diff --git a/packages/services/src/agent-tools/__tests__/prompt.test.ts b/packages/services/src/agent-tools/__tests__/prompt.test.ts
new file mode 100644
index 00000000..23950fd9
--- /dev/null
+++ b/packages/services/src/agent-tools/__tests__/prompt.test.ts
@@ -0,0 +1,76 @@
+import { expect } from "@std/expect";
+import { describe, test } from "@std/testing/bdd";
+
+import {
+  type AgentSystemPromptOptions,
+  buildAgentSystemPrompt,
+} from "../prompt";
+
+const base: AgentSystemPromptOptions = {
+  workspaceName: "Acme",
+  surface: "dashboard",
+  canNotifySubscribers: true,
+};
+
+describe("buildAgentSystemPrompt", () => {
+  test("interpolates the workspace name and leaves no template gaps", () => {
+    const prompt = buildAgentSystemPrompt({
+      ...base,
+      workspaceName: "Acme Inc",
+    });
+    expect(prompt).toContain('workspace "Acme Inc"');
+    expect(prompt).not.toContain("${");
+  });
+
+  test("uses the injected clock for the date anchor", () => {
+    const prompt = buildAgentSystemPrompt({
+      ...base,
+      now: new Date("2026-01-02T03:04:05.000Z"),
+    });
+    expect(prompt).toContain("2026-01-02T03:04:05.000Z");
+  });
+
+  test("slack surface skips the dashboard-only post-tool rule", () => {
+    const prompt = buildAgentSystemPrompt({ ...base, surface: "slack" });
+    expect(prompt).toContain("running inside Slack");
+    expect(prompt).not.toContain("running inside the openstatus dashboard");
+    expect(prompt).not.toContain("already renders a structured view");
+  });
+
+  test("dashboard surface includes the post-tool rendering rule", () => {
+    const prompt = buildAgentSystemPrompt({ ...base, surface: "dashboard" });
+    expect(prompt).toContain("running inside the openstatus dashboard");
+    expect(prompt).toContain("already renders a structured view");
+    expect(prompt).not.toContain("running inside Slack");
+  });
+
+  test("asks about subscriber notifications when the plan allows it", () => {
+    const prompt = buildAgentSystemPrompt({
+      ...base,
+      canNotifySubscribers: true,
+    });
+    expect(prompt).toContain("Should I notify subscribers?");
+  });
+
+  test("forces notify false and drops the question when the plan does not", () => {
+    const prompt = buildAgentSystemPrompt({
+      ...base,
+      canNotifySubscribers: false,
+    });
+    expect(prompt).toContain("notify: false");
+    expect(prompt).not.toContain("Should I notify subscribers?");
+  });
+
+  test("prepends the preamble only when one is provided", () => {
+    const withPreamble = buildAgentSystemPrompt({
+      ...base,
+      preamble: "SLACK CORE.",
+    });
+    expect(withPreamble.startsWith("SLACK CORE.")).toBe(true);
+
+    const withoutPreamble = buildAgentSystemPrompt(base);
+    expect(withoutPreamble.startsWith("You are the openstatus assistant")).toBe(
+      true,
+    );
+  });
+});