fix(core): preserve template replacement tokens (#1402)

* fix(core): preserve template replacement tokens

* fix(core): stringify template variables once

Author: Iams4kura

.changeset/bright-prompts-smile.md

@@ -0,0 +1,5 @@
+---
+"@voltagent/core": patch
+---
+
+Preserve JavaScript replacement tokens in simple template variable values.

packages/core/src/voltops/template-engine.spec.ts

@@ -101,5 +101,27 @@ describe("Template Engine", () => {
 
       expect(result).toBe("Pattern: $1.50 (special)");
     });
+
+    it("should preserve replacement tokens in variable values", () => {
+      const content = "Amount: {{value}}";
+      const variables = { value: "$&" };
+      const result = engine.process(content, variables);
+
+      expect(result).toBe("Amount: $&");
+    });
+
+    it("should convert each variable to a string only once", () => {
+      let conversions = 0;
+      const content = "{{value}}/{{value}}";
+      const variables = {
+        value: {
+          [Symbol.toPrimitive]: () => `value-${++conversions}`,
+        },
+      };
+      const result = engine.process(content, variables);
+
+      expect(result).toBe("value-1/value-1");
+      expect(conversions).toBe(1);
+    });
   });
 });

packages/core/src/voltops/template-engine.ts

@@ -22,7 +22,8 @@ export const createSimpleTemplateEngine = (): TemplateEngine => ({
     let processed = content;
     for (const [key, value] of Object.entries(variables)) {
       const regex = new RegExp(`{{\\s*${key}\\s*}}`, "g");
-      processed = processed.replace(regex, String(value));
+      const replacement = String(value);
+      processed = processed.replace(regex, () => replacement);
     }
     return processed;
   },