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/src/handler/server.rs

@@ -22,7 +22,9 @@ impl<H: ServerHandler> Service<RoleServer> for H {
         request: <RoleServer as ServiceRole>::PeerReq,
         context: RequestContext<RoleServer>,
     ) -> Result<<RoleServer as ServiceRole>::Resp, McpError> {
-        match request {
+        // `context` is moved into the dispatch below, so read the negotiated version first.
+        let protocol_version = context.protocol_version();
+        let result = match request {
             ClientRequest::InitializeRequest(request) => self
                 .initialize(request.params, context)
                 .await
@@ -127,7 +129,18 @@ impl<H: ServerHandler> Service<RoleServer> for H {
                 .cancel_task(request.params, context)
                 .await
                 .map(ServerResult::CancelTaskResult),
-        }
+        };
+        // SEP-2164: peers negotiating 2026-07-28+ get the standard INVALID_PARAMS code for
+        // resource-not-found; older peers keep RESOURCE_NOT_FOUND. ISO `YYYY-MM-DD` versions
+        // compare lexically the same as chronologically.
+        let use_invalid_params =
+            protocol_version.is_some_and(|v| v.as_str() >= ProtocolVersion::V_2026_07_28.as_str());
+        result.map_err(|mut error| {
+            if use_invalid_params && error.code == ErrorCode::RESOURCE_NOT_FOUND {
+                error.code = ErrorCode::INVALID_PARAMS;
+            }
+            error
+        })
     }
 
     async fn handle_notification(

crates/rmcp/src/model.rs

@@ -152,6 +152,7 @@ impl std::fmt::Display for ProtocolVersion {
 }
 
 impl ProtocolVersion {
+    pub const V_2026_07_28: Self = Self(Cow::Borrowed("2026-07-28"));
     pub const V_2025_11_25: Self = Self(Cow::Borrowed("2025-11-25"));
     pub const V_2025_06_18: Self = Self(Cow::Borrowed("2025-06-18"));
     pub const V_2025_03_26: Self = Self(Cow::Borrowed("2025-03-26"));
@@ -164,6 +165,7 @@ impl ProtocolVersion {
         Self::V_2025_03_26,
         Self::V_2025_06_18,
         Self::V_2025_11_25,
+        Self::V_2026_07_28,
     ];
 
     /// Returns the string representation of this protocol version.
@@ -193,6 +195,7 @@ impl<'de> Deserialize<'de> for ProtocolVersion {
             "2025-03-26" => return Ok(ProtocolVersion::V_2025_03_26),
             "2025-06-18" => return Ok(ProtocolVersion::V_2025_06_18),
             "2025-11-25" => return Ok(ProtocolVersion::V_2025_11_25),
+            "2026-07-28" => return Ok(ProtocolVersion::V_2026_07_28),
             _ => {}
         }
         Ok(ProtocolVersion(Cow::Owned(s)))
@@ -541,9 +544,12 @@ impl ErrorData {
             data,
         }
     }
+    /// Resource-not-found error (`-32002`). The server upgrades this to `INVALID_PARAMS`
+    /// (`-32602`) for peers negotiating protocol `2026-07-28` or newer (SEP-2164).
     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 parse_error(message: impl Into<Cow<'static, str>>, data: Option<Value>) -> Self {
         Self::new(ErrorCode::PARSE_ERROR, message, data)
     }

crates/rmcp/src/service.rs

@@ -672,6 +672,16 @@ impl<R: ServiceRole> RequestContext<R> {
     }
 }
 
+#[cfg(feature = "server")]
+impl RequestContext<RoleServer> {
+    /// The protocol version the client negotiated, or `None` before peer info is recorded.
+    pub fn protocol_version(&self) -> Option<crate::model::ProtocolVersion> {
+        self.peer
+            .peer_info()
+            .map(|info| info.protocol_version.clone())
+    }
+}
+
 /// Request execution context
 #[derive(Debug, Clone)]
 #[non_exhaustive]