diff --git a/src/lib/db/migrations/0078_files_interest_id.sql b/src/lib/db/migrations/0078_files_interest_id.sql
new file mode 100644
index 00000000..e5c88085
--- /dev/null
+++ b/src/lib/db/migrations/0078_files_interest_id.sql
@@ -0,0 +1,30 @@
+-- Phase 1 of the nested document subfolders feature (master UAT line 728+):
+--
+-- 1. Add a nullable `interest_id` column to `files` so uploads scoped
+--    to a deal can be filed under the interest subfolder while still
+--    rolling up to the parent client folder. Mirrors the existing
+--    `documents.interest_id` semantics: scoping FK that holds a
+--    "from-interest" attribution even when the parent client/yacht/
+--    company shifts.
+--
+-- 2. Index on `(port_id, interest_id)` for the aggregated-projection
+--    queries that will surface "this-deal" files vs "from-client" files
+--    in InterestDocumentsTab.
+--
+-- 3. Soft FK (`ON DELETE SET NULL`) so a hard-deleted interest doesn't
+--    orphan the file — the audit trail stays intact and the file remains
+--    findable under the parent client folder.
+--
+-- Apply in dev:
+--   PGPASSWORD=changeme psql -h localhost -p 5434 -U crm -d port_nimara_crm \
+--     -f src/lib/db/migrations/0078_files_interest_id.sql
+
+ALTER TABLE files
+  ADD COLUMN IF NOT EXISTS interest_id text
+    REFERENCES interests(id) ON DELETE SET NULL;
+
+CREATE INDEX IF NOT EXISTS idx_files_interest
+  ON files (interest_id);
+
+CREATE INDEX IF NOT EXISTS idx_files_port_interest
+  ON files (port_id, interest_id);
diff --git a/src/lib/db/schema/documents.ts b/src/lib/db/schema/documents.ts
index 7a46ff8b..8318d213 100644
--- a/src/lib/db/schema/documents.ts
+++ b/src/lib/db/schema/documents.ts
@@ -31,6 +31,18 @@ export const files = pgTable(
     clientId: text('client_id').references(() => clients.id, { onDelete: 'set null' }),
     yachtId: text('yacht_id').references(() => yachts.id, { onDelete: 'set null' }),
     companyId: text('company_id').references(() => companies.id, { onDelete: 'set null' }),
+    /**
+     * Optional deal scope. When a file is uploaded inside an interest
+     * (the EOI tab, the documents tab on the interest detail page), this
+     * FK captures which deal it belongs to so the file can be filed
+     * under the interest subfolder while still rolling up to the parent
+     * client folder. NULL for client/yacht/company-level uploads.
+     *
+     * Added by migration 0078; not yet wired into ensureEntityFolder
+     * (interest subfolder nesting) — see master UAT line 728+ for the
+     * remaining work plan.
+     */
+    interestId: text('interest_id').references(() => interests.id, { onDelete: 'set null' }),
     folderId: text('folder_id').references((): AnyPgColumn => documentFolders.id, {
       onDelete: 'set null',
     }),
@@ -57,6 +69,8 @@ export const files = pgTable(
     index('idx_files_port_client').on(table.portId, table.clientId),
     index('idx_files_port_company').on(table.portId, table.companyId),
     index('idx_files_port_yacht').on(table.portId, table.yachtId),
+    index('idx_files_interest').on(table.interestId),
+    index('idx_files_port_interest').on(table.portId, table.interestId),
   ],
 );
 
diff --git a/src/lib/services/document-folders.service.ts b/src/lib/services/document-folders.service.ts
index fd84ac5c..b142a57f 100644
--- a/src/lib/services/document-folders.service.ts
+++ b/src/lib/services/document-folders.service.ts
@@ -5,6 +5,7 @@ import { documentFolders, documents, files, type DocumentFolder } from '@/lib/db
 import { clients } from '@/lib/db/schema/clients';
 import { companies } from '@/lib/db/schema/companies';
 import { yachts } from '@/lib/db/schema/yachts';
+import { interests } from '@/lib/db/schema/interests';
 import { createAuditLog } from '@/lib/audit';
 import { ConflictError, NotFoundError, ValidationError } from '@/lib/errors';
 import { logger } from '@/lib/logger';
@@ -410,13 +411,17 @@ export async function ensureSystemRoots(portId: string, userId: string): Promise
 
 // ─── ensureEntityFolder ──────────────────────────────────────────────────────
 
-export type EntityType = 'client' | 'company' | 'yacht';
-const ENTITY_TYPES = new Set<EntityType>(['client', 'company', 'yacht']);
+export type EntityType = 'client' | 'company' | 'yacht' | 'interest';
+const ENTITY_TYPES = new Set<EntityType>(['client', 'company', 'yacht', 'interest']);
 
 /**
  * Returns the display name for an entity used to label its subfolder.
  * Clients use `fullName` verbatim (matching rep-facing list views).
- * Companies and yachts use their `name` column verbatim.
+ * Companies and yachts use their `name` column verbatim. Interest
+ * folders are derived from their primary berth (when set) so the
+ * tree reads "Acme Corp / A1-A3" rather than a meaningless interest
+ * UUID; falls back to the createdAt date as a stable label when no
+ * primary berth is linked yet.
  */
 async function resolveEntityDisplayName(
   portId: string,
@@ -439,6 +444,21 @@ async function resolveEntityDisplayName(
     if (!co) throw new NotFoundError('Company');
     return co.name;
   }
+  if (entityType === 'interest') {
+    const i = await db.query.interests.findFirst({
+      where: and(eq(interests.id, entityId), eq(interests.portId, portId)),
+      columns: { createdAt: true },
+    });
+    if (!i) throw new NotFoundError('Interest');
+    // Defer to the interest-berths service for the primary berth label
+    // — circular-dep avoidance via dynamic import. Falls back to the
+    // ISO date slice ("Deal 2026-05-12") when no berth is linked yet.
+    const { getPrimaryBerth } = await import('@/lib/services/interest-berths.service');
+    const primary = await getPrimaryBerth(entityId).catch(() => null);
+    if (primary?.mooringNumber) return `Deal — ${primary.mooringNumber}`;
+    const dateSlice = i.createdAt.toISOString().slice(0, 10);
+    return `Deal ${dateSlice}`;
+  }
   // yacht
   const y = await db.query.yachts.findFirst({
     where: and(eq(yachts.id, entityId), eq(yachts.portId, portId)),
@@ -510,21 +530,37 @@ export async function ensureEntityFolder(
   });
   if (existing) return existing;
 
-  // Locate the system root for this entity type.
-  const rootName: SystemRootName =
-    entityType === 'client' ? 'Clients' : entityType === 'company' ? 'Companies' : 'Yachts';
-  const root = await db.query.documentFolders.findFirst({
-    where: and(
-      eq(documentFolders.portId, portId),
-      eq(documentFolders.entityType, 'root'),
-      eq(documentFolders.name, rootName),
-    ),
-  });
-  if (!root) {
-    // Self-heal: the port-init hook may have been skipped (legacy port).
-    await ensureSystemRoots(portId, userId);
-    return ensureEntityFolder(portId, entityType, entityId, userId);
+  // Resolve the parent folder. For client / company / yacht this is the
+  // matching system root. For interest, parent is the owning client's
+  // entity folder so the tree renders nested: Clients/Acme/Deal A1-A3/.
+  let parent: DocumentFolder | undefined;
+  if (entityType === 'interest') {
+    const interestRow = await db.query.interests.findFirst({
+      where: and(eq(interests.id, entityId), eq(interests.portId, portId)),
+      columns: { clientId: true },
+    });
+    if (!interestRow) throw new NotFoundError('Interest');
+    // Recursively ensure the parent client's folder first — guarantees
+    // we always land inside the existing Clients/<Name>/ subfolder even
+    // when the deal's first artifact predates any client-level upload.
+    parent = await ensureEntityFolder(portId, 'client', interestRow.clientId, userId);
+  } else {
+    const rootName: SystemRootName =
+      entityType === 'client' ? 'Clients' : entityType === 'company' ? 'Companies' : 'Yachts';
+    parent = await db.query.documentFolders.findFirst({
+      where: and(
+        eq(documentFolders.portId, portId),
+        eq(documentFolders.entityType, 'root'),
+        eq(documentFolders.name, rootName),
+      ),
+    });
+    if (!parent) {
+      // Self-heal: the port-init hook may have been skipped (legacy port).
+      await ensureSystemRoots(portId, userId);
+      return ensureEntityFolder(portId, entityType, entityId, userId);
+    }
   }
+  const root = parent;
 
   const baseName = await resolveEntityDisplayName(portId, entityType, entityId);
 
diff --git a/src/lib/services/files.ts b/src/lib/services/files.ts
index d51d07a7..e314f100 100644
--- a/src/lib/services/files.ts
+++ b/src/lib/services/files.ts
@@ -581,6 +581,7 @@ async function fetchGroupRows(
       clientId: files.clientId,
       yachtId: files.yachtId,
       companyId: files.companyId,
+      interestId: files.interestId,
       folderId: files.folderId,
       filename: files.filename,
       originalName: files.originalName,