A Wasm runtime is required to load user applications. Please

Error message

A Wasm runtime is required to load user applications. Please enable the `wasmer` or the `wasmtime` feature flags when compiling `linera-storage`.

What it means

linera-storage's load_contract dispatches on the application runtime kind. For Wasm modules it needs a Wasm engine, selected via the wasmer or wasmtime cargo features on linera-storage. If the bytecode is Wasm and neither feature is compiled in, it panics with instructions to enable one of them. This is a build-configuration error surfacing at first Wasm app load, i.e. in get_user_contract during block execution.

Source

Thrown at linera-storage/src/lib.rs:333

        let contract_bytecode = self
            .thread_pool()
            .run_send((), move |()| async move {
                compressed_contract_bytecode.decompress()
            })
            .await
            .await??;
        match application_description.module_id.vm_runtime {
            VmRuntime::Wasm => {
                cfg_if::cfg_if! {
                    if #[cfg(with_wasm_runtime)] {
                        let Some(wasm_runtime) = self.wasm_runtime() else {
                            panic!("A Wasm runtime is required to load user applications.");
                        };
                        Ok(WasmContractModule::new(contract_bytecode, wasm_runtime)
                           .await?
                           .into())
                    } else {
                        panic!(
                            "A Wasm runtime is required to load user applications. \
                             Please enable the `wasmer` or the `wasmtime` feature flags \
                             when compiling `linera-storage`."
                        );
                    }
                }
            }
            VmRuntime::Evm => {
                cfg_if::cfg_if! {
                    if #[cfg(with_revm)] {
                        let evm_runtime = EvmRuntime::Revm;
                        Ok(EvmContractModule::new(contract_bytecode, evm_runtime)?
                           .into())
                    } else {
                        panic!(
                            "An Evm runtime is required to load user applications. \
                             Please enable the `revm` feature flag \
                             when compiling `linera-storage`."

View on GitHub (pinned to 6c226ddcb3)

Solutions

Enable the wasmtime feature (or wasmer) on the linera-storage dependency in your Cargo.toml
Use the standard linera-service build (make build / cargo build -p linera-service), which enables a Wasm runtime by default
If you only deploy EVM applications, keep Wasm apps out of the workload and enable with_revm instead (see the sibling EVM panic)

Example fix

# before (Cargo.toml)
linera-storage = { path = "../linera-storage", default-features = false }

# after
linera-storage = { path = "../linera-storage", default-features = false, features = ["wasmtime", "wasmer"] }

Defensive patterns

Strategy: validation

Validate before calling

// Compile-time guard in the binary that loads applications:
const _: () = {
    #[cfg(not(any(feature = "wasmer", feature = "wasmtime")))]
    compile_error!("enable the wasmer or wasmtime feature on linera-storage to load Wasm contracts");
};

Prevention

Assert required runtime features at compile time instead of discovering the panic in production
Mirror the linera-service feature set in downstream builds
Run a deploy-and-execute smoke test in CI for every runtime your deployment must support

When it happens

Trigger: Calling get_user_contract/load_contract for an application whose bytecode is a Wasm module while the linera-storage crate was built without its wasmer/wasmtime features — e.g. depending on linera-storage with default-features = false, or a downstream binary enabling only with_revm.

Common situations: Custom node/indexer/service binaries assembled from Linera crates with a minimal feature set; CI builds that trim features for speed and then run integration tests deploying example Wasm apps; embedding linera-storage into another project's dependency tree where features get unified away.

Related errors

AI-assisted analysis of linera-io/linera-protocol@6c226ddcb3 (2026-08-22). Data as JSON: /api/errors/5e4c47c93b589383. Report an issue: GitHub.