[kuldeepsinh19]

Summary

Fixes #6215

When a function throws something that isn't an Error instance, such as airtable.js's AirtableError ({ error, message, statusCode }) or a plain string, netlify dev / functions:serve doesn't show the function's error. Instead, the request fails inside the CLI:

TypeError: Cannot read properties of undefined (reading 'map')
    at getNormalizedError (src/lib/functions/synchronous.ts:127:39)
    at handleSynchronousFunction (src/lib/functions/synchronous.ts:42:19)

and the response is Express's default error page with that stack trace.

lambda-local (2.2.0, build/lib/context.js) converts only Error instances into { errorType, errorMessage, stackTrace }, and leaves out stackTrace when the error has no stack. Every other thrown value is passed through unchanged. getNormalizedError() assumed all three properties were always there.

Changes:

• getNormalizedError() (synchronous.ts) takes unknown. It falls back to name/message (or the value itself for a string, inspect() otherwise) and uses an empty stack trace when there's none. Error instances and lambda-local's errors are handled as before.
• formatLambdaError() (utils.ts, used for scheduled and background functions) also threw on a thrown string (Cannot use 'in' operator to search for 'errorType' in boom), so it now handles non-object values too.

With the repro from the issue (an AirtableError-like throw), functions:serve now returns a 500 with Error: Could not find what you are looking for and logs Function hello has returned an error: Could not find what you are looking for.

Tests:

• New tests/unit/lib/functions/synchronous.test.ts calls handleSynchronousFunction with an Error, a lambda-local error, a lambda-local error without a stack trace, a non-Error object (JSON and HTML responses) and a string. The last 3 shapes fail on main with the error above.
• New case in scheduled.test.ts for a thrown string. It fails on main with the in error.
• npm run test:unit (849 tests), npm run typecheck, eslint and oxfmt --check on the changed files pass (Node 24, in a container). I didn't run the integration suites.

This change was written with help from an AI coding assistant (Claude Code). I've reviewed every line and can explain any of it.

---

• Open a bug/issue before writing your code (netlify dev crashes if function throws custom error with no stackTrace property #6215)
• Read the contribution guidelines
• Update or add tests
• Update or add documentation (no docs change needed)
• Make sure the status checks below are successful

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