adding config layer

extensions/diffs/index.test.ts

@@ -1,4 +1,6 @@
+import type { IncomingMessage } from "node:http";
 import { describe, expect, it, vi } from "vitest";
+import { createMockServerResponse } from "../../src/test-utils/mock-http-response.js";
 import plugin from "./index.js";
 
 describe("diffs plugin registration", () => {
@@ -40,4 +42,82 @@ describe("diffs plugin registration", () => {
     expect(on).toHaveBeenCalledTimes(1);
     expect(on.mock.calls[0]?.[0]).toBe("before_prompt_build");
   });
+
+  it("applies plugin-config defaults through registered tool and viewer handler", async () => {
+    let registeredTool:
+      | { execute?: (toolCallId: string, params: Record<string, unknown>) => Promise<unknown> }
+      | undefined;
+    let registeredHttpHandler:
+      | ((
+          req: IncomingMessage,
+          res: ReturnType<typeof createMockServerResponse>,
+        ) => Promise<boolean>)
+      | undefined;
+
+    plugin.register?.({
+      id: "diffs",
+      name: "Diffs",
+      description: "Diffs",
+      source: "test",
+      config: {
+        gateway: {
+          port: 18789,
+          bind: "loopback",
+        },
+      },
+      pluginConfig: {
+        defaults: {
+          theme: "light",
+          background: false,
+          layout: "split",
+        },
+      },
+      runtime: {} as never,
+      logger: {
+        info() {},
+        warn() {},
+        error() {},
+      },
+      registerTool(tool) {
+        registeredTool = typeof tool === "function" ? undefined : tool;
+      },
+      registerHook() {},
+      registerHttpHandler(handler) {
+        registeredHttpHandler = handler as typeof registeredHttpHandler;
+      },
+      registerHttpRoute() {},
+      registerChannel() {},
+      registerGatewayMethod() {},
+      registerCli() {},
+      registerService() {},
+      registerProvider() {},
+      registerCommand() {},
+      resolvePath(input: string) {
+        return input;
+      },
+      on() {},
+    });
+
+    const result = await registeredTool?.execute?.("tool-1", {
+      before: "one\n",
+      after: "two\n",
+    });
+    const viewerPath = String(
+      (result as { details?: Record<string, unknown> } | undefined)?.details?.viewerPath,
+    );
+    const res = createMockServerResponse();
+    const handled = await registeredHttpHandler?.(
+      {
+        method: "GET",
+        url: viewerPath,
+      } as IncomingMessage,
+      res,
+    );
+
+    expect(handled).toBe(true);
+    expect(res.statusCode).toBe(200);
+    expect(String(res.body)).toContain('body data-theme="light"');
+    expect(String(res.body)).toContain('"backgroundEnabled":false');
+    expect(String(res.body)).toContain('"diffStyle":"split"');
+  });
 });