diff --git a/backend/src/cyclone/store.py b/backend/src/cyclone/store.py
index 81a654f..5079658 100644
--- a/backend/src/cyclone/store.py
+++ b/backend/src/cyclone/store.py
@@ -2302,6 +2302,49 @@ class CycloneStore:
                 "updated_at": row.updated_at,
             })
 
+    def update_clearhouse(self, block: Clearhouse) -> Clearhouse:
+        """Replace the singleton clearhouse row. SP25.
+
+        Used by ``PATCH /api/clearhouse`` to flip ``sftp_block.stub``
+        and adjust host/port/paths without touching SQLite directly.
+        Raises ``LookupError`` if the singleton row is missing — the
+        caller is expected to run the lifespan seed first.
+        """
+        from cyclone.db import ClearhouseORM
+        with db.SessionLocal()() as s:
+            row = s.get(ClearhouseORM, 1)
+            if row is None:
+                raise LookupError(
+                    "clearhouse singleton row missing; run the lifespan "
+                    "seed (ensure_clearhouse_seeded) before PATCH /api/clearhouse"
+                )
+            row.name = block.name
+            row.tpid = block.tpid
+            row.submitter_name = block.submitter_name
+            row.submitter_id_qual = block.submitter_id_qual
+            row.submitter_contact_name = block.submitter_contact_name
+            row.submitter_contact_email = block.submitter_contact_email
+            row.filename_block_json = json.loads(
+                json.dumps(block.filename_block.model_dump())
+            )
+            row.sftp_block_json = json.loads(
+                json.dumps(block.sftp_block.model_dump())
+            )
+            row.updated_at = datetime.now(timezone.utc).isoformat()
+            s.commit()
+            return Clearhouse.model_validate({
+                "id": 1,
+                "name": row.name,
+                "tpid": row.tpid,
+                "submitter_name": row.submitter_name,
+                "submitter_id_qual": row.submitter_id_qual,
+                "submitter_contact_name": row.submitter_contact_name,
+                "submitter_contact_email": row.submitter_contact_email,
+                "filename_block": dict(row.filename_block_json),
+                "sftp_block": dict(row.sftp_block_json),
+                "updated_at": row.updated_at,
+            })
+
     def ensure_clearhouse_seeded(self) -> None:
         """Insert the default clearhouse singleton + 3 providers + CO_TXIX payer
         if they don't exist. Idempotent. Called from the API lifespan."""
diff --git a/backend/tests/test_store_update_clearhouse.py b/backend/tests/test_store_update_clearhouse.py
new file mode 100644
index 0000000..51fe765
--- /dev/null
+++ b/backend/tests/test_store_update_clearhouse.py
@@ -0,0 +1,89 @@
+"""SP25 — store.update_clearhouse() round-trip test.
+
+The PATCH /api/clearhouse endpoint needs a way to replace the singleton
+clearhouse row inside one session. This test exercises the method
+directly against a fresh in-memory DB.
+"""
+from __future__ import annotations
+
+import pytest
+
+from cyclone import db
+from cyclone.providers import Clearhouse, SftpBlock
+from cyclone.store import store as cycl_store
+
+
+def _seed_clearhouse():
+    """Insert the default clearhouse row used by SP9's lifespan seed."""
+    cycl_store.ensure_clearhouse_seeded()
+
+
+def test_update_clearhouse_round_trip():
+    """Replace all fields on the singleton row; GET returns the new values."""
+    _seed_clearhouse()
+
+    new_block = SftpBlock(
+        host="mft.gainwelltechnologies.com",
+        port=22,
+        username="colorado-fts\\coxix_prod_11525703",
+        paths={
+            "outbound": "/CO XIX/PROD/coxix_prod_11525703/FromHPE",
+            "inbound": "/CO XIX/PROD/coxix_prod_11525703/ToHPE",
+        },
+        stub=False,
+        staging_dir="./var/sftp/staging",
+        auth={"password_keychain_account": "sftp.gainwell.password"},
+    )
+
+    new_clearhouse = Clearhouse.model_validate({
+        "id": 1,
+        "name": "dzinesco",
+        "tpid": "11525703",
+        "submitter_name": "Dzinesco",
+        "submitter_id_qual": "46",
+        "submitter_contact_name": "Tyler Martinez",
+        "submitter_contact_email": "tyler@dzinesco.com",
+        "filename_block": {
+            "tz": "America/Denver",
+            "outbound_template": "{tpid}-{tx}-{ts_mt}-1of1.{ext}",
+            "inbound_template": "TP{tpid}-{orig_tx}_M{tracking}-{ts}-1of1_{file_type}.x12",
+        },
+        "sftp_block": new_block.model_dump(),
+        "updated_at": "2026-06-24T00:00:00+00:00",
+    })
+
+    cycl_store.update_clearhouse(new_clearhouse)
+
+    fetched = cycl_store.get_clearhouse()
+    assert fetched is not None
+    assert fetched.sftp_block.stub is False
+    assert fetched.sftp_block.host == "mft.gainwelltechnologies.com"
+    assert fetched.sftp_block.auth == {"password_keychain_account": "sftp.gainwell.password"}
+
+
+def test_update_clearhouse_missing_row_raises():
+    """If the singleton row was never seeded, update raises LookupError."""
+    # Wipe the DB.
+    db._reset_for_tests()
+    db.init_db()
+
+    new_block = SftpBlock(
+        host="mft.example.com", port=22, username="u",
+        paths={"outbound": "/o", "inbound": "/i"},
+        stub=True, staging_dir="/tmp", auth={},
+    )
+    new_clearhouse = Clearhouse.model_validate({
+        "id": 1, "name": "x", "tpid": "1",
+        "submitter_name": "X", "submitter_id_qual": "46",
+        "submitter_contact_name": "X", "submitter_contact_email": "x@x",
+        "filename_block": {
+            "tz": "America/Denver",
+            "outbound_template": "{tpid}-{tx}-{ts}-1of1.{ext}",
+            "inbound_template": "TP{tpid}-{tx}_{ts}.x12",
+        },
+        "sftp_block": new_block.model_dump(),
+        "updated_at": "2026-06-24T00:00:00+00:00",
+    })
+
+    with pytest.raises(LookupError):
+        cycl_store.update_clearhouse(new_clearhouse)