> For a complete page index of the Captain API documentation, fetch https://docs.captain.dev/llms.txt?excludeSpec=true

# S3 Access-Key Auth for Syncs

* **S3 auth, two ways**: cross-account assume-role (recommended, no long-lived keys leave your account) or an access key. S3-compatible stores (R2, Supabase, Backblaze) use access-key auth.

Same day: [Sync Tools in the MCP Server](/changelog/2026-08-08-sync-tools-in-the-mcp-server), [Syncs](/changelog/2026-08-08-syncs)