Effect Fiber

You are an Effect TypeScript expert specializing in fiber lifecycle, interruption, and supervision with Fiber, FiberHandle, FiberMap, and FiberSet.

Effect Source Reference

The Effect v4 source is available at ~/.local/share/opencode/repos/github.com/Effect-TS/effect@main/. Browse and read files there directly to look up APIs, types, and implementations.

Reference this for:

• Fiber interface and await/join/interrupt operations (packages/effect/src/Fiber.ts)
• Fork variants, interruption combinators, run* APIs (packages/effect/src/Effect.ts)
• Single-slot supervision (packages/effect/src/FiberHandle.ts)
• Keyed fiber collections (packages/effect/src/FiberMap.ts)
• Grow-only fiber collections (packages/effect/src/FiberSet.ts)
• Runtime internals — FiberImpl, fork/interrupt mechanics (packages/effect/src/internal/effect.ts)
• v3 → v4 fork renames (migration/forking.md), keep-alive changes (migration/fiber-keep-alive.md — partially stale, see section 9)
• Real usage and edge cases (packages/effect/test/FiberHandle.test.ts, FiberMap.test.ts, FiberSet.test.ts)

Core Model

A Fiber<A, E = never> is a handle to a lightweight, cooperatively scheduled execution of an Effect that may still be running or may have completed. It is the unit of concurrency in Effect. A fiber's outcome is an Exit<A, E> — success with A, or failure with a Cause<E> that can contain typed errors, defects, and interruptions.

import {
	Cause,
	Deferred,
	Effect,
	Exit,
	Fiber,
	FiberHandle,
	FiberMap,
	FiberSet,
	Schedule,
	Scope,
	Semaphore
} from 'effect';

Key facts to internalize:

• Structured concurrency. A fiber forked with Effect.forkChild is attached to its parent: when the parent fiber completes (success, failure, or interruption), all still-running children are interrupted before the parent's exit settles. forkScoped/forkIn tie the fiber's lifetime to a Scope instead; forkDetach produces a global fiber with no automatic lifetime.
• Interruption is cooperative. It is observed at effect boundaries; uninterruptible regions and finalizers run to completion first. Interrupting is itself an effect that waits for the target to fully settle.
• Fiber ids are plain numbers in v4 (there is no composite FiberId type). Interruptors are recorded in the Cause as a ReadonlySet<number>.
• Exit is an Effect. You can yield* an Exit directly to propagate its result into the current fiber.
• Useful synchronous members on the fiber object: fiber.id, fiber.pollUnsafe(): Exit<A, E> | undefined, fiber.addObserver(cb): () => void (returns an unsubscribe function), fiber.interruptUnsafe(fiberId?).

High-level concurrency combinators (Effect.all, Effect.forEach, Effect.race*) are covered by the effect-parallelization skill — prefer those when you just need concurrent results; reach for explicit fibers when you need lifecycle control.

1. Forking Fibers

The v3 names are gone: Effect.fork → Effect.forkChild, Effect.forkDaemon → Effect.forkDetach (Effect.forkAll and Effect.forkWithErrorHandler were removed). Four variants exist in v4, differing only in lifetime:

All four return Effect<Fiber<A, E>, never, R> — forking never fails — and all accept the same options object:

{
	readonly startImmediately?: boolean | undefined;
	readonly uninterruptible?: boolean | 'inherit' | undefined;
}

• startImmediately: true evaluates the fiber synchronously up to its first suspension. The default is a deferred start: the fiber is scheduled and begins on the next scheduler tick, after the parent yields.
• uninterruptible: true starts the fiber uninterruptible; 'inherit' copies the parent's current interruptibility; default is interruptible.

const task = Effect.gen(function* () {
	yield* Effect.sleep('2 seconds');
	return 'result';
});

const program = Effect.gen(function* () {
	// data-first and data-last forms both work
	const fiber1 = yield* Effect.forkChild(task);
	const fiber2 = yield* task.pipe(Effect.forkChild);
	const fiber3 = yield* Effect.forkChild(task, { startImmediately: true });
	const fiber4 = yield* task.pipe(Effect.forkChild({ startImmediately: true }));

	const result = yield* Fiber.join(fiber1);
	return result;
});

Forking into a scope:

const scopedFork = Effect.scoped(
	Effect.gen(function* () {
		// tied to the enclosing scope
		const fiber = yield* Effect.forkScoped(task);

		// or fork into an explicit scope you manage
		const scope = yield* Effect.scope;
		const fiber2 = yield* Effect.forkIn(task, scope);

		yield* Effect.sleep('1 second');
		// both fibers are interrupted when the scope closes
	})
);

Detached fibers survive the parent — pair them with explicit interruption or Fiber.runIn:

const daemon = Effect.gen(function* () {
	const fiber = yield* Effect.forkDetach(pollForever);
	// attach a manually managed fiber to a scope after the fact:
	// the scope's close interrupts it (does not wait for it before registering)
	const scope = yield* Effect.scope;
	Fiber.runIn(fiber, scope);
});

Lazy start gotcha

Because forked fibers start on the next tick by default, side effects have not happened immediately after the fork:

const program = Effect.gen(function* () {
	let started = false;
	const fiber = yield* Effect.forkChild(Effect.sync(() => (started = true)));
	// started === false here!
	yield* Effect.yieldNow; // let scheduled fibers run
	// started === true
});

Use { startImmediately: true } when registration must happen before the parent proceeds (e.g. installing an Effect.onInterrupt handler or subscribing to a queue).

2. Joining, Awaiting, and Reading Exits

const program = Effect.gen(function* () {
	const fiber = yield* Effect.forkChild(compute);

	// join: flatten the fiber's result into the current fiber.
	// Failure (or interruption) of the fiber fails the current effect.
	const value = yield* Fiber.join(fiber);

	// await: NEVER fails — always succeeds with the Exit for inspection.
	const exit = yield* Fiber.await(fiber);

	if (Exit.isSuccess(exit)) {
		console.log(exit.value);
	} else if (Cause.hasInterrupts(exit.cause)) {
		// exit is already narrowed to Failure here, so .cause is accessible
		console.log('interrupted by', Cause.interruptors(exit.cause));
	} else {
		console.log('failed:', Cause.squash(exit.cause));
	}
});

Many fibers at once:

// every outcome as data, ordered like the input
const exits = yield* Fiber.awaitAll([fiberA, fiberB]);
// Array<Exit<A, E>>

// all success values; fails fast with the first failed fiber's Cause.
// NOTE: does NOT interrupt the remaining fibers — do that yourself.
const values = yield* Fiber.joinAll([fiberA, fiberB]);

Working with Exit values:

const summary = Exit.match(exit, {
	onSuccess: (value) => `ok: ${value}`,
	onFailure: (cause) => `failed: ${Cause.squash(cause)}`
});

Exit.getSuccess(exit); // Option<A>
Exit.getCause(exit); // Option<Cause<E>>
Exit.hasFails(exit); // has typed errors
Exit.hasDies(exit); // has defects
Exit.hasInterrupts(exit); // has interruptions
Cause.hasInterruptsOnly(cause); // ONLY interruptions (clean cancellation)
// the Exit.has* checks are TYPE GUARDS narrowing to Failure<A, E>; in an
// else-if chain test the cause instead (Cause.hasFails/hasDies/hasInterrupts
// return plain booleans) or the final else narrows `exit` to never

// Exit is itself an Effect: yielding it propagates success/failure
const value = yield* exit;

To capture the exit of an inline effect (no fiber needed) use Effect.exit(effect), which returns Effect<Exit<A, E>, never, R>.

Synchronous inspection (outside or inside effects):

const exit = fiber.pollUnsafe(); // Exit<A, E> | undefined — undefined while running
const cancel = fiber.addObserver((exit) => console.log('done', exit));
cancel(); // unsubscribe

There is no Fiber.poll effect in v4 — pollUnsafe and addObserver are the low-level hooks. In tests, fiber.pollUnsafe() combined with TestClock from effect/testing is the standard way to assert "still running" vs "completed" (see the effect-concurrency-testing skill for the full idiom; effect-testing covers TestClock setup).

3. Interruption

// interrupt one fiber and WAIT for it to fully settle
// (finalizers + uninterruptible regions run to completion first)
yield* Fiber.interrupt(fiber); // Effect<void>

// interrupt many, wait for all of them to settle
yield* Fiber.interruptAll([fiber1, fiber2]);

// record a specific fiber id as the interruptor (diagnostics/tracing)
yield* Fiber.interruptAs(fiber, controllerFiber.id);
yield* Fiber.interruptAllAs([fiber1, fiber2], controllerFiber.id);

// interrupt the CURRENT fiber
yield* Effect.interrupt; // Effect<never>

Notes:

• Fiber.interrupt uses the current fiber's id as the interruptor. The id set ends up in the Cause and is what Effect.onInterrupt finalizers and Cause.interruptors see.
• Fire-and-forget cancellation from synchronous code: fiber.interruptUnsafe() — sends the signal without waiting.
• Self-interruption via Effect.interrupt produces an exit where Cause.hasInterruptsOnly is true; platform runMain runners map that to exit code 130 rather than an error.
• A constructed interrupted exit is available as Exit.interrupt(fiberId?).

Current-fiber accessors:

const self = yield* Effect.fiber; // Effect<Fiber<unknown, unknown>>
const id = yield* Effect.fiberId; // Effect<number>
const eff = Effect.withFiber((fiber) => Effect.succeed(fiber.id)); // low-level constructor
const maybe = Fiber.getCurrent(); // sync: Fiber | undefined (undefined outside a fiber)

4. Uninterruptible Regions and Cleanup

Effect.uninterruptible(critical); // cannot be interrupted inside
Effect.interruptible(effect); // re-enable inside an uninterruptible region

Interruption inside an uninterruptible region is deferred, not dropped: the pending interruption fires the moment the region ends. The canonical acquire/use/release shape protects acquisition and cleanup while keeping the work cancellable:

const withConnection = Effect.uninterruptibleMask((restore) =>
	Effect.gen(function* () {
		const conn = yield* acquireConnection; // protected
		return yield* restore(useConnection(conn)).pipe(
			// cleanup runs on success, failure, AND interruption
			Effect.onExit(() => closeConnection(conn))
		);
	})
);

Effect.interruptibleMask((restore) => ...) is the inverse: the body is interruptible and restore re-applies the outer interruptibility.

Interruption-specific and general finalizers:

// runs ONLY when the effect is interrupted; receives interruptor fiber ids
const guarded = Effect.onInterrupt(longRunning, (interruptors) =>
	Effect.log(`interrupted by: ${[...interruptors].join(', ')}`)
);

// always runs (success / failure / interruption); finalizer cannot fail
const cleaned = Effect.ensuring(task, Effect.log('cleanup'));

// runs with the Exit — inspect how the effect ended
const observed = Effect.onExit(task, (exit) =>
	Effect.log(Exit.isSuccess(exit) ? 'ok' : 'failed/interrupted')
);

// runs only on failure, with the Cause
const (cause) => Effect.log(Cause.squash(cause)));

Bridging interruption to AbortSignal-aware APIs:

const download = Effect.scoped(
	Effect.gen(function* () {
		// fresh AbortController per acquisition; aborted when the scope closes
		const signal = yield* Effect.abortSignal; // Effect<AbortSignal, never, Scope>
		// pass `signal` to fetch(), child processes, etc.
	})
);

5. Structured Concurrency Lifetimes

The most important v4 behavior: when a fiber finishes its work, any children forked with forkChild that are still running are interrupted before the parent's exit is reported. This makes fire-and-forget with forkChild a bug:

// WRONG — the child is interrupted as soon as the parent returns,
// likely before it even starts (lazy start!)
const wrong = Effect.gen(function* () {
	yield* Effect.forkChild(sendAuditLog(event));
	return 'done';
});

Correct options, by intent:

// 1. The work belongs to a longer-lived scope (service, request, app)
const scoped = Effect.gen(function* () {
	yield* Effect.forkScoped(sendAuditLog(event));
	return 'done';
}); // requires Scope — provided by Effect.scoped / a Layer scope

// 2. The parent should wait for children forked during the effect
const awaited = Effect.awaitAllChildren(
	Effect.gen(function* () {
		yield* Effect.forkChild(sendAuditLog(event));
		return 'done';
	})
); // completes only after the audit-log child completes

// 3. Truly detached background work (you own its shutdown)
const detached = Effect.gen(function* () {
	const fiber = yield* Effect.forkDetach(sendAuditLog(event));
	return 'done';
});

Whichever lifetime you pick, a forked fiber's failure is observed by nobody unless you arrange it: join/await the fiber, supervise it via FiberHandle/FiberMap/FiberSet join, or attach Effect.catchCause/Effect.onError plus logging inside the forked effect. v4 removed Effect.forkWithErrorHandler, and the runtime does not log unhandled fiber failures.

Effect.awaitAllChildren only waits for children forked while the wrapped effect runs — children that existed beforehand are not awaited.

forkIn/forkScoped register an interruption finalizer on the scope and remove it when the fiber completes on its own. Forking into an already-closed scope interrupts the new fiber immediately. The same applies to Fiber.runIn(fiber, scope), which only registers the finalizer — it does not wait for the fiber.

Scope mechanics themselves — Effect.scoped, manual Scope.make/close, finalizer ordering — are covered by the effect-scope skill.

6. FiberHandle — Single-Slot Supervision

FiberHandle<A, E> manages at most one fiber. Running a new effect into it interrupts the previous fiber (latest wins); closing the owning scope interrupts the current fiber; completed fibers remove themselves.

const program = Effect.gen(function* () {
	// always pass type parameters — defaults are <unknown, unknown>
	const handle = yield* FiberHandle.make<string, MyError>();
	// Effect<FiberHandle<string, MyError>, never, Scope>

	// fork into the handle; interrupts whatever was there
	const fiber = yield* FiberHandle.run(handle, task);

	// keep the existing fiber; the NEW effect gets an already-interrupted fiber
	yield* FiberHandle.run(handle, otherTask, { onlyIfMissing: true });

	const current = FiberHandle.getUnsafe(handle); // Option<Fiber<string, MyError>>
	const current2 = yield* FiberHandle.get(handle); // Effect-wrapped version

	yield* FiberHandle.clear(handle); // interrupt current fiber, leave handle empty

	yield* FiberHandle.awaitEmpty(handle); // wait for the current fiber to complete
	yield* FiberHandle.join(handle); // see below
}).pipe(Effect.scoped);

run options: { onlyIfMissing?: boolean; propagateInterruption?: boolean }. The type also accepts startImmediately, but it is a no-op — collection fibers are forked via Effect.runForkWith and always start synchronously (see "How collection fibers relate to the caller" in section 8).

join vs awaitEmpty

• FiberHandle.join(handle): Effect<void, E> — fails with the first managed-fiber failure; succeeds (void) only when the handle's scope closes. It never resolves just because a fiber finished successfully. Use it as a supervision watchdog.
• FiberHandle.awaitEmpty(handle): Effect<void, E> — waits for the currently held fiber to complete.

propagateInterruption

By default (false), interruption of a managed fiber — including by an external Fiber.interrupt — does not fail join. With propagateInterruption: true, external interruptions do fail join; interruptions performed internally by the collection itself (replacement, clear, scope close — recorded with internal fiber id -1) never do.

Installing existing fibers

const fiber = Effect.runFork(task);
yield* FiberHandle.set(handle, fiber, { onlyIfMissing: true });
FiberHandle.setUnsafe(handle, fiber); // synchronous variant

Runtime helpers — synchronous runners for callback code

runtime captures the current services and returns a synchronous function that forks effects into the handle:

const handle = yield* FiberHandle.make<void, never>();
const run = yield* FiberHandle.runtime(handle)<MyService>(); // note the curried <R>() call

// plain function — safe to call from event handlers
const fiber = run(taskNeedingMyService, { onlyIfMissing: false });

// Promise-returning runner for an existing handle (rejects with Cause.squash)
const runPromise = yield* FiberHandle.runtimePromise(handle)<MyService>();

Runner options: { signal?: AbortSignal; scheduler?: Scheduler; onlyIfMissing?: boolean; propagateInterruption?: boolean }.

Shortcuts that create the handle and runner together (scoped):

const run = yield* FiberHandle.makeRuntime<never>(); // <R, E, A>
const runPromise = yield* FiberHandle.makeRuntimePromise(); // Promise-returning runner
const promise = runPromise(Effect.succeed('hello')); // rejects with Cause.squash on failure

Closed-handle behavior

After the scope closes: FiberHandle.run interrupts the calling fiber; the captured runtime/makeRuntime runners instead return a shared, already-interrupted fiber. Fibers passed to setUnsafe on a closed handle are interrupted immediately.

7. FiberMap — Keyed Fibers

FiberMap<K, A, E> is a map of running fibers indexed by key. Running a new effect under an existing key interrupts the previous fiber for that key; entries remove themselves on completion; scope close interrupts everything.

const program = Effect.gen(function* () {
	const map = yield* FiberMap.make<string, void, JobError>();
	// Effect<FiberMap<string, void, JobError>, never, Scope>

	// fork under a key — replaces (interrupts) any previous "job-1" fiber
	const fiber = yield* FiberMap.run(map, 'job-1', runJob(1));

	// dedupe: keep the running fiber, new effect gets an interrupted fiber
	yield* FiberMap.run(map, 'job-1', runJob(1), { onlyIfMissing: true });

	yield* FiberMap.has(map, 'job-1'); // Effect<boolean> (hasUnsafe: sync)
	yield* FiberMap.get(map, 'job-1'); // Effect<Option<Fiber<void, JobError>>>
	yield* FiberMap.size(map); // Effect<number>

	yield* FiberMap.remove(map, 'job-1'); // interrupt + remove that key
	yield* FiberMap.clear(map); // interrupt every fiber

	// FiberMap is Iterable<[K, Fiber<A, E>]>
	for (const [key, fiber] of map) {
		console.log(key, fiber.id);
	}

	yield* FiberMap.awaitEmpty(map); // Effect<void, E> — wait until no fibers remain
	yield* FiberMap.join(map); // Effect<void, E> — fail on first fiber failure
}).pipe(Effect.scoped);

run options are the same as FiberHandle's: { onlyIfMissing?, propagateInterruption? } (startImmediately is in the type but is a no-op here too). set/setUnsafe install existing fibers under a key with { onlyIfMissing?, propagateInterruption? }.

Runtime helpers take the key as the first runner argument:

const run = yield* FiberMap.runtime(map)<MyService>();
run('job-2', taskNeedingMyService, { onlyIfMissing: true });
const runPromise = yield* FiberMap.runtimePromise(map)<MyService>(); // Promise runner, key-first

// or create map + runner at once (note generic order: <R, K>)
const run2 = yield* FiberMap.makeRuntime<never, string>();
const runPromise2 = yield* FiberMap.makeRuntimePromise<never, string>();

Closed-map behavior matches FiberHandle: FiberMap.run interrupts the caller; runtime runners return a pre-interrupted fiber.

8. FiberSet — Grow-Only Fiber Collections

FiberSet<A, E> tracks an unkeyed set of fibers. No replacement semantics — every run/add grows the set; completed fibers remove themselves; scope close interrupts all. Nothing limits admission: unbounded run calls on a production ingest path are a hazard — gate them with a Semaphore (see the bounded pattern under Key Patterns).

const program = Effect.gen(function* () {
	const set = yield* FiberSet.make<void, TaskError>();
	// Effect<FiberSet<void, TaskError>, never, Scope>

	const fiber = yield* FiberSet.run(set, handleRequest(req));
	yield* FiberSet.add(set, existingFiber); // track an already-forked fiber
	FiberSet.addUnsafe(set, anotherFiber); // synchronous variant

	yield* FiberSet.size(set); // Effect<number>
	yield* FiberSet.clear(set); // interrupt all members

	// FiberSet is Iterable<Fiber<A, E>>
	const exits = yield* Fiber.awaitAll(set);

	yield* FiberSet.awaitEmpty(set); // Effect<void> — graceful drain
	yield* FiberSet.join(set); // Effect<void, E> — fail on first fiber failure
}).pipe(Effect.scoped);

run options: { propagateInterruption? } (no onlyIfMissing — there is no key; startImmediately is in the type but is a no-op). On a closed set, FiberSet.run returns an already-interrupted fiber (it does not interrupt the caller, unlike FiberHandle/FiberMap).

Runtime helpers mirror the others in shape. FiberSet.runtime and runtimePromise forward propagateInterruption when registering the managed fiber:

const run = yield* FiberSet.runtime(set)<MyService>();
run(taskNeedingMyService, { propagateInterruption: true });
const runPromise = yield* FiberSet.runtimePromise(set)<MyService>(); // Promise runner for an existing set

const run2 = yield* FiberSet.makeRuntime(); // scoped set + runner
const runPromise2 = yield* FiberSet.makeRuntimePromise();

How collection fibers relate to the caller

Fibers forked via FiberHandle/FiberMap/FiberSet.run (and the runtime runners) are created with Effect.runForkWith(parent.context) — they are root fibers carrying the caller's services, not children of the calling fiber. Consequences:

• They start executing immediately and synchronously up to their first suspension (no lazy start, unlike Effect.forkChild).
• The calling fiber's completion does not interrupt them — only key replacement, remove/clear, or the collection's scope close does.

9. Running Fibers at the Program Edge

Effect.runFork starts a root fiber synchronously (it evaluates until the first suspension before returning):

const fiber = Effect.runFork(program, {
	signal: abortController.signal, // interrupt the fiber when aborted
	scheduler: customScheduler, // optional Scheduler service
	uninterruptible: true, // start the fiber uninterruptible
	onFiberStart: (fiber) => console.log('started', fiber.id)
});

fiber.addObserver((exit) => console.log('done', exit));
Effect.runFork(Fiber.interrupt(fiber)); // interruption is itself an effect

All keys of Effect.RunOptions are optional: { signal?, scheduler?, uninterruptible?, onFiberStart? }.

When the effect still needs services, pre-apply a Context:

const runWith = Effect.runForkWith(servicesContext); // <R> pre-applied
const fiber = runWith(effectNeedingServices, { signal });

This is exactly what the FiberHandle/Map/Set runtime helpers wrap for you — prefer those when the forked fibers need supervision.

Keep-alive and runMain

In the current v4 runtime there is no per-fiber keep-alive in the core runtime (it existed earlier in v4 but was removed in beta.80; the migration/fiber-keep-alive.md doc predates the removal). A bare Effect.runFork/Effect.runPromise whose fiber is suspended on a pure Effect primitive (e.g. Deferred.await, Effect.never) does not by itself hold the Node.js process open.

Runtime.makeRunMain-based runners — NodeRuntime.runMain from @effect/platform-node, BunRuntime.runMain, etc. — install a long-interval timer that keeps the process alive until the main fiber completes, and additionally provide SIGINT/SIGTERM handling (interrupting the root fiber gracefully), exit-code mapping (interruption-only causes → 130), and error reporting. Always use runMain for long-lived program entry points:

import { NodeRuntime } from '@effect/platform-node';

NodeRuntime.runMain(program);

See the effect-managed-runtime skill for embedding Effect in existing applications.

Key Patterns

Latest-wins cancellation (FiberHandle + runtime)

Each new request interrupts the in-flight one — autocomplete, file watching, "restart preview on change":

const searchBox = Effect.gen(function* () {
	const handle = yield* FiberHandle.make<void, never>();
	const run = yield* FiberHandle.runtime(handle)<SearchApi>();

	input.addEventListener('input', () => {
		// synchronous call; interrupts the previous search automatically
		run(performSearch(input.value));
	});

	yield* Effect.never; // keep the scope open while the UI lives
}).pipe(Effect.scoped);

Restart-on-crash worker with a supervision watchdog

Effect.retry restarts the worker with backoff; FiberHandle.join propagates a final, unrecovered failure to the supervisor:

const worker = consumeJobs.pipe(
	Effect.retry(
		Schedule.exponential('250 millis').pipe(Schedule.upTo({ times: 10 }))
	)
);

const supervisor = Effect.gen(function* () {
	const handle = yield* FiberHandle.make<never, JobError>();
	yield* FiberHandle.run(handle, worker);

	// blocks until the worker exhausts retries (fails) or the scope closes
	yield* FiberHandle.join(handle);
}).pipe(Effect.scoped);

Keyed jobs with dedupe and per-key cancellation (FiberMap)

const downloads = Effect.gen(function* () {
	const map = yield* FiberMap.make<string, void, DownloadError>();

	const start = (url: string) =>
		// onlyIfMissing dedupes: a second start() for the same url is a no-op
		FiberMap.run(map, url, download(url), { onlyIfMissing: true });

	const cancel = (url: string) => FiberMap.remove(map, url);

	yield* start('https://example.com/a.bin');
	yield* start('https://example.com/a.bin'); // already running — skipped
	yield* cancel('https://example.com/a.bin');

	yield* FiberMap.awaitEmpty(map); // drain remaining downloads
}).pipe(Effect.scoped);

Background task set with graceful drain (FiberSet)

const webhookProcessor = Effect.gen(function* () {
	const tasks = yield* FiberSet.make<void, WebhookError>();

	for (const event of events) {
		yield* FiberSet.run(tasks, handleWebhook(event));
	}

	// graceful shutdown: wait for in-flight tasks instead of interrupting them.
	// Drop this line to let the closing scope interrupt whatever is left.
	yield* FiberSet.awaitEmpty(tasks);
}).pipe(Effect.scoped);

To fail fast when any background task crashes, run the service's main loop against FiberSet.join(tasks) (it fails with the first non-interruption failure).

Bounded background task set (FiberSet + Semaphore)

FiberSet never applies backpressure on its own. Bound admission by taking a semaphore permit before run and releasing it when the task fiber settles:

const boundedProcessor = Effect.gen(function* () {
	const tasks = yield* FiberSet.make<void, WebhookError>();
	const permits = yield* Semaphore.make(16); // at most 16 in flight

	for (const event of events) {
		yield* Semaphore.take(permits, 1); // waits while 16 tasks are in flight
		yield* FiberSet.run(
			tasks,
			handleWebhook(event).pipe(
				// runs on success, failure, AND interruption — permits never leak
				Effect.ensuring(Semaphore.release(permits, 1))
			)
		);
	}

	yield* FiberSet.awaitEmpty(tasks);
}).pipe(Effect.scoped);

See the effect-parallelization skill for Semaphore details (withPermits, withPermitsIfAvailable, PartitionedSemaphore).

Bridging callback APIs into supervised fibers

const socketHandler = Effect.gen(function* () {
	const set = yield* FiberSet.make<void, never>();
	const run = yield* FiberSet.runtime(set)<MessageStore>();

	socket.on('message', (data) => {
		// every message handled on a tracked fiber;
		// all in-flight handlers are interrupted when the scope closes
		run(storeMessage(data));
	});

	yield* Effect.never;
}).pipe(Effect.scoped);

Protected handoff between fibers (Deferred + interruption-safe cleanup)

const handoff = Effect.gen(function* () {
	const deferred = yield* Deferred.make<Result, WorkerError>();

	const producer = yield* Effect.forkScoped(
		produceResult.pipe(
			Effect.onExit((exit) => Deferred.done(deferred, exit)),
			Effect.onInterrupt(() => Effect.log('producer cancelled'))
		)
	);

	// consumer waits without caring which fiber produces
	return yield* Deferred.await(deferred);
});

Common Mistakes

1. Effect.fork / Effect.forkDaemon don't exist in v4 — they are Effect.forkChild and Effect.forkDetach. Effect.forkAll and Effect.forkWithErrorHandler were removed entirely.
2. Fire-and-forget with forkChild silently dies — children are interrupted when the parent fiber completes. Use forkScoped/forkIn for scope-tied work, forkDetach for detached work, or Effect.awaitAllChildren to wait.
3. Asserting right after a fork — forked fibers start on the next scheduler tick by default. yield* Effect.yieldNow first, or fork with { startImmediately: true }.
4. Expecting Fiber.await to fail — it always succeeds with an Exit. Use Fiber.join to propagate the fiber's failure; use Fiber.await to inspect.
5. Looking for Fiber.poll — there is no effectful poll in v4. Use the synchronous fiber.pollUnsafe() (Exit | undefined) or fiber.addObserver.
6. Assuming Fiber.joinAll cancels the rest on failure — it fails fast but leaves the other fibers running. Follow up with Fiber.interruptAll if they should stop.
7. Treating Fiber.interrupt as instant — it waits for the target to fully settle, including finalizers and uninterruptible regions. For a non-blocking signal use fiber.interruptUnsafe().
8. Using join to wait for completion on FiberHandle/FiberMap/FiberSet — join only resolves on a fiber failure or when the scope closes; successful fibers just leave the collection. Use awaitEmpty to wait for work to finish.
9. Expecting external interruption to fail join — by default it doesn't. Pass { propagateInterruption: true } to run/set/add; the collection's own internal interruptions (replacement, clear, scope close) never fail join either way.
10. Expecting onlyIfMissing: true to error when occupied — it succeeds, returning a shared already-interrupted fiber while keeping the existing one. Check Exit.hasInterrupts(yield* Fiber.await(fiber)) to detect the rejected start.
11. Calling run on a closed collection — FiberHandle.run/FiberMap.run interrupt the calling fiber; FiberSet.run and all runtime() runners return a pre-interrupted fiber instead. Neither throws.
12. Assuming collection fibers are children of the caller — they are root fibers created via Effect.runForkWith with the caller's context: they start immediately and survive the calling fiber; only the collection (scope close, replacement, remove/clear) interrupts them.
13. Relying on Effect.runFork/runPromise to keep Node alive — beta.80 removed the core fiber keep-alive; a fiber suspended on Deferred.await/Effect.never won't hold the process open. Use NodeRuntime.runMain (built on Runtime.makeRunMain).
14. Letting interruption leak through acquire/release — wrap the whole sequence in Effect.uninterruptibleMask and restore only the use phase; pending interruption is delivered as soon as the region ends, so cleanup still runs exactly once.
15. Leaving collection type parameters off — FiberHandle.make() defaults to <unknown, unknown>, making join surface unknown errors. Always pass them: FiberHandle.make<A, E>(), FiberMap.make<K, A, E>(), FiberSet.make<A, E>().
16. Using v3 FiberId types — v4 fiber ids are plain numbers; Cause.interruptors(cause) and Effect.onInterrupt finalizers give you ReadonlySet<number>.
17. Assuming someone logs a background fiber's failure — the v4 runtime has no unhandled-fiber-failure reporting and Effect.forkWithErrorHandler is gone. Unobserved failures vanish: join/await the fiber, watch the collection with FiberHandle/FiberMap/FiberSet.join, or build Effect.catchCause/Effect.onError + logging into the forked effect.