Tokens

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

• Input tokens

• Your prompt
• System instructions

• Conversation history

• Output tokens

• The model's response

Billing

• You are billed for both input and output tokens

Context Window

• The number of tokens an LLM can consider 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: 4k tokens
• GPT 5.0: 1M tokens
• Opus/Sonnet 4.6: 1M tokens