The macOS keychain home's write and read have never run against the real security tool. Set stores a value through 'security -i' with the value hex-encoded behind -X on one stdin line (internal/core/credential/keychain.go), and Resolve reads it back with find-generic-password -w; a MaxValueBytes value (4096 bytes) makes that line about 8 KiB. Only the test binary's fake keychain (store_test.go TestMain) has proven the -X in, -w out round trip. If security's non-terminal line reader splits or truncates a long line, the read-back check after the write refuses loudly and echoes nothing, but a wrong item stays in the keychain for the person to remove by hand. One real-machine round trip is owed: a value of MaxValueBytes set through the keychain home, read back byte for byte, then the item removed. Surfaced by the review of the credential store lane (review-cred finding 5, unverified).

The macOS keychain home's write and read have never run against the real security tool. Set stores a value through 'security -i' with the value hex-encoded behind -X on one stdin line (internal/core/credential/keychain.go), and Resolve reads it back with find-generic-password -w; a MaxValueBytes value (4096 bytes) makes that line about 8 KiB. Only the test binary's fake keychain (store_test.go TestMain) has proven the -X in, -w out round trip. If security's non-terminal line reader splits or truncates a long line, the read-back check after the write refuses loudly and echoes nothing, but a wrong item stays in the keychain for the person to remove by hand. One real-machine round trip is owed: a value of MaxValueBytes set through the keychain home, read back byte for byte, then the item removed. Surfaced by the review of the credential store lane (review-cred finding 5, unverified).

Remedy grounds (2026-09-29)

• Why: the gap is evidence, not code; an autonomous run must not write to a real keychain, so the remedy is the one human act (option (a) of the owed item K+) with option (b), the size cap, as the fallback its result would trigger.
• Rejected: capping the size pre-emptively, which would narrow a limit nobody has shown to be wrong.