fix(rmcp-client): detect MCP protocol mismatches

code-rs/rmcp-client/src/rmcp_client.rs

@@ -14,6 +14,7 @@ use mcp_types::InitializeRequestParams;
 use mcp_types::InitializeResult;
 use mcp_types::ListToolsRequestParams;
 use mcp_types::ListToolsResult;
+use mcp_types::MCP_SCHEMA_VERSION;
 use rmcp::model::CallToolRequestParam;
 use rmcp::model::InitializeRequestParam;
 use rmcp::model::PaginatedRequestParam;
@@ -149,20 +150,30 @@ impl RmcpClient {
         };
 
         let service = match timeout {
-            Some(duration) => time::timeout(duration, service_future)
-                .await
-                .map_err(|_| anyhow!("timed out handshaking with MCP server after {duration:?}"))?
-                .map_err(|err| anyhow!("handshaking with MCP server failed: {err}"))?,
-            None => service_future
-                .await
-                .map_err(|err| anyhow!("handshaking with MCP server failed: {err}"))?,
+            Some(duration) => match time::timeout(duration, service_future).await {
+                Ok(Ok(service)) => service,
+                Ok(Err(err)) => return Err(handshake_failed_error(err)),
+                Err(_) => return Err(handshake_timeout_error(duration)),
+            },
+            None => match service_future.await {
+                Ok(service) => service,
+                Err(err) => return Err(handshake_failed_error(err)),
+            },
         };
 
         let initialize_result_rmcp = service
             .peer()
             .peer_info()
             .ok_or_else(|| anyhow!("handshake succeeded but server info was missing"))?;
-        let initialize_result = convert_to_mcp(initialize_result_rmcp)?;
+        let initialize_result: InitializeResult = convert_to_mcp(initialize_result_rmcp)?;
+
+        if initialize_result.protocol_version != MCP_SCHEMA_VERSION {
+            let reported_version = initialize_result.protocol_version.clone();
+            return Err(anyhow!(
+                "MCP server reported protocol version {reported_version}, but this client expects {}. Update either side so both speak the same schema.",
+                MCP_SCHEMA_VERSION
+            ));
+        }
 
         {
             let mut guard = self.state.lock().await;
@@ -217,3 +228,16 @@ impl RmcpClient {
         }
     }
 }
+
+fn handshake_failed_error(err: impl Into<anyhow::Error>) -> anyhow::Error {
+    let err = err.into();
+    anyhow!(
+        "handshaking with MCP server failed: {err} (this client supports MCP schema version {MCP_SCHEMA_VERSION})"
+    )
+}
+
+fn handshake_timeout_error(duration: Duration) -> anyhow::Error {
+    anyhow!(
+        "timed out handshaking with MCP server after {duration:?} (expected MCP schema version {MCP_SCHEMA_VERSION})"
+    )
+}