What problem does it solve?
Teaches Claude Code's concurrency model: isConcurrencySafe() for parallel tool execution, AbortController propagation for cancellation, Promise.all() for parallel initialization, and the StreamingToolExecutor for concurrent progress streams. Use this when writing tools that could benefit from parallel execution, need to respect cancellation, or must stream partial results. Read-only tools that don't declare concurrency safety are needlessly serialized.
Core Features & Use Cases
Key concepts: per-tool isConcurrencySafe(input) for deciding whether a tool can run in parallel; isReadOnly(input) to indicate non-mutating operations; propagation of context.abortController.signal to cancel in-flight I/O; parallel initialization with Promise.all(); streaming partial results via onProgress using a StreamingToolExecutor. Use cases include read-only tools that can run concurrently with others and long-running tools that should be cancellable and stream progress.
Quick Start
Add isConcurrencySafe and isReadOnly checks to your tool, thread cancellation via AbortController signals, and stream progress with onProgress as your tool evolves.
