feat: add max_budget_usd option to Python SDK (#293)

Add support for limiting API costs using the max_budget_usd option,
mirroring the TypeScript SDK functionality. When the budget is exceeded,
query execution stops and returns a result with subtype
'error_max_budget_usd'.

- Add max_budget_usd field to ClaudeAgentOptions
- Pass --max-budget-usd flag to Claude Code CLI
- Add test coverage for budget limit behavior

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

examples/max_budget_usd.py

@@ -0,0 +1,95 @@
+#!/usr/bin/env python3
+"""Example demonstrating max_budget_usd option for cost control."""
+
+import anyio
+
+from claude_agent_sdk import (
+    AssistantMessage,
+    ClaudeAgentOptions,
+    ResultMessage,
+    TextBlock,
+    query,
+)
+
+
+async def without_budget():
+    """Example without budget limit."""
+    print("=== Without Budget Limit ===")
+
+    async for message in query(prompt="What is 2 + 2?"):
+        if isinstance(message, AssistantMessage):
+            for block in message.content:
+                if isinstance(block, TextBlock):
+                    print(f"Claude: {block.text}")
+        elif isinstance(message, ResultMessage):
+            if message.total_cost_usd:
+                print(f"Total cost: ${message.total_cost_usd:.4f}")
+            print(f"Status: {message.subtype}")
+    print()
+
+
+async def with_reasonable_budget():
+    """Example with budget that won't be exceeded."""
+    print("=== With Reasonable Budget ($0.10) ===")
+
+    options = ClaudeAgentOptions(
+        max_budget_usd=0.10,  # 10 cents - plenty for a simple query
+    )
+
+    async for message in query(prompt="What is 2 + 2?", options=options):
+        if isinstance(message, AssistantMessage):
+            for block in message.content:
+                if isinstance(block, TextBlock):
+                    print(f"Claude: {block.text}")
+        elif isinstance(message, ResultMessage):
+            if message.total_cost_usd:
+                print(f"Total cost: ${message.total_cost_usd:.4f}")
+            print(f"Status: {message.subtype}")
+    print()
+
+
+async def with_tight_budget():
+    """Example with very tight budget that will likely be exceeded."""
+    print("=== With Tight Budget ($0.0001) ===")
+
+    options = ClaudeAgentOptions(
+        max_budget_usd=0.0001,  # Very small budget - will be exceeded quickly
+    )
+
+    async for message in query(
+        prompt="Read the README.md file and summarize it", options=options
+    ):
+        if isinstance(message, AssistantMessage):
+            for block in message.content:
+                if isinstance(block, TextBlock):
+                    print(f"Claude: {block.text}")
+        elif isinstance(message, ResultMessage):
+            if message.total_cost_usd:
+                print(f"Total cost: ${message.total_cost_usd:.4f}")
+            print(f"Status: {message.subtype}")
+
+            # Check if budget was exceeded
+            if message.subtype == "error_max_budget_usd":
+                print("⚠️  Budget limit exceeded!")
+                print(
+                    "Note: The cost may exceed the budget by up to one API call's worth"
+                )
+    print()
+
+
+async def main():
+    """Run all examples."""
+    print("This example demonstrates using max_budget_usd to control API costs.\n")
+
+    await without_budget()
+    await with_reasonable_budget()
+    await with_tight_budget()
+
+    print(
+        "\nNote: Budget checking happens after each API call completes,\n"
+        "so the final cost may slightly exceed the specified budget.\n"
+    )
+
+
+if __name__ == "__main__":
+    anyio.run(main)

src/claude_agent_sdk/_internal/transport/subprocess_cli.py

@@ -115,6 +115,9 @@ class SubprocessCLITransport(Transport):
         if self._options.max_turns:
             cmd.extend(["--max-turns", str(self._options.max_turns)])
 
+        if self._options.max_budget_usd is not None:
+            cmd.extend(["--max-budget-usd", str(self._options.max_budget_usd)])
+
         if self._options.disallowed_tools:
             cmd.extend(["--disallowedTools", ",".join(self._options.disallowed_tools)])
 

src/claude_agent_sdk/types.py

@@ -518,6 +518,7 @@ class ClaudeAgentOptions:
     continue_conversation: bool = False
     resume: str | None = None
     max_turns: int | None = None
+    max_budget_usd: float | None = None
     disallowed_tools: list[str] = field(default_factory=list)
     model: str | None = None
     permission_prompt_tool_name: str | None = None

tests/test_integration.py

@@ -212,3 +212,73 @@ class TestIntegration:
                 assert call_kwargs["options"].continue_conversation is True
 
         anyio.run(_test)
+
+    def test_max_budget_usd_option(self):
+        """Test query with max_budget_usd option."""
+
+        async def _test():
+            with patch(
+                "claude_agent_sdk._internal.client.SubprocessCLITransport"
+            ) as mock_transport_class:
+                mock_transport = AsyncMock()
+                mock_transport_class.return_value = mock_transport
+
+                # Mock the message stream that exceeds budget
+                async def mock_receive():
+                    yield {
+                        "type": "assistant",
+                        "message": {
+                            "role": "assistant",
+                            "content": [
+                                {"type": "text", "text": "Starting to read..."}
+                            ],
+                            "model": "claude-opus-4-1-20250805",
+                        },
+                    }
+                    yield {
+                        "type": "result",
+                        "subtype": "error_max_budget_usd",
+                        "duration_ms": 500,
+                        "duration_api_ms": 400,
+                        "is_error": False,
+                        "num_turns": 1,
+                        "session_id": "test-session-budget",
+                        "total_cost_usd": 0.0002,
+                        "usage": {
+                            "input_tokens": 100,
+                            "output_tokens": 50,
+                        },
+                    }
+
+                mock_transport.read_messages = mock_receive
+                mock_transport.connect = AsyncMock()
+                mock_transport.close = AsyncMock()
+                mock_transport.end_input = AsyncMock()
+                mock_transport.write = AsyncMock()
+                mock_transport.is_ready = Mock(return_value=True)
+
+                # Run query with very small budget
+                messages = []
+                async for msg in query(
+                    prompt="Read the readme",
+                    options=ClaudeAgentOptions(max_budget_usd=0.0001),
+                ):
+                    messages.append(msg)
+
+                # Verify results
+                assert len(messages) == 2
+
+                # Check result message
+                assert isinstance(messages[1], ResultMessage)
+                assert messages[1].subtype == "error_max_budget_usd"
+                assert messages[1].is_error is False
+                assert messages[1].total_cost_usd == 0.0002
+                assert messages[1].total_cost_usd is not None
+                assert messages[1].total_cost_usd > 0
+
+                # Verify transport was created with max_budget_usd option
+                mock_transport_class.assert_called_once()
+                call_kwargs = mock_transport_class.call_args.kwargs
+                assert call_kwargs["options"].max_budget_usd == 0.0001
+
+        anyio.run(_test)