Anthropic Vertex AI Incorrectly Rejects ~2MB Requests with 413 'Prompt Is Too Long' When Limit Is 32MB

What this error means

Error code: 413 - Prompt is too long (when payload is only ~2MB, well under documented 32MB limit) is a Anthropic API failure pattern reported for developers trying to fix anthropic vertex ai 413 error rejecting legitimate payloads under 32mb; differentiate between real prompt length limits vs incorrect rate-limit misreporting as 413. Based on the imported evidence, treat this as a tool-specific troubleshooting page rather than a generic API error.

Why this happens

GitHub Issue #1028 on anthropics/anthropic-sdk-python (opened Sep 8, 2025). Author found Vertex AI incorrectly maps Citations API rate limit violations (429) to 413 payload errors. Payloads of 147KB–2MB are rejected with ‘Request exceeds the maximum allowed number of bytes’. Root cause: error misreporting at the Vertex AI layer, not an actual payload size issue. Maps to ‘Anthropic API’ as the error originates from Anthropic’s cloud-serving channel via Vertex AI. High value because document processing workloads in enterprise are completely blocked.

Common causes

• GitHub Issue #1028 on anthropics/anthropic-sdk-python (opened Sep 8, 2025). Author found Vertex AI incorrectly maps Citations API rate limit violations (429) to 413 payload errors. Payloads of 147KB–2MB are rejected with ‘Request exceeds the maximum allowed number of bytes’. Root cause: error misreporting at the Vertex AI layer, not an actual payload size issue. Maps to ‘Anthropic API’ as the error originates from Anthropic’s cloud-serving channel via Vertex AI. High value because document processing workloads in enterprise are completely blocked.

Quick fixes

1. Confirm the exact error signature matches Error code: 413 - Prompt is too long (when payload is only ~2MB, well under documented 32MB limit).
2. Check the Anthropic API account, local tool state, and provider configuration involved in the failing workflow.
3. Reduce request pressure, check quota or plan limits, and retry with backoff instead of immediate repeated requests.

Platform/tool-specific checks

• Verify the command, editor, extension, or API client that produced the error.
• Compare local settings with CI, deployment, or editor-level settings when the error appears in only one environment.
• Avoid deleting credentials, local model data, or project settings until the failing scope is clear.

Step-by-step troubleshooting

1. Capture the exact error message and the command, editor action, or request that triggered it.
2. Check whether the failure is account/auth, quota/rate, model/provider, local runtime, or deployment configuration.
3. Review the source evidence below and compare it with your environment.
4. Apply one change at a time and rerun the smallest failing action.
5. Keep the working fix documented for the team or deployment environment.

How to prevent it

• Keep provider/tool configuration documented.
• Record non-secret diagnostics such as tool version, provider name, model name, and command path.
• Add a lightweight check before CI or production workflows depend on the tool.

Sources checked

• https://github.com/anthropics/anthropic-sdk-python/issues/1028

Evidence note: GitHub Issue #1028 on anthropics/anthropic-sdk-python (opened Sep 8, 2025). Author found Vertex AI incorrectly maps Citations API rate limit violations (429) to 413 payload errors. Payloads of 147KB–2MB are rejected with ‘Request exceeds the maximum allowed number of bytes’. Root cause: error misreporting at the Vertex AI layer, not an actual payload size issue. Maps to ‘Anthropic API’ as the error originates from Anthropic’s cloud-serving channel via Vertex AI. High value because document processing workloads in enterprise are completely blocked.

Related errors

• Anthropic API

FAQ

What should I check first?

Start with the exact Error code: 413 - Prompt is too long (when payload is only ~2MB, well under documented 32MB limit) text and the smallest action that reproduces it.

Can I ignore this error?

No. Treat it as a failed Anthropic API workflow until the root cause is understood.

Is this guaranteed to have one fix?

No. The imported evidence supports the troubleshooting path above, but tool behavior can vary by account, plan, version, provider, and local configuration.

How do I know the fix worked?

Rerun the same command, editor action, or request. The fix is working when that action completes without Error code: 413 - Prompt is too long (when payload is only ~2MB, well under documented 32MB limit).