The agent:bootstrap internal hook fires a "f.toLowerCase is not a function" error on every agent start and periodically during runtime. No stack trace is emitted. Error is non-blocking — agents still respond — but floods logs. 3. Observe logs — error fires on every bootstrap event The agent:bootstrap hook should either succeed silently or, if an error occurs, emit a stack trace identifying the source. Error "f.toLowerCase is not a function" is logged without a stack trace on every agent bootstrap and periodically during runtime. The source cannot be isolated from the log output alone. Agents continue to function despite the error. Error occurs on every agent:bootstrap event: 2026-04-15T10:09:07.076+00:00 [internal-hooks] Hook error [agent:bootstrap]: f.toLowerCase is not a function 2026-04-15T10:11:17.758+00:00 [internal-hooks] Hook error [agent:bootstrap]: f.toLowerCase is not a function 2026-04-17T09:18:26.331+00:00 [internal-hooks] Hook error [agent:bootstrap]: f.toLowerCase is not a function 2026-04-17T09:18:30.816+00:00 [internal-hooks] Hook error [agent:bootstrap]: f.toLowerCase is not a function 2026-04-17T12:08:33.168+00:00 [internal-hooks] Hook error [agent:bootstrap]: f.toLowerCase is not a function

TL;DR

The most likely fix for the "f.toLowerCase is not a function" error on every agent start and periodically during runtime is to apply a defensive null coalescing fix to ensure that the variable being called with toLowerCase() is a string.

Guidance

• Review the code for the agent:bootstrap internal hook to identify where the f.toLowerCase() call is being made and ensure that f is a string before calling toLowerCase() on it.
• Verify that all model configurations and hooks are properly set up with string values to prevent undefined or non-string values from being passed to the agent:bootstrap hook.
• Consider adding a null coalescing fix, similar to the one applied in issue #5062, to handle cases where the variable being called with toLowerCase() might be undefined or null.
• Check the logs again after applying the fix to confirm that the error is no longer occurring.

Example

// Before
const result = f.toLowerCase();

// After (with null coalescing fix)
const result = (f || '').toLowerCase();

Notes

The exact fix may depend on the specific code implementation of the agent:bootstrap hook, which is not provided in the issue. However, applying a defensive null coalescing fix is a safe and reasonable approach to prevent the "f.toLowerCase is not a function" error.

Recommendation

Apply a workaround by adding a null coalescing fix to ensure that the variable being called with toLowerCase() is a string, as this is a safe and reasonable approach to prevent the error without requiring a version upgrade.