An async API-client base for Python

Why headgate

🧰 One class, not a stack

A production client usually grows an HTTP library, a retry library, a rate limiter, a cache, a circuit breaker, an event-stream parser, a mocking library and the glue between them. headgate is all of them, built to cooperate: a cache hit is never retried, a retry waits its rate turn, and one deadline bounds the lot.

🛡️ Secure by default

Credentials stay on your origin on every redirect and next-page link. An https-to-http redirect is refused. An SSRF guard with DNS pinning is one option away. Secrets are redacted from every error, and nothing is read from the environment.

📜 Built on the standards

17 specifications, most followed by default: Retry-After, the IETF RateLimit headers, Problem Details (RFC 9457), Link pagination (RFC 8288), Deprecation and Sunset, and Idempotency-Key once you turn it on.

A client in one screen

A client is a subclass. The configuration lives in __init__, and each endpoint method stays one line. This is part of examples/github_client.py, a tested client for the GitHub REST API:

from headgate import AsyncBaseClient, BearerAuth, Expect, HttpOptions, LinkHeader, Rate, RateBudget, nothing
from headgate.msgspec import decoder

# Readers and pagers are values: build them once, at import, where a bad declaration fails.
REPOSITORY = decoder(Repository)
REPOSITORY_OR_NONE = Expect(REPOSITORY).on(HTTPStatus.NOT_FOUND, nothing)  # a missing or hidden repository
ISSUES = decoder(list[Issue])
ISSUE_PAGES = LinkHeader(items=_issues)


class GitHub(AsyncBaseClient):
    """Read repositories and their open issues from the GitHub REST API."""

    def __init__(self, token: str, *, transport: httpx2.AsyncBaseTransport | None = None) -> None:
        """Authenticate with *token*; *transport* replaces the network, as a test does."""
        super().__init__(
            base_url=API,
            # Well under the 5,000 requests an hour a token gets, and never more than 50 at once.
            rate=RateBudget(Rate.per_second(10), max_in_flight=50),
            auth=BearerAuth(token),
            http=HttpOptions(
                transport=transport,
                headers={"Accept": "application/vnd.github+json", "X-GitHub-Api-Version": "2022-11-28"},
            ),
        )

    async def repository(self, owner: str, name: str) -> Repository | None:
        """Return the repository *owner*/*name*, or ``None`` when it does not exist or the token cannot see it."""
        return await self.get(t"repos/{owner}/{name}", expect=REPOSITORY_OR_NONE)

    def open_issues(self, owner: str, name: str) -> Paginated[list[Issue], Issue]:
        """Return the open issues (and pull requests) of *owner*/*name*, 100 a page, fetched as they are iterated."""
        params = {"state": "open", "per_page": 100}
        return self.paginate(t"repos/{owner}/{name}/issues", expect=ISSUES, pages=ISSUE_PAGES, params=params)

Nothing in those methods mentions retries, pacing, GitHub's X-RateLimit-* headers, timeouts or where the token may go, and nothing needs to. Those come from the base class.

What it replaces

• 🔁 Retries (instead of tenacity, stamina or backoff): RetryPolicy, safe re-sends only, Retry-After obeyed, a budget across calls.
• 🚦 Rate limits (instead of aiolimiter or pyrate-limiter): RateBudget, paced, and paused when the server says so.
• ⏱️ Deadlines (instead of asyncio.timeout around each call, by hand): one deadline over every attempt, wait and buffered read.
• 🔌 Circuit breaking (instead of pybreaker or purgatory): a breaker per origin, charged only for the host's own failures.
• 🗄️ Caching (instead of requests-cache, or hishel wired in by hand): hishel, partitioned by credential.
• 📄 Pagination (instead of a loop per endpoint): paginate(), cursors, offsets, Link headers, next URLs.
• 📡 Server-sent events (instead of httpx-sse): stream().aiter_events(), with resume.
• 🧪 Test doubles (instead of respx or pytest-httpx): headgate.testing, a fake network and a clock that makes waits instant.

Compared with HTTP clients

✅ built in · 🧩 with another library or your own code · ➖ not available

	headgate	requests	httpx	aiohttp
Async	✅	➖	✅	✅
Retries that wait for Retry-After	✅	🧩	🧩	🧩
One deadline over every retry	✅	🧩	🧩	🧩
Rate limiting that obeys the server	✅	🧩	🧩	🧩
Circuit breaker	✅	🧩	🧩	🧩
Every credential kept off other hosts	✅	🧩	🧩	🧩
SSRF guard with DNS pinning	✅	🧩	🧩	🧩
RFC 9457 problem details parsed	✅	➖	➖	➖
Test fakes	✅	🧩	🧩	🧩

The full comparison covers about 30 libraries, and says when each is the better pick.

Install

Python 3.14 or newer.

pip install 'headgate[msgspec]'    # or: uv add 'headgate[msgspec]'. headgate, and the msgspec codec
pip install 'headgate[pydantic]'   # headgate, and the pydantic codec
pip install 'headgate[curl]'       # + AsyncCurlTransport: a browser's TLS fingerprint, via curl_cffi
pip install 'headgate[otel]'       # + OpenTelemetryObserver (opentelemetry-api only)
pip install 'headgate[cache]'      # + what hishel's AsyncSqliteStorage needs (hishel[async]: anysqlite)

pip install headgate alone brings no serialization library: httpx2, hishel and typing-extensions only. Pick a codec as an extra, or bring your own (see codecs).

• Why 3.14: endpoint paths are template strings (t"orders/{order_id}"), so each value is escaped into its own path segment by default: BTC/USD becomes BTC%2FUSD instead of a new path. Template strings exist only from Python 3.14.
• Why httpx2: it is pydantic's maintained fork of httpx, and headgate is built and tested on it alone. Its API is httpx's, so moving a client over is mostly import httpx2 where you had import httpx. Upstream httpx is not supported: passing one of its objects raises a TypeError naming the mistake. See why httpx2.

Quickstart

A walkthrough of a whole client: typed responses, a declared 404, and cursor pagination. A path is a t-string, so every value is escaped as one segment; expect= declares what the endpoint answers.

from http import HTTPStatus
from typing import TYPE_CHECKING

import msgspec

from headgate import AsyncBaseClient, BearerAuth, Cursor, Expect, HttpOptions, Rate, nothing
from headgate.msgspec import decoder

if TYPE_CHECKING:
    import httpx2

    from headgate import Paginated


class Series(msgspec.Struct, frozen=True):
    """A data series: its id, title and observations."""

    id: str
    title: str
    values: list[float]


class Release(msgspec.Struct, frozen=True):
    """A scheduled data release."""

    id: int
    name: str


class ReleasePage(msgspec.Struct, frozen=True):
    """One page of releases, and the cursor of the next page (``None`` on the last)."""

    releases: list[Release]
    next_cursor: str | None = None


def releases_on(page: ReleasePage) -> list[Release]:
    """Return the releases on *page*."""
    return page.releases


def cursor_after(page: ReleasePage) -> str | None:
    """Return the cursor of the page after *page*."""
    return page.next_cursor


# Readers and pagers are values: build them once, at import, where a bad declaration fails.
SERIES = decoder(Series)
SERIES_OR_NONE = Expect(SERIES).on(HTTPStatus.NOT_FOUND, nothing)  # the API documents 404 as "no such series"
RELEASE_PAGES = Cursor(items=releases_on, next=cursor_after, param="cursor")


class CatalogClient(AsyncBaseClient):
    """The catalog API: one method per endpoint, each one line."""

    def __init__(self, api_key: str, *, transport: httpx2.AsyncBaseTransport | None = None) -> None:
        """Authenticate with *api_key*; *transport* replaces the network, as a test does."""
        super().__init__(
            base_url="https://api.catalog.example/v1",
            rate=Rate.per_second(10),
            auth=BearerAuth(api_key),
            http=HttpOptions(transport=transport),
        )

    async def get_series(self, series_id: str) -> Series:
        """Return the series *series_id*; a 404, undeclared here, raises ``UpstreamStatusError``."""
        return await self.get(t"series/{series_id}", expect=SERIES)

    async def find_series(self, series_id: str) -> Series | None:
        """Return the series *series_id*, or ``None`` when the API answers 404."""
        return await self.get(t"series/{series_id}", expect=SERIES_OR_NONE)

    def releases(self) -> Paginated[ReleasePage, Release]:
        """Return every release, fetched a page at a time as the iteration reaches it."""
        return self.paginate("releases", expect=decoder(ReleasePage), pages=RELEASE_PAGES)

Use it with async with, which closes the connection pool on the way out:

async def main(api_key: str) -> tuple[float, Series | None, list[str]]:
    """Read one series, look up one that may not exist, and list every release."""
    async with CatalogClient(api_key) as catalog:
        gdp = await catalog.get_series("GDP")
        retired = await catalog.find_series("RETIRED")  # None: the API answered 404
        names = [release.name async for release in catalog.releases()]
    return gdp.values[-1], retired, names

Nothing in those methods mentions retries, rate limits, timeouts or credentials, and nothing needs to: get_series("GDP") is paced by the client's budget, bounded by a 60-second deadline, retried on a 503 or a dropped connection, refused fast while the origin's breaker is open, and fails with an error that names the client, the request line, the status, the request id and every attempt. examples/ has this client, the same client in both codec flavors, and a fuller one (orders_client.py): a minted OAuth token, an idempotent POST, a declared business refusal, Link-header pages and a streamed export.

What every client gets

Capability	What it does by default
Declared responses	A reader for any 2xx, Expect(...).on(status, reader) for documented non-2xx answers; anything else raises
Retries	3 sends with jittered backoff; idempotent methods after a network failure or a 408, 425, 429, 500, 502, 503, 504; a POST only when provably unprocessed or keyed
Retry budget and hedging	Retries capped at 20% of first attempts (plus 10 a second) per client; opt-in hedged sends for slow idempotent calls
Retry-After	Waited exactly up to 30 s, past that raised with retry_after set
Rate budgets	GCRA pacing, shareable across clients and threads, per-path lanes, an optional in-flight cap
Server feedback	429/503 and RateLimit / X-RateLimit-* headers pause the whole budget; adaptive throttling (AIMD)
Circuit breaker	Per origin, opens after 5 consecutive origin failures, one probe after 60 s
Deadlines and timeouts	60 s for the whole call; each attempt's timeouts cut to what is left
Credentials stay home	Auth headers and params go only to base_url's origin and , on every hop

How the pieces fit together, for contributors and the curious: design.

The contracts

1. Declared answers, or raise. A call returns what its reader read from a 2xx or from a status it declared, and raises a ClientError for anything else. There is no raw mode and no None for "it failed".
2. Transport only. A client does I/O and decoding. Retries, pacing and the breaker live in the transport stack; business decisions belong to the caller.
3. Wire types are data. Responses decode into plain typed values (msgspec structs or pydantic models) with no behavior.
4. No logging; typed events instead. headgate writes no log records. It reports to the observers you install, with sanitized data only, and its errors carry what you need to diagnose a failure.
5. No environment reads. Credentials are passed in. No proxy variable, .netrc or SSL_CERT_FILE is read: the HTTP client and its transport are built with trust_env=False.

Configuration

AsyncBaseClient(*, base_url, rate=UNLIMITED, auth=None, policy=TransportPolicy(), http=HttpOptions()). rate is a Rate (a budget of the client's own) or a RateBudget shared with other clients; without one the client paces nothing itself but still waits out the pauses the server asks for. TransportPolicy says what the stack does to every request, HttpOptions how requests are sent and read. Both are frozen and validate on construction.

class OrdersClient(AsyncBaseClient):
    """The shop's order API."""

    def __init__(
        self,
        client_id: str,
        client_secret: str,
        *,
        observers: tuple[Observer, ...] = (),
        transport: httpx2.AsyncBaseTransport | None = None,
    ) -> None:
        """Mint tokens with *client_id* and *client_secret*; report to *observers*; *transport* replaces the network."""
        super().__init__(
            base_url="https://api.shop.example/v2",
            rate=Rate.per_second(20),
            auth=ShopTokenAuth(client_id, client_secret),
            policy=TransportPolicy(
                retry=RetryPolicy(attempts=4),  # four sends at most, the first included
                deadline=30.0,  # each call, every attempt and wait included
                idempotency=IdempotencyKeys(),  # every POST and PATCH carries a key, so it is retried safely
                rate_limits=(RateRule("/exports", Rate.per_minute(6)),),  # a slower lane for the export endpoint
                routes=(Route("/exports", deadline=300.0),),  # and a longer deadline
            ),
            http=HttpOptions(
                transport=transport,
                timeout=Timeouts(read=20.0),
                user_agent="shop-sync/1.4",
                observers=observers,
            ),
        )

TransportPolicy	Default
retry	RetryPolicy(): 3 attempts
deadline	60.0 seconds per call
breaker	BreakerPolicy(): 5 failures, 60 s
rate_limits	none
cache	none
idempotency	none
routes	none

HttpOptions	Default
timeout	Timeouts(): connect 10 s, read 30 s, write 10 s, pool 15 s
headers	none beyond User-Agent
follow_redirects, max_redirects	True, 5
max_response_bytes	32 MiB
security	SecurityOptions(): credentials to base_url's origin only, SSRF guard off
refusal_check	none
observers, middleware	none

Every option, its validation and the precedence of call, view, route and policy: configuration.

Per-call options

Every verb (get, post, put, patch, delete, head) takes the URL, expect= (on head, nothing unless given), and optionally retry=False or a RetryPolicy, follow_redirects=, cache_ttl=, timeout=, deadline=, idempotency_key=, and the request options params=, headers=, body= (what a codec's encoder wrote), json=, data=, content= and files=. client.with_options(headers=..., timeout=..., deadline=..., retry=...) returns a view with other defaults that shares the pool, the budgets and the breakers; the client it came from owns the pool.

Errors

Every failure is a ClientError, so an application boundary can map the tree onto its own answers:

ClientError                      also: an unparsable URL, a pagination loop
├── TransportError               a network failure that outlasted retries
│   ├── ClientTimeoutError       a timeout, or the call's deadline
│   └── CircuitOpenError         the origin's breaker refused the call unsent
├── UpstreamStatusError          an undeclared status; a 5xx that outlasted retries; a refusal in a body
│   ├── AuthError                401 or 403: our credential was refused
│   └── RateLimitError           429, a quota refusal, or a server pause past the deadline
├── ResponseParseError           a 2xx body the reader could not read
├── ResponseTooLargeError        a body past max_response_bytes
├── ForbiddenDestinationError    a hop the SSRF guard refused, or a token mint sent off its origin
├── PageLimitError               a paginated walk spent its page budget with pages left
└── ClientConfigurationError     our own setup is wrong

Every error carries method, url (credentials redacted), request_id, attempts, retryable, retry_after and a note per attempt (attempt 2/3: 503 Service Unavailable (Retry-After 2.0s) after 0.41s). An UpstreamStatusError adds status_code, the provider's code and its RFC 9457 problem. All of them pickle.

def gateway_status(error: ClientError) -> HTTPStatus:
    """Return the status an API gateway answers with when a call to an upstream failed with *error*."""
    match error:
        case ClientTimeoutError():
            return HTTPStatus.GATEWAY_TIMEOUT  # the upstream was too slow, or the call's deadline passed
        case ClientConfigurationError():
            return HTTPStatus.SERVICE_UNAVAILABLE  # our own setup is wrong; an operator must act
        case _:
            return HTTPStatus.BAD_GATEWAY  # the upstream failed, refused us, or sent something unreadable

More in errors.

Testing your client

headgate.testing tests a client through its real stack, with nothing private patched: a FakeTransport stands in for the network, and a TestClock makes every wait instant.

from quickstart import CatalogClient

from headgate.testing import FakeTransport, Respond, TestClock, isolated_breakers

GDP = {"id": "GDP", "title": "Gross domestic product", "values": [27.4, 28.1]}


async def test_find_series_reads_a_404_as_none() -> None:
    """Return ``None`` for the 404 the endpoint documents, signing the request with the key."""
    fake = FakeTransport().reply("GET", "/v1/series/RETIRED", 404)
    async with CatalogClient("test-key", transport=fake) as catalog:
        assert await catalog.find_series("RETIRED") is None
    assert fake.calls[0].headers["Authorization"] == "Bearer test-key"


async def test_a_503_is_retried_after_the_wait_it_asks_for() -> None:
    """Retry a 503 once its ``Retry-After`` has passed, on a test clock that makes the wait instant."""
    fake = FakeTransport().reply("GET", "/v1/series/GDP", Respond(503, retry_after=2.0), Respond(json=GDP))
    clock = TestClock()
    with clock.install(), isolated_breakers():  # the clients built in the block use them
        catalog = CatalogClient("test-key", transport=fake)
    async with catalog:
        series = await catalog.get_series("GDP")
    assert series.values == [27.4, 28.1]
    assert clock.sleeps == [2.0]

Fault schedules, cassettes and switching the policy off: testing.

Codecs

The core imports no serialization library. headgate.msgspec (headgate[msgspec]) and headgate.pydantic (headgate[pydantic]) are two codecs with the same shortcuts, decode, decoder, encoder and to_builtins, so switching is changing one import. Both decode strictly (a field that changes type fails), ignore unknown fields, and raise a mismatch as DecodeError, which the client turns into ResponseParseError.

Any other library plugs in as a Codec: an object naming its media type, the errors it raises, a decoder and an encoder per type, and a description that never quotes the input. decoder_for(codec, Type) and encoder_for(codec, Type) build a reader and a writer on it, and headgate.testing.check_codec holds it to the promises the built-in codecs keep. See codecs.

Development

uv sync                                   # Python 3.14 and every dev dependency
uv run ruff check && uv run ruff format --check
uv run ty check
uv run python -m pytest --cov             # 100% line and branch coverage is required
uv run python -m pytest benchmarks        # the benchmarks, as CI runs them
uv run mutmut run                         # mutation testing; slow, run it before a release
uv run --only-group docs zensical serve   # preview the docs site at http://localhost:8000

Lint selects every ruff rule, ty treats every rule as an error, and suppression comments are not accepted. See CONTRIBUTING.md for the rules a change must keep, CODE_OF_CONDUCT.md for how we work together, and SECURITY.md to report a vulnerability privately. The changes in each release are in CHANGELOG.md.

License

MIT. src/headgate/_curl.py is derived from httpx-curl-cffi (BSD-3-Clause); its notice is kept in the file.