Normalize 1.0.72 background-agent completion notification wording

The 1.0.72 CLI reworded the background-agent completion system_notification
that the runtime injects as a user message: the status sentence changed from
"has completed successfully." to "has finished processing and is now idle."
and the advice changed from "to retrieve unread results." to "to read the
results, or write_agent to send follow-up messages." Existing snapshots store
the old canonical wording, so the replay proxy could not match live 1.0.72
requests, failing subagent_hooks, should_abort_a_session, and the
background-agent task-details test across every SDK.

Extend the existing task-completion-notification normalization (which already
mapped "retrieve unread results" to "retrieve the full results") into
normalizeAgentCompletionNotification, collapsing every known phrasing of both
sentences to one canonical form on both the request and stored sides.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: e94bab38-c22e-4a38-8d8b-c82c1991a33c

Author: stephentoub

test/harness/replayingCapiProxy.test.ts

@@ -379,6 +379,38 @@ describe("ReplayingCapiProxy", () => {
     expect(result.conversations[0].messages[0].content).toBe(fullNotification);
   });
 
+  test("normalizes the idle-phrasing task completion notification", async () => {
+    const idleNotification = [
+      "<system_notification>",
+      'Agent "sdk-background-agent" (general-purpose) has finished processing and is now idle. Use read_agent with agent_id "sdk-background-agent" to read the results, or write_agent to send follow-up messages.',
+      "</system_notification>",
+    ].join("\n");
+    const fullNotification = [
+      "<system_notification>",
+      'Agent "sdk-background-agent" (general-purpose) has completed successfully. Use read_agent with agent_id "sdk-background-agent" to retrieve the full results.',
+      "</system_notification>",
+    ].join("\n");
+
+    const requestBody = JSON.stringify({
+      messages: [
+        {
+          role: "user",
+          content: idleNotification,
+        },
+      ],
+    });
+    const responseBody = JSON.stringify({
+      choices: [{ message: { role: "assistant", content: "Done" } }],
+    });
+
+    const outputPath = await createProxy([
+      { url: "/chat/completions", requestBody, responseBody },
+    ]);
+
+    const result = await readYamlOutput(outputPath);
+    expect(result.conversations[0].messages[0].content).toBe(fullNotification);
+  });
+
   test("strips agent_instructions from user messages", async () => {
     const requestBody = JSON.stringify({
       messages: [
@@ -1321,6 +1353,67 @@ Always include PINEAPPLE_COCONUT_42.
       }
     });
 
+    test("matches cached completion notification against the idle-phrasing runtime", async () => {
+      const cachePath = path.join(tempDir, "cache.yaml");
+      // Snapshot recorded with the canonical completion wording.
+      const canonicalNotification = [
+        "<system_notification>",
+        'Agent "read-file" (explore) has completed successfully. Use read_agent with agent_id "read-file" to retrieve the full results.',
+        "</system_notification>",
+      ].join("\n");
+      // Newer runtime emits the idle phrasing with write_agent advice.
+      const idleNotification = [
+        "<system_notification>",
+        'Agent "read-file" (explore) has finished processing and is now idle. Use read_agent with agent_id "read-file" to read the results, or write_agent to send follow-up messages.',
+        "</system_notification>",
+      ].join("\n");
+
+      const cacheContent = yaml.stringify({
+        models: ["test-model"],
+        conversations: [
+          {
+            messages: [
+              { role: "system", content: "${system}" },
+              { role: "user", content: "Hello" },
+              { role: "assistant", content: "Hi!" },
+              { role: "user", content: canonicalNotification },
+              { role: "assistant", content: "Read agent completed." },
+            ],
+          },
+        ],
+      } satisfies NormalizedData);
+      await writeFile(cachePath, cacheContent);
+
+      const proxy = new ReplayingCapiProxy(
+        "http://localhost:9999",
+        cachePath,
+        workDir,
+      );
+      const proxyUrl = await proxy.start();
+
+      try {
+        const response = await makeRequest(proxyUrl, "/chat/completions", {
+          body: {
+            model: "test-model",
+            messages: [
+              { role: "system", content: "Be helpful" },
+              { role: "user", content: "Hello" },
+              { role: "assistant", content: "Hi!" },
+              { role: "user", content: idleNotification },
+            ],
+          },
+        });
+
+        expect(response.status).toBe(200);
+        expect(
+          (JSON.parse(response.body) as ChatCompletion).choices[0].message
+            .content,
+        ).toBe("Read agent completed.");
+      } finally {
+        await proxy.stop();
+      }
+    });
+
     test("matches parallel tool results regardless of arrival order", async () => {
       const cachePath = path.join(tempDir, "cache.yaml");
       const cacheContent = yaml.stringify({

test/harness/replayingCapiProxy.ts

@@ -1223,8 +1223,9 @@ function transformOpenAIRequestMessage(
 }
 
 function normalizeUserMessage(content: string): string {
-  return normalizeSkillContextFrontmatter(content)
-    .replace(taskCompletionNotificationPattern, taskCompletionNotificationReplacement)
+  return normalizeAgentCompletionNotification(
+    normalizeSkillContextFrontmatter(content),
+  )
     .replace(/<current_datetime>.*?<\/current_datetime>/g, "")
     .replace(/<reminder>[\s\S]*?<\/reminder>/g, "")
     .replace(/<system_reminder>[\s\S]*?<\/system_reminder>/g, "")
@@ -1237,19 +1238,29 @@ function normalizeUserMessage(content: string): string {
     .trim();
 }
 
-const taskCompletionNotificationPattern =
-  /Use read_agent with agent_id "([^"]+)" to retrieve unread results\./g;
-const taskCompletionNotificationReplacement =
-  'Use read_agent with agent_id "$1" to retrieve the full results.';
+// The runtime's background-agent completion system_notification has been reworded
+// across CLI versions — both the status sentence ("has completed successfully." vs
+// "has finished processing and is now idle.") and the follow-up advice ("to retrieve
+// unread results." vs "to read the results, or write_agent to send follow-up
+// messages."). Collapse every known phrasing to one canonical form so snapshots keep
+// matching regardless of which runtime version recorded them or replays against them.
+function normalizeAgentCompletionNotification(content: string): string {
+  return content
+    .replace(
+      /Agent ("[^"]*" \([^)]*\)) has finished processing and is now idle\./g,
+      "Agent $1 has completed successfully.",
+    )
+    .replace(
+      /Use read_agent with agent_id "([^"]+)" to (?:retrieve unread results|read the results, or write_agent to send follow-up messages)\./g,
+      'Use read_agent with agent_id "$1" to retrieve the full results.',
+    );
+}
 
 function normalizeStoredUserMessages(conversations: NormalizedConversation[]) {
   for (const conversation of conversations) {
     for (const message of conversation.messages) {
       if (message.role === "user" && typeof message.content === "string") {
-        message.content = message.content.replace(
-          taskCompletionNotificationPattern,
-          taskCompletionNotificationReplacement,
-        );
+        message.content = normalizeAgentCompletionNotification(message.content);
       }
     }
   }