@@ -199,8 +199,15 @@ function getLastAssistantUsageInfo(
   return undefined;
 }
 
+const MAX_CACHED_TEXT_UNITS = 1_048_576;
+const MAX_CACHED_TEXT_ENTRIES = 2_048;
+
 export class BpeTokenEstimator implements TokenEstimator {
   private readonly countExactTokens: (text: string) => number;
+  // Content keys remain valid across detached histories, edits and model
+  // projections. Bound retained UTF-16 text as well as entry count.
+  private readonly textTokens = new Map<string, number>();
+  private cachedTextUnits = 0;
 
   /**
    * @param encoder Override the default o200k_base tokenizer. Tests inject
@@ -222,12 +229,29 @@ export class BpeTokenEstimator implements TokenEstimator {
 
   estimateTextTokens(text: string): number {
     if (!text) return 0;
+    const cached = this.textTokens.get(text);
+    if (cached !== undefined) {
+      this.textTokens.delete(text);
+      this.textTokens.set(text, cached);
+      return cached;
+    }
     try {
       if (text.length > MAX_EXACT_TOKENIZER_CHARS && hasOversizedAlphanumericRun(text)) {
         return estimateTextTokensUpperBound(text);
       }
       const tokens = this.countExactTokens(text);
-      return Number.isFinite(tokens) && tokens >= 0 ? tokens : estimateTextTokensUpperBound(text);
+      if (!Number.isFinite(tokens) || tokens < 0) return estimateTextTokensUpperBound(text);
+      if (text.length <= MAX_CACHED_TEXT_UNITS) {
+        while (this.textTokens.size >= MAX_CACHED_TEXT_ENTRIES || this.cachedTextUnits + text.length > MAX_CACHED_TEXT_UNITS) {
+          const oldest = this.textTokens.keys().next().value;
+          if (oldest === undefined) break;
+          this.textTokens.delete(oldest);
+          this.cachedTextUnits -= oldest.length;
+        }
+        this.textTokens.set(text, tokens);
+        this.cachedTextUnits += text.length;
+      }
+      return tokens;
     } catch {
       return estimateTextTokensUpperBound(text);
     }
 
@@ -305,7 +305,7 @@ export function createSessionSystemCanonicalHistoryProvider(
             `Canonical compaction generation verification failed: ${change.sessionId}`,
           );
         }
-        await syncIndex(change.sessionId, paths, true);
+        await syncIndex(change.sessionId, paths, true, committed);
         const result = {
           revision: canonicalActiveHistoryRevision(committed),
           generation: committedGeneration,
@@ -335,7 +335,7 @@ export function createSessionSystemCanonicalHistoryProvider(
       }
       options.activity?.notify(sessionId);
     }
-    await syncIndex(sessionId, paths, true);
+    await syncIndex(sessionId, paths, true, records);
     return historySnapshot(records, allowPendingToolCallTail);
   }
 
@@ -372,8 +372,9 @@ export function createSessionSystemCanonicalHistoryProvider(
     sessionId: string,
     paths: SessionHistoryPaths,
     allowMissingParent = false,
+    verifiedRecords?: readonly CanonicalHistoryEnvelope[],
   ): Promise<void> {
-    const active = await files.readActiveStrict(paths.messages);
+    const active = verifiedRecords ?? (await files.readActiveStrict(paths.messages));
     const adapter = indexes.get(sessionId) ?? createCanonicalHistoryIndexAdapter(paths.sessionDir);
     indexes.set(sessionId, adapter);
     try {
 
@@ -27,9 +27,10 @@ export interface DurableCanonicalHistoryProvider {
   read(sessionId: string): Promise<CanonicalHistorySnapshot>;
   /** Active post-mutation history; the final tool round may still await results. */
   readActive(sessionId: string): Promise<CanonicalHistorySnapshot>;
-  append(change: CanonicalHistoryChange): Promise<unknown>;
-  replace(change: CanonicalHistoryChange): Promise<unknown>;
-  compact?(change: CanonicalHistoryCompactionChange): Promise<unknown>;
+  /** Return the verified post-write snapshot from the same lane, or request a reread with void. */
+  append(change: CanonicalHistoryChange): Promise<CanonicalHistorySnapshot | void>;
+  replace(change: CanonicalHistoryChange): Promise<CanonicalHistorySnapshot | void>;
+  compact?(change: CanonicalHistoryCompactionChange): Promise<CanonicalHistorySnapshot | void>;
   settleTurnTail?(mutation: SettleTurnTailMutation): Promise<TurnHistoryMutationCommit>;
   retractTurn?(mutation: RetractTurnMutation): Promise<TurnHistoryMutationCommit>;
 }
@@ -102,13 +103,17 @@ export class DurableCanonicalHistoryStore implements CanonicalHistoryStore {
   private commit(
     kind: 'append' | 'replace',
     change: CanonicalHistoryChange,
-    mutate: (snapshot: CanonicalHistoryChange) => Promise<unknown>,
+    mutate: (snapshot: CanonicalHistoryChange) => Promise<CanonicalHistorySnapshot | void>,
   ): Promise<CanonicalHistoryCommit> {
     const snapshot = captureSemanticSnapshot(change).value;
     validateCanonicalHistoryChange(snapshot, kind);
     return this.lane.run(snapshot.sessionId, async () => {
-      await mutate(snapshot);
-      return this.readActiveCommitted(snapshot.sessionId);
+      const committed = await mutate(snapshot);
+      // Session-owned providers already reread and validate inside their lane.
+      // Legacy adapters returning void retain the strict post-write read.
+      return committed === undefined
+        ? this.readActiveCommitted(snapshot.sessionId)
+        : this.readProviderSnapshot(committed);
     });
   }
 
 
@@ -1,139 +1,26 @@
-const BLOCK_BYTES = 64;
-const UTF8_CHUNK_CODE_UNITS = 8 * 1_024;
-
-const INITIAL_STATE = [
-  0x6a09e667, 0xbb67ae85, 0x3c6ef372, 0xa54ff53a, 0x510e527f, 0x9b05688c, 0x1f83d9ab, 0x5be0cd19,
-] as const;
+import { createHash } from 'node:crypto';
 
-const ROUND_CONSTANTS = new Uint32Array([
-  0x428a2f98, 0x71374491, 0xb5c0fbcf, 0xe9b5dba5, 0x3956c25b, 0x59f111f1, 0x923f82a4, 0xab1c5ed5,
-  0xd807aa98, 0x12835b01, 0x243185be, 0x550c7dc3, 0x72be5d74, 0x80deb1fe, 0x9bdc06a7, 0xc19bf174,
-  0xe49b69c1, 0xefbe4786, 0x0fc19dc6, 0x240ca1cc, 0x2de92c6f, 0x4a7484aa, 0x5cb0a9dc, 0x76f988da,
-  0x983e5152, 0xa831c66d, 0xb00327c8, 0xbf597fc7, 0xc6e00bf3, 0xd5a79147, 0x06ca6351, 0x14292967,
-  0x27b70a85, 0x2e1b2138, 0x4d2c6dfc, 0x53380d13, 0x650a7354, 0x766a0abb, 0x81c2c92e, 0x92722c85,
-  0xa2bfe8a1, 0xa81a664b, 0xc24b8b70, 0xc76c51a3, 0xd192e819, 0xd6990624, 0xf40e3585, 0x106aa070,
-  0x19a4c116, 0x1e376c08, 0x2748774c, 0x34b0bcb5, 0x391c0cb3, 0x4ed8aa4a, 0x5b9cca4f, 0x682e6ff3,
-  0x748f82ee, 0x78a5636f, 0x84c87814, 0x8cc70208, 0x90befffa, 0xa4506ceb, 0xbef9a3f7, 0xc67178f2,
-]);
+const UTF8_CHUNK_CODE_UNITS = 8 * 1_024;
 
-/**
- * Dependency-free incremental SHA-256 for AgentHost semantic identity.
- * Text is encoded in bounded chunks so hashing one large History string does
- * not allocate another payload-sized Buffer. This is intentionally internal;
- * persistence digests continue to use their owning adapters.
- */
+/** Native SHA-256 with bounded UTF-8 chunks and the existing update boundaries. */
 export class IncrementalSha256 {
-  private readonly state = new Uint32Array(INITIAL_STATE);
-  private readonly block = new Uint8Array(BLOCK_BYTES);
-  private readonly schedule = new Uint32Array(64);
-  private readonly textEncoder = new TextEncoder();
-  private bufferedBytes = 0;
-  private totalBytes = 0;
+  private readonly hash = createHash('sha256');
   private finalized = false;
 
   update(value: string): void {
     if (this.finalized) throw new Error('SHA-256 digest is already finalized.');
     let offset = 0;
     while (offset < value.length) {
       const end = safeUtf8ChunkEnd(value, offset);
-      this.updateBytes(this.textEncoder.encode(value.slice(offset, end)));
+      this.hash.update(value.slice(offset, end), 'utf8');
       offset = end;
     }
   }
 
   digestHex(): string {
     if (this.finalized) throw new Error('SHA-256 digest is already finalized.');
     this.finalized = true;
-    const messageBytes = this.totalBytes;
-    this.block[this.bufferedBytes] = 0x80;
-    this.bufferedBytes += 1;
-    if (this.bufferedBytes > 56) {
-      this.block.fill(0, this.bufferedBytes);
-      this.compress();
-      this.bufferedBytes = 0;
-    }
-    this.block.fill(0, this.bufferedBytes, 56);
-    const messageBits = messageBytes * 8;
-    writeUint32BigEndian(this.block, 56, Math.floor(messageBits / 0x1_0000_0000));
-    writeUint32BigEndian(this.block, 60, messageBits >>> 0);
-    this.compress();
-    return [...this.state].map((word) => word.toString(16).padStart(8, '0')).join('');
-  }
-
-  private updateBytes(bytes: Uint8Array): void {
-    this.totalBytes += bytes.length;
-    let offset = 0;
-    while (offset < bytes.length) {
-      const copied = Math.min(BLOCK_BYTES - this.bufferedBytes, bytes.length - offset);
-      this.block.set(bytes.subarray(offset, offset + copied), this.bufferedBytes);
-      this.bufferedBytes += copied;
-      offset += copied;
-      if (this.bufferedBytes !== BLOCK_BYTES) continue;
-      this.compress();
-      this.bufferedBytes = 0;
-    }
-  }
-
-  private compress(): void {
-    const words = this.schedule;
-    for (let index = 0; index < 16; index += 1) {
-      const offset = index * 4;
-      words[index] =
-        (arrayValue(this.block, offset) << 24) |
-        (arrayValue(this.block, offset + 1) << 16) |
-        (arrayValue(this.block, offset + 2) << 8) |
-        arrayValue(this.block, offset + 3);
-    }
-    for (let index = 16; index < 64; index += 1) {
-      const prior15 = arrayValue(words, index - 15);
-      const prior2 = arrayValue(words, index - 2);
-      const sigma0 = rotateRight(prior15, 7) ^ rotateRight(prior15, 18) ^ (prior15 >>> 3);
-      const sigma1 = rotateRight(prior2, 17) ^ rotateRight(prior2, 19) ^ (prior2 >>> 10);
-      words[index] =
-        (arrayValue(words, index - 16) + sigma0 + arrayValue(words, index - 7) + sigma1) >>> 0;
-    }
-    this.compressRounds(words);
-  }
-
-  private compressRounds(words: Uint32Array): void {
-    let a = arrayValue(this.state, 0);
-    let b = arrayValue(this.state, 1);
-    let c = arrayValue(this.state, 2);
-    let d = arrayValue(this.state, 3);
-    let e = arrayValue(this.state, 4);
-    let f = arrayValue(this.state, 5);
-    let g = arrayValue(this.state, 6);
-    let h = arrayValue(this.state, 7);
-    for (let index = 0; index < 64; index += 1) {
-      const upperSigma1 = rotateRight(e, 6) ^ rotateRight(e, 11) ^ rotateRight(e, 25);
-      const choose = (e & f) ^ (~e & g);
-      const temporary1 =
-        (h +
-          upperSigma1 +
-          choose +
-          arrayValue(ROUND_CONSTANTS, index) +
-          arrayValue(words, index)) >>>
-        0;
-      const upperSigma0 = rotateRight(a, 2) ^ rotateRight(a, 13) ^ rotateRight(a, 22);
-      const majority = (a & b) ^ (a & c) ^ (b & c);
-      const temporary2 = (upperSigma0 + majority) >>> 0;
-      h = g;
-      g = f;
-      f = e;
-      e = (d + temporary1) >>> 0;
-      d = c;
-      c = b;
-      b = a;
-      a = (temporary1 + temporary2) >>> 0;
-    }
-    this.state[0] = (arrayValue(this.state, 0) + a) >>> 0;
-    this.state[1] = (arrayValue(this.state, 1) + b) >>> 0;
-    this.state[2] = (arrayValue(this.state, 2) + c) >>> 0;
-    this.state[3] = (arrayValue(this.state, 3) + d) >>> 0;
-    this.state[4] = (arrayValue(this.state, 4) + e) >>> 0;
-    this.state[5] = (arrayValue(this.state, 5) + f) >>> 0;
-    this.state[6] = (arrayValue(this.state, 6) + g) >>> 0;
-    this.state[7] = (arrayValue(this.state, 7) + h) >>> 0;
+    return this.hash.digest('hex');
   }
 }
 
@@ -152,20 +39,3 @@ function isHighSurrogate(value: number): boolean {
 function isLowSurrogate(value: number): boolean {
   return value >= 0xdc00 && value <= 0xdfff;
 }
-
-function rotateRight(value: number, bits: number): number {
-  return (value >>> bits) | (value << (32 - bits));
-}
-
-function writeUint32BigEndian(target: Uint8Array, offset: number, value: number): void {
-  target[offset] = (value >>> 24) & 0xff;
-  target[offset + 1] = (value >>> 16) & 0xff;
-  target[offset + 2] = (value >>> 8) & 0xff;
-  target[offset + 3] = value & 0xff;
-}
-
-function arrayValue(values: Uint8Array | Uint32Array, index: number): number {
-  const value = values[index];
-  if (value === undefined) throw new RangeError(`SHA-256 array index is out of bounds: ${index}.`);
-  return value;
-}
@@ -5,19 +5,39 @@ export interface SemanticSnapshot<T> {
   readonly fingerprint: string;
 }
 
+const ownedValues = new WeakSet<object>();
+const fingerprints = new WeakMap<object, string>();
+
 /**
  * Detach callback-owned History/event data before it becomes an in-run
  * identity or deferred delivery payload. Unsupported or cyclic values fail
  * closed. The digest is streamed so identity memory does not scale with the
  * encoded payload size.
  */
 export function captureSemanticSnapshot<T>(value: T): SemanticSnapshot<T> {
-  const snapshot = structuredClone(value);
-  const encoder = new SemanticIdentityEncoder(new IncrementalSha256());
-  encodeValue(snapshot, new WeakSet<object>(), encoder);
+  // Clone an external graph in one operation: independent subtree clones can
+  // change aliasing and getter order. Reuse only an already-owned root.
+  const snapshot =
+    typeof value === 'object' && value !== null && ownedValues.has(value)
+      ? value
+      : freezeSemanticValue(structuredClone(value), new WeakSet<object>());
+  let fingerprint: string | undefined;
   return {
-    value: freezeSemanticValue(snapshot, new WeakSet<object>()),
-    fingerprint: encoder.digest(),
+    value: snapshot,
+    // Value-only consumers still validate and detach eagerly, but never encode
+    // or hash the history. Only values frozen by this module may be reused.
+    get fingerprint() {
+      if (fingerprint !== undefined) return fingerprint;
+      const object = typeof snapshot === 'object' && snapshot !== null ? snapshot : undefined;
+      fingerprint = object ? fingerprints.get(object) : undefined;
+      if (fingerprint === undefined) {
+        const encoder = new SemanticIdentityEncoder(new IncrementalSha256());
+        encodeValue(snapshot, new WeakSet<object>(), encoder);
+        fingerprint = encoder.digest();
+        if (object) fingerprints.set(object, fingerprint);
+      }
+      return fingerprint;
+    },
   };
 }
 
@@ -54,13 +74,28 @@ class SemanticIdentityEncoder {
   }
 }
 
-function freezeSemanticValue<T>(value: T, seen: WeakSet<object>): T {
-  if (typeof value !== 'object' || value === null || seen.has(value)) return value;
-  seen.add(value);
-  Reflect.ownKeys(value).forEach((key) => {
-    freezeSemanticValue(Reflect.get(value, key), seen);
-  });
-  return Object.freeze(value);
+function freezeSemanticValue<T>(value: T, ancestors: WeakSet<object>): T {
+  if (typeof value !== 'object' || value === null) {
+    if (!['undefined', 'string', 'boolean', 'number'].includes(typeof value) && value !== null) {
+      throw new TypeError(`Unsupported semantic identity value: ${typeof value}.`);
+    }
+    return value;
+  }
+  if (ownedValues.has(value)) return value;
+  if (ancestors.has(value)) throw new TypeError('Cyclic semantic identity values are unsupported.');
+  const prototype = Object.getPrototypeOf(value);
+  if (!Array.isArray(value) && prototype !== Object.prototype && prototype !== null) {
+    throw new TypeError('Semantic identity values must contain only plain objects and arrays.');
+  }
+  if (Object.getOwnPropertySymbols(value).length > 0) {
+    throw new TypeError('Symbol-keyed semantic identity values are unsupported.');
+  }
+  ancestors.add(value);
+  for (const key of Object.keys(value)) freezeSemanticValue(Reflect.get(value, key), ancestors);
+  ancestors.delete(value);
+  Object.freeze(value);
+  ownedValues.add(value);
+  return value;
 }
 
 function encodeValue(