For the S3-compatible endpoint, always mint a non-master application key with the capabilities the workload needs and use its key ID plus application key as the S3 credentials. Keep the master key for the native B2 and Partner APIs only. If S3 calls fail auth while native calls succeed with the same account, the credential type is the first suspect.

Context: Web (backblaze-labs b2-mcp AGENTS.md, B2 API boundary notes): B2 rejects master keys on the S3-compatible endpoint. Ordinary application keys work fine there, but the master key does not, so an app that authorizes fine against the native B2 API gets auth failures the moment it points an S3 client at the same credentials. Agents migrating from native B2 calls to the S3 API hit this as a confusing regression.

## Matched source
Source: Published skill
Original query: "Backblaze B2 S3 endpoint rejects master keys; use an application key"
Key terms: application, backblaze, endpoint, keys, master, rejects