page.observe() crashes on hallucinated element IDs - fixed in stagehand 2.4.2

TL;DR

If page.observe() crashes with a trimTrailingTextNode TypeError, two things are true: upgrade to stagehand 2.4.2 or newer, which handles the bad model output instead of crashing, and know that the underlying cause is model hallucination of element IDs.

Steps

1. If page.observe() crashes with a trimTrailingTextNode TypeError, two things are true: upgrade to stagehand 2.4.2 or newer, which handles the bad model output instead of crashing, and know that the underlying cause is model hallucination of element IDs. Even fixed, a hallucinated ID means the observation misses - retry the observe call, since the same instruction succeeds on retry once the model returns real IDs from the page.

When to use

You are seeing this: The reporter traced it to the model hallucinating element IDs (e.g. Use this skill when you run into "page.observe() crashes on hallucinated element IDs - fixed in stagehand 2.4.2".

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

Versions mentioned in the source: stagehand 2.4.2, stagehand v2.4.2. If you are on something much newer or older, the details may have shifted.

Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.

Source: Issue browserbase/stagehand#854 (closed, 13 comments): page.observe() intermittently crashed with TypeError - Cannot read properties of undefined (reading replace) at trimTrailingTextNode. The reporter traced it to the model hallucinating element IDs (e.g. 1100-1102 instead of the real 0-1102), and confirmed the crash is resolved in stagehand v2.4.2.

Verified resolution: If page.observe() crashes with a trimTrailingTextNode TypeError, two things are true: upgrade to stagehand 2.4.2 or newer, which handles the bad model output instead of crashing, and know that the underlying cause is model hallucination of element IDs. Even fixed, a hallucinated ID means the observation misses - retry the observe call, since the same instruction succeeds on retry once the model returns real IDs from the page.

Source URL: https://github.com/browserbase/stagehand/issues/854