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fix(mcpserver): advertise capabilities only for registered primitives#2499
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Zelys-DFKH wants to merge 1 commit intomodelcontextprotocol:mainfrom
Open
fix(mcpserver): advertise capabilities only for registered primitives#2499Zelys-DFKH wants to merge 1 commit intomodelcontextprotocol:mainfrom
Zelys-DFKH wants to merge 1 commit intomodelcontextprotocol:mainfrom
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MCPServer unconditionally passed non-None list handlers to the lowlevel Server, which caused it to advertise tools/resources/prompts capabilities even when none of those primitives had been registered. Per the MCP schema spec, a capability entry should only appear when the server actually offers that primitive. Adds a `capability_filter` hook to the lowlevel Server that, if set, post-processes the computed ServerCapabilities before they are returned. MCPServer uses this to suppress tools/resources/prompts entries when the corresponding manager is empty at capability-computation time (i.e. when create_initialization_options() is called, after all decorators have been applied). Fixes modelcontextprotocol#2473. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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Summary
MCPServer.__init__unconditionally passes non-Nonelist handlers tothe lowlevel
Server, causingget_capabilities()to always advertisetools,resources, andprompts, even when none are registered.capability_filterparameter to the lowlevelServerthat post-processes the computed
ServerCapabilities.MCPServerpasses a filter that suppresses each entry when thecorresponding manager is empty at capability-computation time.
Motivation and Context
Per the MCP schema spec and lifecycle docs, a
ServerCapabilitiesentry should appear only when the server actually offers that primitive. Currently, an emptyMCPServeradvertises all three, and a server with only one tool still advertisesresourcesandpromptsit doesn't have.The filter runs when
create_initialization_options()is called at the start ofrun(), after all@mcp.tool()/@mcp.resource()/@mcp.prompt()decorators have been applied. Every transport path (stdio, SSE, streamable HTTP) callsserver.create_initialization_options()→get_capabilities(), so the filter applies without any per-transport changes.How Has This Been Tested?
Six new tests in
tests/server/mcpserver/test_server.pycover: empty server, tool-only, resource-only, resource-template-only, prompt-only, and all-registered cases.Full suite:
./scripts/test— 100% branch coverage.Breaking Changes
None. Servers that previously over-advertised will now send correct capability sets.
Types of changes
Checklist
Closes #2473