Add support for custom env vars (#131)

## Key changes
- Adds env field to `ClaudeCodeOptions`, allowing custom env vars to cli
- Updates tests and examples

## Motivation
Bringing Python SDK to feature parity with TS SDK, which supports custom
env vars

## Notes
- Environment variables are merged in order: system env → user env → SDK
required vars
- This implementation seems slightly more robust than the TypeScript
version, which can exclude OS envs vars if a user passes a minimal env
object
- Some linting changes seem to have been picked up

examples/streaming_mode.py

@@ -188,9 +188,7 @@ async def example_manual_message_handling():
     print("=== Manual Message Handling Example ===")
 
     async with ClaudeSDKClient() as client:
-        await client.query(
-            "List 5 programming languages and their main use cases"
-        )
+        await client.query("List 5 programming languages and their main use cases")
 
         # Manually process messages with custom logic
         languages_found = []
@@ -231,13 +229,14 @@ async def example_with_options():
         allowed_tools=["Read", "Write"],  # Allow file operations
         max_thinking_tokens=10000,
         system_prompt="You are a helpful coding assistant.",
+        env={
+            "ANTHROPIC_MODEL": "claude-3-7-sonnet-20250219",
+        },
     )
 
     async with ClaudeSDKClient(options=options) as client:
         print("User: Create a simple hello.txt file with a greeting message")
-        await client.query(
-            "Create a simple hello.txt file with a greeting message"
-        )
+        await client.query("Create a simple hello.txt file with a greeting message")
 
         tool_uses = []
         async for msg in client.receive_response():
@@ -325,7 +324,9 @@ async def example_bash_command():
                     if isinstance(block, TextBlock):
                         print(f"User: {block.text}")
                     elif isinstance(block, ToolResultBlock):
-                        print(f"Tool Result (id: {block.tool_use_id}): {block.content[:100] if block.content else 'None'}...")
+                        print(
+                            f"Tool Result (id: {block.tool_use_id}): {block.content[:100] if block.content else 'None'}..."
+                        )
 
             elif isinstance(msg, AssistantMessage):
                 # Assistant messages can contain tool use blocks

src/claude_code_sdk/_internal/transport/subprocess_cli.py

@@ -175,13 +175,20 @@ class SubprocessCLITransport(Transport):
             )
 
             # Enable stdin pipe for both modes (but we'll close it for string mode)
+            # Merge environment variables: system -> user -> SDK required
+            process_env = {
+                **os.environ,
+                **self._options.env,  # User-provided env vars
+                "CLAUDE_CODE_ENTRYPOINT": "sdk-py",
+            }
+
             self._process = await anyio.open_process(
                 cmd,
                 stdin=PIPE,
                 stdout=PIPE,
                 stderr=self._stderr_file,
                 cwd=self._cwd,
-                env={**os.environ, "CLAUDE_CODE_ENTRYPOINT": "sdk-py"},
+                env=process_env,
             )
 
             if self._process.stdout:

src/claude_code_sdk/types.py

@@ -137,6 +137,7 @@ class ClaudeCodeOptions:
     cwd: str | Path | None = None
     settings: str | None = None
     add_dirs: list[str | Path] = field(default_factory=list)
+    env: dict[str, str] = field(default_factory=dict)
     extra_args: dict[str, str | None] = field(
         default_factory=dict
     )  # Pass arbitrary CLI flags

tests/test_transport.py

@@ -1,5 +1,7 @@
 """Tests for Claude SDK transport layer."""
 
+import os
+import uuid
 from unittest.mock import AsyncMock, MagicMock, patch
 
 import anyio
@@ -299,3 +301,54 @@ class TestSubprocessCLITransport:
         assert "--mcp-config" in cmd
         mcp_idx = cmd.index("--mcp-config")
         assert cmd[mcp_idx + 1] == json_config
+
+    def test_env_vars_passed_to_subprocess(self):
+        """Test that custom environment variables are passed to the subprocess."""
+
+        async def _test():
+            test_value = f"test-{uuid.uuid4().hex[:8]}"
+            custom_env = {
+                "MY_TEST_VAR": test_value,
+            }
+
+            options = ClaudeCodeOptions(env=custom_env)
+
+            # Mock the subprocess to capture the env argument
+            with patch(
+                "anyio.open_process", new_callable=AsyncMock
+            ) as mock_open_process:
+                mock_process = MagicMock()
+                mock_process.stdout = MagicMock()
+                mock_stdin = MagicMock()
+                mock_stdin.aclose = AsyncMock()  # Add async aclose method
+                mock_process.stdin = mock_stdin
+                mock_process.returncode = None
+                mock_open_process.return_value = mock_process
+
+                transport = SubprocessCLITransport(
+                    prompt="test",
+                    options=options,
+                    cli_path="/usr/bin/claude",
+                )
+
+                await transport.connect()
+
+                # Verify open_process was called with correct env vars
+                mock_open_process.assert_called_once()
+                call_kwargs = mock_open_process.call_args.kwargs
+                assert "env" in call_kwargs
+                env_passed = call_kwargs["env"]
+
+                # Check that custom env var was passed
+                assert env_passed["MY_TEST_VAR"] == test_value
+
+                # Verify SDK identifier is present
+                assert "CLAUDE_CODE_ENTRYPOINT" in env_passed
+                assert env_passed["CLAUDE_CODE_ENTRYPOINT"] == "sdk-py"
+
+                # Verify system env vars are also included with correct values
+                if "PATH" in os.environ:
+                    assert "PATH" in env_passed
+                    assert env_passed["PATH"] == os.environ["PATH"]
+
+        anyio.run(_test)