Effect v4 RC: August 2026 Updates

August was the month Effect v4 crossed the final threshold before stable.

On August 12th, we announced the Effect v4 Release Candidate, and the Effect v4 core package now has zero external dependencies.

What landed in August

Effect v4 Release Candidate

After months of beta releases, real-world usage, and a deep audit of the entire codebase, Effect is ready for 4. No broad breaking changes are planned for the RC cycle, any change that does land will be narrow in scope, clearly communicated, and accompanied by a migration path.

The RC is an invitation to migrate real applications and validate against production workloads. If you find anything that should stop the stable release, we want to know now.

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npm install effect@rc

Read the full RC announcement for context, migration guidance, and what comes next.

Upcoming breaking changes

• MessagePack encoding and RPC serialization were removed. Migrate schema-aware encoding, decoding, and RPC framing to effect/unstable/encoding/SchemaBinary.
• The Mime module moved from @effect/platform-node to effect/unstable/http/Mime, which now provides the built-in MIME registry without the external mime dependency.
• Config’s built-in constructors now use PascalCase, such as Config.String and Config.Number, and Config.mapOrFail is now Config.mapEffect. Update existing call sites to the new names.
• Effect no longer re-exports fast-check, and the Schema.toArbitrary and effect/testing/FastCheck APIs were removed. Use effect/unstable/arbitrary/Arbitrary.schema for Schema-derived generation, or depend on fast-check directly for its other APIs.
• The native PostgreSQL client removed PgClient.fromPool and PgClient.layerFromPool. Pass connection settings to PgClient.make or PgClient.layer instead of wrapping a pg pool.
• The pull-based Socket API removed run, runString, runRaw, and the send queue. Acquire socket.reader in a scope, use socket.writer for writes, and wrap the scoped read loop in Effect.retry to reconnect.

Zero External Dependencies in Core

The effect core package has no external dependencies. This is the result of a sustained effort throughout the RC cycle to internalize or eliminate third-party dependencies, including replacing pg with a native PgProtocol client, removing MessagePack and the MIME dependency from @effect/platform-node, and migrating NodeRedis from ioredis to redis (node-redis).

Effect already replaces 50+ common packages (observability, HTTP, CLI, database drivers, dependency injection), and doing so with no external surface area of its own makes it uniquely resilient to supply chain attacks.

Native PostgreSQL Client

Replaced the pg dependency in @effect/sql-pg with a native PgProtocol client, reducing external dependencies and giving the team full control over the connection lifecycle. PGlite listen behavior was also aligned with standard PostgreSQL semantics.

Pull-based Sockets

The Socket API was reworked around a pull-based reader with end-to-end backpressure:

• reconnects are now as simple as Effect.retry
• the WebSocket client interface is unified across runtimes (Node, Bun, Deno, browser) with typed handshake headers
• fixed the Node platform writing HTTP responses onto already-upgraded WebSocket connections.

Native Schema Arbitraries

Schema’s Arbitrary generation and shrinking were reworked with a native implementation, replacing the previous approach for more predictable and efficient property-based test data generation, with no external dependency.

Schema

New APIs - added partial matching to Schema.TaggedUnion, configurable schema reference policies, and a protocol adapter for the latest MCP specification revision.

Performance - optimized synchronous schema parsing. Fixed projection of checks and annotations through artificial JSON encodings. Fixed Schema class equivalence derivation.

Match

Added Match.fn selector matchers for more ergonomic pattern matching without boilerplate.

RPC

• Made server notifications first-class RPC messages.
• Added HTTP stream backpressure to prevent unbounded memory growth under slow consumers.

Tracing

• Improved tracing performance in span creation and HTTP tracer middleware.
• Shared tracer hex ID generation across spans.
• Documented the public random hex contract.
• Fixed log annotations overwriting active span correlation identifiers.
• Fixed disabled tracer timing still recording span end times.

AI & Streaming Tools

Automatic tool resolution is now interruption-safe when a language model response ends incomplete. Streaming tool handlers still execute eagerly, but only after the stream advances past their tool call; unresolved calls receive synthesized interruption results. Other fixes and improvements:

• preserved encoded AI tool parameters for manual resolution
• fixed Deferred clearing resumes before resuming waiters on completion
• corrected max output tokens for Claude Opus 4.6, 4.7, 4.8 and Sonnet 4.6
• fixed OpenRouter emitting cumulative tool-parameter prefixes as deltas
• fixed OpenRouter reasoning-end metadata casing
• fixed OpenRouter streamed citation end offsets overwriting start offsets
• fixed OpenAI telemetry response attributes using the request namespace type
• fixed batched encrypted event records reusing per-record randomization metadata
• fixed unknown MCP cancellation suppressing a later reused request ID.

MCP

• Fixed MCP conformance issues and added a test suite.
• Emitted MCP tool output schemas.
• Added a protocol adapter for the latest MCP specification revision.

Cluster & Durable Execution

• Fixed a cluster shutdown hang from abandoned requests.
• Fixed defecting execution-plan observers replacing outcomes and leaving events unpaired.
• Added integration coverage for entity shutdown, replay, workflow owner-loss, and cluster transport failure scenarios.
• Bounded teardown interrupt classification.
• Ensured transient routing states are never surfaced for persisted messages.
• Defaulted TCP cluster serialization to SchemaBinary.

STM & Transactions

• Fixed TxPubSub subscriber release interrupting after hub shutdown.
• Fixed TMap.remove/removeAll clearing entire buckets on hash collision.

SQL

• Added D1 batch support.
• Ran SQL persistence cleanup in bounded batches to avoid lock contention.
• Fixed findById resolvers invoking NonEmpty callbacks with empty batches.
• Prevented SQL resolvers from executing empty batches.
• Used migrations for SQL persisted queues.
• Fixed readonly SQLite transactions.
• Propagated failed transaction begin as a typed error.

Platform & Runtime

• Fixed Bun and Deno fileWebResponse ignoring range options.
• Fixed closing an older Bun server not properly tearing down.
• Fixed client-server-client FormData round-trip.
• Fixed NodeStream handling of a zero maxBytes limit.
• Fixed rcRef leaking resources acquired before acquisition failure.
• Fixed scopedRef.set orphaning a replacement when old cleanup defects.
• Fixed buffered worker send failures escaping the WorkerError channel.
• Fixed fiber.joinAll erasing input fiber error types.
• Fixed NodeTerminal readline staying alive between prompts.
• Made a zero-duration Node socket openTimeout fail immediately with a socket open timeout.
• Restored Effect.timeout error message.
• Aligned type IDs with module paths.

Effect Core

• Restored Effect.head, which had been inadvertently removed.
• Normalized numeric collection counts across Array, Chunk, Iterable, and String; TupleOf now falls back to Array for positive fractional lengths.
• Merged effect and finalizer failures during cleanup, preserving other failures alongside Cause.Done.
• Preserved the Context.mapUnsafe accessor when code is compiled with loose object spread transforms.
• Fixed Effect.fromOption inline inference.
• Normalized unbounded PubSub replay capacities.

Schedule

Added support for refinements in Schedule.while.

Graph

Fixed edge mutability and A* heuristics.

CLI

• Added context-based CLI prompt themes for more customizable interactive prompts.
• Fixed quotes escaping in generated shell completion descriptions.

Optics

Added standalone dual Optic functions.

Tooling

Effect LSP rules are now available as custom, type-aware OXLint rules, with support for vite-plus. Effect’s API reference documentation for both v3 and v4 is now available directly on the Effect website, with an improved search experience.

A skill for Effect v3 to v4 migration is now available on skills.sh, enabling near-fully automated migrations with a coding agent.

Documentation

• Recommended Effect.fnUntraced in the AI guide.
• Documented Match and RPC migration changes for teams upgrading from v3.
• Added SQL, HttpApi testing, and CLI schema coverage to the AI docs.
• Fixed KeyValueStore file backend docs.

Keep up with the Effect v4 RC

If you missed the earlier updates, the July recap, the June recap, and the February–May recap cover the full beta period.

For week-by-week updates, follow the This Week in Effect series.

To try the Effect v4 RC:

Happy Effecting. 🚀