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k6 extensions allow you to enhance k6’s functionality by adding custom JavaScript modules, output formats, secret sources, and subcommands. Extensions are built in Go and loaded into k6 using xk6.

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What Are Extensions?

Extensions are Go packages that integrate with k6 to provide additional capabilities beyond the core functionality. They enable you to:
  • Add custom JavaScript APIs
  • Integrate with proprietary systems
  • Create specialized output formats
  • Implement custom authentication sources
  • Add new CLI subcommands
Extensions are compiled into k6 at build time, creating a custom k6 binary with your additional functionality.

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Extension Types

k6 supports four types of extensions (ext/ext.go:26):

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JavaScript Extensions

Add custom JavaScript modules that can be imported in your test scripts. 
import custom from 'k6/x/custom';

export default function () {
  custom.doSomething();
}
Use cases:
  • Protocol implementations (MQTT, Kafka, gRPC)
  • Custom encodings or cryptography
  • Proprietary API clients
  • Specialized data generation

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Output Extensions

Create custom outputs to send test results to any backend or format. 
k6 run --out myoutput=config script.js
Use cases:
  • Proprietary monitoring systems
  • Custom database storage
  • Specialized file formats
  • Real-time streaming platforms
See Creating Custom Outputs for details.

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Secret Source Extensions

Implement custom secret sources for retrieving sensitive configuration values. Use cases:
  • Corporate secret management systems
  • Hardware security modules (HSM)
  • Cloud provider secret managers
  • Custom encryption schemes

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Subcommand Extensions

Add new CLI subcommands to k6. 
k6 mycommand [args]
Use cases:
  • Custom test orchestration
  • Specialized reporting tools
  • Configuration generators
  • Integration utilities

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Extension Registry

Extensions are registered globally using the extension registry (ext/ext.go:18). The registry:
  • Maintains a map of extensions by type
  • Prevents duplicate registrations
  • Tracks module path and version information
  • Provides lookup by extension type

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Registration

Extensions self-register using the Register() function: 
import "go.k6.io/k6/ext"

func init() {
    ext.Register("myextension", ext.JSExtension, &MyExtension{})
}
Attempting to register an extension with a duplicate name will cause a panic. Ensure your extension names are unique.

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Extension Metadata

Each extension contains metadata (ext/ext.go:49): 
type Extension struct {
    Name    string        // Extension name
    Path    string        // Go module path
    Version string        // Module version
    Type    ExtensionType // Extension type
    Module  any           // Extension implementation
}
This metadata is used for:
  • Version tracking and compatibility
  • Debugging and diagnostics
  • Extension discovery
  • Conflict detection

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Viewing Extensions

List all loaded extensions: 
k6 version
The output shows k6 version and all compiled-in extensions with their versions.

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How Extensions Work

1
Build Time
2
Extensions are compiled into k6 using xk6:
3
xk6 build --with github.com/user/xk6-extension@latest
4
Initialization
5
When k6 starts, extension init() functions run and register extensions.
6
Runtime
7
k6 looks up registered extensions when:
8
  • Importing JavaScript modules
  • Processing --out flags
  • Loading secret sources
  • Executing CLI commands
    • 9
    Execution
    10
    Extensions integrate with k6’s core systems:
    11
  • JS extensions expose APIs to test scripts
  • Output extensions receive metric samples
  • Secret sources are queried for values
  • Subcommands execute custom logic

    • ​
    Extension Naming

    Follow these conventions:

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    Repository Names

    Prefix with xk6- for discoverability:
    • xk6-output-kafka
    • xk6-redis
    • xk6-crypto

    ​
    Extension Names

    Use lowercase, descriptive names: 
    ext.Register("redis", ext.JSExtension, module)

    ​
    JavaScript Import Paths

    JS extensions use the k6/x/ namespace: 
    import redis from 'k6/x/redis';
    Consistent naming helps users discover and use your extensions easily.

    ​
    Official Extensions

    • xk6-browser: Browser automation and testing
    • xk6-dashboard: Web-based results dashboard
    • xk6-kubernetes: Kubernetes testing utilities
    • xk6-sql: SQL database testing

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    Community Extensions

    • xk6-kafka: Apache Kafka protocol support
    • xk6-redis: Redis client functionality
    • xk6-mqtt: MQTT protocol implementation
    • xk6-output-influxdb: InfluxDB v2 output
    • xk6-prometheus-remote: Prometheus remote write
    Explore more at k6 Extension Registry.

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    Building Blocks

    Extensions leverage k6’s internal packages:

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    For JavaScript Extensions

    • js/modules: Module registration and lifecycle
    • metrics: Creating and emitting custom metrics

    ​
    For Output Extensions

    • output: Output interface and utilities
    • metrics: Receiving and processing samples

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    Common Packages

    • lib: Core k6 types and utilities
    • stats: Statistical calculations
    • lib/types: Common type definitions

    ​
    Version Compatibility

    Extensions should specify compatible k6 versions: 
    // go.mod
module github.com/user/xk6-myext

go 1.22

require go.k6.io/k6 v0.50.0
    k6’s internal APIs may change between versions. Pin your extension to compatible k6 versions and test thoroughly when upgrading.

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    Performance Considerations

    • JavaScript extensions: Minimize CGO calls and data conversions
    • Output extensions: Use buffering and async processing
    • All extensions: Avoid blocking operations in hot paths
    Badly performing extensions can significantly impact test accuracy by consuming VU execution time.

    ​
    Security Considerations

    • Validate all inputs from JavaScript
    • Sanitize data sent to external systems
    • Handle secrets securely
    • Avoid logging sensitive information
    • Review dependencies for vulnerabilities

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    Extension Development

    Ready to build your own extension?

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