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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Support custom method handlers with custom params and return types on MCPServer","articleBody":"The SDK should provide a clear, documented way to register custom JSON-RPC method handlers on the server side that can:\n\n1. **Receive custom parameter types** — not just the built-in MCP request types, but arbitrary structured params (dicts, Pydantic models, etc.)\n2. **Return custom response types** — not just the built-in MCP result types\n\nThis is important for:\n- Protocol extensions and experimentation\n- Custom server-to-server communication patterns\n- Future spec features (e.g., MRTR) that may introduce new method/response types\n\nThe low-level server was recently refactored to use `on_*` constructor kwargs (PR #1985), which gives a cleaner handler registration pattern. This issue is about making sure there is an explicit, documented escape hatch for methods that are not part of the core MCP spec.\n\n### Open questions\n- Should custom handlers go through the same `on_*` pattern, or a separate `register_method()` API?\n- How do custom params get validated/parsed?\n- How does this interact with middleware (#1233)?\n\nRef: V2 planning discussion","author":{"url":"https://github.com/maxisbey","@type":"Person","name":"maxisbey"},"datePublished":"2026-02-19T20:12:33.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/2105/python-sdk/issues/2105"}