diff --git a/docs/changelog.d/999-report-title-callback-safety.md b/docs/changelog.d/999-report-title-callback-safety.md
new file mode 100644
index 000000000..42e18ba40
--- /dev/null
+++ b/docs/changelog.d/999-report-title-callback-safety.md
@@ -0,0 +1,5 @@
+# Harden generic diagnostics report title callback boundary
+
+## Fixed
+
+- Reject caller-defined `str` subclasses at the public generic diagnostics-report title boundary before truth-value or HTML-escaping callbacks can run, while preserving `None` and an empty exact built-in string as requests for the report-type default title.
diff --git a/python/fast_mlsirm/report.py b/python/fast_mlsirm/report.py
index b8627f83d..450123245 100644
--- a/python/fast_mlsirm/report.py
+++ b/python/fast_mlsirm/report.py
@@ -38,7 +38,9 @@ def render_diagnostics_report(
         raise ValueError("diagnostics JSON must contain an object")
 
     report_type = _diagnostics_type(payload)
-    resolved_title = title or _default_title(report_type)
+    if title is not None and type(title) is not str:
+        raise ValueError("title must be a string or None")
+    resolved_title = title if title else _default_title(report_type)
 
     out = Path(output_path)
     if out.suffix.lower() != ".html":
@@ -217,9 +219,7 @@ def _render_dimensionality_report(payload: dict[str, Any]) -> list[str]:
 def _metric_section(heading: str, metrics: dict[str, Any]) -> str | None:
     """Render a labelled metric-card grid, or ``None`` when there are no metrics."""
     if len(metrics) > MAX_REPORT_METRICS:
-        raise ValueError(
-            f"report metrics exceed the {MAX_REPORT_METRICS}-entry limit"
-        )
+        raise ValueError(f"report metrics exceed the {MAX_REPORT_METRICS}-entry limit")
 
     cards = []
     for key, value in metrics.items():
@@ -443,13 +443,9 @@ def _rows_from_columnar(section: Any) -> list[dict[str, Any]]:
     if row_count == 0:
         return []
     if row_count > MAX_REPORT_ROWS:
-        raise ValueError(
-            f"report table rows exceed the {MAX_REPORT_ROWS}-row limit"
-        )
+        raise ValueError(f"report table rows exceed the {MAX_REPORT_ROWS}-row limit")
     if row_count * len(columns) > MAX_REPORT_CELLS:
-        raise ValueError(
-            f"report table cells exceed the {MAX_REPORT_CELLS}-cell limit"
-        )
+        raise ValueError(f"report table cells exceed the {MAX_REPORT_CELLS}-cell limit")
 
     rows = []
     for index in range(row_count):
@@ -463,9 +459,7 @@ def _rows_from_columnar(section: Any) -> list[dict[str, Any]]:
 def _validate_report_rows(rows: list[dict[str, Any]]) -> None:
     """Reject row-oriented diagnostics that exceed report rendering budgets."""
     if len(rows) > MAX_REPORT_ROWS:
-        raise ValueError(
-            f"report table rows exceed the {MAX_REPORT_ROWS}-row limit"
-        )
+        raise ValueError(f"report table rows exceed the {MAX_REPORT_ROWS}-row limit")
 
     columns: set[str] = set()
     cell_count = 0
@@ -646,7 +640,7 @@ def _css() -> str:
   padding: 32px 20px 48px;
 }
 
-main:focus {
+main:focus:not(:focus-visible) {
   outline: none;
 }
 
@@ -836,7 +830,7 @@ def _css() -> str:
   border-radius: 8px;
 }
 
-.table-wrap:focus {
+.table-wrap:focus:not(:focus-visible) {
   outline: none;
 }
 
@@ -925,6 +919,11 @@ def _css() -> str:
   color: var(--teal);
 }
 
+.exact-values > summary:focus:not(:focus-visible),
+.export-block > summary:focus:not(:focus-visible) {
+  outline: none;
+}
+
 .exact-values > summary:focus-visible,
 .export-block > summary:focus-visible {
   outline: 3px solid var(--teal);
@@ -951,7 +950,7 @@ def _css() -> str:
   white-space: pre;
 }
 
-.export-block pre:focus {
+.export-block pre:focus:not(:focus-visible) {
   outline: none;
 }
 
diff --git a/tests/test_report.py b/tests/test_report.py
index 0fee77e2b..1727a21b3 100644
--- a/tests/test_report.py
+++ b/tests/test_report.py
@@ -7,6 +7,7 @@
 
 
 def test_render_fit_diagnostics_report_has_sections(tmp_path):
+    """Render populated fit diagnostics with the expected report sections."""
     source = tmp_path / "fit_diagnostics.json"
     out = tmp_path / "report.html"
     source.write_text(
@@ -48,6 +49,7 @@ def test_render_fit_diagnostics_report_has_sections(tmp_path):
 
 
 def test_render_dimensionality_report_has_best_candidate(tmp_path):
+    """Render dimensionality diagnostics with the selected candidate."""
     source = tmp_path / "dimension_diagnostics.json"
     out = tmp_path / "dimensions.html"
     source.write_text(
@@ -74,6 +76,7 @@ def test_render_dimensionality_report_has_best_candidate(tmp_path):
 
 
 def test_render_dimensionality_report_summarizes_empty_candidates(tmp_path):
+    """Summarize dimensionality diagnostics when no candidates are available."""
     source = tmp_path / "dimension_diagnostics.json"
     out = tmp_path / "dimensions.html"
     source.write_text(
@@ -97,6 +100,7 @@ def test_render_dimensionality_report_summarizes_empty_candidates(tmp_path):
 
 
 def test_render_report_summarizes_empty_metric_sections(tmp_path):
+    """Summarize an empty metric section without rendering an empty panel."""
     source = tmp_path / "fit_diagnostics.json"
     out = tmp_path / "report.html"
     source.write_text(
@@ -123,6 +127,7 @@ def test_render_report_summarizes_empty_metric_sections(tmp_path):
 
 
 def test_render_dimensionality_report_summarizes_empty_best_candidate(tmp_path):
+    """Summarize a dimensionality report whose best candidate has no metrics."""
     source = tmp_path / "dimension_diagnostics.json"
     out = tmp_path / "dimensions.html"
     source.write_text(
@@ -150,6 +155,7 @@ def test_render_dimensionality_report_summarizes_empty_best_candidate(tmp_path):
 
 
 def test_render_coverage_omits_empty_rendered_tables_column(tmp_path):
+    """Omit the rendered-table coverage column when no table is present."""
     source = tmp_path / "dimension_diagnostics.json"
     out = tmp_path / "dimensions.html"
     source.write_text(
@@ -173,6 +179,7 @@ def test_render_coverage_omits_empty_rendered_tables_column(tmp_path):
 
 
 def test_render_table_section_omits_empty_chart_placeholder(tmp_path):
+    """Avoid an empty chart placeholder when rows have no numeric values."""
     source = tmp_path / "fit_diagnostics.json"
     out = tmp_path / "report.html"
     source.write_text(
@@ -194,6 +201,7 @@ def test_render_table_section_omits_empty_chart_placeholder(tmp_path):
 
 
 def test_render_table_section_charts_later_numeric_rows(tmp_path):
+    """Include a later numeric row when earlier rows are not chartable."""
     source = tmp_path / "fit_diagnostics.json"
     out = tmp_path / "report.html"
     item_ids = list(range(13))
@@ -217,6 +225,7 @@ def test_render_table_section_charts_later_numeric_rows(tmp_path):
 
 
 def test_render_report_rejects_unknown_payload(tmp_path):
+    """Reject diagnostics JSON that has no supported report payload."""
     source = tmp_path / "unknown.json"
     out = tmp_path / "report.html"
     source.write_text(json.dumps({"status": "ok"}), encoding="utf-8")
@@ -226,6 +235,7 @@ def test_render_report_rejects_unknown_payload(tmp_path):
 
 
 def test_render_report_rejects_excessive_json_nesting(tmp_path):
+    """Reject JSON that exceeds the configured nesting safety limit."""
     source = tmp_path / "deep.json"
     out = tmp_path / "report.html"
     depth = MAX_JSON_NESTING_DEPTH + 1
@@ -238,6 +248,7 @@ def test_render_report_rejects_excessive_json_nesting(tmp_path):
 
 
 def test_render_report_requires_html_output(tmp_path):
+    """Reject output paths that do not use the HTML extension."""
     source = tmp_path / "fit_diagnostics.json"
     out = tmp_path / "report.txt"
     source.write_text(json.dumps({"model_fit": {}}), encoding="utf-8")
@@ -247,6 +258,7 @@ def test_render_report_requires_html_output(tmp_path):
 
 
 def test_render_table_region_has_keyboard_focus_style(tmp_path):
+    """Keep keyboard focus styling while suppressing mouse-click outlines."""
     source = tmp_path / "dimension_diagnostics.json"
     out = tmp_path / "dimensions.html"
     source.write_text(
@@ -265,7 +277,7 @@ def test_render_table_region_has_keyboard_focus_style(tmp_path):
     assert 'aria-label="Candidate Comparison diagnostics table"' in html
     assert 'tabindex="0"' in html
     assert ".table-wrap:focus-visible" in html
-    assert ".table-wrap:focus {" in html
+    assert ".table-wrap:focus:not(:focus-visible) {" in html
     assert "tbody tr:hover" in html
     assert '<div class="bar-chart" aria-hidden="true">' in html
     assert '<div class="bar-track" aria-hidden="true">' in html
diff --git a/tests/test_report_focus_contrast.py b/tests/test_report_focus_contrast.py
index 12a32bfb0..daec4c0bb 100644
--- a/tests/test_report_focus_contrast.py
+++ b/tests/test_report_focus_contrast.py
@@ -50,7 +50,9 @@ def test_hover_does_not_dim_unrelated_chart_or_table_content(tmp_path: Path) ->
 
 
 def test_focus_containers_suppress_mouse_click_outlines(tmp_path: Path) -> None:
-    """Semantic focus containers must suppress mouse click outlines."""
+    """Pointer-focus suppression must preserve keyboard-visible focus fallback."""
     html = _render_report(tmp_path)
-    assert ".table-wrap:focus {\n  outline: none;\n}" in html
-    assert ".export-block pre:focus {\n  outline: none;\n}" in html
+    assert ".table-wrap:focus:not(:focus-visible) {\n  outline: none;\n}" in html
+    assert ".export-block pre:focus:not(:focus-visible) {\n  outline: none;\n}" in html
+    assert ".table-wrap:focus {\n  outline: none;\n}" not in html
+    assert ".export-block pre:focus {\n  outline: none;\n}" not in html
diff --git a/tests/test_report_numeric_motion_css.py b/tests/test_report_numeric_motion_css.py
index fff0f1523..be7fca7e4 100644
--- a/tests/test_report_numeric_motion_css.py
+++ b/tests/test_report_numeric_motion_css.py
@@ -61,3 +61,35 @@ def test_rendered_report_uses_tabular_numerals_without_opacity_transitions(tmp_p
     reduced_motion = style.split("@media (prefers-reduced-motion: reduce)", 1)[1]
     reduced_motion = reduced_motion.split("@media (max-width: 720px)", 1)[0]
     assert "transition-duration: 0.01ms !important;" in reduced_motion
+
+
+def test_rendered_report_preserves_keyboard_focus_on_main(tmp_path) -> None:
+    """Pointer focus may be quiet, but keyboard focus on main must stay visible."""
+    source = tmp_path / "fit_diagnostics.json"
+    output = tmp_path / "report.html"
+    source.write_text(
+        json.dumps(
+            {
+                "model_fit": {"loglik": -3.2, "deviance": 6.4},
+                "itemfit": {
+                    "item_id": ["A", "B"],
+                    "outfit_mnsq": [1.0, 1.2],
+                    "observed_count": [4, 4],
+                },
+            }
+        ),
+        encoding="utf-8",
+    )
+
+    render_diagnostics_report(source, output)
+
+    html = output.read_text(encoding="utf-8")
+    style = html.split("<style>", 1)[1].split("</style>", 1)[0]
+
+    pointer_rule = _rule_body(style, "main:focus:not(:focus-visible)")
+    assert "outline: none;" in pointer_rule
+    keyboard_rule = _rule_body(style, "main:focus-visible")
+    assert "outline: 3px solid var(--teal);" in keyboard_rule
+    assert "outline-offset: 3px;" in keyboard_rule
+    assert "outline: none;" not in keyboard_rule
+    assert "\nmain:focus {" not in style
diff --git a/tests/test_report_summary_focus.py b/tests/test_report_summary_focus.py
new file mode 100644
index 000000000..150383361
--- /dev/null
+++ b/tests/test_report_summary_focus.py
@@ -0,0 +1,64 @@
+"""Pointer-versus-keyboard focus regressions for diagnostics disclosures."""
+
+from __future__ import annotations
+
+import json
+from pathlib import Path
+
+from fast_mlsirm.report import render_diagnostics_report
+
+
+def _render_report(tmp_path: Path) -> str:
+    """Render one report containing exact-value and export disclosures."""
+    source = tmp_path / "diagnostics.json"
+    output = tmp_path / "diagnostics.html"
+    source.write_text(
+        json.dumps(
+            {
+                "model_fit": {"loglik": -3.2},
+                "itemfit": {
+                    "item_id": ["item_alpha"],
+                    "outfit_mnsq": [1.0],
+                    "observed_count": [120],
+                },
+            }
+        ),
+        encoding="utf-8",
+    )
+    render_diagnostics_report(source, output, title="Disclosure Focus Review")
+    return output.read_text(encoding="utf-8")
+
+
+def test_disclosure_summary_suppresses_pointer_focus_without_blanket_focus_rule(
+    tmp_path: Path,
+) -> None:
+    """Pointer focus is suppressed without erasing generic keyboard focus."""
+    html = _render_report(tmp_path)
+    selector = (
+        ".exact-values > summary:focus:not(:focus-visible),\n"
+        ".export-block > summary:focus:not(:focus-visible) {"
+    )
+
+    assert html.count(selector) == 1
+    assert ".exact-values > summary:focus,\n.export-block > summary:focus {" not in html
+    rule = html.split(selector, maxsplit=1)[1].split("}", maxsplit=1)[0]
+    assert "outline: none;" in rule
+
+
+def test_disclosure_summary_retains_keyboard_focus_indicator(tmp_path: Path) -> None:
+    """The pointer rule must not erase the explicit keyboard focus treatment."""
+    html = _render_report(tmp_path)
+    pointer_selector = (
+        ".exact-values > summary:focus:not(:focus-visible),\n"
+        ".export-block > summary:focus:not(:focus-visible) {"
+    )
+    keyboard_selector = (
+        ".exact-values > summary:focus-visible,\n"
+        ".export-block > summary:focus-visible {"
+    )
+
+    assert html.count(keyboard_selector) == 1
+    assert html.index(pointer_selector) < html.index(keyboard_selector)
+    rule = html.split(keyboard_selector, maxsplit=1)[1].split("}", maxsplit=1)[0]
+    assert "outline: 3px solid var(--teal);" in rule
+    assert "outline-offset: 2px;" in rule
diff --git a/tests/test_report_title_callback_safety.py b/tests/test_report_title_callback_safety.py
new file mode 100644
index 000000000..e75b20ac1
--- /dev/null
+++ b/tests/test_report_title_callback_safety.py
@@ -0,0 +1,71 @@
+"""Regression tests for the generic diagnostics-report title trust boundary."""
+
+from __future__ import annotations
+
+import json
+
+import pytest
+
+from fast_mlsirm.report import render_diagnostics_report
+
+
+class _HostileTitle(str):
+    """String subclass whose title callbacks must never be dispatched."""
+
+    calls = 0
+
+    def __bool__(self) -> bool:
+        """Record forbidden truth-value dispatch during title resolution."""
+        type(self).calls += 1
+        raise RuntimeError("private title truth callback")
+
+    def replace(self, *args: object, **kwargs: object) -> str:
+        """Record forbidden replacement dispatch during HTML escaping."""
+        type(self).calls += 1
+        raise RuntimeError("private title replacement callback")
+
+
+def _write_fit_payload(path) -> None:
+    """Write one minimal valid fit-diagnostics payload."""
+    path.write_text(json.dumps({"model_fit": {"loglik": -1.0}}), encoding="utf-8")
+
+
+def test_generic_report_rejects_title_subclass_without_callbacks(tmp_path) -> None:
+    """A caller string subclass cannot execute while a report title is admitted."""
+    source = tmp_path / "fit.json"
+    output = tmp_path / "fit.html"
+    _write_fit_payload(source)
+    _HostileTitle.calls = 0
+
+    with pytest.raises(ValueError, match="title must be a string or None"):
+        render_diagnostics_report(source, output, title=_HostileTitle("Private title"))
+
+    assert _HostileTitle.calls == 0
+    assert not output.exists()
+
+
+def test_generic_report_empty_builtin_title_keeps_default(tmp_path) -> None:
+    """An empty exact built-in title preserves the existing default-title contract."""
+    source = tmp_path / "fit.json"
+    output = tmp_path / "fit.html"
+    _write_fit_payload(source)
+
+    render_diagnostics_report(source, output, title="")
+
+    html = output.read_text(encoding="utf-8")
+    assert "<title>Fit Diagnostics Report</title>" in html
+    assert '<h1 id="hero-heading">Fit Diagnostics Report</h1>' in html
+
+
+def test_generic_report_builtin_title_remains_html_escaped(tmp_path) -> None:
+    """An admitted exact built-in title remains escaped in every HTML title surface."""
+    source = tmp_path / "fit.json"
+    output = tmp_path / "fit.html"
+    _write_fit_payload(source)
+
+    render_diagnostics_report(source, output, title='<Admin & "Ops">')
+
+    html = output.read_text(encoding="utf-8")
+    assert "<title>&lt;Admin &amp; &quot;Ops&quot;&gt;</title>" in html
+    assert '<h1 id="hero-heading">&lt;Admin &amp; &quot;Ops&quot;&gt;</h1>' in html
+    assert '<Admin & "Ops">' not in html