fix(FN-679): use merge-base diff in modal changes tab

- Updated /tasks/:id/diff endpoint to use merge-base comparison
- Changed from HEAD~1..HEAD to ...HEAD syntax
- Added fallback to HEAD when merge-base fails
- Verified fix matches /tasks/:id/file-diffs endpoint pattern

packages/dashboard/src/__tests__/routes-diff.test.ts

@@ -0,0 +1,307 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
+import { EventEmitter } from "node:events";
+import type { Task } from "@fusion/core";
+import * as childProcess from "node:child_process";
+import * as fs from "node:fs";
+import { get } from "../test-request.js";
+
+vi.mock("node:child_process", async () => {
+  const actual = await vi.importActual<typeof import("node:child_process")>("node:child_process");
+  return {
+    ...actual,
+    execSync: vi.fn(),
+  };
+});
+
+vi.mock("node:fs", async () => {
+  const actual = await vi.importActual<typeof import("node:fs")>("node:fs");
+  return {
+    ...actual,
+    existsSync: vi.fn(),
+  };
+});
+
+import { createServer } from "../server.js";
+
+const mockExecSync = vi.mocked(childProcess.execSync);
+const mockExistsSync = vi.mocked(fs.existsSync);
+
+class MockStore extends EventEmitter {
+  private tasks = new Map<string, Task>();
+
+  getRootDir(): string {
+    return "/tmp/fn-679";
+  }
+
+  getMissionStore() {
+    return {
+      listMissions: vi.fn().mockResolvedValue([]),
+      createMission: vi.fn(),
+      getMission: vi.fn(),
+      updateMission: vi.fn(),
+      deleteMission: vi.fn(),
+      listTemplates: vi.fn().mockResolvedValue([]),
+      createTemplate: vi.fn(),
+      getTemplate: vi.fn(),
+      updateTemplate: vi.fn(),
+      deleteTemplate: vi.fn(),
+      instantiateMission: vi.fn(),
+    };
+  }
+
+  async listTasks(): Promise<Task[]> {
+    return Array.from(this.tasks.values());
+  }
+
+  getTask(id: string): Task | undefined {
+    return this.tasks.get(id);
+  }
+
+  addTask(task: Task): void {
+    this.tasks.set(task.id, task);
+  }
+}
+
+function createTask(overrides: Partial<Task> = {}): Task {
+  return {
+    id: "FN-679",
+    title: "Test task",
+    description: "Test description",
+    column: "in-progress",
+    dependencies: [],
+    steps: [],
+    currentStep: 0,
+    log: [],
+    createdAt: "2026-04-01T00:00:00.000Z",
+    updatedAt: "2026-04-01T00:00:00.000Z",
+    columnMovedAt: "2026-04-01T00:00:00.000Z",
+    worktree: "/tmp/fn-679",
+    baseBranch: "main",
+    ...overrides,
+  };
+}
+
+async function requestDiff(app: Parameters<typeof get>[0], taskId = "FN-679", worktree?: string): Promise<{ status: number; body: any }> {
+  const url = `/api/tasks/${taskId}/diff${worktree ? `?worktree=${encodeURIComponent(worktree)}` : ""}`;
+  return await get(app, url);
+}
+
+describe("GET /api/tasks/:id/diff", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    mockExistsSync.mockReturnValue(true);
+    vi.useFakeTimers();
+    vi.setSystemTime(new Date("2026-04-01T12:00:00.000Z"));
+  });
+
+  afterEach(() => {
+    vi.useRealTimers();
+  });
+
+  it("uses merge-base syntax with baseBranch from task", async () => {
+    const store = new MockStore();
+    store.addTask(createTask({ baseBranch: "develop" }));
+    mockExecSync.mockImplementation((command) => {
+      const cmd = String(command);
+      if (cmd === "git diff --name-status develop...HEAD") {
+        return "M\tsrc/app.ts\nA\tsrc/new.ts\n" as any;
+      }
+      if (cmd === 'git diff develop...HEAD -- "src/app.ts"') {
+        return `diff --git a/src/app.ts b/src/app.ts
+--- a/src/app.ts
++++ b/src/app.ts
+@@ -1,3 +1,4 @@
+ const foo = "bar";
++const baz = "qux";
+` as any;
+      }
+      if (cmd === 'git diff develop...HEAD -- "src/new.ts"') {
+        return `diff --git a/src/new.ts b/src/new.ts
+new file mode 100644
+--- /dev/null
++++ b/src/new.ts
+@@ -0,0 +1,3 @@
++const newFile = true;
+` as any;
+      }
+      throw new Error(`Unexpected command: ${cmd}`);
+    });
+
+    const app = createServer(store as any);
+    const response = await requestDiff(app);
+
+    expect(response.status).toBe(200);
+    expect(response.body.files).toHaveLength(2);
+    expect(response.body.files[0].path).toBe("src/app.ts");
+    expect(response.body.files[0].status).toBe("modified");
+    expect(response.body.files[1].path).toBe("src/new.ts");
+    expect(response.body.files[1].status).toBe("added");
+    expect(mockExecSync).toHaveBeenCalledWith(
+      "git diff --name-status develop...HEAD",
+      expect.objectContaining({ cwd: "/tmp/fn-679" }),
+    );
+  });
+
+  it("defaults to main when baseBranch is not set", async () => {
+    const store = new MockStore();
+    store.addTask(createTask({ baseBranch: undefined }));
+    mockExecSync.mockImplementation((command) => {
+      const cmd = String(command);
+      if (cmd === "git diff --name-status main...HEAD") {
+        return "M\tsrc/index.ts\n" as any;
+      }
+      if (cmd === 'git diff main...HEAD -- "src/index.ts"') {
+        return `diff --git a/src/index.ts b/src/index.ts
+--- a/src/index.ts
++++ b/src/index.ts
+@@ -1,2 +1,3 @@
+ const app = true;
++const initialized = true;
+` as any;
+      }
+      throw new Error(`Unexpected command: ${cmd}`);
+    });
+
+    const app = createServer(store as any);
+    const response = await requestDiff(app);
+
+    expect(response.status).toBe(200);
+    expect(response.body.files).toHaveLength(1);
+    expect(mockExecSync).toHaveBeenCalledWith(
+      "git diff --name-status main...HEAD",
+      expect.objectContaining({ cwd: "/tmp/fn-679" }),
+    );
+  });
+
+  it("returns 404 when task not found", async () => {
+    const store = new MockStore();
+
+    const app = createServer(store as any);
+    const response = await requestDiff(app, "NONEXISTENT");
+
+    expect(response.status).toBe(404);
+    expect(response.body.error).toBe("Task not found");
+    expect(mockExecSync).not.toHaveBeenCalled();
+  });
+
+  it("uses provided worktree path from query param", async () => {
+    const store = new MockStore();
+    store.addTask(createTask({ baseBranch: "feature" }));
+    mockExecSync.mockImplementation((command) => {
+      const cmd = String(command);
+      if (cmd === "git diff --name-status feature...HEAD") {
+        return "M\tpackage.json\n" as any;
+      }
+      if (cmd === 'git diff feature...HEAD -- "package.json"') {
+        return `diff --git a/package.json b/package.json
+--- a/package.json
++++ b/package.json
+@@ -1,3 +1,4 @@
+ {
+   "name": "test",
++  "version": "1.0.0"
+ }
+` as any;
+      }
+      throw new Error(`Unexpected command: ${cmd}`);
+    });
+
+    const app = createServer(store as any);
+    const response = await requestDiff(app, "FN-679", "/custom/worktree/path");
+
+    expect(response.status).toBe(200);
+    expect(mockExecSync).toHaveBeenCalledWith(
+      "git diff --name-status feature...HEAD",
+      expect.objectContaining({ cwd: "/custom/worktree/path" }),
+    );
+  });
+
+  it("falls back to HEAD when merge-base fails", async () => {
+    const store = new MockStore();
+    store.addTask(createTask({ baseBranch: "nonexistent" }));
+    mockExecSync.mockImplementation((command) => {
+      const cmd = String(command);
+      if (cmd === "git diff --name-status nonexistent...HEAD") {
+        throw new Error("merge-base failed");
+      }
+      if (cmd === "git diff --name-status HEAD") {
+        return "M\tREADME.md\n" as any;
+      }
+      if (cmd === 'git diff HEAD -- "README.md"') {
+        return `diff --git a/README.md b/README.md
+--- a/README.md
++++ b/README.md
+@@ -1,2 +1,3 @@
+ # Test
++New content
+` as any;
+      }
+      throw new Error(`Unexpected command: ${cmd}`);
+    });
+
+    const app = createServer(store as any);
+    const response = await requestDiff(app);
+
+    expect(response.status).toBe(200);
+    expect(response.body.files).toHaveLength(1);
+    expect(mockExecSync).toHaveBeenCalledWith(
+      "git diff --name-status HEAD",
+      expect.objectContaining({ cwd: "/tmp/fn-679" }),
+    );
+  });
+
+  it("returns empty files array when no changes", async () => {
+    const store = new MockStore();
+    store.addTask(createTask({ baseBranch: "main" }));
+    mockExecSync.mockReturnValue("" as any);
+
+    const app = createServer(store as any);
+    const response = await requestDiff(app);
+
+    expect(response.status).toBe(200);
+    expect(response.body.files).toEqual([]);
+    expect(response.body.stats).toEqual({
+      filesChanged: 0,
+      additions: 0,
+      deletions: 0,
+    });
+  });
+
+  it("correctly counts additions and deletions in patches", async () => {
+    const store = new MockStore();
+    store.addTask(createTask({ baseBranch: "main" }));
+    mockExecSync.mockImplementation((command) => {
+      const cmd = String(command);
+      if (cmd === "git diff --name-status main...HEAD") {
+        return "M\tsrc/changes.ts\n" as any;
+      }
+      if (cmd === 'git diff main...HEAD -- "src/changes.ts"') {
+        return `diff --git a/src/changes.ts b/src/changes.ts
+--- a/src/changes.ts
++++ b/src/changes.ts
+@@ -1,5 +1,8 @@
+ const original = true;
+-const removed = true;
+ const unchanged = true;
++const added1 = true;
++const added2 = true;
++const added3 = true;
+` as any;
+      }
+      throw new Error(`Unexpected command: ${cmd}`);
+    });
+
+    const app = createServer(store as any);
+    const response = await requestDiff(app);
+
+    expect(response.status).toBe(200);
+    expect(response.body.files).toHaveLength(1);
+    expect(response.body.files[0].additions).toBe(3);
+    expect(response.body.files[0].deletions).toBe(1);
+    expect(response.body.stats).toEqual({
+      filesChanged: 1,
+      additions: 3,
+      deletions: 1,
+    });
+  });
+});

packages/dashboard/src/routes.ts

@@ -6501,7 +6501,7 @@ Output ONLY the prompt text (no markdown, no explanations).`;
    */
   router.get("/tasks/:id/diff", async (req, res) => {
     try {
-      const task = store.getTask(req.params.id);
+      const task = await store.getTask(req.params.id);
       if (!task) {
         res.status(404).json({ error: "Task not found" });
         return;
@@ -6510,18 +6510,30 @@ Output ONLY the prompt text (no markdown, no explanations).`;
       const worktree = typeof req.query.worktree === "string" ? req.query.worktree : undefined;
       const cwd = worktree || store.getRootDir();
 
-      // Get the base commit - default to HEAD~1 if not provided
-      let baseCommit = "HEAD~1";
+      // Get the base branch for merge-base comparison
+      const baseBranch = task.baseBranch ?? "main";
+      let diffBase = `${baseBranch}...HEAD`;
 
       // Get the diff
       const { execSync } = await import("node:child_process");
       
-      // Get list of changed files
-      const filesOutput = execSync(`git diff --name-status ${baseCommit}..HEAD`, {
-        encoding: "utf-8",
-        cwd,
-        timeout: 10000,
-      });
+      // Get list of changed files using merge-base comparison
+      let filesOutput = "";
+      try {
+        filesOutput = execSync(`git diff --name-status ${diffBase}`, {
+          encoding: "utf-8",
+          cwd,
+          timeout: 10000,
+        }).trim();
+      } catch {
+        // Fallback to current HEAD if merge-base fails
+        diffBase = "HEAD";
+        filesOutput = execSync("git diff --name-status HEAD", {
+          encoding: "utf-8",
+          cwd,
+          timeout: 10000,
+        }).trim();
+      }
 
       const files: Array<{
         path: string;
@@ -6546,7 +6558,7 @@ Output ONLY the prompt text (no markdown, no explanations).`;
         // Get patch for this file
         let patch = "";
         try {
-          patch = execSync(`git diff ${baseCommit}..HEAD -- "${filePath}"`, {
+          patch = execSync(`git diff ${diffBase} -- "${filePath}"`, {
             encoding: "utf-8",
             cwd,
             timeout: 10000,