diff --git a/web/app.py b/web/app.py
index e31f2fd..d85cce0 100644
--- a/web/app.py
+++ b/web/app.py
@@ -23,7 +23,7 @@ sys.path.insert(0, str(Path(__file__).resolve().parent.parent / "mcp-server" / "
 import zipfile
 
 from fastapi import BackgroundTasks, Body, FastAPI, File, Form, HTTPException, UploadFile
-from fastapi.responses import FileResponse, StreamingResponse
+from fastapi.responses import FileResponse, RedirectResponse, StreamingResponse
 from typing import Any, Literal
 from pydantic import BaseModel
 
@@ -31,7 +31,7 @@ import asyncpg
 import httpx
 
 from legal_mcp import config
-from legal_mcp.services import chunker, db, embeddings, extractor, git_sync, metrics as metrics_service, processor, proofreader, research_md
+from legal_mcp.services import chunker, db, embeddings, extractor, git_sync, metrics as metrics_service, processor, proofreader, research_md, storage
 from legal_mcp.tools import cases as cases_tools, search as search_tools, workflow as workflow_tools, drafting as drafting_tools, precedents as precedents_tools
 from legal_mcp.tools.envelope import envelope_unwrap
 
@@ -2769,6 +2769,39 @@ async def api_local_files(case_number: str):
     return result
 
 
+async def serve_blob(
+    path, *, media_type: str, filename: str,
+    bucket=storage.Bucket.DOCUMENTS,
+):
+    """Serve a case blob, routing through the storage layer (#106.5, INV-STG6).
+
+    - ``filesystem`` backend → stream the file from disk (unchanged behaviour).
+    - ``s3``/``dual`` backend → if the object is in MinIO, 302-redirect to a
+      short-lived presigned URL (bytes go browser↔MinIO, never through FastAPI);
+      under ``dual`` a not-yet-migrated file (e.g. the dynamically-built analysis
+      DOCX or research markdown) transparently falls back to its disk copy. This
+      is the tri-model panel's agreed cutover-safety design (2026-06-11): dual +
+      disk-fallback covers files outside the DB-tracked migration set.
+
+    The caller still owns its own existence/path-safety checks; this only
+    decides HOW to serve a path it already validated.
+    """
+    from pathlib import Path as _Path
+    backend = storage.get_storage()
+    if backend.name != "filesystem":
+        try:
+            key = storage.normalize_key(path)
+            if await backend.exists(key, bucket=bucket):
+                url = await backend.presign_get(key, bucket=bucket, download_name=filename)
+                return RedirectResponse(url, status_code=302)
+        except Exception:  # noqa: BLE001 — never 500 on a key/presign hiccup; fall back to disk
+            logger.exception("serve_blob: presign failed for %s; falling back to disk", path)
+        # not in object storage (dual fallback) — serve the disk copy if present
+        if not _Path(path).exists():
+            raise HTTPException(404, "קובץ לא נמצא")
+    return FileResponse(path, media_type=media_type, filename=filename)
+
+
 @app.get("/api/cases/{case_number}/local-files/{folder}/{filename}")
 async def api_read_local_file(case_number: str, folder: str, filename: str):
     """Read contents of a local case file."""
@@ -2781,7 +2814,7 @@ async def api_read_local_file(case_number: str, folder: str, filename: str):
         path = case_dir / "documents" / folder / filename
     if not path.exists() or not path.is_file():
         raise HTTPException(404, "קובץ לא נמצא")
-    return FileResponse(path, media_type="text/plain; charset=utf-8", filename=filename)
+    return await serve_blob(path, media_type="text/plain; charset=utf-8", filename=filename)
 
 
 # ── Research analysis (analysis-and-research.md) — parse + edit ────
@@ -2812,7 +2845,7 @@ async def api_research_analysis_download(case_number: str):
     path = _research_file_path(case_number)
     if not path.exists():
         raise HTTPException(404, "טרם בוצע ניתוח משפטי לתיק זה")
-    return FileResponse(
+    return await serve_blob(
         path,
         media_type="text/markdown; charset=utf-8",
         filename=f"analysis-{case_number}.md",
@@ -2833,7 +2866,7 @@ async def api_research_analysis_export_docx(case_number: str):
     case_dir = config.find_case_dir(case_number)
     if case_dir.exists():
         commit_and_push(case_dir, f"ניתוח משפטי: {path.name}")
-    return FileResponse(
+    return await serve_blob(
         path,
         media_type="application/vnd.openxmlformats-officedocument.wordprocessingml.document",
         filename=path.name,
@@ -3095,7 +3128,7 @@ async def api_download_export(case_number: str, filename: str):
     path = export_dir / filename
     if not path.exists() or not path.parent.samefile(export_dir):
         raise HTTPException(404, "קובץ לא נמצא")
-    return FileResponse(
+    return await serve_blob(
         path,
         media_type="application/vnd.openxmlformats-officedocument.wordprocessingml.document",
         filename=filename,
diff --git a/web/tests/test_serve_blob.py b/web/tests/test_serve_blob.py
new file mode 100644
index 0000000..6ec8373
--- /dev/null
+++ b/web/tests/test_serve_blob.py
@@ -0,0 +1,80 @@
+"""Tests for #106.5 — web.app.serve_blob backend routing (INV-STG6).
+
+Verifies the tri-model-panel cutover-safety design (2026-06-11):
+- filesystem  → FileResponse from disk (behaviour-preserving, current prod).
+- s3/dual     → 302 redirect to a presigned URL when the object is in MinIO.
+- dual + miss → disk fallback (covers files outside the DB-tracked migration set
+  — e.g. dynamically-built analysis DOCX / research markdown).
+- s3 + miss + no disk → 404.
+
+Importing web/app.py needs a few env vars (it wires the Paperclip pool at import);
+they're set before import. Skips cleanly if the heavy import can't be satisfied.
+"""
+
+from __future__ import annotations
+
+import asyncio
+import os
+import sys
+from pathlib import Path
+
+import pytest
+
+os.environ.setdefault("PAPERCLIP_DB_URL", "postgres://x:x@127.0.0.1:54329/paperclip")
+os.environ.setdefault("DATA_DIR", "/home/chaim/legal-ai/data")
+sys.path.insert(0, str(Path(__file__).resolve().parents[1]))                       # web/
+sys.path.insert(0, str(Path(__file__).resolve().parents[2] / "mcp-server" / "src"))  # legal_mcp
+
+app = pytest.importorskip("app", reason="web/app.py import prerequisites unavailable")
+from fastapi.responses import FileResponse, RedirectResponse  # noqa: E402
+from legal_mcp.services import storage  # noqa: E402
+
+DATA_DIR = Path(os.environ["DATA_DIR"])
+
+
+class _FakeBackend:
+    def __init__(self, name: str, has: bool) -> None:
+        self.name, self._has = name, has
+
+    async def exists(self, key, *, bucket) -> bool:  # noqa: ANN001
+        return self._has
+
+    async def presign_get(self, key, *, bucket, download_name=None) -> str:  # noqa: ANN001
+        return f"https://s3.example/{key}"
+
+
+@pytest.fixture()
+def blob(tmp_path_factory, monkeypatch):
+    # a real file UNDER DATA_DIR so storage.normalize_key accepts it
+    d = DATA_DIR / "audit"
+    d.mkdir(parents=True, exist_ok=True)
+    f = d / "_serveblob_pytest.txt"
+    f.write_text("hi")
+    yield f
+    f.unlink(missing_ok=True)
+
+
+def _serve(monkeypatch, name, has, path):
+    monkeypatch.setattr(storage, "get_storage", lambda: _FakeBackend(name, has))
+    return asyncio.new_event_loop().run_until_complete(
+        app.serve_blob(str(path), media_type="text/plain", filename="x.txt"))
+
+
+def test_filesystem_serves_from_disk(blob, monkeypatch):
+    assert isinstance(_serve(monkeypatch, "filesystem", False, blob), FileResponse)
+
+
+def test_dual_in_s3_redirects_presigned(blob, monkeypatch):
+    assert isinstance(_serve(monkeypatch, "dual", True, blob), RedirectResponse)
+
+
+def test_dual_missing_falls_back_to_disk(blob, monkeypatch):
+    # the panel's safety net: a file not yet in MinIO is still served from disk
+    assert isinstance(_serve(monkeypatch, "dual", False, blob), FileResponse)
+
+
+def test_s3_missing_no_disk_404(monkeypatch):
+    from fastapi import HTTPException
+    with pytest.raises(HTTPException) as ei:
+        _serve(monkeypatch, "s3", False, DATA_DIR / "audit" / "_nope.txt")
+    assert ei.value.status_code == 404