The two Convex runtimes

A Convex function runs on one of two runtimes. Nimbus selects the runtime for each module. A "use node"; directive on the first line selects the Node-compatible runtime. Both runtimes use V8, with no separate Node process. They expose different surfaces. The default runtime also applies determinism rules that match Convex.

• Default runtime: a web-standard V8 isolate with fetch, TextEncoder, crypto.subtle, and the other web globals, but no Node builtins. This is where query, mutation, httpAction, and directive-free action modules run.
• Node-compatible runtime: select this runtime with "use node"; at the top of a module. It runs node-compat on V8, not a spawned Node binary. It reaches Node builtins such as node:crypto and node:async_hooks. A "use node" module may contain only actions. Put shared queries and mutations in a directive-free module. Call them with ctx.runQuery or ctx.runMutation.

The runtimes example app runs the same computation on both and compares the results.

Determinism on the default runtime

Convex makes queries and mutations deterministic for replay and caching. Nimbus's default runtime reproduces that behavior. It pins time and randomness for each invocation instead of reading a live clock or entropy source.

Randomness

Math.random() is a seeded pseudorandom number generator (PRNG). It uses a ChaCha20 stream instead of the platform's random source. Nimbus uses two seed methods:

• At module load, Nimbus seeds it from the deployment stamp. A module-scope Math.random() value stays stable across runs and server restarts of the same deployment.
• At each query, paginated query, or mutation start, Nimbus seeds it with fresh per-invocation entropy. Two calls in one handler differ. Each new handler starts with a fresh stream.

Time

Nimbus freezes Date.now() and new Date() at each query, paginated query, or mutation start. They stay constant for the whole handler, including across await points. During module evaluation, they read the deployment timestamp. Actions are exempt and see a live, side-channel-coarsened host clock.

Nimbus fixes performance.now() during import and inside queries. It increments inside mutations and actions. Nimbus pins performance.timeOrigin to the deployment timestamp for every invocation kind.

Where the deployment timestamp comes from

Nimbus derives the deployment stamp from the deployed bundle entrypoint's modification time. A deployment rewrites that file. If Nimbus cannot read the modification time, it uses the first-observation time. The bundle SHA-256 seeds module-load randomness. Both values stay stable for a given deployed bundle. This stability keeps import-time values steady across restarts.

The modification time is a proxy, not a recorded deploy event. Copying or restoring a bundle can change the entrypoint modification time. Such a change shifts the deployment timestamp that functions observe.

It is parity, not a sandbox boundary

These rules reproduce Convex's observable behavior. They are not a security control. A default-runtime function shares a trust domain with its own seed. Determinism provides behavioral parity, not isolation. Treat it as a compatibility feature, not a confinement feature.

fetch

fetch is available in actions only. Calling it from a query or mutation fails closed with the same message Convex uses:

Can't use fetch() in queries and mutations. Please consider using an action.

The tenant's deny-by-default egress policy still applies to an action that passes this check. The query or mutation gate does not authorize a network destination. The egress policy decides that separately.

process.env

The default runtime exposes only process.env on process. It does not expose versions, release, cwd, or Buffer. It is not a Node environment. Reads use the same capability-gated environment proxy as the Node lanes. Nimbus denies access to variables without runtime grants instead of returning undefined.

Async context

The default runtime provides node:async_hooks, but only with AsyncLocalStorage and AsyncResource. It also accepts the bare async_hooks specifier. This is not the full Node surface. Nimbus does not provide createHook or executionAsyncId. V8's async-context frame backs the implementation directly.

An AsyncLocalStorage store does not propagate into ctx.runQuery, ctx.runMutation, or ctx.runAction. Those host calls run outside the invocation's async context. The called function cannot see a store that you set with .run(...). Pass the value as an explicit argument.

WebAssembly

The WebAssembly API provides WebAssembly.instantiate, WebAssembly.Module, and WebAssembly.Instance. Nimbus disables shared WebAssembly memory to reduce side channels. A WebAssembly.Memory constructor with shared: true throws.

The global surface is a superset

Nimbus's default runtime adds several globals to the web-standard scope. They include WebSocket, URL, structuredClone, and timer functions such as setTimeout. This is a superset of the globals that Convex documents for its default runtime. Code can behave differently if it detects a runtime by an absent global. Code that uses only expected globals behaves the same.

Related pages

• Migrate a Convex app to Nimbus: what carries over and what to check first.
• Convex API compatibility: the full supported-surface matrix, including the runtime semantics summarized here.
• Convex example apps: runnable apps, including the two-runtime runtimes demo.