Reject null input in agent tool parameter validation

JSON null unmarshals into an empty string, so the presence-only
key check let {"input":null} through, running the nested agent
with a blank user message.

Signed-off-by: Bryan Frimin <bryan@getprobo.com>

pkg/agent/agent_tool.go

@@ -83,7 +83,8 @@ func (t *agentTool) Execute(ctx context.Context, arguments string) (ToolResult,
 		}, nil
 	}
 
-	if _, ok := fields["input"]; !ok {
+	raw, ok := fields["input"]
+	if !ok || string(raw) == "null" {
 		return ToolResult{
 			Content: "Missing required parameters: input",
 			IsError: true,

pkg/agent/agent_tool_test.go

@@ -215,6 +215,27 @@ func TestAgentTool_Execute(t *testing.T) {
 		},
 	)
 
+	t.Run(
+		"null input returns tool error for missing input",
+		func(t *testing.T) {
+			t.Parallel()
+
+			ag := agent.New(
+				"sub",
+				newTestClient(&mockProvider{}),
+				agent.WithModel("test-model"),
+			)
+
+			tool := ag.AsTool("sub_tool", "A sub-agent tool.")
+			result, err := tool.Execute(context.Background(), `{"input":null}`)
+
+			require.NoError(t, err)
+			assert.True(t, result.IsError)
+			assert.Contains(t, result.Content, "Missing required parameters")
+			assert.Contains(t, result.Content, "input")
+		},
+	)
+
 	t.Run(
 		"sub-agent error propagates as Go error",
 		func(t *testing.T) {