fix(core): allow rewind after compressed history

packages/core/src/utils/environmentContext.test.ts

@@ -616,6 +616,69 @@ describe('getStartupContextLength', () => {
     );
     expect(getStartupContextLength([merged])).toBe(0);
   });
+
+  // Compressed history prefix: composePostCompactHistory produces
+  // [user(summary), model(ack), user(postAckParts)?, ...]. The ack sentinel
+  // is distinct from the legacy ack above.
+
+  it('is 2 for a compressed prefix without attachments', () => {
+    const history: Content[] = [
+      {
+        role: 'user',
+        parts: [{ text: 'summary text\n\nResume the prior task...' }],
+      },
+      {
+        role: 'model',
+        parts: [{ text: 'Got it. Thanks for the additional context!' }],
+      },
+    ];
+    expect(getStartupContextLength(history)).toBe(2);
+  });
+
+  it('is 3 for a compressed prefix with post-compact attachments', () => {
+    const history: Content[] = [
+      {
+        role: 'user',
+        parts: [{ text: 'summary text\n\nResume the prior task...' }],
+      },
+      {
+        role: 'model',
+        parts: [{ text: 'Got it. Thanks for the additional context!' }],
+      },
+      {
+        role: 'user',
+        parts: [
+          { text: 'file restoration content' },
+          { text: '<system-reminder>\nplan mode\n</system-reminder>' },
+        ],
+      },
+    ];
+    expect(getStartupContextLength(history)).toBe(3);
+  });
+
+  it('is 2 for a degraded compression fallback with merged reminders', () => {
+    const history: Content[] = [
+      {
+        role: 'user',
+        parts: [
+          { text: 'summary\n\nResume the prior task...' },
+          { text: '<system-reminder>\nplan mode active\n</system-reminder>' },
+        ],
+      },
+      {
+        role: 'model',
+        parts: [
+          { text: 'Got it. Thanks for the additional context!' },
+          { functionCall: { name: 'fn', args: {} } },
+        ],
+      },
+      {
+        role: 'user',
+        parts: [{ functionResponse: { name: 'fn', response: {} } }],
+      },
+    ];
+    expect(getStartupContextLength(history)).toBe(2);
+  });
 });
 
 describe('buildAvailableSkillsReminder', () => {

packages/core/src/utils/environmentContext.ts

@@ -571,10 +571,18 @@ export async function getInitialChatHistory(
 }
 
 /**
- * Returns the number of initial API entries occupied by the startup reminder
- * (0 or 1). A single user message wrapped in <system-reminder> is the only
- * shape getInitialChatHistory currently produces, but routes through this
- * helper so detection stays consistent across the CLI and ACP integration.
+ * Returns the number of initial API entries occupied by structural context
+ * that should be skipped when counting real user turns:
+ *
+ *  - The startup reminder prelude (0 or 1 entry) — a single user message
+ *    wrapped in `<system-reminder>…</system-reminder>`, produced by
+ *    `getInitialChatHistory`.
+ *  - The legacy ack-pair prelude (2 entries) — sessions saved before the
+ *    startup context moved into system reminders.
+ *  - The compressed-history prefix (2 or 3 entries) — summary, ack, and
+ *    optionally a post-compact attachments entry produced by
+ *    `composePostCompactHistory`. These synthetic entries must not be
+ *    counted as real user prompts for rewind indexing.
  */
 export function getStartupContextLength(history: Content[]): number {
   const firstEntry = history[0];
@@ -601,9 +609,29 @@ export function getStartupContextLength(history: Content[]): number {
   ) {
     return 2;
   }
+  // Post-compression prefix: composePostCompactHistory always produces
+  // [user(summary), model(ack)] and optionally appends user(postAckParts)
+  // when file restorations or state reminders accompany the summary. The
+  // summary is plain prose (not <system-reminder>-wrapped) so it would pass
+  // isUserTextContent and corrupt rewind turn-counting. Detected via the
+  // compression-specific ack sentinel — distinct from the legacy ack above.
+  if (
+    history[1]?.role === 'model' &&
+    history[1]?.parts?.[0]?.text ===
+      'Got it. Thanks for the additional context!'
+  ) {
+    if (isUserTextContextEntry(history[2])) return 3;
+    return 2;
+  }
   return 0;
 }
 
+function isUserTextContextEntry(content: Content | undefined): boolean {
+  if (content?.role !== 'user') return false;
+  const parts = content.parts ?? [];
+  return parts.length > 0 && !parts.some((part) => 'functionResponse' in part);
+}
+
 /**
  * True when `content` is a *pure* system-reminder entry: it has parts and
  * EVERY part is a text part wrapped in `<system-reminder>…</system-reminder>`.