diff --git a/README.md b/README.md
index a74d626..d440a3b 100644
--- a/README.md
+++ b/README.md
@@ -195,3 +195,29 @@ repository carries implementation and does not carry decisions.
 - `02-DECISIONS/0039-the-link-is-the-security-boundary.md` — a node owns no password
 - `02-DECISIONS/0041-the-host-depends-on-nothing.md` — why this is a static binary, and Go
 - `04-ISSUES/007-an-installed-package-is-not-a-capability` — why detection works this way
+
+## Checks that cross into the control plane's repository
+
+Two things are agreed between this repository and `novox/mesh-control`, and each is a separate
+struct on each side. A field renamed on one of them fails **silently** — the crossing succeeds and
+something is simply absent — so both are checked by handing one side's real output to the other's
+real parser. Neither runs by default; each skips with a reason, because a repository that fails
+without its neighbour checked out is a repository nobody can build.
+
+**What the mesh sends, read by this host:**
+
+```
+mesh-control:  ./build/mesh-control plan <node> --json > /tmp/d.json
+mesh-host:     MESH_EMITTED=/tmp/d.json go test ./internal/declaration/ -v
+```
+
+**What this node says when it joins, read by the mesh:**
+
+```
+mesh-host:     MESH_ENROL_OUT=/tmp/enrol.json go test ./internal/link/
+mesh-control:  MESH_ENROL=/tmp/enrol.json make check
+```
+
+The second writes the private half of the sealing key beside the request, so the mesh's suite can
+prove that what it sealed is openable rather than merely present. A key that is correctly named
+and simply *wrong* passes every check that only looks at the message.
diff --git a/internal/link/enrol_shape_test.go b/internal/link/enrol_shape_test.go
new file mode 100644
index 0000000..3100a4a
--- /dev/null
+++ b/internal/link/enrol_shape_test.go
@@ -0,0 +1,62 @@
+package link
+
+import (
+	"encoding/json"
+	"os"
+	"testing"
+
+	"github.com/novox/mesh-host/internal/identity"
+)
+
+// What this node says when it joins, written out so the mesh's own suite can accept it.
+//
+// The two ends are separate structs in separate repositories. Every field here is one somebody
+// could rename on one side, and the failure would be silent: enrolment succeeds, a key is simply
+// absent, and the node looks joined until the first thing sealed to it cannot be opened. That is
+// exactly the shape of fault this project keeps finding late.
+//
+// Skipped unless MESH_ENROL_OUT names a file, so this is a check somebody runs deliberately
+// rather than a dependency between two repositories.
+func TestWhatThisNodeSaysWhenItJoins(t *testing.T) {
+	path := os.Getenv("MESH_ENROL_OUT")
+	if path == "" {
+		t.Skip("set MESH_ENROL_OUT to write the enrolment request the mesh's suite reads")
+	}
+
+	// A real one. Generated the way enrolment generates them rather than typed as literals, so a
+	// key that stopped being a key would be caught here rather than travelling.
+	mine, err := identity.Generate("workstation")
+	if err != nil {
+		t.Fatal(err)
+	}
+	overlay, err := identity.GenerateOverlayKey()
+	if err != nil {
+		t.Fatal(err)
+	}
+	sealing, err := identity.GenerateSealingKey()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	request := EnrolRequest{
+		Node:       "workstation",
+		Secret:     "a-one-time-secret",
+		PublicKey:  mine.Public,
+		OverlayKey: overlay.Public,
+		SealingKey: sealing.Public,
+		Profile:    map[string]any{"seat": true},
+	}
+	body, err := json.MarshalIndent(request, "", "  ")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(path, append(body, '\n'), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	// The private half of the sealing key goes beside it, so the mesh's suite can prove what it
+	// sealed is openable rather than merely present.
+	if err := os.WriteFile(path+".sealing-private", []byte(sealing.Private), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("wrote %s", path)
+}