diff --git a/src/state/index.ts b/src/state/index.ts
index 3b68b3e4..cf7c16f3 100644
--- a/src/state/index.ts
+++ b/src/state/index.ts
@@ -122,6 +122,17 @@ function refusedUnannounced(name: string): void {
   }
 }
 
+/** The size of one chunk of the Go side's put and get sessions, the most a call carries (design 47 G §518, A-21). */
+export const CHUNK_BYTES = 512 * 1024;
+
+/** A TypeScript bundle runs no put or get session in G (design 47 G §643): a value over one call's limit fails at once, naming it. */
+function withinOneCall(key: string, value: unknown): void {
+  const size = Buffer.byteLength(JSON.stringify(value) ?? "");
+  if (size > CHUNK_BYTES) {
+    throw new Error(`the value of "${key}" is ${size} bytes, over the ${CHUNK_BYTES} bytes (512 KiB) one call carries: a TypeScript bundle puts no value larger than that`);
+  }
+}
+
 /** A state this module keeps (by its local name) or reads (`<module>.<name>`). */
 export function state<T = unknown>(name: string): State<T> {
   const bound = (): StateChannel => {
@@ -130,8 +141,10 @@ export function state<T = unknown>(name: string): State<T> {
   };
   return {
     get: async (key) => (await bound().ask("mesh/state.get", { state: name, key })) as StateEntry<T> | null,
-    put: async (key, value) =>
-      ((await bound().ask("mesh/state.put", { state: name, key, value })) as { revision: number }).revision,
+    put: async (key, value) => {
+      withinOneCall(key, value);
+      return ((await bound().ask("mesh/state.put", { state: name, key, value })) as { revision: number }).revision;
+    },
     delete: async (key) => {
       await bound().ask("mesh/state.delete", { state: name, key });
     },
diff --git a/test/sdk.test.ts b/test/sdk.test.ts
index 2ca4e66b..3db384a9 100644
--- a/test/sdk.test.ts
+++ b/test/sdk.test.ts
@@ -1005,3 +1005,8 @@ test("a bundle reads its tool runner's capability words once, reads an unknown c
   if (was === undefined) delete process.env.MESH_NODE;
   else process.env.MESH_NODE = was;
 });
+
+test("a TypeScript bundle's value over one call's limit fails at once, naming the limit (design 47 G §643)", async () => {
+  const { state, CHUNK_BYTES } = await import("../dist/state/index.js");
+  await assert.rejects(state("s").put("k", { text: "x".repeat(CHUNK_BYTES) }), /512 KiB/);
+});