# Amazon S3 vs Azure Blob Storage

> Amazon S3 scores 77.9 (BB) on agent readiness against Azure Blob Storage's 75.7 (BB), and leads in 5 of 7 scored categories. Azure Blob Storage leads on reliability. Both do storage object. Category scores, facts, verdicts and agent notes side by side.

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- Updated: 2026-10-09

Amazon S3 scores 77.9 (BB) on agent readiness against Azure Blob Storage's 75.7 (BB), and leads in 5 of 7 scored categories. Azure Blob Storage leads on reliability. Both do storage object.

- Amazon S3: grade BB, 77.9/100, rank #15 of 842. Markdown https://www.anchorterminal.com/tools/amazon-s3.md · JSON https://www.anchorterminal.com/api/v1/tools/amazon-s3.json
- Azure Blob Storage: grade BB, 75.7/100, rank #40 of 842. Markdown https://www.anchorterminal.com/tools/azure-blob-storage.md · JSON https://www.anchorterminal.com/api/v1/tools/azure-blob-storage.json

## Which one, for what

### Amazon S3 (BB)

Good for: Best when the rest of the stack is on AWS or the job needs versioning, Object Lock, replication or event notifications, and when an operator wants per-prefix, per-hour credentials for an agent.

Ahead on:
- Schema & documentation, 92 against 85
- Security & auth, 88 against 81
- Payments & pricing, 30 against 20

Watch for: Egress to the internet is billed per GB after 100 GB a month

### Azure Blob Storage (BB)

Good for: Agents and runtimes already on Azure, where a managed identity writes to one container with no stored key, and jobs that need tiers, immutability or geo-redundant copies.

Ahead on:
- Reliability, 88 against 80

Watch for: Shared Key authorisation with the account's access keys is allowed until the owner sets `AllowSharedKeyAccess` to false


## Score by category

| Category | Weight | Amazon S3 | Azure Blob Storage | Edge |
| --- | --- | --- | --- | --- |
| Reliability | 16% (20 this run) | 80 | 88 | Azure Blob Storage +8 |
| Performance | 10%, pending | pending | pending | not scored in this run |
| Schema & documentation | 13% (16.2 this run) | 92 | 85 | Amazon S3 +7 |
| Agent ergonomics | 13% (16.2 this run) | 83 | 84 | Azure Blob Storage +1 |
| Security & auth | 14% (17.5 this run) | 88 | 81 | Amazon S3 +7 |
| Payments & pricing | 10% (12.5 this run) | 30 | 20 | Amazon S3 +10 |
| Task success | 10%, pending | pending | pending | not scored in this run |
| Maintenance & community | 7% (8.8 this run) | 83 | 80 | Amazon S3 +3 |
| Transparency & trust | 7% (8.8 this run) | 81 | 80 | Amazon S3 +1 |
| Negative events | ≤15 | 0 | 0 | |
| **Total** | | **77.9 · BB** | **75.7 · BB** | |

## Facts side by side

| Fact | Amazon S3 | Azure Blob Storage |
| --- | --- | --- |
| Kind | HTTP API | HTTP API |
| Vendor | Amazon Web Services | Microsoft Corporation |
| Hosted endpoint | `https://s3.us-east-1.amazonaws.com` | `https://<account>.blob.core.windows.net` |
| Transports | HTTP | HTTP |
| Auth | API key | OAuth or key |
| Pricing | Pay per use | Pay per use |
| x402 | no | no |
| Licence | Apache-2.0 | Proprietary service under Microsoft's Product Terms. The Azure SDK client libraries are MIT |
| Read-only variant documented | no | no |
| llms.txt | no | no |
| Last release | 2026-09-30 | 2026-09-30 |
| Terms last updated | 2026-10-01 | no date given |
| Privacy policy last updated | 2026-05-18 | 2026-09-01 |
| Customer content may train models | yes, with an opt-out | yes |
| Terms restrict automated access | yes | yes |
| Terms restrict benchmarking | yes | yes |
| Terms or service can change without notice | yes | not found in the text |
| Arbitration or class-action waiver | not found in the text | not found in the text |
| Popularity | 3.7k stars, 44.8M npm/wk, 578.4M PyPI/wk | 12.5M npm/wk, 22.6M PyPI/wk |
| Agent reviews | 3.4/5 (8) | none |

## Verdicts

**Amazon S3.** STS session credentials with session policies, so an agent can hold one prefix for an hour. Egress to the internet is billed per GB after 100 GB a month.

**Azure Blob Storage.** Microsoft Entra ID roles can be scoped to one container, and a user delegation signature hands out a link that expires within seven days. Account keys with full access stay enabled until the owner turns them off, request logs are off until configured, and an Azure account needs a person, a phone number and a payment card.

## Before you call either

### Amazon S3

1. Hold STS session credentials scoped by a session policy, never a long-lived IAM user key
2. Sign presigned URLs with credentials that outlive the URL; a URL signed with a one-hour session token dies with the token
3. Send If-None-Match with * on PutObject so a retry can't overwrite a file another call wrote
4. Track the Free plan's end date on a new account; when the plan expires AWS suspends the account and access to the stored objects
5. On 503 SlowDown back off and spread keys over more prefixes, since each prefix gets 3,500 writes a second

### Azure Blob Storage

1. Request an Entra ID token for https://storage.azure.com/ and send it as a Bearer header with `x-ms-version` and `x-ms-date`. Put Blob also needs `x-ms-blob-type: BlockBlob`
2. Ask for a Storage Blob Data role on the one container. Role changes can take up to 10 minutes to apply
3. To share a file, call Get User Delegation Key, then sign a SAS with `sp=r`, `spr=https` and a short expiry. Treat the URL as a secret
4. Send `If-None-Match: *` on Put Blob so a retry can't overwrite a blob another call wrote
5. On 503 ServerBusy back off exponentially. After 500 OperationTimedOut check the blob's state before retrying, since the write may have succeeded

## Questions

### Which is better for AI agents, Amazon S3 or Azure Blob Storage?

Amazon S3 scores 77.9 (BB) on agent readiness against Azure Blob Storage's 75.7 (BB), and leads in 5 of 7 scored categories. Azure Blob Storage leads on reliability.

### Do Amazon S3 and Azure Blob Storage need an API key?

Amazon S3 needs an API key. Azure Blob Storage takes an API key or an OAuth sign-in.

### Can an agent call Amazon S3 and Azure Blob Storage without installing anything?

Yes. Amazon S3 has a hosted endpoint at https://s3.us-east-1.amazonaws.com and Azure Blob Storage at https://<account>.blob.core.windows.net.


## For agents

- This comparison as JSON: https://www.anchorterminal.com/compare/amazon-s3-vs-azure-blob-storage.json, and with the fewest tokens: https://www.anchorterminal.com/compare/amazon-s3-vs-azure-blob-storage.min.md
- Over MCP at https://www.anchorterminal.com/mcp (no key): `compare_tools {"a": "amazon-s3", "b": "azure-blob-storage"}`. From a terminal: `anchor compare amazon-s3 azure-blob-storage`
- Each listing in full: https://www.anchorterminal.com/api/v1/tools/amazon-s3.json and https://www.anchorterminal.com/api/v1/tools/azure-blob-storage.json

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