diff --git a/backend/fluksio/api/routes/flows.py b/backend/fluksio/api/routes/flows.py
index 2e8be16..4303578 100644
--- a/backend/fluksio/api/routes/flows.py
+++ b/backend/fluksio/api/routes/flows.py
@@ -1,7 +1,9 @@
 """The flow API. Everything the editor can do is available here first."""
 
 import asyncio
+import os
 import time
+from pathlib import Path
 from typing import Any
 
 import orjson
@@ -400,6 +402,34 @@ def _check_flavors(definition: FlowDef) -> None:
         )
 
 
+def _check_origin(definition: FlowDef) -> None:
+    """Refuse an ``origin.repo`` that is not a project's own directory.
+
+    Every run of a code-defined flow hashes the python files below it, so
+    ``/`` or a home directory makes each run walk far more than the project,
+    and a relative path names nothing the engine can find.
+    """
+    repo = definition.origin.repo if definition.origin is not None else ""
+    if not repo:
+        return
+    if not os.path.isabs(repo):
+        problem = "is a relative path"
+    elif os.path.realpath(repo) == "/":
+        problem = "is the filesystem root"
+    elif os.path.realpath(repo) == os.path.realpath(Path.home()):
+        problem = "is the home directory"
+    else:
+        return
+    raise HTTPException(
+        status_code=422,
+        detail=(
+            f"origin.repo '{repo}' {problem}. It has to be the absolute path "
+            "of the repository the flow is declared in, since every run hashes "
+            "the python files below it. Sync from inside that repository."
+        ),
+    )
+
+
 def _worker_env(node: NodeDef | None) -> dict[str, str]:
     return node.resources.env if node is not None and node.resources else {}
 
@@ -468,6 +498,7 @@ async def save_flow(
         raise HTTPException(status_code=400, detail="Node names must be unique")
 
     _check_flavors(definition)
+    _check_origin(definition)
     if not user.is_superuser:
         _refuse_code_changes(controller, name, definition)
 
diff --git a/backend/tests/api/routes/test_sync.py b/backend/tests/api/routes/test_sync.py
index 79c7985..0dbb977 100644
--- a/backend/tests/api/routes/test_sync.py
+++ b/backend/tests/api/routes/test_sync.py
@@ -9,7 +9,7 @@ from fastapi.testclient import TestClient
 
 from fluksio.core.config import settings
 from fluksio.sdk import FLOWS, MARKER, SyncError
-from fluksio.sdk.client import Client, sync
+from fluksio.sdk.client import ApiError, Client, sync
 
 EXAMPLES = Path(__file__).parents[4] / "examples"
 PREFIX = settings.API_V1_STR
@@ -47,6 +47,15 @@ def commits() -> int:
     return int(result.stdout.strip()) if result.returncode == 0 else 0
 
 
+@pytest.mark.parametrize("repo", ["/", str(Path.home()), "examples"])
+def test_sync_refuses_a_repo_every_run_would_walk_the_disk_for(api, flows, repo):
+    with pytest.raises(ApiError) as refused:
+        sync([flows["train"]], api, origin={**ORIGIN, "repo": repo})
+
+    assert refused.value.status == 422
+    assert "origin.repo" in str(refused.value.detail)
+
+
 def test_sync_stores_a_runnable_flow_with_its_origin(api, flows):
     reports = sync([flows["train"]], api, origin=ORIGIN)
 
diff --git a/docs/code/cli.md b/docs/code/cli.md
index 34a8cd9..cbf3b1b 100644
--- a/docs/code/cli.md
+++ b/docs/code/cli.md
@@ -164,6 +164,11 @@ The generated node bodies import your functions. Keep the project available
 to each machine running them; sync does not copy your package to a remote
 worker. Uploading source requires a superuser token.
 
+The flow records the git repository it was declared in, or the directory when
+there is none, and each run hashes the Python files below it. The engine
+refuses `/`, its home directory and relative paths there, so run sync from
+inside the project's repository.
+
 | Flag | Purpose |
 |---|---|
 | `--dry-run` | Print flow documents and node bodies without uploading |