Conversation history unexpectedly syncs across devices without user consent

Describe the bug

Claude Code conversation history is syncing across devices when logged into the same Anthropic account. A conversation started on one machine (Windows 11) was visible in full on a separate machine (macOS) — including older segments — with no prior knowledge or opt-in from the user.

Expected behavior

Project files and conversation history should remain local to each device, as implied by Claude Code's local-first design. Cross-device sync, if intentional, should require explicit user consent and clear documentation.

Actual behavior

Full conversation history (including older segments) appeared on a separate macOS device logged into the same Anthropic account. The user was unaware this syncing was happening.

Steps to reproduce

1. Start a conversation in Claude Code on Device A (Windows 11)
2. Log into the same Anthropic account on Device B (macOS)
3. Open Claude Code on Device B
4. Observe that Device A's conversation history is visible

Environment

• Device A: Windows 11
• Device B: macOS
• Claude Code: latest version