Closes #517.

Changes

Adds optional cancellation and timeoutSeconds arguments to Client::callTool(). Existing calls remain valid. An explicit per-call timeout replaces the configured request timeout and uses one deadline across protocol exchanges.

• Check interruption before sending, after synchronous I/O returns, and after a suspended request resumes. An interrupted call cannot return a buffered success.
• Clear pending state, discard late responses, and leave the connection available for subsequent calls.
• Send notifications/cancelled over STDIO and handshake-era HTTP. HTTP protocol 2026-07-28 uses response-stream closure instead.
• Log notification-delivery failures without replacing the original interruption. Discard notification response bodies so they cannot replace an active SSE stream.
• Document the API and transport tradeoffs in docs/client/transports.md.

Async behavior and limitations

The API remains synchronous and framework-agnostic. No async HTTP client or event loop is added.

HTTP cancellation is cooperative. Blocking PSR-18 requests and PSR-7 body reads must return before the SDK can observe interruption. A legacy HTTP cancellation POST can itself block. Per-call deadlines are therefore not hard HTTP wall-clock limits; callers still need HTTP-client network timeouts. Cancellation does not guarantee that server-side work stops or rolls back.

Validation

• make ci: passed, 1,775 tests and 4,646 assertions, with 7 existing Inspector skips. Those skips are explicitly declared in HttpClientCommunicationTest for logging/sampling limitations.
• make docs-guides: strict build passed.
• make conformance-tests: client matched its expected-failure baseline, 12 checks passed and 42 expected failures. The initial server run was blocked by fixture-directory permissions; after applying the setup used by CI, make conformance-server passed all 80 checks.
• Hatfield integration against this SDK: 189 MCP tests and 727 assertions passed.

New regression coverage uses protocol/HTTP unit tests and a real STDIO integration fixture. It covers synchronous JSON interruption, cancellation during body reads, interrupted suspended requests with buffered replies, version-dependent signaling, notification failures, late-response cleanup, and connection reuse. No new Inspector scenarios were added.