diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 0000000..4bad64b
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,53 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+jobs:
+  lint:
+    name: Lint
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v6
+        with:
+          python-version: "3.13"
+          cache: pip
+
+      - name: Install dependencies
+        run: pip install .[dev]
+
+      - name: Ruff
+        run: ruff check .
+
+      - name: Mypy
+        run: mypy src tests
+
+  test:
+    name: Test
+    runs-on: ubuntu-latest
+    needs: lint
+    strategy:
+      fail-fast: false
+      matrix:
+        python-version: ["3.11", "3.12", "3.13"]
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v6
+        with:
+          python-version: ${{ matrix.python-version }}
+          cache: pip
+
+      - name: Install dependencies
+        run: pip install .[dev]
+
+      - name: Pytest
+        run: pytest
diff --git a/examples/custom_tracer.py b/examples/custom_tracer.py
index 57844ce..09ec8ac 100644
--- a/examples/custom_tracer.py
+++ b/examples/custom_tracer.py
@@ -7,7 +7,7 @@
 from pathlib import Path
 from typing import Any
 
-from tsunagi import Pipeline, Tracer, step
+from tsunagi import Pipeline, step
 
 
 class JSONLTracer:
diff --git a/pyproject.toml b/pyproject.toml
index 182e91d..545646d 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -51,6 +51,10 @@ strict = true
 warn_return_any = true
 warn_unused_configs = true
 
+[[tool.mypy.overrides]]
+module = "tests.*"
+disable_error_code = ["arg-type"]
+
 [tool.ruff]
 target-version = "py311"
 line-length = 100
diff --git a/src/tsunagi/agent.py b/src/tsunagi/agent.py
index 1511fd7..bbafa5b 100644
--- a/src/tsunagi/agent.py
+++ b/src/tsunagi/agent.py
@@ -16,12 +16,16 @@
 
 import json
 from dataclasses import dataclass, field
-from typing import Any, AsyncIterator, Protocol, runtime_checkable
+from typing import TYPE_CHECKING, Any, Protocol, runtime_checkable
 
 from tsunagi.context import Context
-from tsunagi.tool import Tool
 from tsunagi.tracer import NullTracer, Tracer
 
+if TYPE_CHECKING:
+    from collections.abc import AsyncIterator
+
+    from tsunagi.tool import Tool
+
 
 @dataclass
 class Message:
@@ -88,9 +92,10 @@ def __init__(
 
     def _get_client(self) -> Any:
         if self._client is None:
-            from anthropic import AsyncAnthropic
+            import importlib
 
-            self._client = AsyncAnthropic(**self.client_kwargs)
+            anthropic = importlib.import_module("anthropic")
+            self._client = anthropic.AsyncAnthropic(**self.client_kwargs)
         return self._client
 
     async def send(
@@ -148,9 +153,10 @@ def __init__(
 
     def _get_client(self) -> Any:
         if self._client is None:
-            from openai import AsyncOpenAI
+            import importlib
 
-            self._client = AsyncOpenAI(**self.client_kwargs)
+            openai_module = importlib.import_module("openai")
+            self._client = openai_module.AsyncOpenAI(**self.client_kwargs)
         return self._client
 
     async def send(
@@ -247,10 +253,7 @@ async def chat(self, user_message: str) -> str:
                     await self._tracer.on_step_start(ctx, tc.name, tc.arguments)
                     try:
                         result = await tool_obj(**tc.arguments)
-                        if isinstance(result, str):
-                            result_str = result
-                        else:
-                            result_str = json.dumps(result)
+                        result_str = result if isinstance(result, str) else json.dumps(result)
                         await self._tracer.on_step_end(ctx, tc.name, result_str)
                     except Exception as e:  # pragma: no cover - exercised in tests
                         result_str = json.dumps({"error": str(e)})
diff --git a/src/tsunagi/pipeline.py b/src/tsunagi/pipeline.py
index c34ca8e..03c2397 100644
--- a/src/tsunagi/pipeline.py
+++ b/src/tsunagi/pipeline.py
@@ -142,7 +142,7 @@ async def run_parallel(
             step_inputs = [input] * len(steps)
 
         try:
-            tasks = [s.execute(inp) for s, inp in zip(steps, step_inputs)]
+            tasks = [s.execute(inp) for s, inp in zip(steps, step_inputs, strict=True)]
             return list(await asyncio.gather(*tasks))
         except Exception as e:  # pragma: no cover - handled in pipeline tests
             raise PipelineError("parallel", e) from e
diff --git a/src/tsunagi/retry.py b/src/tsunagi/retry.py
index 10214aa..67921ad 100644
--- a/src/tsunagi/retry.py
+++ b/src/tsunagi/retry.py
@@ -8,7 +8,10 @@
 
 import random
 from dataclasses import dataclass
-from typing import Callable
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from collections.abc import Callable
 
 
 @dataclass(frozen=True)
diff --git a/src/tsunagi/step.py b/src/tsunagi/step.py
index 3a1c774..16d6e8e 100644
--- a/src/tsunagi/step.py
+++ b/src/tsunagi/step.py
@@ -15,12 +15,15 @@
 import asyncio
 import functools
 import inspect
-from typing import Any, Awaitable, Callable, TypeVar, overload
+from typing import TYPE_CHECKING, Any, TypeVar, overload
 
 from tsunagi.errors import StepError
 from tsunagi.errors import TimeoutError as TsunagiTimeoutError
 from tsunagi.retry import NO_RETRY, RetryConfig
 
+if TYPE_CHECKING:
+    from collections.abc import Awaitable, Callable
+
 T = TypeVar("T")
 
 
@@ -70,19 +73,23 @@ async def execute(self, *args: Any, **kwargs: Any) -> Any:
         """
         last_error: Exception | None = None
 
+        timeout_seconds = self.timeout_seconds
+
         for attempt in range(self.retry.max_attempts):
             try:
-                if self.timeout_seconds is not None:
+                if timeout_seconds is not None:
                     result = await asyncio.wait_for(
                         self.fn(*args, **kwargs),
-                        timeout=self.timeout_seconds,
+                        timeout=timeout_seconds,
                     )
                 else:
                     result = await self.fn(*args, **kwargs)
                 return result
 
-            except asyncio.TimeoutError:
-                raise TsunagiTimeoutError(self.name, self.timeout_seconds) from None
+            except TimeoutError:
+                if timeout_seconds is None:
+                    raise
+                raise TsunagiTimeoutError(self.name, timeout_seconds) from None
 
             except Exception as e:
                 last_error = e
@@ -150,7 +157,7 @@ async def bound_fn(*a: Any, **kw: Any) -> Any:
         try:
             return await self._original.execute(*args)
         finally:
-            self._original.fn = original_fn  # type: ignore[assignment]
+            self._original.fn = original_fn
 
 
 class StepSequence:
diff --git a/src/tsunagi/tool.py b/src/tsunagi/tool.py
index 2aaa491..c4f7120 100644
--- a/src/tsunagi/tool.py
+++ b/src/tsunagi/tool.py
@@ -8,7 +8,10 @@
 from __future__ import annotations
 
 import inspect
-from typing import Any, Awaitable, Callable, get_type_hints
+from typing import TYPE_CHECKING, Any, get_type_hints
+
+if TYPE_CHECKING:
+    from collections.abc import Awaitable, Callable
 
 
 class Tool:
diff --git a/src/tsunagi/tracer.py b/src/tsunagi/tracer.py
index add0247..2359004 100644
--- a/src/tsunagi/tracer.py
+++ b/src/tsunagi/tracer.py
@@ -7,9 +7,10 @@
 from __future__ import annotations
 
 import sys
-from typing import Any, Protocol, runtime_checkable
+from typing import TYPE_CHECKING, Any, Protocol, runtime_checkable
 
-from tsunagi.context import Context
+if TYPE_CHECKING:
+    from tsunagi.context import Context
 
 
 @runtime_checkable
diff --git a/tests/__init__.py b/tests/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/test_agent.py b/tests/test_agent.py
index 8b8f135..eff8642 100644
--- a/tests/test_agent.py
+++ b/tests/test_agent.py
@@ -1,11 +1,14 @@
 from __future__ import annotations
 
 import json
+from typing import TYPE_CHECKING, Any
 
 import pytest
 
 from tsunagi import Agent, AgentEvent, ToolCall, tool
-from tsunagi.agent import Message
+
+if TYPE_CHECKING:
+    from tsunagi.agent import Message
 
 
 class TextAdapter:
@@ -88,7 +91,9 @@ async def test_agent_unknown_tool() -> None:
     assert result == "final"
 
     tool_result_message = adapter.seen_messages[-1][-1]
-    content = tool_result_message.content[0]["content"]
+    assert isinstance(tool_result_message.content, list)
+    content_list: list[dict[str, Any]] = tool_result_message.content
+    content = content_list[0]["content"]
     assert "Unknown tool" in content
 
 
@@ -101,7 +106,7 @@ async def test_agent_stream() -> None:
     agent = Agent(adapter)
     agent.register(adder)
 
-    events = [event async for event in agent.stream("calc")]  # type: ignore[misc]
+    events = [event async for event in agent.stream("calc")]
 
     assert events[0] == AgentEvent(
         type="tool_call", tool="adder", args={"a": 1, "b": 2}
diff --git a/tests/test_integration.py b/tests/test_integration.py
index 7f55bf2..e741d04 100644
--- a/tests/test_integration.py
+++ b/tests/test_integration.py
@@ -2,8 +2,8 @@
 
 import pytest
 
-from tsunagi import Pipeline
 from tests.conftest import add_one, double, to_string
+from tsunagi import Pipeline
 
 
 @pytest.mark.asyncio
diff --git a/tests/test_pipeline.py b/tests/test_pipeline.py
index afa225c..26e7a37 100644
--- a/tests/test_pipeline.py
+++ b/tests/test_pipeline.py
@@ -2,8 +2,8 @@
 
 import pytest
 
-from tsunagi import NullTracer, Pipeline, PipelineError
 from tests.conftest import add_one, always_fail, double, to_string
+from tsunagi import NullTracer, Pipeline, PipelineError
 
 
 class MockTracer:
diff --git a/tests/test_retry.py b/tests/test_retry.py
index 9d12f43..12a128c 100644
--- a/tests/test_retry.py
+++ b/tests/test_retry.py
@@ -2,9 +2,9 @@
 
 import pytest
 
-from tsunagi import NO_RETRY, RetryConfig
 from tests import conftest
 from tests.conftest import fail_twice_then_succeed
+from tsunagi import NO_RETRY, RetryConfig
 
 
 def test_no_retry_default() -> None:
diff --git a/tests/test_step.py b/tests/test_step.py
index a2dd0d5..ccb9840 100644
--- a/tests/test_step.py
+++ b/tests/test_step.py
@@ -2,8 +2,8 @@
 
 import pytest
 
-from tsunagi import StepError, StepSequence, step
 from tests.conftest import add_one, always_fail, double, to_string
+from tsunagi import StepError, StepSequence, step
 
 
 @pytest.mark.asyncio