feat(streams): add writer session seam (#3832)

## Summary & Motivation

Gives one in-memory stream writer a stable identity and its own sequence space, so a transport can preserve chunk ordering across a mid-stream HTTP/WebSocket transition. `Streamer.streams.createWriteSession` is optional — Worlds that don't implement it keep using `write`/`writeMulti`/`close` unchanged.

Abort disposes the session rather than closing it, since a producer failure is transport cleanup, not stream completion.

## Test Plan

Tests added, plus the full `@workflow/world-vercel` suite and package builds/typecheck pass. Root build/typecheck is blocked locally by a missing Rust toolchain for the unrelated `@workflow/swc-plugin`.

Author: alangenfeld

.changeset/ws-stream-writer-session.md

@@ -0,0 +1,6 @@
+---
+'@workflow/core': patch
+'@workflow/world': minor
+---
+
+Add an optional stateful stream writer-session seam with stable writer identity and sequence tracking.

packages/core/src/serialization.ts

@@ -6,6 +6,7 @@ import {
   WorkflowRuntimeError,
 } from '@workflow/errors';
 import { once } from '@workflow/utils';
+import type { StreamWriteSession } from '@workflow/world';
 import { envNumber } from '@workflow/world/env-config';
 import { parse, stringify, unflatten } from 'devalue';
 import { monotonicFactory } from 'ulid';
@@ -1310,6 +1311,20 @@ export class WorkflowServerWritableStream extends WritableStream<Uint8Array> {
       }
     };
 
+    // One stable identity and sequence space per in-memory sink. Start session
+    // construction as soon as the run-ready barrier permits, so transports may
+    // negotiate eagerly without allowing a write to overtake run creation.
+    // This eager chain cannot strand a new rejection: ensureRunReady absorbs its
+    // ordering-only failure, and every path that can observe worldPromise also
+    // awaits this session promise before it writes, closes, or disposes.
+    const writerId = `wrtr_${defaultUlid()}` as const;
+    const writeSessionPromise: Promise<StreamWriteSession | undefined> =
+      ensureRunReady().then(async () => {
+        const world = await worldPromise;
+        return world.streams.createWriteSession?.(runId, name, { writerId });
+      });
+    let nextChunkSeq = 0;
+
     // ------------------------------------------------------------------
     // Group-commit buffering.
     //
@@ -1427,15 +1442,24 @@ export class WorkflowServerWritableStream extends WritableStream<Uint8Array> {
     ): Promise<void> => {
       await ensureRunReady();
       const world = await worldPromise;
+      const session = await writeSessionPromise;
       const dispatchAt = Date.now();
-      if (typeof world.streams.writeMulti === 'function' && group.length > 1) {
+      if (session) {
+        await session.write(nextChunkSeq, group);
+      } else if (
+        typeof world.streams.writeMulti === 'function' &&
+        group.length > 1
+      ) {
         await world.streams.writeMulti(runId, name, group);
       } else {
         // Fall back to sequential writes
         for (const chunk of group) {
           await world.streams.write(runId, name, chunk);
         }
       }
+      // `inFlight` admits only one dispatch loop, so no second group can read
+      // this sequence space until the current group has advanced it.
+      nextChunkSeq += group.length;
       if (groupT0 !== undefined) {
         recordStreamWriteFlush(
           groupT0,
@@ -1635,8 +1659,13 @@ export class WorkflowServerWritableStream extends WritableStream<Uint8Array> {
         await ensureRunReady();
 
         const world = await worldPromise;
+        const session = await writeSessionPromise;
         const closeStart = Date.now();
-        await world.streams.close(runId, name);
+        if (session) {
+          await session.close();
+        } else {
+          await world.streams.close(runId, name);
+        }
         recordStreamClose(closeStart, runId, name);
       },
       async abort(reason) {
@@ -1671,6 +1700,11 @@ export class WorkflowServerWritableStream extends WritableStream<Uint8Array> {
         // Reject blocked writers and drain waiters so nothing leaks or
         // hangs on a promise whose timer was just cleared.
         rejectWaiters(sinkError);
+        // Abort is transport cleanup, not semantic stream completion. A
+        // stateful World releases its socket without sending close; stateless
+        // Worlds keep the existing no-op behavior.
+        const session = await writeSessionPromise.catch(() => undefined);
+        await session?.dispose?.();
       },
     });
 

packages/core/src/writable-stream.test.ts

@@ -31,6 +31,7 @@ async function waitFor(
 
 describe('WorkflowServerWritableStream', () => {
   let mockStreams: {
+    createWriteSession?: ReturnType<typeof vi.fn>;
     write: ReturnType<typeof vi.fn>;
     writeMulti: ReturnType<typeof vi.fn>;
     close: ReturnType<typeof vi.fn>;
@@ -516,6 +517,82 @@ describe('WorkflowServerWritableStream', () => {
     });
   });
 
+  describe('stateful writer sessions', () => {
+    it('uses one stable writer id and writer-local sequence across groups', async () => {
+      const session = {
+        write: vi.fn().mockResolvedValue(undefined),
+        close: vi.fn().mockResolvedValue(undefined),
+      };
+      mockStreams.createWriteSession = vi.fn(() => session);
+
+      const stream = new WorkflowServerWritableStream('run-123', 'test-stream');
+      const writer = stream.getWriter();
+      await writer.write(new Uint8Array([1]));
+      await waitFor(() => expect(session.write).toHaveBeenCalledTimes(1));
+      await writer.write(new Uint8Array([2]));
+      await waitFor(() => expect(session.write).toHaveBeenCalledTimes(2));
+      await writer.close();
+
+      expect(mockStreams.createWriteSession).toHaveBeenCalledTimes(1);
+      expect(mockStreams.createWriteSession).toHaveBeenCalledWith(
+        'run-123',
+        'test-stream',
+        {
+          writerId: expect.stringMatching(
+            /^wrtr_[0123456789ABCDEFGHJKMNPQRSTVWXYZ]{26}$/
+          ),
+        }
+      );
+      expect(session.write).toHaveBeenNthCalledWith(1, 0, [
+        new Uint8Array([1]),
+      ]);
+      expect(session.write).toHaveBeenNthCalledWith(2, 1, [
+        new Uint8Array([2]),
+      ]);
+      expect(session.close).toHaveBeenCalledTimes(1);
+      expect(mockStreams.write).not.toHaveBeenCalled();
+      expect(mockStreams.writeMulti).not.toHaveBeenCalled();
+      expect(mockStreams.close).not.toHaveBeenCalled();
+    });
+
+    it('creates a new writer id for each in-memory sink lifetime', async () => {
+      const writerIds: string[] = [];
+      mockStreams.createWriteSession = vi.fn((_runId, _name, options) => {
+        writerIds.push(options.writerId);
+        return {
+          write: vi.fn().mockResolvedValue(undefined),
+          close: vi.fn().mockResolvedValue(undefined),
+        };
+      });
+
+      new WorkflowServerWritableStream('run-123', 'test-stream');
+      new WorkflowServerWritableStream('run-123', 'test-stream');
+      await waitFor(() => expect(writerIds).toHaveLength(2));
+      expect(new Set(writerIds).size).toBe(2);
+    });
+  });
+
+  describe('abort cleanup', () => {
+    it('drains accepted chunks then disposes without semantic close', async () => {
+      const session = {
+        write: vi.fn().mockResolvedValue(undefined),
+        close: vi.fn().mockResolvedValue(undefined),
+        dispose: vi.fn().mockResolvedValue(undefined),
+      };
+      mockStreams.createWriteSession = vi.fn(() => session);
+
+      const stream = new WorkflowServerWritableStream('run-123', 'test-stream');
+      const writer = stream.getWriter();
+      await writer.write(new Uint8Array([1]));
+      await writer.abort(new Error('producer failed'));
+
+      expect(session.write).toHaveBeenCalledWith(0, [new Uint8Array([1])]);
+      expect(session.dispose).toHaveBeenCalledTimes(1);
+      expect(session.close).not.toHaveBeenCalled();
+      expect(mockStreams.close).not.toHaveBeenCalled();
+    });
+  });
+
   describe('error handling', () => {
     it('surfaces a dispatch failure at the durability barrier (close) and poisons later writes', async () => {
       mockStreams.write.mockRejectedValueOnce(new Error('write error'));
@@ -549,6 +626,28 @@ describe('WorkflowServerWritableStream', () => {
       expect(mockStreams.write).toHaveBeenCalledTimes(1);
     });
 
+    it('keeps a session failure sticky without advancing its sequence', async () => {
+      const session = {
+        write: vi.fn().mockRejectedValueOnce(new Error('session failed')),
+        close: vi.fn().mockResolvedValue(undefined),
+      };
+      mockStreams.createWriteSession = vi.fn(() => session);
+
+      const writer = new WorkflowServerWritableStream(
+        'run-123',
+        'test-stream'
+      ).getWriter();
+      await writer.write(new Uint8Array([1]));
+      await waitFor(() => expect(session.write).toHaveBeenCalledTimes(1));
+      await expect(writer.write(new Uint8Array([2]))).rejects.toThrow(
+        'session failed'
+      );
+      await expect(writer.close()).rejects.toThrow();
+      expect(session.write).toHaveBeenCalledTimes(1);
+      expect(session.write).toHaveBeenCalledWith(0, [new Uint8Array([1])]);
+      expect(session.close).not.toHaveBeenCalled();
+    });
+
     it('should propagate close errors', async () => {
       mockStreams.close.mockRejectedValueOnce(new Error('close error'));
 

packages/world/src/interfaces.ts

@@ -40,6 +40,25 @@ import type {
   StepWithoutData,
 } from './steps.js';
 
+export interface StreamWriteSession {
+  /**
+   * Write one ordered group from this in-memory writer lifetime.
+   * `chunkSeq` is writer-local and identifies the first chunk in `chunks`.
+   */
+  write(chunkSeq: number, chunks: (string | Uint8Array)[]): Promise<void>;
+
+  /** Close this writer lifetime after all prior writes are durable. */
+  close(): Promise<void>;
+
+  /** Release transport resources without semantically closing the stream. */
+  dispose?(): Promise<void> | void;
+}
+
+export interface CreateStreamWriteSessionOptions {
+  /** Stable observational id for this in-memory writer lifetime. */
+  writerId: `wrtr_${string}`;
+}
+
 export interface Streamer {
   /**
    * Number of milliseconds a stream waits for additional chunks to arrive
@@ -57,6 +76,17 @@ export interface Streamer {
   streamFlushIntervalMs?: number;
 
   streams: {
+    /**
+     * Optionally create a stateful writer session. Core creates at most one
+     * session per in-memory WritableStream and otherwise uses the stateless
+     * write/writeMulti/close methods below unchanged.
+     */
+    createWriteSession?(
+      runId: string,
+      name: string,
+      options: CreateStreamWriteSessionOptions
+    ): StreamWriteSession;
+
     write(
       runId: string,
       name: string,