Merge latest invariant registration gate

# Conflicts:
#	docs/event-producer-consumer.md

packages/session-persistence/session-persistence-jsonl/src/format.ts

@@ -12,8 +12,20 @@ import { createHash } from 'node:crypto'
 import { join } from 'node:path'
 import type { SessionEvent, SessionHeader, SessionId } from '@deepseek-ai/dsh-session'
 
+/** Physical encoding selected for JSONL session artifacts. */
+export type JsonlCompression = 'zstd' | 'none'
+
 /**
- * The first line of a session's `.jsonl` file: the immutable
+ * Return the artifact suffix for one physical encoding.
+ * @param compression - configured JSONL artifact encoding.
+ * @returns `.jsonl.zstd` for Zstandard or `.jsonl` for plaintext.
+ */
+export function logSuffix(compression: JsonlCompression): '.jsonl.zstd' | '.jsonl' {
+  return compression === 'zstd' ? '.jsonl.zstd' : '.jsonl'
+}
+
+/**
+ * The first JSONL record of a session artifact: the immutable
  * {@link SessionHeader} tagged as a `session` record so a reader can tell it
  * apart from an event line.
  */
@@ -25,6 +37,7 @@ export interface HeaderLine {
   cwd?: string
   parentSession?: SessionId
   seedLength?: number
+  delegationDepth: number
 }
 
 /**
@@ -41,6 +54,7 @@ export function toHeaderLine(header: SessionHeader): HeaderLine {
     ...header.cwd !== undefined ? { cwd: header.cwd } : {},
     ...header.parentSession !== undefined ? { parentSession: header.parentSession } : {},
     ...header.seedLength !== undefined ? { seedLength: header.seedLength } : {},
+    delegationDepth: header.delegationDepth ?? 0,
   }
 }
 
@@ -57,6 +71,7 @@ export function fromHeaderLine(line: HeaderLine): SessionHeader {
     ...line.cwd !== undefined ? { cwd: line.cwd } : {},
     ...line.parentSession !== undefined ? { parentSession: line.parentSession } : {},
     ...line.seedLength !== undefined ? { seedLength: line.seedLength } : {},
+    delegationDepth: line.delegationDepth,
   }
 }
 
@@ -68,6 +83,10 @@ function isHeaderLine(value: unknown): value is HeaderLine {
     && typeof (value as { version?: unknown }).version === 'number'
     && typeof (value as { id?: unknown }).id === 'string'
     && typeof (value as { createdAt?: unknown }).createdAt === 'number'
+    && typeof (value as { delegationDepth?: unknown }).delegationDepth === 'number'
+    && Number.isSafeInteger((value as { delegationDepth: number }).delegationDepth)
+    && (value as { delegationDepth: number }).delegationDepth >= 0
+    && !Object.is((value as { delegationDepth: number }).delegationDepth, -0)
   )
 }
 
@@ -119,10 +138,16 @@ export function sessionDir(root: string, cwd: string | undefined): string {
  * @param root - the backend's session root directory.
  * @param cwd - the session's project directory (picks the per-cwd bucket; `undefined` → `_no-cwd`).
  * @param id - the session id, path-encoded via {@link encodeSegment} before filesystem use.
- * @returns the session's `.jsonl` log file path.
+ * @param compression - physical artifact encoding and filename suffix.
+ * @returns the session's configured JSONL artifact path.
  */
-export function logPath(root: string, cwd: string | undefined, id: SessionId): string {
-  return join(sessionDir(root, cwd), `${encodeSegment(id)}.jsonl`)
+export function logPath(
+  root: string,
+  cwd: string | undefined,
+  id: SessionId,
+  compression: JsonlCompression,
+): string {
+  return join(sessionDir(root, cwd), `${encodeSegment(id)}${logSuffix(compression)}`)
 }
 
 /**

packages/session-persistence/session-persistence-jsonl/src/index.ts

@@ -17,8 +17,20 @@ import {
 } from '@deepseek-ai/dsh-session-persistence'
 import type { SessionEvent, SessionId, SessionHeader } from '@deepseek-ai/dsh-session'
 import {
-  encodeSegment, eventLine, logPath, parseHeaderMeta, scanLog, sessionDir, toHeaderLine,
+  encodeSegment, eventLine, logPath, logSuffix, parseHeaderMeta, scanLog, sessionDir, toHeaderLine,
+  type JsonlCompression,
 } from './format.ts'
+import { compressZstdFrame, decompressZstdFrame, scanZstdFrames } from './zstd.ts'
+
+export type { JsonlCompression } from './format.ts'
+
+const DEFAULT_COMPRESSION: JsonlCompression = 'zstd'
+
+/** Loader schema for the JSONL artifact's physical encoding. */
+export const JsonlCompressionSchema: z<JsonlCompression> = z.union([
+  z.const('zstd'),
+  z.const('none'),
+]).default(DEFAULT_COMPRESSION)
 
 /** Plugin config: where the JSONL backend keeps its session logs (`root` is required — no default). */
 export interface Config {
@@ -28,6 +40,14 @@ export interface Config {
    * (bash calls, subprocesses). Sessions group under per-cwd subdirectories.
    */
   root: string
+  /** Physical encoding; defaults to checksummed Zstandard frames. */
+  compression?: JsonlCompression
+}
+
+/** Opaque coordinator token for replacing bytes recovered from a torn frame. */
+interface JsonlTornMarker {
+  truncateTo: number
+  recoveredEvents: SessionEvent[]
 }
 
 /** Whether a filesystem error means absence; every non-ENOENT failure must surface. */
@@ -38,13 +58,15 @@ function isENOENT(error: unknown): boolean {
 /**
  * The JSONL persistence backend. Load as a plugin; it registers as
  * `ctx.sessionPersistence` and (via the coordinator) installs the write-path
- * listeners. Its torn-tail marker is the byte offset to truncate the log to.
+ * listeners. Its torn-tail marker carries the byte offset and any events
+ * recovered from an incomplete final Zstandard frame.
  */
-export class SessionPersistenceJsonl extends SessionPersistence implements PersistenceBackend<number> {
+export class SessionPersistenceJsonl extends SessionPersistence implements PersistenceBackend<JsonlTornMarker> {
   static inject = ['sessions']
 
   static Config: z<Config> = z.object({
     root: z.string().required(),
+    compression: JsonlCompressionSchema,
   })
 
   /**
@@ -55,7 +77,9 @@ export class SessionPersistenceJsonl extends SessionPersistence implements Persi
   override readonly name = 'session-persistence-jsonl'
 
   private root: string
-  private coordinator: PersistenceCoordinator<number>
+  private compression: JsonlCompression
+  private coordinator: PersistenceCoordinator<JsonlTornMarker>
+  private rootEncodingCheck: Promise<void> | undefined
 
   /** Runtime host platform used to decide whether directory sync is supported. */
   readonly internals: { platform: NodeJS.Platform } = { platform: process.platform }
@@ -64,7 +88,8 @@ export class SessionPersistenceJsonl extends SessionPersistence implements Persi
     super(ctx)
     // Resolve once so later process.cwd() changes cannot split one backend across roots.
     this.root = resolve(config.root)
-    this.coordinator = new PersistenceCoordinator<number>(this.ctx, this)
+    this.compression = config.compression ?? DEFAULT_COMPRESSION
+    this.coordinator = new PersistenceCoordinator<JsonlTornMarker>(this.ctx, this)
   }
 
   // Each backend keeps the typed service surface beside its storage hooks;
@@ -74,7 +99,7 @@ export class SessionPersistenceJsonl extends SessionPersistence implements Persi
 
   /** Resolve the absolute target path without touching the filesystem. */
   locate(meta: SessionHeader): SessionLocation {
-    return { kind: 'jsonl', path: logPath(this.root, meta.cwd, meta.id) }
+    return { kind: 'jsonl', path: logPath(this.root, meta.cwd, meta.id, this.compression) }
   }
 
   create(meta: SessionHeader): Promise<void> {
@@ -96,7 +121,8 @@ export class SessionPersistenceJsonl extends SessionPersistence implements Persi
   // --- PersistenceBackend hooks (the file-bytes storage primitives) ---
 
   /** Read a stored prefix by id across all cwd buckets when cwd is unknown. */
-  async loadStored(id: SessionId): Promise<StoredPrefix<number> | undefined> {
+  async loadStored(id: SessionId): Promise<StoredPrefix<JsonlTornMarker> | undefined> {
+    await this.ensureRootEncoding()
     const file = await this.findLog(id)
     if (file === undefined) return undefined
     return this.readPrefix(file.path)
@@ -106,28 +132,85 @@ export class SessionPersistenceJsonl extends SessionPersistence implements Persi
    * Read a stored prefix within one cwd for HMR adoption. `undefined` names the
    * no-cwd bucket rather than an unknown cwd, so this never scans other buckets.
    */
-  async loadLive(id: SessionId, cwd: string | undefined): Promise<StoredPrefix<number> | undefined> {
-    const path = logPath(this.root, cwd, id)
-    if (!await this.exists(path)) return undefined
+  async loadLive(id: SessionId, cwd: string | undefined): Promise<StoredPrefix<JsonlTornMarker> | undefined> {
+    await this.ensureRootEncoding()
+    const path = logPath(this.root, cwd, id, this.compression)
+    if (!await this.exists(path)) {
+      await this.rejectOppositeArtifact(cwd, id)
+      return undefined
+    }
     return this.readPrefix(path)
   }
 
   /**
-   * Read a stored prefix and convert torn-tail state to the byte offset the
-   * coordinator can round-trip without knowing the file format.
+   * Read a stored prefix and convert torn-tail state to the opaque marker the
+   * coordinator can round-trip without knowing the physical encoding.
    */
-  private async readPrefix(path: string): Promise<StoredPrefix<number>> {
+  private async readPrefix(path: string): Promise<StoredPrefix<JsonlTornMarker>> {
     const buffer = await readFile(path)
+    if (this.compression === 'zstd') return this.readZstdPrefix(buffer)
     const { meta, events, committedBytes } = scanLog(buffer)
     return {
       meta,
       events,
-      ...committedBytes < buffer.byteLength ? { tornMarker: committedBytes } : {},
+      ...committedBytes < buffer.byteLength
+        ? { tornMarker: { truncateTo: committedBytes, recoveredEvents: [] } }
+        : {},
+    }
+  }
+
+  /** Decode complete frames and retain complete JSONL records from a torn final frame. */
+  private async readZstdPrefix(buffer: Buffer): Promise<StoredPrefix<JsonlTornMarker>> {
+    const { frames, tornStart } = scanZstdFrames(buffer)
+    if (frames.length === 0) throw new Error('empty or header-less Zstandard session log')
+
+    const plaintextFrames: Buffer[] = []
+    for (const frame of frames) {
+      try {
+        plaintextFrames.push(await decompressZstdFrame(buffer.subarray(frame.start, frame.end)))
+      } catch (error) {
+        throw new Error(`corrupt Zstandard session log: frame at byte ${frame.start} failed validation`, { cause: error })
+      }
+    }
+
+    const headerFrame = plaintextFrames[0]
+    if (headerFrame === undefined || headerFrame.length === 0 || headerFrame.indexOf(0x0A) !== headerFrame.length - 1) {
+      throw new Error('corrupt Zstandard session log: first frame is not exactly one header line')
+    }
+    const completePlaintext = Buffer.concat(plaintextFrames)
+    const completePrefix = scanLog(completePlaintext)
+    if (completePrefix.committedBytes !== completePlaintext.length) {
+      throw new Error('corrupt Zstandard session log: complete frame contains a torn JSONL record')
+    }
+    if (tornStart === undefined) {
+      return { meta: completePrefix.meta, events: completePrefix.events }
+    }
+
+    let recoveredPlaintext: Buffer = Buffer.alloc(0)
+    try {
+      recoveredPlaintext = await decompressZstdFrame(buffer.subarray(tornStart))
+    } catch {
+      // A structurally incomplete final frame may end before Node's decoder can
+      // emit any plaintext; the complete prior frames remain recoverable.
+    }
+    const recoveredPrefix = scanLog(Buffer.concat([completePlaintext, recoveredPlaintext]))
+    /* v8 ignore next 3 -- appending plaintext cannot shorten the already-scanned complete prefix */
+    if (recoveredPrefix.events.length < completePrefix.events.length) {
+      throw new Error('corrupt Zstandard session log: recovered prefix does not extend complete frames')
+    }
+    return {
+      meta: recoveredPrefix.meta,
+      events: recoveredPrefix.events,
+      tornMarker: {
+        truncateTo: tornStart,
+        recoveredEvents: recoveredPrefix.events.slice(completePrefix.events.length),
+      },
     }
   }
 
   /** Durably append a batch, lazily materializing the file when not yet present. */
   async appendBatch(meta: SessionHeader, events: readonly SessionEvent[], isMaterialized: boolean): Promise<void> {
+    await this.ensureRootEncoding()
     if (isMaterialized) {
       await this.appendLines(meta, events)
     } else {
@@ -136,22 +219,30 @@ export class SessionPersistenceJsonl extends SessionPersistence implements Persi
   }
 
   /**
-   * Make a crash repair durable: truncate the torn tail to `tornMarker` bytes (if
-   * any), then append the synthetic `closers` (if any). Two fsync'd steps — the
-   * seam does not require this to be atomic.
+   * Make a crash repair durable: truncate a torn tail, restore complete events
+   * decoded from it, then append synthetic closers. Two fsync'd steps — the seam
+   * does not require this to be atomic.
    */
-  async commitRepair(meta: SessionHeader, tornMarker: number | undefined, closers: readonly SessionEvent[]): Promise<void> {
-    if (tornMarker !== undefined) await this.repair(meta, tornMarker)
-    if (closers.length > 0) await this.appendLines(meta, closers)
+  async commitRepair(
+    meta: SessionHeader,
+    tornMarker: JsonlTornMarker | undefined,
+    closers: readonly SessionEvent[],
+  ): Promise<void> {
+    if (tornMarker !== undefined) await this.repair(meta, tornMarker.truncateTo)
+    const repairedEvents = [...(tornMarker?.recoveredEvents ?? []), ...closers]
+    if (repairedEvents.length > 0) await this.appendLines(meta, repairedEvents)
   }
 
   /** List all stored sessions' metadata (header line only — no full-log parse). */
   async list(): Promise<SessionHeader[]> {
+    await this.ensureRootEncoding()
     const metas: SessionHeader[] = []
     for (const dir of await this.listCwdDirs()) {
-      for (const name of await this.listJsonl(dir)) {
+      for (const name of await this.listArtifacts(dir)) {
         // Read only headers so listing scales with session count, not log size.
-        const first = await this.readFirstLine(`${dir}/${name}`)
+        const first = this.compression === 'zstd'
+          ? await this.readFirstZstdLine(`${dir}/${name}`)
+          : await this.readFirstLine(`${dir}/${name}`)
         if (first === undefined) continue // empty/half-written file
         const meta = parseHeaderMeta(first)
         if (meta === undefined) continue // not a session header
@@ -170,15 +261,14 @@ export class SessionPersistenceJsonl extends SessionPersistence implements Persi
     await this.syncDir(dirname(this.root))
     await mkdir(dir, { recursive: true, mode: 0o700 })
     await this.syncDir(this.root)
-    const finalPath = logPath(this.root, meta.cwd, meta.id)
+    const finalPath = logPath(this.root, meta.cwd, meta.id, this.compression)
     // Materialization is the first write; an existing log is an id collision.
     /* v8 ignore next 3 -- createCore guards collisions before materialize; this is a TOCTOU backstop */
     if (await this.exists(finalPath)) {
       throw new Error(`refusing to materialize "${meta.id}": a log already exists on disk (load/resume it instead)`)
     }
-    const header = JSON.stringify(toHeaderLine(meta))
-    const body = events.map(eventLine).join('\n')
-    const content = header + '\n' + body + '\n'
+    await this.rejectOppositeArtifact(meta.cwd, meta.id)
+    const content = await this.encodeMaterialization(meta, events)
 
     const tmp = `${finalPath}.${randomBytes(6).toString('hex')}.tmp`
     const handle = await open(tmp, 'wx', 0o600)
@@ -211,6 +301,22 @@ export class SessionPersistenceJsonl extends SessionPersistence implements Persi
     }
   }
 
+  /** Encode the header and first batch without combining their frame boundaries. */
+  private async encodeMaterialization(meta: SessionHeader, events: readonly SessionEvent[]): Promise<Buffer | string> {
+    const header = JSON.stringify(toHeaderLine(meta)) + '\n'
+    const body = events.map(eventLine).join('\n') + '\n'
+    if (this.compression === 'none') return header + body
+    const headerFrame = await compressZstdFrame(header)
+    const eventFrame = await compressZstdFrame(body)
+    return Buffer.concat([headerFrame, eventFrame])
+  }
+
+  /** Encode one durable append batch in the configured physical representation. */
+  private async encodeEventBatch(events: readonly SessionEvent[]): Promise<Buffer | string> {
+    const body = events.map(eventLine).join('\n') + '\n'
+    return this.compression === 'zstd' ? compressZstdFrame(body) : body
+  }
+
   /** fsync a directory when the host exposes that durability primitive. */
   private async syncDir(dir: string): Promise<void> {
     const handle = await open(dir, 'r')
@@ -234,12 +340,13 @@ export class SessionPersistenceJsonl extends SessionPersistence implements Persi
    * batch; leaving partial bytes would create duplicate sequence numbers.
    */
   private async appendLines(meta: SessionHeader, events: readonly SessionEvent[]): Promise<void> {
-    const path = logPath(this.root, meta.cwd, meta.id)
+    const content = await this.encodeEventBatch(events)
+    const path = logPath(this.root, meta.cwd, meta.id, this.compression)
     const handle = await open(path, 'a')
     try {
       const { size: before } = await handle.stat()
       try {
-        await handle.writeFile(events.map(eventLine).join('\n') + '\n')
+        await handle.writeFile(content)
         await handle.sync()
       } catch (error) {
         // Roll back whatever bytes landed so a retry starts from a clean EOF.
@@ -254,7 +361,7 @@ export class SessionPersistenceJsonl extends SessionPersistence implements Persi
 
   /** Truncate the log file to `offset` bytes and fsync (discard the crash tail). */
   private async repair(meta: SessionHeader, offset: number): Promise<void> {
-    const path = logPath(this.root, meta.cwd, meta.id)
+    const path = logPath(this.root, meta.cwd, meta.id, this.compression)
     await truncate(path, offset)
     const handle = await open(path, 'r+')
     try {
@@ -292,17 +399,47 @@ export class SessionPersistenceJsonl extends SessionPersistence implements Persi
     }
   }
 
+  /** Read and validate only the independently compressed header frame. */
+  private async readFirstZstdLine(path: string): Promise<string | undefined> {
+    const handle = await open(path, 'r')
+    try {
+      let content = Buffer.alloc(0)
+      const chunk = Buffer.alloc(8192)
+      for (;;) {
+        const { bytesRead } = await handle.read(chunk, 0, chunk.length, null)
+        if (bytesRead === 0) return undefined
+        content = Buffer.concat([content, chunk.subarray(0, bytesRead)])
+        const first = scanZstdFrames(content, 1).frames[0]
+        if (first === undefined) continue
+        let plaintext: Buffer
+        try {
+          plaintext = await decompressZstdFrame(content.subarray(first.start, first.end))
+        } catch (error) {
+          throw new Error('corrupt Zstandard session log: header frame failed validation', { cause: error })
+        }
+        if (plaintext.length === 0 || plaintext.indexOf(0x0A) !== plaintext.length - 1) {
+          throw new Error('corrupt Zstandard session log: first frame is not exactly one header line')
+        }
+        return plaintext.subarray(0, -1).toString('utf8')
+      }
+    } finally {
+      await handle.close()
+    }
+  }
+
   /**
    * Find a session by id across cwd buckets for resume. Cwd-scoped HMR adoption
    * bypasses this scan so a no-cwd session cannot claim another bucket.
    */
   private async findLog(id: SessionId): Promise<{ path: string; cwd: string | undefined } | undefined> {
-    const target = encodeSegment(id) + '.jsonl'
+    const target = encodeSegment(id) + logSuffix(this.compression)
     for (const dir of await this.listCwdDirs()) {
       const path = `${dir}/${target}`
+      const opposite = `${dir}/${encodeSegment(id)}${logSuffix(this.oppositeCompression())}`
+      if (await this.exists(opposite)) throw this.encodingMismatch(opposite)
       if (await this.exists(path)) {
         // Recover the cwd from the header so the caller has the session's bucket.
-        const { meta } = scanLog(await readFile(path))
+        const { meta } = await this.readPrefix(path)
         return { path, cwd: meta.cwd }
       }
     }
@@ -321,9 +458,45 @@ export class SessionPersistenceJsonl extends SessionPersistence implements Persi
     }
   }
 
-  private async listJsonl(dir: string): Promise<string[]> {
+  private async listArtifacts(dir: string): Promise<string[]> {
     const entries = await readdir(dir)
-    return entries.filter(n => n.endsWith('.jsonl'))
+    const oppositeSuffix = logSuffix(this.oppositeCompression())
+    const incompatible = entries.find(name => name.endsWith(oppositeSuffix))
+    if (incompatible !== undefined) throw this.encodingMismatch(`${dir}/${incompatible}`)
+    const suffix = logSuffix(this.compression)
+    return entries.filter(name => name.endsWith(suffix))
+  }
+
+  /** Reject a root that already belongs to the other physical encoding. */
+  private ensureRootEncoding(): Promise<void> {
+    this.rootEncodingCheck ??= this.checkRootEncoding()
+    return this.rootEncodingCheck
+  }
+
+  private async checkRootEncoding(): Promise<void> {
+    const oppositeSuffix = logSuffix(this.oppositeCompression())
+    for (const dir of await this.listCwdDirs()) {
+      const entries = await readdir(dir)
+      const incompatible = entries.find(name => name.endsWith(oppositeSuffix))
+      if (incompatible !== undefined) throw this.encodingMismatch(`${dir}/${incompatible}`)
+    }
+  }
+
+  private async rejectOppositeArtifact(cwd: string | undefined, id: SessionId): Promise<void> {
+    const path = logPath(this.root, cwd, id, this.oppositeCompression())
+    if (await this.exists(path)) throw this.encodingMismatch(path)
+  }
+
+  private oppositeCompression(): JsonlCompression {
+    return this.compression === 'zstd' ? 'none' : 'zstd'
+  }
+
+  private encodingMismatch(path: string): Error {
+    return new Error(
+      `session artifact ${JSON.stringify(path)} uses ${logSuffix(this.oppositeCompression())}, `
+      + `but this backend is configured for compression ${JSON.stringify(this.compression)}; `
+      + 'use a separate root or select the matching compression mode',
+    )
   }
 
   private async exists(path: string): Promise<boolean> {

packages/session-persistence/session-persistence-jsonl/src/zstd.ts

@@ -0,0 +1,116 @@
+/**
+ * Zstandard frame primitives for the JSONL persistence backend. The backend
+ * owns a concatenated-frame container so it can append and recover batches
+ * without exposing compression mechanics through the persistence seam.
+ * @module dsh-session-persistence-jsonl/zstd
+ */
+
+import { constants, zstdCompress, zstdDecompress, type ZstdOptions } from 'node:zlib'
+import { promisify } from 'node:util'
+
+const ZSTD_MAGIC = 0xFD2FB528
+const zstdCompressAsync = promisify(zstdCompress)
+const zstdDecompressAsync = promisify(zstdDecompress)
+const CHECKSUM_OPTIONS: ZstdOptions = {
+  params: { [constants.ZSTD_c_checksumFlag]: 1 },
+}
+
+/** Byte range occupied by one structurally complete Zstandard frame. */
+export interface ZstdFrameRange {
+  /** Inclusive frame start. */
+  start: number
+  /** Exclusive frame end. */
+  end: number
+}
+
+/** Structural scan result for a concatenated Zstandard stream. */
+export interface ZstdFrameScan {
+  /** Complete frames in file order. */
+  frames: ZstdFrameRange[]
+  /** Start of an incomplete final frame, when EOF interrupts one. */
+  tornStart?: number
+}
+
+/**
+ * Locate complete frames without decompressing their blocks. Invalid complete
+ * structure rejects; EOF inside the final frame returns its start for repair.
+ * @param buffer - complete bytes currently present in the session artifact.
+ * @param maxFrames - optional complete-frame limit for metadata-only readers.
+ * @returns complete frame ranges and an optional incomplete-final-frame start.
+ */
+export function scanZstdFrames(buffer: Buffer, maxFrames = Number.POSITIVE_INFINITY): ZstdFrameScan {
+  const frames: ZstdFrameRange[] = []
+  let offset = 0
+
+  while (offset < buffer.length) {
+    const start = offset
+    if (buffer.length - offset < 4) return { frames, tornStart: start }
+    if (buffer.readUInt32LE(offset) !== ZSTD_MAGIC) {
+      throw new Error(`corrupt Zstandard session log: invalid frame magic at byte ${offset}`)
+    }
+    offset += 4
+
+    if (offset === buffer.length) return { frames, tornStart: start }
+    const descriptor = buffer.readUInt8(offset)
+    offset += 1
+    if ((descriptor & 0x18) !== 0) {
+      throw new Error(`corrupt Zstandard session log: reserved frame-header bit at byte ${offset - 1}`)
+    }
+
+    const contentSizeFlag = descriptor >>> 6
+    const singleSegment = (descriptor & 0x20) !== 0
+    const checksum = (descriptor & 0x04) !== 0
+    const dictionaryFlag = descriptor & 0x03
+    const dictionaryBytes = dictionaryFlag === 3 ? 4 : dictionaryFlag
+    const contentSizeBytes = contentSizeFlag === 0
+      ? (singleSegment ? 1 : 0)
+      : 1 << contentSizeFlag
+    const remainingHeaderBytes = (singleSegment ? 0 : 1) + dictionaryBytes + contentSizeBytes
+    if (buffer.length - offset < remainingHeaderBytes) return { frames, tornStart: start }
+    offset += remainingHeaderBytes
+
+    for (;;) {
+      if (buffer.length - offset < 3) return { frames, tornStart: start }
+      const blockHeader = buffer.readUIntLE(offset, 3)
+      offset += 3
+      const lastBlock = (blockHeader & 1) !== 0
+      const blockType = (blockHeader >>> 1) & 0x03
+      const blockSize = blockHeader >>> 3
+      if (blockType === 0x03) {
+        throw new Error(`corrupt Zstandard session log: reserved block type at byte ${offset - 3}`)
+      }
+      const payloadBytes = blockType === 0x01 ? 1 : blockSize
+      if (buffer.length - offset < payloadBytes) return { frames, tornStart: start }
+      offset += payloadBytes
+      if (lastBlock) break
+    }
+
+    if (checksum) {
+      if (buffer.length - offset < 4) return { frames, tornStart: start }
+      offset += 4
+    }
+    frames.push({ start, end: offset })
+    if (frames.length === maxFrames) return { frames }
+  }
+
+  return { frames }
+}
+
+/**
+ * Compress one independently decodable, checksummed Zstandard frame.
+ * @param input - JSONL bytes for a header or durable event batch.
+ * @returns the complete encoded frame.
+ */
+export async function compressZstdFrame(input: Buffer | string): Promise<Buffer> {
+  return zstdCompressAsync(input, CHECKSUM_OPTIONS)
+}
+
+/**
+ * Decompress one complete frame or the available prefix of a torn final frame.
+ * Complete-frame checksums are validated by Node's decoder.
+ * @param input - bytes beginning at a Zstandard frame boundary.
+ * @returns plaintext produced from the available input.
+ */
+export async function decompressZstdFrame(input: Buffer): Promise<Buffer> {
+  return zstdDecompressAsync(input)
+}