Anthropic Vertex AI 413 Payload Too Large: Incorrect rejection of requests well under documented 32MB limit

What this error means

Error code: 413 - Request exceeds the maximum allowed number of bytes. The maximum request size is 32 MB (but payloads of ~2MB are rejected) is a Anthropic API failure pattern reported for developers trying to developer sends ~2mb document payload to claude sonnet 4 via vertex ai and gets 413 error despite being well under the documented 32mb limit — debugging difficult because error misreports the cause. 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 7, 2025 by abhiwebshar. Detailed root-cause analysis reveals Vertex AI incorrectly maps Citations API rate limit violations (actual 429 rate_limit_error) to misleading 413 ‘Prompt is too long’ errors. Payloads as small as 147KB are rejected with 413. Root cause: error misreporting, not actual payload size limit. Affects production use of large-context capabilities on Vertex AI. Also reports 403 errors around 200k tokens blocking full context window usage.

Common causes

• GitHub Issue #1028 on anthropics/anthropic-sdk-python opened Sep 7, 2025 by abhiwebshar. Detailed root-cause analysis reveals Vertex AI incorrectly maps Citations API rate limit violations (actual 429 rate_limit_error) to misleading 413 ‘Prompt is too long’ errors. Payloads as small as 147KB are rejected with 413. Root cause: error misreporting, not actual payload size limit. Affects production use of large-context capabilities on Vertex AI. Also reports 403 errors around 200k tokens blocking full context window usage.

Quick fixes

1. Confirm the exact error signature matches Error code: 413 - Request exceeds the maximum allowed number of bytes. The maximum request size is 32 MB (but payloads of ~2MB are rejected).
2. Check the Anthropic API account, local tool state, and provider configuration involved in the failing workflow.
3. Compare the failing environment with a known working setup, then change one configuration value at a time.

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 7, 2025 by abhiwebshar. Detailed root-cause analysis reveals Vertex AI incorrectly maps Citations API rate limit violations (actual 429 rate_limit_error) to misleading 413 ‘Prompt is too long’ errors. Payloads as small as 147KB are rejected with 413. Root cause: error misreporting, not actual payload size limit. Affects production use of large-context capabilities on Vertex AI. Also reports 403 errors around 200k tokens blocking full context window usage.

Related errors

• Anthropic API

FAQ

What should I check first?

Start with the exact Error code: 413 - Request exceeds the maximum allowed number of bytes. The maximum request size is 32 MB (but payloads of ~2MB are rejected) 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 - Request exceeds the maximum allowed number of bytes. The maximum request size is 32 MB (but payloads of ~2MB are rejected).