[venshine]

On macOS, switching accounts could report success while claude auth status reported signed out. Keychain writes copied JSON containing newlines; Apple's security -w returns hex for values with non-printable bytes, which Claude cannot parse as JSON. A stale .credentials.json could also cause ccswitch to select a different backend from Claude.

Default macOS to Keychain, and serialize Keychain writes as compact ASCII JSON without a trailing newline. Reject invalid JSON and non-finite numbers before replacing a live credential. Explicit CCSWITCH_BACKEND=file remains available. Update the security fixture to reproduce Apple's output behavior and keep the file smoke suite portable on macOS Bash 3.2.

Validation:

• Corrected fixture reproduced the original unreadable-credential/signed-out warning before the fix.
• 26 Keychain/integration checks passed with local Claude Code 2.1.305, including both accounts' login status and email, shared/isolated MCP credentials, Unicode/escaped newlines and invalid-write preservation.
• ShellCheck 0.11, all six Bash syntax checks and version consistency passed.
• Smoke suite: 88 passed; two process-inspection assertions fail because this host denies ps/pgrep. They still require CI verification.
• User confirmed real macOS switching works after applying the fix.

The automated tests use synthetic credentials and a mocked security(1); real-account confirmation was provided by the user. Apple's output behavior is documented in its source.