feat: add tool_choice support

Adds first_turn_tool_choice and ensure_tool_usage options to
  ClaudeAgentOptions for controlling tool usage in agentic loops.

src/claude_agent_sdk/_internal/transport/subprocess_cli.py

@@ -225,6 +225,14 @@ class SubprocessCLITransport(Transport):
                 ["--max-thinking-tokens", str(self._options.max_thinking_tokens)]
             )
 
+        if self._options.first_turn_tool_choice is not None:
+            cmd.extend(
+                ["--first-turn-tool-choice", json.dumps(self._options.first_turn_tool_choice)]
+            )
+
+        if self._options.ensure_tool_usage:
+            cmd.append("--ensure-tool-usage")
+
         # Check if command line is too long (Windows limitation)
         cmd_str = " ".join(cmd)
         if len(cmd_str) > _CMD_LENGTH_LIMIT and self._options.agents:

src/claude_agent_sdk/types.py

@@ -17,6 +17,9 @@ PermissionMode = Literal["default", "acceptEdits", "plan", "bypassPermissions"]
 # Agent definitions
 SettingSource = Literal["user", "project", "local"]
 
+# Tool choice types
+ToolChoice = Literal["auto", "any", "none"] | dict[str, str]
+
 
 class SystemPromptPreset(TypedDict):
     """System prompt preset configuration."""
@@ -557,6 +560,10 @@ class ClaudeAgentOptions:
     plugins: list[SdkPluginConfig] = field(default_factory=list)
     # Max tokens for thinking blocks
     max_thinking_tokens: int | None = None
+    # Tool choice for first turn only
+    first_turn_tool_choice: ToolChoice | None = None
+    # Ensure at least one tool is used before conversation ends
+    ensure_tool_usage: bool = False
 
 
 # SDK Control Protocol