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Spike the MCP 2026-07-28 modern server on Restlet (gate 0) - #1662
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Gate 0 for MCP 2026-07-28 (#1640): the MCP SDK 2.2.0-M2 modern stateless server behind a test-only Restlet transport that mirrors the SDK's servlet transport: POST only, header checks against the body, JSON replies, errors mapped to their HTTP status, streaming replies over core SSE. Requests are shaped as the revision requires: server/discover, tools/list and tools/call with plain and base64 Mcp-Name, header, version and method errors, legacy GET, notification and initialize. The SDK moves to 2.2.0-M2, a milestone whose modern API is a preview; the classic API ext.mcp uses is unchanged and its tests pass. Co-Authored-By: Claude Opus 5.5 <[email protected]>
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🟡 Changes recommended
The tests do not exercise the modern SDK’s streaming response path over Restlet, leaving SSE content type, notification ordering, final response ID, and stream completion unverified.
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What changed in this PR
Adds a gate-0 spike for hosting the MCP 2026-07-28 modern stateless server on Restlet, supporting the R6 transport investigation. The transport remains test-only; the SDK upgrade affects the production extension.
Changes:
- Upgrades the MCP Java SDK from 2.1.0 to 2.2.0-M2.
- Adds HTTP tests for discovery, tool calls, protocol errors, legacy traffic, and required client capabilities.
- Implements a test-only Restlet transport with header validation, JSON replies, and an SSE response path.
| File | Description |
|---|---|
pom.xml |
Updates the shared MCP SDK version to 2.2.0-M2. |
org.restlet.ext.mcp/src/test/java/org/restlet/ext/mcp/McpModernSpikeTestCase.java |
Adds the modern-server transport spike and HTTP interoperability tests. |
Review effort: Balanced
A tool reporting progress before its result is answered on an SSE stream: the progress notification first, then the result answering the request, then the end of the stream. Co-Authored-By: Claude Opus 5.5 <[email protected]>
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jlouvel
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Nov 7, 2026
McpServerAdapter now also serves MCP revision 2026-07-28 on the same endpoint: a request whose MCP-Protocol-Version header names it goes to the MCP SDK's modern stateless server (mcp-core 2.2.0-M2), any other request keeps the 2025-11-25 path unchanged. - The tools, resources, templates and prompts registered on the adapter are served to both revisions, bridged through JSON between the two SDK schemas; getModernTools() adds tools on the modern types, served to 2026-07-28 clients only, for input_required and streamed progress. - The interceptors wrap each 2026-07-28 request and its final result or error; progress streamed before the result passes without them. - The Origin allow-list, message size, call limit, request timeout, stop and peer disconnect apply to both revisions; the tool filters apply to 2026-07-28 listings, modern tools included. - A streamed reply goes over core SSE through a bounded queue, with heartbeats; leaving it early, on a timeout, a disconnect or a stop, cancels the SDK's stream and ends the client's with an error answering the request. - Mcp-Param headers are passed to the SDK, which validates x-mcp-header arguments against them. - input_required state is sealed bound to the authenticated user; setRequestStateKey() shares the key between instances. McpRequestContext reports the revision of its request, and McpProtocol.MODERN_VERSION names 2026-07-28. Not yet for 2026-07-28: stdio, subscriptions/listen, modern resources and prompts, the client. The test-only spike of #1662 is replaced by McpModernServerTestCase. Co-Authored-By: Claude Opus 5.5 <[email protected]>
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Gate 0 for MCP revision 2026-07-28 (#1640), the revision Ikanos speaks today. Test-only, apart from the SDK version.
SDK.
lib-mcp-versionmoves from 2.1.0 to 2.2.0-M2 (2026-11-02), the first Java SDK line with a 2026-07-28 server:io.modelcontextprotocol.modern, a stateless server with a transport-neutralhandle(context, request). It is a milestone, and its modern API is marked preview until 3.0. The classic API thatMcpServerAdapteruses is unchanged: the existing MCP tests pass on M2, the session spike included.Spike.
McpModernSpikeTestCaseputs the SDK's modernMcpServer, with one tool, behind a test-onlyRestletthat does what the SDK'sHttpServletMcpTransportdoes: POST only, JSON content type,Mcp-Method/MCP-Protocol-Version/Mcp-Namechecked against the body (base64 values included), 202 for notifications, JSON replies, errors mapped to their HTTP status, and streaming replies over core SSE (EventStreamRepresentation). A JDKHttpClientsends requests as the revision requires (the Java SDK has no modern client yet):server/discoveradvertises["2026-07-28"], the tools capability,resultType: completeand_metaserverInfo.tools/listreturns the tool withttlMsandcacheScope;tools/callworks with a plain and a base64Mcp-Name.data.supported, an unknown method 404 / -32601.initialize400 / -32602._metaclient capabilities is refused with 400 / -32602.Findings against Ikanos' implementation of the revision:
_meta; the SDK requires the client capabilities too, as the revision does, so Ikanos' client can't call an SDK-based server until it sends them (empty is enough).initializewith -32022; the SDK with -32602. Both refuse it.mvn -pl org.restlet.ext.mcp verify: 42 tests, coverage and SpotBugs pass.🤖 Generated with Claude Code