diff --git a/packages/paperclip-runner/src/drivers/acpx/runtime-host.test.ts b/packages/paperclip-runner/src/drivers/acpx/runtime-host.test.ts
index 2db1cb51c64..f6b637a4f16 100644
--- a/packages/paperclip-runner/src/drivers/acpx/runtime-host.test.ts
+++ b/packages/paperclip-runner/src/drivers/acpx/runtime-host.test.ts
@@ -309,11 +309,18 @@ describe("ACPX runtime host", () => {
           { ...options, runtimeContext: context },
           dependencies,
         );
-        const message = JSON.stringify({
+        let message = JSON.stringify({
           schema: "paperclip.native-model-envelope.v2",
           task: { description: "Use /assigned", prompt: "A new direct user request" },
           interactionResponses: index ? [{ response: { status: "accepted" } }] : [],
         });
+        if (index === 1) {
+          // The provider command is internal overhead, not part of the caller's
+          // 1 MiB allowance. Keep the exact envelope even at that boundary.
+          message += " ".repeat(1024 * 1024 - Buffer.byteLength(message));
+          expect(() => host.startTurn({ text: `${message} `, requestId: "oversized" }))
+            .toThrow("turn text exceeds its bounded size");
+        }
         host.startTurn({ text: message, requestId: `skill-turn-${index}` });
         expect(providerStartTurn).toHaveBeenLastCalledWith({
           text: `/assigned ${message}`, requestId: `skill-turn-${index}`,
diff --git a/packages/paperclip-runner/src/drivers/acpx/runtime-host.ts b/packages/paperclip-runner/src/drivers/acpx/runtime-host.ts
index a400499f0db..efc09052a5b 100644
--- a/packages/paperclip-runner/src/drivers/acpx/runtime-host.ts
+++ b/packages/paperclip-runner/src/drivers/acpx/runtime-host.ts
@@ -649,9 +649,12 @@ export class AcpxRuntimeHost {
       throw new Error("ACPX runtime host already has an active turn");
     }
     const requestId = boundedRequestId(input.requestId);
-    const text = boundedTurnText(claudeNativeSkillPrompt(
+    // Bound caller input before adding the provider's assigned-skill command.
+    // The command is internal framing; it must not consume the envelope's
+    // allowance or cause an otherwise valid envelope to be truncated/rejected.
+    const text = claudeNativeSkillPrompt(
       boundedTurnText(input.text), this.#claudeSkillNames,
-    ));
+    );
     const turn = this.#runtime.startTurn({
       text,
       requestId,