MobX autorun side-effect ordering across overlapping reactions needs diagnosis

Short answer

No. MobX does not guarantee a deterministic execution order for side effects when multiple autorun reactions observe overlapping observables. Reactions are flushed after the current synchronous batch or action completes, and their relative order is implementation-dependent rather than a public contract. Async work returned from an autorun callback is fire-and-forget: MobX does not await it, so it does not block other reactions or act as a completion barrier.

Confirmed behavior versus uncertainty

Confirmed behavior:

• Side effects do not interleave with a synchronous mutation; they run after the outermost action or transaction ends.
• MobX does not await promises returned by autorun callbacks.
• Relative ordering among overlapping reactions is unspecified and must not be used for state consistency.

Uncertainty: I cannot confirm a scheduling change between MobX 6.x and 7.x from available knowledge. React 18 concurrent batching may change observed timing, but that interaction is version- and environment-specific and is not a MobX ordering guarantee.

Steps for this case

1. Check the installed MobX version in package.json; do not assume a 6.x-to-7.x difference exists.
2. Create a minimal repro with two autorun reactions reading the same observable, mutate inside runInAction, and log each reaction.
3. Run the repro repeatedly. If the log order changes, treat reaction order as nondeterministic.
4. If async work must coordinate, perform the mutation in an explicit action after the promise resolves, or use a derived value instead of relying on autorun ordering.

Verification snippet

import { autorun, observable, runInAction } from "mobx";

const state = observable.box(0);
const logs = [];

autorun(() => { logs.push("A:" + state.get()); });
autorun(() => { logs.push("B:" + state.get()); });

runInAction(() => { state.set(1); });
console.log(logs);

If logs is unstable across runs, do not depend on reaction order for consistency.