diff --git a/.github/workflows/backend-ci.yml b/.github/workflows/backend-ci.yml
new file mode 100644
index 0000000..e662f2c
--- /dev/null
+++ b/.github/workflows/backend-ci.yml
@@ -0,0 +1,53 @@
+name: Backend CI
+
+on:
+  pull_request:
+    paths:
+      - "backend/**"
+      - ".github/workflows/backend-ci.yml"
+  push:
+    branches:
+      - main
+    paths:
+      - "backend/**"
+      - ".github/workflows/backend-ci.yml"
+
+permissions:
+  contents: read
+
+jobs:
+  test:
+    name: Python ${{ matrix.python-version }}
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        python-version:
+          - "3.11"
+          - "3.12"
+
+    defaults:
+      run:
+        working-directory: backend
+
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: ${{ matrix.python-version }}
+          cache: pip
+          cache-dependency-path: backend/requirements.txt
+
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install -r requirements.txt
+
+      - name: Compile Python sources
+        run: python -m compileall app tests
+
+      - name: Run backend regression tests
+        run: python -m unittest discover -s tests -v
diff --git a/backend/tests/test_main.py b/backend/tests/test_main.py
new file mode 100644
index 0000000..2ffeff7
--- /dev/null
+++ b/backend/tests/test_main.py
@@ -0,0 +1,126 @@
+import asyncio
+import os
+import tempfile
+import unittest
+from pathlib import Path
+from unittest.mock import patch
+
+from fastapi import HTTPException
+from pydantic import ValidationError
+
+from app.main import (
+    RunRequest,
+    apply_edits,
+    github_headers,
+    health,
+    parse_json_object,
+    safe_branch_name,
+    safe_repo_name,
+)
+
+
+class UtilitySafetyTests(unittest.TestCase):
+    def test_safe_repo_name_normalizes_git_url(self) -> None:
+        self.assertEqual(
+            safe_repo_name("https://github.com/example/project-name.git"),
+            "project-name",
+        )
+
+    def test_safe_branch_name_normalizes_unsafe_input(self) -> None:
+        self.assertEqual(
+            safe_branch_name("  feat//unsafe branch  "),
+            "feat/unsafe-branch",
+        )
+
+    def test_parse_json_object_accepts_fenced_json(self) -> None:
+        payload = parse_json_object('```json\n{"summary":"ok","plan":[]}\n```')
+        self.assertEqual(payload, {"summary": "ok", "plan": []})
+
+    def test_apply_edits_updates_exact_match(self) -> None:
+        with tempfile.TemporaryDirectory() as temp_dir:
+            repo = Path(temp_dir)
+            target = repo / "src" / "example.txt"
+            target.parent.mkdir(parents=True)
+            target.write_text("before\n", encoding="utf-8")
+
+            ok, message = apply_edits(
+                repo,
+                {
+                    "edits": [
+                        {
+                            "path": "src/example.txt",
+                            "old": "before",
+                            "new": "after",
+                        }
+                    ]
+                },
+            )
+
+            self.assertTrue(ok, message)
+            self.assertEqual(target.read_text(encoding="utf-8"), "after\n")
+
+    def test_apply_edits_rejects_path_escape(self) -> None:
+        with tempfile.TemporaryDirectory() as temp_dir:
+            repo = Path(temp_dir)
+            ok, message = apply_edits(
+                repo,
+                {
+                    "edits": [
+                        {
+                            "path": "../escape.txt",
+                            "old": "before",
+                            "new": "after",
+                        }
+                    ]
+                },
+            )
+
+            self.assertFalse(ok)
+            self.assertIn("escapes repository", message)
+
+    def test_apply_edits_is_atomic_when_later_edit_is_invalid(self) -> None:
+        with tempfile.TemporaryDirectory() as temp_dir:
+            repo = Path(temp_dir)
+            first = repo / "first.txt"
+            second = repo / "second.txt"
+            first.write_text("alpha", encoding="utf-8")
+            second.write_text("beta", encoding="utf-8")
+
+            ok, _ = apply_edits(
+                repo,
+                {
+                    "edits": [
+                        {"path": "first.txt", "old": "alpha", "new": "changed"},
+                        {"path": "second.txt", "old": "missing", "new": "changed"},
+                    ]
+                },
+            )
+
+            self.assertFalse(ok)
+            self.assertEqual(first.read_text(encoding="utf-8"), "alpha")
+            self.assertEqual(second.read_text(encoding="utf-8"), "beta")
+
+    def test_github_headers_requires_token(self) -> None:
+        with patch.dict(os.environ, {}, clear=True):
+            with self.assertRaises(HTTPException) as context:
+                github_headers()
+
+        self.assertEqual(context.exception.status_code, 400)
+
+    def test_run_request_rejects_invalid_iteration_count(self) -> None:
+        with self.assertRaises(ValidationError):
+            RunRequest(
+                repo_url="https://example.test/repo.git",
+                task="fix",
+                max_iterations=0,
+            )
+
+    def test_health_contract(self) -> None:
+        self.assertEqual(
+            asyncio.run(health()),
+            {"status": "ok", "service": "PatchPilot", "version": "0.4.0"},
+        )
+
+
+if __name__ == "__main__":
+    unittest.main()