diff --git a/backend/text_coverage.py b/backend/text_coverage.py
new file mode 100644
index 0000000..508b46e
--- /dev/null
+++ b/backend/text_coverage.py
@@ -0,0 +1,135 @@
+"""text_coverage.py - deterministic extraction-coverage measurement.
+
+The coverage guarantee: for any page with a usable text layer, measure how
+much of that layer ended up represented in extracted objects. Pages below
+the floor route into the extraction retry ladder (agents/extractors.py and
+pipeline/extractor.py). fallback_objects() is the last rung: stub objects
+segmented straight from the text layer so no text-bearing page goes dark.
+"""
+
+import re
+from typing import Dict, List, Optional
+
+MIN_LINE_CHARS = 12
+_TICK_RE = re.compile(r"^[\d\s'\"/.,-]+$")
+_WORD_RE = re.compile(r"[a-z0-9]+")
+
+
+def _meaningful_lines(text: str) -> List[str]:
+    lines = []
+    for raw in (text or "").splitlines():
+        line = " ".join(raw.split())
+        if len(line) < MIN_LINE_CHARS or _TICK_RE.match(line):
+            continue
+        lines.append(line)
+    return lines
+
+
+def _norm(text: str) -> str:
+    return " ".join(_WORD_RE.findall((text or "").lower()))
+
+
+def text_coverage(page_text: str, objects: List[Dict]) -> Dict:
+    """Fraction of meaningful text-layer lines whose normalized form appears
+    in the concatenated normalized source_text of extracted objects."""
+    lines = _meaningful_lines(page_text)
+    if not lines:
+        return {"total_lines": 0, "covered_lines": 0, "ratio": 1.0}
+    haystack = " ".join(
+        _norm(str(o.get("source_text") or o.get("object_description")
+                  or o.get("value") or ""))
+        for o in objects if isinstance(o, dict)
+    )
+    covered = sum(1 for ln in lines if _norm(ln) and _norm(ln) in haystack)
+    return {
+        "total_lines": len(lines),
+        "covered_lines": covered,
+        "ratio": covered / len(lines) if lines else 1.0,
+    }
+
+
+def segment_text_layer(text: str) -> List[str]:
+    """Segment a page text layer into note-sized blocks."""
+    segments: List[str] = []
+    buf: List[str] = []
+    number_re = re.compile(r"^(\d{1,2}[.)]?|[A-Z]\d{0,2}[.)]?)\s*$")
+
+    def flush():
+        joined = " ".join(buf).strip()
+        if len(joined) >= MIN_LINE_CHARS:
+            segments.append(joined)
+        buf.clear()
+
+    for raw in (text or "").splitlines():
+        line = raw.strip()
+        if not line:
+            flush()
+            continue
+        if number_re.match(line):
+            flush()
+            buf.append(line.rstrip(".)"))
+            continue
+        buf.append(line)
+        if line.endswith(".") and len(" ".join(buf)) > 120:
+            flush()
+    flush()
+    return segments
+
+
+def fallback_objects(page_text: str, page_number: int,
+                     max_objects: int = 200) -> List[Dict]:
+    """Last-rung deterministic extraction: one stub object per text segment,
+    source_text verbatim from the text layer."""
+    objs = []
+    for idx, seg in enumerate(segment_text_layer(page_text)[:max_objects]):
+        objs.append({
+            "object_id": f"p{page_number}-tl{idx}",
+            "object_type": "general_note",
+            "category": "general",
+            "tag": None,
+            "name": seg[:80],
+            "description": seg,
+            "attributes": {},
+            "location_key": {},
+            "source_text": seg,
+            "graphical_basis": None,
+            "review_uses": ["code_review", "constructability_review"],
+            "confidence": "low",
+            "grounding": "text_layer_fallback",
+        })
+    return objs
+
+
+def merge_objects(vision_objs: List[Dict], text_objs: List[Dict]) -> List[Dict]:
+    """Union of vision and text-structured objects. Vision results come first
+    and are never dropped. Text objects are appended unless their normalized
+    source_text is already represented."""
+    merged = list(vision_objs or [])
+    seen = {_norm(str(o.get("source_text") or ""))
+            for o in merged if isinstance(o, dict)}
+    seen.discard("")
+    for obj in text_objs or []:
+        if not isinstance(obj, dict):
+            continue
+        key = _norm(str(obj.get("source_text") or ""))
+        if key and key in seen:
+            continue
+        seen.add(key)
+        merged.append(obj)
+    return merged
+
+
+_SHEET_ID_RE = re.compile(r"\b([A-Z]{1,2}\d{2,3}(?:\.\d+)?)\b")
+
+
+def recover_sheet_number(page_text: str) -> Optional[str]:
+    """Deterministic sheet id from the text layer: prefer candidates in the
+    last ~15% of the page (title block lives at the drawing edge)."""
+    text = page_text or ""
+    cands = _SHEET_ID_RE.findall(text)
+    if not cands:
+        return None
+    tail = text[int(len(text) * 0.85):]
+    for cand in reversed(_SHEET_ID_RE.findall(tail)):
+        return cand
+    return cands[0]
diff --git a/tests/test_text_coverage.py b/tests/test_text_coverage.py
new file mode 100644
index 0000000..0e10ab1
--- /dev/null
+++ b/tests/test_text_coverage.py
@@ -0,0 +1,76 @@
+from backend.text_coverage import (text_coverage, segment_text_layer,
+                                   fallback_objects, merge_objects,
+                                   recover_sheet_number)
+
+
+def test_coverage_full():
+    text = "NOTE 1\nALL LUMBER NO. 2 SOUTHERN PINE\nNOTE 2\nUSE 5/8\" PLYWOOD"
+    objects = [{"source_text": "ALL LUMBER NO. 2 SOUTHERN PINE"},
+               {"source_text": "USE 5/8\" PLYWOOD"}]
+    cov = text_coverage(text, objects)
+    assert cov["covered_lines"] == 2
+    assert cov["total_lines"] == 2
+    assert cov["ratio"] == 1.0
+
+
+def test_coverage_zero_on_empty_objects():
+    cov = text_coverage("LINE ALPHA CONTENT\nLINE BETA CONTENT\nLINE GAMMA CONTENT", [])
+    assert cov["ratio"] == 0.0 and cov["total_lines"] == 3
+
+
+def test_coverage_ignores_short_and_numeric_noise_lines():
+    text = "15\"\n19\"\nA\nB\nREAL NOTE ABOUT FRAMING HERE"
+    cov = text_coverage(text, [{"source_text": "REAL NOTE ABOUT FRAMING HERE"}])
+    assert cov["total_lines"] == 1 and cov["ratio"] == 1.0
+
+
+def test_segment_notes_and_rows():
+    text = "WOOD CONSTRUCTION\n1. \nALL SAWN LUMBER TO BE SOUTHERN PINE.\n2. \nROOF SHEATHING 5/8\" PLYWOOD."
+    segs = segment_text_layer(text)
+    assert any("ALL SAWN LUMBER" in s for s in segs)
+    assert any("ROOF SHEATHING" in s for s in segs)
+
+
+def test_fallback_objects_verbatim_and_stamped():
+    objs = fallback_objects("1. \nALL SAWN LUMBER TO BE SOUTHERN PINE.", page_number=8)
+    assert len(objs) == 1
+    assert objs[0]["source_text"] == "1 ALL SAWN LUMBER TO BE SOUTHERN PINE."
+    assert objs[0]["grounding"] == "text_layer_fallback"
+    assert objs[0]["confidence"] == "low"
+
+
+def test_merge_objects_keeps_vision_and_unions_text():
+    vision = [
+        {"source_text": "2X6 WD STUD @ 16\" O.C.", "object_type": "wall"},
+        {"source_text": None, "graphical_basis": "light fixture symbol, grid C-4",
+         "object_type": "lighting_fixture"},
+    ]
+    text = [
+        {"source_text": "2X6 WD STUD @ 16\" O.C.", "object_type": "wall"},
+        {"source_text": "ALL LUMBER NO. 2 SOUTHERN PINE", "object_type": "general_note"},
+    ]
+    merged = merge_objects(vision, text)
+    assert len(merged) == 3
+    assert any(o.get("graphical_basis") for o in merged)
+    assert merged[0]["object_type"] == "wall"
+
+
+def test_merge_objects_dedupes_by_normalized_text():
+    a = [{"source_text": "RTU-1: 5 TON, 1600 CFM"}]
+    b = [{"source_text": "rtu 1 5 ton 1600 cfm"}]
+    assert len(merge_objects(a, b)) == 1
+
+
+def test_recover_sheet_number_from_title_block():
+    text = ("WALL SECTIONS\n...\nSheet Information\nS301\n"
+            "Issue Date 05.29.26\nProject Number 25177")
+    assert recover_sheet_number(text) == "S301"
+
+
+def test_recover_sheet_number_none_when_absent():
+    assert recover_sheet_number("just some notes about lumber") is None
+
+
+def test_recover_prefers_discipline_pattern_over_dates():
+    text = "Issue Date 05.29.26\nProject Number 25177\nA102 REFLECTED CEILING PLAN"
+    assert recover_sheet_number(text) == "A102"