fix(ai-messages): avoid duplicate checkpoint copy

docs/architecture/agent-message-ux-catalog.md

@@ -40,6 +40,8 @@ The current column exercises these visible cases in one trace:
 - subagent progress and nested output
 - Todo progress
 - changed-file diff summary
+- a Codex turn-start checkpoint whose expanded recovery control does not repeat
+  the accordion title
 - single-line system notices, rate-limit guidance, plugin installation, and a
   multi-line provider warning whose expanded body contains detail without
   repeating the row title
@@ -118,7 +120,8 @@ construction even though they are not separate `NormalizedProviderEvent` types.
   such as `Context window at 62%` must not repeat the same sentence inside a
   nested body. For a multi-line generic system notice, the first non-empty line
   is the title and only the remaining lines belong in the expanded body;
-  compact checkpoints keep their specialized recovery surface.
+  compact and turn-start checkpoints keep their specialized recovery surface,
+  without repeating the title inside it.
 - Tool results update the existing tool row by id. A result must never create a
   second row that repeats the header or input.
 - Stave-owned MCP tools use the Stave icon and an action-oriented title. Only

src/components/session/chat-panel-message-parts.tsx

@@ -226,6 +226,7 @@ export function MessagePartRenderer(args: {
       const compactBoundaryTrigger =
         part.compactBoundary?.trigger ?? compactedMatch?.[1];
       const compactBoundaryGitRef = part.compactBoundary?.gitRef;
+      const isTurnStartCheckpoint = compactBoundaryTrigger === "turn_start";
       const handleRestoreCompactBoundary = () => {
         if (!compactBoundaryGitRef || isRestoringCompactBoundary) {
           return;
@@ -241,10 +242,11 @@ export function MessagePartRenderer(args: {
           setIsRestoringCompactBoundary(false);
         });
       };
-      if (compactedMatch) {
+      if (part.compactBoundary != null || compactedMatch) {
         return (
           <ContextCompactedCheckpoint
-            trigger={compactBoundaryTrigger}
+            label={isTurnStartCheckpoint ? "Workspace checkpoint" : undefined}
+            trigger={isTurnStartCheckpoint ? undefined : compactBoundaryTrigger}
             onRestore={handleRestoreCompactBoundary}
             restorePending={isRestoringCompactBoundary}
             restoreDisabled={!compactBoundaryGitRef}

src/dev/agent-preview/fixtures.ts

@@ -147,6 +147,11 @@ function commonParts(args: { reasoningStreaming: boolean }): MessagePart[] {
       type: "system_event",
       content: "Context window at 62% — compaction not required.",
     },
+    {
+      type: "system_event",
+      content: "Checkpoint captured before Codex turn.",
+      compactBoundary: { trigger: "turn_start", gitRef: "preview-checkpoint" },
+    },
     {
       type: "system_event",
       content: "Approaching rate limit (72% used). Consider pacing requests.",

tests/assistant-message-body.test.tsx

@@ -200,6 +200,28 @@ describe("AssistantMessageBody", () => {
     });
   });
 
+  test("does not repeat a Codex turn checkpoint inside its accordion", async () => {
+    const { AssistantMessageBody } = await loadAssistantMessageBodies();
+    const checkpointNotice = "Checkpoint captured before Codex turn.";
+    const html = renderToStaticMarkup(createElement(AssistantMessageBody, {
+      message: createAssistantMessage({
+        isStreaming: true,
+        parts: [{
+          type: "system_event",
+          content: checkpointNotice,
+          compactBoundary: { trigger: "turn_start", gitRef: "abc123" },
+        }],
+      }),
+      taskId: "task-1",
+      messageId: "message-1",
+      streamingEnabled: true,
+    }));
+
+    expect(html.split(checkpointNotice).length - 1).toBe(1);
+    expect(html).toContain("Workspace checkpoint");
+    expect(html).toContain("Restore");
+  });
+
   test("uses Stave-specific copy and icon without rewriting external MCP names", async () => {
     const { AssistantMessageBody } = await loadAssistantMessageBodies();
     const html = renderToStaticMarkup(createElement(AssistantMessageBody, {