feat: implement bypassMultiToolsLimit for vertex_ai_search

core/src/commonMain/kotlin/com/google/adk/kt/agents/LlmAgentTurn.kt

@@ -40,7 +40,10 @@ import com.google.adk.kt.tools.BaseTool
 import com.google.adk.kt.tools.GoogleSearchAgentTool
 import com.google.adk.kt.tools.GoogleSearchTool
 import com.google.adk.kt.tools.ToolContext
+import com.google.adk.kt.tools.VertexAiSearchAgentTool
+import com.google.adk.kt.tools.VertexAiSearchTool
 import com.google.adk.kt.tools.createGoogleSearchAgent
+import com.google.adk.kt.tools.createVertexAiSearchAgent
 import com.google.adk.kt.types.UsageMetadata
 import kotlin.time.Clock
 import kotlinx.coroutines.CancellationException
@@ -184,6 +187,8 @@ internal class LlmAgentTurn(
         when {
           tool is GoogleSearchTool && tool.bypassMultiToolsLimit ->
             GoogleSearchAgentTool(createGoogleSearchAgent(agent.model))
+          tool is VertexAiSearchTool && tool.bypassMultiToolsLimit ->
+            VertexAiSearchAgentTool(createVertexAiSearchAgent(agent.model, tool))
           else -> tool
         }
       }

core/src/commonMain/kotlin/com/google/adk/kt/tools/VertexAiSearchAgentTool.kt

@@ -0,0 +1,61 @@
+/*
+ * Copyright 2026 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.kt.tools
+
+import com.google.adk.kt.agents.Instruction
+import com.google.adk.kt.agents.LlmAgent
+import com.google.adk.kt.models.Model
+
+/** Name of the sub-agent (and resulting function tool) created by [createVertexAiSearchAgent]. */
+internal const val VERTEX_AI_SEARCH_AGENT_NAME = "vertex_ai_search_agent"
+
+/**
+ * Creates a sub-agent whose only tool is the given [VertexAiSearchTool].
+ *
+ * Used by the multi-tools-limit workaround so that `vertex_ai_search` can be exposed to the parent
+ * agent as a function tool (via [VertexAiSearchAgentTool]) and therefore coexist with other tools.
+ * The original [tool] is reused as the sub-agent's only tool; since the sub-agent has a single
+ * tool, it is never itself rewritten by the workaround.
+ */
+internal fun createVertexAiSearchAgent(model: Model, tool: VertexAiSearchTool): LlmAgent =
+  LlmAgent(
+    name = VERTEX_AI_SEARCH_AGENT_NAME,
+    model = model,
+    description = "An agent for searching using the `vertex_ai_search` tool",
+    instruction =
+      Instruction.Text(
+        "You are a specialized Vertex AI Search agent.\n\n" +
+          "When given a search query, use the `vertex_ai_search` tool to find the related " +
+          "information."
+      ),
+    tools = listOf(tool),
+  )
+
+/**
+ * A tool that wraps a sub-agent which only uses a [VertexAiSearchTool].
+ *
+ * Lets `vertex_ai_search` be used alongside other tools (built-in tools cannot be combined with
+ * other tools in a single request). The built-in is wrapped in a sub-agent and exposed as a
+ * function tool, and the sub-agent's grounding metadata is propagated back to the parent.
+ *
+ * NOTE: this intentionally diverges from the Python ADK, which instead converts the
+ * `VertexAiSearchTool` into a `DiscoveryEngineSearchTool` that queries the Discovery Engine API
+ * directly. That public Discovery Engine client is not available as a dependency here, so this
+ * reuses the same sub-agent mechanism as [GoogleSearchAgentTool].
+ */
+internal class VertexAiSearchAgentTool(agent: LlmAgent) :
+  AgentTool(agent = agent, propagateGroundingMetadata = true)

core/src/commonMain/kotlin/com/google/adk/kt/tools/VertexAiSearchTool.kt

@@ -37,6 +37,9 @@ import com.google.adk.kt.types.VertexAISearchDataStoreSpec
  *   `projects/{project}/locations/{location}/collections/{collection}/engines/{engine}`.
  * @property filter The filter to apply to the search results.
  * @property maxResults The maximum number of results to return.
+ * @property bypassMultiToolsLimit When true, allows this built-in tool to be used alongside other
+ *   tools: it is exposed as a function tool (see [VertexAiSearchAgentTool]) since built-in tools
+ *   cannot otherwise be combined with other tools in a single request.
  * @property model Deprecated and unused. Tool support is verified by the backend.
  */
 class VertexAiSearchTool(
@@ -45,6 +48,7 @@ class VertexAiSearchTool(
   val searchEngineId: String? = null,
   val filter: String? = null,
   val maxResults: Int? = null,
+  val bypassMultiToolsLimit: Boolean = false,
   @Deprecated(
     "Model-based tool gating has been removed; tool support is verified by the backend. " +
       "This parameter is unused and will be removed in a future release."

core/src/commonTest/kotlin/com/google/adk/kt/agents/LlmAgentMultiToolsLimitTest.kt

@@ -24,6 +24,7 @@ import com.google.adk.kt.testing.DummyTool
 import com.google.adk.kt.testing.modelMessage
 import com.google.adk.kt.testing.userMessage
 import com.google.adk.kt.tools.GoogleSearchTool
+import com.google.adk.kt.tools.VertexAiSearchTool
 import kotlin.test.Test
 import kotlin.test.assertNotNull
 import kotlin.test.assertNull
@@ -110,4 +111,35 @@ class LlmAgentMultiToolsLimitTest {
     // Did not opt in: the built-in google_search is left as-is (not swapped).
     assertNotNull(tools.firstOrNull { it.googleSearch != null })
   }
+
+  @Test
+  fun multipleTools_vertexAiSearchWithBypass_swappedToFunctionTool() = runTest {
+    var captured: LlmRequest? = null
+    val model =
+      DummyModel("m") { request ->
+        captured = request
+        flowOf(LlmResponse(content = modelMessage("done")))
+      }
+    val agent =
+      LlmAgent(
+        name = "main",
+        model = model,
+        tools =
+          listOf(
+            VertexAiSearchTool(dataStoreId = "ds1", bypassMultiToolsLimit = true),
+            DummyTool("calc"),
+          ),
+      )
+    val runner = InMemoryRunner(agent = agent)
+
+    val unused =
+      runner.runAsync(userId = "u", sessionId = "s", newMessage = userMessage("hi")).toList()
+
+    val tools = captured?.config?.tools
+    assertNotNull(tools)
+    // The built-in vertex_ai_search retrieval is replaced and re-exposed as a function tool.
+    assertNull(tools.firstOrNull { it.retrieval != null })
+    val declaredNames = tools.flatMap { it.functionDeclarations ?: emptyList() }.map { it.name }
+    assertTrue(declaredNames.contains("vertex_ai_search_agent"))
+  }
 }

core/src/commonTest/kotlin/com/google/adk/kt/tools/VertexAiSearchAgentToolTest.kt

@@ -0,0 +1,47 @@
+/*
+ * Copyright 2026 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.kt.tools
+
+import com.google.adk.kt.testing.DummyModel
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertIs
+import kotlin.test.assertTrue
+
+class VertexAiSearchAgentToolTest {
+
+  @Test
+  fun createVertexAiSearchAgent_buildsAgentWithVertexAiSearchTool() {
+    val tool = VertexAiSearchTool(dataStoreId = "ds1", bypassMultiToolsLimit = true)
+
+    val agent = createVertexAiSearchAgent(DummyModel("test"), tool)
+
+    assertEquals(VERTEX_AI_SEARCH_AGENT_NAME, agent.name)
+    assertEquals(1, agent.tools.size)
+    assertIs<VertexAiSearchTool>(agent.tools[0])
+  }
+
+  @Test
+  fun vertexAiSearchAgentTool_declaration_usesAgentName() {
+    val tool = VertexAiSearchTool(dataStoreId = "ds1", bypassMultiToolsLimit = true)
+
+    val agentTool = VertexAiSearchAgentTool(createVertexAiSearchAgent(DummyModel("test"), tool))
+
+    assertEquals(VERTEX_AI_SEARCH_AGENT_NAME, agentTool.declaration().name)
+    assertTrue(agentTool.propagateGroundingMetadata)
+  }
+}