Now I understand the full picture. Let me analyze:

1. compatibility-date.capnp change: $experimental -> $compatEnableDate("2026-04-21") — This graduates the enhanced_error_serialization flag from experimental to a standard compat flag with a future enable date of 2026-04-21. This is a standard graduation pattern.

2. streams-test.wd-test: Adds "enhanced_error_serialization" to the explicit compatibility flags. Since the default test variant uses compat date 2000-01-01 (well before 2026-04-21), the flag must be explicitly listed for the test to use it. This is correct.

3. streams-test.js: The ExampleError test case changes:

   • Old: expectError: 'Error: ExampleError: foo bar' with errorType: Error — Without enhanced error serialization, the custom error class isn't preserved through serialization; it becomes a plain Error wrapping the original string.
   • New: expectError: 'ExampleError: foo bar' with errorType removed — With enhanced error serialization, the custom name property is preserved, so String(e) gives 'ExampleError: foo bar' instead of 'Error: ExampleError: foo bar'.

The removal of errorType: Error makes sense because with enhanced error serialization, the error's name is preserved but the deserialized object may not literally be an ExampleError instance (since ExampleError is a custom class that can't be reconstructed). The instanceof Error check would still pass, but the test just doesn't assert a specific type anymore — which is fine since the String(e) comparison already validates the name.

This is a clean, minimal graduation of an experimental compat flag. The enable date is ~25 days in the future from the PR creation date, giving adequate lead time. The test adjustment correctly reflects the changed behavior when the flag is enabled.

LGTM

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