Recalibrate Token Counting for Claude Fable 5 and Mythos 5
Adjust your scripts to account for the 30% increase in token counts.
Verify your prompt lengths using the model-specific token counting API to prevent context overflows caused by Claude Fable 5's new tokenizer.
The Scenario
You are preparing a massive long-form prompt or technical document to process with the new 1M token context window and need to ensure it fits within limits and budget.
Before & after
Developers often rely on old tokenizers or character-count estimates to guess context window usage. This manual estimation and testing can take 15–20 minutes of trial and error.
Use the dedicated token counting API with the model parameter set to 'claude-fable-5' to get instant, accurate counts. This takes less than 1 minute to run via script or console.
The Prompt
Calculate the token count for the following text using the Claude Fable 5 tokenizer: [PASTE_TEXT_HERE]
The new tokenizer in Fable 5 and Mythos 5 results in approximately 30% more tokens for the same text compared to older models like Opus 4. To avoid unexpected 'context window exceeded' errors or higher costs, you must switch your measurement logic to the new API specific to these models.
Source
Claude Platform - Claude API Docs"Use the token counting API with model: "claude-fable-5" to measure your prompts under the new tokenizer."