Function countChatMessageTokens

core/llm/countTokens.ts:179–218

(
  modelName: string,
  chatMessage: ChatMessage,
)

}

function countChatMessageTokens(
  modelName: string,
  chatMessage: ChatMessage,
): number {
  // Doing simpler, safer version of what is here:
  // https://github.com/openai/openai-cookbook/blob/main/examples/How_to_count_tokens_with_tiktoken.ipynb
  // every message follows <|im_start|>{role/name}\n{content}<|end|>\n
  const BASE_TOKENS = 4;
  const TOOL_CALL_EXTRA_TOKENS = 10;
  const TOOL_OUTPUT_EXTRA_TOKENS = 10;
  let tokens = BASE_TOKENS;

  if (chatMessage.content) {
    tokens += countTokens(chatMessage.content, modelName);
  }

  if ("toolCalls" in chatMessage && chatMessage.toolCalls) {
    for (const call of chatMessage.toolCalls) {
      tokens += TOOL_CALL_EXTRA_TOKENS;
      tokens += countTokens(JSON.stringify(call), modelName); // TODO hone this
    }
  }

  if (chatMessage.role === "thinking") {
    if (chatMessage.redactedThinking) {
      tokens += countTokens(chatMessage.redactedThinking, modelName);
    }
    if (chatMessage.signature) {
      tokens += countTokens(chatMessage.signature, modelName);
    }
  }

  if (chatMessage.role === "tool") {
    tokens += TOOL_OUTPUT_EXTRA_TOKENS; // safety
    if (chatMessage.toolCallId) {
      tokens += countTokens(chatMessage.toolCallId, modelName);
    }
  }
  return tokens;
}

/**
 * Extracts and validates the tool call sequence from the end of a message array.