diff --git a/graphify/cache.py b/graphify/cache.py
index 03e62d3ec..8094ef9c2 100644
--- a/graphify/cache.py
+++ b/graphify/cache.py
@@ -1,12 +1,53 @@
 # per-file extraction cache - skip unchanged files on re-run
 from __future__ import annotations
 
+import functools
 import hashlib
 import json
 import os
 from pathlib import Path
 
 
+@functools.lru_cache(maxsize=None)
+def _git_root(directory: Path) -> Path | None:
+    """Return the git repository root for *directory*, or None if not in a repo."""
+    current = directory.resolve()
+    while True:
+        if (current / ".git").exists():
+            return current
+        parent = current.parent
+        if parent == current:
+            return None
+        current = parent
+
+
+@functools.lru_cache(maxsize=None)
+def _git_common_dir(directory: Path) -> Path | None:
+    """Return the shared .git directory — identical across all worktrees of a repo.
+
+    Regular clone  → <repo>/.git/
+    git worktree   → reads <worktree>/.git file, returns <repo>/.git/
+    Not a git repo → None
+    """
+    root = _git_root(directory)
+    if root is None:
+        return None
+    git_path = root / ".git"
+    if git_path.is_dir():
+        return git_path
+    if git_path.is_file():
+        content = git_path.read_text(encoding="utf-8").strip()
+        if content.startswith("gitdir:"):
+            worktree_git = Path(content[len("gitdir:"):].strip())
+            if not worktree_git.is_absolute():
+                worktree_git = (root / worktree_git).resolve()
+            # .git/worktrees/<branch>/ → .git/
+            common = worktree_git.parent.parent
+            if (common / "HEAD").exists():
+                return common
+    return None
+
+
 def _body_content(content: bytes) -> bytes:
     """Strip YAML frontmatter from Markdown content, returning only the body."""
     text = content.decode(errors="replace")
@@ -42,8 +83,18 @@ def file_hash(path: Path, root: Path = Path(".")) -> str:
 
 
 def cache_dir(root: Path = Path(".")) -> Path:
-    """Returns graphify-out/cache/ - creates it if needed."""
-    d = Path(root) / "graphify-out" / "cache"
+    """Return the cache directory, creating it if needed.
+
+    When inside a git repository, the cache lives in <git-common-dir>/graphify-cache/
+    so all worktrees of the same repo share one cache — a worktree for a feature
+    branch that touches 3 of 500 files gets 497 cache hits instead of a cold start.
+    Outside a git repo, falls back to <root>/graphify-out/cache/.
+    """
+    common = _git_common_dir(Path(root).resolve())
+    if common is not None:
+        d = common / "graphify-cache"
+    else:
+        d = Path(root) / "graphify-out" / "cache"
     d.mkdir(parents=True, exist_ok=True)
     return d
 
diff --git a/tests/test_cache.py b/tests/test_cache.py
index fd57cad19..be2c95430 100644
--- a/tests/test_cache.py
+++ b/tests/test_cache.py
@@ -1,7 +1,20 @@
 """Tests for graphify/cache.py."""
 import pytest
 from pathlib import Path
-from graphify.cache import file_hash, cache_dir, load_cached, save_cached, cached_files, clear_cache, _body_content
+from graphify.cache import (
+    file_hash, cache_dir, load_cached, save_cached, cached_files, clear_cache,
+    _body_content, _git_root, _git_common_dir,
+)
+
+
+@pytest.fixture(autouse=True)
+def _clear_git_lookup_caches():
+    """lru_cache on _git_root/_git_common_dir persists between tests; clear it."""
+    _git_root.cache_clear()
+    _git_common_dir.cache_clear()
+    yield
+    _git_root.cache_clear()
+    _git_common_dir.cache_clear()
 
 
 @pytest.fixture
@@ -67,11 +80,99 @@ def test_cached_files(tmp_path, cache_root):
 
 
 def test_clear_cache(tmp_file, cache_root):
-    """clear_cache removes all .json files from graphify-out/cache/."""
+    """clear_cache removes all .json files from the cache directory."""
     save_cached(tmp_file, {"nodes": [], "edges": []}, root=cache_root)
-    assert len(list((cache_root / "graphify-out" / "cache").glob("*.json"))) > 0
+    cdir = cache_dir(cache_root)
+    assert len(list(cdir.glob("*.json"))) > 0
     clear_cache(cache_root)
-    assert len(list((cache_root / "graphify-out" / "cache").glob("*.json"))) == 0
+    assert len(list(cdir.glob("*.json"))) == 0
+
+
+def test_cache_dir_falls_back_when_not_in_git_repo(tmp_path):
+    """Outside a git repo, cache_dir returns <root>/graphify-out/cache/ (legacy)."""
+    d = cache_dir(tmp_path)
+    assert d == tmp_path / "graphify-out" / "cache"
+    assert d.exists()
+
+
+def test_cache_dir_uses_git_common_dir(tmp_path):
+    """Inside a git repo (regular clone), cache_dir returns <.git>/graphify-cache/."""
+    repo = tmp_path / "repo"
+    repo.mkdir()
+    (repo / ".git").mkdir()  # simulate a regular clone
+    (repo / ".git" / "HEAD").write_text("ref: refs/heads/main\n")
+
+    d = cache_dir(repo)
+    assert d == repo / ".git" / "graphify-cache"
+    assert d.exists()
+
+
+def test_cache_dir_shared_between_main_and_worktree(tmp_path):
+    """Main tree and worktree resolve to the SAME cache directory."""
+    repo = tmp_path / "repo"
+    repo.mkdir()
+    git_dir = repo / ".git"
+    git_dir.mkdir()
+    (git_dir / "HEAD").write_text("ref: refs/heads/main\n")
+    # Create the per-worktree git dir that a `git worktree add` would create
+    wt_git = git_dir / "worktrees" / "feature"
+    wt_git.mkdir(parents=True)
+    (wt_git / "HEAD").write_text("ref: refs/heads/feature\n")
+    # The worktree itself (separate directory, .git is a *file*)
+    worktree = tmp_path / "worktrees" / "feature"
+    worktree.mkdir(parents=True)
+    (worktree / ".git").write_text(f"gitdir: {wt_git}\n")
+
+    main_cache = cache_dir(repo)
+    wt_cache = cache_dir(worktree)
+
+    assert main_cache == wt_cache == git_dir / "graphify-cache"
+
+
+def test_cache_hit_across_worktrees(tmp_path):
+    """A file cached from the main tree is reused when that file appears in a worktree."""
+    repo = tmp_path / "repo"
+    (repo / "src").mkdir(parents=True)
+    git_dir = repo / ".git"
+    git_dir.mkdir()
+    (git_dir / "HEAD").write_text("ref: refs/heads/main\n")
+
+    # Same relative path + same content, different absolute paths.
+    main_file = repo / "src" / "foo.py"
+    main_file.write_text("def greet(): return 'hello'\n")
+
+    # Save cache from the main tree
+    result = {"nodes": [{"id": "greet"}], "edges": []}
+    save_cached(main_file, result, root=repo)
+
+    # Now set up the worktree with the SAME file content at the SAME relative path
+    wt_git = git_dir / "worktrees" / "feature"
+    wt_git.mkdir(parents=True)
+    (wt_git / "HEAD").write_text("ref: refs/heads/feature\n")
+    worktree = tmp_path / "worktrees" / "feature"
+    (worktree / "src").mkdir(parents=True)
+    (worktree / ".git").write_text(f"gitdir: {wt_git}\n")
+    wt_file = worktree / "src" / "foo.py"
+    wt_file.write_text("def greet(): return 'hello'\n")
+
+    # Loading from the worktree should hit the cache the main tree wrote.
+    loaded = load_cached(wt_file, root=worktree)
+    assert loaded == result
+
+
+def test_file_hash_portable_across_worktrees(tmp_path):
+    """Same content + same relative path → same hash, even at different absolute paths."""
+    repo_main = tmp_path / "main"
+    repo_wt = tmp_path / "worktree"
+    (repo_main / "src").mkdir(parents=True)
+    (repo_wt / "src").mkdir(parents=True)
+
+    f_main = repo_main / "src" / "foo.py"
+    f_wt = repo_wt / "src" / "foo.py"
+    f_main.write_text("x = 1\n")
+    f_wt.write_text("x = 1\n")
+
+    assert file_hash(f_main, root=repo_main) == file_hash(f_wt, root=repo_wt)
 
 
 def test_md_frontmatter_only_change_same_hash(tmp_path):