diff --git a/tests/fixtures/legacy-anthropic-0200.json b/tests/fixtures/legacy-anthropic-0200.json
new file mode 100644
index 0000000000..589d6a80e8
--- /dev/null
+++ b/tests/fixtures/legacy-anthropic-0200.json
@@ -0,0 +1,175 @@
+{
+  "provenance": "Synthetic body decoded by the pinned 0.1.200 kernel; no private corpus content",
+  "body": {
+    "messages": [
+      {
+        "role": "user",
+        "content": "Summarize this synthetic lookup."
+      },
+      {
+        "role": "assistant",
+        "content": [
+          {
+            "type": "tool_use",
+            "id": "synthetic-call-1",
+            "name": "lookup",
+            "input": {
+              "key": "invented"
+            }
+          }
+        ]
+      },
+      {
+        "role": "user",
+        "content": [
+          {
+            "type": "tool_result",
+            "tool_use_id": "synthetic-call-1",
+            "content": "invented lookup result"
+          }
+        ]
+      },
+      {
+        "role": "user",
+        "content": "Synthetic tail 0."
+      },
+      {
+        "role": "assistant",
+        "content": "Synthetic tail 1."
+      },
+      {
+        "role": "user",
+        "content": "Synthetic tail 2."
+      },
+      {
+        "role": "assistant",
+        "content": "Synthetic tail 3."
+      },
+      {
+        "role": "user",
+        "content": "Synthetic tail 4."
+      },
+      {
+        "role": "assistant",
+        "content": "Synthetic tail 5."
+      },
+      {
+        "role": "user",
+        "content": "Synthetic tail 6."
+      },
+      {
+        "role": "assistant",
+        "content": "Synthetic tail 7."
+      },
+      {
+        "role": "user",
+        "content": "Synthetic tail 8."
+      },
+      {
+        "role": "assistant",
+        "content": "Synthetic tail 9."
+      }
+    ]
+  },
+  "legacyMessages": [
+    {
+      "id": "h_dd37bf0d01f1f2c2",
+      "role": "user",
+      "contentType": "text",
+      "text": "Summarize this synthetic lookup.",
+      "wireIndex": 0
+    },
+    {
+      "id": "h_ad375fbf578d70f4",
+      "role": "assistant",
+      "contentType": "tool-call",
+      "toolName": "lookup",
+      "toolCallId": "synthetic-call-1",
+      "text": "{\"key\":\"invented\"}",
+      "wireIndex": 1
+    },
+    {
+      "id": "h_5acf06bd84245dbd",
+      "role": "tool",
+      "contentType": "tool-result",
+      "toolCallId": "synthetic-call-1",
+      "text": "invented lookup result",
+      "toolName": "lookup",
+      "wireIndex": 2
+    },
+    {
+      "id": "h_dd7c34c8f76f8de0",
+      "role": "user",
+      "contentType": "text",
+      "text": "Synthetic tail 0.",
+      "wireIndex": 3
+    },
+    {
+      "id": "h_ecbe503530c507a1",
+      "role": "assistant",
+      "contentType": "text",
+      "text": "Synthetic tail 1.",
+      "wireIndex": 4
+    },
+    {
+      "id": "h_91cd90107009cd4c",
+      "role": "user",
+      "contentType": "text",
+      "text": "Synthetic tail 2.",
+      "wireIndex": 5
+    },
+    {
+      "id": "h_cc36cc6011d0bf55",
+      "role": "assistant",
+      "contentType": "text",
+      "text": "Synthetic tail 3.",
+      "wireIndex": 6
+    },
+    {
+      "id": "h_39dc1e2dc520545c",
+      "role": "user",
+      "contentType": "text",
+      "text": "Synthetic tail 4.",
+      "wireIndex": 7
+    },
+    {
+      "id": "h_fa8ed6aa56f9e474",
+      "role": "assistant",
+      "contentType": "text",
+      "text": "Synthetic tail 5.",
+      "wireIndex": 8
+    },
+    {
+      "id": "h_f5b2ff5752bc2354",
+      "role": "user",
+      "contentType": "text",
+      "text": "Synthetic tail 6.",
+      "wireIndex": 9
+    },
+    {
+      "id": "h_3309f696e463ee67",
+      "role": "assistant",
+      "contentType": "text",
+      "text": "Synthetic tail 7.",
+      "wireIndex": 10
+    },
+    {
+      "id": "h_377cfa6ea7c6261e",
+      "role": "user",
+      "contentType": "text",
+      "text": "Synthetic tail 8.",
+      "wireIndex": 11
+    },
+    {
+      "id": "h_ec73340115ef30cb",
+      "role": "assistant",
+      "contentType": "text",
+      "text": "Synthetic tail 9.",
+      "wireIndex": 12
+    }
+  ],
+  "coveredIds": [
+    "h_ad375fbf578d70f4",
+    "h_5acf06bd84245dbd"
+  ]
+}
\ No newline at end of file
diff --git a/tests/legacy-anthropic-identity-replay.test.ts b/tests/legacy-anthropic-identity-replay.test.ts
new file mode 100644
index 0000000000..c632c9ce2b
--- /dev/null
+++ b/tests/legacy-anthropic-identity-replay.test.ts
@@ -0,0 +1,74 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { readFileSync } from 'node:fs';
+import { anthropicToCore, type AnthropicRequestBody } from 'acp-kernel/wire';
+import { createInitialState, prune, type CoreMessage } from 'acp-kernel';
+import { normalizedIdentity, normalizedIdentityNoToolCallId, reconcileFoldCoverage } from '../src/fold-reconcile.js';
+import type { Session } from '../src/session.js';
+
+const fixture = JSON.parse(readFileSync(new URL('./fixtures/legacy-anthropic-0200.json', import.meta.url), 'utf8')) as {
+    body: AnthropicRequestBody;
+    legacyMessages: CoreMessage[];
+    coveredIds: string[];
+};
+
+function legacySession(): Session {
+    // A valid legacy coverage shape with no surviving anchors for this old pair.
+    // The 13-message resend clears the auxiliary-side-request guard. Persisted
+    // lastMessages is empty, as in the affected legacy envelope. No private
+    // messages, anchors, summaries or identifiers are included in this fixture.
+    const state = createInitialState();
+    state.blocks.push({
+        blockId: 'b1', runId: 'synthetic-fold', tier: 1,
+        summary: 'The synthetic lookup completed.',
+        directMessageIds: [...fixture.coveredIds],
+        effectiveMessageIds: [...fixture.coveredIds], directBlockIds: [],
+        compressedTokens: 20, createdAt: 1700000000000,
+        survivedCount: 0, generation: 'old', active: true,
+    });
+    return {
+        id: 'synthetic-legacy-replay',
+        state,
+        metadata: { foldAnchorOrder: fixture.legacyMessages.map(message => message.id), foldAnchors: {} },
+        lastMessages: [],
+    } as unknown as Session;
+}
+
+test('legacy IDs still match persisted coverage without surviving anchors', () => {
+    const session = legacySession();
+    const messages = structuredClone(fixture.legacyMessages);
+    const result = reconcileFoldCoverage(session, messages, { mode: 'repair' });
+    assert.equal(result.missing, 0);
+    assert.equal(result.unmatched, 0);
+    assert.equal(prune(messages, session.state).filter(message => message.toolCallId === 'synthetic-call-1').length, 0);
+});
+
+test('synthetic old and current decoders preserve the same message payloads', () => {
+    const current = anthropicToCore(structuredClone(fixture.body)).msgs;
+    const withoutIds = (messages: CoreMessage[]) => messages.map(({ id: _id, ...payload }) => payload);
+    assert.deepEqual(withoutIds(current), withoutIds(fixture.legacyMessages));
+    assert.equal(current.length, 13);
+    assert.equal(fixture.coveredIds.length, 2);
+});
+
+test('surviving tool anchors recover the same current replay', () => {
+    const session = legacySession();
+    session.metadata.foldAnchors = Object.fromEntries(fixture.legacyMessages.map(message => [message.id, {
+        n: normalizedIdentity(message), r: message.role, b: message.text?.length ?? 0,
+        ...(message.toolCallId ? { t: message.toolCallId, m: normalizedIdentityNoToolCallId(message) } : {}),
+    }]));
+    const current = anthropicToCore(structuredClone(fixture.body)).msgs;
+    const result = reconcileFoldCoverage(session, current, { mode: 'repair' });
+    assert.equal(result.unmatched, 0);
+    assert.equal(prune(current, session.state).filter(message => message.toolCallId === 'synthetic-call-1').length, 0);
+});
+
+test('unchanged Anthropic replay keeps legacy folded tool coverage after upgrade', () => {
+    const session = legacySession();
+    const current = anthropicToCore(structuredClone(fixture.body)).msgs;
+    const result = reconcileFoldCoverage(session, current, { mode: 'repair' });
+    assert.equal(prune(current, session.state).filter(message => message.toolCallId === 'synthetic-call-1').length, 0,
+        'already-folded tool messages must stay folded on unchanged replay');
+    assert.equal(result.unmatched, 0,
+        'unchanged tool-call/result payloads must not re-enter the wire unfolded after upgrading persisted legacy coverage');
+});