feat(mcp): switch to YAML serialization for tool outputs

- Replaced serde_json with serde_yaml in all handler endpoints.

- This drastically reduces token consumption for large list/graph responses by removing JSON braces and quotes.

Cargo.lock

@@ -2654,6 +2654,7 @@ dependencies = [
  "schemars",
  "serde",
  "serde_json",
+ "serde_yaml",
  "tantivy",
  "tempfile",
  "thiserror 2.0.20",
@@ -4342,6 +4343,19 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "serde_yaml"
+version = "0.9.34+deprecated"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6a8b1a1a2ebf674015cc02edccce75287f1a0130d394307b36743c2f5d504b47"
+dependencies = [
+ "indexmap",
+ "itoa",
+ "ryu",
+ "serde",
+ "unsafe-libyaml",
+]
+
 [[package]]
 name = "sha1"
 version = "0.10.7"
@@ -5438,6 +5452,12 @@ version = "0.1.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "39ec24b3121d976906ece63c9daad25b85969647682eee313cb5779fdd69e14e"
 
+[[package]]
+name = "unsafe-libyaml"
+version = "0.2.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "673aac59facbab8a9007c7f6108d11f63b603f7cabff99fabf650fea5c32b861"
+
 [[package]]
 name = "untrusted"
 version = "0.9.0"

server/Cargo.toml

@@ -51,6 +51,7 @@ tokio-stream = { version = "0.1", features = ["sync"] }
 chrono = { version = "0.4.45", features = ["serde"] }
 ocrs = "0.13.1"
 rten = "0.26.0"
+serde_yaml = "0.9.34"
 
 [build-dependencies]
 chrono = "0.4.45"

server/src/error.rs

@@ -40,6 +40,9 @@ pub enum AppError {
 
     #[error("I/O Error: {0}")]
     IoError(#[from] std::io::Error),
+
+    #[error("YAML Serialization Error: {0}")]
+    YamlError(#[from] serde_yaml::Error),
 }
 
 impl From<AppError> for String {
@@ -81,6 +84,10 @@ impl IntoResponse for AppError {
                 StatusCode::INTERNAL_SERVER_ERROR,
                 format!("I/O Error: {}", e),
             ),
+            AppError::YamlError(e) => (
+                StatusCode::INTERNAL_SERVER_ERROR,
+                format!("YAML Serialization Error: {}", e),
+            ),
         };
 
         let body = Json(json!({

server/src/handlers/ast.rs

@@ -504,7 +504,7 @@ impl McpTool for FindSymbolReferencesHandler {
             matches.extend(disk_matches);
         }
 
-        Ok(serde_json::to_string_pretty(&matches)?)
+        Ok(serde_yaml::to_string(&matches)?)
     }
 }
 
@@ -560,7 +560,7 @@ impl McpTool for GetCallersHandler {
             callers.extend(disk_callers);
         }
 
-        Ok(serde_json::to_string_pretty(&callers)?)
+        Ok(serde_yaml::to_string(&callers)?)
     }
 }
 
@@ -649,7 +649,7 @@ impl McpTool for AnalyzeImpactHandler {
             }
         });
 
-        Ok(serde_json::to_string_pretty(&result)?)
+        Ok(serde_yaml::to_string(&result)?)
     }
 }
 

server/src/handlers/env.rs

@@ -70,7 +70,7 @@ impl McpTool for EnvironmentHandler {
                     .env_fingerprints
                     .read_with(|fps| fps.get(&ns).cloned());
                 if let Some(fp) = data {
-                    let data = Ok::<String, crate::error::AppError>(serde_json::to_string(&fp)?)?;
+                    let data = Ok::<String, crate::error::AppError>(serde_yaml::to_string(&fp)?)?;
                     Ok(data)
                 } else {
                     Ok("{}".to_string())
@@ -118,7 +118,7 @@ impl McpTool for EnvironmentHandler {
             EnvAction::GetDetails => {
                 let data = state.env.environments.read_with(|envs| {
                     let filtered: Vec<_> = envs.iter().filter(|e| e.namespace == ns).collect();
-                    Ok::<String, crate::error::AppError>(serde_json::to_string(&filtered)?)
+                    Ok::<String, crate::error::AppError>(serde_yaml::to_string(&filtered)?)
                 })?;
                 Ok(data)
             }

server/src/handlers/git.rs

@@ -87,7 +87,7 @@ impl McpTool for GetActiveWorktreeContextHandler {
             .await
             .map_err(|e| crate::error::AppError::Internal(format!("Task panic: {}", e)))??;
 
-        Ok::<String, crate::error::AppError>(serde_json::to_string_pretty(&result)?)
+        Ok::<String, crate::error::AppError>(serde_yaml::to_string(&result)?)
     }
 }
 
@@ -168,7 +168,7 @@ impl McpTool for QueryGitDiffsHandler {
             matches.extend(git_matches);
         }
 
-        Ok(serde_json::to_string_pretty(&matches)?)
+        Ok(serde_yaml::to_string(&matches)?)
     }
 }
 

server/src/handlers/graph.rs

@@ -542,7 +542,7 @@ impl McpTool for ReadGraphHandler {
                 })
             };
 
-            let mut result_json = serde_json::to_string(&serialized)?;
+            let mut result_json = serde_yaml::to_string(&serialized)?;
 
             if let Some(max_tok) = max_tokens {
                 let max_chars = max_tok * 4;
@@ -579,7 +579,7 @@ impl McpTool for ReadGraphHandler {
                                 obj.insert("has_more".to_string(), serde_json::Value::Bool(true));
                             }
 
-                        if let Ok(trunc_str) = serde_json::to_string(&truncated_val)
+                        if let Ok(trunc_str) = serde_yaml::to_string(&truncated_val)
                             && (trunc_str.len() <= max_chars || keep_count == 1) {
                                 result_json = trunc_str;
                                 break;
@@ -638,7 +638,7 @@ impl McpTool for SearchNodesHandler {
                     }
                 }
             }
-            Ok::<String, crate::error::AppError>(serde_json::to_string(&matched_entities)?)
+            Ok::<String, crate::error::AppError>(serde_yaml::to_string(&matched_entities)?)
         })?;
         Ok(data)
     }
@@ -679,7 +679,7 @@ impl McpTool for OpenNodesHandler {
                     result.entities.insert(name, e);
                 }
             }
-            Ok::<String, crate::error::AppError>(serde_json::to_string(&result)?)
+            Ok::<String, crate::error::AppError>(serde_yaml::to_string(&result)?)
         })?;
         Ok(data)
     }
@@ -954,7 +954,7 @@ impl McpTool for FindOrphansHandler {
                 .cloned()
                 .collect::<Vec<String>>()
         });
-        let data = Ok::<String, crate::error::AppError>(serde_json::to_string(&orphans)?)?;
+        let data = Ok::<String, crate::error::AppError>(serde_yaml::to_string(&orphans)?)?;
         Ok(data)
     }
 }
@@ -1047,7 +1047,7 @@ impl McpTool for GetSubgraphHandler {
             "relations": matched_relations,
         });
 
-        Ok(serde_json::to_string_pretty(&result)?)
+        Ok(serde_yaml::to_string(&result)?)
     }
 }
 
@@ -1146,7 +1146,7 @@ impl McpTool for SweepGraphHealthHandler {
             "health_score": if orphans.is_empty() && duplicates.is_empty() && stale_entities.is_empty() { "100%" } else { "Needs Maintenance" }
         });
 
-        Ok(serde_json::to_string_pretty(&report)?)
+        Ok(serde_yaml::to_string(&report)?)
     }
 }
 
@@ -1203,7 +1203,7 @@ impl McpTool for ResolveStaleSymbolsHandler {
             "status": "success"
         });
 
-        Ok(serde_json::to_string_pretty(&report)?)
+        Ok(serde_yaml::to_string(&report)?)
     }
 }
 

server/src/handlers/meta.rs

@@ -142,7 +142,7 @@ impl McpTool for SearchErrorFixesHandler {
                     })
                     .collect();
 
-                Ok::<String, crate::error::AppError>(serde_json::to_string_pretty(&suggestions)?)
+                Ok::<String, crate::error::AppError>(serde_yaml::to_string(&suggestions)?)
             })?;
             return Ok(data);
         }
@@ -169,7 +169,7 @@ impl McpTool for SearchErrorFixesHandler {
                     }
                 })
                 .collect();
-            Ok::<String, crate::error::AppError>(serde_json::to_string_pretty(&filtered)?)
+            Ok::<String, crate::error::AppError>(serde_yaml::to_string(&filtered)?)
         })?;
         Ok(data)
     }
@@ -358,7 +358,7 @@ impl McpTool for QueryRecentChangesHandler {
                 .take(limit)
                 .cloned()
                 .collect();
-            Ok::<String, crate::error::AppError>(serde_json::to_string(&filtered)?)
+            Ok::<String, crate::error::AppError>(serde_yaml::to_string(&filtered)?)
         })?;
         Ok(data)
     }
@@ -575,7 +575,7 @@ impl McpTool for DecisionsHandler {
                         .collect();
 
                     if include_body {
-                        Ok::<String, crate::error::AppError>(serde_json::to_string(&filtered)?)
+                        Ok::<String, crate::error::AppError>(serde_yaml::to_string(&filtered)?)
                     } else {
                         let compact: Vec<_> = filtered
                             .iter()
@@ -592,7 +592,7 @@ impl McpTool for DecisionsHandler {
                                 })
                             })
                             .collect();
-                        Ok::<String, crate::error::AppError>(serde_json::to_string(&compact)?)
+                        Ok::<String, crate::error::AppError>(serde_yaml::to_string(&compact)?)
                     }
                 })?;
                 Ok(data)
@@ -738,7 +738,7 @@ impl McpTool for TechDebtHandler {
                             }),
                         })
                         .collect();
-                    let mut json_str = serde_json::to_string(&filtered)?;
+                    let mut json_str = serde_yaml::to_string(&filtered)?;
                     if let Some(max_t) = req.max_tokens {
                         let char_limit = max_t * 4;
                         if json_str.len() > char_limit {
@@ -1305,7 +1305,7 @@ impl McpTool for ManageCheckpointHandler {
                 for s in snaps {
                     list.push(serde_json::json!({"type": "snapshot", "id": s.id, "description": s.description, "namespace": s.namespace}));
                 }
-                Ok(serde_json::to_string_pretty(&list)?)
+                Ok(serde_yaml::to_string(&list)?)
             }
             CheckpointAction::Delete => {
                 let name = req.name_or_id.ok_or_else(|| {
@@ -1427,7 +1427,7 @@ impl McpTool for QueryLineageHandler {
             "timeline": timeline
         });
 
-        Ok(serde_json::to_string_pretty(&res)?)
+        Ok(serde_yaml::to_string(&res)?)
     }
 }
 
@@ -1486,7 +1486,7 @@ impl McpTool for GetNextActionableTasksHandler {
             "tasks": actionable
         });
 
-        Ok(serde_json::to_string_pretty(&res)?)
+        Ok(serde_yaml::to_string(&res)?)
     }
 }
 
@@ -1555,7 +1555,7 @@ impl McpTool for HypothesesHandler {
                     })
                     .collect();
 
-                Ok(serde_json::to_string_pretty(&filtered)?)
+                Ok(serde_yaml::to_string(&filtered)?)
             }
         }
     }
@@ -1634,7 +1634,7 @@ impl McpTool for GetPreflightContextHandler {
             "recent_activities": recent_activities
         });
 
-        Ok(serde_json::to_string_pretty(&preflight)?)
+        Ok(serde_yaml::to_string(&preflight)?)
     }
 }
 
@@ -1742,7 +1742,7 @@ impl McpTool for AgentSignalsHandler {
                         .collect::<Vec<_>>()
                 });
 
-                Ok(serde_json::to_string_pretty(&filtered)?)
+                Ok(serde_yaml::to_string(&filtered)?)
             }
         }
     }

server/src/handlers/notes.rs

@@ -60,7 +60,7 @@ impl McpTool for HandoffMemosHandler {
                             }
                         })
                         .collect();
-                    Ok::<String, crate::error::AppError>(serde_json::to_string(&filtered)?)
+                    Ok::<String, crate::error::AppError>(serde_yaml::to_string(&filtered)?)
                 })?;
                 Ok(data)
             }

server/src/handlers/tasks.rs

@@ -297,12 +297,12 @@ impl McpTool for TasksHandler {
                             _ => serde_json::json!({ "id": t.id, "title": t.title, "status": t.status, "description": t.description, "git_branch": t.git_branch }),
                         })
                         .collect();
-                    let mut json_str = serde_json::to_string(&filtered)?;
+                    let mut json_str = serde_yaml::to_string(&filtered)?;
                     if let Some(max_t) = req.max_tokens {
                         let char_limit = max_t * 4;
                         while json_str.len() > char_limit && !filtered.is_empty() {
                             filtered.pop();
-                            json_str = serde_json::to_string(&filtered)?;
+                            json_str = serde_yaml::to_string(&filtered)?;
                         }
                     }
                     Ok::<String, crate::error::AppError>(json_str)
@@ -514,7 +514,7 @@ impl McpTool for MilestonesHandler {
                             }
                         })
                         .collect();
-                    Ok::<String, crate::error::AppError>(serde_json::to_string(&filtered)?)
+                    Ok::<String, crate::error::AppError>(serde_yaml::to_string(&filtered)?)
                 })?;
                 Ok(data)
             }

server/src/handlers/vision.rs

@@ -758,7 +758,7 @@ impl McpTool for ClipboardHandler {
                 );
 
                 if let Some(img) = item {
-                    Ok(serde_json::to_string_pretty(&img)?)
+                    Ok(serde_yaml::to_string(&img)?)
                 } else {
                     Ok(json!({
                         "status": "no_image_in_clipboard",
@@ -786,7 +786,7 @@ impl McpTool for ClipboardHandler {
 
                 state.record_activity("clipboard", "Retrieved text from clipboard cache", None);
                 if let Some(txt) = item {
-                    Ok(serde_json::to_string_pretty(&txt)?)
+                    Ok(serde_yaml::to_string(&txt)?)
                 } else {
                     Ok(json!({
                         "text": "",
@@ -818,7 +818,7 @@ impl McpTool for ClipboardHandler {
                 };
 
                 state.record_activity("clipboard", "Viewed clipboard history ring buffer", None);
-                Ok(serde_json::to_string_pretty(&history)?)
+                Ok(serde_yaml::to_string(&history)?)
             }
             ClipboardAction::Clear => {
                 tokio::task::spawn_blocking(|| {
@@ -905,7 +905,7 @@ impl McpTool for ClipboardHandler {
                 .map_err(|e| crate::error::AppError::Internal(format!("Task panic: {}", e)))??;
 
                 state.record_activity("clipboard", "Read contents from OS clipboard", None);
-                Ok::<String, crate::error::AppError>(serde_json::to_string_pretty(&Value::Object(
+                Ok::<String, crate::error::AppError>(serde_yaml::to_string(&Value::Object(
                     out,
                 ))?)
             }

server/src/handlers/workspaces.rs

@@ -122,7 +122,7 @@ impl McpTool for SnippetsHandler {
                             .total_cmp(&a["score"].as_f64().unwrap_or(0.0))
                     });
                     scored.truncate(10);
-                    Ok(serde_json::to_string_pretty(&scored)?)
+                    Ok(serde_yaml::to_string(&scored)?)
                 } else {
                     let data = state.code.snippets.read_with(|snippets| {
                         let results: Vec<_> = snippets
@@ -153,7 +153,7 @@ impl McpTool for SnippetsHandler {
                                 })
                             })
                             .collect::<Vec<_>>();
-                        serde_json::to_string_pretty(&results)
+                        serde_yaml::to_string(&results)
                     })?;
                     Ok(data)
                 }