fix: pass --thinking flag for adaptive/disabled instead of budget tokens (#796)

## Summary

`thinking={"type":"adaptive"}` was being lowered to
`--max-thinking-tokens 32000`, which puts the CLI into fixed-budget mode
rather than adaptive mode. `disabled` similarly mapped to
`--max-thinking-tokens 0`.

The CLI exposes an explicit `--thinking <adaptive|enabled|disabled>`
flag that takes precedence over `--max-thinking-tokens`. The TypeScript
SDK already uses it. This change mirrors that behavior:

- `adaptive` → `--thinking adaptive`
- `disabled` → `--thinking disabled`
- `enabled` → `--max-thinking-tokens <budget_tokens>`
- deprecated `max_thinking_tokens` is only emitted when `thinking` is
unset

## Test plan

- `pytest tests/test_transport.py` (59 passed)
- New parametrized test for all three `thinking` types
- New precedence test: `thinking` overrides `max_thinking_tokens`

Author: hackyon-anthropic

src/claude_agent_sdk/_internal/transport/subprocess_cli.py

@@ -300,20 +300,20 @@ def _build_command(self) -> list[str]:
                 # Flag with value
                 cmd.extend([f"--{flag}", str(value)])
 
-        # Resolve thinking config → --max-thinking-tokens
+        # Resolve thinking config -> --thinking / --max-thinking-tokens
         # `thinking` takes precedence over the deprecated `max_thinking_tokens`
-        resolved_max_thinking_tokens = self._options.max_thinking_tokens
         if self._options.thinking is not None:
             t = self._options.thinking
             if t["type"] == "adaptive":
-                if resolved_max_thinking_tokens is None:
-                    resolved_max_thinking_tokens = 32_000
+                cmd.extend(["--thinking", "adaptive"])
             elif t["type"] == "enabled":
-                resolved_max_thinking_tokens = t["budget_tokens"]
+                cmd.extend(["--max-thinking-tokens", str(t["budget_tokens"])])
             elif t["type"] == "disabled":
-                resolved_max_thinking_tokens = 0
-        if resolved_max_thinking_tokens is not None:
-            cmd.extend(["--max-thinking-tokens", str(resolved_max_thinking_tokens)])
+                cmd.extend(["--thinking", "disabled"])
+        elif self._options.max_thinking_tokens is not None:
+            cmd.extend(
+                ["--max-thinking-tokens", str(self._options.max_thinking_tokens)]
+            )
 
         if self._options.effort is not None:
             cmd.extend(["--effort", self._options.effort])

tests/test_transport.py

@@ -233,6 +233,44 @@ def test_build_command_with_max_thinking_tokens(self):
         assert "--max-thinking-tokens" in cmd
         assert "5000" in cmd
 
+    @pytest.mark.parametrize(
+        ("thinking", "expected", "absent"),
+        [
+            ({"type": "adaptive"}, ["--thinking", "adaptive"], "--max-thinking-tokens"),
+            (
+                {"type": "enabled", "budget_tokens": 5000},
+                ["--max-thinking-tokens", "5000"],
+                "--thinking",
+            ),
+            ({"type": "disabled"}, ["--thinking", "disabled"], "--max-thinking-tokens"),
+        ],
+    )
+    def test_build_command_with_thinking(self, thinking, expected, absent):
+        """Test building CLI command with thinking option."""
+        transport = SubprocessCLITransport(
+            prompt="test",
+            options=make_options(thinking=thinking),
+        )
+
+        cmd = transport._build_command()
+        idx = cmd.index(expected[0])
+        assert cmd[idx : idx + 2] == expected
+        assert absent not in cmd
+
+    def test_build_command_thinking_precedence_over_max_thinking_tokens(self):
+        """thinking takes precedence over deprecated max_thinking_tokens."""
+        transport = SubprocessCLITransport(
+            prompt="test",
+            options=make_options(
+                thinking={"type": "adaptive"}, max_thinking_tokens=9999
+            ),
+        )
+
+        cmd = transport._build_command()
+        idx = cmd.index("--thinking")
+        assert cmd[idx : idx + 2] == ["--thinking", "adaptive"]
+        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