diff --git a/packages/testmap/src/testmap/cli.py b/packages/testmap/src/testmap/cli.py
index 4e9e3aa..bb04369 100644
--- a/packages/testmap/src/testmap/cli.py
+++ b/packages/testmap/src/testmap/cli.py
@@ -8,6 +8,7 @@
 from pathlib import Path
 
 from testmap.discover import discover
+from testmap.gitdiff import changed_files
 from testmap.report import build_report, load_config, render
 
 # The skill directory name; also the slash-command name (`/testmap-do`).
@@ -80,6 +81,15 @@ def main() -> None:
         default=Path("pyproject.toml"),
         help="pyproject.toml holding [tool.testmap] (default: ./pyproject.toml)",
     )
+    report.add_argument(
+        "--diff",
+        nargs="?",
+        const="HEAD",
+        default=None,
+        metavar="REF",
+        help="only gate generated features whose source changed since REF "
+        "(default: HEAD; requires the config directory to be a git repo)",
+    )
     args = parser.parse_args()
 
     if args.command == "install-skill":
@@ -90,10 +100,13 @@ def main() -> None:
 
     config = load_config(args.config)
     tests = _load_records(args.path, args.config)
-    result = build_report(tests, config)
+    changed = changed_files(args.diff, args.config.resolve().parent) if args.diff else None
+    result = build_report(tests, config, changed)
     print(json.dumps(result, indent=2) if args.json else render(result, config))
 
-    # Exit non-zero when any feature is missing a required kind, so `testmap
+    # Exit non-zero when a gated feature is missing a required kind, so `testmap
     # report` doubles as a coverage gate (CI, pre-commit) with no extra flag.
-    if any(not data["complete"] for data in result["features"].values()):
+    # Without --diff every feature is gated; with it, only features whose
+    # generated source changed count, so unrelated legacy gaps don't block a PR.
+    if any(data["gated"] and not data["complete"] for data in result["features"].values()):
         raise SystemExit(1)
diff --git a/packages/testmap/src/testmap/discover.py b/packages/testmap/src/testmap/discover.py
index 83f13d0..2e35fde 100644
--- a/packages/testmap/src/testmap/discover.py
+++ b/packages/testmap/src/testmap/discover.py
@@ -58,20 +58,22 @@ def _records_from_file(path: Path, root: Path) -> list[dict[str, str]]:
     return records
 
 
-def generate_features(generators: Iterable[dict[str, str]], root: Path) -> list[str]:
-    """Derive feature names from source by globbing files and reading their AST.
+def generate_feature_sources(generators: Iterable[dict[str, str]], root: Path) -> dict[str, str]:
+    """Derive features from source by globbing files and reading their AST.
 
     Each generator is `{select, from, where}`: `select` is "functions" or
     "classes", `from` is a glob (Linux rules, `**` allowed) resolved against
     `root`, and `where` filters by visibility ("public", "private", "all").
     Only top-level definitions count — the natural notion of a file's public API.
-    The feature name is the symbol name; the returned list is sorted and deduped.
+    Returns `{feature_name: source_file}`, the file given as a `root`-relative
+    posix path so it lines up with `git diff --name-only` output for
+    diff-scoped gating (the first file a duplicated name is found in wins).
     """
     node_types = {
         "functions": (ast.FunctionDef, ast.AsyncFunctionDef),
         "classes": (ast.ClassDef,),
     }
-    features: set[str] = set()
+    sources: dict[str, str] = {}
     for gen in generators:
         select, pattern = gen["select"], gen["from"]
         if select not in node_types:
@@ -93,8 +95,13 @@ def generate_features(generators: Iterable[dict[str, str]], root: Path) -> list[
                 private = node.name.startswith("_")
                 if (where == "public" and private) or (where == "private" and not private):
                     continue
-                features.add(node.name)
-    return sorted(features)
+                sources.setdefault(node.name, file.relative_to(root).as_posix())
+    return sources
+
+
+def generate_features(generators: Iterable[dict[str, str]], root: Path) -> list[str]:
+    """Feature names derived from source (see `generate_feature_sources`)."""
+    return sorted(generate_feature_sources(generators, root))
 
 
 def discover(paths: Iterable[Path], root: Path | None = None) -> list[dict[str, str]]:
diff --git a/packages/testmap/src/testmap/gitdiff.py b/packages/testmap/src/testmap/gitdiff.py
new file mode 100644
index 0000000..d5d24a6
--- /dev/null
+++ b/packages/testmap/src/testmap/gitdiff.py
@@ -0,0 +1,22 @@
+"""Changed-file discovery for `testmap report --diff`."""
+
+import subprocess
+from pathlib import Path
+
+
+def changed_files(against: str, root: Path) -> set[str]:
+    """Paths (root-relative, posix) changed since `against`, tracked + untracked.
+
+    Untracked files count as "changed" too, since a brand-new feature's source
+    has nothing to diff against yet but should still be in scope.
+    """
+
+    def git(*args: str) -> list[str]:
+        out = subprocess.run(
+            ["git", *args], cwd=root, capture_output=True, text=True, check=True
+        ).stdout
+        return [line for line in out.splitlines() if line]
+
+    tracked = git("diff", "--name-only", against)
+    untracked = git("ls-files", "--others", "--exclude-standard")
+    return {*tracked, *untracked}
diff --git a/packages/testmap/src/testmap/report.py b/packages/testmap/src/testmap/report.py
index c5adbeb..acbe49e 100644
--- a/packages/testmap/src/testmap/report.py
+++ b/packages/testmap/src/testmap/report.py
@@ -8,7 +8,7 @@
 from dataclasses import dataclass
 from pathlib import Path
 
-from testmap.discover import generate_features
+from testmap.discover import generate_feature_sources
 
 # The Status column symbols, used when [tool.testmap.statuses] is absent. At
 # least one of each state so the table renders without any config.
@@ -25,6 +25,7 @@ class Config:
     excludes: dict[str, list[str]]  # per-feature kinds dropped from requirements
     statuses: dict[str, str]  # Status-column symbol per state (complete/incomplete)
     generated: list[str]  # feature names derived from source via [tool.testmap.generate]
+    generated_sources: dict[str, str]  # generated feature -> its source file (for --diff)
 
     def excluded_for(self, feature: str) -> list[str]:
         return self.excludes.get(feature, [])
@@ -39,6 +40,15 @@ def required_for(self, feature: str) -> list[str]:
     def status_symbol(self, complete: bool) -> str:
         return self.statuses["complete" if complete else "incomplete"]
 
+    def in_diff_scope(self, feature: str, changed: set[str]) -> bool:
+        """Whether `feature`'s source changed, for `--diff` gating.
+
+        A feature with no known source (not from [tool.testmap.generate]) can't
+        be scoped, so it stays in scope rather than silently skipping its gate.
+        """
+        source = self.generated_sources.get(feature)
+        return source is None or source in changed
+
 
 def load_config(pyproject: Path) -> Config:
     """Load `[tool.testmap]` from a pyproject.toml.
@@ -88,7 +98,9 @@ def load_config(pyproject: Path) -> Config:
     # Generators derive the expected feature universe from source, anchored to
     # the pyproject's directory, so a feature with no tests at all still surfaces
     # (as missing every required kind) instead of being invisible.
-    generated = generate_features(data.get("generate", []), pyproject.resolve().parent)
+    generated_sources = generate_feature_sources(
+        data.get("generate", []), pyproject.resolve().parent
+    )
 
     return Config(
         kinds=kinds,
@@ -96,16 +108,23 @@ def load_config(pyproject: Path) -> Config:
         features=features,
         excludes=excludes,
         statuses=statuses,
-        generated=generated,
+        generated=sorted(generated_sources),
+        generated_sources=generated_sources,
     )
 
 
-def build_report(tests: list[dict[str, str]], config: Config) -> dict:
+def build_report(
+    tests: list[dict[str, str]], config: Config, changed: set[str] | None = None
+) -> dict:
     """Aggregate `{feature, kind}` records into the feature x kind matrix.
 
     Raises on a kind not declared in `config.kinds` (no silent fallback). Only
     features with at least one test appear; a feature is complete when none of
     its required kinds are missing.
+
+    `changed` is the `--diff` file set: when given, a feature is "gated" (counts
+    toward the exit-code gate) only if its source changed, so unrelated legacy
+    gaps don't block a PR that never touched them.
     """
     matrix: dict[str, dict[str, int]] = {}
     # Seed source-derived features first so those with zero tests still appear
@@ -128,6 +147,7 @@ def build_report(tests: list[dict[str, str]], config: Config) -> dict:
             "missing": missing,
             "excluded": excluded,
             "complete": not missing,
+            "gated": changed is None or config.in_diff_scope(feature, changed),
         }
     return {"features": features}
 
@@ -155,6 +175,7 @@ def fmt(cells: list[str]) -> str:
     missing = [
         f"  • {feature}: {kind}"
         for feature, data in report["features"].items()
+        if data["gated"]
         for kind in data["missing"]
     ]
     if missing:
diff --git a/packages/testmap/tests/test_cli.py b/packages/testmap/tests/test_cli.py
index 9dce977..755ac34 100644
--- a/packages/testmap/tests/test_cli.py
+++ b/packages/testmap/tests/test_cli.py
@@ -1,5 +1,6 @@
 import json
 import runpy
+import subprocess
 from pathlib import Path
 
 import pytest
@@ -73,6 +74,73 @@ def test_default_paths_missing_pyproject(tmp_path) -> None:
     assert _default_paths(tmp_path / "nope.toml") == [Path(".")]
 
 
+DIFF_PYPROJECT = (
+    "[tool.pytest.ini_options]\n"
+    'testpaths = ["tests"]\n'
+    "[tool.testmap]\n"
+    'kinds = ["unit", "integration"]\n'
+    'required = ["unit", "integration"]\n'
+    "[[tool.testmap.generate]]\n"
+    'select = "functions"\nfrom = "src/**/*.py"\nwhere = "public"\n'
+)
+
+DIFF_SUITE = (
+    "from pytest_testmap import testmap\n\n"
+    '@testmap(feature="parse", kind="unit")\n'
+    "def test_a(): ...\n\n"
+    '@testmap(feature="parse", kind="integration")\n'
+    "def test_b(): ...\n"
+)
+
+
+@pytest.fixture
+def diff_project(tmp_path, monkeypatch):
+    # `render` is left with zero tests, so it's incomplete from the first commit
+    # onward — a "legacy gap" --diff should ignore once its source is untouched.
+    (tmp_path / "pyproject.toml").write_text(DIFF_PYPROJECT)
+    (tmp_path / "src").mkdir()
+    (tmp_path / "src" / "mod.py").write_text("def parse(): ...\ndef render(): ...\n")
+    tests = tmp_path / "tests"
+    tests.mkdir()
+    (tests / "test_suite.py").write_text(DIFF_SUITE)
+
+    def git(*args: str) -> None:
+        subprocess.run(["git", *args], cwd=tmp_path, check=True, capture_output=True)
+
+    git("init")
+    git("config", "user.email", "test@example.com")
+    git("config", "user.name", "Test")
+    git("add", "-A")
+    git("commit", "-m", "initial")
+
+    monkeypatch.chdir(tmp_path)
+    return tmp_path
+
+
+def test_report_diff_ignores_unchanged_incomplete_feature(diff_project, monkeypatch) -> None:
+    # render is incomplete but untouched since HEAD -> --diff lets the gate pass.
+    monkeypatch.setattr("sys.argv", ["testmap", "report", "--diff"])
+    main()  # no SystemExit -> exit code 0
+
+
+def test_report_without_diff_still_gates_everything(diff_project, monkeypatch) -> None:
+    monkeypatch.setattr("sys.argv", ["testmap", "report"])
+    with pytest.raises(SystemExit) as exc:
+        main()
+    assert exc.value.code == 1
+
+
+def test_report_diff_gates_changed_source(diff_project, monkeypatch) -> None:
+    # Touching mod.py brings render back into scope for --diff.
+    (diff_project / "src" / "mod.py").write_text(
+        "def parse(): ...\ndef render(): ...\ndef validate(): ...\n"
+    )
+    monkeypatch.setattr("sys.argv", ["testmap", "report", "--diff"])
+    with pytest.raises(SystemExit) as exc:
+        main()
+    assert exc.value.code == 1
+
+
 def test_report_ingests_json_file(project, capsys, monkeypatch) -> None:
     records = {"tests": [{"feature": "parser", "kind": "integration"}]}
     (project / "in.json").write_text(json.dumps(records))
diff --git a/packages/testmap/tests/test_discover.py b/packages/testmap/tests/test_discover.py
index 5b33557..c72f532 100644
--- a/packages/testmap/tests/test_discover.py
+++ b/packages/testmap/tests/test_discover.py
@@ -1,5 +1,5 @@
 import pytest
-from testmap.discover import discover, generate_features
+from testmap.discover import discover, generate_feature_sources, generate_features
 
 SOURCE = """
 from pytest_testmap import testmap
@@ -108,3 +108,12 @@ def test_generate_features_globs_recursively_and_dedupes(tmp_path) -> None:
 def test_generate_features_rejects_unknown_select(tmp_path) -> None:
     with pytest.raises(ValueError, match="unknown select"):
         generate_features([{"select": "modules", "from": "*.py"}], tmp_path)
+
+
+def test_generate_feature_sources_maps_feature_to_file(tmp_path) -> None:
+    (tmp_path / "pkg").mkdir()
+    (tmp_path / "pkg" / "mod.py").write_text("def parse(): ...\n")
+
+    sources = generate_feature_sources([{"select": "functions", "from": "**/*.py"}], tmp_path)
+
+    assert sources == {"parse": "pkg/mod.py"}
diff --git a/packages/testmap/tests/test_report.py b/packages/testmap/tests/test_report.py
index e0bbfba..291d3a4 100644
--- a/packages/testmap/tests/test_report.py
+++ b/packages/testmap/tests/test_report.py
@@ -8,6 +8,7 @@
     excludes={},
     statuses={"complete": "✓", "incomplete": "✗"},
     generated=[],
+    generated_sources={},
 )
 
 TESTS = [
@@ -115,6 +116,7 @@ def test_exclude_makes_feature_complete_and_renders_na() -> None:
         excludes={"auth": ["perf"]},
         statuses={"complete": "✓", "incomplete": "✗"},
         generated=[],
+        generated_sources={},
     )
     tests = [{"feature": "auth", "kind": "unit"}, {"feature": "auth", "kind": "integration"}]
     report = build_report(tests, config)
@@ -183,7 +185,43 @@ def test_load_config_generates_features_from_source(tmp_path) -> None:
     )
     config = load_config(pyproject)
     assert config.generated == ["parse"]
+    assert config.generated_sources == {"parse": "src/mod.py"}
     # A generated feature with no tests surfaces as an all-missing row.
     report = build_report([], config)["features"]
     assert report["parse"]["missing"] == ["unit"]
     assert report["parse"]["complete"] is False
+
+
+def test_in_diff_scope_gates_only_changed_generated_features() -> None:
+    config = Config(
+        kinds=["unit"],
+        required=["unit"],
+        features={},
+        excludes={},
+        statuses={"complete": "✓", "incomplete": "✗"},
+        generated=["parse"],
+        generated_sources={"parse": "src/mod.py"},
+    )
+    assert config.in_diff_scope("parse", {"src/mod.py"}) is True
+    assert config.in_diff_scope("parse", {"src/other.py"}) is False
+    # A feature with no known source (not from [tool.testmap.generate]) can't
+    # be scoped, so it always stays in scope.
+    assert config.in_diff_scope("manual", set()) is True
+
+
+def test_build_report_marks_ungated_features_when_diff_scoped() -> None:
+    config = Config(
+        kinds=["unit"],
+        required=["unit"],
+        features={},
+        excludes={},
+        statuses={"complete": "✓", "incomplete": "✗"},
+        generated=["parse", "render"],
+        generated_sources={"parse": "src/a.py", "render": "src/b.py"},
+    )
+    report = build_report([], config, changed={"src/a.py"})["features"]
+    assert report["parse"]["gated"] is True
+    assert report["render"]["gated"] is False
+    # render is still shown as missing in the table, just excluded from the
+    # Missing section / exit-code gate since its source wasn't touched.
+    assert "render: unit" not in render({"features": report}, config)