wasmerio/wasmer · error

`wasmer validate` only validates WebAssembly files

Error message

`wasmer validate` only validates WebAssembly files

What it means

`wasmer validate` checks that a file is valid WebAssembly, but first it verifies the file even *is* WebAssembly using the `is_wasm` magic-bytes check. If the file does not start with the wasm binary magic (`\0asm`) or the text-format, validation is refused because the engine's `Module::validate` only handles wasm input. The error means the input file is not WebAssembly at all, not that it contains invalid wasm code.

Source

Thrown at lib/cli/src/commands/validate.rs:29

pub struct Validate {
    /// File to validate as WebAssembly
    #[clap(name = "FILE")]
    path: PathBuf,

    #[clap(flatten)]
    rt: RuntimeOptions,
}

impl Validate {
    /// Runs logic for the `validate` subcommand
    pub fn execute(&self) -> Result<()> {
        self.inner_execute()
            .context(format!("failed to validate `{}`", self.path.display()))
    }
    fn inner_execute(&self) -> Result<()> {
        let module_contents = std::fs::read(&self.path)?;
        if !is_wasm(&module_contents) {
            bail!("`wasmer validate` only validates WebAssembly files");
        }

        let engine = self
            .rt
            .get_engine_for_module(&module_contents, &Target::default())?;
        Module::validate(&engine, &module_contents)?;
        eprintln!("Validation passed for `{}`.", self.path.display());
        Ok(())
    }
}

View on GitHub (pinned to 8c4b9ee9d3)

Solutions

1. Check the file: `xxd <path> | head -1` — it should start with `00 61 73 6d` (`\0asm`) for binary wasm
2. If the file is WAT text, compile it first, e.g. `wat2wasm file.wat -o file.wasm`, then validate the .wasm
3. Re-download or rebuild the module if the file is truncated/corrupted
4. Verify you passed the right path (the wrapper also reports "failed to validate `<path>`" with the actual file)

Example fix

// before
wasmer validate app.wat
// after
wat2wasm app.wat -o app.wasm && wasmer validate app.wasm

let bytes = std::fs::read(path)?;
fn is_wasm(bytes: &[u8]) -> bool {
    bytes.starts_with(b"\0asm")
}
if !is_wasm(&bytes) {
    eprintln!("{path:?} is not a WebAssembly binary (missing \\0asm magic)");
    std::process::exit(1);
}

fn is_wasm_file(bytes: &[u8]) -> bool {
    bytes.len() >= 4 && bytes[..4] == [0x00, 0x61, 0x73, 0x6d]
}

match wasmer.validate(path) {
    Ok(()) => println!("valid wasm"),
    Err(e) if e.to_string().contains("only validates WebAssembly files") => {
        eprintln!("Not a wasm binary — compile WAT first (wat2wasm)");
    }
    Err(e) => return Err(e.into()),
}

When it happens

Trigger: Running `wasmer validate <path>` where std::fs::read succeeds but is_wasm(&module_contents) returns false — i.e. the file lacks wasm magic bytes: a WAT text file without proper encoding, a directory dump, a partially downloaded/truncated file, or any non-wasm binary passed by mistake.

Common situations: Pointing validate at a .wat source file, an accident where the wrong path is passed (e.g. a .js glue file or tarball), or a corrupted download where the header bytes are missing.

Related errors

• global #{} is a constant
• Global index must be valid
• unimplemented operator {operator:?}
• Unable to determine the wasmer dir: {e}
• Cannot load reference type

AI-assisted analysis of wasmerio/wasmer@8c4b9ee9d3 (2026-09-01). Data as JSON: /api/errors/61f0ad549556431f. Report an issue: GitHub.