Set default accept-encoding to compression algorithms supported by the client

noextras/test/test_compression_default.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import pytest
+from connectrpc._compression import get_accept_encoding
 from example.eliza_connect import (
     ElizaService,
     ElizaServiceASGIApplication,
@@ -94,3 +95,13 @@ async def say(self, request, ctx):
         str(exc_info.value)
         == f"Unsupported compression method: {compression}. Available methods: gzip, identity"
     )
+
+
+def test_accept_encoding_only_includes_available_compressions():
+    """Verify Accept-Encoding only advertises compressions that are actually available.
+
+    When brotli and zstandard are not installed (as in the noextras environment),
+    the Accept-Encoding header should not include 'br' or 'zstd'.
+    """
+    accept_encoding = get_accept_encoding()
+    assert accept_encoding == "gzip", f"Expected 'gzip' only, got '{accept_encoding}'"

src/connectrpc/_client_shared.py

@@ -6,7 +6,12 @@
 
 from . import _compression
 from ._codec import CODEC_NAME_JSON, CODEC_NAME_JSON_CHARSET_UTF8, Codec
-from ._compression import Compression, get_available_compressions, get_compression
+from ._compression import (
+    Compression,
+    get_accept_encoding,
+    get_available_compressions,
+    get_compression,
+)
 from ._protocol import ConnectWireError
 from ._protocol_connect import (
     CONNECT_PROTOCOL_VERSION,
@@ -88,7 +93,7 @@ def create_request_context(
     if accept_compression is not None:
         headers[accept_compression_header] = ", ".join(accept_compression)
     else:
-        headers[accept_compression_header] = "gzip, br, zstd"
+        headers[accept_compression_header] = get_accept_encoding()
     if send_compression is not None:
         headers[compression_header] = send_compression.name()
     else:

src/connectrpc/_compression.py

@@ -91,6 +91,10 @@ def decompress(self, data: bytes | bytearray) -> bytes:
 _identity = IdentityCompression()
 _compressions["identity"] = _identity
 
+# Preferred compression names for Accept-Encoding header, in order of preference.
+# Excludes 'identity' since it's an implicit fallback.
+DEFAULT_ACCEPT_ENCODING_COMPRESSIONS = ("gzip", "br", "zstd")
+
 
 def get_compression(name: str) -> Compression | None:
     return _compressions.get(name.lower())
@@ -101,6 +105,17 @@ def get_available_compressions() -> KeysView:
     return _compressions.keys()
 
 
+def get_accept_encoding() -> str:
+    """Returns Accept-Encoding header value with available compressions in preference order.
+
+    This excludes 'identity' since it's an implicit fallback, and returns
+    only compressions that are actually available (i.e., their dependencies are installed).
+    """
+    return ", ".join(
+        name for name in DEFAULT_ACCEPT_ENCODING_COMPRESSIONS if name in _compressions
+    )
+
+
 def negotiate_compression(accept_encoding: str) -> Compression:
     for accept in accept_encoding.split(","):
         compression = _compressions.get(accept.strip())