diff --git a/tests/__init__.py b/tests/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/conftest.py b/tests/conftest.py
new file mode 100644
index 0000000..4bfaa07
--- /dev/null
+++ b/tests/conftest.py
@@ -0,0 +1,36 @@
+# pyright: reportMissingImports=false, reportUnknownMemberType=false, reportUntypedFunctionDecorator=false, reportUnknownParameterType=false, reportMissingParameterType=false, reportUnknownVariableType=false, reportUnknownArgumentType=false
+
+import pytest
+
+
+@pytest.fixture
+def simple_messages():
+    return [{"role": "user", "content": "hello world"}]
+
+
+@pytest.fixture
+def complex_messages():
+    return [
+        {
+            "role": "user",
+            "content": [
+                {"type": "image", "source": {"type": "base64", "data": "abc"}},
+                {"type": "text", "text": "hello world"},
+            ],
+        }
+    ]
+
+
+@pytest.fixture
+def basic_api_kwargs(simple_messages):
+    return {
+        "system": [
+            {
+                "type": "text",
+                "text": "You are Claude Code, Anthropic's official CLI for Claude.\nStay helpful.",
+            },
+            {"type": "text", "text": "Extra system guidance"},
+        ],
+        "messages": [dict(message) for message in simple_messages],
+        "model": "claude-opus-4-6-20260101",
+    }
diff --git a/tests/test_bypass.py b/tests/test_bypass.py
new file mode 100644
index 0000000..ff5d936
--- /dev/null
+++ b/tests/test_bypass.py
@@ -0,0 +1,95 @@
+# pyright: reportPrivateUsage=false, reportUnknownParameterType=false, reportMissingParameterType=false, reportUnknownArgumentType=false, reportUnknownVariableType=false, reportUnknownMemberType=false, reportArgumentType=false
+
+import copy
+
+from anthropic_billing_bypass import (
+    _SYSTEM_IDENTITY,
+    _fix_temperature_for_oauth_adaptive,
+    apply_claude_code_bypass,
+)
+
+
+def test_apply_claude_code_bypass_injects_billing_header_and_preserves_identity(
+    basic_api_kwargs,
+):
+    apply_claude_code_bypass(basic_api_kwargs, "2.1.90")
+
+    system = basic_api_kwargs["system"]
+    assert system[0]["text"].startswith("x-anthropic-billing-header: ")
+    assert system[1]["text"] == _SYSTEM_IDENTITY
+
+
+def test_apply_claude_code_bypass_relocates_non_identity_system_text_to_first_user_message(
+    basic_api_kwargs,
+):
+    apply_claude_code_bypass(basic_api_kwargs, "2.1.90")
+
+    user_content = basic_api_kwargs["messages"][0]["content"]
+    assert isinstance(user_content, list)
+    text = user_content[0]["text"]
+    assert "<system-reminder>\nStay helpful.\n</system-reminder>" in text
+    assert "<system-reminder>\nExtra system guidance\n</system-reminder>" in text
+    assert text.endswith("hello world")
+
+
+def test_apply_claude_code_bypass_is_idempotent(basic_api_kwargs):
+    apply_claude_code_bypass(basic_api_kwargs, "2.1.90")
+    once = copy.deepcopy(basic_api_kwargs)
+
+    apply_claude_code_bypass(basic_api_kwargs, "2.1.90")
+
+    assert len(basic_api_kwargs["system"]) == 2
+    assert basic_api_kwargs["system"][0]["text"].startswith(
+        "x-anthropic-billing-header: "
+    )
+    assert basic_api_kwargs["system"][1]["text"] == _SYSTEM_IDENTITY
+    assert basic_api_kwargs["messages"] == once["messages"]
+
+
+def test_apply_claude_code_bypass_normalizes_string_system(simple_messages):
+    api_kwargs = {
+        "system": "plain system",
+        "messages": [dict(message) for message in simple_messages],
+        "model": "claude-opus-4-6-20260101",
+    }
+
+    apply_claude_code_bypass(api_kwargs, "2.1.90")
+
+    assert isinstance(api_kwargs["system"], list)
+    assert api_kwargs["system"][1]["text"] == _SYSTEM_IDENTITY
+    assert (
+        "<system-reminder>\nplain system\n</system-reminder>"
+        in api_kwargs["messages"][0]["content"][0]["text"]
+    )
+
+
+def test_apply_claude_code_bypass_without_messages_is_noop():
+    api_kwargs = {"system": "plain system", "model": "claude-opus-4-6-20260101"}
+
+    apply_claude_code_bypass(api_kwargs, "2.1.90")
+
+    assert api_kwargs == {"system": "plain system", "model": "claude-opus-4-6-20260101"}
+
+
+def test_fix_temperature_for_oauth_adaptive_removes_non_default_temperature():
+    api_kwargs = {"model": "claude-opus-4-6-20260101", "temperature": 0.2}
+    _fix_temperature_for_oauth_adaptive(api_kwargs, site="test")
+    assert "temperature" not in api_kwargs
+
+
+def test_fix_temperature_for_oauth_adaptive_keeps_temperature_one():
+    api_kwargs = {"model": "claude-opus-4-6-20260101", "temperature": 1}
+    _fix_temperature_for_oauth_adaptive(api_kwargs, site="test")
+    assert api_kwargs["temperature"] == 1
+
+
+def test_fix_temperature_for_oauth_adaptive_keeps_temperature_for_other_models():
+    api_kwargs = {"model": "claude-3-7-sonnet", "temperature": 0.2}
+    _fix_temperature_for_oauth_adaptive(api_kwargs, site="test")
+    assert api_kwargs["temperature"] == 0.2
+
+
+def test_fix_temperature_for_oauth_adaptive_without_temperature_is_noop():
+    api_kwargs = {"model": "claude-opus-4-6-20260101"}
+    _fix_temperature_for_oauth_adaptive(api_kwargs, site="test")
+    assert api_kwargs == {"model": "claude-opus-4-6-20260101"}
diff --git a/tests/test_hook.py b/tests/test_hook.py
new file mode 100644
index 0000000..4b27a2b
--- /dev/null
+++ b/tests/test_hook.py
@@ -0,0 +1,101 @@
+# pyright: reportMissingImports=false, reportUnknownMemberType=false, reportUntypedFunctionDecorator=false, reportUnknownParameterType=false, reportMissingParameterType=false, reportUnusedParameter=false, reportArgumentType=false, reportCallIssue=false, reportUnknownVariableType=false, reportUnknownArgumentType=false, reportAttributeAccessIssue=false
+
+import importlib
+import importlib.machinery
+import importlib.util
+import sys
+import types
+
+import pytest
+
+
+def _clear_bypass_finders():
+    sys.meta_path[:] = [
+        finder
+        for finder in sys.meta_path
+        if finder.__class__.__name__ != "_ClaudeCodeBypassFinder"
+    ]
+
+
+@pytest.fixture
+def hook_module(monkeypatch):
+    _clear_bypass_finders()
+    module = importlib.import_module("sitecustomize_hook")
+    module = importlib.reload(module)
+    _clear_bypass_finders()
+    yield module
+    _clear_bypass_finders()
+
+
+def test_install_hook_registers_finder(hook_module):
+    hook_module._install_hook()
+
+    assert any(
+        finder.__class__.__name__ == "_ClaudeCodeBypassFinder"
+        for finder in sys.meta_path
+    )
+
+
+def test_finder_only_targets_agent_anthropic_adapter(hook_module, monkeypatch):
+    calls = []
+
+    class Loader:
+        def exec_module(self, module):
+            module.loaded = True
+
+    spec = importlib.machinery.ModuleSpec(hook_module._TARGET_MODULE, Loader())
+
+    def fake_find_spec(fullname):
+        calls.append(fullname)
+        return spec if fullname == hook_module._TARGET_MODULE else None
+
+    monkeypatch.setattr(importlib.util, "find_spec", fake_find_spec)
+    hook_module._install_hook()
+    finder = next(
+        finder
+        for finder in sys.meta_path
+        if finder.__class__.__name__ == "_ClaudeCodeBypassFinder"
+    )
+
+    assert finder.find_spec("some.other.module") is None
+    found_spec = finder.find_spec(hook_module._TARGET_MODULE)
+
+    assert found_spec is spec
+    assert calls == [hook_module._TARGET_MODULE]
+
+
+def test_finder_sets_patched_flag_and_stops_repatching(hook_module, monkeypatch):
+    applied = []
+
+    class Loader:
+        def exec_module(self, module):
+            module.loaded = True
+
+    spec = importlib.machinery.ModuleSpec(hook_module._TARGET_MODULE, Loader())
+
+    def fake_find_spec(fullname):
+        return spec if fullname == hook_module._TARGET_MODULE else None
+
+    bypass_module = types.SimpleNamespace(
+        apply_patches=lambda module: applied.append(module.__name__)
+    )
+
+    monkeypatch.setattr(importlib.util, "find_spec", fake_find_spec)
+    monkeypatch.setitem(sys.modules, "anthropic_billing_bypass", bypass_module)
+
+    hook_module._install_hook()
+    finder = next(
+        finder
+        for finder in sys.meta_path
+        if finder.__class__.__name__ == "_ClaudeCodeBypassFinder"
+    )
+
+    found_spec = finder.find_spec(hook_module._TARGET_MODULE)
+    assert found_spec is spec
+
+    module = types.ModuleType(hook_module._TARGET_MODULE)
+    found_spec.loader.exec_module(module)
+
+    assert finder._patched is True
+    assert applied == [hook_module._TARGET_MODULE]
+    assert finder.find_spec(hook_module._TARGET_MODULE) is None
diff --git a/tests/test_signing.py b/tests/test_signing.py
new file mode 100644
index 0000000..c953ea5
--- /dev/null
+++ b/tests/test_signing.py
@@ -0,0 +1,76 @@
+# pyright: reportPrivateUsage=false, reportUnknownParameterType=false, reportMissingParameterType=false, reportUnknownArgumentType=false, reportUnknownVariableType=false
+
+import hashlib
+
+from anthropic_billing_bypass import (
+    _build_billing_header_value,
+    _compute_cch,
+    _compute_version_suffix,
+    _extract_first_user_message_text,
+)
+
+
+def test_extract_first_user_message_text_with_string_content(simple_messages):
+    assert _extract_first_user_message_text(simple_messages) == "hello world"
+
+
+def test_extract_first_user_message_text_with_text_block(complex_messages):
+    assert _extract_first_user_message_text(complex_messages) == "hello world"
+
+
+def test_extract_first_user_message_text_with_image_block_only_returns_empty_string():
+    messages = [{"role": "user", "content": [{"type": "image", "source": {}}]}]
+    assert _extract_first_user_message_text(messages) == ""
+
+
+def test_extract_first_user_message_text_with_no_user_message_returns_empty_string():
+    messages = [{"role": "assistant", "content": "hello"}]
+    assert _extract_first_user_message_text(messages) == ""
+
+
+def test_extract_first_user_message_text_uses_first_user_message_only():
+    messages = [
+        {"role": "user", "content": "first"},
+        {"role": "user", "content": "second"},
+    ]
+    assert _extract_first_user_message_text(messages) == "first"
+
+
+def test_extract_first_user_message_text_with_empty_messages_returns_empty_string():
+    assert _extract_first_user_message_text([]) == ""
+
+
+def test_compute_cch_known_values():
+    assert _compute_cch("hello world") == "b94d2"
+    assert _compute_cch("") == "e3b0c"
+
+
+def test_compute_version_suffix_pads_short_text():
+    expected = hashlib.sha256(b"59cf53e54c78e002.1.90").hexdigest()[:3]
+    assert _compute_version_suffix("abcde", "2.1.90") == expected
+
+
+def test_compute_version_suffix_samples_long_enough_text():
+    text = "abcdefghijklmnopqrstuvwxyz"
+    sampled = f"{text[4]}{text[7]}{text[20]}"
+    expected = hashlib.sha256(
+        f"59cf53e54c78{sampled}2.1.90".encode("utf-8")
+    ).hexdigest()[:3]
+    assert _compute_version_suffix(text, "2.1.90") == expected
+
+
+def test_compute_version_suffix_known_value_for_hello_world():
+    expected = hashlib.sha256(b"59cf53e54c78oo02.1.90").hexdigest()[:3]
+    assert _compute_version_suffix("hello world", "2.1.90") == expected
+
+
+def test_build_billing_header_value_format(simple_messages):
+    version = "2.1.90"
+    entrypoint = "cli"
+    suffix = _compute_version_suffix("hello world", version)
+    cch = _compute_cch("hello world")
+
+    assert _build_billing_header_value(simple_messages, version, entrypoint) == (
+        f"x-anthropic-billing-header: cc_version={version}.{suffix}; "
+        f"cc_entrypoint={entrypoint}; cch={cch};"
+    )