Fix MCP error wrapping for DRC payloads

src/tools/design-rules.ts

@@ -7,6 +7,7 @@
 import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
 import { z } from "zod";
 import { logger } from "../logger.js";
+import { formatKicadResult } from "./tool-response.js";
 
 // Command function type for KiCAD script calls
 type CommandFunction = (command: string, params: Record<string, unknown>) => Promise<any>;
@@ -52,14 +53,7 @@ export function registerDesignRuleTools(server: McpServer, callKicadScript: Comm
       logger.debug("Setting design rules");
       const result = await callKicadScript("set_design_rules", params);
 
-      return {
+      return formatKicadResult(result);
-        content: [
-          {
-            type: "text",
-            text: JSON.stringify(result),
-          },
-        ],
-      };
     },
   );
 
@@ -74,14 +68,7 @@ export function registerDesignRuleTools(server: McpServer, callKicadScript: Comm
       logger.debug("Getting design rules");
       const result = await callKicadScript("get_design_rules", {});
 
-      return {
+      return formatKicadResult(result);
-        content: [
-          {
-            type: "text",
-            text: JSON.stringify(result),
-          },
-        ],
-      };
     },
   );
 
@@ -98,14 +85,7 @@ export function registerDesignRuleTools(server: McpServer, callKicadScript: Comm
       logger.debug("Running DRC check");
       const result = await callKicadScript("run_drc", { reportPath });
 
-      return {
+      return formatKicadResult(result);
-        content: [
-          {
-            type: "text",
-            text: JSON.stringify(result),
-          },
-        ],
-      };
     },
   );
 
@@ -162,14 +142,7 @@ export function registerDesignRuleTools(server: McpServer, callKicadScript: Comm
         nets,
       });
 
-      return {
+      return formatKicadResult(result);
-        content: [
-          {
-            type: "text",
-            text: JSON.stringify(result),
-          },
-        ],
-      };
     },
   );
 
@@ -190,14 +163,7 @@ export function registerDesignRuleTools(server: McpServer, callKicadScript: Comm
         netClass,
       });
 
-      return {
+      return formatKicadResult(result);
-        content: [
-          {
-            type: "text",
-            text: JSON.stringify(result),
-          },
-        ],
-      };
     },
   );
 
@@ -224,14 +190,7 @@ export function registerDesignRuleTools(server: McpServer, callKicadScript: Comm
         minViaDrill,
       });
 
-      return {
+      return formatKicadResult(result);
-        content: [
-          {
-            type: "text",
-            text: JSON.stringify(result),
-          },
-        ],
-      };
     },
   );
 
@@ -284,14 +243,7 @@ export function registerDesignRuleTools(server: McpServer, callKicadScript: Comm
         item2,
       });
 
-      return {
+      return formatKicadResult(result);
-        content: [
-          {
-            type: "text",
-            text: JSON.stringify(result),
-          },
-        ],
-      };
     },
   );
 
@@ -311,14 +263,7 @@ export function registerDesignRuleTools(server: McpServer, callKicadScript: Comm
       logger.debug("Getting DRC violations");
       const result = await callKicadScript("get_drc_violations", { severity });
 
-      return {
+      return formatKicadResult(result);
-        content: [
-          {
-            type: "text",
-            text: JSON.stringify(result),
-          },
-        ],
-      };
     },
   );
 

src/tools/tool-response.ts

@@ -0,0 +1,30 @@
+export type McpTextResult = {
+  content: Array<{
+    type: "text";
+    text: string;
+  }>;
+  isError?: true;
+};
+
+function isKicadFailure(result: unknown): boolean {
+  return (
+    typeof result === "object" &&
+    result !== null &&
+    "success" in result &&
+    (result as { success?: unknown }).success === false
+  );
+}
+
+export function formatKicadResult(result: unknown): McpTextResult {
+  const text = JSON.stringify(result) ?? String(result);
+
+  return {
+    content: [
+      {
+        type: "text",
+        text,
+      },
+    ],
+    ...(isKicadFailure(result) ? { isError: true as const } : {}),
+  };
+}

tests/test_mcp_error_wrapping_static.py

@@ -0,0 +1,51 @@
+"""
+Regression tests for MCP error wrapping in TypeScript tool adapters.
+
+KiCad backend commands may report domain failures as JSON payloads such as
+{"success": false, "message": "No board is loaded"}. The MCP tool result must
+also be marked with isError so clients do not treat the failed command as OK.
+"""
+
+from pathlib import Path
+
+import pytest
+
+ROOT = Path(__file__).parent.parent
+TOOLS_DIR = ROOT / "src" / "tools"
+DESIGN_RULES_TS = TOOLS_DIR / "design-rules.ts"
+TOOL_RESPONSE_TS = TOOLS_DIR / "tool-response.ts"
+
+
+@pytest.mark.unit
+class TestMcpErrorWrapping:
+    def test_helper_marks_only_explicit_failure_payloads_as_mcp_errors(self):
+        """A KiCad success:false payload must become an MCP isError result."""
+        helper = TOOL_RESPONSE_TS.read_text(encoding="utf-8")
+
+        assert "export function formatKicadResult" in helper
+        assert "success === false" in helper
+        assert "isError: true" in helper
+
+    def test_design_rule_tools_use_shared_error_wrapper(self):
+        """DRC/design-rule wrappers must not return success:false as a plain OK result."""
+        source = DESIGN_RULES_TS.read_text(encoding="utf-8")
+
+        assert 'import { formatKicadResult } from "./tool-response.js";' in source
+
+        for command in (
+            "set_design_rules",
+            "get_design_rules",
+            "run_drc",
+            "add_net_class",
+            "assign_net_to_class",
+            "set_layer_constraints",
+            "check_clearance",
+            "get_drc_violations",
+        ):
+            marker = f'callKicadScript("{command}"'
+            command_index = source.find(marker)
+            assert command_index != -1, f"{command} wrapper not found"
+
+            next_tool_index = source.find("server.tool(", command_index + len(marker))
+            wrapper_body = source[command_index : next_tool_index if next_tool_index != -1 else None]
+            assert "return formatKicadResult(result);" in wrapper_body