diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 0000000..a728edf
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,23 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  test:
+    name: test
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up pixi
+        uses: prefix-dev/setup-pixi@v0.8.8
+        with:
+          manifest-path: pixi.toml
+          locked: true
+
+      - name: Run tests
+        run: pixi run test
diff --git a/conftest.py b/conftest.py
new file mode 100644
index 0000000..7b4f0b9
--- /dev/null
+++ b/conftest.py
@@ -0,0 +1,84 @@
+"""Shared fixtures: fake Venmo client/transaction objects so tests never touch the network."""
+
+from types import SimpleNamespace
+
+import pytest
+
+
+def make_user(user_id, username="user", display_name="User"):
+    return SimpleNamespace(id=user_id, username=username, display_name=display_name)
+
+
+def make_txn(
+    *,
+    txn_id="1",
+    payment_type="pay",
+    amount=10.0,
+    note="note",
+    status="settled",
+    audience="private",
+    date_created="1780887063",
+    actor=None,
+    target=None,
+):
+    return SimpleNamespace(
+        id=txn_id,
+        payment_type=payment_type,
+        amount=amount,
+        note=note,
+        status=status,
+        audience=audience,
+        date_created=date_created,
+        actor=actor,
+        target=target,
+    )
+
+
+class FakeUserApi:
+    """Records which read method was called and returns a canned page (a plain list)."""
+
+    def __init__(self, txns):
+        self._txns = txns
+        self.calls = []
+
+    def get_user_transactions(self, user_id=None, limit=50, before_id=None):
+        self.calls.append(
+            {"method": "get_user_transactions", "user_id": user_id, "limit": limit, "before_id": before_id}
+        )
+        return list(self._txns)
+
+    def get_transaction_between_two_users(
+        self, user_id_one=None, user_id_two=None, limit=50, before_id=None
+    ):
+        self.calls.append(
+            {
+                "method": "get_transaction_between_two_users",
+                "user_id_one": user_id_one,
+                "user_id_two": user_id_two,
+                "limit": limit,
+                "before_id": before_id,
+            }
+        )
+        return list(self._txns)
+
+
+class FakeClient:
+    def __init__(self, txns, my_id):
+        self.user = FakeUserApi(txns)
+        self._my_id = my_id
+
+    def my_profile(self):
+        return SimpleNamespace(id=self._my_id)
+
+
+@pytest.fixture
+def fake_client_factory(monkeypatch):
+    """Patch server.get_client to return a FakeClient; hand the client back for assertions."""
+    import server
+
+    def _factory(txns, my_id="me"):
+        client = FakeClient(txns, my_id)
+        monkeypatch.setattr(server, "get_client", lambda: client)
+        return client
+
+    return _factory
diff --git a/pixi.lock b/pixi.lock
index e0b4047..30ff236 100644
--- a/pixi.lock
+++ b/pixi.lock
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       - pypi: https://files.pythonhosted.org/packages/78/b6/6307fbef88d9b5ee7421e68d78a9f162e0da4900bc5f5793f6d3d0e34fb8/annotated_types-0.7.0-py3-none-any.whl
@@ -222,7 +255,6 @@ environments:
       - pypi: https://files.pythonhosted.org/packages/2b/60/19f7884db5d5603edf3c6bce35408f45ad3e97e10007df0e17dd57af18f8/rpds_py-0.30.0-cp314-cp314-macosx_11_0_arm64.whl
       - pypi: https://files.pythonhosted.org/packages/f8/7f/3de5402f39890ac5660b86bcf5c03f9d855dad5c4ed764866d7b592b46fd/sse_starlette-3.3.4-py3-none-any.whl
       - pypi: https://files.pythonhosted.org/packages/0b/c9/584bc9651441b4ba60cc4d557d8a547b5aff901af35bda3a4ee30c819b82/starlette-1.0.0-py3-none-any.whl
-      - pypi: https://files.pythonhosted.org/packages/18/67/36e9267722cc04a6b9f15c7f3441c2363321a3ea07da7ae0c0707beb2a9c/typing_extensions-4.15.0-py3-none-any.whl
       - pypi: https://files.pythonhosted.org/packages/dc/9b/47798a6c91d8bdb567fe2698fe81e0c6b7cb7ef4d13da4114b41d239f65d/typing_inspection-0.4.2-py3-none-any.whl
       - pypi: https://files.pythonhosted.org/packages/39/08/aaaad47bc4e9dc8c725e68f9d04865dbcb2052843ff09c97b08904852d84/urllib3-2.6.3-py3-none-any.whl
       - pypi: https://files.pythonhosted.org/packages/55/df/0cf5b0c451602748fdc7a702d4667f6e209bf96aa6e3160d754234445f2a/uvicorn-0.43.0-py3-none-any.whl
@@ -382,6 +414,17 @@ packages:
   requires_dist:
   - colorama ; sys_platform == 'win32'
   requires_python: '>=3.10'
+- conda: https://conda.anaconda.org/conda-forge/noarch/colorama-0.4.6-pyhd8ed1ab_1.conda
+  sha256: ab29d57dc70786c1269633ba3dff20288b81664d3ff8d21af995742e2bb03287
+  md5: 962b9857ee8e7018c22f2776ffa0b2d7
+  depends:
+  - python >=3.9
+  license: BSD-3-Clause
+  license_family: BSD
+  purls:
+  - pkg:pypi/colorama?source=hash-mapping
+  size: 27011
+  timestamp: 1733218222191
 - pypi: https://files.pythonhosted.org/packages/34/71/1ea5a7352ae516d5512d17babe7e1b87d9db5150b21f794b1377eac1edc0/cryptography-46.0.6-cp311-abi3-manylinux_2_28_x86_64.whl
   name: cryptography
   version: 46.0.6
@@ -472,6 +515,17 @@ packages:
   - check-sdist ; extra == 'pep8test'
   - click>=8.0.1 ; extra == 'pep8test'
   requires_python: '>=3.8,!=3.9.0,!=3.9.1'
+- conda: https://conda.anaconda.org/conda-forge/noarch/exceptiongroup-1.3.1-pyhd8ed1ab_0.conda
+  sha256: ee6cf346d017d954255bbcbdb424cddea4d14e4ed7e9813e429db1d795d01144
+  md5: 8e662bd460bda79b1ea39194e3c4c9ab
+  depends:
+  - python >=3.10
+  - typing_extensions >=4.6.0
+  license: MIT and PSF-2.0
+  purls:
+  - pkg:pypi/exceptiongroup?source=hash-mapping
+  size: 21333
+  timestamp: 1763918099466
 - pypi: https://files.pythonhosted.org/packages/04/4b/29cac41a4d98d144bf5f6d33995617b185d14b22401f75ca86f384e87ff1/h11-0.16.0-py3-none-any.whl
   name: h11
   version: 0.16.0
@@ -555,6 +609,17 @@ packages:
   - pytest>=8.3.2 ; extra == 'all'
   - flake8>=7.1.1 ; extra == 'all'
   requires_python: '>=3.8'
+- conda: https://conda.anaconda.org/conda-forge/noarch/iniconfig-2.3.0-pyhd8ed1ab_0.conda
+  sha256: e1a9e3b1c8fe62dc3932a616c284b5d8cbe3124bbfbedcf4ce5c828cb166ee19
+  md5: 9614359868482abba1bd15ce465e3c42
+  depends:
+  - python >=3.10
+  license: MIT
+  license_family: MIT
+  purls:
+  - pkg:pypi/iniconfig?source=hash-mapping
+  size: 13387
+  timestamp: 1760831448842
 - pypi: https://files.pythonhosted.org/packages/69/90/f63fb5873511e014207a475e2bb4e8b2e570d655b00ac19a9a0ca0a385ee/jsonschema-4.26.0-py3-none-any.whl
   name: jsonschema
   version: 4.26.0
@@ -1077,6 +1142,18 @@ packages:
   purls: []
   size: 3104268
   timestamp: 1769556384749
+- conda: https://conda.anaconda.org/conda-forge/noarch/packaging-26.2-pyhc364b38_0.conda
+  sha256: 3906abfb6511a3bb309e39b9b1b7bc38f50a723971de2395489fd1f379255890
+  md5: 4c06a92e74452cfa53623a81592e8934
+  depends:
+  - python >=3.8
+  - python
+  license: Apache-2.0
+  license_family: APACHE
+  purls:
+  - pkg:pypi/packaging?source=hash-mapping
+  size: 91574
+  timestamp: 1777103621679
 - conda: https://conda.anaconda.org/conda-forge/noarch/pip-26.0.1-pyh145f28c_0.conda
   sha256: 5f66ea31d62188c266c5a8752119b0cc90a5bf05963f665cf48a33e0ec58d39c
   md5: 09a970fbf75e8ed1aa633827ded6aa4f
@@ -1088,6 +1165,18 @@ packages:
   - pkg:pypi/pip?source=compressed-mapping
   size: 1180743
   timestamp: 1770270312477
+- conda: https://conda.anaconda.org/conda-forge/noarch/pluggy-1.6.0-pyhf9edf01_1.conda
+  sha256: e14aafa63efa0528ca99ba568eaf506eb55a0371d12e6250aaaa61718d2eb62e
+  md5: d7585b6550ad04c8c5e21097ada2888e
+  depends:
+  - python >=3.9
+  - python
+  license: MIT
+  license_family: MIT
+  purls:
+  - pkg:pypi/pluggy?source=hash-mapping
+  size: 25877
+  timestamp: 1764896838868
 - pypi: https://files.pythonhosted.org/packages/0c/c3/44f3fbbfa403ea2a7c779186dc20772604442dde72947e7d01069cbe98e3/pycparser-3.0-py3-none-any.whl
   name: pycparser
   version: '3.0'
@@ -1149,6 +1238,17 @@ packages:
   - tomli>=2.0.1 ; extra == 'toml'
   - pyyaml>=6.0.1 ; extra == 'yaml'
   requires_python: '>=3.10'
+- conda: https://conda.anaconda.org/conda-forge/noarch/pygments-2.20.0-pyhd8ed1ab_0.conda
+  sha256: cf70b2f5ad9ae472b71235e5c8a736c9316df3705746de419b59d442e8348e86
+  md5: 16c18772b340887160c79a6acc022db0
+  depends:
+  - python >=3.10
+  license: BSD-2-Clause
+  license_family: BSD
+  purls:
+  - pkg:pypi/pygments?source=hash-mapping
+  size: 893031
+  timestamp: 1774796815820
 - pypi: https://files.pythonhosted.org/packages/e5/7a/8dd906bd22e79e47397a61742927f6747fe93242ef86645ee9092e610244/pyjwt-2.12.1-py3-none-any.whl
   name: pyjwt
   version: 2.12.1
@@ -1169,6 +1269,27 @@ packages:
   - coverage[toml]==7.10.7 ; extra == 'tests'
   - pytest>=8.4.2,<9.0.0 ; extra == 'tests'
   requires_python: '>=3.9'
+- conda: https://conda.anaconda.org/conda-forge/noarch/pytest-9.0.3-pyhc364b38_1.conda
+  sha256: 960f59442173eee0731906a9077bd5ccf60f4b4226f05a22d1728ab9a21a879c
+  md5: 6a991452eadf2771952f39d43615bb3e
+  depends:
+  - colorama >=0.4
+  - pygments >=2.7.2
+  - python >=3.10
+  - iniconfig >=1.0.1
+  - packaging >=22
+  - pluggy >=1.5,<2
+  - tomli >=1
+  - exceptiongroup >=1
+  - python
+  constrains:
+  - pytest-faulthandler >=2
+  license: MIT
+  license_family: MIT
+  purls:
+  - pkg:pypi/pytest?source=hash-mapping
+  size: 299984
+  timestamp: 1775644472530
 - conda: https://conda.anaconda.org/conda-forge/linux-64/python-3.14.3-h32b2ec7_101_cp314.conda
   build_number: 101
   sha256: cb0628c5f1732f889f53a877484da98f5a0e0f47326622671396fb4f2b0cd6bd
@@ -1460,11 +1581,18 @@ packages:
   purls: []
   size: 3127137
   timestamp: 1769460817696
-- pypi: https://files.pythonhosted.org/packages/18/67/36e9267722cc04a6b9f15c7f3441c2363321a3ea07da7ae0c0707beb2a9c/typing_extensions-4.15.0-py3-none-any.whl
-  name: typing-extensions
-  version: 4.15.0
-  sha256: f0fa19c6845758ab08074a0cfa8b7aecb71c999ca73d62883bc25cc018c4e548
-  requires_python: '>=3.9'
+- conda: https://conda.anaconda.org/conda-forge/noarch/tomli-2.4.1-pyhcf101f3_0.conda
+  sha256: 91cafdb64268e43e0e10d30bd1bef5af392e69f00edd34dfaf909f69ab2da6bd
+  md5: b5325cf06a000c5b14970462ff5e4d58
+  depends:
+  - python >=3.10
+  - python
+  license: MIT
+  license_family: MIT
+  purls:
+  - pkg:pypi/tomli?source=hash-mapping
+  size: 21561
+  timestamp: 1774492402955
 - pypi: https://files.pythonhosted.org/packages/dc/9b/47798a6c91d8bdb567fe2698fe81e0c6b7cb7ef4d13da4114b41d239f65d/typing_inspection-0.4.2-py3-none-any.whl
   name: typing-inspection
   version: 0.4.2
@@ -1472,6 +1600,18 @@ packages:
   requires_dist:
   - typing-extensions>=4.12.0
   requires_python: '>=3.9'
+- conda: https://conda.anaconda.org/conda-forge/noarch/typing_extensions-4.15.0-pyhcf101f3_0.conda
+  sha256: 032271135bca55aeb156cee361c81350c6f3fb203f57d024d7e5a1fc9ef18731
+  md5: 0caa1af407ecff61170c9437a808404d
+  depends:
+  - python >=3.10
+  - python
+  license: PSF-2.0
+  license_family: PSF
+  purls:
+  - pkg:pypi/typing-extensions?source=hash-mapping
+  size: 51692
+  timestamp: 1756220668932
 - conda: https://conda.anaconda.org/conda-forge/noarch/tzdata-2025c-hc9c84f9_1.conda
   sha256: 1d30098909076af33a35017eed6f2953af1c769e273a0626a04722ac4acaba3c
   md5: ad659d0a2b3e47e38d829aa8cad2d610
diff --git a/pixi.toml b/pixi.toml
index 563afc4..34605be 100644
--- a/pixi.toml
+++ b/pixi.toml
@@ -8,6 +8,7 @@ platforms = ["osx-arm64", "osx-64", "linux-64", "linux-aarch64"]
 [dependencies]
 python = "==3.14.3"
 pip = "==26.0.1"
+pytest = ">=9.0.3,<10"
 
 [pypi-dependencies]
 venmo-api = "==0.3.1"
@@ -16,3 +17,4 @@ mcp = "==1.26.0"
 [tasks]
 serve = "python server.py"
 auth = "python auth.py"
+test = "pytest -q"
diff --git a/tests/test_get_transactions.py b/tests/test_get_transactions.py
new file mode 100644
index 0000000..ecdac6a
--- /dev/null
+++ b/tests/test_get_transactions.py
@@ -0,0 +1,73 @@
+"""Tests for the get_transactions tool: routing and filtering, with a faked client."""
+
+import json
+
+import server
+from conftest import make_txn, make_user
+
+
+def _txns():
+    return [
+        make_txn(
+            txn_id="1",
+            note="May 29 cleaning",
+            actor=make_user("me", "andy", "Andy"),
+            target=make_user("maria", "mariaandcrew", "Maria Perez"),
+        ),
+        make_txn(
+            txn_id="2",
+            note="Outlet, light",
+            actor=make_user("me", "andy", "Andy"),
+            target=make_user("jose", "jose", "Jose"),
+        ),
+    ]
+
+
+def test_default_uses_user_transactions(fake_client_factory):
+    client = fake_client_factory(_txns(), my_id="me")
+    out = json.loads(server.get_transactions())
+    assert isinstance(out, list) and len(out) == 2
+    methods = [c["method"] for c in client.user.calls]
+    assert methods == ["get_user_transactions"]
+
+
+def test_with_user_id_routes_to_between_two_users(fake_client_factory):
+    client = fake_client_factory(_txns(), my_id="me")
+    server.get_transactions(with_user_id="maria")
+    call = client.user.calls[0]
+    assert call["method"] == "get_transaction_between_two_users"
+    assert call["user_id_one"] == "me"
+    assert call["user_id_two"] == "maria"
+
+
+def test_note_contains_filters_case_insensitively(fake_client_factory):
+    fake_client_factory(_txns(), my_id="me")
+    out = json.loads(server.get_transactions(note_contains="CLEANING"))
+    assert len(out) == 1
+    assert out[0]["note"] == "May 29 cleaning"
+
+
+def test_note_contains_no_match_returns_empty(fake_client_factory):
+    fake_client_factory(_txns(), my_id="me")
+    out = json.loads(server.get_transactions(note_contains="zzz"))
+    assert out == []
+
+
+def test_limit_and_before_id_are_passed_through(fake_client_factory):
+    client = fake_client_factory(_txns(), my_id="me")
+    server.get_transactions(limit=5, before_id="abc")
+    call = client.user.calls[0]
+    assert call["limit"] == 5
+    assert call["before_id"] == "abc"
+
+
+def test_empty_before_id_becomes_none(fake_client_factory):
+    client = fake_client_factory(_txns(), my_id="me")
+    server.get_transactions()  # before_id defaults to ""
+    assert client.user.calls[0]["before_id"] is None
+
+
+def test_direction_reflects_authenticated_user(fake_client_factory):
+    fake_client_factory(_txns(), my_id="me")
+    out = json.loads(server.get_transactions())
+    assert all(t["direction"] == "sent" for t in out)
diff --git a/tests/test_helpers.py b/tests/test_helpers.py
new file mode 100644
index 0000000..cfd3f04
--- /dev/null
+++ b/tests/test_helpers.py
@@ -0,0 +1,69 @@
+"""Unit tests for the pure serialization helpers in server.py (no network, no money)."""
+
+import server
+from conftest import make_txn, make_user
+
+
+class TestEpochToIso:
+    def test_known_epoch_formats_as_utc(self):
+        # 1780887063 -> 2026-06-08 02:51:03 UTC
+        assert server._epoch_to_iso("1780887063") == "2026-06-08 02:51:03Z"
+
+    def test_accepts_int(self):
+        assert server._epoch_to_iso(1780887063) == "2026-06-08 02:51:03Z"
+
+    def test_none_and_empty_return_none(self):
+        assert server._epoch_to_iso(None) is None
+        assert server._epoch_to_iso("") is None
+
+    def test_garbage_falls_back_to_str(self):
+        assert server._epoch_to_iso("not-a-number") == "not-a-number"
+
+
+class TestUserBrief:
+    def test_none_user(self):
+        assert server._user_brief(None) is None
+
+    def test_brief_fields(self):
+        u = make_user("42", "alice", "Alice A")
+        assert server._user_brief(u) == {"id": "42", "username": "alice", "display_name": "Alice A"}
+
+
+class TestSerializeTxnDirection:
+    def test_pay_as_actor_is_sent(self):
+        t = make_txn(payment_type="pay", actor=make_user("me"), target=make_user("them"))
+        assert server._serialize_txn(t, "me")["direction"] == "sent"
+
+    def test_pay_as_target_is_received(self):
+        t = make_txn(payment_type="pay", actor=make_user("them"), target=make_user("me"))
+        assert server._serialize_txn(t, "me")["direction"] == "received"
+
+    def test_charge_as_actor_is_you_requested(self):
+        t = make_txn(payment_type="charge", actor=make_user("me"), target=make_user("them"))
+        assert server._serialize_txn(t, "me")["direction"] == "you_requested"
+
+    def test_charge_as_target_is_requested_from_you(self):
+        t = make_txn(payment_type="charge", actor=make_user("them"), target=make_user("me"))
+        assert server._serialize_txn(t, "me")["direction"] == "requested_from_you"
+
+
+class TestSerializeTxnShape:
+    def test_flattens_all_fields(self):
+        t = make_txn(
+            txn_id="99",
+            amount=365.0,
+            note="May 29 cleaning",
+            status="settled",
+            audience="private",
+            actor=make_user("me", "andy", "Andy"),
+            target=make_user("maria", "mariaandcrew", "Maria Perez"),
+        )
+        out = server._serialize_txn(t, "me")
+        assert out["id"] == "99"
+        assert out["amount"] == 365.0
+        assert out["note"] == "May 29 cleaning"
+        assert out["status"] == "settled"
+        assert out["audience"] == "private"
+        assert out["actor"]["username"] == "andy"
+        assert out["target"]["username"] == "mariaandcrew"
+        assert out["date"] == "2026-06-08 02:51:03Z"