[sk-20231]

Fixes #1984

Problem

HandlePostRequestAsync reads some request values as strings while it validates the headers against the body. Two kinds of input make those reads throw InvalidOperationException, which the handler does not catch (it only catches JsonException), so the request returns 500:

• A string that is not valid UTF-8. Utf8JsonReader accepts the bytes inside a string token and only validates them when the token is converted to a .NET string.
• A value that is not a string where a string is expected, for example "name": 5 on a tools/call request. JsonValue.GetValue<string>() throws for it.

Affected reads:

Value	Validation	Before	After
params.protocolVersion on initialize (with an MCP-Protocol-Version header)	protocol version envelope	500	400, HeaderMismatch (-32020)
_meta/io.modelcontextprotocol/protocolVersion	protocol version envelope	500	400, InvalidParams (-32602)
Body value compared with Mcp-Name (invalid UTF-8 or not a string)	ValidateMcpHeaders	500	400, HeaderMismatch (-32020)
Argument annotated with x-mcp-header	ValidateCustomParamHeaders	500	400, HeaderMismatch (-32020)

Change

• Added a private TryGetStringValue helper that returns false instead of throwing when a node is not a JSON string or the string is not valid UTF-8, and a TryGetJsonNodeStringProperty wrapper that replaces GetJsonNodeStringProperty.
• Each check now rejects an unreadable value with the error code it already uses for a bad value of that field, instead of adding one generic code. An unreadable _meta protocol version is handled like a missing one, the same as a non-string value today.
• Every response echoes the request id, as the existing error paths do.
• No public API changes. McpHeaderEncoder is unchanged. The x-mcp-header check validates the argument before calling ConvertToHeaderValue.

On the question in the issue (-32600 vs -32700): the body is well-formed JSON and is parsed into a valid JSON-RPC request with a readable id, so I used the codes each validation step already returns. Happy to switch to a different code if you prefer.

Tests

Five tests in HttpHeaderConformanceTests. Each one failed with 500 before the change. Each now checks the status, the JSON-RPC error code, and the echoed id:

• Server_RejectsInvalidUtf8InProtocolVersionMeta_WithBadRequest
• Server_RejectsInvalidUtf8InInitializeProtocolVersion_WithBadRequest
• Server_RejectsInvalidUtf8InMcpNameBodyValue_WithBadRequest
• Server_RejectsInvalidUtf8InCustomParamHeaderArgument_WithBadRequest
• Server_RejectsNonStringMcpNameBodyValue_WithBadRequest

Local results:

• Full solution build: 0 errors
• ModelContextProtocol.AspNetCore.Tests (net10.0): 759 passed, 0 failed, 45 skipped
• ModelContextProtocol.Tests (net472): 3299 passed, 0 failed, 290 skipped

Note

This PR was written with help from an AI coding assistant (Claude Code). I reviewed the change, reproduced the bug, and ran the build and tests locally.