diff --git a/README.md b/README.md
index da7019f3..79d7232a 100644
--- a/README.md
+++ b/README.md
@@ -1462,6 +1462,13 @@ let router = axum::Router::new().nest_service("/mcp", service);
 > Because there is no per-session state, the `service_factory` runs per request.
 > Keep shared state (DB pools, caches) in a `Clone` handle captured by the
 > closure; don't rely on in-memory state surviving between requests.
+>
+> Modern-only servers can additionally call
+> `with_stateless_protocol_metadata_required(true)` to reject the compatibility
+> fallback for requests missing their per-request protocol signals. rmcp clients
+> negotiated below `2026-07-28` do not attach that body metadata and will be
+> rejected, so pair this option with a `supported_protocol_versions`
+> implementation that advertises only `2026-07-28` and later.
 
 ### Client-side
 
diff --git a/crates/rmcp/src/transport/streamable_http_server/tower.rs b/crates/rmcp/src/transport/streamable_http_server/tower.rs
index f574ea99..f1fef585 100644
--- a/crates/rmcp/src/transport/streamable_http_server/tower.rs
+++ b/crates/rmcp/src/transport/streamable_http_server/tower.rs
@@ -127,6 +127,32 @@ pub struct StreamableHttpServerConfig {
     /// chunked transfer encoding, or HTTP version. Oversized payloads receive
     /// a `413 Payload Too Large` response.
     pub max_request_body_bytes: usize,
+    /// Require stateless JSON-RPC request POSTs to carry per-request protocol
+    /// signals before handler dispatch.
+    ///
+    /// Non-initialize requests must carry `MCP-Protocol-Version`; ordinary
+    /// non-discovery requests must also carry
+    /// `_meta.io.modelcontextprotocol/protocolVersion`. `server/discover`
+    /// retains its existing request-metadata validation. For `2026-07-28`
+    /// requests, the server handler continues to require the remaining
+    /// per-request metadata, including `clientCapabilities`. Initialize,
+    /// notifications, and other message kinds retain their existing rules.
+    ///
+    /// This option applies to requests routed statelessly. Set
+    /// `legacy_session_mode` to `false` to ensure every request uses that path.
+    /// Legacy session routing and its error precedence remain unchanged.
+    ///
+    /// The validator checks metadata presence rather than applying a version
+    /// allowlist. However, rmcp clients negotiated below `2026-07-28` do not
+    /// attach per-request protocol metadata, so enabling this option rejects
+    /// their ordinary requests. Servers using this option should normally
+    /// override
+    /// [`ServerHandler::supported_protocol_versions`](crate::ServerHandler::supported_protocol_versions)
+    /// to advertise only `2026-07-28` and later.
+    ///
+    /// Default is `false`, preserving today's legacy behavior where an absent
+    /// header is treated as protocol version `2025-03-26`.
+    pub stateless_protocol_metadata_required: bool,
 }
 
 impl std::fmt::Debug for dyn SessionStore {
@@ -147,6 +173,7 @@ impl Default for StreamableHttpServerConfig {
             allowed_origins: vec![],
             session_store: None,
             max_request_body_bytes: DEFAULT_MAX_REQUEST_BODY_BYTES,
+            stateless_protocol_metadata_required: false,
         }
     }
 }
@@ -206,6 +233,18 @@ impl StreamableHttpServerConfig {
         self.max_request_body_bytes = bytes;
         self
     }
+
+    /// Require per-request protocol signals on stateless JSON-RPC request
+    /// POSTs.
+    ///
+    /// See [`StreamableHttpServerConfig::stateless_protocol_metadata_required`].
+    pub fn with_stateless_protocol_metadata_required(
+        mut self,
+        stateless_protocol_metadata_required: bool,
+    ) -> Self {
+        self.stateless_protocol_metadata_required = stateless_protocol_metadata_required;
+        self
+    }
 }
 
 #[expect(
@@ -504,6 +543,77 @@ fn validate_request_protocol_version_meta(
     Ok(())
 }
 
+/// When `stateless_protocol_metadata_required` is enabled in stateless mode,
+/// every non-initialize Streamable HTTP JSON-RPC request POST must carry the
+/// `MCP-Protocol-Version` HTTP header. A missing header is rejected with
+/// HTTP 400 / JSON-RPC `-32020` before handler dispatch. `server/discover`
+/// is included so the seam aligns with the per-POST header contract; its
+/// body-metadata rule is preserved unchanged.
+#[expect(
+    clippy::result_large_err,
+    reason = "BoxResponse is intentionally large; matches other handlers in this file"
+)]
+fn validate_required_protocol_header(
+    config: &StreamableHttpServerConfig,
+    headers: &HeaderMap,
+    message: &ClientJsonRpcMessage,
+) -> Result<(), BoxResponse> {
+    if !config.stateless_protocol_metadata_required {
+        return Ok(());
+    }
+    let ClientJsonRpcMessage::Request(request) = message else {
+        // Notifications, response messages, and error messages are exempt.
+        return Ok(());
+    };
+    if matches!(&request.request, ClientRequest::InitializeRequest(_)) {
+        // Initialize keeps its own header-matching rule.
+        return Ok(());
+    }
+    if headers.contains_key(HEADER_MCP_PROTOCOL_VERSION) {
+        return Ok(());
+    }
+    Err(header_mismatch_jsonrpc_response(
+        Some(request.id.clone()),
+        "Missing MCP-Protocol-Version header for request requiring per-request protocol metadata",
+    ))
+}
+
+/// When `stateless_protocol_metadata_required` is enabled in stateless mode,
+/// every non-initialize, non-discover Streamable HTTP JSON-RPC request must
+/// carry `io.modelcontextprotocol/protocolVersion` in `_meta`. A missing entry
+/// is rejected with HTTP 400 / JSON-RPC `-32602` (invalid_params). `initialize`,
+/// `server/discover` (whose body-metadata rule is already enforced by
+/// `validate_request_protocol_version_meta`), notifications, and other message
+/// kinds are exempt.
+#[expect(
+    clippy::result_large_err,
+    reason = "BoxResponse is intentionally large; matches other handlers in this file"
+)]
+fn validate_required_protocol_meta(
+    config: &StreamableHttpServerConfig,
+    message: &ClientJsonRpcMessage,
+) -> Result<(), BoxResponse> {
+    if !config.stateless_protocol_metadata_required {
+        return Ok(());
+    }
+    let ClientJsonRpcMessage::Request(request) = message else {
+        return Ok(());
+    };
+    if matches!(
+        &request.request,
+        ClientRequest::InitializeRequest(_) | ClientRequest::DiscoverRequest(_)
+    ) {
+        return Ok(());
+    }
+    if request.request.get_meta().protocol_version().is_some() {
+        return Ok(());
+    }
+    Err(invalid_params_jsonrpc_response(
+        Some(request.id.clone()),
+        "Invalid params: request requires protocolVersion in request _meta",
+    ))
+}
+
 fn jsonrpc_http_status(message: &ServerJsonRpcMessage) -> http::StatusCode {
     let ServerJsonRpcMessage::Error(error) = message else {
         return http::StatusCode::OK;
@@ -1809,6 +1919,10 @@ where
             // Stateless mode:
             // - on initialize: the header (if present) must match `params.protocolVersion`
             // - on every other request: the header must name a known version.
+            //
+            // The opt-in seam applies only here so legacy session routing and
+            // its error precedence remain unchanged.
+            validate_required_protocol_header(&self.config, &part.headers, &message)?;
             let has_per_request_version = message_has_per_request_protocol_version(&message);
             match &message {
                 ClientJsonRpcMessage::Request(req) => {
@@ -1829,6 +1943,7 @@ where
             // Validate SEP-2243 standard headers against the body
             validate_standard_headers(&part.headers, &message, |name| self.tool_schema(name))?;
             validate_request_protocol_version_meta(&part.headers, &message)?;
+            validate_required_protocol_meta(&self.config, &message)?;
             let service = self
                 .get_service()
                 .map_err(internal_error_response("get service"))?;
diff --git a/crates/rmcp/tests/test_streamable_http_protocol_version.rs b/crates/rmcp/tests/test_streamable_http_protocol_version.rs
index f1bf1912..fcbeb41a 100644
--- a/crates/rmcp/tests/test_streamable_http_protocol_version.rs
+++ b/crates/rmcp/tests/test_streamable_http_protocol_version.rs
@@ -1,9 +1,15 @@
 #![cfg(not(feature = "local"))]
 //! Streamable HTTP protocol-version and request-metadata validation tests.
-use std::sync::Arc;
+use std::sync::{
+    Arc,
+    atomic::{AtomicUsize, Ordering},
+};
 
-use rmcp::transport::streamable_http_server::{
-    StreamableHttpServerConfig, StreamableHttpService, session::local::LocalSessionManager,
+use rmcp::{
+    ServerHandler,
+    transport::streamable_http_server::{
+        StreamableHttpServerConfig, StreamableHttpService, session::local::LocalSessionManager,
+    },
 };
 use serde_json::{Value, json};
 use tokio_util::sync::CancellationToken;
@@ -380,3 +386,468 @@ async fn stateless_request_accepts_missing_optional_meta_client_info() {
     assert_eq!(response.status(), 200);
     ct.cancel();
 }
+
+// ---------------------------------------------------------------------------
+// Opt-in seam: `with_stateless_protocol_metadata_required(true)`
+// ---------------------------------------------------------------------------
+// In stateless mode, every non-initialize Streamable HTTP JSON-RPC request
+// POST must carry the `MCP-Protocol-Version` HTTP header (missing → HTTP 400 /
+// JSON-RPC `-32020` HeaderMismatch before dispatch). Every non-initialize,
+// non-discover request must additionally carry a per-request
+// `io.modelcontextprotocol/protocolVersion` in `_meta` (missing → HTTP 400 /
+// JSON-RPC `-32602`). The existing rule that requires `server/discover` to
+// carry `_meta.protocolVersion` is preserved unchanged.
+//
+// Non-dispatch is proven with an explicit invocation counter (`AtomicUsize`).
+
+#[derive(Clone)]
+struct CountingServer {
+    lists: Arc<AtomicUsize>,
+}
+
+impl CountingServer {
+    fn new() -> (Self, Arc<AtomicUsize>) {
+        let lists = Arc::new(AtomicUsize::new(0));
+        (
+            Self {
+                lists: lists.clone(),
+            },
+            lists,
+        )
+    }
+}
+
+impl ServerHandler for CountingServer {
+    fn get_info(&self) -> rmcp::model::ServerInfo {
+        rmcp::model::ServerInfo::new(
+            rmcp::model::ServerCapabilities::builder()
+                .enable_tools()
+                .build(),
+        )
+    }
+
+    fn list_tools(
+        &self,
+        _request: Option<rmcp::model::PaginatedRequestParams>,
+        _context: rmcp::service::RequestContext<rmcp::RoleServer>,
+    ) -> impl std::future::Future<Output = Result<rmcp::model::ListToolsResult, rmcp::ErrorData>>
+    + Send
+    + '_ {
+        self.lists.fetch_add(1, Ordering::SeqCst);
+        std::future::ready(Ok(rmcp::model::ListToolsResult::default()))
+    }
+}
+
+async fn spawn_counting(
+    config: StreamableHttpServerConfig,
+) -> (reqwest::Client, String, CancellationToken, Arc<AtomicUsize>) {
+    let ct = config.cancellation_token.clone();
+    let (server, lists) = CountingServer::new();
+    let service: StreamableHttpService<CountingServer, LocalSessionManager> =
+        StreamableHttpService::new(move || Ok(server.clone()), 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,
+        lists,
+    )
+}
+
+fn modern_required_config() -> StreamableHttpServerConfig {
+    StreamableHttpServerConfig::default()
+        .with_legacy_session_mode(false)
+        .with_json_response(true)
+        .with_sse_keep_alive(None)
+        .with_stateless_protocol_metadata_required(true)
+        .with_cancellation_token(CancellationToken::new())
+}
+
+async fn post_seam(
+    client: &reqwest::Client,
+    url: &str,
+    body: &str,
+    header_version: Option<&str>,
+    extra_headers: &[(&str, &str)],
+) -> reqwest::Response {
+    let mut req = client
+        .post(url)
+        .header("Content-Type", "application/json")
+        .header("Accept", "application/json, text/event-stream")
+        .body(body.to_owned());
+    if let Some(h) = header_version {
+        req = req.header("MCP-Protocol-Version", h);
+    }
+    for (k, v) in extra_headers {
+        req = req.header(*k, *v);
+    }
+    req.send().await.expect("send request")
+}
+
+// With the seam disabled, explicit stateless mode still dispatches a request
+// without protocol metadata, preserving backwards compatibility.
+#[tokio::test]
+async fn seam_disabled_preserves_stateless_compatibility() -> anyhow::Result<()> {
+    let (client, url, ct, lists) = spawn_counting(stateless_json_config()).await;
+
+    let body = r#"{"jsonrpc":"2.0","id":1,"method":"tools/list","params":{}}"#;
+    let response = post_seam(&client, &url, body, None, &[]).await;
+    assert_eq!(response.status(), 200);
+    assert_eq!(
+        lists.load(Ordering::SeqCst),
+        1,
+        "seam-disabled stateless config must dispatch exactly once"
+    );
+
+    ct.cancel();
+    Ok(())
+}
+
+// With legacy compatibility enabled, the seam applies only to requests that
+// are routed statelessly: an established legacy session remains unchanged,
+// while a self-identifying modern request is still enforced.
+#[tokio::test]
+async fn seam_mixed_mode_enforces_only_stateless_routed_requests() -> anyhow::Result<()> {
+    let config = StreamableHttpServerConfig::default()
+        .with_legacy_session_mode(true)
+        .with_json_response(true)
+        .with_sse_keep_alive(None)
+        .with_stateless_protocol_metadata_required(true)
+        .with_cancellation_token(CancellationToken::new());
+    let (client, url, ct, lists) = spawn_counting(config).await;
+
+    let legacy_body = r#"{"jsonrpc":"2.0","id":2,"method":"tools/list","params":{}}"#;
+    let unknown_response = post_seam(
+        &client,
+        &url,
+        legacy_body,
+        None,
+        &[("Mcp-Session-Id", "unknown-session")],
+    )
+    .await;
+    assert_eq!(
+        unknown_response.status(),
+        reqwest::StatusCode::NOT_FOUND,
+        "the stateless seam must not shadow the legacy unknown-session boundary"
+    );
+    assert_eq!(lists.load(Ordering::SeqCst), 0);
+
+    let initialize = post_init(&client, &url, None, "2025-11-25").await;
+    assert_eq!(initialize.status(), 200);
+    let session_id = initialize
+        .headers()
+        .get("Mcp-Session-Id")
+        .and_then(|value| value.to_str().ok())
+        .expect("legacy initialize response must include a session id")
+        .to_owned();
+
+    let legacy_response = post_seam(
+        &client,
+        &url,
+        legacy_body,
+        None,
+        &[("Mcp-Session-Id", &session_id)],
+    )
+    .await;
+    assert_eq!(legacy_response.status(), 200);
+    let _ = legacy_response.bytes().await?;
+    assert_eq!(
+        lists.load(Ordering::SeqCst),
+        1,
+        "legacy session routing must remain unchanged"
+    );
+
+    let delete_response = client
+        .delete(&url)
+        .header("Mcp-Session-Id", &session_id)
+        .send()
+        .await?;
+    assert_eq!(delete_response.status(), reqwest::StatusCode::ACCEPTED);
+
+    let terminated_response = post_seam(
+        &client,
+        &url,
+        legacy_body,
+        None,
+        &[("Mcp-Session-Id", &session_id)],
+    )
+    .await;
+    assert_eq!(
+        terminated_response.status(),
+        reqwest::StatusCode::NOT_FOUND,
+        "the stateless seam must not shadow the legacy terminated-session boundary"
+    );
+    assert_eq!(lists.load(Ordering::SeqCst), 1);
+
+    let modern_body = r#"{"jsonrpc":"2.0","id":3,"method":"tools/list","params":{}}"#;
+    let modern_response = post_seam(
+        &client,
+        &url,
+        modern_body,
+        Some("2026-07-28"),
+        &[("Mcp-Method", "tools/list")],
+    )
+    .await;
+    assert_eq!(modern_response.status(), 400);
+    let payload: serde_json::Value = modern_response.json().await?;
+    assert_eq!(payload["error"]["code"], -32602);
+    assert_eq!(
+        lists.load(Ordering::SeqCst),
+        1,
+        "stateless metadata rejection must happen before another handler dispatch"
+    );
+
+    ct.cancel();
+    Ok(())
+}
+
+// Both signals absent → missing header / -32020 before dispatch.
+#[tokio::test]
+async fn seam_opt_in_rejects_missing_header_before_dispatch() -> anyhow::Result<()> {
+    let (client, url, ct, lists) = spawn_counting(modern_required_config()).await;
+
+    let without_meta = r#"{"jsonrpc":"2.0","id":2,"method":"tools/list","params":{}}"#;
+    let response = post_seam(&client, &url, without_meta, None, &[]).await;
+    assert_eq!(response.status(), 400, "missing header must yield HTTP 400");
+    let payload: serde_json::Value = response.json().await?;
+    assert_eq!(payload["error"]["code"], -32020);
+    assert_eq!(
+        lists.load(Ordering::SeqCst),
+        0,
+        "handler invocation counter must stay at 0 when the header is missing"
+    );
+
+    ct.cancel();
+    Ok(())
+}
+
+// Header present but `_meta.protocolVersion` absent → -32602 before
+// dispatch. Counter must equal 0.
+#[tokio::test]
+async fn seam_opt_in_rejects_missing_meta_before_dispatch() -> anyhow::Result<()> {
+    let (client, url, ct, lists) = spawn_counting(modern_required_config()).await;
+
+    let body = r#"{"jsonrpc":"2.0","id":4,"method":"tools/list","params":{}}"#;
+    let response = post_seam(
+        &client,
+        &url,
+        body,
+        Some("2026-07-28"),
+        &[("Mcp-Method", "tools/list")],
+    )
+    .await;
+    assert_eq!(response.status(), 400);
+    let payload: serde_json::Value = response.json().await?;
+    assert_eq!(payload["error"]["code"], -32602);
+    assert_eq!(
+        lists.load(Ordering::SeqCst),
+        0,
+        "handler invocation counter must stay at 0 when _meta.protocolVersion is missing"
+    );
+
+    ct.cancel();
+    Ok(())
+}
+
+// A 2026 request with protocolVersion but no clientCapabilities reaches the
+// existing inline-metadata validator and is rejected before method dispatch.
+#[tokio::test]
+async fn seam_opt_in_rejects_missing_client_capabilities_before_dispatch() -> anyhow::Result<()> {
+    let (client, url, ct, lists) = spawn_counting(modern_required_config()).await;
+
+    let body = r#"{"jsonrpc":"2.0","id":5,"method":"tools/list","params":{"_meta":{"io.modelcontextprotocol/protocolVersion":"2026-07-28"}}}"#;
+    let response = post_seam(
+        &client,
+        &url,
+        body,
+        Some("2026-07-28"),
+        &[("Mcp-Method", "tools/list")],
+    )
+    .await;
+    assert_eq!(response.status(), 400);
+    let payload: serde_json::Value = response.json().await?;
+    assert_eq!(payload["error"]["code"], -32602);
+    assert_eq!(
+        lists.load(Ordering::SeqCst),
+        0,
+        "handler invocation counter must stay at 0 when clientCapabilities is missing"
+    );
+
+    ct.cancel();
+    Ok(())
+}
+
+// Both signals present at the current version, plus the routing header
+// required by `validate_standard_headers` for `>= STANDARD_HEADERS`. The
+// seam must dispatch and the handler must run exactly once.
+#[tokio::test]
+async fn seam_opt_in_dispatches_when_header_and_meta_present() -> anyhow::Result<()> {
+    let (client, url, ct, lists) = spawn_counting(modern_required_config()).await;
+
+    let body = r#"{"jsonrpc":"2.0","id":6,"method":"tools/list","params":{"_meta":{"io.modelcontextprotocol/protocolVersion":"2026-07-28","io.modelcontextprotocol/clientInfo":{"name":"test","version":"1.0"},"io.modelcontextprotocol/clientCapabilities":{}}}}"#;
+    let response = post_seam(
+        &client,
+        &url,
+        body,
+        Some("2026-07-28"),
+        &[("Mcp-Method", "tools/list")],
+    )
+    .await;
+    assert_eq!(response.status(), 200);
+    let payload: serde_json::Value = response.json().await?;
+    assert!(payload.get("result").is_some());
+    assert_eq!(
+        lists.load(Ordering::SeqCst),
+        1,
+        "handler invocation counter must equal 1 on successful dispatch"
+    );
+
+    ct.cancel();
+    Ok(())
+}
+
+// rmcp clients negotiated below 2026-07-28 send the protocol header but do
+// not attach per-request protocol metadata. Requiring the metadata therefore
+// rejects their real request shape even if the handler supports that version.
+#[tokio::test]
+async fn seam_opt_in_rejects_older_rmcp_client_request_shape() -> anyhow::Result<()> {
+    let (client, url, ct, lists) = spawn_counting(modern_required_config()).await;
+
+    let body = r#"{"jsonrpc":"2.0","id":7,"method":"tools/list","params":{}}"#;
+    let response = post_seam(&client, &url, body, Some("2025-11-25"), &[]).await;
+    assert_eq!(response.status(), 400);
+    let payload: serde_json::Value = response.json().await?;
+    assert_eq!(payload["error"]["code"], -32602);
+    assert_eq!(
+        lists.load(Ordering::SeqCst),
+        0,
+        "an older client request without per-request metadata must not dispatch"
+    );
+
+    ct.cancel();
+    Ok(())
+}
+
+// When the 2026+ header is present but `Mcp-Method` is missing, the
+// existing `validate_standard_headers` rule must fire first with -32020,
+// before the seam's -32602 missing-meta check.
+#[tokio::test]
+async fn seam_opt_in_preserves_standard_header_precedence() -> anyhow::Result<()> {
+    let (client, url, ct, lists) = spawn_counting(modern_required_config()).await;
+
+    let body = r#"{"jsonrpc":"2.0","id":7,"method":"tools/list","params":{"_meta":{"io.modelcontextprotocol/protocolVersion":"2026-07-28"}}}"#;
+    let response = post_seam(&client, &url, body, Some("2026-07-28"), &[]).await;
+    assert_eq!(response.status(), 400);
+    let payload: serde_json::Value = response.json().await?;
+    assert_eq!(
+        payload["error"]["code"], -32020,
+        "standard-headers routing-header error must precede the seam's meta check"
+    );
+    assert_eq!(
+        lists.load(Ordering::SeqCst),
+        0,
+        "handler invocation counter must stay at 0 when a routing header is missing"
+    );
+
+    ct.cancel();
+    Ok(())
+}
+
+// `initialize` is exempt from the new required-header check while retaining
+// its existing optional-header and header/body consistency rules.
+#[tokio::test]
+async fn seam_opt_in_preserves_initialize_rules() -> anyhow::Result<()> {
+    let (client, url, ct) = spawn_server(modern_required_config()).await;
+
+    let response = post_init(&client, &url, None, "2025-11-25").await;
+    assert_eq!(response.status(), 200);
+
+    let response = post_init(&client, &url, Some("2025-11-25"), "2025-11-25").await;
+    assert_eq!(response.status(), 200);
+
+    let response = post_init(&client, &url, Some("2025-03-26"), "2025-11-25").await;
+    assert_eq!(response.status(), 400);
+    let payload: serde_json::Value = response.json().await?;
+    assert_eq!(payload["error"]["code"], -32600);
+
+    ct.cancel();
+    Ok(())
+}
+
+// `notifications/initialized` returns HTTP 202 (Accepted) without
+// requiring any protocol metadata, even under the seam.
+#[tokio::test]
+async fn seam_opt_in_notifications_return_202() -> anyhow::Result<()> {
+    let (client, url, ct) = spawn_server(modern_required_config()).await;
+
+    let body = r#"{"jsonrpc":"2.0","method":"notifications/initialized"}"#;
+    let response = post_seam(&client, &url, body, None, &[]).await;
+    assert_eq!(
+        response.status(),
+        reqwest::StatusCode::ACCEPTED,
+        "notifications must surface HTTP 202 regardless of metadata"
+    );
+
+    ct.cancel();
+    Ok(())
+}
+
+// `server/discover` is still subject to the new HTTP-header precheck.
+#[tokio::test]
+async fn seam_opt_in_discover_rejects_missing_header() -> anyhow::Result<()> {
+    let (client, url, ct) = spawn_server(modern_required_config()).await;
+
+    let with_meta = r#"{"jsonrpc":"2.0","id":8,"method":"server/discover","params":{"_meta":{"io.modelcontextprotocol/protocolVersion":"2025-11-25","io.modelcontextprotocol/clientInfo":{"name":"test","version":"1.0"},"io.modelcontextprotocol/clientCapabilities":{}}}}"#;
+    let without_meta = r#"{"jsonrpc":"2.0","id":9,"method":"server/discover","params":{}}"#;
+    for body in [with_meta, without_meta] {
+        let response = post_seam(
+            &client,
+            &url,
+            body,
+            None,
+            &[("Mcp-Method", "server/discover")],
+        )
+        .await;
+        assert_eq!(response.status(), 400);
+        let payload: serde_json::Value = response.json().await?;
+        assert_eq!(payload["error"]["code"], -32020);
+    }
+
+    let response = post_seam(
+        &client,
+        &url,
+        without_meta,
+        Some("2025-11-25"),
+        &[("Mcp-Method", "server/discover")],
+    )
+    .await;
+    assert_eq!(response.status(), 400);
+    let payload: serde_json::Value = response.json().await?;
+    assert_eq!(payload["error"]["code"], -32602);
+
+    let response = post_seam(
+        &client,
+        &url,
+        with_meta,
+        Some("2025-11-25"),
+        &[("Mcp-Method", "server/discover")],
+    )
+    .await;
+    assert_eq!(response.status(), 200);
+
+    ct.cancel();
+    Ok(())
+}