feat: Add OpenAITokenCounter using their input tokens api

[sjrl]

Related Issues

• fixes #issue-number

Proposed Changes:

Follow up to #12195

Add OpenAITokenCounter, which uses OpenAI's input token counting API to return model-specific counts for
Haystack ChatMessage objects and optional tool schemas. Unlike local estimates, it supports OpenAI's exact
accounting for request formatting, images, files, and tools.

Here is an example:

from haystack.dataclasses import ChatMessage
from haystack.token_counters import OpenAITokenCounter

counter = OpenAITokenCounter("gpt-5-mini")
count = counter.count([ChatMessage.from_user("Hello!")])

I also added docs pages for the new token counter.

How did you test it?

Added new tests.

Notes for the reviewer

Checklist

• I have read the contributors guidelines and the code of conduct.
• I have updated the related issue with new insights and changes.
• I have added unit tests and updated the docstrings.
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test: and added ! in case the PR includes breaking changes.
• I have documented my code.
• I have added a release note file, following the contributors guidelines.
• I have run pre-commit hooks and fixed any issue.

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
haystack-docs	Ignored	Preview	Aug 4, 2026 2:00pm

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Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
haystack/core/pipeline
pipeline.py
haystack/token_counters
__init__.py
openai_counter.py					104
Project Total

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[anakin87]

Only a minor comment