diff --git a/packages/core/src/agents/delegate-to-agent-tool.test.ts b/packages/core/src/agents/delegate-to-agent-tool.test.ts
index 89cd1babdb..bc29869c0e 100644
--- a/packages/core/src/agents/delegate-to-agent-tool.test.ts
+++ b/packages/core/src/agents/delegate-to-agent-tool.test.ts
@@ -121,19 +121,22 @@ describe('DelegateToAgentTool', () => {
     );
   });
 
-  it('should throw helpful error when agent_name does not exist', async () => {
-    // We allow validation to pass now, checking happens in execute.
-    const invocation = tool.build({
-      agent_name: 'non_existent_agent',
-    } as DelegateParams);
-
-    await expect(() =>
-      invocation.execute(new AbortController().signal),
-    ).rejects.toThrow(
+  it('should throw helpful error when agent_name does not exist', () => {
+    expect(() =>
+      tool.build({
+        agent_name: 'non_existent_agent',
+      } as DelegateParams),
+    ).toThrow(
       "Agent 'non_existent_agent' not found. Available agents are: 'test_agent' (A test agent), 'remote_agent' (A remote agent). Please choose a valid agent_name.",
     );
   });
 
+  it('should throw error when agent_name is missing', () => {
+    expect(() => tool.build({} as DelegateParams)).toThrow(
+      "Missing 'agent_name' parameter. Please specify the agent you want to delegate to.",
+    );
+  });
+
   it('should validate correct arguments', async () => {
     const invocation = tool.build({
       agent_name: 'test_agent',
@@ -152,28 +155,24 @@ describe('DelegateToAgentTool', () => {
     );
   });
 
-  it('should throw helpful error for missing required argument', async () => {
-    const invocation = tool.build({
-      agent_name: 'test_agent',
-      arg2: 123,
-    } as DelegateParams);
-
-    await expect(() =>
-      invocation.execute(new AbortController().signal),
-    ).rejects.toThrow(
+  it('should throw helpful error for missing required argument', () => {
+    expect(() =>
+      tool.build({
+        agent_name: 'test_agent',
+        arg2: 123,
+      } as DelegateParams),
+    ).toThrow(
       `Invalid arguments for agent 'test_agent': params must have required property 'arg1'. Input schema: ${JSON.stringify(mockAgentDef.inputConfig.inputSchema)}.`,
     );
   });
 
-  it('should throw helpful error for invalid argument type', async () => {
-    const invocation = tool.build({
-      agent_name: 'test_agent',
-      arg1: 123,
-    } as DelegateParams);
-
-    await expect(() =>
-      invocation.execute(new AbortController().signal),
-    ).rejects.toThrow(
+  it('should throw helpful error for invalid argument type', () => {
+    expect(() =>
+      tool.build({
+        agent_name: 'test_agent',
+        arg1: 123,
+      } as DelegateParams),
+    ).toThrow(
       `Invalid arguments for agent 'test_agent': params/arg1 must be string. Input schema: ${JSON.stringify(mockAgentDef.inputConfig.inputSchema)}.`,
     );
   });
diff --git a/packages/core/src/agents/delegate-to-agent-tool.ts b/packages/core/src/agents/delegate-to-agent-tool.ts
index 064428940d..8b79e923a0 100644
--- a/packages/core/src/agents/delegate-to-agent-tool.ts
+++ b/packages/core/src/agents/delegate-to-agent-tool.ts
@@ -121,11 +121,33 @@ export class DelegateToAgentTool extends BaseDeclarativeTool<
     );
   }
 
-  override validateToolParams(_params: DelegateParams): string | null {
-    // We override the default schema validation because the generic JSON schema validation
-    // produces poor error messages for discriminated unions (anyOf).
-    // Instead, we perform detailed, agent-specific validation in the `execute` method
-    // to provide rich error messages that help the LLM self-heal.
+  override validateToolParams(params: DelegateParams): string | null {
+    if (!params.agent_name) {
+      return "Missing 'agent_name' parameter. Please specify the agent you want to delegate to.";
+    }
+
+    const definition = this.registry.getDefinition(params.agent_name);
+    if (!definition) {
+      const availableAgents = this.registry
+        .getAllDefinitions()
+        .map((def) => `'${def.name}' (${def.description})`)
+        .join(', ');
+
+      return `Agent '${params.agent_name}' not found. Available agents are: ${availableAgents}. Please choose a valid agent_name.`;
+    }
+
+    const { agent_name: _agent_name, ...agentArgs } = params;
+
+    // Validate specific agent arguments here using SchemaValidator to generate helpful error messages.
+    const validationError = SchemaValidator.validate(
+      definition.inputConfig.inputSchema,
+      agentArgs,
+    );
+
+    if (validationError) {
+      return `Invalid arguments for agent '${definition.name}': ${validationError}. Input schema: ${JSON.stringify(definition.inputConfig.inputSchema)}.`;
+    }
+
     return null;
   }
 
@@ -173,8 +195,8 @@ class DelegateInvocation extends BaseToolInvocation<
   override async shouldConfirmExecute(
     abortSignal: AbortSignal,
   ): Promise<ToolCallConfirmationDetails | false> {
-    const definition = this.registry.getDefinition(this.params.agent_name);
-    if (!definition || definition.kind !== 'remote') {
+    const definition = this.registry.getDefinition(this.params.agent_name)!;
+    if (definition.kind !== 'remote') {
       // Local agents should execute without confirmation. Inner tool calls will bubble up their own confirmations to the user.
       return false;
     }
@@ -191,32 +213,9 @@ class DelegateInvocation extends BaseToolInvocation<
     signal: AbortSignal,
     updateOutput?: (output: string | AnsiOutput) => void,
   ): Promise<ToolResult> {
-    const definition = this.registry.getDefinition(this.params.agent_name);
-    if (!definition) {
-      const availableAgents = this.registry
-        .getAllDefinitions()
-        .map((def) => `'${def.name}' (${def.description})`)
-        .join(', ');
-
-      throw new Error(
-        `Agent '${this.params.agent_name}' not found. Available agents are: ${availableAgents}. Please choose a valid agent_name.`,
-      );
-    }
-
+    const definition = this.registry.getDefinition(this.params.agent_name)!;
     const { agent_name: _agent_name, ...agentArgs } = this.params;
 
-    // Validate specific agent arguments here using SchemaValidator to generate helpful error messages.
-    const validationError = SchemaValidator.validate(
-      definition.inputConfig.inputSchema,
-      agentArgs,
-    );
-
-    if (validationError) {
-      throw new Error(
-        `Invalid arguments for agent '${definition.name}': ${validationError}. Input schema: ${JSON.stringify(definition.inputConfig.inputSchema)}.`,
-      );
-    }
-
     const invocation = this.buildSubInvocation(
       definition,
       agentArgs as AgentInputs,