diff --git a/.github/skills/add-disc-fixture/SKILL.md b/.github/skills/add-disc-fixture/SKILL.md
index 79258b4..b69ba5a 100644
--- a/.github/skills/add-disc-fixture/SKILL.md
+++ b/.github/skills/add-disc-fixture/SKILL.md
@@ -70,6 +70,11 @@ If counts don't match → proceed to step 4 (debug).
 See [debugging guide](./references/debug-analysis.md) for systematic
 investigation of episode/special count mismatches.
 
+**Important**: When fixing mismatches, prefer structural signals over
+numeric thresholds.  Study chapter durations, IG menu structure, and
+navigation data across multiple fixtures — don't just look at the one
+that broke.  The debugging guide's "How to Fix" section has details.
+
 ### 5. Extract ICS Menu Data
 
 Find the menu clip (usually a short m2ts with IG streams, often clip 00003
diff --git a/.github/skills/add-disc-fixture/references/debug-analysis.md b/.github/skills/add-disc-fixture/references/debug-analysis.md
index e19652e..815b96c 100644
--- a/.github/skills/add-disc-fixture/references/debug-analysis.md
+++ b/.github/skills/add-disc-fixture/references/debug-analysis.md
@@ -121,3 +121,44 @@ for h in sorted(ig_raw, key=lambda x: (x.page_id, x.button_id)):
 - **Multi-feature playlists** with register-based chapter selection
   (SET reg2 before JumpTitle) are supported, but only when `imm_op2=True`
   (immediate value). Register-indirect chapter indices are not resolved.
+
+## How to Fix Mismatches — Structural Signals, Not Thresholds
+
+When analysis returns wrong counts, resist the urge to add a numeric
+threshold or ratio guard that fixes the immediate disc.  Thresholds are
+"just happens to work" — they break on the next disc.
+
+### The right process
+
+1. **Dump data across fixtures** — compare the failing disc against
+   fixtures that work.  Key data to examine:
+   - Chapter durations (look for repeating OP/body/ED cycles)
+   - IG menu buttons per page (episode pages ~5 buttons, scene grids ~10)
+   - IG chapter marks (JT + reg2 patterns)
+   - Segment labels, play item structure, title counts
+
+2. **Find a structural signal** — something the disc data says about
+   itself.  Ask: "What makes the working discs structurally different
+   from the failing disc?"
+
+3. **Require positive evidence** — the code should ask "does the data
+   say this IS an episode compilation?" not "does the data say this is
+   NOT a movie?".  Positive detection produces zero false positives
+   when the signal is absent.
+
+4. **Validate across ALL fixtures** — run the new logic against every
+   fixture, not just the one that broke.
+
+### Anti-patterns to avoid
+
+- `if count <= N: return []` — arbitrary threshold, will break
+- `if ratio > X: return []` — same problem
+- Lowering/raising an existing threshold to accommodate one more disc
+- Any fix that only looks at the failing disc without comparing others
+
+### Example: Chapter-split detection
+
+Bad (threshold): `if chapters_per_episode > 7: don't split`
+Good (structural): detect repeating OP/body/ED chapter cycle via
+`_detect_episode_periodicity()` — only split when positive evidence
+of episode structure exists.
diff --git a/AGENTS.md b/AGENTS.md
index e39bafc..6ce865a 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -175,9 +175,34 @@ Output includes: `schema_version`, `disc`, `playlists`, `episodes`, `special_fea
 - Special feature detection is in `_detect_special_features()` — uses IG JumpTitle buttons pointing to non-episode playlists
 - `JumpTitle(N)` in HDMV commands is **1-based** — convert to 0-based index title with `N - 1`
 - Chapter-split features: when a button sets `reg2` before `JumpTitle`, it selects a chapter within the target playlist (multi-feature playlists)
-- Playlist classifications are heuristic-based; new disc patterns may need new rules
 - Segment keys use quantization (default ±250ms) to handle tiny timing variances
 
+### Fixing Analysis Mismatches — Structural Signals over Thresholds
+
+When a new disc produces wrong episode or special counts, **do not** add numeric
+thresholds or ratio guards.  Instead:
+
+1. **Study the data** — dump chapter durations, IG menu buttons, segment labels,
+   and MovieObject navigation across the failing disc AND existing fixtures that
+   work correctly.  Look for structural patterns that differentiate the two cases.
+2. **Identify a structural signal** — something the disc data tells you about its
+   own content type (e.g. repeating OP/body/ED chapter cycle for episodes,
+   IG button-per-page counts matching chapters-per-episode, title-hint references
+   in navigation commands).
+3. **Require positive evidence** — the code should ask "does the data say this IS
+   X?" rather than "does the data say this is NOT X?".  Negative guards based on
+   thresholds (like `max_chapters_per_episode = 7`) are brittle and will break on
+   the next disc that doesn't match the assumed range.
+4. **Combine signals** — when one signal isn't sufficient alone, combine multiple
+   independent signals (e.g. IG marks + chapter periodicity + button-per-page).
+   Each signal lowers the confidence bar, but at least one must be present.
+
+Examples of structural signals already in use:
+- **Chapter periodicity** (`_detect_episode_periodicity`): detects repeating
+  OP (~90 s) / body / ED (~90 s) / preview (~30 s) cycle in chapter durations
+- **IG chapter marks**: JT + reg2 buttons directly encode episode boundaries
+- **Digital archive multi-signal**: item count + title hint + no-audio streams
+
 ## Copyright & Fixture Guidelines
 - **NEVER commit copyrighted media content** (m2ts video/audio streams, full disc images, cover art, subtitle tracks, etc.) to the repository.
 - **Test fixtures** in `tests/fixtures/` contain only small structural metadata files (MPLS, CLPI, index.bdmv, MovieObject.bdmv, ICS segments) — these are binary headers/indexes, not audiovisual content.
diff --git a/bdpl/analyze/ordering.py b/bdpl/analyze/ordering.py
index 6f09d52..c0af3d1 100644
--- a/bdpl/analyze/ordering.py
+++ b/bdpl/analyze/ordering.py
@@ -104,6 +104,77 @@ def _episodes_from_play_all(
     return episodes
 
 
+def _chapter_durations_s(playlist: Playlist) -> list[float]:
+    """Return chapter durations in seconds for a playlist."""
+    ch_times = [ticks_to_ms(ch.timestamp) for ch in playlist.chapters]
+    total_ms = playlist.duration_ms
+    durs: list[float] = []
+    for i in range(len(ch_times)):
+        end = ch_times[i + 1] if i + 1 < len(ch_times) else total_ms
+        durs.append((end - ch_times[i]) / 1000)
+    return durs
+
+
+# Anime episode chapter structure ranges (in seconds)
+_OP_MIN_S, _OP_MAX_S = 45, 160  # opening theme
+_BODY_MIN_S_CH = 180  # body segment (scene)
+_ED_MIN_S, _ED_MAX_S = 45, 160  # ending theme
+
+
+def _detect_episode_periodicity(
+    ch_durs_s: list[float],
+) -> tuple[int, int, float] | None:
+    """Detect repeating episode structure in chapter durations.
+
+    Anime episode compilations embed a fixed structure per episode:
+    OP (~90 s) → Body segments → ED (~90 s) [→ Preview (~30 s)].  This
+    creates a periodic pattern visible in the chapter durations.
+
+    Tries periods 4–7 (chapters per episode).  For each candidate period,
+    partitions chapters into groups and checks whether each group matches
+    the expected structure (OP-length first chapter, at least one long body
+    chapter, ED-length chapter near the end).
+
+    Returns ``(period, n_episodes, confidence)`` for the best match, where
+    *confidence* is the fraction of groups that match.  Returns ``None``
+    when no period achieves ≥ 75 % match with ≥ 2 groups.
+    """
+    n = len(ch_durs_s)
+    best: tuple[int, int, float] | None = None
+
+    for period in range(4, 8):
+        # Allow total chapters to be within ±1 of period × n_groups
+        for n_groups in range(2, n // period + 2):
+            total_expected = n_groups * period
+            if abs(total_expected - n) > 1:
+                continue
+
+            groups_matched = 0
+            for g in range(n_groups):
+                start = g * period
+                end = min(start + period, n)
+                group = ch_durs_s[start:end]
+                if len(group) < 3:
+                    continue
+
+                op_ok = _OP_MIN_S <= group[0] <= _OP_MAX_S
+                body_ok = any(d > _BODY_MIN_S_CH for d in group[1:-1])
+                ed_ok = (_ED_MIN_S <= group[-1] <= _ED_MAX_S) or (
+                    len(group) >= 3 and _ED_MIN_S <= group[-2] <= _ED_MAX_S
+                )
+
+                if op_ok and body_ok and ed_ok:
+                    groups_matched += 1
+
+            if n_groups >= 2:
+                score = groups_matched / n_groups
+                if score >= 0.75:
+                    if best is None or score > best[2] or (score == best[2] and n_groups > best[1]):
+                        best = (period, n_groups, score)
+
+    return best
+
+
 def _episodes_from_chapters(
     playlist: Playlist,
     ig_chapter_marks: list[int] | None = None,
@@ -113,44 +184,48 @@ def _episodes_from_chapters(
     Used when a playlist contains one (or few) very long play item(s) with
     multiple episodes encoded back-to-back, distinguishable only by chapters.
 
-    When *ig_chapter_marks* are provided (from IG menu buttons), they serve as
-    structural confirmation that the playlist contains multiple episodes.
-    Without such evidence, a minimum of 3 estimated episodes is required —
-    an ``est_count`` of 2 is ambiguous (could be a single ~50 min movie).
+    **Decision to split** requires positive structural evidence from at least
+    one of two signals:
+
+    1. **IG chapter marks** — buttons in the disc menu directly encode episode
+       start chapters (e.g. reg2 = [0, 5, 10, 15]).  Definitive.
+    2. **Chapter periodicity** — chapter durations show a repeating
+       OP / body / ED cycle characteristic of anime episode compilations.
 
-    Heuristic: group consecutive chapters into blocks whose total duration
-    falls within episode range (10–45 min). When a running block exceeds the
-    expected episode length, start a new episode at the chapter boundary.
+    Without either signal the playlist is assumed to be a single movie or OVA
+    and is *not* split, regardless of total duration.
+
+    Splitting uses a greedy algorithm that groups consecutive chapters into
+    blocks whose total duration approaches the target episode length.
     """
     if not playlist.chapters or len(playlist.chapters) < 4:
         return []
 
-    # Only consider chapters on the main play item (item_ref=0 typically)
-    # Build list of (chapter_index, start_time_ms)
     main_item = playlist.play_items[0]
-    ticks_to_ms(main_item.in_time)
 
     ch_times: list[float] = []
     for ch in playlist.chapters:
-        ch_ms = ticks_to_ms(ch.timestamp)
-        ch_times.append(ch_ms)
+        ch_times.append(ticks_to_ms(ch.timestamp))
 
-    # Compute total playlist duration
     total_dur_ms = playlist.duration_ms
-    # Estimate episode count from total duration
-    # Typical anime episode: 22–26 min; try to find the best fit
     est_ep_dur_ms = 25 * 60 * 1000  # 25 minutes as starting estimate
     est_count = max(1, round(total_dur_ms / est_ep_dur_ms))
 
     if est_count <= 1:
         return []  # Not worth splitting
 
-    # IG chapter marks provide structural evidence of multiple episodes.
-    # Without such evidence, require est_count >= 3 because est_count == 2
-    # (~50 min total) is ambiguous — could be a single movie.
+    # --- Require positive structural evidence before splitting ---
     has_ig_confirmation = ig_chapter_marks is not None and len(ig_chapter_marks) >= 2
-    if est_count <= 2 and not has_ig_confirmation:
-        return []
+    if not has_ig_confirmation:
+        ch_durs = _chapter_durations_s(playlist)
+        periodicity = _detect_episode_periodicity(ch_durs)
+        if periodicity is None:
+            return []  # No structural evidence of episodes
+        # Use the detected episode count from periodicity when it differs
+        # from the duration-based estimate.
+        _, periodic_count, _ = periodicity
+        if abs(periodic_count - est_count) <= 1:
+            est_count = periodic_count
 
     # Target duration per episode
     target_dur_ms = total_dur_ms / est_count
diff --git a/tests/conftest.py b/tests/conftest.py
index 6a3ef83..ad154dd 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -268,6 +268,30 @@ def disc19_analysis(disc19_path):
     return _analyze_fixture(disc19_path)
 
 
+@pytest.fixture(scope="session")
+def disc20_path() -> Path:
+    """Return path to bundled disc20 fixture."""
+    return _fixture_path("disc20")
+
+
+@pytest.fixture(scope="session")
+def disc20_analysis(disc20_path):
+    """Run and cache full analysis for the bundled disc20 fixture."""
+    return _analyze_fixture(disc20_path)
+
+
+@pytest.fixture(scope="session")
+def disc21_path() -> Path:
+    """Return path to bundled disc21 fixture."""
+    return _fixture_path("disc21")
+
+
+@pytest.fixture(scope="session")
+def disc21_analysis(disc21_path):
+    """Run and cache full analysis for the bundled disc21 fixture."""
+    return _analyze_fixture(disc21_path)
+
+
 @pytest.fixture
 def cli_runner() -> Callable[..., subprocess.CompletedProcess[str]]:
     """Return helper to invoke `python -m bdpl.cli` consistently in tests."""
diff --git a/tests/fixtures/disc20/CLIPINF/00000.clpi b/tests/fixtures/disc20/CLIPINF/00000.clpi
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diff --git a/tests/fixtures/disc20/META/DL/bdmt_eng.xml b/tests/fixtures/disc20/META/DL/bdmt_eng.xml
new file mode 100644
index 0000000..7b76388
--- /dev/null
+++ b/tests/fixtures/disc20/META/DL/bdmt_eng.xml
@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<disclib>
+<di:discinfo xmlns:di="urn:BDA:bdmv;discinfo">
+<di:title><di:name>TEST DISC 20</di:name></di:title>
+</di:discinfo>
+</disclib>
diff --git a/tests/fixtures/disc20/MovieObject.bdmv b/tests/fixtures/disc20/MovieObject.bdmv
new file mode 100644
index 0000000..9e4becd
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diff --git a/tests/fixtures/disc20/PLAYLIST/00000.mpls b/tests/fixtures/disc20/PLAYLIST/00000.mpls
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diff --git a/tests/fixtures/disc21/META/DL/bdmt_eng.xml b/tests/fixtures/disc21/META/DL/bdmt_eng.xml
new file mode 100644
index 0000000..48a1786
--- /dev/null
+++ b/tests/fixtures/disc21/META/DL/bdmt_eng.xml
@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<disclib>
+<di:discinfo xmlns:di="urn:BDA:bdmv;discinfo">
+<di:title><di:name>TEST DISC 21</di:name></di:title>
+</di:discinfo>
+</disclib>
diff --git a/tests/fixtures/disc21/MovieObject.bdmv b/tests/fixtures/disc21/MovieObject.bdmv
new file mode 100644
index 0000000..46f30df
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diff --git a/tests/fixtures/disc21/PLAYLIST/00000.mpls b/tests/fixtures/disc21/PLAYLIST/00000.mpls
new file mode 100644
index 0000000..9541ca8
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diff --git a/tests/fixtures/disc21/PLAYLIST/00001.mpls b/tests/fixtures/disc21/PLAYLIST/00001.mpls
new file mode 100644
index 0000000..92bc632
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diff --git a/tests/fixtures/disc21/PLAYLIST/00002.mpls b/tests/fixtures/disc21/PLAYLIST/00002.mpls
new file mode 100644
index 0000000..aa48c08
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diff --git a/tests/fixtures/disc21/PLAYLIST/00003.mpls b/tests/fixtures/disc21/PLAYLIST/00003.mpls
new file mode 100644
index 0000000..32f5dd0
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diff --git a/tests/fixtures/disc21/ics_menu.bin b/tests/fixtures/disc21/ics_menu.bin
new file mode 100644
index 0000000..879f245
Binary files /dev/null and b/tests/fixtures/disc21/ics_menu.bin differ
diff --git a/tests/fixtures/disc21/index.bdmv b/tests/fixtures/disc21/index.bdmv
new file mode 100644
index 0000000..cee9035
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diff --git a/tests/test_disc20_scan.py b/tests/test_disc20_scan.py
new file mode 100644
index 0000000..13a43ec
--- /dev/null
+++ b/tests/test_disc20_scan.py
@@ -0,0 +1,44 @@
+"""Tests for disc20 fixture — single compilation movie with scene chapters."""
+
+from __future__ import annotations
+
+import pytest
+
+from bdpl.model import DiscAnalysis
+
+pytestmark = pytest.mark.integration
+
+
+class TestDisc20Episodes:
+    def test_episode_count(self, disc20_analysis: DiscAnalysis) -> None:
+        assert len(disc20_analysis.episodes) == 1
+
+    def test_episode_playlist(self, disc20_analysis: DiscAnalysis) -> None:
+        assert disc20_analysis.episodes[0].playlist == "00002.mpls"
+
+    def test_episode_duration_is_movie_length(self, disc20_analysis: DiscAnalysis) -> None:
+        dur_min = disc20_analysis.episodes[0].duration_ms / 60000
+        assert 100 < dur_min < 140, f"Movie duration {dur_min:.1f}min out of range"
+
+    def test_not_chapter_split(self, disc20_analysis: DiscAnalysis) -> None:
+        """Movie with 41 scene chapters must NOT be split into episodes."""
+        assert len(disc20_analysis.episodes) == 1
+        assert disc20_analysis.episodes[0].confidence == 1.0
+
+
+class TestDisc20Specials:
+    def test_special_feature_count(self, disc20_analysis: DiscAnalysis) -> None:
+        assert len(disc20_analysis.special_features) == 1
+
+    def test_special_category(self, disc20_analysis: DiscAnalysis) -> None:
+        sf = disc20_analysis.special_features[0]
+        assert sf.category == "extra"
+        assert sf.playlist == "00003.mpls"
+
+    def test_special_visible(self, disc20_analysis: DiscAnalysis) -> None:
+        assert disc20_analysis.special_features[0].menu_visible
+
+
+class TestDisc20Metadata:
+    def test_disc_title(self, disc20_analysis: DiscAnalysis) -> None:
+        assert disc20_analysis.disc_title == "TEST DISC 20"
diff --git a/tests/test_disc21_scan.py b/tests/test_disc21_scan.py
new file mode 100644
index 0000000..8ce018a
--- /dev/null
+++ b/tests/test_disc21_scan.py
@@ -0,0 +1,39 @@
+"""Integration tests for the disc21 fixture — special disc with OVA + digital archive."""
+
+from __future__ import annotations
+
+import pytest
+
+from bdpl.model import DiscAnalysis
+
+pytestmark = pytest.mark.integration
+
+
+class TestDisc21Episodes:
+    def test_episode_count(self, disc21_analysis: DiscAnalysis) -> None:
+        assert len(disc21_analysis.episodes) == 1
+
+    def test_episode_playlist(self, disc21_analysis: DiscAnalysis) -> None:
+        assert disc21_analysis.episodes[0].playlist == "00002.mpls"
+
+    def test_episode_duration(self, disc21_analysis: DiscAnalysis) -> None:
+        dur_min = disc21_analysis.episodes[0].duration_ms / 60000
+        assert 44.0 < dur_min < 44.2, f"OVA duration {dur_min:.2f}min, expected ~44:03"
+
+
+class TestDisc21Specials:
+    def test_special_feature_count(self, disc21_analysis: DiscAnalysis) -> None:
+        assert len(disc21_analysis.special_features) == 1
+
+    def test_digital_archive(self, disc21_analysis: DiscAnalysis) -> None:
+        sf = disc21_analysis.special_features[0]
+        assert sf.category == "digital_archive"
+        assert sf.playlist == "00003.mpls"
+
+    def test_digital_archive_visible(self, disc21_analysis: DiscAnalysis) -> None:
+        assert disc21_analysis.special_features[0].menu_visible
+
+
+class TestDisc21Metadata:
+    def test_disc_title(self, disc21_analysis: DiscAnalysis) -> None:
+        assert disc21_analysis.disc_title == "TEST DISC 21"
diff --git a/tests/test_disc_matrix.py b/tests/test_disc_matrix.py
index 8ed2b8a..1a02ffe 100644
--- a/tests/test_disc_matrix.py
+++ b/tests/test_disc_matrix.py
@@ -30,6 +30,8 @@
         ("disc17_analysis", 1, ["00002.mpls"]),
         ("disc18_analysis", 1, ["00002.mpls"]),
         ("disc19_analysis", 1, ["00002.mpls"]),
+        ("disc20_analysis", 1, ["00002.mpls"]),
+        ("disc21_analysis", 1, ["00002.mpls"]),
     ],
 )
 def test_disc_episode_expectation_matrix(
@@ -65,6 +67,8 @@ def test_disc_episode_expectation_matrix(
         ("disc17_analysis", 1, 1),  # 1 digital archive
         ("disc18_analysis", 2, 2),  # 1 extra + 1 creditless ED
         ("disc19_analysis", 1, 1),  # 1 digital archive (hint-backed)
+        ("disc20_analysis", 1, 1),  # 1 extra (trailer)
+        ("disc21_analysis", 1, 1),  # 1 digital archive
     ],
 )
 def test_disc_special_visibility_expectation_matrix(
@@ -104,6 +108,8 @@ def test_disc_special_visibility_expectation_matrix(
         "disc17_analysis",
         "disc18_analysis",
         "disc19_analysis",
+        "disc20_analysis",
+        "disc21_analysis",
     ],
 )
 def test_disc_episode_segment_boundaries_matrix(
@@ -147,6 +153,8 @@ def test_disc_episode_segment_boundaries_matrix(
         "disc17_analysis",
         "disc18_analysis",
         "disc19_analysis",
+        "disc20_analysis",
+        "disc21_analysis",
     ],
 )
 def test_disc_special_boundary_semantics_matrix(
@@ -197,6 +205,8 @@ def test_disc_special_boundary_semantics_matrix(
         ("disc17_analysis", 0),
         ("disc18_analysis", 0),
         ("disc19_analysis", 0),
+        ("disc20_analysis", 0),
+        ("disc21_analysis", 0),
     ],
 )
 def test_disc_special_chapter_split_expectation_matrix(
@@ -230,6 +240,8 @@ def test_disc_special_chapter_split_expectation_matrix(
         ("disc17_analysis", "TEST DISC 17"),
         ("disc18_analysis", "TEST DISC 18"),
         ("disc19_analysis", "TEST DISC 19"),
+        ("disc20_analysis", "TEST DISC 20"),
+        ("disc21_analysis", "TEST DISC 21"),
     ],
 )
 def test_disc_title_extraction_matrix(