feat(compat): GRPC client compatible with 0.3 servers.

Author: bartek-w

src/a2a/client/client_factory.py

@@ -7,6 +7,8 @@
 
 import httpx
 
+from packaging.version import InvalidVersion, Version
+
 from a2a.client.base_client import BaseClient
 from a2a.client.card_resolver import A2ACardResolver
 from a2a.client.client import Client, ClientConfig, Consumer
@@ -21,6 +23,8 @@
     AgentInterface,
 )
 from a2a.utils.constants import (
+    PROTOCOL_VERSION_0_3,
+    PROTOCOL_VERSION_1_0,
     PROTOCOL_VERSION_CURRENT,
     VERSION_HEADER,
     TransportProtocol,
@@ -33,6 +37,12 @@
     GrpcTransport = None  # type: ignore # pyright: ignore
 
 
+try:
+    from a2a.compat.v0_3.grpc_transport import CompatGrpcTransport
+except ImportError:
+    CompatGrpcTransport = None  # type: ignore # pyright: ignore
+
+
 logger = logging.getLogger(__name__)
 
 
@@ -109,10 +119,102 @@ def _register_defaults(self, supported: list[str]) -> None:
                     'To use GrpcClient, its dependencies must be installed. '
                     'You can install them with \'pip install "a2a-sdk[grpc]"\''
                 )
+
+            def grpc_transport_producer(
+                card: AgentCard,
+                url: str,
+                config: ClientConfig,
+                interceptors: list[ClientCallInterceptor],
+            ) -> ClientTransport:
+                # The interface has already been selected and passed as `url`.
+                # We determine its version to use the appropriate transport implementation.
+                interface = ClientFactory._find_best_interface(
+                    list(card.supported_interfaces),
+                    protocol_bindings=[TransportProtocol.GRPC],
+                    url=url,
+                )
+                version = (
+                    interface.protocol_version
+                    if interface
+                    else PROTOCOL_VERSION_CURRENT
+                )
+
+                compat_transport = CompatGrpcTransport
+                if version and compat_transport is not None:
+                    try:
+                        v = Version(version)
+                        if (
+                            Version(PROTOCOL_VERSION_0_3)
+                            <= v
+                            < Version(PROTOCOL_VERSION_1_0)
+                        ):
+                            return compat_transport.create(
+                                card, url, config, interceptors
+                            )
+                    except InvalidVersion:
+                        pass
+
+                grpc_transport = GrpcTransport
+                if grpc_transport is not None:
+                    return grpc_transport.create(
+                        card, url, config, interceptors
+                    )
+
+                raise ImportError(
+                    'GrpcTransport is not available. '
+                    'You can install it with \'pip install "a2a-sdk[grpc]"\''
+                )
+
             self.register(
                 TransportProtocol.GRPC,
-                GrpcTransport.create,
+                grpc_transport_producer,
+            )
+
+    @staticmethod
+    def _find_best_interface(
+        interfaces: list[AgentInterface],
+        protocol_bindings: list[str] | None = None,
+        url: str | None = None,
+    ) -> AgentInterface | None:
+        """Finds the best interface based on protocol version priorities."""
+        candidates = [
+            i
+            for i in interfaces
+            if (
+                protocol_bindings is None
+                or i.protocol_binding in protocol_bindings
             )
+            and (url is None or i.url == url)
+        ]
+
+        if not candidates:
+            return None
+
+        # Prefer interface with version 1.0
+        for i in candidates:
+            if i.protocol_version == PROTOCOL_VERSION_1_0:
+                return i
+
+        best_gt_1_0 = None
+        best_ge_0_3 = None
+        best_no_version = None
+
+        for i in candidates:
+            if not i.protocol_version:
+                if best_no_version is None:
+                    best_no_version = i
+                continue
+
+            try:
+                v = Version(i.protocol_version)
+                if best_gt_1_0 is None and v > Version(PROTOCOL_VERSION_1_0):
+                    best_gt_1_0 = i
+                if best_ge_0_3 is None and v >= Version(PROTOCOL_VERSION_0_3):
+                    best_ge_0_3 = i
+            except InvalidVersion:
+                pass
+
+        return best_gt_1_0 or best_ge_0_3 or best_no_version
 
     @classmethod
     async def connect(  # noqa: PLR0913
@@ -220,13 +322,9 @@ def create(
         selected_interface = None
         if self._config.use_client_preference:
             for protocol_binding in client_set:
-                selected_interface = next(
-                    (
-                        si
-                        for si in card.supported_interfaces
-                        if si.protocol_binding == protocol_binding
-                    ),
-                    None,
+                selected_interface = ClientFactory._find_best_interface(
+                    list(card.supported_interfaces),
+                    protocol_bindings=[protocol_binding],
                 )
                 if selected_interface:
                     transport_protocol = protocol_binding
@@ -235,7 +333,10 @@ def create(
             for supported_interface in card.supported_interfaces:
                 if supported_interface.protocol_binding in client_set:
                     transport_protocol = supported_interface.protocol_binding
-                    selected_interface = supported_interface
+                    selected_interface = ClientFactory._find_best_interface(
+                        list(card.supported_interfaces),
+                        protocol_bindings=[transport_protocol],
+                    )
                     break
         if not transport_protocol or not selected_interface:
             raise ValueError('no compatible transports found.')