diff --git a/engine/antigravity_engine/hub/_merkle.py b/engine/antigravity_engine/hub/_merkle.py
new file mode 100644
index 000000000..eee29f2e5
--- /dev/null
+++ b/engine/antigravity_engine/hub/_merkle.py
@@ -0,0 +1,197 @@
+"""Content-hash change detection for incremental refresh.
+
+A Merkle-style content hash tree over the workspace at file -> module -> repo
+granularity.  Persisting a snapshot lets a later refresh diff the current tree
+against the previous one and learn *which modules changed* without trusting
+git state — it sees uncommitted edits, survives branch switches, and works in
+non-git directories.
+
+This module is deliberately self-contained and side-effect-free apart from the
+explicit snapshot read/write helpers.  It is NOT yet wired into the refresh
+pipeline: detecting change here has no behavioural effect on its own.  The
+follow-up step consumes :func:`diff_trees` to skip unchanged modules and
+recompute only a changed module plus its graph impact closure.
+
+Why module-grained hashes (not Cursor-style per-chunk):  Cursor re-embeds only
+changed files because an embedding is a context-free function of one chunk.
+Antigravity's per-module knowledge is *interpreted* and cross-references other
+modules, so the eventual cache key is ``hash(own files + dependency closure)``
+— but the raw per-file/per-module hashes built here are the inputs that key is
+computed from.
+"""
+from __future__ import annotations
+
+import hashlib
+import json
+import logging
+from dataclasses import dataclass, field
+from pathlib import Path
+from typing import Mapping
+
+logger = logging.getLogger(__name__)
+
+# Bump when the on-disk snapshot schema changes; an older snapshot then loads
+# as ``None`` and the caller rebuilds from scratch instead of misreading it.
+SNAPSHOT_VERSION = 1
+
+# Default file name for a persisted tree, written under the .antigravity dir.
+SNAPSHOT_FILENAME = "merkle.json"
+
+# Separator placed between a path and its hash inside a hashed line. NUL never
+# appears in file paths, so it cannot be forged by a crafted path.
+_SEP = "\0"
+
+
+def compute_content_hash(content: str | bytes) -> str:
+    """Return the SHA-256 hex digest of file content."""
+    data = content.encode("utf-8") if isinstance(content, str) else content
+    return hashlib.sha256(data).hexdigest()
+
+
+@dataclass(frozen=True)
+class ModuleNode:
+    """One module's hash plus the per-file hashes it was derived from."""
+
+    hash: str
+    files: dict[str, str] = field(default_factory=dict)  # rel_path -> file hash
+
+
+@dataclass(frozen=True)
+class MerkleTree:
+    """A workspace content-hash tree: root hash over all module hashes."""
+
+    root: str
+    modules: dict[str, ModuleNode] = field(default_factory=dict)
+
+    def to_dict(self) -> dict:
+        return {
+            "version": SNAPSHOT_VERSION,
+            "root": self.root,
+            "modules": {
+                module_id: {"hash": node.hash, "files": node.files}
+                for module_id, node in self.modules.items()
+            },
+        }
+
+    @classmethod
+    def from_dict(cls, data: Mapping) -> "MerkleTree":
+        modules = {
+            module_id: ModuleNode(
+                hash=str(entry.get("hash", "")),
+                files=dict(entry.get("files", {})),
+            )
+            for module_id, entry in data.get("modules", {}).items()
+        }
+        return cls(root=str(data.get("root", "")), modules=modules)
+
+
+@dataclass(frozen=True)
+class MerkleDiff:
+    """Modules that appeared, changed, or vanished between two trees."""
+
+    added: list[str]
+    modified: list[str]
+    removed: list[str]
+
+    @property
+    def changed_modules(self) -> list[str]:
+        """Modules whose knowledge must be (re)generated: added + modified."""
+        return sorted(set(self.added) | set(self.modified))
+
+    @property
+    def is_empty(self) -> bool:
+        return not (self.added or self.modified or self.removed)
+
+
+def _hash_lines(lines: list[str]) -> str:
+    """Hash a set of ``name<SEP>hash`` lines order-independently."""
+    return hashlib.sha256("\n".join(sorted(lines)).encode("utf-8")).hexdigest()
+
+
+def build_tree(module_file_hashes: Mapping[str, Mapping[str, str]]) -> MerkleTree:
+    """Build a :class:`MerkleTree` from precomputed per-file hashes.
+
+    Pure: takes ``{module_id: {rel_path: file_hash}}`` and rolls hashes up
+    deterministically (a module hash covers its file paths *and* contents, so
+    renames register; the root covers all module ids and hashes).
+    """
+    modules: dict[str, ModuleNode] = {}
+    for module_id, file_hashes in module_file_hashes.items():
+        files = dict(file_hashes)
+        module_hash = _hash_lines(
+            [f"{rel_path}{_SEP}{files[rel_path]}" for rel_path in files]
+        )
+        modules[module_id] = ModuleNode(hash=module_hash, files=files)
+
+    root = _hash_lines(
+        [f"{module_id}{_SEP}{node.hash}" for module_id, node in modules.items()]
+    )
+    return MerkleTree(root=root, modules=modules)
+
+
+def build_workspace_tree(workspace: Path) -> MerkleTree:
+    """Build the content-hash tree for ``workspace``.
+
+    Uses the same module detection and file-loading the refresh pipeline uses,
+    so a "changed module" here corresponds exactly to a unit refresh would
+    regenerate.  Performs file reads only — no LLM, no network.
+    """
+    from antigravity_engine.hub.module_grouping import load_module_files
+    from antigravity_engine.hub.scanner import detect_modules, resolve_module_path
+
+    module_file_hashes: dict[str, dict[str, str]] = {}
+    for module_id in detect_modules(workspace):
+        module_path = resolve_module_path(workspace, module_id)
+        if module_path is None or not Path(module_path).is_dir():
+            continue
+        files = load_module_files(module_path, workspace)
+        module_file_hashes[module_id] = {
+            source_file.rel_path: compute_content_hash(source_file.content)
+            for source_file in files
+        }
+    return build_tree(module_file_hashes)
+
+
+def diff_trees(previous: MerkleTree | None, current: MerkleTree) -> MerkleDiff:
+    """Compare two trees and report added / modified / removed modules."""
+    if previous is None:
+        return MerkleDiff(added=sorted(current.modules), modified=[], removed=[])
+
+    prev_ids = set(previous.modules)
+    cur_ids = set(current.modules)
+    added = sorted(cur_ids - prev_ids)
+    removed = sorted(prev_ids - cur_ids)
+    modified = sorted(
+        module_id
+        for module_id in (cur_ids & prev_ids)
+        if current.modules[module_id].hash != previous.modules[module_id].hash
+    )
+    return MerkleDiff(added=added, modified=modified, removed=removed)
+
+
+def save_snapshot(tree: MerkleTree, path: Path) -> None:
+    """Persist ``tree`` to ``path`` as JSON (creating parent dirs)."""
+    path.parent.mkdir(parents=True, exist_ok=True)
+    path.write_text(
+        json.dumps(tree.to_dict(), indent=2, sort_keys=True), encoding="utf-8"
+    )
+
+
+def load_snapshot(path: Path) -> MerkleTree | None:
+    """Load a previously saved tree, or ``None`` if missing/unreadable/stale.
+
+    Returns ``None`` (rather than raising) on a missing file, malformed JSON,
+    or a snapshot written by a different :data:`SNAPSHOT_VERSION`, so callers
+    transparently fall back to a full rebuild.
+    """
+    if not path.is_file():
+        return None
+    try:
+        data = json.loads(path.read_text(encoding="utf-8"))
+    except (ValueError, OSError) as exc:
+        logger.warning("Ignoring unreadable merkle snapshot %s: %s", path, exc)
+        return None
+    if not isinstance(data, dict) or data.get("version") != SNAPSHOT_VERSION:
+        logger.info("Discarding merkle snapshot %s: schema version mismatch", path)
+        return None
+    return MerkleTree.from_dict(data)
diff --git a/engine/tests/test_hub_merkle.py b/engine/tests/test_hub_merkle.py
new file mode 100644
index 000000000..9dae144a2
--- /dev/null
+++ b/engine/tests/test_hub_merkle.py
@@ -0,0 +1,130 @@
+"""Tests for hub._merkle content-hash change detection."""
+from pathlib import Path
+
+from antigravity_engine.hub._merkle import (
+    SNAPSHOT_VERSION,
+    MerkleTree,
+    build_tree,
+    build_workspace_tree,
+    compute_content_hash,
+    diff_trees,
+    load_snapshot,
+    save_snapshot,
+)
+
+
+# --- hashing primitives ---------------------------------------------------
+
+
+def test_content_hash_str_and_bytes_agree() -> None:
+    assert compute_content_hash("hello") == compute_content_hash(b"hello")
+    assert compute_content_hash("a") != compute_content_hash("b")
+
+
+def test_build_tree_is_deterministic_and_order_independent() -> None:
+    a = build_tree({"m1": {"x.py": "h1", "y.py": "h2"}, "m2": {"z.py": "h3"}})
+    b = build_tree({"m2": {"z.py": "h3"}, "m1": {"y.py": "h2", "x.py": "h1"}})
+    assert a.root == b.root
+    assert a.modules["m1"].hash == b.modules["m1"].hash
+
+
+def test_build_tree_changes_when_a_file_hash_changes() -> None:
+    base = build_tree({"m1": {"x.py": "h1"}})
+    changed = build_tree({"m1": {"x.py": "h1-CHANGED"}})
+    assert base.root != changed.root
+    assert base.modules["m1"].hash != changed.modules["m1"].hash
+
+
+def test_build_tree_root_changes_when_a_module_is_added() -> None:
+    base = build_tree({"m1": {"x.py": "h1"}})
+    plus = build_tree({"m1": {"x.py": "h1"}, "m2": {"y.py": "h2"}})
+    assert base.root != plus.root
+    assert base.modules["m1"].hash == plus.modules["m1"].hash  # unchanged module stable
+
+
+# --- diff -----------------------------------------------------------------
+
+
+def test_diff_against_none_marks_everything_added() -> None:
+    cur = build_tree({"m1": {"x.py": "h1"}, "m2": {"y.py": "h2"}})
+    d = diff_trees(None, cur)
+    assert d.added == ["m1", "m2"]
+    assert d.modified == [] and d.removed == []
+    assert d.changed_modules == ["m1", "m2"]
+
+
+def test_diff_detects_modified_added_removed() -> None:
+    prev = build_tree({"keep": {"a.py": "h"}, "gone": {"b.py": "h"}, "edit": {"c.py": "h1"}})
+    cur = build_tree({"keep": {"a.py": "h"}, "edit": {"c.py": "h2"}, "new": {"d.py": "h"}})
+    d = diff_trees(prev, cur)
+    assert d.added == ["new"]
+    assert d.modified == ["edit"]
+    assert d.removed == ["gone"]
+    assert d.changed_modules == ["edit", "new"]  # removed is not "to recompute"
+
+
+def test_diff_identical_trees_is_empty() -> None:
+    t = build_tree({"m1": {"x.py": "h1"}})
+    again = build_tree({"m1": {"x.py": "h1"}})
+    d = diff_trees(t, again)
+    assert d.is_empty
+    assert d.changed_modules == []
+
+
+# --- snapshot persistence -------------------------------------------------
+
+
+def test_snapshot_round_trip(tmp_path: Path) -> None:
+    tree = build_tree({"m1": {"x.py": "h1", "y.py": "h2"}, "m2": {"z.py": "h3"}})
+    snap = tmp_path / "merkle.json"
+    save_snapshot(tree, snap)
+
+    loaded = load_snapshot(snap)
+    assert loaded is not None
+    assert loaded.root == tree.root
+    assert loaded.modules["m1"].files == {"x.py": "h1", "y.py": "h2"}
+    assert diff_trees(loaded, tree).is_empty
+
+
+def test_load_snapshot_missing_returns_none(tmp_path: Path) -> None:
+    assert load_snapshot(tmp_path / "nope.json") is None
+
+
+def test_load_snapshot_bad_json_returns_none(tmp_path: Path) -> None:
+    bad = tmp_path / "merkle.json"
+    bad.write_text("{not valid json", encoding="utf-8")
+    assert load_snapshot(bad) is None
+
+
+def test_load_snapshot_version_mismatch_returns_none(tmp_path: Path) -> None:
+    stale = tmp_path / "merkle.json"
+    stale.write_text(
+        '{"version": %d, "root": "r", "modules": {}}' % (SNAPSHOT_VERSION + 1),
+        encoding="utf-8",
+    )
+    assert load_snapshot(stale) is None
+
+
+# --- real workspace end to end (file reads only, no LLM) ------------------
+
+
+def test_build_workspace_tree_detects_content_change(tmp_path: Path) -> None:
+    (tmp_path / "main.go").write_text("package main\nfunc main() {}\n", encoding="utf-8")
+    internal = tmp_path / "internal"
+    internal.mkdir()
+    service = internal / "service.go"
+    service.write_text("package internal\nfunc Service() {}\n", encoding="utf-8")
+
+    before = build_workspace_tree(tmp_path)
+    assert len(before.root) == 64  # sha256 hex
+    assert before.modules  # at least one module detected
+
+    # Edit one file's content; structure (module set) is unchanged.
+    service.write_text("package internal\nfunc Service() { return }\n", encoding="utf-8")
+    after = build_workspace_tree(tmp_path)
+
+    assert after.root != before.root
+    d = diff_trees(before, after)
+    assert not d.is_empty
+    assert d.added == [] and d.removed == []
+    assert d.changed_modules  # the edited module shows up as modified