diff --git a/src/host/backfill.ts b/src/host/backfill.ts
index e0be9e1..f898efc 100644
--- a/src/host/backfill.ts
+++ b/src/host/backfill.ts
@@ -93,8 +93,15 @@ interface ConnectionHostFace {
  */
 const YIELD_MS = 100
 
-/** The `name` dsh raises for a durable log its migration surface refuses. */
-const UNSUPPORTED_MIGRATION = 'SessionFormatUnsupportedMigrationError'
+/**
+ * The `name` family dsh raises for a durable log it cannot interpret. The
+ * format edge throws `SessionFormatUnsupportedMigrationError`, but the
+ * persistence seam translates it — together with every other "intact yet not
+ * interpretable" refusal — into `SessionFormatUnsupportedError` before the
+ * error escapes (issue #75): classify by the family prefix, since either
+ * exact name may be the one that arrives.
+ */
+const UNSUPPORTED_FORMAT = 'SessionFormatUnsupported'
 
 function asRecord(value: unknown): Record<string, unknown> | null {
   return value !== null && typeof value === 'object' ? value as Record<string, unknown> : null
@@ -170,15 +177,15 @@ function isLive(sessions: Record<string, unknown> | null, id: string): boolean {
 }
 
 /**
- * Whether the cold read failed because the running harness refuses to
- * migrate this log — a permanent, source-side refusal (the artifact is left
- * unchanged, so retrying cannot succeed). Matched by the error's documented
- * `name`, so no dsh symbol needs importing and every supported baseline
- * classifies alike: an unknown error shape just falls to the warn path.
+ * Whether the cold read failed because the running harness refuses this log's
+ * format — a permanent, source-side refusal (the artifact is left unchanged,
+ * so retrying cannot succeed). Matched by the error's documented `name`, so no
+ * dsh symbol needs importing and every supported baseline classifies alike: an
+ * unknown error shape just falls to the warn path.
  */
-function isUnsupportedMigration(error: unknown): boolean {
+function isUnsupportedFormat(error: unknown): boolean {
   try {
-    return asRecord(error)?.name === UNSUPPORTED_MIGRATION
+    return String(asRecord(error)?.name).startsWith(UNSUPPORTED_FORMAT)
   } catch {
     return false
   }
@@ -271,7 +278,7 @@ export function watchActivityBackfill(ctx: Context): () => void {
           ;(cache as unknown as ProjectionCacheLike).coldSnapshot(log.header, log.inheritedEventCount, log.events)
           folded++
         } catch (error: unknown) {
-          if (isUnsupportedMigration(error)) {
+          if (isUnsupportedFormat(error)) {
             // Permanent and source-side: the per-session detail drops to
             // debug; the summary below is the pass's only above-debug word.
             refused++
diff --git a/tests/host/backfill.spec.ts b/tests/host/backfill.spec.ts
index 2176250..598c876 100644
--- a/tests/host/backfill.spec.ts
+++ b/tests/host/backfill.spec.ts
@@ -59,10 +59,19 @@ function captureLogs(ctx: Context): LogLine[] {
   return lines
 }
 
-/** The migration-refusal error dsh raises for a legacy log it cannot migrate. */
-function unsupportedMigrationError(): Error {
-  const error = new Error('subagent/descriptor 0 uses unsupported descriptor version 2; source v0 artifact remains unchanged')
-  error.name = 'SessionFormatUnsupportedMigrationError'
+/**
+ * One format-refusal face of the persistence seam. The format edge throws
+ * `SessionFormatUnsupportedMigrationError`, but the seam translates it — with
+ * the "source vN artifact remains unchanged (raw log: …)" suffix — into
+ * `SessionFormatUnsupportedError` before the error escapes (issue #75): the
+ * pass must classify BOTH faces as the same refusal.
+ */
+function unsupportedFormatError(name: string): Error {
+  const error = new Error(
+    'subagent/descriptor 0 uses unsupported descriptor version 2'
+    + '; source v0 artifact remains unchanged (raw log: C:\\Users\\u\\.dsh\\sessions\\log.jsonl.zstd)',
+  )
+  error.name = name
   return error
 }
 
@@ -435,8 +444,11 @@ describe('watchActivityBackfill', () => {
       { header: { id: 'legacy-2', cwd: '/repo/legacy-2' } },
       { header: { id: 'good', cwd: '/repo/good' } },
     ])
-    state.failReads.set('legacy-1', unsupportedMigrationError())
-    state.failReads.set('legacy-2', unsupportedMigrationError())
+    // One refusal per face of the seam's translation: the translated name a
+    // real dsh delivers, and the untranslated format-edge name (either may
+    // arrive) — both classify alike, one summary for the pair.
+    state.failReads.set('legacy-1', unsupportedFormatError('SessionFormatUnsupportedError'))
+    state.failReads.set('legacy-2', unsupportedFormatError('SessionFormatUnsupportedMigrationError'))
     const route = arm(ctx, state)
     const lines = captureLogs(ctx)
     const dispose = watchActivityBackfill(ctx)