@@ -0,0 +1,162 @@
+"""FET- family evidence trail view schema.
+
+Per-family aggregation of all evidence schema references into a timeline view.
+Shows which schemas are present, missing, and when produced.
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+
+REQUIRED_TRAIL_SCHEMA_TYPES: frozenset[str] = frozenset({
+    "BTI", "BEG", "SAT", "PCC",
+    "BSP", "SEG", "ECC", "RSR", "PQT",
+})
+
+VALID_TRAIL_COMPLETENESS_GRADES: frozenset[str] = frozenset({
+    "complete", "partial", "empty",
+})
+
+
+@dataclass
+class FamilySchemaEvent:
+    schema_type: str
+    artifact_id: str
+    created_at: str
+
+
+@dataclass
+class FamilyEvidenceTrailView:
+    fet_id: str
+    candidate_family_id: str
+    batch_id: str
+    pipeline_version: str
+    schema_events: list[FamilySchemaEvent]
+    present_schema_types: list[str]
+    missing_schema_types: list[str]
+    n_schemas_present: int
+    n_schemas_required: int
+    trail_completeness_grade: str
+    dry_lab_only: bool
+    limitations: list[str]
+    created_at: str
+
+
+def _compute_grade(n_present: int, n_required: int) -> str:
+    if n_present == 0:
+        return "empty"
+    if n_present == n_required:
+        return "complete"
+    return "partial"
+
+
+def validate_family_evidence_trail_view(view: FamilyEvidenceTrailView) -> None:
+    if not view.fet_id.startswith("FET-"):
+        raise ValueError(f"fet_id must start with 'FET-': {view.fet_id!r}")
+    if view.candidate_family_id.startswith("TOY-"):
+        raise ValueError(
+            f"candidate_family_id must not start with 'TOY-': {view.candidate_family_id!r}"
+        )
+    if not view.candidate_family_id:
+        raise ValueError("candidate_family_id must be non-empty")
+    if not view.batch_id:
+        raise ValueError("batch_id must be non-empty")
+    if not view.pipeline_version:
+        raise ValueError("pipeline_version must be non-empty")
+    for event in view.schema_events:
+        if event.schema_type not in REQUIRED_TRAIL_SCHEMA_TYPES:
+            raise ValueError(
+                f"schema_type {event.schema_type!r} not in REQUIRED_TRAIL_SCHEMA_TYPES"
+            )
+        if not event.artifact_id:
+            raise ValueError(
+                f"artifact_id must be non-empty for schema_type {event.schema_type!r}"
+            )
+    if view.trail_completeness_grade not in VALID_TRAIL_COMPLETENESS_GRADES:
+        raise ValueError(
+            f"trail_completeness_grade {view.trail_completeness_grade!r} not in "
+            f"VALID_TRAIL_COMPLETENESS_GRADES"
+        )
+    if not view.dry_lab_only:
+        raise ValueError("dry_lab_only must be True")
+    if not view.limitations:
+        raise ValueError("limitations must be non-empty")
+    if not view.created_at:
+        raise ValueError("created_at must be non-empty")
+    n_req = len(REQUIRED_TRAIL_SCHEMA_TYPES)
+    if view.n_schemas_required != n_req:
+        raise ValueError(
+            f"n_schemas_required must be {n_req}, got {view.n_schemas_required}"
+        )
+    if view.n_schemas_present != len(view.present_schema_types):
+        raise ValueError("n_schemas_present must equal len(present_schema_types)")
+    expected_missing = sorted(
+        REQUIRED_TRAIL_SCHEMA_TYPES - set(view.present_schema_types)
+    )
+    if view.missing_schema_types != expected_missing:
+        raise ValueError(
+            f"missing_schema_types mismatch: expected {expected_missing}, "
+            f"got {view.missing_schema_types}"
+        )
+
+
+def build_family_evidence_trail_view(
+    *,
+    fet_id: str,
+    candidate_family_id: str,
+    batch_id: str,
+    pipeline_version: str,
+    schema_event_dicts: list[dict],
+    limitations: list[str],
+    created_at: str,
+) -> FamilyEvidenceTrailView:
+    events = [
+        FamilySchemaEvent(
+            schema_type=d["schema_type"],
+            artifact_id=d["artifact_id"],
+            created_at=d["created_at"],
+        )
+        for d in schema_event_dicts
+    ]
+    present_types = sorted({e.schema_type for e in events})
+    missing_types = sorted(REQUIRED_TRAIL_SCHEMA_TYPES - set(present_types))
+    n_required = len(REQUIRED_TRAIL_SCHEMA_TYPES)
+    n_present = len(present_types)
+    grade = _compute_grade(n_present, n_required)
+    view = FamilyEvidenceTrailView(
+        fet_id=fet_id,
+        candidate_family_id=candidate_family_id,
+        batch_id=batch_id,
+        pipeline_version=pipeline_version,
+        schema_events=events,
+        present_schema_types=present_types,
+        missing_schema_types=missing_types,
+        n_schemas_present=n_present,
+        n_schemas_required=n_required,
+        trail_completeness_grade=grade,
+        dry_lab_only=True,
+        limitations=limitations,
+        created_at=created_at,
+    )
+    validate_family_evidence_trail_view(view)
+    return view
+
+
+def format_family_evidence_trail_view(view: FamilyEvidenceTrailView) -> str:
+    lines = [
+        f"Family Evidence Trail View — {view.fet_id}",
+        f"Family: {view.candidate_family_id}  |  Batch: {view.batch_id}  |  Pipeline: {view.pipeline_version}",
+        f"Grade: {view.trail_completeness_grade}  |  Present: {view.n_schemas_present}/{view.n_schemas_required}",
+    ]
+    if view.present_schema_types:
+        lines.append(f"Present: {', '.join(view.present_schema_types)}")
+    if view.missing_schema_types:
+        lines.append(f"Missing: {', '.join(view.missing_schema_types)}")
+    lines.append(f"Created: {view.created_at}")
+    if view.schema_events:
+        lines.append("Events:")
+        for e in view.schema_events:
+            lines.append(f"  {e.schema_type}: {e.artifact_id} @ {e.created_at}")
+    lines.append(f"Limitations: {'; '.join(view.limitations)}")
+    lines.append(f"dry_lab_only: {view.dry_lab_only}")
+    return "\n".join(lines)