diff --git a/backend/src/cyclone/api.py b/backend/src/cyclone/api.py
index 5d74285..091f9bf 100644
--- a/backend/src/cyclone/api.py
+++ b/backend/src/cyclone/api.py
@@ -971,6 +971,20 @@ def get_ack_endpoint(ack_id: int) -> dict:
         )
     body = _ack_to_ui(row)
     body["raw_json"] = row.raw_json
+    # Regenerate the X12 text from raw_json so the operator can download
+    # the actual 999 file. (SP3 P3 follow-up: list endpoint doesn't carry
+    # the regenerated text to keep payloads small; detail does.)
+    if row.raw_json:
+        try:
+            from cyclone.parsers.models_999 import ParseResult999
+            regenerated = ParseResult999.model_validate(row.raw_json)
+            icn = regenerated.envelope.control_number or "000000001"
+            body["raw_999_text"] = serialize_999(regenerated, interchange_control_number=icn)
+        except Exception as exc:  # noqa: BLE001 — never 500 on a regen failure
+            log.warning("Could not regenerate 999 for ack %s: %s", ack_id, exc)
+            body["raw_999_text"] = None
+    else:
+        body["raw_999_text"] = None
     return body
 
 
diff --git a/backend/tests/test_api_999.py b/backend/tests/test_api_999.py
index 61ce51c..e3d69d5 100644
--- a/backend/tests/test_api_999.py
+++ b/backend/tests/test_api_999.py
@@ -108,3 +108,35 @@ def test_parse_999_rejected_fixture_surfaces_r_code(client: TestClient):
     assert body["ack"]["ack_code"] == "R"
     assert body["ack"]["rejected_count"] == 1
     assert body["ack"]["accepted_count"] == 0
+
+
+def test_get_ack_detail_regenerates_x12_text(client: TestClient) -> None:
+    """SP3 P3 follow-up: GET /api/acks/{id} must include `raw_999_text`
+    regenerated from raw_json so the operator can actually download the
+    999 file. The P3 subagent shipped `raw_999_text: None` as a stub."""
+    text = REJECTED.read_text()
+    post_resp = client.post(
+        "/api/parse-999",
+        files={"file": ("rejected_999.txt", text, "text/plain")},
+        headers={"Accept": "application/json"},
+    )
+    assert post_resp.status_code == 200
+    ack_id = post_resp.json()["ack"]["id"]
+    detail_resp = client.get(f"/api/acks/{ack_id}", headers={"Accept": "application/json"})
+    assert detail_resp.status_code == 200, detail_resp.text
+    body = detail_resp.json()
+    # raw_999_text must be present and non-empty X12.
+    assert "raw_999_text" in body
+    assert body["raw_999_text"] is not None
+    assert body["raw_999_text"].startswith("ISA*")
+    assert "AK1*" in body["raw_999_text"]
+    assert "AK9*" in body["raw_999_text"]
+    # IEA segment exists in output; serializer writes `IEA*1*<icn>~`.
+    assert "IEA*" in body["raw_999_text"]
+    assert body["raw_999_text"].rstrip().endswith("~")
+
+
+def test_get_ack_404_for_missing(client: TestClient) -> None:
+    """GET /api/acks/{id} returns 404 for a missing id (not 500)."""
+    resp = client.get("/api/acks/9999", headers={"Accept": "application/json"})
+    assert resp.status_code == 404