ci/test_core.sh ran 'dart test test/ipc test/pty ... test/pql' in the default parallel pool, so the real-PTY tests in test/pty competed for fds + CPU with the other suites and the reader isolate was starved — intermittently failing 'write sends keystrokes to child' under load. Split into a serial --tags pty pass and a parallel --exclude-tags pty pass, mirroring ci/test.sh. Same 571 tests; pty pass verified stable across repeated runs. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>