Language Selection

Auto-Detection

When no --language flag is provided, wasmrun detects the project language by checking for marker files:

File	Detected Language	Runtime
package.json	nodejs	QuickJS (from wasmhub)
requirements.txt	python	RustPython (from wasmhub)
pyproject.toml	python	RustPython (from wasmhub)
Cargo.toml	rust	Rust WASM runtime
go.mod	go	Go WASM runtime

Detection is checked in this order. The first match wins.

# Has package.json → detected as nodejs
wasmrun os ./my-node-app
# 🔍 Detected: nodejs project

# Has requirements.txt → detected as python
wasmrun os ./my-python-app
# 🔍 Detected: python project

If no marker files are found:

Could not auto-detect language for project: ./my-project
Please specify --language

Manual Override

Use -l or --language to force a specific language:

wasmrun os ./my-app --language nodejs
wasmrun os ./my-app -l python

Supported Values

Value	Aliases	Runtime
nodejs	node, js, javascript	QuickJS
python	py	RustPython

# All equivalent
wasmrun os ./app --language nodejs
wasmrun os ./app --language node
wasmrun os ./app --language js
wasmrun os ./app --language javascript

When to Override

Override is useful when:

• The project has multiple language markers (e.g., both package.json and requirements.txt)
• Auto-detection picks the wrong language
• You want to test the same project with a different runtime

# Project has both package.json and requirements.txt
# Force Python
wasmrun os ./hybrid-project --language python

Runtime Fetching

Language runtimes are WASM binaries from wasmhub. They're fetched on first use and cached at ~/.wasmrun/runtimes/.

~/.wasmrun/runtimes/
├── quickjs.wasm          # Node.js/JavaScript runtime
├── quickjs.wasm.sha256   # Checksum
├── rustpython.wasm       # Python runtime
└── rustpython.wasm.sha256

Language Name Mapping

wasmrun maps detected language names to wasmhub runtime names:

Detected Language	wasmhub Runtime
nodejs	quickjs
python	rustpython
rust	rust
go	go

Cache Management

Runtimes are cached indefinitely. To force a re-download:

# Remove cached runtime
rm ~/.wasmrun/runtimes/quickjs.wasm*

# Next run will re-fetch
wasmrun os ./my-app

Integrity

Every download is validated with a SHA-256 checksum. If a cached file is corrupted, wasmrun detects the mismatch and re-downloads automatically.

Runtime API

Check runtime status via the REST API:

# Get detected language and cache status
curl http://localhost:8420/api/runtimes

{
  "detected_language": "nodejs",
  "wasmhub_runtime": "quickjs",
  "cached": true,
  "cached_version": "0.1.4",
  "available_languages": ["quickjs", "rustpython", "rust", "go"]
}

# Download the runtime binary directly
curl http://localhost:8420/api/runtime/nodejs -o runtime.wasm

See Also

• Running Projects — startup flow and UI
• Server Options — port, CORS, watch configuration
• Plugins — language plugins for server mode (different from OS mode runtimes)