diff --git a/docs/plans/2026-08-05-inter-app-comms-implementation.md b/docs/plans/2026-08-05-inter-app-comms-implementation.md
index 921deeb0..021bef75 100644
--- a/docs/plans/2026-08-05-inter-app-comms-implementation.md
+++ b/docs/plans/2026-08-05-inter-app-comms-implementation.md
@@ -520,6 +520,20 @@ describe("defineService", () => {
     ).toThrow(/procedure name/i);
   });
 
+  test("a hand-built procedure is frozen too, not just builder output", () => {
+    // AnyProcedures accepts any ProcedureDef shape; the guarantee must not
+    // depend on the caller having used procedure.build().
+    const contract = defineService({
+      name: "demo",
+      procedures: { ping: { permission: "demo:read" } },
+    });
+    expect(Object.isFrozen(contract.procedures.ping)).toBe(true);
+    expect(() => {
+      (contract.procedures.ping as { permission?: string }).permission = "hacked";
+    }).toThrow();
+    expect(contract.procedures.ping.permission).toBe("demo:read");
+  });
+
   test("the builder is immutable — reusing a base does not cross-contaminate", () => {
     const base = procedure.permission("a:read");
     const one = base.idempotent().build();
@@ -609,7 +623,18 @@ export function defineService<Procedures extends AnyProcedures>(def: {
       );
     }
   }
-  return Object.freeze({ name: def.name, procedures: Object.freeze({ ...def.procedures }) });
+  // Freeze each procedure, not just the map. AnyProcedures accepts any object
+  // of ProcedureDef shape, so a hand-built def that never went through
+  // procedure.build() would otherwise stay mutable and the "single source of
+  // truth" guarantee would rest on every call site remembering the builder.
+  const frozen: Record<string, ProcedureDef> = {};
+  for (const [name, value] of Object.entries(def.procedures)) {
+    frozen[name] = Object.freeze({ ...value });
+  }
+  return Object.freeze({
+    name: def.name,
+    procedures: Object.freeze(frozen) as Procedures,
+  });
 }
 ```