wasmtime

Wasmtime Development

Wasmtime is a standalone, fast, secure WebAssembly runtime from the Bytecode Alliance. It implements the WebAssembly standard and extensions including WASI (WebAssembly System Interface) and the Component Model.

When to Use This Skill

Activate when:

• Compiling Rust or Zig code to WebAssembly targets
• Embedding a WebAssembly runtime in Rust or Elixir applications
• Working with WASI for system interface access
• Using the Component Model and WIT interfaces
• Building plugin systems with WebAssembly sandboxing
• Optimizing wasm module size or runtime performance

Core Concepts

Concept	Purpose
Engine	Shared compilation environment and configuration
Store	Per-instance state container (fuel, epoch, host data)
Module	Compiled .wasm binary (core module)
Component	Compiled component with typed interfaces (WIT-based)
Instance	Runtime instantiation of a module or component
Linker	Resolves imports by name, defines host functions
WIT	WebAssembly Interface Type language for component contracts

Supported Languages

Guest Languages (compile TO wasm)

Language	Target	Tooling	Reference
Rust	wasm32-wasip1, wasm32-wasip2, wasm32-unknown-unknown	cargo-component, wit-bindgen	guest-rust.md
Zig	wasm32-wasi, wasm32-freestanding	zig build, build.zig	guest-zig.md

Host Languages (embed wasmtime IN)

Language	Crate/Package	Async Support	Reference
Rust	wasmtime crate	Yes (tokio)	host-rust.md
Elixir	wasmex hex package	Via GenServer	host-elixir.md

WASI Versions

Version	Status	Key Differences
WASIp1 (Preview 1)	Stable, widely supported	POSIX-like, fd_* functions, linear memory I/O
WASIp2 (Preview 2)	Current standard	Component Model-based, typed streams, async-ready

Use WASIp2 for new projects. WASIp1 remains supported for existing code. See overview.md for migration details.

Reference Index

Reference	Contents
overview.md	Installation, core concepts detail, Component Model, WIT syntax, WASI deep dive, resource limits, AOT compilation, debugging
guest-rust.md	Rust wasm targets, cargo-component, wit-bindgen, binary size optimization, testing strategies
guest-zig.md	Zig wasm targets, build.zig configuration, allocator patterns, WASI imports, exports
host-rust.md	Wasmtime Rust API, WASI context setup, Component Model bindgen!, async support, plugin system patterns
host-elixir.md	Wasmex API, GenServer integration, memory access, host callbacks, supervision patterns

WebAssembly instrumentation (whamm)

whamm! instruments compiled .wasm modules by inserting probes through bytecode rewriting, using a DTrace-inspired .mm DSL for profiling, opcode counting, cache simulation, and call graph analysis. Run instrumented modules on wasmtime by preloading the whamm core library: wasmtime run --env TO_CONSOLE=true --preload whamm_core=whamm_core.wasm out.wasm. For CLI installation (mise github backend), the .mm language, and instrumentation strategies, see the whamm and whamm-dsl skills in the whamm plugin (/plugin install whamm@vinnie357).

Common Pitfalls

• Wrong target triple: Use wasm32-wasip2 for Component Model, wasm32-wasip1 for legacy WASI, wasm32-unknown-unknown for bare modules
• Missing WASI context: Host must add WASI to the linker before instantiating WASI-dependent modules
• Store lifetime: Each Store owns instance state — do not share stores across threads without synchronization
• Fuel exhaustion: Enable fuel metering for untrusted code and handle OutOfFuel traps
• Component vs Module: Components use WIT-typed interfaces; core modules use raw numeric imports/exports — do not mix APIs
• Linear memory bounds: Always validate pointer+length pairs when passing data through linear memory