fix(web): make root->dev-ui redirect reverse-proxy-prefix-aware

dev/src/main/java/com/google/adk/web/AdkWebServer.java

@@ -143,12 +143,15 @@ public void addResourceHandlers(ResourceHandlerRegistry registry) {
   }
 
   /**
-   * Configures simple automated controllers: - Redirects the root path "/" to "/dev-ui". - Forwards
-   * requests to "/dev-ui" to "/dev-ui/index.html" so the ResourceHandler serves it.
+   * Configures simple automated controllers: - Redirects the root path "/" to "dev-ui" (context-
+   * relative, not "/dev-ui", so it stays reverse-proxy-prefix-aware when this app runs behind a
+   * proxy that strips a path prefix and forwards it via server.forward-headers-strategy /
+   * X-Forwarded-Prefix). - Forwards requests to "/dev-ui" to "/dev-ui/index.html" so the
+   * ResourceHandler serves it.
    */
   @Override
   public void addViewControllers(ViewControllerRegistry registry) {
-    registry.addRedirectViewController("/", "/dev-ui");
+    registry.addRedirectViewController("/", "dev-ui");
     registry.addViewController("/dev-ui").setViewName("forward:/index.html");
     registry.addViewController("/dev-ui/").setViewName("forward:/index.html");
   }

dev/src/test/java/com/google/adk/web/AdkWebServerReverseProxyTest.java

@@ -0,0 +1,72 @@
+/*
+ * Copyright 2025 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.adk.web;
+
+import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.redirectedUrl;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.redirectedUrlPattern;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.boot.webmvc.test.autoconfigure.AutoConfigureMockMvc;
+import org.springframework.test.web.servlet.MockMvc;
+
+/**
+ * Verifies that the root ("/") -> "dev-ui" redirect (see {@link AdkWebServer#addViewControllers})
+ * is reverse-proxy-prefix-aware once Spring's ForwardedHeaderFilter is enabled via {@code
+ * server.forward-headers-strategy=framework}.
+ *
+ * <p>This is a regression test for the redirect target being context-relative ("dev-ui") rather
+ * than root-relative ("/dev-ui"): only a context-relative sendRedirect() target is passed through
+ * {@link org.springframework.util.StringUtils#applyRelativePath}, which is what makes {@code
+ * ForwardedHeaderFilter} splice an {@code X-Forwarded-Prefix} header's value back into the redirect
+ * Location. A root-relative target bypasses that codepath entirely and would leave this test's
+ * "with X-Forwarded-Prefix" case failing.
+ *
+ * <p>Uses a dedicated test class (rather than adding to {@link AdkWebServerUITest}) because {@code
+ * server.forward-headers-strategy} is a server-wide property, kept separate here so enabling it
+ * doesn't affect the default context {@link AdkWebServerUITest} runs against.
+ */
+@SpringBootTest(properties = "server.forward-headers-strategy=framework")
+@AutoConfigureMockMvc
+public class AdkWebServerReverseProxyTest {
+
+  @Autowired private MockMvc mockMvc;
+
+  /**
+   * With no Forwarded/X-Forwarded-* headers on the request, Spring's ForwardedHeaderFilter doesn't
+   * wrap the response at all (verified by running this test), so behavior matches the property
+   * being off entirely: the redirect is the raw, unresolved "dev-ui" string.
+   */
+  @Test
+  public void rootRedirect_withoutForwardedHeaders_isUnchanged() throws Exception {
+    mockMvc
+        .perform(get("/"))
+        .andExpect(status().is3xxRedirection())
+        .andExpect(redirectedUrl("dev-ui"));
+  }
+
+  @Test
+  public void rootRedirect_withForwardedPrefixHeader_restoresPathPrefix() throws Exception {
+    mockMvc
+        .perform(get("/").header("X-Forwarded-Prefix", "/my-app-prefix"))
+        .andExpect(status().is3xxRedirection())
+        .andExpect(redirectedUrlPattern("http://*/my-app-prefix/dev-ui"));
+  }
+}

dev/src/test/java/com/google/adk/web/AdkWebServerUITest.java

@@ -45,7 +45,7 @@ public void rootShouldRedirectToDevUi() throws Exception {
     mockMvc
         .perform(get("/"))
         .andExpect(status().is3xxRedirection())
-        .andExpect(redirectedUrl("/dev-ui"));
+        .andExpect(redirectedUrl("dev-ui"));
   }
 
   @ParameterizedTest