Add regression tests for caching with mounted server prefixes (#2762)

tests/server/middleware/test_caching.py

@@ -521,3 +521,96 @@ class TestCachableToolResult:
         assert cached_tool_result.content == tool_result.content
         assert cached_tool_result.structured_content == tool_result.structured_content
         assert cached_tool_result.meta == tool_result.meta
+
+
+class TestCachingWithImportedServerPrefixes:
+    """Test that caching preserves prefixes from imported servers.
+
+    Regression tests for issue #2300: ResponseCachingMiddleware was losing
+    prefix information when caching components from imported servers.
+    """
+
+    @pytest.fixture
+    async def parent_with_imported_child(self, tracking_calculator: TrackingCalculator):
+        """Create a parent server with an imported child server (prefixed)."""
+        child = FastMCP("child")
+        tracking_calculator.add_tools(fastmcp=child)
+        tracking_calculator.add_resources(fastmcp=child)
+        tracking_calculator.add_prompts(fastmcp=child)
+
+        parent = FastMCP("parent")
+        parent.add_middleware(ResponseCachingMiddleware())
+        parent.mount(child, namespace="child")
+
+        return parent
+
+    async def test_tool_prefixes_preserved_after_cache_hit(
+        self, parent_with_imported_child: FastMCP
+    ):
+        """Tool names should retain prefix after being served from cache."""
+        async with Client(parent_with_imported_child) as client:
+            # First call populates cache
+            tools_first = await client.list_tools()
+            tool_names_first = [t.name for t in tools_first]
+
+            # Second call should come from cache
+            tools_cached = await client.list_tools()
+            tool_names_cached = [t.name for t in tools_cached]
+
+            # All tools should have prefix in both calls
+            assert all(name.startswith("child_") for name in tool_names_first)
+            assert all(name.startswith("child_") for name in tool_names_cached)
+            assert tool_names_first == tool_names_cached
+
+    async def test_resource_prefixes_preserved_after_cache_hit(
+        self, parent_with_imported_child: FastMCP
+    ):
+        """Resource URIs should retain prefix after being served from cache."""
+        async with Client(parent_with_imported_child) as client:
+            # First call populates cache
+            resources_first = await client.list_resources()
+            resource_uris_first = [str(r.uri) for r in resources_first]
+
+            # Second call should come from cache
+            resources_cached = await client.list_resources()
+            resource_uris_cached = [str(r.uri) for r in resources_cached]
+
+            # All resources should have prefix in URI path in both calls
+            # Resources get path-style prefix: resource://child/path
+            assert all("://child/" in uri for uri in resource_uris_first)
+            assert all("://child/" in uri for uri in resource_uris_cached)
+            assert resource_uris_first == resource_uris_cached
+
+    async def test_prompt_prefixes_preserved_after_cache_hit(
+        self, parent_with_imported_child: FastMCP
+    ):
+        """Prompt names should retain prefix after being served from cache."""
+        async with Client(parent_with_imported_child) as client:
+            # First call populates cache
+            prompts_first = await client.list_prompts()
+            prompt_names_first = [p.name for p in prompts_first]
+
+            # Second call should come from cache
+            prompts_cached = await client.list_prompts()
+            prompt_names_cached = [p.name for p in prompts_cached]
+
+            # All prompts should have prefix in both calls
+            assert all(name.startswith("child_") for name in prompt_names_first)
+            assert all(name.startswith("child_") for name in prompt_names_cached)
+            assert prompt_names_first == prompt_names_cached
+
+    async def test_prefixed_tool_callable_after_cache_hit(
+        self,
+        parent_with_imported_child: FastMCP,
+        tracking_calculator: TrackingCalculator,
+    ):
+        """Prefixed tools should be callable after cache populates."""
+        async with Client(parent_with_imported_child) as client:
+            # Trigger cache population
+            await client.list_tools()
+            await client.list_tools()  # From cache
+
+            # Tool should be callable with prefixed name
+            result = await client.call_tool("child_add", {"a": 5, "b": 3})
+            assert not result.is_error
+            assert tracking_calculator.add_calls == 1