diff --git a/src/server/responses/encrypted-payload.ts b/src/server/responses/encrypted-payload.ts
index 9de44cd2a2..3df72f769a 100644
--- a/src/server/responses/encrypted-payload.ts
+++ b/src/server/responses/encrypted-payload.ts
@@ -175,7 +175,24 @@ function textWithoutFernetRuns(payload: string, runs: readonly FernetTokenRun[])
   return `${text}${payload.slice(last)}`;
 }
 
-export const AGENT_MESSAGE_ROUTING_ENVELOPE = /(?:^|\n)Message Type\s*:\s*NEW_TASK[^\n]*\nTask name\s*:[^\n]*\nSender\s*:[^\n]*\nPayload\s*:\s*(?:\n|$)/gi;
+/**
+ * The routing header codex-rs writes above a delegated agent payload.
+ *
+ * `MESSAGE` is matched as well as `NEW_TASK`, and only for the unreadability CHECK --
+ * recovery stays NEW_TASK-only. #3021 reported a subagent `MESSAGE` arriving in the
+ * parent conversation as raw `gAAAA...` ciphertext after an `adapter_eof`. The detector
+ * decides "unreadable" by stripping the envelope and asking whether any plaintext
+ * survives, so an envelope shape it does not recognise counts as surviving text: a
+ * `MESSAGE` whose entire body is one Fernet token measured as READABLE and was forwarded
+ * verbatim.
+ *
+ * Widening the strip is not the same as widening recovery. Recovery decrypts, and
+ * decrypting a `MESSAGE` on the parent's behalf would build a plaintext oracle out of a
+ * payload the parent's session may have no right to read. This only lets the proxy
+ * NOTICE that what it is about to forward is unreadable ciphertext, which is what the
+ * report asks for: fail closed with a structured error rather than paste the token.
+ */
+export const AGENT_MESSAGE_ROUTING_ENVELOPE = /(?:^|\n)Message Type\s*:\s*(?:NEW_TASK|MESSAGE)[^\n]*\nTask name\s*:[^\n]*\nSender\s*:[^\n]*\nPayload\s*:\s*(?:\n|$)/gi;
 
 // CXC is the compatibility-hook control namespace. Strip only the tagged paragraph:
 // later untagged paragraphs may be genuine task text. Repeated CXC paragraphs are
diff --git a/tests/v2-agent-message-failfast.test.ts b/tests/v2-agent-message-failfast.test.ts
index 5501c3362b..9e218dec11 100644
--- a/tests/v2-agent-message-failfast.test.ts
+++ b/tests/v2-agent-message-failfast.test.ts
@@ -32,6 +32,10 @@ const ROUTING_ENVELOPE = [
   "",
 ].join("\n");
 
+// The same envelope a delegated agent uses to REPLY, as opposed to being spawned.
+// #3021 saw one of these reach the parent conversation as raw `gAAAA...` text.
+const MESSAGE_ROUTING_ENVELOPE = ROUTING_ENVELOPE.replace("NEW_TASK", "MESSAGE");
+
 afterEach(() => {
   globalThis.fetch = originalFetch;
 });
@@ -135,6 +139,46 @@ describe("V2 routed agent-message ciphertext guard", () => {
     ]))).toBe(true);
   });
 
+  /**
+   * #3021: a delegated subagent's MESSAGE reply reached the parent conversation as
+   * raw `gAAAA...` ciphertext after an `adapter_eof`.
+   *
+   * The detector decides "unreadable" by stripping the routing envelope and asking
+   * whether any plaintext survives, so an envelope shape it does not recognise counts
+   * as surviving text. The envelope pattern matched only NEW_TASK, so a MESSAGE whose
+   * entire body was one Fernet token measured as READABLE and was forwarded verbatim.
+   *
+   * This is the detection half only. Recovery stays NEW_TASK-only on purpose:
+   * decrypting a MESSAGE on the parent's behalf would build a plaintext oracle out of
+   * a payload the parent's session may not be entitled to read.
+   */
+  test("blocks a MESSAGE reply envelope followed only by a Fernet payload", () => {
+    expect(hasUnreadableEncryptedAgentTask(agentMessage([
+      { type: "input_text", text: MESSAGE_ROUTING_ENVELOPE },
+      { type: "encrypted_content", encrypted_content: FERNET_TASK },
+    ]))).toBe(true);
+  });
+
+  test("blocks a MESSAGE envelope carried inside the encrypted slot itself", () => {
+    // The shape the report describes: header and ciphertext arrive as one
+    // encrypted_content string rather than as separate parts.
+    expect(hasUnreadableEncryptedAgentTask(agentMessage([
+      {
+        type: "encrypted_content",
+        encrypted_content: `${MESSAGE_ROUTING_ENVELOPE}${FERNET_TASK}`,
+      },
+    ]))).toBe(true);
+  });
+
+  test("a MESSAGE reply that carries real text stays readable", () => {
+    // The control. Widening the envelope must not turn every agent reply into a
+    // blocked one -- only the ones with nothing left after the header comes off.
+    expect(hasUnreadableEncryptedAgentTask(agentMessage([
+      { type: "input_text", text: `${MESSAGE_ROUTING_ENVELOPE}the worker finished the migration` },
+      { type: "encrypted_content", encrypted_content: FERNET_TASK },
+    ]))).toBe(false);
+  });
+
   test("blocks a control preamble mixed into the Fernet slot before sanitization", async () => {
     const input = agentMessage([
       { type: "input_text", text: ROUTING_ENVELOPE },