diff --git a/okf/src/reference_agent/viewer/generator.py b/okf/src/reference_agent/viewer/generator.py
index 86aae8b..62197f2 100644
--- a/okf/src/reference_agent/viewer/generator.py
+++ b/okf/src/reference_agent/viewer/generator.py
@@ -6,7 +6,7 @@
 from pathlib import Path
 from typing import Any
 
-from reference_agent.bundle.document import OKFDocument, OKFDocumentError
+from reference_agent.bundle.document import OKFDocument
 
 _INDEX_NAME = "index.md"
 _LINK_RE = re.compile(r"\]\(([^)\s]+\.md)(?:#[A-Za-z0-9_\-]*)?\)")
@@ -75,7 +75,14 @@ def _walk_concepts(bundle_root: Path) -> list[Concept]:
         concept_id = "/".join(rel.parts)
         try:
             doc = OKFDocument.parse(md_path.read_text(encoding="utf-8"))
-        except OKFDocumentError:
+        except (ValueError, OSError):
+            # Skip a single malformed/unreadable file rather than aborting the
+            # whole visualization. ValueError covers OKFDocument parse failures
+            # (OKFDocumentError) and invalid UTF-8 (UnicodeDecodeError) — both
+            # ValueError subclasses — plus PyYAML's *bare* ValueError for an
+            # out-of-range frontmatter value such as a bad timestamp; OSError
+            # covers read errors. bundle.index._load_doc tolerates the same
+            # files via a broader `except Exception`.
             continue
         fm = doc.frontmatter or {}
         tags = fm.get("tags") or []
diff --git a/okf/tests/test_viewer.py b/okf/tests/test_viewer.py
index cdb0b18..9fa626e 100644
--- a/okf/tests/test_viewer.py
+++ b/okf/tests/test_viewer.py
@@ -160,6 +160,40 @@ def test_node_colors_match_palette(tmp_path: Path):
     assert by_id["references/metrics/dau"]["color"] == "#10b981"
 
 
+def test_unreadable_concept_file_is_skipped(tmp_path: Path):
+    # A single file with invalid UTF-8 bytes must not abort the whole
+    # visualization — it should be skipped like a malformed-frontmatter file.
+    bundle = tmp_path / "bundle"
+    _make_bundle(bundle)
+    corrupt = bundle / "tables" / "corrupt.md"
+    corrupt.parent.mkdir(parents=True, exist_ok=True)
+    corrupt.write_bytes(b"---\ntype: BigQuery Table\n\xff\xfe not utf-8\n---\n")
+    out = tmp_path / "viz.html"
+    stats = generate_visualization(bundle, out)
+    assert out.exists()
+    # The 4 well-formed concepts still render; the corrupt file is skipped.
+    assert stats["concepts"] == 4
+
+
+def test_malformed_timestamp_concept_file_is_skipped(tmp_path: Path):
+    # A valid-UTF-8 file whose frontmatter has an out-of-range timestamp makes
+    # PyYAML raise a *bare* ValueError (not an OKFDocumentError); it must be
+    # skipped like any other malformed file, not abort the whole viz.
+    bundle = tmp_path / "bundle"
+    _make_bundle(bundle)
+    bad = bundle / "tables" / "badts.md"
+    bad.parent.mkdir(parents=True, exist_ok=True)
+    bad.write_text(
+        "---\ntype: BigQuery Table\ntimestamp: 2026-13-45\n---\nbody\n",
+        encoding="utf-8",
+    )
+    out = tmp_path / "viz.html"
+    stats = generate_visualization(bundle, out)
+    assert out.exists()
+    # The 4 well-formed concepts still render; the bad-timestamp file is skipped.
+    assert stats["concepts"] == 4
+
+
 def test_raises_when_bundle_missing(tmp_path: Path):
     with pytest.raises(FileNotFoundError):
         generate_visualization(tmp_path / "nope", tmp_path / "viz.html")