diff --git a/extensions/qa-matrix/src/runners/contract/scenario-runtime-shared.ts b/extensions/qa-matrix/src/runners/contract/scenario-runtime-shared.ts
index d27b2e7b664..a655ccde885 100644
--- a/extensions/qa-matrix/src/runners/contract/scenario-runtime-shared.ts
+++ b/extensions/qa-matrix/src/runners/contract/scenario-runtime-shared.ts
@@ -87,12 +87,10 @@ export function buildMatrixBlockStreamingPrompt(
   secondText: string,
 ) {
   return [
-    sutUserId,
-    "Block streaming QA check:",
-    "emit exactly two assistant message blocks in order.",
-    `First exact marker: \`${firstText}\`.`,
-    `Second exact marker: \`${secondText}\`.`,
-  ].join(" ");
+    `${sutUserId} Block streaming QA check: reply with exactly this two-line body and no extra text:`,
+    firstText,
+    secondText,
+  ].join("\n");
 }
 
 export function isMatrixQaMessageLikeKind(kind: MatrixQaObservedEvent["kind"]) {
diff --git a/extensions/qa-matrix/src/runners/contract/scenarios.test.ts b/extensions/qa-matrix/src/runners/contract/scenarios.test.ts
index 2d6170c112e..951e93201fe 100644
--- a/extensions/qa-matrix/src/runners/contract/scenarios.test.ts
+++ b/extensions/qa-matrix/src/runners/contract/scenarios.test.ts
@@ -2246,10 +2246,11 @@ describe("matrix live qa scenarios", () => {
   it("preserves separate finalized block events when Matrix block streaming is enabled", async () => {
     const primeRoom = vi.fn().mockResolvedValue("driver-sync-start");
     const sendTextMessage = vi.fn().mockResolvedValue("$block-stream-trigger");
-    const readBlockText = (label: "First" | "Second") =>
-      new RegExp(`${label} exact marker: \`([^\\\`]+)\``).exec(
-        String(sendTextMessage.mock.calls[0]?.[0]?.body),
-      )?.[1] ?? `MATRIX_QA_BLOCK_${label.toUpperCase()}_FIXED`;
+    const readBlockText = (label: "ONE" | "TWO") =>
+      String(sendTextMessage.mock.calls[0]?.[0]?.body)
+        .split("\n")
+        .find((line) => line.startsWith(`MATRIX_QA_BLOCK_${label}_`)) ??
+      `MATRIX_QA_BLOCK_${label}_FIXED`;
     const waitForRoomEvent = vi
       .fn()
       .mockImplementationOnce(async () => ({
@@ -2259,7 +2260,7 @@ describe("matrix live qa scenarios", () => {
           eventId: "$block-one",
           sender: "@sut:matrix-qa.test",
           type: "m.room.message",
-          body: readBlockText("First"),
+          body: readBlockText("ONE"),
         },
         since: "driver-sync-block-one",
       }))
@@ -2270,7 +2271,7 @@ describe("matrix live qa scenarios", () => {
           eventId: "$block-two",
           sender: "@sut:matrix-qa.test",
           type: "m.room.message",
-          body: readBlockText("Second"),
+          body: readBlockText("TWO"),
         },
         since: "driver-sync-next",
       }));
@@ -2333,6 +2334,10 @@ describe("matrix live qa scenarios", () => {
       mentionUserIds: ["@sut:matrix-qa.test"],
       roomId: "!block:matrix-qa.test",
     });
+    const body = String(sendTextMessage.mock.calls[0]?.[0]?.body);
+    expect(body).toMatch(
+      /reply with exactly this two-line body and no extra text:\nMATRIX_QA_BLOCK_ONE_[A-F0-9]{8}\nMATRIX_QA_BLOCK_TWO_[A-F0-9]{8}$/,
+    );
     expect(waitForRoomEvent).toHaveBeenNthCalledWith(
       2,
       expect.objectContaining({