diff --git a/packages/runtime/src/__tests__/history-compact-checkpoint.test.ts b/packages/runtime/src/__tests__/history-compact-checkpoint.test.ts
index db635d0fcf..b983d2d004 100644
--- a/packages/runtime/src/__tests__/history-compact-checkpoint.test.ts
+++ b/packages/runtime/src/__tests__/history-compact-checkpoint.test.ts
@@ -29,6 +29,7 @@ import {
   historyCompactCheckpointToModelMessage,
   historyCompactCheckpointToRuntimeEvent,
   isProviderHistoryCompactCheckpoint,
+  isSupersededHistoryCompactCheckpoint,
   matchHistoryCompactCheckpointPrefix,
   validateHistoryCompactCheckpointShape,
 } from '../history-compact-checkpoint.js';
@@ -594,6 +595,38 @@ describe('history compact checkpoint', () => {
     assert.equal(validateHistoryCompactCheckpointShape(unmarked, 'session-1'), false);
   });
 
+  test('an unmarked 0.1.x checkpoint reads as superseded, a damaged one does not', () => {
+    // Diagnostics have to tell "this Session predates the current summary
+    // contract" apart from "this record is broken". Both fail shape validation,
+    // so the only separator is whether restoring the stamp would make it valid.
+    const stamped = buildHistoryCompactCheckpoint({
+      sessionId: 'session-1',
+      coveredRuntimeEvents: [textEvent(0)],
+      summary: STRUCTURED_SUMMARY,
+    });
+    const { summaryFormat: _summaryFormat, ...unmarked } = stamped;
+
+    assert.equal(isSupersededHistoryCompactCheckpoint(unmarked, 'session-1'), true);
+    assert.equal(isSupersededHistoryCompactCheckpoint(stamped, 'session-1'), false);
+    assert.equal(
+      isSupersededHistoryCompactCheckpoint(
+        { ...stamped, source: { ...stamped.source, policyVersion: 'maka.older_policy.v0' } },
+        'session-1',
+      ),
+      true,
+    );
+    assert.equal(
+      isSupersededHistoryCompactCheckpoint({ ...unmarked, coverage: undefined }, 'session-1'),
+      false,
+    );
+    assert.equal(
+      isSupersededHistoryCompactCheckpoint({ ...unmarked, summary: '' }, 'session-1'),
+      false,
+    );
+    // A record from another Session is not this Session's history.
+    assert.equal(isSupersededHistoryCompactCheckpoint(unmarked, 'session-2'), false);
+  });
+
   test('a marked checkpoint is held to the complete predicate at load', async () => {
     // A section-less summary written through a seam that bypassed the write
     // gates (direct recorder, older copy) but carrying the sectioned marker
diff --git a/packages/runtime/src/execution-inspect.ts b/packages/runtime/src/execution-inspect.ts
index 22b8eeb72d..335bd5b2b7 100644
--- a/packages/runtime/src/execution-inspect.ts
+++ b/packages/runtime/src/execution-inspect.ts
@@ -295,18 +295,18 @@ function inspectCompactionCheckpoints(
     if (event.type !== 'history_compact_checkpoint_recorded') continue;
     const checkpoint = event.data?.checkpoint;
     if (!validateHistoryCompactCheckpointShape(checkpoint, invocation.sessionId)) {
-      // A checkpoint recorded under an older source policy is expected history,
-      // not corruption: the ledger keeps every checkpoint it ever wrote, and
-      // every consumer fails open on it. Reporting it as an error would drown
-      // out the records that really are damaged.
-      const superseded = isSupersededHistoryCompactCheckpoint(checkpoint);
+      // A checkpoint recorded under an older contract is expected history, not
+      // corruption: the ledger keeps every checkpoint it ever wrote, and every
+      // consumer fails open on it. Reporting it as an error would drown out the
+      // records that really are damaged.
+      const superseded = isSupersededHistoryCompactCheckpoint(checkpoint, invocation.sessionId);
       diagnostics.push(
         diagnostic(
           invocation,
           superseded ? 'compaction_checkpoint_superseded' : 'compaction_checkpoint_invalid',
           superseded ? 'info' : 'error',
           superseded
-            ? 'AgentRun contains a durable Compaction checkpoint from a superseded source policy; it is ignored and re-created on demand.'
+            ? 'AgentRun contains a durable Compaction checkpoint from a superseded summary contract; it is ignored and re-created on demand.'
             : 'AgentRun contains an invalid durable Compaction checkpoint record.',
           event.id,
         ),
diff --git a/packages/runtime/src/history-compact-checkpoint.ts b/packages/runtime/src/history-compact-checkpoint.ts
index e19f64d4f2..24cd9f3873 100644
--- a/packages/runtime/src/history-compact-checkpoint.ts
+++ b/packages/runtime/src/history-compact-checkpoint.ts
@@ -491,24 +491,45 @@ export function validateHistoryCompactCheckpointShape(
   );
 }
 
-/** Accept forward progress, or a compare-and-swap rewrite of the exact same source coverage. */
 /**
  * A recorded checkpoint that this Runtime no longer holds to its own contract
- * because it was minted under an older source policy — not a corrupt record.
+ * because it was minted under an older contract — not a corrupt record.
  * Every consumer already fails open on it (compaction re-summarizes, copy
  * drops it); diagnostics use this to say so instead of crying corruption.
+ *
+ * Two generations qualify. A checkpoint carrying an older source policy names
+ * itself. A v0.1.x text checkpoint cannot: it predates both the source policy
+ * and the summary format stamp, so the only thing that separates it from a
+ * damaged record is that restoring the stamp would make it valid.
  */
-export function isSupersededHistoryCompactCheckpoint(value: unknown): boolean {
+export function isSupersededHistoryCompactCheckpoint(value: unknown, sessionId?: string): boolean {
   if (!value || typeof value !== 'object') return false;
   const checkpoint = value as Partial<HistoryCompactCheckpoint>;
+  if (checkpoint.kind !== 'maka.history_compact_checkpoint') return false;
   const policyVersion = checkpoint.source?.policyVersion as unknown;
-  return (
-    checkpoint.kind === 'maka.history_compact_checkpoint' &&
+  if (
     typeof policyVersion === 'string' &&
     policyVersion !== HISTORY_COMPACT_SOURCE_POLICY_VERSION
+  ) {
+    return true;
+  }
+  return isRetiredFreeformTextCheckpoint(checkpoint, sessionId);
+}
+
+/** The unmarked v0.1.x summary contract this Runtime stopped admitting. */
+function isRetiredFreeformTextCheckpoint(
+  checkpoint: Partial<HistoryCompactCheckpoint>,
+  sessionId?: string,
+): boolean {
+  const text = checkpoint as Partial<TextHistoryCompactCheckpoint>;
+  if (checkpoint.version !== 2 || text.summaryFormat !== undefined) return false;
+  return validateHistoryCompactCheckpointShape(
+    { ...checkpoint, summaryFormat: SECTIONED_SUMMARY_FORMAT },
+    sessionId,
   );
 }
 
+/** Accept forward progress, or a compare-and-swap rewrite of the exact same source coverage. */
 export function canReplaceHistoryCompactCheckpoint(
   current: HistoryCompactCheckpoint | undefined,
   candidate: HistoryCompactCheckpoint,