diff --git a/ast_rag/services/graph_updater_service.py b/ast_rag/services/graph_updater_service.py
index cf954fc..d2dd3aa 100644
--- a/ast_rag/services/graph_updater_service.py
+++ b/ast_rag/services/graph_updater_service.py
@@ -345,17 +345,19 @@ def update_from_git(
             if lang is None:
                 continue
 
-            # Load new content (may no longer exist if deleted)
-            if not os.path.exists(file_path):
-                # File was deleted: all old nodes from this file need expiration.
-                # For PoC: we mark them by querying Neo4j.
+            rel_path = os.path.relpath(file_path, repo_path)
+
+            # Read the new side at new_commit, not from disk: this writes to
+            # the graph, so a dirty tree would persist code that never existed
+            # at the commit it gets stamped with. Uncommitted work has its own
+            # path in get_workspace_diff.
+            new_source = _read_blob(repo, new_commit, rel_path)
+            if new_source is None:
+                # Absent at new_commit: expire everything this file defined.
                 _expire_file_nodes(driver, file_path, new_commit)
                 continue
 
-            with open(file_path, "rb") as fh:
-                new_source = fh.read()
-
-            # Parse the NEW version
+            # Parse the version at new_commit
             new_tree = pm.parse_file(file_path, source=new_source)
             if new_tree is None:
                 continue
@@ -365,7 +367,6 @@ def update_from_git(
             )
 
             # Load old version from git at old_commit
-            rel_path = os.path.relpath(file_path, repo_path)
             old_source_bytes = _read_blob(repo, old_commit, rel_path)
             old_nodes: list[ASTNode] = []
             old_edges: list[ASTEdge] = []
@@ -509,11 +510,13 @@ def compute_diff_for_commits(
         if lang is None:
             continue
 
-        # Load new content (may no longer exist if deleted)
-        if not os.path.exists(file_path):
-            # File was deleted - we still process it to mark nodes for expiration
-            # Load old version from git
-            rel_path = os.path.relpath(file_path, repo_path)
+        rel_path = os.path.relpath(file_path, repo_path)
+
+        # Both sides come out of the object store. Reading the new side from
+        # disk would describe the working tree while stamping it to_commit.
+        new_source = _read_blob(repo, to_commit, rel_path)
+        if new_source is None:
+            # Absent at to_commit: expire whatever it defined at from_commit.
             old_source_bytes = _read_blob(repo, from_commit, rel_path)
             if old_source_bytes is not None:
                 old_tree = pm.parse_file(file_path, source=old_source_bytes)
@@ -532,10 +535,7 @@ def compute_diff_for_commits(
                     agg_diff.deleted_edge_ids.extend(file_diff.deleted_edge_ids)
             continue
 
-        with open(file_path, "rb") as fh:
-            new_source = fh.read()
-
-        # Parse the NEW version
+        # Parse the version at to_commit
         new_tree = pm.parse_file(file_path, source=new_source)
         if new_tree is None:
             continue
@@ -543,7 +543,6 @@ def compute_diff_for_commits(
         new_edges = pm.extract_edges(new_tree, new_nodes, file_path, lang, new_source, to_commit)
 
         # Load old version from git at from_commit
-        rel_path = os.path.relpath(file_path, repo_path)
         old_source_bytes = _read_blob(repo, from_commit, rel_path)
         old_nodes: list[ASTNode] = []
         old_edges: list[ASTEdge] = []
diff --git a/tests/test_diff_reads_the_named_commit.py b/tests/test_diff_reads_the_named_commit.py
new file mode 100644
index 0000000..cefa997
--- /dev/null
+++ b/tests/test_diff_reads_the_named_commit.py
@@ -0,0 +1,116 @@
+"""``A..B`` must describe B, not whatever is checked out right now.
+
+Both git-diff paths read the *old* side from the object store with
+``_read_blob`` and the *new* side with ``open(file_path)`` -- the working
+tree. ``compute_diff_for_commits`` promises "added_nodes: New AST nodes in
+to_commit", and ``ast-rag update`` takes ``--from-commit`` and ``--to-commit``
+as required options, so a user naming any pair other than
+``<something>..HEAD`` on a clean tree gets the current source parsed and
+stamped with ``to_commit``.
+
+``update_from_git`` has the same line, and that one writes to Neo4j: the
+graph ends up holding code that never existed at the commit it is labelled
+with. Uncommitted work already has its own path in ``get_workspace_diff``, so
+the two are not meant to be the same thing.
+"""
+
+from __future__ import annotations
+
+import subprocess
+from pathlib import Path
+
+import pytest
+
+from ast_rag.services.graph_updater_service import compute_diff_for_commits
+
+pytest.importorskip("git", reason="GitPython is needed to build the fixture repo")
+
+
+def _git(repo: Path, *args: str) -> str:
+    return subprocess.run(
+        ["git", *args],
+        cwd=repo,
+        check=True,
+        capture_output=True,
+        text=True,
+    ).stdout.strip()
+
+
+def _commit(repo: Path, message: str) -> str:
+    _git(repo, "add", "-A")
+    _git(repo, "commit", "-q", "-m", message)
+    return _git(repo, "rev-parse", "HEAD")
+
+
+@pytest.fixture
+def repo(tmp_path: Path) -> dict:
+    """A repo where `alpha` sits on a different line in A, B and the tree."""
+    root = tmp_path / "repo"
+    root.mkdir()
+    _git(root, "init", "-q")
+    _git(root, "config", "user.email", "test@example.com")
+    _git(root, "config", "user.name", "Test")
+
+    module = root / "mod.py"
+
+    # A: alpha on line 1
+    module.write_text("def alpha():\n    return 1\n")
+    commit_a = _commit(root, "a")
+
+    # B: one comment pushes alpha to line 2, and the body changes so the
+    # node is genuinely part of the diff rather than untouched.
+    module.write_text("# header\ndef alpha():\n    return 2\n")
+    commit_b = _commit(root, "b")
+
+    # Working tree only: five more comments push alpha to line 7. Never committed.
+    module.write_text("# header\n# 1\n# 2\n# 3\n# 4\n# 5\ndef alpha():\n    return 2\n")
+
+    return {"path": str(root), "a": commit_a, "b": commit_b, "file": module}
+
+
+def _alpha(diff) -> object:
+    nodes = [
+        n
+        for n in (*diff.added_nodes, *diff.updated_nodes)
+        if n.name == "alpha" and n.kind.value in {"Function", "Method"}
+    ]
+    assert nodes, (
+        "diff reported no `alpha` node at all; "
+        f"added={[n.name for n in diff.added_nodes]} "
+        f"updated={[n.name for n in diff.updated_nodes]}"
+    )
+    return nodes[0]
+
+
+def test_working_tree_is_not_mistaken_for_the_target_commit(repo):
+    """A..B must parse B's blob, not the file on disk."""
+    diff = compute_diff_for_commits(repo["path"], repo["a"], repo["b"])
+    alpha = _alpha(diff)
+
+    assert alpha.start_line == 2, (
+        f"`alpha` is on line 2 at to_commit and line 7 in the working tree; "
+        f"the diff reported line {alpha.start_line}, so it parsed the working tree"
+    )
+
+
+def test_diff_is_stable_while_the_working_tree_moves(repo):
+    """The same A..B must not change answer when an unrelated edit lands."""
+    before = _alpha(compute_diff_for_commits(repo["path"], repo["a"], repo["b"])).start_line
+
+    repo["file"].write_text("# a\n# b\n# c\n" + repo["file"].read_text())
+    after = _alpha(compute_diff_for_commits(repo["path"], repo["a"], repo["b"])).start_line
+
+    assert before == after, (
+        f"editing an uncommitted file changed the reported diff of two fixed "
+        f"commits ({before} -> {after})"
+    )
+
+
+def test_source_text_comes_from_the_target_commit(repo):
+    """The body parsed must be B's body, not the tree's."""
+    diff = compute_diff_for_commits(repo["path"], repo["a"], repo["b"])
+    alpha = _alpha(diff)
+
+    assert alpha.end_line == 3, (
+        f"`alpha` spans lines 2-3 at to_commit; got {alpha.start_line}-{alpha.end_line}"
+    )