diff --git a/src/_canary/testspec.py b/src/_canary/testspec.py
index 86ab2ab6..8bf05fca 100644
--- a/src/_canary/testspec.py
+++ b/src/_canary/testspec.py
@@ -60,7 +60,9 @@ def unmasked(cls) -> "Mask":
 @dataclasses.dataclass
 class BaseSpec(Generic[T]):
     file_root: Path
+    """The search path contiaining the generated spec; typically the some version-control root"""
     file_path: Path
+    """The path to the test file relative to `file_root`"""
     family: str = ""
     stdout: str = "canary-out.txt"
     stderr: str | None = None  # combine stdout/stderr by default
@@ -131,6 +133,7 @@ def from_dict(cls: Type[T], d: dict, lookup: dict[str, T]) -> T:
 
     @cached_property
     def file(self) -> Path:
+        """Path to the test specification file"""
         return self.file_root / self.file_path
 
     @property
@@ -482,30 +485,18 @@ def _generate_dependency_patterns(
 
 
 class _GlobalSpecCache:
-    _file_hash: dict[Path, str] = {}
-    _repo_root: dict[Path, Path] = {}
-    _lock = threading.Lock()
+    """Simple cache for storing re-used data feeding the spec ID"""
 
-    @classmethod
-    def file_hash(cls, path: Path) -> str:
-        path = path.absolute()
-        try:
-            return cls._file_hash[path]
-        except KeyError:
-            pass
-        h = hashlib.blake2b(digest_size=16)
-        h.update(path.read_bytes())
-        digest = h.hexdigest()
-        with cls._lock:
-            return cls._file_hash.setdefault(path, digest)
+    _key: dict[Path, Path] = {}
+    """Maps the input file path to a key index (absolute path)"""
+
+    _file_hash: dict[Path, bytes] = {}
+    _repo_root: dict[Path, bytes] = {}
+    _rel_repo: dict[Path, bytes] = {}
+    _lock = threading.Lock()
 
     @classmethod
-    def repo_root(cls, path: Path) -> Path:
-        path = path.absolute()
-        try:
-            return cls._repo_root[path]
-        except KeyError:
-            pass
+    def _compute_repo_root(cls, path: Path) -> Path:
         d = path.parent
         while d.parent != d:
             if (d / ".git").exists() or (d / ".repo").exists():
@@ -514,8 +505,41 @@ def repo_root(cls, path: Path) -> Path:
             d = d.parent
         else:
             root = path.parent
+        return root
+
+    @classmethod
+    def populate_cache(cls, path: Path) -> Path:
+        try:
+            return cls._key[path]
+        except KeyError:
+            pass
+
+        key = path.absolute()
+        h = hashlib.blake2b(digest_size=16)
+        h.update(key.read_bytes())
+        root = cls._compute_repo_root(key)
+        rel = key.relative_to(root)
+
         with cls._lock:
-            return cls._repo_root.setdefault(path, root)
+            cls._repo_root[key] = str(root).encode()
+            cls._rel_repo[key] = str(rel).encode()
+            cls._file_hash[key] = h.hexdigest().encode()
+            return cls._key.setdefault(path, key)
+
+    @classmethod
+    def file_hash(cls, path: Path) -> bytes:
+        key = cls.populate_cache(path)
+        return cls._file_hash[key]
+
+    @classmethod
+    def repo_root(cls, path: Path) -> bytes:
+        key = cls.populate_cache(path)
+        return cls._repo_root[key]
+
+    @classmethod
+    def rel_repo(cls, path: Path) -> bytes:
+        key = cls.populate_cache(path)
+        return cls._rel_repo[key]
 
 
 def build_spec_id(spec: BaseSpec) -> str:
@@ -526,8 +550,9 @@ def build_spec_id(spec: BaseSpec) -> str:
     if parameters:
         for p in sorted(parameters):
             hasher.update(f"{p}={stringify(parameters[p], float_fmt='%.16e')}".encode())
-    hasher.update(_GlobalSpecCache.file_hash(spec.file).encode())
-    hasher.update(str(_GlobalSpecCache.repo_root(spec.file)).encode())
+    hasher.update(_GlobalSpecCache.file_hash(spec.file))
+    hasher.update(_GlobalSpecCache.repo_root(spec.file))
+    hasher.update(_GlobalSpecCache.rel_repo(spec.file))
     return hasher.hexdigest()
 
 
diff --git a/src/_canary/workspace.py b/src/_canary/workspace.py
index b26ba1d9..1943f06c 100644
--- a/src/_canary/workspace.py
+++ b/src/_canary/workspace.py
@@ -13,6 +13,7 @@
 from pathlib import Path
 from typing import Any
 from typing import Callable
+from typing import Iterable
 from typing import Sequence
 from typing import TypeVar
 
@@ -1169,14 +1170,14 @@ def delete_selection(self, tag: str) -> bool:
             self.connection.execute("DELETE FROM selections WHERE tag = ?", (tag,))
         return True
 
-    def get_updownstream_ids(self, seeds: list[str] | None = None) -> tuple[set[str], set[str]]:
+    def get_updownstream_ids(self, seeds: Iterable[str] | None = None) -> tuple[set[str], set[str]]:
         if seeds is None:
             return set(), set()
         downstream = self.get_downstream_ids(seeds)
-        upstream = self.get_upstream_ids(list(downstream.union(seeds)))
+        upstream = self.get_upstream_ids(downstream.union(seeds))
         return upstream, downstream
 
-    def get_downstream_ids(self, seeds: list[str]) -> set[str]:
+    def get_downstream_ids(self, seeds: Iterable[str]) -> set[str]:
         """Return dependencies in instantiation order."""
         if not seeds:
             return set()
@@ -1198,7 +1199,7 @@ def get_downstream_ids(self, seeds: list[str]) -> set[str]:
         rows = self.connection.execute(query, tuple(seeds)).fetchall()
         return {r[0] for r in rows}
 
-    def get_upstream_ids(self, seeds: list[str]) -> set[str]:
+    def get_upstream_ids(self, seeds: Iterable[str]) -> set[str]:
         """Return dependents in reverse instantiation order."""
         if not seeds:
             return set()