diff --git a/sdk/typescript/src/agent/messaging.ts b/sdk/typescript/src/agent/messaging.ts
index 040a55ac..039639ef 100644
--- a/sdk/typescript/src/agent/messaging.ts
+++ b/sdk/typescript/src/agent/messaging.ts
@@ -89,9 +89,26 @@ export interface SendMessageResult {
 }
 
 /**
- * Sends a message to `recipient` (a @handle, cryptoId, or base64 key). The body
- * is Signal-encrypted by the client before it leaves the process when encryption
- * is configured (the recommended setup); otherwise it is sent as plaintext.
+ * The E2E facade REQUIRES a client with encryption configured (`encryption: { store }`
+ * + a signer). A client without it would relay the body as plaintext, which the backend
+ * rejects — a plaintext JSON body trips its `looksLikeJSON` guard with `HTTP 400: body
+ * must be encrypted ciphertext`, surfacing far from the misconfiguration (a client built
+ * without a signer/store, e.g. an under-provisioned daemon). Fail fast with a clear cause
+ * instead of leaking plaintext. Callers that want plain transport use `client.messages.send`.
+ */
+function assertEncryptionEnabled(client: TinyPlaceClient): void {
+  if (client.encryptionEnabled) return;
+  throw new Error(
+    "agent messaging requires encryption: construct the client with " +
+      "`encryption: { store }` and a signer. Sending a plaintext body over the " +
+      'E2E channel is rejected by the relay ("body must be encrypted ciphertext").',
+  );
+}
+
+/**
+ * Sends a message to `recipient` (a @handle, cryptoId, or base64 key). The body is
+ * Signal-encrypted by the client before it leaves the process; encryption must be
+ * configured (see {@link assertEncryptionEnabled}).
  */
 export async function sendMessage(
   client: TinyPlaceClient,
@@ -99,6 +116,7 @@ export async function sendMessage(
   recipient: string,
   text: string,
 ): Promise<SendMessageResult> {
+  assertEncryptionEnabled(client);
   const to = await resolveRecipientKey(client, recipient);
   const envelope: MessageEnvelope = {
     id: messageId(),
diff --git a/sdk/typescript/tests/agent-messaging.test.ts b/sdk/typescript/tests/agent-messaging.test.ts
index 600e3c5e..9d3f8e41 100644
--- a/sdk/typescript/tests/agent-messaging.test.ts
+++ b/sdk/typescript/tests/agent-messaging.test.ts
@@ -162,4 +162,16 @@ describe("sendMessage / readMessages round-trip", () => {
     // Consumed on read.
     expect(await readMessages(bob.client, bob.signer)).toHaveLength(0);
   });
+
+  it("refuses to send when the client has no encryption configured", async () => {
+    // A client built without `encryption: { store }` would relay the body as
+    // plaintext, which the backend rejects with `400: body must be encrypted
+    // ciphertext`. The facade must fail fast at the misconfiguration instead.
+    const signer = await LocalSigner.generate();
+    const plain = new TinyPlaceClient({ baseUrl: "https://relay.test", signer });
+    expect(plain.encryptionEnabled).toBe(false);
+    await expect(
+      sendMessage(plain, signer, signer.agentId, "hi"),
+    ).rejects.toThrow(/requires encryption/);
+  });
 });