feat: Update model resolution logic for LLM agents

core/src/main/java/com/google/adk/agents/BaseAgentConfig.java

@@ -26,9 +26,7 @@
 public class BaseAgentConfig {
   private String name;
   private String description = "";
-  // TODO: Add agentClassType enum to the config and handle different values from user
-  // input.e.g.LLM_AGENT, LlmAgent
-  private String agentClass = null;
+  private String agentClass;
 
   @JsonProperty(value = "name", required = true)
   public String name() {

core/src/main/java/com/google/adk/agents/LlmAgent.java

@@ -16,7 +16,6 @@
 
 package com.google.adk.agents;
 
-import static com.google.common.base.Strings.isNullOrEmpty;
 import static com.google.common.base.Strings.nullToEmpty;
 import static java.util.stream.Collectors.joining;
 
@@ -47,7 +46,7 @@
 import com.google.adk.flows.llmflows.BaseLlmFlow;
 import com.google.adk.flows.llmflows.SingleFlow;
 import com.google.adk.models.BaseLlm;
-import com.google.adk.models.Gemini;
+import com.google.adk.models.LlmRegistry;
 import com.google.adk.models.Model;
 import com.google.adk.tools.BaseTool;
 import com.google.adk.tools.BaseToolset;
@@ -835,8 +834,17 @@ public Model resolvedModel() {
    */
   private Model resolveModelInternal() {
     if (this.model.isPresent()) {
-      if (this.model().isPresent()) {
-        return this.model.get();
+      Model currentModel = this.model.get();
+
+      if (currentModel.model().isPresent()) {
+        return currentModel;
+      }
+
+      if (currentModel.modelName().isPresent()) {
+        String modelName = currentModel.modelName().get();
+        BaseLlm resolvedLlm = LlmRegistry.getLlm(modelName);
+
+        return Model.builder().modelName(modelName).model(resolvedLlm).build();
       }
     }
     BaseAgent current = this.parentAgent();
@@ -879,44 +887,8 @@ public static LlmAgent fromConfig(LlmAgentConfig config, String configAbsPath)
             .description(nullToEmpty(config.description()))
             .instruction(config.instruction());
 
-    // Set optional model configuration
     if (config.model() != null && !config.model().trim().isEmpty()) {
-      logger.info("Configuring model: {}", config.model());
-
-      // TODO: resolve model name
-      if (config.model().startsWith("gemini")) {
-        try {
-          // Check for API key in system properties (for testing) or environment variables
-          String apiKey = System.getProperty("GOOGLE_API_KEY");
-          if (isNullOrEmpty(apiKey)) {
-            apiKey = System.getProperty("GEMINI_API_KEY");
-          }
-          if (isNullOrEmpty(apiKey)) {
-            apiKey = System.getenv("GOOGLE_API_KEY");
-          }
-          if (isNullOrEmpty(apiKey)) {
-            apiKey = System.getenv("GEMINI_API_KEY");
-          }
-
-          Gemini.Builder geminiBuilder = Gemini.builder().modelName(config.model());
-          if (apiKey != null && !apiKey.isEmpty()) {
-            geminiBuilder.apiKey(apiKey);
-          }
-
-          BaseLlm model = geminiBuilder.build();
-          builder.model(model);
-          logger.debug("Successfully configured Gemini model: {}", config.model());
-        } catch (RuntimeException e) {
-          logger.warn(
-              "Failed to create Gemini model '{}'. The agent will use the default LLM. Error: {}",
-              config.model(),
-              e.getMessage());
-        }
-      } else {
-        logger.warn(
-            "Model '{}' is not a supported Gemini model. The agent will use the default LLM.",
-            config.model());
-      }
+      builder.model(config.model());
     }
 
     // Set optional transfer configuration

core/src/test/java/com/google/adk/agents/ConfigAgentUtilsTest.java

@@ -178,4 +178,69 @@ public void fromConfig_missingRequiredFields_throwsException() throws IOExceptio
     assertThat(exception).hasMessageThat().contains("Failed to create agent from config");
     assertThat(exception.getCause()).isNotNull();
   }
+
+  @Test
+  public void fromConfig_withModel_setsModelOnAgent() throws IOException, ConfigurationException {
+    File configFile = tempFolder.newFile("with_model.yaml");
+    Files.writeString(
+        configFile.toPath(),
+        "name: modelAgent\n"
+            + "description: Agent with a model\n"
+            + "instruction: test instruction\n"
+            + "agent_class: LlmAgent\n"
+            + "model: \"gemini-pro\"\n");
+    String configPath = configFile.getAbsolutePath();
+
+    BaseAgent agent = ConfigAgentUtils.fromConfig(configPath);
+
+    assertThat(agent).isInstanceOf(LlmAgent.class);
+    LlmAgent llmAgent = (LlmAgent) agent;
+    assertThat(llmAgent.model()).isPresent();
+    assertThat(llmAgent.model().get().modelName()).hasValue("gemini-pro");
+  }
+
+  @Test
+  public void fromConfig_withEmptyModel_doesNotSetModelOnAgent()
+      throws IOException, ConfigurationException {
+    File configFile = tempFolder.newFile("empty_model.yaml");
+    Files.writeString(
+        configFile.toPath(),
+        "name: emptyModelAgent\n"
+            + "description: Agent with an empty model\n"
+            + "instruction: test instruction\n"
+            + "agent_class: LlmAgent\n"
+            + "model: \"\"\n");
+    String configPath = configFile.getAbsolutePath();
+
+    BaseAgent agent = ConfigAgentUtils.fromConfig(configPath);
+
+    assertThat(agent).isInstanceOf(LlmAgent.class);
+    LlmAgent llmAgent = (LlmAgent) agent;
+    assertThat(llmAgent.model()).isEmpty();
+  }
+
+  @Test
+  public void fromConfig_withInvalidModel_throwsExceptionOnModelResolution()
+      throws IOException, ConfigurationException {
+    File configFile = tempFolder.newFile("invalid_model.yaml");
+    Files.writeString(
+        configFile.toPath(),
+        """
+        name: invalidModelAgent
+        description: Agent with an invalid model
+        instruction: test instruction
+        agent_class: LlmAgent
+        model: "invalid-model-name"
+        """);
+    String configPath = configFile.getAbsolutePath();
+
+    BaseAgent agent = ConfigAgentUtils.fromConfig(configPath);
+
+    assertThat(agent).isInstanceOf(LlmAgent.class);
+    LlmAgent llmAgent = (LlmAgent) agent;
+
+    IllegalArgumentException exception =
+        assertThrows(IllegalArgumentException.class, llmAgent::resolvedModel);
+    assertThat(exception).hasMessageThat().contains("invalid-model-name");
+  }
 }

core/src/test/java/com/google/adk/agents/LlmAgentTest.java

@@ -27,7 +27,9 @@
 import static org.junit.Assert.assertThrows;
 
 import com.google.adk.events.Event;
+import com.google.adk.models.LlmRegistry;
 import com.google.adk.models.LlmResponse;
+import com.google.adk.models.Model;
 import com.google.adk.testing.TestLlm;
 import com.google.adk.testing.TestUtils.EchoTool;
 import com.google.adk.tools.BaseTool;
@@ -289,4 +291,16 @@ public void testCanonicalGlobalInstruction_providerInstructionInjectsContext() {
 
     assertThat(canonicalInstruction).isEqualTo(instruction + invocationContext.invocationId());
   }
+
+  @Test
+  public void resolveModel_withModelName_resolvesFromRegistry() {
+    String modelName = "test-model";
+    TestLlm testLlm = createTestLlm(LlmResponse.builder().build());
+    LlmRegistry.registerLlm(modelName, (name) -> testLlm);
+    LlmAgent agent = createTestAgentBuilder(testLlm).model(modelName).build();
+    Model resolvedModel = agent.resolvedModel();
+
+    assertThat(resolvedModel.modelName()).hasValue(modelName);
+    assertThat(resolvedModel.model()).hasValue(testLlm);
+  }
 }