fix: Validate that the primary URL specified in the AgentCard matches the URL for the preferred transport (a2aproject#306)

fixes a2aproject#272

Author: skyllarr

server-common/src/main/java/io/a2a/server/AgentCardValidator.java

@@ -84,6 +84,22 @@ static void validateTransportConfiguration(AgentCard agentCard, Set<String> avai
             // Following the GitHub issue suggestion to use an error instead of warning
             throw new IllegalStateException(errorMessage);
         }
+
+        // check that the primary URL matches the URL for the preferred transport
+        if (agentCard.additionalInterfaces() != null) {
+            agentCard.additionalInterfaces().stream()
+                    .filter(agentInterface -> agentInterface.transport().equals(agentCard.preferredTransport()))
+                    .findFirst()
+                    .ifPresent(preferredTransportAgentInterface -> {
+                if (!preferredTransportAgentInterface.url().equals(agentCard.url())) {
+                    LOGGER.warning(String.format(
+                            "AgentCard's URL=%s does not correspond to the URL of the preferred transport=%s.",
+                            agentCard.url(),
+                            preferredTransportAgentInterface.url()
+                    ));
+                }
+            });
+        }
     }
 
     /**

server-common/src/test/java/io/a2a/server/AgentCardValidatorTest.java

@@ -229,6 +229,37 @@ void testSkipPropertiesFilterWarnings() {
         }
     }
 
+    @Test
+    void testPrimaryURLMatchesPreferredTransport() {
+        // Create a simple AgentCard (with preferred HTTP_JSON transport even though primary URL points to JSONRPC transport)
+        AgentCard agentCard = createTestAgentCardBuilder()
+                .preferredTransport(TransportProtocol.HTTP_JSON.asString())
+                .additionalInterfaces(List.of(
+                        new AgentInterface(TransportProtocol.JSONRPC.asString(), "http://localhost:9999"),
+                        new AgentInterface(TransportProtocol.GRPC.asString(), "http://localhost:9000"),
+                        new AgentInterface(TransportProtocol.HTTP_JSON.asString(), "http://localhost:8000")
+                ))
+                .build();
+
+        // Define available transports
+        Set<String> availableTransports = Set.of(TransportProtocol.JSONRPC.asString(), TransportProtocol.GRPC.asString(), TransportProtocol.HTTP_JSON.asString());
+
+        // Capture logs
+        Logger logger = Logger.getLogger(AgentCardValidator.class.getName());
+        TestLogHandler testLogHandler = new TestLogHandler();
+        logger.addHandler(testLogHandler);
+
+        try {
+            AgentCardValidator.validateTransportConfiguration(agentCard, availableTransports);
+        } finally {
+            logger.removeHandler(testLogHandler);
+        }
+
+        // Test that the warning was logged
+        assertTrue(testLogHandler.getLogMessages().stream()
+                .anyMatch(msg -> msg.contains("AgentCard's URL=http://localhost:9999 does not correspond to the URL of the preferred transport=http://localhost:8000.")));
+    }
+
     // A simple log handler for testing
     private static class TestLogHandler extends Handler {
         private final List<String> logMessages = new java.util.ArrayList<>();