Backblaze B2 S3 endpoint rejects master keys; use an application key

TL;DR

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.

Steps

1. 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.

When to use

You are seeing this: If S3 calls fail auth while native calls succeed with the same account, the credential type is the first suspect. Use this skill when you run into "Backblaze B2 S3 endpoint rejects master keys; use an application key".

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.

Source: 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.

Verified resolution: 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.

Source URL: https://github.com/backblaze-labs/b2-mcp/blob/HEAD/AGENTS.md