diff --git a/src/bedrock_agentcore/runtime/a2a.py b/src/bedrock_agentcore/runtime/a2a.py
index 4a05d871..ad6a4204 100644
--- a/src/bedrock_agentcore/runtime/a2a.py
+++ b/src/bedrock_agentcore/runtime/a2a.py
@@ -252,9 +252,10 @@ def build_a2a_app(
         executor: An ``AgentExecutor`` that implements the agent logic.
         agent_card: Optional ``a2a.types.AgentCard`` describing the agent.
             If ``None``, one is built automatically by introspecting the executor.
-        runtime_url: URL advertised by an automatically generated agent card.
-            Defaults to ``http://localhost:9000/``. ``AGENTCORE_RUNTIME_URL``
-            takes precedence when set.
+        runtime_url: URL advertised by the agent card. When an explicit card is
+            supplied, its JSON-RPC URL is updated when this argument is set.
+            Defaults to ``http://localhost:9000/`` for automatically generated
+            cards. ``AGENTCORE_RUNTIME_URL`` takes precedence when set.
         task_store: Optional ``TaskStore``; defaults to ``InMemoryTaskStore``.
         context_builder: Optional ``ServerCallContextBuilder``; defaults to
             ``BedrockCallContextBuilder``.
@@ -273,13 +274,14 @@ def build_a2a_app(
     from starlette.responses import JSONResponse
     from starlette.routing import Route
 
-    runtime_url = os.environ.get(AGENTCORE_RUNTIME_URL_ENV, runtime_url or "http://localhost:9000/")
+    runtime_url_override = os.environ.get(AGENTCORE_RUNTIME_URL_ENV) or runtime_url
+    advertised_url = runtime_url_override or "http://localhost:9000/"
     is_a2a_v1 = _is_a2a_v1()
 
     if agent_card is None:
-        agent_card = _build_agent_card(executor, runtime_url)
-    elif os.environ.get(AGENTCORE_RUNTIME_URL_ENV):
-        _set_jsonrpc_url(agent_card, runtime_url)
+        agent_card = _build_agent_card(executor, advertised_url)
+    elif runtime_url_override:
+        _set_jsonrpc_url(agent_card, advertised_url)
 
     if task_store is None:
         task_store = InMemoryTaskStore()
diff --git a/tests/bedrock_agentcore/runtime/test_a2a.py b/tests/bedrock_agentcore/runtime/test_a2a.py
index 6b804d8f..dd3c0062 100644
--- a/tests/bedrock_agentcore/runtime/test_a2a.py
+++ b/tests/bedrock_agentcore/runtime/test_a2a.py
@@ -236,8 +236,15 @@ def test_agent_card_url_auto_populated_from_env(self):
             build_a2a_app(_EchoExecutor(), card)
         assert _card_url(card) == "https://deployed.example.com/"
 
+    def test_agent_card_url_uses_explicit_runtime_url(self):
+        """A supplied runtime_url overrides the URL on an explicit card."""
+        card = _make_agent_card()
+        with patch.dict("os.environ", {}, clear=True):
+            build_a2a_app(_EchoExecutor(), card, runtime_url="http://localhost:9002/")
+        assert _card_url(card) == "http://localhost:9002/"
+
     def test_agent_card_url_unchanged_without_env(self):
-        """When AGENTCORE_RUNTIME_URL is not set, the agent card URL stays as-is."""
+        """Without a runtime URL override, an explicit card URL stays as-is."""
         card = _make_agent_card()
         original_url = _card_url(card)
         with patch.dict("os.environ", {}, clear=False):
@@ -466,11 +473,41 @@ def test_port_from_environment(self, mock_uvicorn_run):
         assert mock_uvicorn_run.call_args.kwargs["port"] == 9001
         assert _card_response_url(response.json()) == "http://localhost:9001/"
 
+    @patch("uvicorn.run")
+    def test_port_from_environment_updates_explicit_card(self, mock_uvicorn_run):
+        with patch.dict("os.environ", {"PORT": "9002"}, clear=True):
+            serve_a2a(_EchoExecutor(), _make_agent_card())
+
+        app = mock_uvicorn_run.call_args.args[0]
+        response = TestClient(app).get("/.well-known/agent-card.json")
+        assert mock_uvicorn_run.call_args.kwargs["port"] == 9002
+        assert _card_response_url(response.json()) == "http://localhost:9002/"
+
     @patch("uvicorn.run")
     def test_explicit_port_overrides_environment(self, mock_uvicorn_run):
         with patch.dict("os.environ", {"PORT": "9001"}):
             serve_a2a(_EchoExecutor(), _make_agent_card(), port=8888)
+        app = mock_uvicorn_run.call_args.args[0]
+        response = TestClient(app).get("/.well-known/agent-card.json")
         assert mock_uvicorn_run.call_args.kwargs["port"] == 8888
+        assert _card_response_url(response.json()) == "http://localhost:8888/"
+
+    @patch("uvicorn.run")
+    def test_runtime_url_environment_overrides_port_for_card(self, mock_uvicorn_run):
+        with patch.dict(
+            "os.environ",
+            {
+                "PORT": "9002",
+                "AGENTCORE_RUNTIME_URL": "https://deployed.example.com/",
+            },
+            clear=True,
+        ):
+            serve_a2a(_EchoExecutor(), _make_agent_card())
+
+        app = mock_uvicorn_run.call_args.args[0]
+        response = TestClient(app).get("/.well-known/agent-card.json")
+        assert mock_uvicorn_run.call_args.kwargs["port"] == 9002
+        assert _card_response_url(response.json()) == "https://deployed.example.com/"
 
     @patch("uvicorn.run")
     def test_docker_detection_dockerenv(self, mock_uvicorn_run):