feat!: add SEP-2243 HTTP standard headers (#907)

* feat: add SEP-2243 HTTP standard headers

* feat!: validate Mcp-Param-* headers on server

* fix: emit Mcp-Method on reinit initialized POST

* fix: align header mismatch error code with draft spec

* chore: remove redundant streamable HTTP client comments

Author: DaleSeo

crates/rmcp/Cargo.toml

@@ -139,12 +139,13 @@ server-side-http = [
   "dep:bytes",
   "dep:sse-stream",
   "tower",
+  "base64",
 ]
 
 transport-worker = ["dep:tokio-stream"]
 
 # SSE stream parsing utilities (used by streamable HTTP client for SSE-formatted responses)
-client-side-sse = ["dep:sse-stream", "dep:http"]
+client-side-sse = ["dep:sse-stream", "dep:http", "base64"]
 
 # Streamable HTTP client
 transport-streamable-http-client = ["client-side-sse", "transport-worker"]
@@ -293,6 +294,11 @@ name = "test_protocol_version_negotiation"
 required-features = ["server", "client"]
 path = "tests/test_protocol_version_negotiation.rs"
 
+[[test]]
+name = "test_streamable_http_standard_headers"
+required-features = ["server", "client", "transport-streamable-http-server", "reqwest"]
+path = "tests/test_streamable_http_standard_headers.rs"
+
 [[test]]
 name = "test_streamable_http_4xx_error_body"
 required-features = ["transport-streamable-http-client", "transport-streamable-http-client-reqwest"]

crates/rmcp/src/model.rs

@@ -168,6 +168,9 @@ impl ProtocolVersion {
     pub const V_2024_11_05: Self = Self(Cow::Borrowed("2024-11-05"));
     pub const LATEST: Self = Self::V_2025_11_25;
 
+    /// First protocol version that requires SEP-2243 standard HTTP headers.
+    pub const STANDARD_HEADERS: Self = Self::V_2026_07_28;
+
     /// All protocol versions known to this SDK.
     pub const KNOWN_VERSIONS: &[Self] = &[
         Self::V_2024_11_05,
@@ -512,6 +515,7 @@ pub struct JsonRpcNotification<N = Notification> {
 pub struct ErrorCode(pub i32);
 
 impl ErrorCode {
+    pub const HEADER_MISMATCH: Self = Self(-32020);
     pub const RESOURCE_NOT_FOUND: Self = Self(-32002);
     pub const INVALID_REQUEST: Self = Self(-32600);
     pub const METHOD_NOT_FOUND: Self = Self(-32601);
@@ -557,7 +561,9 @@ impl ErrorData {
     pub fn resource_not_found(message: impl Into<Cow<'static, str>>, data: Option<Value>) -> Self {
         Self::new(ErrorCode::RESOURCE_NOT_FOUND, message, data)
     }
-
+    pub fn header_mismatch(message: impl Into<Cow<'static, str>>, data: Option<Value>) -> Self {
+        Self::new(ErrorCode::HEADER_MISMATCH, message, data)
+    }
     pub fn parse_error(message: impl Into<Cow<'static, str>>, data: Option<Value>) -> Self {
         Self::new(ErrorCode::PARSE_ERROR, message, data)
     }

crates/rmcp/src/transport/common.rs

@@ -3,6 +3,9 @@ pub mod server_side_http;
 
 pub mod http_header;
 
+#[cfg(any(feature = "client-side-sse", feature = "server-side-http"))]
+pub mod mcp_headers;
+
 #[cfg(feature = "__reqwest")]
 mod reqwest;
 

crates/rmcp/src/transport/common/http_header.rs

@@ -4,6 +4,15 @@ pub const HEADER_MCP_PROTOCOL_VERSION: &str = "MCP-Protocol-Version";
 pub const EVENT_STREAM_MIME_TYPE: &str = "text/event-stream";
 pub const JSON_MIME_TYPE: &str = "application/json";
 
+// SEP-2243 standard headers, gated on protocol version >= 2026-07-28.
+pub const HEADER_MCP_METHOD: &str = "Mcp-Method";
+pub const HEADER_MCP_NAME: &str = "Mcp-Name";
+pub const HEADER_MCP_PARAM_PREFIX: &str = "Mcp-Param-";
+
+/// Sentinel wrapping a Base64-encoded SEP-2243 header value (`=?base64?<b64>?=`).
+pub const BASE64_HEADER_PREFIX: &str = "=?base64?";
+pub const BASE64_HEADER_SUFFIX: &str = "?=";
+
 /// Reserved headers that must not be overridden by user-supplied custom headers.
 /// `MCP-Protocol-Version` is in this list but is allowed through because the worker
 /// injects it after initialization.