feat: Update model resolution logic for LLM agents

This change refactors the model resolution logic within the LlmAgent to utilize the LlmRegistry for looking up model instances by name. It also introduces a canonicalModel() method to ensure a BaseLlm instance is consistently available, simplifying model access in flows.

PiperOrigin-RevId: 792361836

Author: Doris26

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);
+  }
 }