diff --git a/.gitattributes b/.gitattributes
index 1d4232b23296..2024db716b8a 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -6,3 +6,11 @@ Resources/markdown-viewer/diff-viewer/**/*.mjs -whitespace
 # source, so exclude them from GitHub Linguist's language stats.
 Resources/markdown-viewer/** linguist-vendored
 Resources/markdown-viewer/diff-viewer-app/** linguist-generated
+
+# Xcode string catalogs are one large JSON object keyed by string id. Two
+# branches that each add a different key collide positionally, producing dozens
+# of conflict hunks with no semantic disagreement. scripts/merge-xcstrings.py
+# merges per key and defers to the default driver whenever both sides changed
+# the same key. Register it with scripts/install-git-hooks.sh (run by setup.sh);
+# without that config git silently uses the default driver.
+*.xcstrings merge=xcstrings
diff --git a/.github/workflows/ci-guards.yml b/.github/workflows/ci-guards.yml
index d0c2dfae9af2..4b38f45b748a 100644
--- a/.github/workflows/ci-guards.yml
+++ b/.github/workflows/ci-guards.yml
@@ -120,6 +120,10 @@ jobs:
         if: ${{ matrix.group == 'preflight' }}
         run: python3 tests/test_localizable_xcstrings_structure.py
 
+      - name: Validate .xcstrings merge driver
+        if: ${{ matrix.group == 'preflight' }}
+        run: python3 tests/test_merge_xcstrings.py
+
       - name: Validate Python test harness syntax
         if: ${{ matrix.group == 'preflight' }}
         run: git ls-files 'tests/*.py' 'tests_v2/*.py' 'scripts/*.py' | xargs python3 -m py_compile
diff --git a/scripts/install-git-hooks.sh b/scripts/install-git-hooks.sh
index dc5e66f65554..2b42cdaf7db5 100755
--- a/scripts/install-git-hooks.sh
+++ b/scripts/install-git-hooks.sh
@@ -10,3 +10,9 @@ cd "$REPO_ROOT"
 git config core.hooksPath scripts/git-hooks
 chmod +x scripts/git-hooks/*
 echo "==> Git hooks installed (core.hooksPath = scripts/git-hooks)."
+
+# Merge drivers named by .gitattributes have to be defined per clone; git will
+# not run a driver it cannot resolve, it just falls back to the default one.
+git config merge.xcstrings.name "Xcode string catalog (key-wise three-way merge)"
+git config merge.xcstrings.driver "python3 scripts/merge-xcstrings.py %O %A %B %P"
+echo "==> .xcstrings merge driver installed (merge.xcstrings.driver)."
diff --git a/scripts/merge-xcstrings.py b/scripts/merge-xcstrings.py
new file mode 100755
index 000000000000..e76a56b484f1
--- /dev/null
+++ b/scripts/merge-xcstrings.py
@@ -0,0 +1,120 @@
+#!/usr/bin/env python3
+"""Git merge driver for Xcode string catalogs (.xcstrings).
+
+A string catalog is one large JSON object keyed by string id. Two pull requests
+that each add a new key collide positionally even though the keys are disjoint,
+because the additions land in the same region of the file. On
+Resources/Localizable.xcstrings (~6,700 entries, ~541k lines) that produced 42
+conflict hunks in a single pull request, none of which were semantic.
+
+This driver merges per key instead of per line. It is deliberately conservative:
+when the same key is changed on both sides it exits non-zero and lets git write
+normal conflict markers, so a real disagreement is never resolved silently.
+
+Formatting is preserved because the catalog round-trips byte-identically through
+`json.dumps(..., ensure_ascii=False, indent=2)` plus a trailing newline; the
+driver asserts that on the inputs before writing.
+
+Usage (git passes these): merge-xcstrings.py %O %A %B %P
+"""
+from __future__ import annotations
+
+import json
+import sys
+from pathlib import Path
+
+SERIALIZED_SUFFIX = "\n"
+
+
+def render(document: dict) -> str:
+    return json.dumps(document, ensure_ascii=False, indent=2) + SERIALIZED_SUFFIX
+
+
+def load(path: Path) -> tuple[dict, bool]:
+    """Return the parsed catalog and whether it re-renders byte-identically."""
+    text = path.read_text(encoding="utf-8")
+    document = json.loads(text)
+    return document, render(document) == text
+
+
+def merge_mapping(base: dict, ours: dict, theirs: dict, label: str) -> tuple[dict, list[str]]:
+    """Three-way merge one mapping. Returns (merged, conflicting keys)."""
+    merged: dict = {}
+    conflicts: list[str] = []
+    # Preserve ours' order, then append keys only theirs introduced.
+    for key in list(ours) + [k for k in theirs if k not in ours]:
+        in_base, in_ours, in_theirs = key in base, key in ours, key in theirs
+        ours_value = ours.get(key)
+        theirs_value = theirs.get(key)
+        base_value = base.get(key)
+        ours_changed = ours_value != base_value if in_base else in_ours
+        theirs_changed = theirs_value != base_value if in_base else in_theirs
+        if not in_ours and not in_theirs:
+            continue
+        if ours_changed and theirs_changed:
+            if ours_value == theirs_value:
+                if in_ours:
+                    merged[key] = ours_value
+                continue
+            conflicts.append(f"{label}.{key}")
+            continue
+        if ours_changed:
+            if in_ours:
+                merged[key] = ours_value
+            continue
+        if theirs_changed:
+            if in_theirs:
+                merged[key] = theirs_value
+            continue
+        merged[key] = ours_value
+    return merged, conflicts
+
+
+def merge_catalog(base: dict, ours: dict, theirs: dict) -> tuple[dict, list[str]]:
+    strings, conflicts = merge_mapping(
+        base.get("strings", {}), ours.get("strings", {}), theirs.get("strings", {}), "strings"
+    )
+    top_base = {k: v for k, v in base.items() if k != "strings"}
+    top_ours = {k: v for k, v in ours.items() if k != "strings"}
+    top_theirs = {k: v for k, v in theirs.items() if k != "strings"}
+    merged, top_conflicts = merge_mapping(top_base, top_ours, top_theirs, "catalog")
+    merged["strings"] = strings
+    ordered = {k: merged[k] for k in ("sourceLanguage", "strings", "version") if k in merged}
+    ordered.update({k: v for k, v in merged.items() if k not in ordered})
+    return ordered, conflicts + top_conflicts
+
+
+def main(argv: list[str]) -> int:
+    if len(argv) < 4:
+        print("usage: merge-xcstrings.py %O %A %B [%P]", file=sys.stderr)
+        return 2
+    base_path, ours_path, theirs_path = (Path(p) for p in argv[1:4])
+    name = argv[4] if len(argv) > 4 else str(ours_path)
+    try:
+        base, base_exact = load(base_path)
+        ours, ours_exact = load(ours_path)
+        theirs, theirs_exact = load(theirs_path)
+    except (OSError, ValueError) as error:
+        print(f"merge-xcstrings: {name}: cannot parse ({error}); falling back", file=sys.stderr)
+        return 1
+    if not (base_exact and ours_exact and theirs_exact):
+        print(
+            f"merge-xcstrings: {name}: input is not canonically serialized; "
+            "falling back to the default driver so formatting is not rewritten",
+            file=sys.stderr,
+        )
+        return 1
+    merged, conflicts = merge_catalog(base, ours, theirs)
+    if conflicts:
+        print(
+            f"merge-xcstrings: {name}: {len(conflicts)} key(s) changed on both sides; "
+            "leaving them to the default driver: " + ", ".join(conflicts[:5]),
+            file=sys.stderr,
+        )
+        return 1
+    ours_path.write_text(render(merged), encoding="utf-8")
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main(sys.argv))
diff --git a/tests/test_merge_xcstrings.py b/tests/test_merge_xcstrings.py
new file mode 100644
index 000000000000..15c7bd773321
--- /dev/null
+++ b/tests/test_merge_xcstrings.py
@@ -0,0 +1,117 @@
+#!/usr/bin/env python3
+"""Contracts for the .xcstrings git merge driver.
+
+The driver must merge disjoint key additions (the common case) and must refuse
+to resolve a key that both sides changed differently, so a real disagreement
+still reaches the author as a normal git conflict.
+"""
+
+import json
+import subprocess
+import sys
+import tempfile
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parents[1]
+DRIVER = ROOT / "scripts" / "merge-xcstrings.py"
+
+
+def unit(value):
+    return {"localizations": {"en": {"stringUnit": {"state": "translated", "value": value}}}}
+
+
+def catalog(strings):
+    return {"sourceLanguage": "en", "strings": strings, "version": "1.0"}
+
+
+def render(document):
+    return json.dumps(document, ensure_ascii=False, indent=2) + "\n"
+
+
+def run(base, ours, theirs):
+    with tempfile.TemporaryDirectory() as directory:
+        paths = {}
+        for name, document in (("O", base), ("A", ours), ("B", theirs)):
+            path = Path(directory) / f"{name}.json"
+            path.write_text(document if isinstance(document, str) else render(document), encoding="utf-8")
+            paths[name] = path
+        result = subprocess.run(
+            [sys.executable, str(DRIVER), str(paths["O"]), str(paths["A"]), str(paths["B"]), "Localizable.xcstrings"],
+            capture_output=True,
+            text=True,
+        )
+        merged = paths["A"].read_text(encoding="utf-8")
+        return result.returncode, merged, result.stderr
+
+
+def test_disjoint_additions_merge():
+    base = catalog({"a": unit("A")})
+    ours = catalog({"a": unit("A"), "b": unit("B")})
+    theirs = catalog({"a": unit("A"), "c": unit("C")})
+    code, merged, _ = run(base, ours, theirs)
+    assert code == 0, "disjoint additions must merge"
+    strings = json.loads(merged)["strings"]
+    assert set(strings) == {"a", "b", "c"}, strings.keys()
+
+
+def test_same_key_same_value_is_not_a_conflict():
+    base = catalog({"a": unit("old")})
+    ours = catalog({"a": unit("new")})
+    theirs = catalog({"a": unit("new")})
+    code, merged, _ = run(base, ours, theirs)
+    assert code == 0
+    assert json.loads(merged)["strings"]["a"] == unit("new")
+
+
+def test_same_key_diverging_falls_back_to_git():
+    base = catalog({"a": unit("old")})
+    ours = catalog({"a": unit("ours")})
+    theirs = catalog({"a": unit("theirs")})
+    code, merged, stderr = run(base, ours, theirs)
+    assert code == 1, "a real disagreement must not be resolved silently"
+    assert "strings.a" in stderr, stderr
+    assert json.loads(merged)["strings"]["a"] == unit("ours"), "ours must be left untouched for git"
+
+
+def test_one_sided_delete_applies():
+    base = catalog({"a": unit("A"), "b": unit("B")})
+    ours = catalog({"a": unit("A"), "b": unit("B")})
+    theirs = catalog({"a": unit("A")})
+    code, merged, _ = run(base, ours, theirs)
+    assert code == 0
+    assert set(json.loads(merged)["strings"]) == {"a"}
+
+
+def test_delete_versus_modify_conflicts():
+    base = catalog({"a": unit("A")})
+    ours = catalog({"a": unit("changed")})
+    theirs = catalog({})
+    code, _, stderr = run(base, ours, theirs)
+    assert code == 1, stderr
+
+
+def test_non_canonical_input_falls_back():
+    base = catalog({"a": unit("A")})
+    ours = json.dumps(catalog({"a": unit("A"), "b": unit("B")}), indent=4)  # wrong indent
+    theirs = catalog({"a": unit("A"), "c": unit("C")})
+    code, _, stderr = run(base, ours, theirs)
+    assert code == 1, "must not rewrite a catalog it cannot reproduce byte-for-byte"
+    assert "canonically serialized" in stderr, stderr
+
+
+def test_unparseable_input_falls_back():
+    code, _, stderr = run(catalog({}), "{not json", catalog({}))
+    assert code == 1
+    assert "cannot parse" in stderr, stderr
+
+
+def main():
+    tests = [value for name, value in sorted(globals().items()) if name.startswith("test_")]
+    for test in tests:
+        test()
+        print(f"ok {test.__name__}")
+    print(f"\n{len(tests)} tests passed")
+
+
+if __name__ == "__main__":
+    main()