Consider enabling HTTP2 by default

Confirm this is a feature request for the Python library and not the underlying OpenAI API.

• This is a feature request for the Python library

Describe the feature or improvement you're requesting

Motivation

The OpenAI Python SDK uses httpx as its HTTP client, but HTTP/2 is disabled by default (http2=False in httpx.Client.__init__). This means every SDK user is stuck with HTTP/1.1 unless they manually configure a custom httpx.Client(http2=True) — a detail most users never discover.

Checked against v2.44.0 (latest), the _DefaultHttpxClient (src/openai/_base_client.py:834) still doesn't pass http2=True, and pyproject.toml only declares httpx>=0.23.0, <1 with no [http2] extra:

class _DefaultHttpxClient(httpx.Client):
    def __init__(self, **kwargs: Any) -> None:
        kwargs.setdefault("timeout", DEFAULT_TIMEOUT)
        kwargs.setdefault("limits", DEFAULT_CONNECTION_LIMITS)
        kwargs.setdefault("follow_redirects", True)
        super().__init__(**kwargs)  # http2 defaults to False

Proposed Change

class _DefaultHttpxClient(httpx.Client):
    def __init__(self, **kwargs: Any) -> None:
        kwargs.setdefault("timeout", DEFAULT_TIMEOUT)
        kwargs.setdefault("limits", DEFAULT_CONNECTION_LIMITS)
        kwargs.setdefault("follow_redirects", True)
+       kwargs.setdefault("http2", True)
        super().__init__(**kwargs)

And optionally add an [http2] extra in pyproject.toml:

[project.optional-dependencies]
http2 = ["httpx[http2]"]

Prior Art

• Anthropic Python SDK (v0.104.1) already passes http2 through to the httpx transport (_base_client.py:871)
• httpx has supported HTTP/2 since v0.13 (current declared dep is httpx>=0.23.0)
• The h2 package is lightweight, pure-Python, and widely used
• HTTP/2 is supported across all major LLM API providers

Compatibility

• If the user provides a custom http_client, their configuration takes precedence — no breaking change
• If the server doesn't support HTTP/2, httpx gracefully downgrades to HTTP/1.1 (this has been handled by httpx for years)
• Connection limits already set by the SDK are fully compatible with HTTP/2
• No API changes needed — purely internal client configuration

Why Not Just Document It

Users can already do this manually:

import httpx
from openai import OpenAI
client = OpenAI(http_client=httpx.Client(http2=True))

But most users never find this knob. Making it the default means everyone benefits, especially developers using the SDK for production workloads with high request volumes. The downside is negligible: users who don't have h2 installed would get an ImportError at client construction (easily debuggable), or we mitigate this by making h2 optional and wrapping the import.

Additional context

Many popular LLM API providers already support HTTP/2 (confirmed via ALPN h2):

• OpenAI (api.openai.com)
• DeepSeek (api.deepseek.com)
• Anthropic (api.anthropic.com)

Performance Impact

HTTP/2 brings meaningful improvements, especially for chat completions (frequent, streaming requests):

Metric	HTTP/1.1	HTTP/2
TLS handshake	~100ms	~67ms (1-RTT)
Single request	~134ms	~123ms
Header compression	❌	✅ HPACK
Multiplexing	❌ (6 conn/browser limit)	✅ (1-2 connections)
Connections for 10 requests	10	1-2

The biggest win is connection reuse + multiplexing. In HTTP/1.1, every concurrent or back-to-back request opens a new TCP+TLS handshake. With HTTP/2, a single persistent connection handles multiple concurrent requests — critical for streaming chat.completions.create(stream=True) where the SDK opens many chunks over the same connection. Real-world test on DeepSeek API showed TLS handshake dropping from ~100ms to ~67ms just from HTTP/2's 1-RTT TLS.

Start with _DefaultHttpxClient in src/openai/_base_client.py and the dependency declarations in pyproject.toml. Determine how enabling HTTP/2 should handle the h2 dependency and existing custom http_client behavior; the work is done when the default client uses HTTP/2 without disrupting HTTP/1.1 fallback or user-provided clients.