diff --git a/.changeset/clean-birds-fetch.md b/.changeset/clean-birds-fetch.md
new file mode 100644
index 000000000..c54b0da2f
--- /dev/null
+++ b/.changeset/clean-birds-fetch.md
@@ -0,0 +1,5 @@
+---
+"@browserbasehq/stagehand": patch
+---
+
+Enforce <number>-<number> regex validation on act/observe for elementId
diff --git a/packages/core/lib/inference.ts b/packages/core/lib/inference.ts
index 7d60b4ae2..0779900b0 100644
--- a/packages/core/lib/inference.ts
+++ b/packages/core/lib/inference.ts
@@ -245,6 +245,7 @@ export async function observe({
         z.object({
           elementId: z
             .string()
+            .regex(/^\d+-\d+$/)
             .describe(
               "the ID string associated with the element. Never include surrounding square brackets. This field must follow the format of 'number-number'.",
             ),
@@ -382,6 +383,7 @@ export async function act({
   const actSchema = z.object({
     elementId: z
       .string()
+      .regex(/^\d+-\d+$/)
       .describe(
         "the ID string associated with the element. Never include surrounding square brackets. This field must follow the format of 'number-number'.",
       ),