# AN-2026-0571: Anthropic API: 400 invalid_request_error "prompt is too long" = the input alone exceeds the model's context window (1M or 200k tokens); retrying will not help

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- id: AN-2026-0571 (version 1; cite https://breakwire.org/claim/AN-2026-0571/v/1)
- kind: gotcha · status: active · review: editor
- source: <https://platform.claude.com/docs/en/build-with-claude/context-windows#context-window-overflow-behavior>
- more sources: <https://platform.claude.com/docs/en/build-with-claude/context-windows.md> <https://github.com/anthropics/claude-cookbooks/blob/bfe50e59dbb4f5546a0af9ca36ebb77e637c9ce1/extended_thinking/extended_thinking.ipynb>
- services: anthropic-api
- severity: medium
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. Anthropic's Context windows documentation (Context window overflow behavior) says that if the input alone already exceeds the model's context window, the API returns a 400 invalid_request_error ("prompt is too long") on every model.  [✓0 ✗0]
1. The same page says that on Claude 4.5 models and newer, a request whose input tokens plus max_tokens exceed the context window is accepted, and generation that reaches the limit stops with stop_reason model_context_window_exceeded; on earlier models the API returns a validation error instead (opt in with the model-context-window-exceeded-2025-08-26 beta header).  [✓0 ✗0]
2. Context window sizes by model (same page): the Claude 5.x family, Opus 4.6 to 4.8, Sonnet 4.6, Haiku 5.5 and Claude Mythos Preview have a 1M-token context window by default; other Claude models, including Claude Sonnet 4.5 (deprecated), have 200k. Cached prompt prefixes still count toward the window.  [✓0 ✗0]
3. Anthropic's official claude-cookbooks extended-thinking notebook shows the full error body: Error code: 400, type invalid_request_error, message of the form prompt is too long: <input tokens> tokens > <limit> maximum.  [✓0 ✗0]

## Fix
```text
Shrink the input below the model's window before resending: count tokens first with the token counting API, drop or summarize old turns (server-side compaction is Anthropic's recommended strategy for long conversations), clear old thinking/tool results, and trim large documents or base64 payloads. Or switch to a 1M-context model if you are on a 200k one. Do not retry the same request unchanged.
```

## Error signatures
- substring: `prompt is too long` (verbatim in Anthropic's Context windows docs (overflow behavior, opened 2026-10-29); full API message prompt is too long: N tokens > N maximum shown in anthropics/claude-cookbooks @bfe50e5. Scoped to )

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0571/reports_

## History
- v1 2026-10-29 by wire-editor: filed from primary source opened 2026-10-28