Relax legacy request metadata validation - #1852
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Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
halter73
requested review from
jeffhandley and
tarekgh
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a lite review from Copilot
September 3, 2026 23:45
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Copilot review overview
🟡 Changes recommended
Legacy requests with a malformed _meta/io.modelcontextprotocol/protocolVersion still throw InvalidParams due to unconditional parsing, which contradicts the intended “ignore malformed future metadata on legacy” behavior.
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Review tier: Lite
Findings: 1
New issues introduced by this change (1)
| Severity | Finding |
|---|---|
 |
src/ModelContextProtocol.Core/Server/McpServerImpl.cs — ReadRequestMetadata calls GetProtocolVersionMeta unconditionally, which throws InvalidParams if… |
What changed in this PR
This PR relaxes validation of legacy per-request _meta so legacy clients (notably ChatGPT negotiating 2025-11-25) aren’t rejected when they include auxiliary metadata introduced in later protocol eras, while keeping modern (2026-07-28+) protocol/version claims and required metadata strict. It also adjusts server/discover capability advertisement to avoid claiming deprecated logging on modern discover responses.
Changes:
- Reworks server-side
_metaparsing/projection and protocol boundary validation to treat known legacy-era metadata as advisory while enforcing strictness for modern envelopes and version switching. - Omits deprecated
loggingcapability from2026-07-28server/discoverresponses while retaining it for legacyinitialize. - Adds regression coverage across server/client behavior, including legacy session isolation vs per-request metadata and malformed metadata handling.
| File | Description |
|---|---|
| tests/ModelContextProtocol.Tests/Server/TaskProtocolGatingTests.cs | Updates legacy task-gating expectation to tolerate per-request capability metadata without enabling tasks. |
| tests/ModelContextProtocol.Tests/Server/NegotiatedProtocolVersionTests.cs | Expands coverage for advisory legacy version metadata, legacy session isolation, malformed metadata handling, and modern version switching. |
| tests/ModelContextProtocol.Tests/Server/McpServerTests.cs | Validates legacy transport header remains authoritative over advisory _meta protocol version; asserts legacy initialize includes logging capability. |
| tests/ModelContextProtocol.Tests/Client/McpClientMetaTests.cs | Adds coverage that legacy per-request client metadata does not override initialized session identity/capabilities. |
| tests/ModelContextProtocol.AspNetCore.Tests/RawHttpConformanceTests.cs | Adds conformance checks for discover capability suppression and legacy raw POST tolerance of auxiliary per-request metadata. |
| src/ModelContextProtocol.Core/Server/McpServerImpl.cs | Implements the relaxed legacy _meta semantics, strict modern validation, and logging capability suppression on discover. |
| src/ModelContextProtocol.Core/Protocol/JsonRpcMessageContext.cs | Updates documentation to clarify authoritative protocol version semantics and advisory legacy _meta behavior. |
| src/ModelContextProtocol.Core/McpSessionHandler.cs | Removes session-handler _meta projection now handled by server pipeline filter logic. |
Suppressed comments (1)
src/ModelContextProtocol.Core/Server/McpServerImpl.cs:318
- ProjectInitializeRequestMetadata also uses GetProtocolVersionMeta, so an
initializerequest carrying a malformed_meta/io.modelcontextprotocol/protocolVersion(or other invalid value type) will currently fail with InvalidParams. Since initialize is a legacy handshake path and_metais meant to be extensible there, consider treating a malformedprotocolVersionentry as advisory/absent rather than rejecting the request.
JsonObject? meta = GetRequestMeta(request);
string? metadataProtocolVersion = GetProtocolVersionMeta(meta, out bool hasProtocolVersionMeta);
string? transportProtocolVersion = request.Context?.ProtocolVersion;
ValidateProtocolVersionMatch(transportProtocolVersion, metadataProtocolVersion);
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| private void ReadRequestMetadata(JsonRpcRequest request) | ||
| { | ||
| JsonObject? meta = GetRequestMeta(request); | ||
| string? metadataProtocolVersion = GetProtocolVersionMeta(meta, out bool hasProtocolVersionMeta); | ||
| string? transportProtocolVersion = request.Context?.ProtocolVersion; | ||
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| ValidateProtocolVersionMatch(transportProtocolVersion, metadataProtocolVersion); | ||
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| bool establishedModernProtocol = McpProtocolVersions.RequiresPerRequestMetadata(_negotiatedProtocolVersion); | ||
| bool transportClaimsModernProtocol = | ||
| transportProtocolVersion is not null && | ||
| !McpProtocolVersions.SupportsInitializeHandshake(transportProtocolVersion); | ||
| bool metadataClaimsModernProtocol = | ||
| hasProtocolVersionMeta && | ||
| !McpProtocolVersions.SupportsInitializeHandshake(metadataProtocolVersion); | ||
| bool serverRequiresModernProtocol = | ||
| _initializeHandshakeProtocolVersions.Length == 0 && | ||
| _perRequestMetadataProtocolVersions.Length > 0; | ||
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ChatGPT currently negotiates
2025-11-25but includes metadata introduced in2026-07-28. Legacy_metais extensible, so rejecting these requests is unnecessarily strict and prevents otherwise compatible clients from using the SDK.This change:
initializefor stateful legacy sessions while exposing well-formed auxiliary metadata to request filters and stateless handlers.loggingfrom2026-07-28server/discoverresponses while retaining it for legacyinitializeresponses.Added regression coverage for the ChatGPT request shape, advisory legacy version metadata, malformed metadata boundaries, initialized-session isolation, modern version switching, capability spoofing, and protocol-specific logging advertisement.
Fixes: #1783
Fixes: #1774