cases run in a single command with parallel provider execution.

Executes identical test suites against multiple LLM providers (OpenAI, Anthropic, Google, AWS Bedrock, Ollama, etc.) and generates side-by-side comparison reports. The provider system (src/providers/) implements a unified interface with provider-specific adapters that handle authentication, request formatting, and response normalization. Results are aggregated with metrics like latency, cost, and quality scores to enable direct model comparison.

Unique: Implements a provider registry pattern (src/providers/index.ts) with unified Provider interface that abstracts away vendor-specific API differences (OpenAI function calling vs Anthropic tool_use vs Bedrock invoke formats). Enables swapping providers without test config changes and supports custom HTTP providers for private/self-hosted models.

vs alternatives: Faster than manually testing each model separately because a single test run evaluates all providers in parallel, and more comprehensive than individual provider dashboards because it normalizes metrics across different pricing and response formats.