Tokens

• 1 token ≈ 3/4 of an English word, 1,000 tokens ≈ 750 words

• Input tokens

• Your prompt
• System instructions
• Conversation history
• Tool results

• File contents

• Output tokens

• The model's response

Billing

• You are billed for both input and output tokens

Context Window (Input tokens)

• The number of tokens an LLM can consider as input when generating text (the "size" of your prompt)
• Large context windows require more memory and processing power

• These days (2026) it's common to have a 1 million context window limit

• Limits

• GPT 3.5 (Nov 2022): 4k tokens - alongside ChatGPT launch
• GPT 5.0 (Aug 2025): 1M tokens
• Opus/Sonnet 4.6 (Fev 2026): 1M tokens