What problem does it solve? Effect's collection combinators run sequentially by default, and its concurrency APIs differ substantially from v3 and from common training data, so agents frequently write sequential code, use wrong option keys, or misuse semaphores and latches. This Skill provides verified guidance for declarative concurrency in Effect v4. ## Core Features & Use Cases - Bounded fan-out: Use Effect.all and Effect.forEach with explicit concurrency options, discard, and mode: 'result' for per-item outcomes without fail-fast interruption. - Racing and timeouts: Choose between race/raceAll (first success) and raceFirst/raceAllFirst (first completion), plus firstSuccessOf for sequential fallback and timeout combinators. - Coordination primitives: Apply Semaphore for global rate limits, PartitionedSemaphore for per-key round-robin fairness, and Latch for gating fibers on startup signals. - Use Case: Sync thousands of users against an external API that allows only 5 in-flight requests by wrapping calls in a service-held Semaphore while fanning out with Effect.forEach at unbounded concurrency. ## Quick Start Ask the agent to fetch a list of user IDs concurrently with a limit of 8 in flight using Effect.forEach, collecting per-item failures with Effect.partition.
