diff --git a/docs/accountability-root-manifest.md b/docs/accountability-root-manifest.md
index 34fff48..95a3a63 100644
--- a/docs/accountability-root-manifest.md
+++ b/docs/accountability-root-manifest.md
@@ -45,6 +45,12 @@ Ownership review schema:
 schemas/accountability-ownership-review.schema.yaml
 ```
 
+Update delta schema:
+
+```text
+schemas/accountability-update-delta.schema.yaml
+```
+
 ## Required Sections
 
 - `netkingdom`: root id, name, and king actor.
@@ -142,3 +148,19 @@ railiance-fabric review-identity identity:repository:example-repo \
 Reviewer decisions are keyed by stable identity key. Later rescans apply the
 latest decision for that key, so ordinary evidence refreshes do not lose
 reviewed ownership choices.
+
+## Update Deltas
+
+To compare the current run with previous identity and ownership-review outputs:
+
+```bash
+railiance-fabric discover-roots \
+  --delta \
+  --previous-identity-projection previous-identities.json \
+  --previous-ownership-review previous-ownership.json
+```
+
+The delta separates candidate graph node changes, candidate graph edge changes,
+ownership changes, containment changes, review-state changes, and blocker
+changes. When `summary.promotion_needed` is `false`, the update loop can skip
+promotion because the durable evidence produced no meaningful Fabric change.
diff --git a/docs/financial-fabric-operator-guide.md b/docs/financial-fabric-operator-guide.md
index f881b19..e87e937 100644
--- a/docs/financial-fabric-operator-guide.md
+++ b/docs/financial-fabric-operator-guide.md
@@ -76,6 +76,14 @@ railiance-fabric review-identity <stable-key> \
   --fabric-id fabric.railiance.primary
 ```
 
+To compare a new run with saved review outputs:
+
+```bash
+railiance-fabric discover-roots --delta \
+  --previous-identity-projection previous-identities.json \
+  --previous-ownership-review previous-ownership.json
+```
+
 The financial export must satisfy these invariants:
 
 - every accepted node has resolvable ownership;
diff --git a/railiance_fabric/accountability_roots.py b/railiance_fabric/accountability_roots.py
index 41d4b73..5c53250 100644
--- a/railiance_fabric/accountability_roots.py
+++ b/railiance_fabric/accountability_roots.py
@@ -246,6 +246,105 @@ def build_ownership_review(
     return review
 
 
+def build_update_delta(
+    current_identity_projection: dict[str, Any],
+    current_ownership_review: dict[str, Any],
+    *,
+    previous_identity_projection: dict[str, Any] | None = None,
+    previous_ownership_review: dict[str, Any] | None = None,
+) -> dict[str, Any]:
+    previous_identity_projection = previous_identity_projection or {}
+    previous_ownership_review = previous_ownership_review or {}
+    current_nodes = {
+        item["stable_key"]: item
+        for item in current_identity_projection.get("identity_candidates", [])
+        if isinstance(item, dict) and item.get("stable_key")
+    }
+    previous_nodes = {
+        item["stable_key"]: item
+        for item in previous_identity_projection.get("identity_candidates", [])
+        if isinstance(item, dict) and item.get("stable_key")
+    }
+    current_edges = {
+        item["id"]: item
+        for item in current_identity_projection.get("candidate_graph", {}).get("edges", [])
+        if isinstance(item, dict) and item.get("id")
+    }
+    previous_edges = {
+        item["id"]: item
+        for item in previous_identity_projection.get("candidate_graph", {}).get("edges", [])
+        if isinstance(item, dict) and item.get("id")
+    }
+    current_review = {
+        item["stable_key"]: item
+        for item in current_ownership_review.get("items", [])
+        if isinstance(item, dict) and item.get("stable_key")
+    }
+    previous_review = {
+        item["stable_key"]: item
+        for item in previous_ownership_review.get("items", [])
+        if isinstance(item, dict) and item.get("stable_key")
+    }
+
+    node_delta = _delta_sets(previous_nodes, current_nodes)
+    edge_delta = _delta_sets(previous_edges, current_edges)
+    ownership_changes = _field_changes(previous_review, current_review, "ownership")
+    containment_changes = _field_changes(previous_review, current_review, "containment")
+    review_state_changes = [
+        key
+        for key in sorted(set(previous_review) & set(current_review))
+        if previous_review[key].get("review_state") != current_review[key].get("review_state")
+    ]
+    blocker_changes = _field_changes(previous_review, current_review, "blockers")
+    meaningful_changes = _unique_strings(
+        [
+            *node_delta["added"],
+            *node_delta["changed"],
+            *node_delta["removed"],
+            *edge_delta["added"],
+            *edge_delta["changed"],
+            *edge_delta["removed"],
+            *ownership_changes,
+            *containment_changes,
+            *review_state_changes,
+            *blocker_changes,
+        ]
+    )
+    delta = {
+        "apiVersion": "railiance.fabric/v1alpha2",
+        "kind": "AccountabilityUpdateDelta",
+        "generated_at": _utc_now(),
+        "current": current_identity_projection.get("evidence_run", {}),
+        "previous": previous_identity_projection.get("evidence_run", {}),
+        "node_delta": node_delta,
+        "edge_delta": edge_delta,
+        "change_sets": {
+            "ownership": ownership_changes,
+            "containment": containment_changes,
+            "review_state": review_state_changes,
+            "blockers": blocker_changes,
+        },
+        "summary": {
+            "nodes_added": len(node_delta["added"]),
+            "nodes_changed": len(node_delta["changed"]),
+            "nodes_removed": len(node_delta["removed"]),
+            "nodes_unchanged": len(node_delta["unchanged"]),
+            "edges_added": len(edge_delta["added"]),
+            "edges_changed": len(edge_delta["changed"]),
+            "edges_removed": len(edge_delta["removed"]),
+            "edges_unchanged": len(edge_delta["unchanged"]),
+            "meaningful_change_count": len(meaningful_changes),
+            "promotion_needed": bool(meaningful_changes),
+        },
+    }
+    validator = draft202012_validator(repo_root() / "schemas" / "accountability-update-delta.schema.yaml")
+    errors = sorted(validator.iter_errors(delta), key=lambda error: list(error.path))
+    if errors:
+        location = ".".join(str(part) for part in errors[0].path) or "<root>"
+        raise ValueError(f"invalid accountability update delta at {location}: {errors[0].message}")
+    return delta
+
+
 @dataclass(frozen=True)
 class AccountabilityEvidenceStore:
     path: Path
@@ -741,6 +840,44 @@ def _ownership_item(
     return item
 
 
+def _delta_sets(previous: dict[str, dict[str, Any]], current: dict[str, dict[str, Any]]) -> dict[str, list[str]]:
+    previous_keys = set(previous)
+    current_keys = set(current)
+    common = previous_keys & current_keys
+    changed = [
+        key
+        for key in sorted(common)
+        if short_fingerprint(_stable_payload(previous[key]), length=16)
+        != short_fingerprint(_stable_payload(current[key]), length=16)
+    ]
+    return {
+        "added": sorted(current_keys - previous_keys),
+        "changed": changed,
+        "removed": sorted(previous_keys - current_keys),
+        "unchanged": sorted(common - set(changed)),
+    }
+
+
+def _field_changes(
+    previous: dict[str, dict[str, Any]],
+    current: dict[str, dict[str, Any]],
+    field: str,
+) -> list[str]:
+    return [
+        key
+        for key in sorted(set(previous) & set(current))
+        if previous[key].get(field) != current[key].get(field)
+    ]
+
+
+def _stable_payload(value: dict[str, Any]) -> dict[str, Any]:
+    return {
+        key: data
+        for key, data in value.items()
+        if key not in {"generated_at", "decision"}
+    }
+
+
 def _add_identity_candidate(
     candidates: dict[str, dict[str, Any]],
     *,
diff --git a/railiance_fabric/cli.py b/railiance_fabric/cli.py
index aefd199..ca172a4 100644
--- a/railiance_fabric/cli.py
+++ b/railiance_fabric/cli.py
@@ -19,6 +19,7 @@ from .accountability_roots import (
     AccountabilityEvidenceStore,
     build_identity_projection,
     build_ownership_review,
+    build_update_delta,
     collect_accountability_root_evidence,
     load_accountability_root_manifest,
 )
@@ -125,6 +126,9 @@ def build_parser() -> argparse.ArgumentParser:
     discover_roots.add_argument("--max-items-per-root", type=int, default=200)
     discover_roots.add_argument("--identity-projection", action="store_true", help="Print normalized identity candidates instead of raw evidence.")
     discover_roots.add_argument("--ownership-review", action="store_true", help="Print ownership resolution and review blockers.")
+    discover_roots.add_argument("--delta", action="store_true", help="Print a delta against previous identity/ownership review files.")
+    discover_roots.add_argument("--previous-identity-projection", type=Path, default=None)
+    discover_roots.add_argument("--previous-ownership-review", type=Path, default=None)
     discover_roots.add_argument("--store-db", type=Path, default=None, help="Persist evidence and identity candidates in a SQLite store.")
 
     review_identity = sub.add_parser(
@@ -363,9 +367,27 @@ def main(argv: list[str] | None = None) -> int:
         store = AccountabilityEvidenceStore(args.store_db) if args.store_db else None
         decisions = store.latest_review_decisions() if store else {}
         ownership_review = build_ownership_review(projection, manifest, review_decisions=decisions)
+        update_delta = build_update_delta(
+            projection,
+            ownership_review,
+            previous_identity_projection=_load_json_file(args.previous_identity_projection)
+            if args.previous_identity_projection
+            else None,
+            previous_ownership_review=_load_json_file(args.previous_ownership_review)
+            if args.previous_ownership_review
+            else None,
+        )
         if args.store_db:
             store.add_evidence_run(payload, projection)
-        output = ownership_review if args.ownership_review else projection if args.identity_projection else payload
+        output = (
+            update_delta
+            if args.delta
+            else ownership_review
+            if args.ownership_review
+            else projection
+            if args.identity_projection
+            else payload
+        )
         print(json.dumps(output, indent=2, sort_keys=True))
         return 0
 
@@ -1648,6 +1670,13 @@ def _load_graph_or_exit(paths: list[Path]) -> FabricGraph:
     return graph
 
 
+def _load_json_file(path: Path) -> dict[str, Any]:
+    payload = json.loads(path.read_text(encoding="utf-8"))
+    if not isinstance(payload, dict):
+        raise ValueError(f"JSON file must contain an object: {path}")
+    return payload
+
+
 def _print_providers(graph: FabricGraph, capability: str) -> None:
     providers = graph.providers(capability)
     if not providers:
diff --git a/schemas/accountability-update-delta.schema.yaml b/schemas/accountability-update-delta.schema.yaml
new file mode 100644
index 0000000..dd8734d
--- /dev/null
+++ b/schemas/accountability-update-delta.schema.yaml
@@ -0,0 +1,120 @@
+$schema: "https://json-schema.org/draft/2020-12/schema"
+$id: "https://railiance.local/fabric/schemas/accountability-update-delta.schema.yaml"
+title: "AccountabilityUpdateDelta"
+type: object
+additionalProperties: false
+required:
+  - apiVersion
+  - kind
+  - generated_at
+  - current
+  - previous
+  - node_delta
+  - edge_delta
+  - change_sets
+  - summary
+properties:
+  apiVersion:
+    type: string
+    const: "railiance.fabric/v1alpha2"
+  kind:
+    type: string
+    const: AccountabilityUpdateDelta
+  generated_at:
+    type: string
+    format: date-time
+  current:
+    type: object
+    additionalProperties: true
+  previous:
+    type: object
+    additionalProperties: true
+  node_delta:
+    $ref: "#/$defs/deltaSets"
+  edge_delta:
+    $ref: "#/$defs/deltaSets"
+  change_sets:
+    type: object
+    additionalProperties: false
+    required:
+      - ownership
+      - containment
+      - review_state
+      - blockers
+    properties:
+      ownership:
+        $ref: "#/$defs/stringList"
+      containment:
+        $ref: "#/$defs/stringList"
+      review_state:
+        $ref: "#/$defs/stringList"
+      blockers:
+        $ref: "#/$defs/stringList"
+  summary:
+    type: object
+    additionalProperties: false
+    required:
+      - nodes_added
+      - nodes_changed
+      - nodes_removed
+      - nodes_unchanged
+      - edges_added
+      - edges_changed
+      - edges_removed
+      - edges_unchanged
+      - meaningful_change_count
+      - promotion_needed
+    properties:
+      nodes_added:
+        type: integer
+        minimum: 0
+      nodes_changed:
+        type: integer
+        minimum: 0
+      nodes_removed:
+        type: integer
+        minimum: 0
+      nodes_unchanged:
+        type: integer
+        minimum: 0
+      edges_added:
+        type: integer
+        minimum: 0
+      edges_changed:
+        type: integer
+        minimum: 0
+      edges_removed:
+        type: integer
+        minimum: 0
+      edges_unchanged:
+        type: integer
+        minimum: 0
+      meaningful_change_count:
+        type: integer
+        minimum: 0
+      promotion_needed:
+        type: boolean
+
+$defs:
+  stringList:
+    type: array
+    items:
+      type: string
+
+  deltaSets:
+    type: object
+    additionalProperties: false
+    required:
+      - added
+      - changed
+      - removed
+      - unchanged
+    properties:
+      added:
+        $ref: "#/$defs/stringList"
+      changed:
+        $ref: "#/$defs/stringList"
+      removed:
+        $ref: "#/$defs/stringList"
+      unchanged:
+        $ref: "#/$defs/stringList"
diff --git a/tests/test_accountability_root_adapters.py b/tests/test_accountability_root_adapters.py
index b040062..6dd31c2 100644
--- a/tests/test_accountability_root_adapters.py
+++ b/tests/test_accountability_root_adapters.py
@@ -5,6 +5,7 @@ from railiance_fabric.accountability_roots import (
     AccountabilityEvidenceStore,
     build_identity_projection,
     build_ownership_review,
+    build_update_delta,
     collect_accountability_root_evidence,
     load_accountability_root_manifest,
 )
@@ -191,6 +192,78 @@ def test_review_identity_cli_persists_decision_for_ownership_review(tmp_path: Pa
     assert repo_item["decision"]["reviewer"] == "tester"
 
 
+def test_update_delta_detects_ownership_changes_and_unchanged_runs(tmp_path: Path) -> None:
+    manifest_path = _fixture_manifest(tmp_path)
+    manifest = load_accountability_root_manifest(manifest_path)
+    projection = build_identity_projection(collect_accountability_root_evidence(manifest_path), manifest)
+    review = build_ownership_review(projection, manifest)
+
+    unchanged = build_update_delta(
+        projection,
+        review,
+        previous_identity_projection=projection,
+        previous_ownership_review=review,
+    )
+    validator = draft202012_validator(Path("schemas/accountability-update-delta.schema.yaml"))
+    assert list(validator.iter_errors(unchanged)) == []
+    assert unchanged["summary"]["promotion_needed"] is False
+    assert unchanged["node_delta"]["unchanged"]
+
+    store = AccountabilityEvidenceStore(tmp_path / "evidence.sqlite3")
+    store.add_review_decision(
+        stable_key="identity:repository:fixture-repo",
+        decision="accept",
+        reviewer="tester",
+        owner_actor_id="actor.fixture.lord",
+        fabric_id="fabric.fixture.primary",
+    )
+    accepted_review = build_ownership_review(
+        projection,
+        manifest,
+        review_decisions=store.latest_review_decisions(),
+    )
+    changed = build_update_delta(
+        projection,
+        accepted_review,
+        previous_identity_projection=projection,
+        previous_ownership_review=review,
+    )
+    assert changed["summary"]["promotion_needed"] is True
+    assert "identity:repository:fixture-repo" in changed["change_sets"]["ownership"]
+    assert "identity:repository:fixture-repo" in changed["change_sets"]["review_state"]
+
+
+def test_discover_roots_cli_can_emit_delta(tmp_path: Path, capsys) -> None:
+    manifest = _fixture_manifest(tmp_path)
+    manifest_data = load_accountability_root_manifest(manifest)
+    projection = build_identity_projection(collect_accountability_root_evidence(manifest), manifest_data)
+    review = build_ownership_review(projection, manifest_data)
+    projection_path = tmp_path / "previous-identities.json"
+    review_path = tmp_path / "previous-ownership.json"
+    projection_path.write_text(json.dumps(projection), encoding="utf-8")
+    review_path.write_text(json.dumps(review), encoding="utf-8")
+
+    assert (
+        cli_main(
+            [
+                "discover-roots",
+                "--manifest",
+                str(manifest),
+                "--delta",
+                "--previous-identity-projection",
+                str(projection_path),
+                "--previous-ownership-review",
+                str(review_path),
+            ]
+        )
+        == 0
+    )
+
+    payload = json.loads(capsys.readouterr().out)
+    assert payload["kind"] == "AccountabilityUpdateDelta"
+    assert payload["summary"]["promotion_needed"] is False
+
+
 def _fixture_manifest(tmp_path: Path) -> Path:
     workspace = tmp_path / "workspace"
     repo = workspace / "fixture-repo"
diff --git a/workplans/RAIL-FAB-WP-0018-accountability-root-discovery-update-loop.md b/workplans/RAIL-FAB-WP-0018-accountability-root-discovery-update-loop.md
index e143219..f83dce8 100644
--- a/workplans/RAIL-FAB-WP-0018-accountability-root-discovery-update-loop.md
+++ b/workplans/RAIL-FAB-WP-0018-accountability-root-discovery-update-loop.md
@@ -231,7 +231,7 @@ Result:
 
 ```task
 id: RAIL-FAB-WP-0018-T05
-status: todo
+status: done
 priority: medium
 state_hub_task_id: "c2f28b34-de32-4090-8782-5d00541b9018"
 ```
@@ -257,6 +257,25 @@ Done when:
   changes are highlighted;
 - unchanged sources are not needlessly promoted.
 
+Result:
+
+- Added `schemas/accountability-update-delta.schema.yaml` for
+  `AccountabilityUpdateDelta` payloads.
+- Added `build_update_delta()` to compare current and previous identity
+  projections plus ownership reviews.
+- Deltas distinguish candidate graph node additions/changes/removals,
+  candidate graph edge additions/changes/removals, ownership changes,
+  containment changes, review-state changes, blocker changes, and unchanged
+  nodes/edges.
+- Added `railiance-fabric discover-roots --delta` with optional
+  `--previous-identity-projection` and `--previous-ownership-review` inputs.
+- Added tests proving unchanged runs do not require promotion and ownership
+  review changes are highlighted.
+- Documented update deltas in the manifest and operator docs.
+- Verified with `python3 -m pytest tests/test_accountability_root_adapters.py -q`,
+  `python3 -m railiance_fabric.cli discover-roots --max-items-per-root 5 --delta`,
+  and full `python3 -m pytest`.
+
 ## T06 - Bootstrap The Current Railiance Rebuild
 
 ```task