redis/redis-py · error · ConnectionError

Cannot retrieve information about server version

Error message

Cannot retrieve information about server version

What it means

Raised in CacheProxyConnection.connect (connection.py:1727-1734) when the underlying connection's handshake_metadata lacks a 'server' or 'version' field. Client-side caching (CacheProxyConnection) needs to know the server software and version to decide compatibility, and that metadata comes from the RESP3 HELLO response. Without HELLO (e.g. protocol=2, or a server/proxy that doesn't populate handshake metadata) the version cannot be determined and the cache proxy refuses to connect.

Solutions

Use protocol=3 (RESP3) so HELLO populates handshake_metadata with server/version.
Ensure no proxy strips the HELLO response fields.
Upgrade Redis to a version that reports server name and version in HELLO.
Disable client-side caching if you cannot use RESP3.

Example fix

# before
r = redis.Redis(host=h, port=p, protocol=2, cache=my_cache)
# after
r = redis.Redis(host=h, port=p, protocol=3, cache=my_cache)

Defensive patterns

Strategy: validation

Validate before calling

# Client-side caching requires RESP3 (HELLO) to populate handshake metadata
assert int(protocol or 3) == 3, 'client-side caching needs protocol=3'
r = redis.Redis(host=h, port=p, protocol=3, cache=my_cache)
# verify metadata is populated:
with r.connection_pool.get_connection('HELLO') as c:
    assert c.handshake_metadata.get(b'version') or c.handshake_metadata.get('version')

Type guard

null

Try / catch

from redis.exceptions import ConnectionError
try:
    r = redis.Redis(host=h, port=p, protocol=3, cache=my_cache)
    r.ping()
except ConnectionError as e:
    if 'Cannot retrieve information about server version' in str(e):
        # strip the cache; keep the client working
        r = redis.Redis(host=h, port=p, protocol=3)
    else:
        raise

Prevention

Always pair client-side caching with protocol=3.
Remove any proxy that strips HELLO response fields.
Smoke-test HELLO output: redis-cli -3 HELLO should show server+version.

When it happens

Trigger: Enabling client-side caching (cache=...) while using protocol=2, or against a server/intermediary that does not return server & version in HELLO. handshake_metadata is only populated by the HELLO handshake (RESP3).

Common situations: Turning on the client-side cache without switching to RESP3; proxy in front that strips HELLO metadata; OSS Redis older than the version reporting fields.

Related errors

AI-assisted analysis of redis/redis-py@6a6b581b48 (2026-08-10). Data as JSON: /api/errors/402a0f2a55aa50b2. Report an issue: GitHub.

Appendix: source

Thrown at redis/connection.py:1734

        if isinstance(self._conn, MaintNotificationsAbstractConnection):
            self._conn.set_maint_notifications_cluster_handler_for_connection(
                oss_cluster_maint_notifications_handler
            )

    def get_protocol(self):
        return self._conn.get_protocol()

    def connect(self):
        self._conn.connect()

        server_name = self._conn.handshake_metadata.get(b"server", None)
        if server_name is None:
            server_name = self._conn.handshake_metadata.get("server", None)
        server_ver = self._conn.handshake_metadata.get(b"version", None)
        if server_ver is None:
            server_ver = self._conn.handshake_metadata.get("version", None)
        if server_ver is None or server_name is None:
            raise ConnectionError("Cannot retrieve information about server version")

        server_ver = ensure_string(server_ver)
        server_name = ensure_string(server_name)

        if (
            server_name != self.DEFAULT_SERVER_NAME
            or compare_versions(server_ver, self.MIN_ALLOWED_VERSION) == 1
        ):
            raise ConnectionError(
                "To maximize compatibility with all Redis products, client-side caching is supported by Redis 7.4 or later"  # noqa: E501
            )

    def on_connect(self):
        self._conn.on_connect()

    def disconnect(self, *args, **kwargs):
        with self._cache_lock:
            self._cache.flush()

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