diff --git a/src/claude_agent_sdk/_internal/transport/subprocess_cli.py b/src/claude_agent_sdk/_internal/transport/subprocess_cli.py
index 7c142f69..95611fc2 100644
--- a/src/claude_agent_sdk/_internal/transport/subprocess_cli.py
+++ b/src/claude_agent_sdk/_internal/transport/subprocess_cli.py
@@ -308,7 +308,7 @@ def _build_command(self) -> list[str]:
             elif t["type"] == "enabled":
                 resolved_max_thinking_tokens = t["budget_tokens"]
             elif t["type"] == "disabled":
-                resolved_max_thinking_tokens = 0
+                cmd.append("--thinking-disabled")
         if resolved_max_thinking_tokens is not None:
             cmd.extend(["--max-thinking-tokens", str(resolved_max_thinking_tokens)])
 
diff --git a/tests/test_transport.py b/tests/test_transport.py
index 7d09ecee..84ea4c36 100644
--- a/tests/test_transport.py
+++ b/tests/test_transport.py
@@ -160,6 +160,17 @@ def test_build_command_with_max_thinking_tokens(self):
         assert "--max-thinking-tokens" in cmd
         assert "5000" in cmd
 
+    def test_build_command_with_thinking_disabled(self):
+        """Test that thinking=disabled sends --thinking-disabled flag, not --max-thinking-tokens 0."""
+        transport = SubprocessCLITransport(
+            prompt="test",
+            options=make_options(thinking={"type": "disabled"}),
+        )
+
+        cmd = transport._build_command()
+        assert "--thinking-disabled" in cmd
+        assert "--max-thinking-tokens" not in cmd
+
     def test_build_command_with_add_dirs(self):
         """Test building CLI command with add_dirs option."""
         from pathlib import Path