feat(agent): data-grounding rule in the agent system prompt (never fabricate)

lib/chat/__tests__/buildAgentSystemPrompt.test.ts

@@ -4,7 +4,7 @@ import { buildAgentSystemPrompt } from "@/lib/chat/buildAgentSystemPrompt";
 describe("buildAgentSystemPrompt", () => {
   it("emits only customInstructions when no cwd is provided", () => {
     const prompt = buildAgentSystemPrompt({ customInstructions: "hello" });
-    expect(prompt).toBe("hello");
+    expect(prompt).toContain("hello");
     expect(prompt).not.toMatch(/Working directory/);
   });
 
@@ -26,7 +26,9 @@ describe("buildAgentSystemPrompt", () => {
     expect(customIdx).toBeGreaterThan(envIdx);
   });
 
-  it("returns empty string when all options are empty", () => {
-    expect(buildAgentSystemPrompt({})).toBe("");
+  it("always includes the data-grounding no-fabrication rule, even with empty options", () => {
+    const prompt = buildAgentSystemPrompt({});
+    expect(prompt).toMatch(/never fabricate/i);
+    expect(prompt).toMatch(/sample|estimate|industry average/i);
   });
 });

lib/chat/buildAgentSystemPrompt.ts

@@ -3,6 +3,15 @@ const ENVIRONMENT_SECTION = `# Environment
 Working directory: . (workspace root)
 Use workspace-relative paths for all file operations.`;
 
+const DATA_GROUNDING_SECTION = `# Data grounding — never fabricate
+
+State only figures, statistics, or facts you retrieved from a **successful tool
+call in this run**. If a data call fails, returns empty, or the source isn't
+connected, say so plainly (e.g. "no YouTube data connected for this artist") and
+omit the metric — send a shorter, honest report or stop. **Never** estimate, use
+"industry averages", or fill gaps with sample/placeholder/illustrative numbers. A
+short accurate report always beats a padded one built on invented data.`;
+
 export type BuildAgentSystemPromptOptions = {
   /**
    * Sandbox working directory. Triggers inclusion of the Environment
@@ -39,7 +48,8 @@ export type BuildAgentSystemPromptOptions = {
  * generated branches).
  */
 export function buildAgentSystemPrompt(options: BuildAgentSystemPromptOptions): string {
-  const parts: string[] = [];
+  // Always first: the no-fabrication rule applies to every agent run.
+  const parts: string[] = [DATA_GROUNDING_SECTION];
 
   if (options.cwd) {
     parts.push(ENVIRONMENT_SECTION);