fix: sync dev fixes into v2 and improve regression reports (#6821)

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Closes #6604

apps/nextjs/src/app/api/mcp/_protocol.ts

@@ -4,6 +4,7 @@ import { callMcpTool } from "@homarr/api/mcp";
 import type { McpTool } from "@homarr/api/mcp";
 
 import { toAssistantToolOutput } from "../assistant/chat/assistant-tool-output";
+import { serializeMcpToolResult } from "./_serialize-result";
 
 interface CreateMcpProtocolHandlerOptions {
   caller: unknown;
@@ -53,7 +54,7 @@ export const createMcpProtocolHandler = ({
                 content: [
                   {
                     type: "text" as const,
-                    text: JSON.stringify(toAssistantToolOutput(result)),
+                    text: serializeMcpToolResult(toAssistantToolOutput(result)),
                   },
                 ],
               };

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";
+}