0.15.0

Description

This release makes inbound message handling concurrent after the initialization handshake, adds SSE heartbeats and elicitation schema validation, and fixes several Streamable HTTP response and session-lifecycle bugs.

Breaking Changes

Concurrent inbound dispatch by @devcrocod in #884

Inbound messages were processed serially on the transport read loop, so one slow or peer-awaiting handler (sampling, elicitation, roots) blocked every later message on the connection, including the responses and notifications/cancelled those handlers depend on. After the handshake, handlers may now run concurrently; processing stays serial during initialization.

• ProtocolOptions / ClientOptions / ServerOptions gain a handlerCoroutineContext parameter (defaults to Dispatchers.Default). Concurrency is bounded internally; there is no opt-out flag by design.
• RequestHandlerExtra is now a CoroutineContext.Element and is no longer user-constructible. It exposes requestId, method, sendRequest, and sendNotification, and can be read inside a handler via the new currentRequestHandlerExtra().

- val extra = RequestHandlerExtra()
+ val extra = currentRequestHandlerExtra()

Duplicate feature names are rejected at registration by @devcrocod in #883

addTool/addPrompt/addResource/addResourceTemplate silently replaced an existing entry and emitted a spurious list_changed notification. They now throw IllegalArgumentException and leave the existing registration intact. The batch variants (addTools/addPrompts/addResources) are all-or-nothing. To replace a feature, remove it first.

- server.addTool(name = "search", ...)  // silently overwrote
+ server.removeTool("search")
+ server.addTool(name = "search", ...)

eventStore removed from mcpStatelessStreamableHttp by @devcrocod in #909

The parameter was never read or written: a stateless endpoint answers GET with 405, so no stream exists to store or replay events on. The old overload is retained at DeprecationLevel.ERROR with a ReplaceWith migration hint. Use mcpStreamableHttp when you need resumability.

  application.mcpStatelessStreamableHttp(
      path = "/mcp",
-     eventStore = myEventStore,
  ) { server }

Features

• Optional SSE heartbeats for Streamable HTTP servers via mcpStreamableHttp(sseHeartbeatConfig = { ... }), keeping long-lived streams alive against clients that disconnect on idle. Heartbeats stay off by default, by @UnscientificJsZhai in #761
• Accepted form-mode elicitation content is now validated against requestedSchema; a mismatch fails createElicitation with McpException (INVALID_PARAMS) instead of reaching server code unchecked, by @rea9r in #896
• The Streamable HTTP client now sends the standard Mcp-Method and Mcp-Name POST headers, by @AndreKalberer in #894

Fixed

• Stateless Streamable HTTP now closes its per-request session, fixing unbounded growth of the session registry and notification subscriptions, by @KlyneChrysler in #872
• Streamable HTTP responses keep their status and body when the client accepts only text/event-stream, by @devcrocod in #911
• The Accept header is parsed as media ranges, so clients sending */* or application/* are matched correctly, by @devcrocod in #912
• Malformed request params are reported as Invalid params (-32602) instead of Internal error, by @jstar0 in #886
• Duplicate initialize errors preserve the JSON-RPC request id, by @jstar0 in #868
• Connection teardown clears the recently-cancelled request id buffer, by @devcrocod in #892

Maintenance

• Create the server session before connecting the client in tests, avoiding a ChannelTransport startup race, by @devcrocod in #885

Dependencies

• Kotlin to v2.4.0 in #834
• Ktor to v3.5.1 in #832
• kotlinx-collections-immutable to v0.5.1 in #873
• Netty BOM to v4.2.16.Final in #889
• Kotest to v6.2.3 in #888, #915
• logback-classic to v1.6.0 in #890, #897, #914
• detekt-gradle-plugin to v2.0.0-alpha.5 in #849
• Kover to v0.9.9 in #902
• junit-jupiter to v6.1.2 in #897
• Shadow plugin to v9.6.1 and anthropic-java to v2.52.0 in #882, #914
• Sample projects updated to Kotlin 2.4.x, Ktor BOM 3.5.1, and mcp-kotlin 0.14.0 in #819, #822, #825, #827, #829, #859, #860, #861, #862, #863, #875, #876, #877, #878, #879, #899
• Test harness and CI updates (TypeScript integration-test dependencies, actions/setup-node v7) in #880, #891, #898, #906, #907, #910

New Contributors

• @UnscientificJsZhai made their first contribution in #761
• @KlyneChrysler made their first contribution in #872
• @AndreKalberer made their first contribution in #894

Full Changelog: 0.14.0...0.15.0