feat: standardize resource-not-found error code (SEP-2164) (#899)

* feat: implement SEP-2164 resource not found errors

* test: update protocol version utility expectations

* feat: gate not-found code at server boundary

---------

Co-authored-by: Michael Neale <michael.neale@gmail.com>

crates/rmcp/tests/test_custom_headers.rs

@@ -866,16 +866,18 @@ async fn test_server_rejects_unsupported_protocol_version() {
 fn test_protocol_version_utilities() {
     use rmcp::model::ProtocolVersion;
 
+    assert_eq!(ProtocolVersion::V_2026_07_28.as_str(), "2026-07-28");
     assert_eq!(ProtocolVersion::V_2025_11_25.as_str(), "2025-11-25");
     assert_eq!(ProtocolVersion::V_2025_06_18.as_str(), "2025-06-18");
     assert_eq!(ProtocolVersion::V_2025_03_26.as_str(), "2025-03-26");
     assert_eq!(ProtocolVersion::V_2024_11_05.as_str(), "2024-11-05");
 
-    assert_eq!(ProtocolVersion::KNOWN_VERSIONS.len(), 4);
+    assert_eq!(ProtocolVersion::KNOWN_VERSIONS.len(), 5);
     assert!(ProtocolVersion::KNOWN_VERSIONS.contains(&ProtocolVersion::V_2024_11_05));
     assert!(ProtocolVersion::KNOWN_VERSIONS.contains(&ProtocolVersion::V_2025_03_26));
     assert!(ProtocolVersion::KNOWN_VERSIONS.contains(&ProtocolVersion::V_2025_06_18));
     assert!(ProtocolVersion::KNOWN_VERSIONS.contains(&ProtocolVersion::V_2025_11_25));
+    assert!(ProtocolVersion::KNOWN_VERSIONS.contains(&ProtocolVersion::V_2026_07_28));
 }
 
 /// Integration test: Verify server validates only the Host header for DNS rebinding protection

crates/rmcp/tests/test_resource_not_found_version.rs

@@ -0,0 +1,91 @@
+//! SEP-2164: the resource-not-found error code follows the negotiated protocol version.
+//!
+//! `2026-07-28` and newer get the standard `INVALID_PARAMS` (-32602); older versions
+//! keep the legacy `RESOURCE_NOT_FOUND` (-32002).
+#![cfg(not(feature = "local"))]
+#![cfg(feature = "client")]
+
+use rmcp::{
+    ClientHandler, RoleServer, ServerHandler, ServiceError, ServiceExt,
+    model::{
+        ClientInfo, ErrorCode, ErrorData, ProtocolVersion, ReadResourceRequestParams,
+        ReadResourceResult,
+    },
+    service::RequestContext,
+};
+
+#[derive(Debug, Clone, Default)]
+struct ResourceServer;
+
+impl ServerHandler for ResourceServer {
+    async fn read_resource(
+        &self,
+        _request: ReadResourceRequestParams,
+        _context: RequestContext<RoleServer>,
+    ) -> Result<ReadResourceResult, ErrorData> {
+        Err(ErrorData::resource_not_found("resource not found", None))
+    }
+}
+
+#[derive(Debug, Clone)]
+struct VersionedClient {
+    protocol_version: ProtocolVersion,
+}
+
+impl ClientHandler for VersionedClient {
+    fn get_info(&self) -> ClientInfo {
+        let mut info = ClientInfo::default();
+        info.protocol_version = self.protocol_version.clone();
+        info
+    }
+}
+
+async fn not_found_code(client_version: ProtocolVersion) -> ErrorCode {
+    let (server_transport, client_transport) = tokio::io::duplex(4096);
+
+    let server_handle = tokio::spawn(async move {
+        ResourceServer
+            .serve(server_transport)
+            .await?
+            .waiting()
+            .await?;
+        anyhow::Ok(())
+    });
+
+    let client = VersionedClient {
+        protocol_version: client_version,
+    }
+    .serve(client_transport)
+    .await
+    .expect("client should connect");
+
+    let error = client
+        .read_resource(ReadResourceRequestParams::new("missing://resource"))
+        .await
+        .expect_err("missing resource should error");
+
+    let code = match error {
+        ServiceError::McpError(data) => data.code,
+        other => panic!("expected McpError, got: {other:?}"),
+    };
+
+    client.cancel().await.expect("client should cancel");
+    server_handle.await.expect("server task").expect("server");
+    code
+}
+
+#[tokio::test]
+async fn legacy_version_gets_resource_not_found_code() {
+    assert_eq!(
+        not_found_code(ProtocolVersion::V_2025_11_25).await,
+        ErrorCode::RESOURCE_NOT_FOUND,
+    );
+}
+
+#[tokio::test]
+async fn sep_2164_version_gets_invalid_params_code() {
+    assert_eq!(
+        not_found_code(ProtocolVersion::V_2026_07_28).await,
+        ErrorCode::INVALID_PARAMS,
+    );
+}