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Implementation for SEP-2243 Http Standardization (#1553)

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Co-authored-by: Tarek Mahmoud Sayed <tarekms@microsoft.com>
Co-authored-by: Tarek Mahmoud Sayed <tarekms@ntdev.microsoft.com>
Co-authored-by: Tarek Mahmoud Sayed <10833894+tarekgh@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Mike Kistler 2026-05-14 17:39:37 -05:00 • committed by GitHub
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25
docs/concepts/tools/tools.md
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@ -315,3 +315,28 @@ public static string Search(
// Schema will include descriptions and default value for maxResults
}
```
### Custom HTTP headers from tool parameters
When using the Streamable HTTP transport, tool parameters can be mirrored as HTTP headers so that network infrastructure (load balancers, proxies, gateways) can make routing decisions without parsing the JSON-RPC request body. Apply the <xref:ModelContextProtocol.Server.McpHeaderAttribute> to a parameter to opt it in:
```csharp
[McpServerTool, Description("Executes a SQL query in a specific region")]
public static string ExecuteSql(
[McpHeader("Region"), Description("Target datacenter region")] string region,
[Description("The SQL query to execute")] string query)
{
// Clients will send an additional HTTP header:
// Mcp-Param-Region: <region value>
}
```
When the tool's schema is generated, the annotated parameter includes an `x-mcp-header` extension property. Clients read this annotation and automatically add the corresponding `Mcp-Param-{Name}` header on outgoing `tools/call` requests. The server validates that the header value matches the value in the JSON-RPC body.
Rules and constraints:
- Only primitive parameter types (`string`, numeric types, `bool`) are supported.
- The header name must contain only visible ASCII characters (0x21–0x7E) excluding colon (`:`).
- Values containing non-ASCII characters, control characters, or leading/trailing whitespace are Base64-encoded using the `=?base64?{value}?=` wrapper.
- Header names must be case-insensitively unique within the tool's input schema.
- Header validation is enforced only for protocol versions that support the HTTP Standardization feature (currently `DRAFT-2026-v1` and later).