clockworklabs/SpacetimeDB · error · anyhow::Error

getrandom usage detected. It seems like either you or a…

Error message

getrandom usage detected.

It seems like either you or a crate in your dependency tree is depending on
the `getrandom` crate for random number generation. getrandom is the default
randomness source for the `rand` crate, and is used when you call
`rand::random()` or `rand::thread_rng()`. If this is you, you should instead
use `ctx.rng()` on a `ReducerContext`. If this is a crate in your
tree, you should try to see if the crate provides a way to pass in a custom
`Rng` type, and pass it the rng returned from `ctx.rng()`.

What it means

SpacetimeDB modules are deterministic and sandboxed: OS entropy is not available, so the `getrandom` backend that the `rand` crate uses by default cannot be satisfied by the host. check_for_issues detects the `__getrandom_custom` import in the compiled wasm and rejects the module, directing you to the host-provided RNG on ReducerContext. Reducers that need randomness must use ctx.rng().

Solutions

Replace rand::random()/thread_rng() with the reducer's ReducerContext RNG: `use rand::Rng; let n = ctx.rng().gen();`
For dependencies, pass randomness in: construct them with an API that accepts a custom Rng and give it `ctx.rng()` (or StdRng::from_rng(ctx.rng()))
Disable random-sourcing features (e.g. uuid's `v4`) and generate the IDs from ctx.rng() yourself
Rebuild and publish

Example fix

// before
let roll: u32 = rand::random();

// after
use rand::Rng;
let roll: u32 = ctx.rng().gen(); // ctx: ReducerContext

Defensive patterns

Strategy: validation

Validate before calling

#!/usr/bin/env bash
# module code must not source OS entropy directly
if grep -RnE 'rand::random|thread_rng|OsRng' --include='*.rs' src/; then
  echo 'ERROR: use ctx.rng() instead of OS randomness' >&2
  exit 1
fi

Prevention

Always take ReducerContext in reducers that need randomness and use ctx.rng()
Disable default random features (uuid's v4, etc.) on module dependencies
Prefer APIs that accept an injected Rng when choosing helper crates
Grep for rand usage in code review of ported code

When it happens

Trigger: Module code calling rand::random() or rand::thread_rng(); or a dependency that internally sources entropy itself (uuid::Uuid::new_v4 with default `v4` features, nanoid-style crates, anything constructing OsRng) rather than accepting an injected Rng.

Common situations: Porting game code that rolls dice with rand::random(); generating UUIDs server-side in a reducer; a helper crate that defaults to ThreadRng/OsRng internally.

Related errors

AI-assisted analysis of clockworklabs/SpacetimeDB@6dee26c6ef (2026-08-20). Data as JSON: /api/errors/e633513e0fad29ce. Report an issue: GitHub.

Appendix: source

Thrown at crates/cli/src/tasks/rust.rs:133

    };
    let Ok(module) = wasmbin::Module::decode_from(&mut io::BufReader::new(file)) else {
        return Ok(());
    };
    if has_wasm_bindgen(&module) {
        anyhow::bail!(
            "wasm-bindgen detected.\n\
             \n\
             It seems like either you or a crate in your dependency tree is depending on\n\
             wasm-bindgen. wasm-bindgen is only for webassembly modules that target the web\n\
             platform, and will not work in the context of SpacetimeDB.\n\
             \n\
             To find the offending dependency, run `cargo tree -i wasm-bindgen`. Try checking\n\
             its cargo features for 'js' or 'web' or 'wasm-bindgen' to see if there's a way\n\
             to disable it."
        )
    }
    if has_getrandom(&module) {
        anyhow::bail!(
            "getrandom usage detected.\n\
             \n\
             It seems like either you or a crate in your dependency tree is depending on\n\
             the `getrandom` crate for random number generation. getrandom is the default\n\
             randomness source for the `rand` crate, and is used when you call\n\
             `rand::random()` or `rand::thread_rng()`. If this is you, you should instead\n\
             use `ctx.rng()` on a `ReducerContext`. If this is a crate in your\n\
             tree, you should try to see if the crate provides a way to pass in a custom\n\
             `Rng` type, and pass it the rng returned from `ctx.rng()`."
        )
    }
    Ok(())
}

const WBINDGEN_PREFIX: &str = "__wbindgen";
fn has_wasm_bindgen(module: &wasmbin::Module) -> bool {
    let check_import = |import: &wasmbin::sections::Import| {
        import.path.module.starts_with(WBINDGEN_PREFIX) || import.path.name.starts_with(WBINDGEN_PREFIX)

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