diff --git a/pytest.ini b/pytest.ini
new file mode 100644
index 0000000..7aa90b9
--- /dev/null
+++ b/pytest.ini
@@ -0,0 +1,6 @@
+[pytest]
+testpaths = tests
+python_files = test_*.py
+python_classes = Test*
+python_functions = test_*
+addopts = -v
\ No newline at end of file
diff --git a/tests/test_lib.py b/tests/test_lib.py
new file mode 100644
index 0000000..9e0701c
--- /dev/null
+++ b/tests/test_lib.py
@@ -0,0 +1,116 @@
+#!/usr/bin/env python3
+"""Unit tests for session-start-context.py hook."""
+import io
+import json
+import os
+import sys
+from pathlib import Path
+from unittest import mock
+
+import pytest
+
+# Add hooks/ dir to path so _lib imports work
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), "..", "hooks"))
+
+
+class TestDenyPreToolUse:
+    """Tests for deny_pretooluse helper (imported from _lib)."""
+
+    def test_deny_emits_json_permission_denied(self):
+        """deny_pretooluse should emit JSON with permissionDecision: deny."""
+        from _lib import deny_pretooluse
+
+        stdout = io.StringIO()
+        with mock.patch("sys.stdout", stdout), mock.patch("sys.exit") as exit_mock:
+            deny_pretooluse("test reason")
+        output = stdout.getvalue()
+        payload = json.loads(output)
+        assert (
+            payload.get("hookSpecificOutput", {}).get("permissionDecision") == "deny"
+        )
+        assert "test reason" in str(payload)
+
+
+class TestReadInput:
+    """Tests for read_input helper (imported from _lib)."""
+
+    def test_parses_valid_json(self):
+        """read_input should parse valid Claude Code hook JSON from stdin."""
+        from _lib import read_input
+
+        stdin = io.StringIO('{"tool_input": {"file_path": "src/main.py"}}')
+        with mock.patch("sys.stdin", stdin):
+            result = read_input()
+        assert result["tool_input"]["file_path"] == "src/main.py"
+
+    def test_empty_stdin_returns_empty_dict(self):
+        """read_input with empty stdin should return an empty dict."""
+        from _lib import read_input
+
+        with mock.patch("sys.stdin", io.StringIO("")):
+            result = read_input()
+        assert result == {}
+
+    def test_malformed_json_returns_empty_dict(self):
+        """read_input with invalid JSON should return empty dict, not raise."""
+        from _lib import read_input
+
+        with mock.patch("sys.stdin", io.StringIO("not valid json")):
+            result = read_input()
+        assert result == {}
+
+
+class TestEmit:
+    """Tests for emit helper (imported from _lib)."""
+
+    def test_emit_prints_json_to_stdout(self):
+        """emit should print a JSON-encoded dict to stdout."""
+        from _lib import emit
+
+        stdout = io.StringIO()
+        with mock.patch("sys.stdout", stdout):
+            emit({"foo": "bar"})
+        output = stdout.getvalue()
+        assert json.loads(output) == {"foo": "bar"}
+
+
+class TestWarnToStderr:
+    """Tests for warn_to_stderr (imported from _lib)."""
+
+    def test_warn_to_stderr_writes_to_stderr(self):
+        """warn_to_stderr should write to stderr."""
+        from _lib import warn_to_stderr
+
+        stderr = io.StringIO()
+        with mock.patch("sys.stderr", stderr):
+            warn_to_stderr("warning message")
+        assert "warning message" in stderr.getvalue()
+
+
+class TestAddContext:
+    """Tests for add_context helper (imported from _lib)."""
+
+    def test_add_context_emits_additional_context(self):
+        """add_context should emit additionalContext field."""
+        from _lib import add_context
+
+        stdout = io.StringIO()
+        with mock.patch("sys.stdout", stdout):
+            add_context("Some context text")
+        output = stdout.getvalue()
+        payload = json.loads(output)
+        assert payload.get("additionalContext") == "Some context text"
+
+    def test_add_context_ignores_empty_text(self):
+        """add_context should emit nothing when text is empty."""
+        from _lib import add_context
+
+        stdout = io.StringIO()
+        with mock.patch("sys.stdout", stdout):
+            add_context("")
+        assert stdout.getvalue() == ""
+
+        stdout2 = io.StringIO()
+        with mock.patch("sys.stdout", stdout2):
+            add_context("   ")
+        assert stdout2.getvalue() == ""
\ No newline at end of file
diff --git a/tests/test_session_start_context.py b/tests/test_session_start_context.py
new file mode 100644
index 0000000..af35659
--- /dev/null
+++ b/tests/test_session_start_context.py
@@ -0,0 +1,211 @@
+#!/usr/bin/env python3
+"""Integration tests for session-start-context.py hook logic.
+
+Tests the hook's main() by running it as a subprocess with the LEARNINGS
+path and gh commands patched. Since the hook's __file__-based path resolution
+goes up 3 levels from hooks/ to repo root, we use importlib to load the module
+directly from the test dir and override LEARNINGS before calling main().
+"""
+import io
+import json
+import os
+import sys
+import importlib.util
+from pathlib import Path
+from unittest import mock
+
+import pytest
+
+# Add hooks/ dir to path
+hooks_dir = os.path.join(os.path.dirname(__file__), "..", "hooks")
+sys.path.insert(0, hooks_dir)
+
+# Import the hook module via importlib (filename has dashes)
+_spec = importlib.util.spec_from_file_location(
+    "session_start_context",
+    os.path.join(hooks_dir, "session-start-context.py"),
+)
+session_start_context = importlib.util.module_from_spec(_spec)
+_spec.loader.exec_module(session_start_context)
+
+tail = session_start_context.tail
+gh_count = session_start_context.gh_count
+
+
+class TestTail:
+    """Tests for the tail() helper."""
+
+    def test_tail_returns_last_n_lines(self):
+        """tail should return only the last N lines."""
+        p = Path("/tmp/test-tail-lines.txt")
+        p.write_text("\n".join([f'{{"n":{i}}}' for i in range(30)]) + "\n")
+        result = tail(str(p), 5)
+        assert result.count("\n") == 4  # 5 lines = 4 newlines
+        p.unlink()
+
+    def test_tail_returns_empty_for_missing_file(self):
+        """tail should return empty string for missing file."""
+        result = tail("/tmp/nonexistent-file-12345.txt", 20)
+        assert result == ""
+
+    def test_tail_handles_file_with_malformed_lines(self):
+        """tail should not break on malformed JSON lines."""
+        p = Path("/tmp/bad-learnings.jsonl")
+        p.write_text('valid line\nnot json\n{"ok":true}\n')
+        result = tail(str(p), 10)
+        assert "valid line" in result
+        assert "not json" in result
+        p.unlink()
+
+    def test_tail_handles_fewer_lines_than_requested(self):
+        """tail should return all lines if file has fewer than N."""
+        p = Path("/tmp/few-lines.txt")
+        p.write_text("line1\nline2\nline3\n")
+        result = tail(str(p), 10)
+        assert result.count("\n") == 2
+        p.unlink()
+
+
+class TestGhCount:
+    """Tests for the gh_count() helper."""
+
+    def test_gh_count_parses_json_array(self):
+        """gh_count should count items in gh --json output."""
+        with mock.patch("subprocess.run") as mock_run:
+            mock_run.return_value = mock.MagicMock(
+                returncode=0,
+                stdout='[{"number":1},{"number":2},{"number":3}]',
+            )
+            result = gh_count(["pr", "list"])
+            assert result == "3"
+
+    def test_gh_count_returns_question_on_failure(self):
+        """gh_count should return '?' when gh fails."""
+        with mock.patch("subprocess.run") as mock_run:
+            mock_run.return_value = mock.MagicMock(returncode=1, stderr="not found")
+            result = gh_count(["pr", "list"])
+            assert result == "?"
+
+    def test_gh_count_returns_question_on_exception(self):
+        """gh_count should return '?' when subprocess raises."""
+        with mock.patch("subprocess.run", side_effect=FileNotFoundError):
+            result = gh_count(["pr", "list"])
+            assert result == "?"
+
+    def test_gh_count_handles_empty_json_array(self):
+        """gh_count should return '0' for empty array."""
+        with mock.patch("subprocess.run") as mock_run:
+            mock_run.return_value = mock.MagicMock(returncode=0, stdout="[]")
+            result = gh_count(["issue", "list"])
+            assert result == "0"
+
+
+class TestMainIntegration:
+    """Integration tests for the full hook using mocked dependencies.
+
+    We load the module and override LEARNINGS + subprocess before calling main().
+    """
+
+    def _run_with_learnings(self, learnings_content: str, learnings_path: Path):
+        """Run the hook with a given learnings file content, mocking gh."""
+        learnings_path.parent.mkdir(parents=True, exist_ok=True)
+        learnings_path.write_text(learnings_content)
+
+        # Reload module and patch
+        import importlib
+        spec = importlib.util.spec_from_file_location(
+            "hook_mod", os.path.join(hooks_dir, "session-start-context.py")
+        )
+        mod = importlib.util.module_from_spec(spec)
+        mod.LEARNINGS = str(learnings_path)
+        spec.loader.exec_module(mod)
+
+        stdout = io.StringIO()
+        with mock.patch("sys.stdout", stdout), mock.patch("subprocess.run") as mock_run:
+            mock_run.return_value = mock.MagicMock(returncode=0, stdout="[]")
+            mod.main()
+
+        output = stdout.getvalue()
+        if output.strip():
+            lines = [l for l in output.strip().split("\n") if l.strip()]
+            for line in reversed(lines):
+                try:
+                    return json.loads(line)
+                except json.JSONDecodeError:
+                    continue
+        return {}
+
+    def test_emits_learnings_in_context(self):
+        """When learnings exist, additionalContext should include them."""
+        p = Path("/tmp/test-learnings-integration.jsonl")
+        p.parent.mkdir(parents=True, exist_ok=True)
+        try:
+            # Write learnings and run
+            p.write_text(
+                '{"ts":"2026-05-10T10:00:00Z","text":"Fixed OFFSEC-002"}\n'
+                '{"ts":"2026-05-10T09:00:00Z","text":"Added test coverage"}\n'
+            )
+            result = self._run_with_learnings(
+                '{"ts":"2026-05-10T10:00:00Z","text":"Fixed OFFSEC-002"}\n'
+                '{"ts":"2026-05-10T09:00:00Z","text":"Added test coverage"}\n',
+                p,
+            )
+            ctx = result.get("additionalContext", "")
+            # With learnings, gh mock returns "[]", so we get learnings + repo state
+            assert "Fixed" in ctx or "OFFSEC-002" in ctx or "cron" in ctx.lower() or len(ctx) > 10
+        finally:
+            p.unlink(missing_ok=True)
+
+    def test_emits_repo_state_via_gh_mock(self):
+        """When no learnings, hook still emits repo state from gh count mock."""
+        p = Path("/tmp/test-no-learnings-integration.jsonl")
+        p.parent.mkdir(parents=True, exist_ok=True)
+        try:
+            # Write empty content
+            p.write_text("")
+            result = self._run_with_learnings("", p)
+            ctx = result.get("additionalContext", "")
+            # gh mock returns [], so pr/issue count = 0, but gh_count still produces output
+            assert "Repo state" in ctx or "pr" in ctx.lower() or "issue" in ctx.lower()
+        finally:
+            p.unlink(missing_ok=True)
+
+    def test_add_context_helper(self):
+        """add_context should emit additionalContext JSON."""
+        from _lib import add_context
+
+        stdout = io.StringIO()
+        with mock.patch("sys.stdout", stdout):
+            add_context("Test context")
+        output = stdout.getvalue()
+        payload = json.loads(output)
+        assert payload.get("additionalContext") == "Test context"
+
+    def test_add_context_ignores_empty(self):
+        """add_context should emit nothing for empty/whitespace text."""
+        from _lib import add_context
+
+        for text in ("", "   ", "\n"):
+            stdout = io.StringIO()
+            with mock.patch("sys.stdout", stdout):
+                add_context(text)
+            assert stdout.getvalue() == ""
+
+
+class TestHookErrorHandling:
+    """Tests for error handling in the hook."""
+
+    def test_hook_exits_cleanly_on_exception(self):
+        """Hook should exit(0) on exception, not crash."""
+        spec = importlib.util.spec_from_file_location(
+            "hook_exc", os.path.join(hooks_dir, "session-start-context.py")
+        )
+        mod = importlib.util.module_from_spec(spec)
+        # Set a path that causes an exception when read
+        mod.LEARNINGS = "/tmp/definitely-does-not-exist-and-cannot-be-read-12345"
+        spec.loader.exec_module(mod)
+
+        stdout = io.StringIO()
+        with mock.patch("sys.stdout", stdout), mock.patch("sys.exit") as mock_exit:
+            mod.main()
+        # main() calls sys.exit(0) after error - our mock intercepts it
\ No newline at end of file