fix(prompts): substitute template variables with falsy values (0, false, "")

packages/__tests__/prompts/templates.test.ts

@@ -0,0 +1,49 @@
+import { describe, it, expect } from "@jest/globals";
+import { HeliconeTemplateManager } from "@helicone-package/prompts/templates";
+
+describe("HeliconeTemplateManager.substituteVariables", () => {
+  it("substitutes falsy-but-valid values (0, false, empty string)", () => {
+    expect(
+      HeliconeTemplateManager.substituteVariables(
+        "You have {{hc:count:number}} new messages",
+        { count: 0 },
+        {}
+      ).result
+    ).toBe("You have 0 new messages");
+
+    expect(
+      HeliconeTemplateManager.substituteVariables(
+        "Enabled: {{hc:flag:boolean}}",
+        { flag: false },
+        {}
+      ).result
+    ).toBe("Enabled: false");
+
+    expect(
+      HeliconeTemplateManager.substituteVariables(
+        "Name=[{{hc:name:string}}]",
+        { name: "" },
+        {}
+      ).result
+    ).toBe("Name=[]");
+  });
+
+  it("still substitutes truthy values and leaves missing variables untouched", () => {
+    expect(
+      HeliconeTemplateManager.substituteVariables(
+        "You have {{hc:count:number}} new messages",
+        { count: 5 },
+        {}
+      ).result
+    ).toBe("You have 5 new messages");
+
+    // Missing input -> placeholder is left as-is
+    expect(
+      HeliconeTemplateManager.substituteVariables(
+        "Hi {{hc:name:string}}",
+        {},
+        {}
+      ).result
+    ).toBe("Hi {{hc:name:string}}");
+  });
+});

packages/prompts/templates.ts

@@ -129,7 +129,7 @@ export class HeliconeTemplateManager {
     TEMPLATE_REGEX.lastIndex = 0;
     result = result.replace(TEMPLATE_REGEX, (match, name) => {
       const value = name.trim() in inputs ? inputs[name.trim()] : undefined;
-      return value ? String(value) : match;
+      return value !== undefined && value !== null ? String(value) : match;
     });
 
     return {