diff --git a/src/git_commit_guard/__init__.py b/src/git_commit_guard/__init__.py
index df7b7f2..05a9975 100644
--- a/src/git_commit_guard/__init__.py
+++ b/src/git_commit_guard/__init__.py
@@ -411,6 +411,8 @@ def _resolve_github_username(rev, email):
         except urllib.error.HTTPError as e:
             if e.code == HTTPStatus.NOT_FOUND:
                 commits_api_404 = True
+            elif e.code in (HTTPStatus.UNAUTHORIZED, HTTPStatus.FORBIDDEN):
+                raise
         except (urllib.error.URLError, TimeoutError):
             pass
     if username is None:
@@ -454,6 +456,24 @@ def check_signature(rev, result):
             "git operation timed out — cannot verify signature",
             check=Check.SIGNATURE,
         )
+    except urllib.error.HTTPError as e:
+        if e.code == HTTPStatus.UNAUTHORIZED:
+            result.error(
+                "GitHub API rejected token (HTTP 401) — "
+                "GITHUB_TOKEN may be invalid or expired",
+                check=Check.SIGNATURE,
+            )
+        elif e.code == HTTPStatus.FORBIDDEN:
+            result.error(
+                "GitHub API forbidden (HTTP 403) — GITHUB_TOKEN may lack "
+                "'repo' scope, or you are unauthenticated and rate-limited",
+                check=Check.SIGNATURE,
+            )
+        else:
+            result.error(
+                f"GitHub API error (HTTP {e.code}) — cannot verify signature",
+                check=Check.SIGNATURE,
+            )
     except (urllib.error.URLError, TimeoutError):
         result.error(
             "GitHub API unreachable — cannot verify signature",
diff --git a/tests/test_git_commit_guard.py b/tests/test_git_commit_guard.py
index d58046f..0c96f9e 100644
--- a/tests/test_git_commit_guard.py
+++ b/tests/test_git_commit_guard.py
@@ -929,6 +929,84 @@ def test_commits_api_non_404_http_error_falls_through(self):
             check_signature("abc123", r)
         assert r.ok
 
+    def test_commits_api_401_surfaces_token_message(self):
+        r = Result()
+        with (
+            patch(
+                "git_commit_guard._get_author_email", return_value="user@example.com"
+            ),
+            patch(
+                "git_commit_guard._get_github_remote_info",
+                return_value=("owner", "repo"),
+            ),
+            patch(
+                "git_commit_guard._fetch_github_commit_author",
+                side_effect=urllib.error.HTTPError(
+                    url="", code=401, msg="Unauthorized", hdrs=None, fp=None
+                ),
+            ),
+        ):
+            check_signature("abc123", r)
+        assert not r.ok
+        assert any("rejected token (HTTP 401)" in msg for _, _, msg in r.errors)
+
+    def test_commits_api_403_surfaces_token_message(self):
+        r = Result()
+        with (
+            patch(
+                "git_commit_guard._get_author_email", return_value="user@example.com"
+            ),
+            patch(
+                "git_commit_guard._get_github_remote_info",
+                return_value=("owner", "repo"),
+            ),
+            patch(
+                "git_commit_guard._fetch_github_commit_author",
+                side_effect=urllib.error.HTTPError(
+                    url="", code=403, msg="Forbidden", hdrs=None, fp=None
+                ),
+            ),
+        ):
+            check_signature("abc123", r)
+        assert not r.ok
+        assert any("forbidden (HTTP 403)" in msg for _, _, msg in r.errors)
+
+    def test_search_api_403_surfaces_rate_limit_message(self):
+        r = Result()
+        with (
+            patch(
+                "git_commit_guard._get_author_email", return_value="corp@example.com"
+            ),
+            patch("git_commit_guard._get_github_remote_info", return_value=None),
+            patch(
+                "git_commit_guard._fetch_github_username",
+                side_effect=urllib.error.HTTPError(
+                    url="", code=403, msg="Forbidden", hdrs=None, fp=None
+                ),
+            ),
+        ):
+            check_signature("abc123", r)
+        assert not r.ok
+        assert any("forbidden (HTTP 403)" in msg for _, _, msg in r.errors)
+
+    def test_other_http_error_uses_generic_http_message(self):
+        r = Result()
+        with (
+            patch(
+                "git_commit_guard._get_author_email", return_value="corp@example.com"
+            ),
+            patch("git_commit_guard._get_github_remote_info", return_value=None),
+            patch(
+                "git_commit_guard._fetch_github_username",
+                side_effect=urllib.error.HTTPError(
+                    url="", code=500, msg="Server Error", hdrs=None, fp=None
+                ),
+            ),
+        ):
+            check_signature("abc123", r)
+        assert not r.ok
+        assert any("GitHub API error (HTTP 500)" in msg for _, _, msg in r.errors)
+
     def test_url_error_fails(self):
         r = Result()
         with patch(