Token count discrepancy between tiktoken and API response when messages contain tool calls

Description

I've encountered a significant discrepancy between token counts calculated by tiktoken locally and the actual token usage returned by the OpenAI API, specifically when messages contain tool calls.

Environment

• tiktoken version: 3.11
• Python version: 0.7.0
• Model: gpt-4.1-mini

Steps to Reproduce

1. Prepare a message payload that includes tool calls (function calling)
2. Calculate token count using tiktoken locally
3. Send the same payload to OpenAI API
4. Compare tiktoken's result with the usage field in API response

Expected Behavior

The token count calculated by tiktoken should be close to (or exactly match) the token usage reported by the API.

Actual Behavior

There is a large discrepancy between tiktoken's calculation and the API's reported token usage when tool calls are present in the message body.

tiktoken calculation

encoding = tiktoken.encoding_for_model("gpt-4")

Result

tiktoken_count = 47,194

API response shows: api_count = 140,384

Difference: -93,190

No repository file or test is identified in the report. Start by reproducing the tool-call case using the payload described, compare tiktoken.encoding_for_model("gpt-4") with the API response's usage field, and determine the source of the discrepancy. Done means the reported mismatch is explained and the relevant counting behavior is corrected or documented with a reproducible test.