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Context-window fit checker

Will your prompt fit — and how much room is left for the answer? Exact for OpenAI models, a labeled estimate for everything else.
exact counts need the AI worker — unavailable here; estimate mode still works · registry snapshot 2026-08-26Check fitClearListed OpenAI model (exact)Other model (estimate)Model

o200k_base encoding · max output 128,000 tokens

Paste a prompt, then press Check fit.

- OpenAI rows are exact: the count comes from the model's own encoding, cross-checked against the installed tokenizer package's model catalog. Windows come from that same catalog (snapshot 2026-08-26 ).
- Claude and Gemini have no offline tokenizer here — their rows are a characters÷4 heuristic, labeled estimate, and this tool never calls a vendor API to refine them. Use the custom mode with the vendor's published window.
- Providers count differently: some bill the system prompt, tools, and message framing separately, and chat wrappers add tokens per message. Treat every number as a planning check, not an invoice.

- gpt-5.6-sol1,050,000 in
- gpt-5.6-terra1,050,000 in
- gpt-5.6-luna1,050,000 in
- gpt-5.6-cyber400,000 in
- gpt-5.51,050,000 in
- gpt-5.5-pro1,050,000 in
- gpt-5.41,050,000 in
- gpt-5.4-mini400,000 in
- gpt-5.4-nano400,000 in
- gpt-5.4-pro1,050,000 in
- … 33 models in the dated snapshot
Nothing in yet

Paste, drop, or type to begin. Everything stays on this device.

nothing checks until you press Check fit

Next door in AI

- Tokenizer playground (/ai/tokenizer)
- API cost estimator (/ai/cost-estimator)
All AI tools (/ai)

Canonical HTML: https://nutter.tools/ai/context-fit
Markdown version: https://nutter.tools/ai/context-fit/index.md
Plain-text version: https://nutter.tools/ai/context-fit/index.txt
Agent index: https://nutter.tools/llms.txt