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diff --git a/src/features/heartbeat/ui/PromptEditor.test.ts b/src/features/heartbeat/ui/PromptEditor.test.ts
new file mode 100644
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--- /dev/null
+++ b/src/features/heartbeat/ui/PromptEditor.test.ts
@@ -0,0 +1,168 @@
+import { render, screen } from "@testing-library/svelte";
+import userEvent from "@testing-library/user-event";
+import { describe, expect, it, vi } from "vitest";
+import type {
+  HeartbeatConfigPatch,
+  HeartbeatConfigResult,
+  SaveHeartbeatConfig,
+} from "../logic/types";
+import PromptEditor from "./PromptEditor.svelte";
+
+// Fakes for the injected ports.
+
+function fakeLoadVariables() {
+  return vi.fn(async () => ({ ok: true, variables: [] }) as const);
+}
+
+function fakeLoadDefaultPrompt(template = "You are a helpful assistant.") {
+  return vi.fn(async () => ({ ok: true, template }) as const);
+}
+
+/** A capturing saveConfig that resolves ok, echoing the merged config shape. */
+function fakeSaveConfig(): {
+  calls: HeartbeatConfigPatch[];
+  impl: SaveHeartbeatConfig;
+} {
+  const calls: HeartbeatConfigPatch[] = [];
+  const impl: SaveHeartbeatConfig = async (patch) => {
+    calls.push(patch);
+    // Echo a config that reflects the persisted patch (so onSaved sync is realistic).
+    const config = {
+      enabled: false,
+      inactiveOnly: true,
+      systemPrompt: patch.systemPrompt ?? "",
+      taskPrompt: patch.taskPrompt ?? "",
+      intervalMinutes: 30,
+      model: "openai/gpt-4o",
+      reasoningEffort: null,
+    };
+    return { ok: true, config } satisfies HeartbeatConfigResult;
+  };
+  return { calls, impl };
+}
+
+const baseProps = (overrides: Record<string, unknown> = {}) => ({
+  systemPrompt: "",
+  taskPrompt: "",
+  loadVariables: fakeLoadVariables(),
+  loadDefaultPrompt: fakeLoadDefaultPrompt(),
+  saveConfig: fakeSaveConfig().impl,
+  onSaved: vi.fn(),
+  onClose: vi.fn(),
+  ...overrides,
+});
+
+describe("PromptEditor save flow", () => {
+  it("persists an edited system prompt and clears the unsaved state (regression: save flickered + reverted)", async () => {
+    const user = userEvent.setup();
+    const save = fakeSaveConfig();
+    const onSaved = vi.fn();
+    render(PromptEditor, {
+      props: baseProps({
+        // Start inheriting (empty override); the default pre-fills.
+        systemPrompt: "",
+        saveConfig: save.impl,
+        onSaved,
+      }),
+    });
+
+    // Wait for the default to load + pre-fill the system textarea.
+    const systemBox = await screen.findByLabelText("Heartbeat system prompt");
+    expect(systemBox).toHaveValue("You are a helpful assistant.");
+
+    // Save is disabled while it matches the default (no explicit edit).
+    expect(screen.getByRole("button", { name: "Save" })).toBeDisabled();
+
+    // Edit the system prompt → an override.
+    await user.clear(systemBox);
+    await user.type(systemBox, "custom override");
+
+    // Save is now enabled.
+    const saveBtn = screen.getByRole("button", { name: "Save" });
+    expect(saveBtn).toBeEnabled();
+    await user.click(saveBtn);
+
+    // The save port was called with the override persisted verbatim.
+    expect(save.calls).toHaveLength(1);
+    expect(save.calls[0]?.systemPrompt).toBe("custom override");
+    expect(onSaved).toHaveBeenCalledWith("custom override", "");
+
+    // THE REGRESSION: after save, hasChanges must clear (Save disabled again)
+    // and the "Saved." confirmation shows — NOT "Unsaved changes".
+    await vi.waitFor(() => {
+      expect(screen.getByRole("button", { name: "Save" })).toBeDisabled();
+    });
+    expect(screen.getByText("Saved.")).toBeInTheDocument();
+    expect(screen.queryByText(/Unsaved changes/i)).not.toBeInTheDocument();
+  });
+
+  it("persisting text that matches the default sends '' (inherit) and clears unsaved state", async () => {
+    const user = userEvent.setup();
+    const save = fakeSaveConfig();
+    render(PromptEditor, {
+      props: baseProps({
+        // Start with an override.
+        systemPrompt: "old override",
+        saveConfig: save.impl,
+      }),
+    });
+
+    const systemBox = await screen.findByLabelText("Heartbeat system prompt");
+    expect(systemBox).toHaveValue("old override");
+
+    // Reset to default → text matches the default → saving inherits ("").
+    await user.click(screen.getByRole("button", { name: "Reset to default" }));
+    expect(systemBox).toHaveValue("You are a helpful assistant.");
+
+    const saveBtn = screen.getByRole("button", { name: "Save" });
+    expect(saveBtn).toBeEnabled();
+    await user.click(saveBtn);
+
+    expect(save.calls).toHaveLength(1);
+    expect(save.calls[0]?.systemPrompt).toBe(""); // inherit
+    await vi.waitFor(() => {
+      expect(screen.getByRole("button", { name: "Save" })).toBeDisabled();
+    });
+    expect(screen.getByText("Saved.")).toBeInTheDocument();
+  });
+
+  it("editing the task prompt saves + clears unsaved state", async () => {
+    const user = userEvent.setup();
+    const save = fakeSaveConfig();
+    render(PromptEditor, {
+      props: baseProps({ saveConfig: save.impl }),
+    });
+
+    const taskBox = await screen.findByLabelText("Heartbeat task prompt");
+    await user.type(taskBox, "do the thing");
+
+    const saveBtn = screen.getByRole("button", { name: "Save" });
+    expect(saveBtn).toBeEnabled();
+    await user.click(saveBtn);
+
+    expect(save.calls[0]?.taskPrompt).toBe("do the thing");
+    await vi.waitFor(() => {
+      expect(screen.getByRole("button", { name: "Save" })).toBeDisabled();
+    });
+    expect(screen.getByText("Saved.")).toBeInTheDocument();
+  });
+
+  it("a failed save surfaces the error and keeps the edit unsaved", async () => {
+    const user = userEvent.setup();
+    const failingSave: SaveHeartbeatConfig = async () => ({ ok: false, error: "boom" });
+    render(PromptEditor, {
+      props: baseProps({ saveConfig: failingSave }),
+    });
+
+    const systemBox = await screen.findByLabelText("Heartbeat system prompt");
+    await user.clear(systemBox);
+    await user.type(systemBox, "custom");
+
+    await user.click(screen.getByRole("button", { name: "Save" }));
+
+    expect(screen.getByText("boom")).toBeInTheDocument();
+    // Still unsaved (Save stays enabled), no success badge.
+    expect(screen.getByRole("button", { name: "Save" })).toBeEnabled();
+    expect(screen.queryByText("Saved.")).not.toBeInTheDocument();
+  });
+});