What does GOOS=wasip1 GOARCH=wasm produce, and what can that module no longer do at run time?

## The target WASI, the WebAssembly System Interface, is a standard set of host functions — open a file, read a clock, write to a descriptor — that a WebAssembly module can import so it can behave like a program rather than a pure function. `GOOS=wasip1 GOARCH=wasm` targets preview 1 of that interface. The output is a single `.wasm` file whose import section names the WASI functions; any host runtime that implements preview 1 can load and run it. The practical contrast with the browser target is worth stating plainly. `GOOS=js` produces a module whose imports are satisfied by `wasm_exec.js`, the JavaScript glue shipped with the toolchain, and it exists to run inside a page. `GOOS=wasip1` produces a module whose imports are satisfied by the host runtime itself, and it exists to run outside a browser — as a plugin inside another process, at an edge, or as a portable CLI artefact. Browsers do not implement WASI, so the two are not interchangeable. There is no `syscall/js` on the wasip1 target. ## Command versus reactor A default build is a WASI *command*: the module exports a start function, the host calls it, `main` runs, the program exits, and the instance is spent. That is the right shape for a batch tool. If you want the host to call *into* the module repeatedly — the plugin shape — you build a reactor instead, with `-buildmode=c-shared`, which exports an initialise function and leaves the runtime alive so exported functions stay callable afterwards. ## The sandbox: what is actually denied The headline is that WASI is **capability-based**. A native process starts with ambient authority over the whole filesystem, subject only to permissions. A WASI module starts with none, and receives capabilities as preopened file descriptors from the host. So: - **Files.** `os.Open("/etc/hosts")` succeeds only if the host preopened a directory containing that path, and mapped it into the module's view. Otherwise it fails as though the path did not exist. Code that assumes a config file is somewhere absolute breaks; code that takes its paths from arguments generally survives. - **Network.** Preview 1 has no API for making an outbound connection, so `net.Dial` and anything built on it — an HTTP client, a database driver — cannot work. Accepting connections is the one direction that can work, and only when the host hands the module an already-listening socket to accept on. - **Processes and signals.** No `fork`, no `os/exec`, no signal delivery. There is no process tree to speak of. - **Threads.** Preview 1 has no threads. The Go runtime runs every goroutine on the module's single execution thread. Goroutines, channels and the scheduler all work exactly as written, and concurrency is real — but parallelism is not, so a CPU-bound goroutine starves everything else until it yields or blocks. - **Environment and arguments.** These do exist; the host passes them in, and `os.Args` and `os.Getenv` read them. Clock and randomness likewise come from host imports. ## Consequences for how you write the code Because capabilities arrive from the host, wasip1 rewards programs that take their inputs explicitly: read paths from arguments, take the working directory from a preopen, and stream input on standard input rather than reaching around the filesystem. It punishes libraries that reach for ambient resources during package initialisation — a package that opens a well-known path in `init` will fail at start-up under a host that did not preopen it, and the failure looks like a bug in your program rather than a missing capability. And because there is no dialling, a service you were hoping to port often turns out to depend on outbound calls. Discovering that early is the point of trying a wasip1 build before promising one. ## Testing `go build` produces the module, and `go test` for this target runs the compiled test binary under a WebAssembly runtime configured for the toolchain, which is the cheapest way to find out which parts of a package silently depend on ambient authority.