fix(gateway): preserve structured tool results

Author: Xuleileon

crates/mcpmux-gateway/src/mcp/handler.rs

@@ -860,15 +860,13 @@ impl ServerHandler for McpMuxGatewayHandler {
             .await
             .map_err(|e| McpError::internal_error(format!("Tool call failed: {}", e), None))?;
 
-        // Convert ToolCallResult to MCP CallToolResult
-        let content: Vec<Content> = tool_result
-            .content
-            .into_iter()
-            .filter_map(|v| serde_json::from_value(v).ok())
-            .collect();
+        // Convert ToolCallResult to MCP CallToolResult without dropping
+        // structuredContent or protocol-level _meta from the upstream server.
+        let result = tool_result.into_mcp_result();
 
         // Log result summary - show content types and approximate sizes
-        let content_summary: Vec<String> = content
+        let content_summary: Vec<String> = result
+            .content
             .iter()
             .map(|c| {
                 // Content is Annotated<RawContent>, serialize to inspect type
@@ -907,17 +905,11 @@ impl ServerHandler for McpMuxGatewayHandler {
             .collect();
         debug!(
             tool = %params.name,
-            is_error = tool_result.is_error,
+            is_error = result.is_error.unwrap_or(false),
             content = ?content_summary,
             "call_tool result"
         );
 
-        let result = if tool_result.is_error {
-            CallToolResult::error(content)
-        } else {
-            CallToolResult::success(content)
-        };
-
         Ok(result)
     }
 

crates/mcpmux-gateway/src/pool/routing.rs

@@ -12,7 +12,7 @@ use std::time::Duration;
 
 use anyhow::{anyhow, Result};
 use mcpmux_core::{FeatureType, LogLevel, LogSource, ServerLog, ServerLogManager};
-use rmcp::model::CallToolRequestParams;
+use rmcp::model::{CallToolRequestParams, CallToolResult, Content, Meta};
 use serde_json::Value;
 use tracing::{debug, info, warn};
 use uuid::Uuid;
@@ -52,6 +52,39 @@ pub struct RoutedResource {
 pub struct ToolCallResult {
     pub content: Vec<Value>,
     pub is_error: bool,
+    pub structured_content: Option<Value>,
+    pub meta: Option<Meta>,
+}
+
+impl ToolCallResult {
+    fn from_mcp_result(result: CallToolResult) -> Self {
+        Self {
+            content: result
+                .content
+                .into_iter()
+                .map(|item| serde_json::to_value(item).unwrap_or(Value::Null))
+                .collect(),
+            is_error: result.is_error.unwrap_or(false),
+            structured_content: result.structured_content,
+            meta: result.meta,
+        }
+    }
+
+    pub(crate) fn into_mcp_result(self) -> CallToolResult {
+        let content: Vec<Content> = self
+            .content
+            .into_iter()
+            .filter_map(|item| serde_json::from_value(item).ok())
+            .collect();
+        let mut result = if self.is_error {
+            CallToolResult::error(content)
+        } else {
+            CallToolResult::success(content)
+        };
+        result.structured_content = self.structured_content;
+        result.meta = self.meta;
+        result
+    }
 }
 
 /// Default timeout for MCP tool calls (60 seconds)
@@ -284,16 +317,7 @@ impl RoutingService {
                         .map_err(|_| anyhow!("Tool call timed out after {:?}", TOOL_CALL_TIMEOUT))?
                         .map_err(|e| anyhow!("MCP call failed: {}", e))?;
 
-                    let content: Vec<Value> = res
-                        .content
-                        .into_iter()
-                        .map(|c| serde_json::to_value(c).unwrap_or(Value::Null))
-                        .collect();
-
-                    Ok(ToolCallResult {
-                        content,
-                        is_error: res.is_error.unwrap_or(false),
-                    })
+                    Ok(ToolCallResult::from_mcp_result(res))
                 }
                 None => Err(anyhow!("Server instance has no active client")),
             }
@@ -726,3 +750,29 @@ impl RoutingService {
         false
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::ToolCallResult;
+    use rmcp::model::{CallToolResult, Content, Meta};
+    use serde_json::json;
+
+    #[test]
+    fn tool_result_round_trip_preserves_structured_content_and_meta() {
+        let structured = json!({ "matches": [{ "message": "found" }] });
+        let mut meta = Meta::new();
+        meta.0.insert("traceId".to_string(), json!("trace-123"));
+
+        let mut upstream = CallToolResult::structured(structured.clone());
+        upstream.content = vec![Content::text("search completed")];
+        upstream.meta = Some(meta.clone());
+
+        let routed = ToolCallResult::from_mcp_result(upstream);
+        let forwarded = routed.into_mcp_result();
+
+        assert_eq!(forwarded.content, vec![Content::text("search completed")]);
+        assert_eq!(forwarded.structured_content, Some(structured));
+        assert_eq!(forwarded.meta, Some(meta));
+        assert_eq!(forwarded.is_error, Some(false));
+    }
+}