fix(mcp): return valid string for void tool results (#6703)

apps/nextjs/src/app/api/mcp/[transport]/route.ts

@@ -14,6 +14,7 @@ import { db } from "@homarr/db";
 
 import { getPackageVersion } from "~/versions/package-reader";
 import { extractMcpTools } from "../_extract-tools";
+import { serializeMcpToolResult } from "../_serialize-result";
 
 const logger = createLogger({ module: "mcpRoute" });
 
@@ -203,7 +204,7 @@ const mcpHandler = createMcpHandler(
         const input = args && Object.keys(args).length > 0 ? args : undefined;
         const result = await procedure(input);
         return {
-          content: [{ type: "text" as const, text: JSON.stringify(result) }],
+          content: [{ type: "text" as const, text: serializeMcpToolResult(result) }],
         };
       } catch (error) {
         const message = sanitizeErrorMessage(error);

apps/nextjs/src/app/api/mcp/_serialize-result.spec.ts

@@ -0,0 +1,23 @@
+import { describe, expect, it } from "vitest";
+
+import { serializeMcpToolResult } from "./_serialize-result";
+
+describe("serializeMcpToolResult", () => {
+  it("serializes objects as JSON strings", () => {
+    expect(serializeMcpToolResult({ boardId: "1" })).toBe('{"boardId":"1"}');
+  });
+
+  it("serializes primitive values", () => {
+    expect(serializeMcpToolResult("hello")).toBe('"hello"');
+    expect(serializeMcpToolResult(42)).toBe("42");
+    expect(serializeMcpToolResult(true)).toBe("true");
+  });
+
+  it("serializes null as the string null", () => {
+    expect(serializeMcpToolResult(null)).toBe("null");
+  });
+
+  it("serializes void results as the string null so MCP content text is always a string", () => {
+    expect(serializeMcpToolResult(undefined)).toBe("null");
+  });
+});

apps/nextjs/src/app/api/mcp/_serialize-result.ts

@@ -0,0 +1,3 @@
+export function serializeMcpToolResult(result: unknown): string {
+  return JSON.stringify(result) ?? "null";
+}