diff --git a/.gitignore b/.gitignore
index 6fbe4b4..c2324cd 100644
--- a/.gitignore
+++ b/.gitignore
@@ -23,3 +23,4 @@
 # Python bytecode
 __pycache__/
 *.py[cod]
+.pytest_cache/
diff --git a/hooks/_lib.py b/hooks/_lib.py
index 1d0555a..e8fa9a2 100755
--- a/hooks/_lib.py
+++ b/hooks/_lib.py
@@ -4,9 +4,47 @@ Hooks receive JSON on stdin per the Claude Code hook spec, and may emit
 JSON on stdout or exit with code 2 to block. This module wraps both.
 """
 import json
+import os
 import sys
 
 
+def get_repo_root(hook_file: str) -> str:
+    """Return the repo root given a hook's __file__ path.
+
+    The plugin is installed at <repo>/hooks/<hook>.py. We walk up three levels
+    from __file__ to find the workspace root: hooks/ → repo/ → workspace/.
+    When __file__ is absolute (Claude Code invokes hooks with absolute paths),
+    the naive dirname chain overshoots by one level.
+
+    Distinguishes the production layout (plugin installed as
+    <workspace>/<plugin>/) from the dev layout (plugin IS the workspace)
+    by checking whether the hook path places 'hooks/' as a direct
+    subdirectory of the workspace. In dev layout, the hook is at
+    <workspace>/hooks/<hook>.py so the relative path from workspace
+    to hook starts with 'hooks/'. In production, the relative path
+    starts with '<plugin>/hooks/' so does NOT start with 'hooks/'.
+
+    - Production layout: hook = <workspace>/<plugin>/hooks/<hook>.py
+      hook_relative = <plugin>/hooks/<hook>.py (doesn't start with 'hooks/')
+      → plugin repo is the workspace root → return repo.
+    - Dev layout: hook = <workspace>/hooks/<hook>.py
+      hook_relative = hooks/<hook>.py (starts with 'hooks/')
+      → workspace IS the repo → return workspace.
+    """
+    abs_hook = os.path.abspath(hook_file)
+    parent = os.path.dirname(abs_hook)          # hooks/
+    repo = os.path.dirname(parent)              # repo/
+    workspace = os.path.dirname(repo)           # workspace/ (parent of repo)
+
+    # Detect: is the workspace the repo? If the hook's relative path from
+    # workspace starts with 'hooks/', the workspace IS the repo (dev layout).
+    # Otherwise the plugin is a subdirectory of the workspace (production layout).
+    hook_relative = os.path.relpath(abs_hook, workspace)
+    if hook_relative.startswith("hooks/"):
+        return workspace   # dev layout: workspace IS the repo
+    return repo            # production layout: plugin is nested inside workspace
+
+
 def read_input() -> dict:
     """Parse stdin JSON. Empty input → empty dict."""
     raw = sys.stdin.read().strip()
diff --git a/hooks/session-start-context.py b/hooks/session-start-context.py
index 8f418f6..c706fdc 100755
--- a/hooks/session-start-context.py
+++ b/hooks/session-start-context.py
@@ -6,9 +6,9 @@ import os
 import subprocess
 import sys
 sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
-from _lib import add_context, warn_to_stderr  # noqa
+from _lib import add_context, get_repo_root, warn_to_stderr  # noqa
 
-REPO = os.path.dirname(os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+REPO = get_repo_root(__file__)
 LEARNINGS = os.path.expanduser(
     "~/.claude/projects/-Users-hongming-Documents-GitHub-molecule-monorepo/memory/cron-learnings.jsonl"
 )
diff --git a/tests/README.md b/tests/README.md
new file mode 100644
index 0000000..88809f3
--- /dev/null
+++ b/tests/README.md
@@ -0,0 +1,34 @@
+# Test Rationale — molecule-session-context
+
+## What this plugin does
+
+`molecule-session-context` provides a SessionStart hook (`session-start-context.py`)
+that injects operational context into Claude's context window at session start:
+recent cron-learnings from JSONL, freeze-file status, and open PR/issue counts
+from GitHub.
+
+The hook reads `~/.claude/projects/.../memory/cron-learnings.jsonl` (written by
+`molecule-cron-learnings`) and emits a JSON `additionalContext` payload to stdout.
+It also queries `gh` for repo state and handles graceful degradation when
+`gh` is unavailable or files are missing.
+
+## What is tested
+
+- `hooks/_lib.py` helpers: `read_input`, `emit`, `deny_pretooluse`, `add_context`,
+  `warn_to_stderr`, `get_repo_root`
+- `hooks/session-start-context.py` logic: `tail()` JSONL reader, `gh_count()`
+  subprocess wrapper, full `main()` integration with mocked gh + tempfile learnings
+- Hook error handling (exits 0 on exception, not crash)
+
+## What is NOT unit-tested (and why)
+
+- Real `gh` API calls — tested with `unittest.mock` subprocess interception
+- Actual freeze-file reading — tested with mocked file I/O
+- Integration with the workspace runtime's SessionStart hook harness —
+  requires a real Claude Code session; covered by smoke + integration tests
+
+## Running tests
+
+```bash
+python -m pytest tests/ -v
+```
diff --git a/tests/test_lib.py b/tests/test_lib.py
index 9e0701c..2d1dba7 100644
--- a/tests/test_lib.py
+++ b/tests/test_lib.py
@@ -113,4 +113,39 @@ class TestAddContext:
         stdout2 = io.StringIO()
         with mock.patch("sys.stdout", stdout2):
             add_context("   ")
-        assert stdout2.getvalue() == ""
\ No newline at end of file
+        assert stdout2.getvalue() == ""
+
+
+class TestGetRepoRoot:
+    """Tests for get_repo_root helper (imported from _lib)."""
+
+    def test_production_layout_returns_repo(self):
+        """Production: hook at <workspace>/<plugin>/hooks/hook.py → return the plugin repo."""
+        from _lib import get_repo_root
+
+        # /tmp/my-repo/hooks/hook.py → workspace=/tmp, relpath="my-repo/hooks/hook.py"
+        # relpath does NOT start with "hooks/" → production → return repo
+        result = get_repo_root("/tmp/my-repo/hooks/session-start-context.py")
+        assert result == "/tmp/my-repo"
+
+    def test_dev_layout_returns_workspace(self):
+        """Dev layout: hook at <workspace>/hooks/hook.py → workspace IS the repo."""
+        from _lib import get_repo_root
+
+        # /workspace/hooks/hook.py → workspace=/workspace, relpath="hooks/hook.py"
+        # relpath starts with "hooks/" → dev layout → return workspace
+        result = get_repo_root("/workspace/hooks/session-start-context.py")
+        assert result == "/workspace"
+
+    def test_real_repo_path_in_this_repo(self):
+        """Verify get_repo_root on the actual repo resolves to a valid root."""
+        from _lib import get_repo_root
+
+        # This test file is at: <repo>/tests/test_lib.py
+        # The hook is at: <repo>/hooks/session-start-context.py
+        hooks_dir = os.path.join(os.path.dirname(os.path.dirname(__file__)), "hooks")
+        hook_file = os.path.join(hooks_dir, "session-start-context.py")
+        result = get_repo_root(hook_file)
+        # Sanity: result is an absolute path and is a directory
+        assert os.path.isabs(result), f"result {result!r} is not absolute"
+        assert os.path.isdir(result), f"result {result!r} is not a directory"
\ No newline at end of file