diff --git a/dotbot/cli/_fw_sources.py b/dotbot/cli/_fw_sources.py
index 485be125..afa976ad 100644
--- a/dotbot/cli/_fw_sources.py
+++ b/dotbot/cli/_fw_sources.py
@@ -431,6 +431,7 @@ def write_build_manifest(
     for f in files:
         entries[f.name] = {
             "sha256": sha256(f),
+            "repo": str(Path(repo).resolve()),
             "git_sha": sha,
             "dirty": dirty,
             "build_config": config,
@@ -453,6 +454,18 @@ def write_build_manifest(
     return manifest
 
 
+def recorded_repo(out_dir: Path) -> Path | None:
+    """The source folder a built set's manifest names, or None."""
+    try:
+        manifest = json.loads((out_dir / MANIFEST_NAME).read_text())
+    except (OSError, ValueError):
+        return None
+    if not isinstance(manifest, dict) or manifest.get("kind") != "build":
+        return None
+    repo = manifest.get("repo")
+    return Path(repo) if repo else None
+
+
 def collect(files: Iterable[Path], out_dir: Path) -> list[Path]:
     """Copy `files` flat into `out_dir` under their own names.
 
diff --git a/dotbot/cli/fw.py b/dotbot/cli/fw.py
index 4dfe4e2e..372d02ba 100644
--- a/dotbot/cli/fw.py
+++ b/dotbot/cli/fw.py
@@ -222,6 +222,15 @@ def _source_repo(source: str, path: Path | None, explicit: bool) -> Path:
     default=False,
     help="Print where each artifact would be collected, without building.",
 )
+@click.option(
+    "--force",
+    is_flag=True,
+    default=False,
+    help=(
+        "Build into an --as set that was built from another source folder, "
+        "which then names this one. Its other images stay as they were built."
+    ),
+)
 @_verbose_option
 @click.pass_context
 def build(
@@ -236,6 +245,7 @@ def build(
     set_name,
     rebuild,
     print_path,
+    force,
     verbose,
 ):
     """Build firmware from your source folders into the cache.
@@ -329,6 +339,19 @@ def build(
             planned.append((source, src_repo, cfg, target, files, df_apps))
 
     root = artifacts_dir()
+    for source, src_repo, *_ in planned:
+        out = fs.set_dir(source, set_name, root)
+        recorded = fs.recorded_repo(out)
+        if force or recorded is None or recorded == src_repo.resolve():
+            continue
+        raise click.ClickException(
+            f"The '{set_name}' {source} set was built from {recorded}, not "
+            f"{src_repo.resolve()}: images from two source folders in one set "
+            "can disagree, e.g. on the swarmit NSC table.\n"
+            f"  - build from the set's folder: --path {recorded}\n"
+            "  - or build into another set: --as <name>\n"
+            "  - or mix them anyway: --force"
+        )
     if print_path:
         for source, _, _, _, files, _ in planned:
             out = fs.set_dir(source, set_name, root)
@@ -340,8 +363,11 @@ def build(
     t0 = time.perf_counter()
     for source, src_repo, cfg, board, _, selection in planned:
         mode = "rebuild" if rebuild or source != APP_SOURCE else "incremental"
+        origin = "--path" if path is not None else "configured source"
         click.echo(
-            f"Building {_LABELS[source]} for {board} ({cfg}, {mode})...", err=True
+            f"Building {_LABELS[source]} for {board} ({cfg}, {mode}) from "
+            f"{src_repo} ({origin})...",
+            err=True,
         )
         if source == "swarmit":
             files = fs.build_swarmit(
diff --git a/dotbot/tests/test_fw_sources.py b/dotbot/tests/test_fw_sources.py
index e2d5c324..ddc41130 100644
--- a/dotbot/tests/test_fw_sources.py
+++ b/dotbot/tests/test_fw_sources.py
@@ -958,3 +958,29 @@ def test_list_survives_a_malformed_manifest(isolated, manifest):
     assert result.exit_code == 0, result.output
     assert "swarmit-odd" in result.output
     assert "  netcore-nrf5340-net.hex" in result.output
+
+
+# --- the set's source folder ----------------------------------------------
+
+
+def test_build_into_a_set_from_another_folder_is_refused(
+    isolated, swarmit_repo, fake_build
+):
+    other = isolated / "wt-swarmit"
+    shutil.copytree(swarmit_repo, other)
+    assert build("swarmit-sandbox", "--as", "bench").exit_code == 0
+    result = build("swarmit-sandbox", "--path", str(other), "--as", "bench")
+    assert result.exit_code != 0
+    assert str(swarmit_repo.resolve()) in result.output
+    assert str(other.resolve()) in result.output
+    assert "--force" in result.output
+    forced = build("swarmit-sandbox", "--path", str(other), "--as", "bench", "--force")
+    assert forced.exit_code == 0, forced.output
+    assert f"from {other}" in forced.output
+
+
+def test_build_says_which_source_folder_it_builds_from(
+    isolated, firmware_repo, fake_make
+):
+    result = build("dotbot")
+    assert f"from {firmware_repo} (configured source)" in result.output