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

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.

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

@@ -0,0 +1,654 @@
+//! SEP-2243 HTTP header standardization.
+//!
+//! Builds and validates the `Mcp-Method`, `Mcp-Name`, and `Mcp-Param-*` headers
+//! so middle boxes can route Streamable HTTP traffic without parsing the body.
+//! All emission/validation is gated by the negotiated protocol version
+//! (`>= ProtocolVersion::STANDARD_HEADERS`) at the call sites.
+
+// Which helpers are reachable depends on the client/server feature combination,
+// mirroring `server_side_http`.
+#![allow(dead_code)]
+
+use serde_json::Value;
+
+use super::http_header::{
+    BASE64_HEADER_PREFIX, BASE64_HEADER_SUFFIX, HEADER_MCP_METHOD, HEADER_MCP_NAME,
+    HEADER_MCP_PARAM_PREFIX,
+};
+use crate::model::JsonObject;
+
+/// Methods whose `Mcp-Name` is sourced from `params.name`.
+const NAME_FROM_NAME: &[&str] = &["tools/call", "prompts/get"];
+/// Methods whose `Mcp-Name` is sourced from `params.uri`.
+const NAME_FROM_URI: &[&str] = &[
+    "resources/read",
+    "resources/subscribe",
+    "resources/unsubscribe",
+];
+
+/// Returns the `Mcp-Name` value for a request, if the method carries one.
+fn extract_name(method: &str, params: Option<&Value>) -> Option<String> {
+    let params = params?;
+    let key = if NAME_FROM_NAME.contains(&method) {
+        "name"
+    } else if NAME_FROM_URI.contains(&method) {
+        "uri"
+    } else {
+        return None;
+    };
+    params.get(key)?.as_str().map(str::to_owned)
+}
+
+/// Converts a JSON primitive to its SEP-2243 string form. Non-primitives yield `None`.
+fn primitive_to_string(value: &Value) -> Option<String> {
+    match value {
+        Value::String(s) => Some(s.clone()),
+        Value::Bool(b) => Some(b.to_string()),
+        Value::Number(n) => Some(n.to_string()),
+        _ => None,
+    }
+}
+
+/// True if `value` must be Base64-wrapped to survive as an HTTP header value:
+/// leading/trailing space or tab, control/non-ASCII characters, or a value that
+/// already looks like the `=?base64?...?=` sentinel.
+#[cfg(feature = "client-side-sse")]
+fn requires_base64(value: &str) -> bool {
+    if value.is_empty() {
+        return false;
+    }
+    let bytes = value.as_bytes();
+    if matches!(bytes.first(), Some(b' ' | b'\t')) || matches!(bytes.last(), Some(b' ' | b'\t')) {
+        return true;
+    }
+    if value
+        .chars()
+        .any(|c| (c as u32) < 0x20 || (c as u32) > 0x7E)
+    {
+        return true;
+    }
+    value.starts_with(BASE64_HEADER_PREFIX) && value.ends_with(BASE64_HEADER_SUFFIX)
+}
+
+/// RFC 9110 §5.6.2 token character.
+#[cfg(feature = "client-side-sse")]
+fn is_tchar(c: char) -> bool {
+    c.is_ascii_alphanumeric()
+        || matches!(
+            c,
+            '!' | '#'
+                | '$'
+                | '%'
+                | '&'
+                | '\''
+                | '*'
+                | '+'
+                | '-'
+                | '.'
+                | '^'
+                | '_'
+                | '`'
+                | '|'
+                | '~'
+        )
+}
+
+/// Top-level properties carrying an `x-mcp-header` annotation, as `(property, header)` pairs.
+fn param_header_annotations(input_schema: &JsonObject) -> Vec<(String, String)> {
+    let mut out = Vec::new();
+    if let Some(Value::Object(props)) = input_schema.get("properties") {
+        for (prop, schema) in props {
+            if let Some(Value::String(header)) = schema.get("x-mcp-header") {
+                if !header.is_empty() {
+                    out.push((prop.clone(), header.clone()));
+                }
+            }
+        }
+    }
+    out
+}
+
+/// Validates the `x-mcp-header` annotations in a tool input schema.
+///
+/// Annotations must be non-empty RFC 9110 tokens, case-insensitively unique,
+/// applied only to top-level primitive (`string`/`integer`/`boolean`) properties.
+/// Returns the offending reason on the first violation.
+#[cfg(feature = "client-side-sse")]
+pub(crate) fn validate_param_header_annotations(input_schema: &JsonObject) -> Result<(), String> {
+    let Some(Value::Object(props)) = input_schema.get("properties") else {
+        return Ok(());
+    };
+    let mut seen = std::collections::HashSet::new();
+    for (prop, schema) in props {
+        reject_nested_annotations(schema, prop)?;
+        let Some(raw) = schema.get("x-mcp-header") else {
+            continue;
+        };
+        let Value::String(header) = raw else {
+            return Err(format!("property `{prop}`: x-mcp-header must be a string"));
+        };
+        if header.is_empty() {
+            return Err(format!("property `{prop}`: x-mcp-header must not be empty"));
+        }
+        if !header.chars().all(is_tchar) {
+            return Err(format!(
+                "property `{prop}`: x-mcp-header `{header}` is not a valid HTTP token"
+            ));
+        }
+        if !seen.insert(header.to_ascii_lowercase()) {
+            return Err(format!(
+                "property `{prop}`: duplicate x-mcp-header `{header}` (case-insensitive)"
+            ));
+        }
+        match schema.get("type").and_then(Value::as_str) {
+            Some("string" | "integer" | "boolean") => {}
+            other => {
+                return Err(format!(
+                    "property `{prop}`: x-mcp-header requires a primitive type \
+                     (string/integer/boolean), got {other:?}"
+                ));
+            }
+        }
+    }
+    Ok(())
+}
+
+/// Rejects `x-mcp-header` on nested properties (only top-level promotion is supported).
+#[cfg(feature = "client-side-sse")]
+fn reject_nested_annotations(schema: &Value, path: &str) -> Result<(), String> {
+    if let Some(Value::Object(nested)) = schema.get("properties") {
+        for (key, value) in nested {
+            if value.get("x-mcp-header").is_some() {
+                return Err(format!(
+                    "property `{path}.{key}`: x-mcp-header is not supported on nested properties"
+                ));
+            }
+            reject_nested_annotations(value, &format!("{path}.{key}"))?;
+        }
+    }
+    Ok(())
+}
+
+/// Wraps a value as `=?base64?<b64>?=` when it cannot travel as a bare header value.
+#[cfg(feature = "client-side-sse")]
+fn encode_header_value(value: &str) -> String {
+    use base64::{Engine, prelude::BASE64_STANDARD};
+    if requires_base64(value) {
+        format!(
+            "{BASE64_HEADER_PREFIX}{}{BASE64_HEADER_SUFFIX}",
+            BASE64_STANDARD.encode(value)
+        )
+    } else {
+        value.to_owned()
+    }
+}
+
+/// Reverses [`encode_header_value`]. Returns `None` if the sentinel wraps invalid Base64/UTF-8.
+#[cfg(feature = "server-side-http")]
+fn decode_header_value(value: &str) -> Option<String> {
+    use base64::{Engine, prelude::BASE64_STANDARD};
+    match value
+        .strip_prefix(BASE64_HEADER_PREFIX)
+        .and_then(|inner| inner.strip_suffix(BASE64_HEADER_SUFFIX))
+    {
+        Some(inner) => {
+            let bytes = BASE64_STANDARD.decode(inner).ok()?;
+            String::from_utf8(bytes).ok()
+        }
+        None => Some(value.to_owned()),
+    }
+}
+
+/// Builds the SEP-2243 headers for an outgoing request from its JSON form.
+///
+/// `tool_schema` is the cached input schema of the called tool, used to promote
+/// annotated `tools/call` arguments to `Mcp-Param-*` headers.
+#[cfg(feature = "client-side-sse")]
+pub(crate) fn standard_request_headers(
+    request: &Value,
+    tool_schema: Option<&JsonObject>,
+) -> Vec<(http::HeaderName, http::HeaderValue)> {
+    use http::{HeaderName, HeaderValue};
+
+    let mut out = Vec::new();
+    let Some(method) = request.get("method").and_then(Value::as_str) else {
+        return out;
+    };
+    let params = request.get("params");
+
+    let mut push = |name: &str, value: &str| {
+        if let (Ok(name), Ok(value)) = (
+            HeaderName::from_bytes(name.as_bytes()),
+            HeaderValue::from_str(value),
+        ) {
+            out.push((name, value));
+        }
+    };
+
+    push(HEADER_MCP_METHOD, method);
+    if let Some(name) = extract_name(method, params) {
+        push(HEADER_MCP_NAME, &encode_header_value(&name));
+    }
+
+    if method == "tools/call" {
+        if let (Some(schema), Some(arguments)) =
+            (tool_schema, params.and_then(|p| p.get("arguments")))
+        {
+            for (prop, header) in param_header_annotations(schema) {
+                let Some(arg) = arguments.get(&prop) else {
+                    continue;
+                };
+                let Some(encoded) = primitive_to_string(arg).map(|s| encode_header_value(&s))
+                else {
+                    continue;
+                };
+                push(&format!("{HEADER_MCP_PARAM_PREFIX}{header}"), &encoded);
+            }
+        }
+    }
+    out
+}
+
+/// Validates incoming SEP-2243 headers against the request body.
+///
+/// Returns `Err(reason)` when a required header is missing or its value does not
+/// match the body; the caller maps this to a JSON-RPC `-32020` error (HTTP 400).
+#[cfg(feature = "server-side-http")]
+pub(crate) fn validate_request_headers(
+    headers: &http::HeaderMap,
+    request: &Value,
+    tool_schema: Option<&JsonObject>,
+) -> Result<(), String> {
+    let Some(method) = request.get("method").and_then(Value::as_str) else {
+        return Ok(());
+    };
+    let params = request.get("params");
+
+    let header_method = header_str(headers, HEADER_MCP_METHOD);
+    match header_method {
+        None => return Err("missing required Mcp-Method header".to_owned()),
+        Some(value) if value != method => {
+            return Err(format!(
+                "Mcp-Method header `{value}` does not match body method `{method}`"
+            ));
+        }
+        Some(_) => {}
+    }
+
+    if let Some(expected) = extract_name(method, params) {
+        match header_str(headers, HEADER_MCP_NAME) {
+            None => return Err(format!("missing required Mcp-Name header for `{method}`")),
+            Some(raw) => {
+                let decoded = decode_header_value(raw)
+                    .ok_or_else(|| "Mcp-Name header is not valid Base64".to_owned())?;
+                if decoded != expected {
+                    return Err(format!(
+                        "Mcp-Name header `{decoded}` does not match body value `{expected}`"
+                    ));
+                }
+            }
+        }
+    }
+
+    if method == "tools/call" {
+        if let Some(schema) = tool_schema {
+            let arguments = params.and_then(|p| p.get("arguments"));
+            for (prop, header) in param_header_annotations(schema) {
+                let full = format!("{HEADER_MCP_PARAM_PREFIX}{header}");
+                let header_value = header_str(headers, &full);
+                let arg = arguments.and_then(|a| a.get(&prop));
+                let body_value = arg.filter(|v| !v.is_null()).and_then(primitive_to_string);
+
+                match (header_value, body_value) {
+                    (None, None) => {}
+                    (Some(_), None) => {
+                        return Err(format!(
+                            "unexpected {full} header for absent or null `{prop}`"
+                        ));
+                    }
+                    (None, Some(_)) => {
+                        return Err(format!("missing {full} header for `{prop}`"));
+                    }
+                    (Some(raw), Some(expected)) => {
+                        let decoded = decode_header_value(raw)
+                            .ok_or_else(|| format!("{full} header is not valid Base64"))?;
+                        if decoded != expected {
+                            return Err(format!(
+                                "{full} header `{decoded}` does not match body value `{expected}`"
+                            ));
+                        }
+                    }
+                }
+            }
+        }
+    }
+    Ok(())
+}
+
+/// Case-insensitive header lookup returning the value as `&str`, if present and valid UTF-8.
+#[cfg(feature = "server-side-http")]
+fn header_str<'a>(headers: &'a http::HeaderMap, name: &str) -> Option<&'a str> {
+    headers.get(name).and_then(|value| value.to_str().ok())
+}
+
+#[cfg(all(test, feature = "client-side-sse", feature = "server-side-http"))]
+mod tests {
+    use std::collections::HashMap;
+
+    use http::{HeaderMap, HeaderName, HeaderValue};
+    use serde_json::json;
+
+    use super::*;
+
+    fn schema_with(properties: serde_json::Value) -> JsonObject {
+        json!({ "type": "object", "properties": properties })
+            .as_object()
+            .unwrap()
+            .clone()
+    }
+
+    fn header_map(pairs: &[(&str, &str)]) -> HeaderMap {
+        let mut map = HeaderMap::new();
+        for (name, value) in pairs {
+            map.insert(
+                HeaderName::from_bytes(name.as_bytes()).unwrap(),
+                HeaderValue::from_str(value).unwrap(),
+            );
+        }
+        map
+    }
+
+    fn assert_wrapped(value: &str) {
+        let encoded = encode_header_value(value);
+        assert!(
+            encoded.starts_with(BASE64_HEADER_PREFIX),
+            "expected {value:?} to be Base64-wrapped, got {encoded:?}"
+        );
+    }
+
+    mod encode_header_value {
+        use super::*;
+
+        #[test]
+        fn passes_plain_ascii_through() {
+            assert_eq!(encode_header_value("us-west1"), "us-west1");
+        }
+
+        #[test]
+        fn passes_internal_spaces_through() {
+            assert_eq!(encode_header_value("a b c"), "a b c");
+        }
+
+        #[test]
+        fn wraps_non_ascii() {
+            assert_wrapped("café");
+        }
+
+        #[test]
+        fn wraps_leading_whitespace() {
+            assert_wrapped(" padded");
+        }
+
+        #[test]
+        fn wraps_trailing_whitespace() {
+            assert_wrapped("trailing ");
+        }
+
+        #[test]
+        fn wraps_control_characters() {
+            assert_wrapped("line1\nline2");
+        }
+
+        #[test]
+        fn wraps_crlf_injection_attempt() {
+            assert_wrapped("a\r\nEvil: 1");
+        }
+
+        #[test]
+        fn wraps_sentinel_collision() {
+            assert_wrapped(&format!("{BASE64_HEADER_PREFIX}x{BASE64_HEADER_SUFFIX}"));
+        }
+    }
+
+    mod decode_header_value {
+        use super::*;
+
+        #[test]
+        fn round_trips_with_encode() {
+            for value in ["us-west1", "café", " padded ", "line1\nline2", "true", "42"] {
+                let encoded = encode_header_value(value);
+                assert_eq!(
+                    decode_header_value(&encoded).as_deref(),
+                    Some(value),
+                    "round-trip failed for {value:?}"
+                );
+            }
+        }
+
+        #[test]
+        fn rejects_invalid_base64() {
+            let bad = format!("{BASE64_HEADER_PREFIX}!!!not-base64!!!{BASE64_HEADER_SUFFIX}");
+            assert_eq!(decode_header_value(&bad), None);
+        }
+    }
+
+    mod extract_name {
+        use super::*;
+
+        #[test]
+        fn from_name_for_tools_call() {
+            let params = json!({ "name": "my_tool" });
+            assert_eq!(
+                extract_name("tools/call", Some(&params)).as_deref(),
+                Some("my_tool")
+            );
+        }
+
+        #[test]
+        fn from_name_for_prompts_get() {
+            let params = json!({ "name": "my_prompt" });
+            assert_eq!(
+                extract_name("prompts/get", Some(&params)).as_deref(),
+                Some("my_prompt")
+            );
+        }
+
+        #[test]
+        fn from_uri_for_resources_read() {
+            let params = json!({ "uri": "file:///x" });
+            assert_eq!(
+                extract_name("resources/read", Some(&params)).as_deref(),
+                Some("file:///x")
+            );
+        }
+
+        #[test]
+        fn none_for_unrelated_method() {
+            let params = json!({ "name": "my_tool" });
+            assert_eq!(extract_name("ping", Some(&params)), None);
+        }
+
+        #[test]
+        fn none_when_params_absent() {
+            assert_eq!(extract_name("tools/call", None), None);
+        }
+    }
+
+    mod validate_param_header_annotations {
+        use super::*;
+
+        #[test]
+        fn accepts_primitive_types() {
+            let schema = schema_with(json!({
+                "region": { "type": "string", "x-mcp-header": "Region" },
+                "count": { "type": "integer", "x-mcp-header": "Count" },
+                "flag": { "type": "boolean", "x-mcp-header": "Flag" },
+            }));
+            assert!(validate_param_header_annotations(&schema).is_ok());
+        }
+
+        #[test]
+        fn rejects_number_type() {
+            let schema = schema_with(json!({ "n": { "type": "number", "x-mcp-header": "N" } }));
+            assert!(validate_param_header_annotations(&schema).is_err());
+        }
+
+        #[test]
+        fn rejects_complex_type() {
+            let schema = schema_with(json!({ "a": { "type": "array", "x-mcp-header": "A" } }));
+            assert!(validate_param_header_annotations(&schema).is_err());
+        }
+
+        #[test]
+        fn rejects_empty_header_name() {
+            let schema = schema_with(json!({ "r": { "type": "string", "x-mcp-header": "" } }));
+            assert!(validate_param_header_annotations(&schema).is_err());
+        }
+
+        #[test]
+        fn rejects_non_token_header_name() {
+            let schema =
+                schema_with(json!({ "r": { "type": "string", "x-mcp-header": "bad:name" } }));
+            assert!(validate_param_header_annotations(&schema).is_err());
+        }
+
+        #[test]
+        fn rejects_case_insensitive_duplicate() {
+            let schema = schema_with(json!({
+                "a": { "type": "string", "x-mcp-header": "Region" },
+                "b": { "type": "string", "x-mcp-header": "region" },
+            }));
+            assert!(validate_param_header_annotations(&schema).is_err());
+        }
+
+        #[test]
+        fn rejects_nested_annotation() {
+            let schema = schema_with(json!({
+                "outer": {
+                    "type": "object",
+                    "properties": { "inner": { "type": "string", "x-mcp-header": "Inner" } }
+                }
+            }));
+            assert!(validate_param_header_annotations(&schema).is_err());
+        }
+    }
+
+    mod standard_request_headers {
+        use super::*;
+
+        fn tools_call_headers() -> HashMap<String, String> {
+            let schema = schema_with(json!({
+                "region": { "type": "string", "x-mcp-header": "Region" },
+            }));
+            let request = json!({
+                "jsonrpc": "2.0", "id": 1, "method": "tools/call",
+                "params": { "name": "deploy", "arguments": { "region": "us-west1" } }
+            });
+            super::super::standard_request_headers(&request, Some(&schema))
+                .into_iter()
+                .map(|(name, value)| (name.as_str().to_owned(), value.to_str().unwrap().to_owned()))
+                .collect()
+        }
+
+        #[test]
+        fn sets_method_header() {
+            assert_eq!(
+                tools_call_headers().get("mcp-method").map(String::as_str),
+                Some("tools/call")
+            );
+        }
+
+        #[test]
+        fn sets_name_header() {
+            assert_eq!(
+                tools_call_headers().get("mcp-name").map(String::as_str),
+                Some("deploy")
+            );
+        }
+
+        #[test]
+        fn sets_annotated_param_header() {
+            assert_eq!(
+                tools_call_headers()
+                    .get("mcp-param-region")
+                    .map(String::as_str),
+                Some("us-west1")
+            );
+        }
+    }
+
+    mod validate_request_headers {
+        use super::*;
+
+        fn tools_call_request() -> Value {
+            json!({
+                "jsonrpc": "2.0", "id": 1, "method": "tools/call",
+                "params": { "name": "deploy" }
+            })
+        }
+
+        #[test]
+        fn accepts_matching_method_and_name() {
+            let headers = header_map(&[("Mcp-Method", "tools/call"), ("Mcp-Name", "deploy")]);
+            assert!(validate_request_headers(&headers, &tools_call_request(), None).is_ok());
+        }
+
+        #[test]
+        fn rejects_method_mismatch() {
+            let headers = header_map(&[("Mcp-Method", "tools/list"), ("Mcp-Name", "deploy")]);
+            assert!(validate_request_headers(&headers, &tools_call_request(), None).is_err());
+        }
+
+        #[test]
+        fn rejects_missing_method() {
+            let headers = header_map(&[("Mcp-Name", "deploy")]);
+            assert!(validate_request_headers(&headers, &tools_call_request(), None).is_err());
+        }
+
+        #[test]
+        fn rejects_name_mismatch() {
+            let headers = header_map(&[("Mcp-Method", "tools/call"), ("Mcp-Name", "other")]);
+            assert!(validate_request_headers(&headers, &tools_call_request(), None).is_err());
+        }
+
+        #[test]
+        fn rejects_missing_name() {
+            let headers = header_map(&[("Mcp-Method", "tools/call")]);
+            assert!(validate_request_headers(&headers, &tools_call_request(), None).is_err());
+        }
+
+        #[test]
+        fn accepts_matching_param() {
+            let schema = schema_with(json!({
+                "region": { "type": "string", "x-mcp-header": "Region" },
+            }));
+            let request = json!({
+                "jsonrpc": "2.0", "id": 1, "method": "tools/call",
+                "params": { "name": "deploy", "arguments": { "region": "us-west1" } }
+            });
+            let headers = header_map(&[
+                ("Mcp-Method", "tools/call"),
+                ("Mcp-Name", "deploy"),
+                ("Mcp-Param-Region", "us-west1"),
+            ]);
+            assert!(validate_request_headers(&headers, &request, Some(&schema)).is_ok());
+        }
+
+        #[test]
+        fn rejects_param_mismatch() {
+            let schema = schema_with(json!({
+                "region": { "type": "string", "x-mcp-header": "Region" },
+            }));
+            let request = json!({
+                "jsonrpc": "2.0", "id": 1, "method": "tools/call",
+                "params": { "name": "deploy", "arguments": { "region": "us-west1" } }
+            });
+            let headers = header_map(&[
+                ("Mcp-Method", "tools/call"),
+                ("Mcp-Name", "deploy"),
+                ("Mcp-Param-Region", "eu-central1"),
+            ]);
+            assert!(validate_request_headers(&headers, &request, Some(&schema)).is_err());
+        }
+    }
+}

crates/rmcp/src/transport/streamable_http_client.rs

@@ -17,17 +17,82 @@ use super::common::client_side_sse::{ExponentialBackoff, SseRetryPolicy, SseStre
 use crate::{
     RoleClient,
     model::{
-        ClientJsonRpcMessage, ClientNotification, ErrorData, InitializedNotification, RequestId,
+        ClientJsonRpcMessage, ClientNotification, ErrorData, InitializedNotification, JsonObject,
-        ServerJsonRpcMessage, ServerResult,
+        ProtocolVersion, RequestId, ServerJsonRpcMessage, ServerResult,
     },
     transport::{
-        common::client_side_sse::SseAutoReconnectStream,
+        common::{client_side_sse::SseAutoReconnectStream, mcp_headers},
         worker::{Worker, WorkerQuitReason, WorkerSendRequest, WorkerTransport},
     },
 };
 
 type BoxedSseStream = BoxStream<'static, Result<Sse, SseError>>;
 
+fn build_request_headers(
+    base: &HashMap<HeaderName, HeaderValue>,
+    message: &ClientJsonRpcMessage,
+    tool_cache: &HashMap<String, Arc<JsonObject>>,
+    version: &ProtocolVersion,
+) -> HashMap<HeaderName, HeaderValue> {
+    use serde_json::Value;
+
+    let mut headers = base.clone();
+    if *version >= ProtocolVersion::STANDARD_HEADERS {
+        if let Ok(value) = serde_json::to_value(message) {
+            let schema = value
+                .get("method")
+                .and_then(Value::as_str)
+                .filter(|method| *method == "tools/call")
+                .and_then(|_| value.get("params"))
+                .and_then(|params| params.get("name"))
+                .and_then(Value::as_str)
+                .and_then(|name| tool_cache.get(name))
+                .map(Arc::as_ref);
+            for (name, val) in mcp_headers::standard_request_headers(&value, schema) {
+                headers.insert(name, val);
+            }
+        }
+    }
+    headers
+}
+
+fn cache_tools_from_response(
+    cache: &mut HashMap<String, Arc<JsonObject>>,
+    message: &ServerJsonRpcMessage,
+) {
+    if let ServerJsonRpcMessage::Response(response) = message {
+        if let ServerResult::ListToolsResult(list) = &response.result {
+            for tool in &list.tools {
+                if let Err(reason) =
+                    mcp_headers::validate_param_header_annotations(&tool.input_schema)
+                {
+                    tracing::warn!(tool = %tool.name, "ignoring x-mcp-header annotations: {reason}");
+                    continue;
+                }
+                cache.insert(tool.name.to_string(), tool.input_schema.clone());
+            }
+        }
+    }
+}
+
+fn negotiate_version_headers(
+    init_response: &ServerJsonRpcMessage,
+    base: HashMap<HeaderName, HeaderValue>,
+) -> (ProtocolVersion, HashMap<HeaderName, HeaderValue>) {
+    let mut version = ProtocolVersion::default();
+    let mut headers = base;
+    if let ServerJsonRpcMessage::Response(response) = init_response {
+        if let ServerResult::InitializeResult(init_result) = &response.result {
+            version = init_result.protocol_version.clone();
+            // HeaderName::from_static requires lowercase
+            if let Ok(hv) = HeaderValue::from_str(init_result.protocol_version.as_str()) {
+                headers.insert(HeaderName::from_static("mcp-protocol-version"), hv);
+            }
+        }
+    }
+    (version, headers)
+}
+
 #[derive(Debug)]
 #[non_exhaustive]
 pub struct AuthRequiredError {
@@ -542,17 +607,8 @@ impl<C: StreamableHttpClient> StreamableHttpClientWorker<C> {
 
         let new_session_id: Option<Arc<str>> = new_session_id_str.map(|s| Arc::from(s.as_str()));
 
-        // Start from custom_headers, then inject the negotiated MCP-Protocol-Version
+        let (negotiated_version, new_protocol_headers) =
-        // so all subsequent requests carry the right version (MCP 2025-06-18 spec).
+            negotiate_version_headers(&init_msg, custom_headers);
-        let mut new_protocol_headers = custom_headers;
-        if let ServerJsonRpcMessage::Response(response) = &init_msg {
-            if let ServerResult::InitializeResult(init_result) = &response.result {
-                if let Ok(hv) = HeaderValue::from_str(init_result.protocol_version.as_str()) {
-                    new_protocol_headers
-                        .insert(HeaderName::from_static("mcp-protocol-version"), hv);
-                }
-            }
-        }
 
         let initialized_notification = ClientJsonRpcMessage::notification(
             ClientNotification::InitializedNotification(InitializedNotification {
@@ -560,13 +616,20 @@ impl<C: StreamableHttpClient> StreamableHttpClientWorker<C> {
                 extensions: Default::default(),
             }),
         );
+        // SEP-2243: notifications carry no Mcp-Param-*, so an empty tool cache suffices.
+        let initialized_headers = build_request_headers(
+            &new_protocol_headers,
+            &initialized_notification,
+            &HashMap::new(),
+            &negotiated_version,
+        );
         client
             .post_message(
                 uri,
                 initialized_notification,
                 new_session_id.clone(),
                 auth_header,
-                new_protocol_headers.clone(),
+                initialized_headers,
             )
             .await?
             .expect_accepted_or_json::<C::Error>()?;
@@ -642,21 +705,11 @@ impl<C: StreamableHttpClient> Worker for StreamableHttpClientWorker<C> {
             }
             None
         };
-        // Extract the negotiated protocol version from the init response
+        let (negotiated_version, mut protocol_headers) =
-        // and build a custom headers map that includes MCP-Protocol-Version
+            negotiate_version_headers(&message, config.custom_headers.clone());
-        // for all subsequent HTTP requests (per MCP 2025-06-18 spec).
+        // SEP-2243: tool input schemas (name -> schema) cached from tools/list responses,
-        let mut protocol_headers = {
+        // used to promote annotated tools/call arguments to Mcp-Param-* headers.
-            let mut headers = config.custom_headers.clone();
+        let mut tool_header_cache: HashMap<String, Arc<JsonObject>> = HashMap::new();
-            if let ServerJsonRpcMessage::Response(response) = &message {
-                if let ServerResult::InitializeResult(init_result) = &response.result {
-                    if let Ok(hv) = HeaderValue::from_str(init_result.protocol_version.as_str()) {
-                        // HeaderName::from_static requires lowercase
-                        headers.insert(HeaderName::from_static("mcp-protocol-version"), hv);
-                    }
-                }
-            }
-            headers
-        };
 
         // Store session info for cleanup when run() exits (not spawned, so cleanup completes before close() returns)
         let mut session_cleanup_info = session_id.as_ref().map(|sid| SessionCleanupInfo {
@@ -670,13 +723,19 @@ impl<C: StreamableHttpClient> Worker for StreamableHttpClientWorker<C> {
         context.send_to_handler(message).await?;
         let initialized_notification = context.recv_from_handler().await?;
         // expect a initialized response
+        let initialized_headers = build_request_headers(
+            &protocol_headers,
+            &initialized_notification.message,
+            &tool_header_cache,
+            &negotiated_version,
+        );
         self.client
             .post_message(
                 config.uri.clone(),
                 initialized_notification.message,
                 session_id.clone(),
                 config.auth_header.clone(),
-                protocol_headers.clone(),
+                initialized_headers,
             )
             .await
             .map_err(WorkerQuitReason::fatal_context(
@@ -788,6 +847,12 @@ impl<C: StreamableHttpClient> Worker for StreamableHttpClientWorker<C> {
                     // Pass a clone to the first attempt so `message` is retained for a
                     // potential re-init retry. `post_message` takes ownership and the
                     // trait cannot be changed, so the clone is unavoidable.
+                    let request_headers = build_request_headers(
+                        &protocol_headers,
+                        &message,
+                        &tool_header_cache,
+                        &negotiated_version,
+                    );
                     let response = self
                         .client
                         .post_message(
@@ -795,7 +860,7 @@ impl<C: StreamableHttpClient> Worker for StreamableHttpClientWorker<C> {
                             message.clone(),
                             session_id.clone(),
                             config.auth_header.clone(),
-                            protocol_headers.clone(),
+                            request_headers,
                         )
                         .await;
                     let send_result = match response {
@@ -908,6 +973,12 @@ impl<C: StreamableHttpClient> Worker for StreamableHttpClientWorker<C> {
                                         });
                                         }
 
+                                        let retry_headers = build_request_headers(
+                                            &protocol_headers,
+                                            &message,
+                                            &tool_header_cache,
+                                            &negotiated_version,
+                                        );
                                         let retry_response = self
                                             .client
                                             .post_message(
@@ -915,7 +986,7 @@ impl<C: StreamableHttpClient> Worker for StreamableHttpClientWorker<C> {
                                                 message,
                                                 session_id.clone(),
                                                 config.auth_header.clone(),
-                                                protocol_headers.clone(),
+                                                retry_headers,
                                             )
                                             .await;
                                         match retry_response {
@@ -931,6 +1002,10 @@ impl<C: StreamableHttpClient> Worker for StreamableHttpClientWorker<C> {
                                                 Ok(())
                                             }
                                             Ok(StreamableHttpPostResponse::Json(msg, ..)) => {
+                                                cache_tools_from_response(
+                                                    &mut tool_header_cache,
+                                                    &msg,
+                                                );
                                                 context.send_to_handler(msg).await?;
                                                 Ok(())
                                             }
@@ -973,6 +1048,7 @@ impl<C: StreamableHttpClient> Worker for StreamableHttpClientWorker<C> {
                             Ok(())
                         }
                         Ok(StreamableHttpPostResponse::Json(message, ..)) => {
+                            cache_tools_from_response(&mut tool_header_cache, &message);
                             context.send_to_handler(message).await?;
                             Ok(())
                         }
@@ -1007,6 +1083,7 @@ impl<C: StreamableHttpClient> Worker for StreamableHttpClientWorker<C> {
                         &mut pending_stream_response_ids,
                         &json_rpc_message,
                     );
+                    cache_tools_from_response(&mut tool_header_cache, &json_rpc_message);
                     // send the message to the handler
                     if let Err(e) = context.send_to_handler(json_rpc_message).await {
                         break 'main_loop Err(e);

crates/rmcp/src/transport/streamable_http_server/tower.rs

@@ -18,7 +18,7 @@ use crate::{
     model::{
         ClientCapabilities, ClientJsonRpcMessage, ClientNotification, ClientRequest, ErrorData,
         GetExtensions, Implementation, InitializeRequest, InitializeRequestParams,
-        InitializedNotification, JsonRpcError, ProtocolVersion, RequestId,
+        InitializedNotification, JsonObject, JsonRpcError, ProtocolVersion, RequestId,
     },
     serve_server,
     service::serve_directly,
@@ -29,6 +29,7 @@ use crate::{
                 EVENT_STREAM_MIME_TYPE, HEADER_LAST_EVENT_ID, HEADER_MCP_PROTOCOL_VERSION,
                 HEADER_SESSION_ID, JSON_MIME_TYPE,
             },
+            mcp_headers,
             server_side_http::{
                 BoxResponse, ServerSseMessage, accepted_response, expect_json,
                 internal_error_response, sse_stream_response, unexpected_message_response,
@@ -260,6 +261,71 @@ fn validate_header_matches_init_body(
     Ok(())
 }
 
+fn header_mismatch_jsonrpc_response(
+    id: Option<RequestId>,
+    message: impl Into<Cow<'static, str>>,
+) -> BoxResponse {
+    let err = JsonRpcError::new(id, ErrorData::header_mismatch(message, None));
+    let body = serde_json::to_vec(&err).expect("serialize JsonRpcError");
+    Response::builder()
+        .status(http::StatusCode::BAD_REQUEST)
+        .header(http::header::CONTENT_TYPE, JSON_MIME_TYPE)
+        .body(Full::new(Bytes::from(body)).boxed())
+        .expect("valid response")
+}
+
+/// Validates SEP-2243 `Mcp-Method` / `Mcp-Name` / `Mcp-Param-*` headers against the body.
+///
+/// Only enforced when the request declares a protocol version `>= STANDARD_HEADERS`.
+/// The `initialize` handshake is exempt: clients emit these headers only after the
+/// version has been negotiated. `tool_schema` supplies the called tool's input schema
+/// so annotated `Mcp-Param-*` headers can be checked (no schema => those are skipped).
+#[expect(
+    clippy::result_large_err,
+    reason = "BoxResponse is intentionally large; matches other handlers in this file"
+)]
+fn validate_standard_headers(
+    headers: &HeaderMap,
+    message: &ClientJsonRpcMessage,
+    tool_schema: impl Fn(&str) -> Option<Arc<JsonObject>>,
+) -> Result<(), BoxResponse> {
+    let version_requires_headers = headers
+        .get(HEADER_MCP_PROTOCOL_VERSION)
+        .and_then(|value| value.to_str().ok())
+        .is_some_and(|version| version >= ProtocolVersion::STANDARD_HEADERS.as_str());
+    if !version_requires_headers {
+        return Ok(());
+    }
+
+    let request_id = match message {
+        ClientJsonRpcMessage::Request(req) => {
+            if matches!(&req.request, ClientRequest::InitializeRequest(_)) {
+                return Ok(());
+            }
+            Some(req.id.clone())
+        }
+        ClientJsonRpcMessage::Notification(_) => None,
+        _ => return Ok(()),
+    };
+
+    let Ok(value) = serde_json::to_value(message) else {
+        return Ok(());
+    };
+    // For tools/call, look up the tool schema so Mcp-Param-* headers are validated.
+    let schema = value
+        .get("method")
+        .and_then(|method| method.as_str())
+        .filter(|method| *method == "tools/call")
+        .and_then(|_| value.get("params"))
+        .and_then(|params| params.get("name"))
+        .and_then(|name| name.as_str())
+        .and_then(tool_schema);
+    if let Err(reason) = mcp_headers::validate_request_headers(headers, &value, schema.as_deref()) {
+        return Err(header_mismatch_jsonrpc_response(request_id, reason));
+    }
+    Ok(())
+}
+
 fn forbidden_response(message: impl Into<String>) -> BoxResponse {
     Response::builder()
         .status(http::StatusCode::FORBIDDEN)
@@ -555,6 +621,10 @@ pub struct StreamableHttpService<S, M> {
     pending_restores: Option<
         Arc<tokio::sync::RwLock<HashMap<SessionId, tokio::sync::watch::Sender<Option<bool>>>>>,
     >,
+    /// Caches tool input schemas by name for SEP-2243 `Mcp-Param-*` validation.
+    /// Populated lazily via `get_tool` so the service factory runs at most once
+    /// per tool name. `None` value means the tool exposes no schema.
+    tool_schemas: Arc<std::sync::RwLock<HashMap<String, Option<Arc<JsonObject>>>>>,
 }
 
 impl<S, M> Clone for StreamableHttpService<S, M> {
@@ -564,6 +634,7 @@ impl<S, M> Clone for StreamableHttpService<S, M> {
             session_manager: self.session_manager.clone(),
             service_factory: self.service_factory.clone(),
             pending_restores: self.pending_restores.clone(),
+            tool_schemas: self.tool_schemas.clone(),
         }
     }
 }
@@ -571,7 +642,7 @@ impl<S, M> Clone for StreamableHttpService<S, M> {
 impl<RequestBody, S, M> tower_service::Service<Request<RequestBody>> for StreamableHttpService<S, M>
 where
     RequestBody: Body + Send + 'static,
-    S: crate::Service<RoleServer> + Send + 'static,
+    S: crate::ServerHandler + Send + 'static,
     M: SessionManager,
     RequestBody::Error: Display,
     RequestBody::Data: Send + 'static,
@@ -625,7 +696,7 @@ impl Drop for PendingRestoreGuard {
 
 impl<S, M> StreamableHttpService<S, M>
 where
-    S: crate::Service<RoleServer> + Send + 'static,
+    S: crate::ServerHandler + Send + 'static,
     M: SessionManager,
 {
     pub fn new(
@@ -644,12 +715,33 @@ where
             session_manager,
             service_factory: Arc::new(service_factory),
             pending_restores,
+            tool_schemas: Arc::new(std::sync::RwLock::new(HashMap::new())),
         }
     }
     fn get_service(&self) -> Result<S, std::io::Error> {
         (self.service_factory)()
     }
 
+    /// Returns the cached input schema for `name`, constructing a service once
+    /// per name to read its `ServerHandler::get_tool` definition. Used to
+    /// validate SEP-2243 `Mcp-Param-*` headers against the request body.
+    fn tool_schema(&self, name: &str) -> Option<Arc<JsonObject>> {
+        if let Ok(cache) = self.tool_schemas.read() {
+            if let Some(schema) = cache.get(name) {
+                return schema.clone();
+            }
+        }
+        let schema = self
+            .get_service()
+            .ok()
+            .and_then(|service| service.get_tool(name))
+            .map(|tool| tool.input_schema);
+        if let Ok(mut cache) = self.tool_schemas.write() {
+            cache.insert(name.to_owned(), schema.clone());
+        }
+        schema
+    }
+
     /// Spawn a task that runs `serve_server` for the given session, waits for
     /// it to finish, and then calls `close_session`.
     ///
@@ -664,7 +756,7 @@ where
         transport: M::Transport,
         init_done_tx: Option<tokio::sync::oneshot::Sender<()>>,
     ) where
-        S: crate::Service<RoleServer> + Send + 'static,
+        S: crate::ServerHandler + Send + 'static,
         M: SessionManager,
     {
         tokio::spawn(async move {
@@ -707,7 +799,7 @@ where
         parts: &http::request::Parts,
     ) -> Result<bool, std::io::Error>
     where
-        S: crate::Service<RoleServer> + Send + 'static,
+        S: crate::ServerHandler + Send + 'static,
         M: SessionManager,
     {
         // Both fields are Some iff a session store is configured.
@@ -1083,6 +1175,8 @@ where
 
                 // Validate MCP-Protocol-Version header (per 2025-06-18 spec)
                 validate_protocol_version_header(&part.headers)?;
+                // Validate SEP-2243 standard headers against the body
+                validate_standard_headers(&part.headers, &message, |name| self.tool_schema(name))?;
 
                 // inject request part to extensions
                 match &mut message {
@@ -1235,6 +1329,8 @@ where
                     validate_protocol_version_header(&part.headers)?;
                 }
             }
+            // Validate SEP-2243 standard headers against the body
+            validate_standard_headers(&part.headers, &message, |name| self.tool_schema(name))?;
             let service = self
                 .get_service()
                 .map_err(internal_error_response("get service"))?;

crates/rmcp/tests/test_streamable_http_standard_headers.rs

@@ -0,0 +1,292 @@
+#![cfg(not(feature = "local"))]
+//! SEP-2243 server-side validation of `Mcp-Method` / `Mcp-Name` / `Mcp-Param-*` headers.
+use std::sync::Arc;
+
+use rmcp::{
+    ServerHandler,
+    model::{ServerCapabilities, ServerInfo, Tool},
+    transport::streamable_http_server::{
+        StreamableHttpServerConfig, StreamableHttpService, session::local::LocalSessionManager,
+    },
+};
+use tokio_util::sync::CancellationToken;
+
+const SEP_VERSION: &str = "2026-07-28";
+
+/// Server exposing one tool whose `region` argument is promoted to `Mcp-Param-Region`.
+#[derive(Clone, Default)]
+struct HeaderValidationServer;
+
+impl ServerHandler for HeaderValidationServer {
+    fn get_info(&self) -> ServerInfo {
+        ServerInfo::new(ServerCapabilities::builder().enable_tools().build())
+    }
+
+    fn get_tool(&self, name: &str) -> Option<Tool> {
+        if name != "deploy" {
+            return None;
+        }
+        let schema = serde_json::json!({
+            "type": "object",
+            "properties": { "region": { "type": "string", "x-mcp-header": "Region" } }
+        });
+        let schema = schema.as_object().expect("object schema").clone();
+        Some(Tool::new("deploy", "deploy a thing", Arc::new(schema)))
+    }
+}
+
+async fn spawn_server() -> (reqwest::Client, String, CancellationToken) {
+    let config = StreamableHttpServerConfig::default()
+        .with_stateful_mode(false)
+        .with_json_response(true)
+        .with_sse_keep_alive(None)
+        .with_cancellation_token(CancellationToken::new());
+    let ct = config.cancellation_token.clone();
+    let service: StreamableHttpService<HeaderValidationServer, LocalSessionManager> =
+        StreamableHttpService::new(|| Ok(HeaderValidationServer), Default::default(), config);
+
+    let router = axum::Router::new().nest_service("/mcp", service);
+    let tcp_listener = tokio::net::TcpListener::bind("127.0.0.1:0").await.unwrap();
+    let addr = tcp_listener.local_addr().unwrap();
+    tokio::spawn({
+        let ct = ct.clone();
+        async move {
+            let _ = axum::serve(tcp_listener, router)
+                .with_graceful_shutdown(async move { ct.cancelled_owned().await })
+                .await;
+        }
+    });
+    (reqwest::Client::new(), format!("http://{addr}/mcp"), ct)
+}
+
+/// POSTs a `tools/call` with the given protocol-version and optional SEP-2243 headers.
+async fn post_tool_call(
+    client: &reqwest::Client,
+    url: &str,
+    version: &str,
+    tool_name: &str,
+    arguments: serde_json::Value,
+    mcp_method: Option<&str>,
+    mcp_name: Option<&str>,
+    param_region: Option<&str>,
+) -> reqwest::Response {
+    let body = serde_json::json!({
+        "jsonrpc": "2.0",
+        "id": 1,
+        "method": "tools/call",
+        "params": {
+            "name": tool_name,
+            "arguments": arguments,
+        }
+    });
+    let mut req = client
+        .post(url)
+        .header("Content-Type", "application/json")
+        .header("Accept", "application/json, text/event-stream")
+        .header("MCP-Protocol-Version", version)
+        .body(body.to_string());
+    if let Some(method) = mcp_method {
+        req = req.header("Mcp-Method", method);
+    }
+    if let Some(name) = mcp_name {
+        req = req.header("Mcp-Name", name);
+    }
+    if let Some(region) = param_region {
+        req = req.header("Mcp-Param-Region", region);
+    }
+    req.send().await.expect("send tools/call request")
+}
+
+#[tokio::test]
+async fn accepts_matching_standard_headers() -> anyhow::Result<()> {
+    let (client, url, ct) = spawn_server().await;
+
+    // Matching headers pass validation and reach dispatch. (Stateless mode without a
+    // prior initialize yields an unrelated -32601, which still proves -32020 was not raised.)
+    let response = post_tool_call(
+        &client,
+        &url,
+        SEP_VERSION,
+        "sum",
+        serde_json::json!({ "a": 1, "b": 2 }),
+        Some("tools/call"),
+        Some("sum"),
+        None,
+    )
+    .await;
+    let body: serde_json::Value = response.json().await?;
+    assert_ne!(
+        body["error"]["code"], -32020,
+        "matching headers must not be rejected as a header mismatch, got: {body}"
+    );
+
+    ct.cancel();
+    Ok(())
+}
+
+#[tokio::test]
+async fn rejects_method_mismatch_with_32020() -> anyhow::Result<()> {
+    let (client, url, ct) = spawn_server().await;
+
+    let response = post_tool_call(
+        &client,
+        &url,
+        SEP_VERSION,
+        "sum",
+        serde_json::json!({ "a": 1, "b": 2 }),
+        Some("tools/list"),
+        Some("sum"),
+        None,
+    )
+    .await;
+    assert_eq!(response.status(), 400);
+    let body: serde_json::Value = response.json().await?;
+    assert_eq!(body["error"]["code"], -32020);
+
+    ct.cancel();
+    Ok(())
+}
+
+#[tokio::test]
+async fn rejects_missing_method_header_with_32020() -> anyhow::Result<()> {
+    let (client, url, ct) = spawn_server().await;
+
+    let response = post_tool_call(
+        &client,
+        &url,
+        SEP_VERSION,
+        "sum",
+        serde_json::json!({ "a": 1, "b": 2 }),
+        None,
+        Some("sum"),
+        None,
+    )
+    .await;
+    assert_eq!(response.status(), 400);
+    let body: serde_json::Value = response.json().await?;
+    assert_eq!(body["error"]["code"], -32020);
+
+    ct.cancel();
+    Ok(())
+}
+
+#[tokio::test]
+async fn rejects_name_mismatch_with_32020() -> anyhow::Result<()> {
+    let (client, url, ct) = spawn_server().await;
+
+    let response = post_tool_call(
+        &client,
+        &url,
+        SEP_VERSION,
+        "sum",
+        serde_json::json!({ "a": 1, "b": 2 }),
+        Some("tools/call"),
+        Some("product"),
+        None,
+    )
+    .await;
+    assert_eq!(response.status(), 400);
+    let body: serde_json::Value = response.json().await?;
+    assert_eq!(body["error"]["code"], -32020);
+
+    ct.cancel();
+    Ok(())
+}
+
+#[tokio::test]
+async fn skips_validation_for_pre_sep_version() -> anyhow::Result<()> {
+    let (client, url, ct) = spawn_server().await;
+
+    // Older version: headers are not enforced even when absent.
+    let response = post_tool_call(
+        &client,
+        &url,
+        "2025-11-25",
+        "sum",
+        serde_json::json!({ "a": 1, "b": 2 }),
+        None,
+        None,
+        None,
+    )
+    .await;
+    let body: serde_json::Value = response.json().await?;
+    assert_ne!(
+        body["error"]["code"], -32020,
+        "pre-SEP versions must skip header validation, got: {body}"
+    );
+
+    ct.cancel();
+    Ok(())
+}
+
+#[tokio::test]
+async fn accepts_matching_param_header() -> anyhow::Result<()> {
+    let (client, url, ct) = spawn_server().await;
+
+    let response = post_tool_call(
+        &client,
+        &url,
+        SEP_VERSION,
+        "deploy",
+        serde_json::json!({ "region": "us-west1" }),
+        Some("tools/call"),
+        Some("deploy"),
+        Some("us-west1"),
+    )
+    .await;
+    let body: serde_json::Value = response.json().await?;
+    assert_ne!(
+        body["error"]["code"], -32020,
+        "matching Mcp-Param-* must not be rejected, got: {body}"
+    );
+
+    ct.cancel();
+    Ok(())
+}
+
+#[tokio::test]
+async fn rejects_param_mismatch_with_32020() -> anyhow::Result<()> {
+    let (client, url, ct) = spawn_server().await;
+
+    let response = post_tool_call(
+        &client,
+        &url,
+        SEP_VERSION,
+        "deploy",
+        serde_json::json!({ "region": "us-west1" }),
+        Some("tools/call"),
+        Some("deploy"),
+        Some("eu-central1"),
+    )
+    .await;
+    assert_eq!(response.status(), 400);
+    let body: serde_json::Value = response.json().await?;
+    assert_eq!(body["error"]["code"], -32020);
+
+    ct.cancel();
+    Ok(())
+}
+
+#[tokio::test]
+async fn rejects_missing_param_header_with_32020() -> anyhow::Result<()> {
+    let (client, url, ct) = spawn_server().await;
+
+    // `region` argument is present but the annotated `Mcp-Param-Region` header is absent.
+    let response = post_tool_call(
+        &client,
+        &url,
+        SEP_VERSION,
+        "deploy",
+        serde_json::json!({ "region": "us-west1" }),
+        Some("tools/call"),
+        Some("deploy"),
+        None,
+    )
+    .await;
+    assert_eq!(response.status(), 400);
+    let body: serde_json::Value = response.json().await?;
+    assert_eq!(body["error"]["code"], -32020);
+
+    ct.cancel();
+    Ok(())
+}