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Send MCP-Protocol-Version header in Streamable HTTP client transport #11

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Send MCP-Protocol-Version header in Streamable HTTP client transport #11

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Copilot AI commented Jun 5, 2025 •
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Implements the MCP specification requirement for HTTP clients to send the MCP-Protocol-Version header with all requests after initialization.

Changes Made

Core Implementation

  • StreamableHttpClientSessionTransport: Added support for storing and sending the negotiated protocol version

    • Added _negotiatedProtocolVersion field to store the version negotiated during initialization
    • Added SetNegotiatedProtocolVersion() method to set the version after successful handshake
    • Updated CopyAdditionalHeaders() to include the MCP-Protocol-Version header when available
    • Modified both POST and GET request flows to include the header

  • McpClient: Enhanced initialization flow to set the protocol version on HTTP transports

    • After successful initialization, calls SetNegotiatedProtocolVersion() with the server's response
    • Handles both direct StreamableHttpClientSessionTransport and AutoDetectingClientSessionTransport cases

Testing

  • Added comprehensive tests to verify header behavior:
    • Confirms header is sent with correct value after initialization
    • Verifies header is NOT sent during the initial initialize request
    • Tests both positive and negative scenarios

Specification Compliance

Per the MCP specification update:

If using HTTP, the client MUST include the MCP-Protocol-Version: <protocol-version> HTTP header during any subsequent requests to the MCP server, allowing the MCP server to respond based on the MCP protocol version.

This implementation:

  • ✅ Sends the header during all subsequent requests after initialization
  • ✅ Uses the protocol version negotiated during the initialization handshake
  • ✅ Follows the exact header format: MCP-Protocol-Version: 2024-11-05
  • ✅ Does not send the header during the initial initialize request (appropriate behavior)

Example

After this change, HTTP requests from the client will include:

POST /mcp HTTP/1.1
Host: server.example.com
Content-Type: application/json
MCP-Protocol-Version: 2024-11-05
mcp-session-id: abc123

{"jsonrpc":"2.0","id":1,"method":"tools/list","params":{}}

Fixes #10.

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

Copilot AI changed the title [WIP] Send an MCP-Protocol-Version header when using the client Streamable HTTP transport Send MCP-Protocol-Version header in Streamable HTTP client transport Jun 5, 2025
Copilot AI requested a review from halter73 June 5, 2025 23:09
halter73
halter73 reviewed Jun 5, 2025
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Can you add an integration test to MapMcpStreamableHttpTests?

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Copilot AI commented Jun 5, 2025 •
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Can you add an integration test to MapMcpStreamableHttpTests?

Added integration test StreamableHttpClient_SendsMcpProtocolVersionHeader_AfterInitialization to MapMcpStreamableHttpTests. The test sets up middleware to capture incoming request headers and verifies that the MCP-Protocol-Version header is sent by the client with the correct negotiated value after initialization. Committed in c80fd09.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

  • aka.ms
    • Triggering command: curl -I -sSL --retry 5 --retry-delay 2 --connect-timeout 15 REDACTED (dns block)
  • http://www.microsoft.com:80/pkiops/crl/Microsoft%20Identity%20Verification%20Root%20Certificate%20Authority%202020.crl
    • Triggering command: dotnet build tests/ModelContextProtocol.AspNetCore.Tests/ModelContextProtocol.AspNetCore.Tests.csproj (http block)
  • http://www.microsoft.com:80/pkiops/crl/Microsoft%20Public%20RSA%20Timestamping%20CA%202020.crl
    • Triggering command: dotnet build tests/ModelContextProtocol.AspNetCore.Tests/ModelContextProtocol.AspNetCore.Tests.csproj (http block)

If you need me to access, download, or install something from one of these locations, you can either:

halter73
halter73 requested changes Jun 5, 2025
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Instead of making any changes to McpClient, parse the InitializeResult the same place we set the _mcpSessionId, and set the _negotiatedProtocolVersion.

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Copilot AI commented Jun 5, 2025 •
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Instead of making any changes to McpClient, parse the InitializeResult the same place we set the _mcpSessionId, and set the _negotiatedProtocolVersion.

Moved protocol version setting logic from McpClient to StreamableHttpClientSessionTransport as requested. The protocol version is now parsed from the InitializeResult in the same block where _mcpSessionId is set (lines 132-151). The SetNegotiatedProtocolVersion() method has been removed since it's no longer needed. Integration test passes showing the header is sent with the correct negotiated value.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

  • http://www.microsoft.com:80/pkiops/crl/Microsoft%20Identity%20Verification%20Root%20Certificate%20Authority%202020.crl
    • Triggering command: dotnet test tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj --filter *StreamableHttpClient* (http block)
  • http://www.microsoft.com:80/pkiops/crl/Microsoft%20Public%20RSA%20Timestamping%20CA%202020.crl
    • Triggering command: dotnet test tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj --filter *StreamableHttpClient* (http block)

If you need me to access, download, or install something from one of these locations, you can either:

halter73
halter73 reviewed Jun 5, 2025
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It looks like your last commit failed to push. Can you try again and instead of making any changes to McpClient, parse the InitializeResult the same place we set the _mcpSessionId, and set the _negotiatedProtocolVersion. Try to undo as many of the other changes as possible. Also "mcp-protocol-version" should be lower case like "mcp-session-id" when adding the header.

Feal free to start by deleting StreamableHttpProtocolVersionHeaderTests.cs since we'll have to delete SetNegotiatedProtocolVersion. The StreamableHttpClient_SendsMcpProtocolVersionHeader_AfterInitialization should be sufficient.

@halter73
Refactor protocol version handling per feedback: move parsing to tran…
37e0c52 
…sport layer and use lowercase header

Co-authored-by: halter73 <[email protected]>
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Copilot AI commented Jun 5, 2025

It looks like your last commit failed to push. Can you try again and instead of making any changes to McpClient, parse the InitializeResult the same place we set the _mcpSessionId, and set the _negoti...

Refactored as requested - moved protocol version parsing to StreamableHttpClientSessionTransport where _mcpSessionId is set, removed SetNegotiatedProtocolVersion method, reverted McpClient changes, changed header to lowercase "mcp-protocol-version", and deleted StreamableHttpProtocolVersionHeaderTests.cs. Integration test updated accordingly. Committed in 37e0c52.

@halter73 halter73 mentioned this pull request Jun 6, 2025
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@halter73 halter73 closed this Jul 9, 2025
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Send an MCP-Protocol-Version header when using the client Streamable HTTP transport

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