Add Run#getWritable() for appending to another run's stream

.changeset/run-get-writable.md

@@ -0,0 +1,6 @@
+---
+'@workflow/core': minor
+'workflow': minor
+---
+
+Add `Run#getWritable()` to append to an existing run's stream.

docs/content/docs/v4/api-reference/workflow-api/get-run.mdx

@@ -67,6 +67,16 @@ import type { WorkflowReadableStreamOptions } from "workflow/api";
 export default WorkflowReadableStreamOptions;`}
 />
 
+#### WorkflowRunWritableStreamOptions
+
+<TSDoc
+definition={`
+import type { WorkflowRunWritableStreamOptions } from "workflow/api";
+export default WorkflowRunWritableStreamOptions;`}
+/>
+
+See [Writing to another run's stream](/docs/foundations/streaming#writing-to-another-runs-stream) for usage and lifecycle details.
+
 #### StopSleepOptions
 
 <TSDoc

docs/content/docs/v4/foundations/streaming.mdx

@@ -313,6 +313,34 @@ export async function POST(request: Request) {
 }
 ```
 
+## Writing to another run's stream
+
+`getRun(runId).getWritable()` appends to a stream owned by another run. This lets short-lived runs contribute to a long-lived holder run's stream using only its ID.
+
+```typescript title="workflows/turn.ts" lineNumbers
+import { getRun } from "workflow/api";
+
+type SessionEvent = { turn: number; text: string };
+
+async function runTurn(holderRunId: string, turn: number) {
+  "use step";
+
+  const writable = await getRun(holderRunId).getWritable<SessionEvent>(); // [!code highlight]
+  const writer = writable.getWriter();
+
+  await writer.write({ turn, text: "done" });
+  writer.releaseLock(); // [!code highlight]
+}
+```
+
+Pass `{ namespace: "name" }` to target a [namespaced stream](#namespaced-streams). The writable can also be forwarded through `start()` and into steps.
+
+<Callout type="warn">
+Contributors should call `releaseLock()`, which flushes pending writes. Calling `close()` closes the shared stream for every writer.
+</Callout>
+
+The API grants append access, not read access or additional authorization. The owning run controls the stream's lifecycle, and an unknown run rejects with `WorkflowRunNotFoundError`.
+
 ## Common patterns
 
 ### Progress updates for long-running tasks

docs/content/docs/v5/api-reference/workflow-api/get-run.mdx

@@ -65,6 +65,16 @@ import type { WorkflowReadableStreamOptions } from "workflow/api";
 export default WorkflowReadableStreamOptions;`}
 />
 
+#### WorkflowRunWritableStreamOptions
+
+<TSDoc
+definition={`
+import type { WorkflowRunWritableStreamOptions } from "workflow/api";
+export default WorkflowRunWritableStreamOptions;`}
+/>
+
+See [Writing to another run's stream](/docs/foundations/streaming#writing-to-another-runs-stream) for usage and lifecycle details.
+
 #### StopSleepOptions
 
 <TSDoc

docs/content/docs/v5/foundations/streaming.mdx

@@ -314,6 +314,34 @@ export async function POST(request: Request) {
 }
 ```
 
+## Writing to another run's stream
+
+`getRun(runId).getWritable()` appends to a stream owned by another run. This lets short-lived runs contribute to a long-lived holder run's stream using only its ID.
+
+```typescript title="workflows/turn.ts" lineNumbers
+import { getRun } from "workflow/api";
+
+type SessionEvent = { turn: number; text: string };
+
+async function runTurn(holderRunId: string, turn: number) {
+  "use step";
+
+  const writable = await getRun(holderRunId).getWritable<SessionEvent>(); // [!code highlight]
+  const writer = writable.getWriter();
+
+  await writer.write({ turn, text: "done" });
+  writer.releaseLock(); // [!code highlight]
+}
+```
+
+Pass `{ namespace: "name" }` to target a [namespaced stream](#namespaced-streams). The writable can also be forwarded through `start()` and into steps.
+
+<Callout type="warn">
+Contributors should call `releaseLock()`, which flushes pending writes. Calling `close()` closes the shared stream for every writer.
+</Callout>
+
+The API grants append access, not read access or additional authorization. The owning run controls the stream's lifecycle, and an unknown run rejects with `WorkflowRunNotFoundError`.
+
 ## Common patterns
 
 ### Progress updates for long-running tasks

packages/core/src/runtime.ts

@@ -161,6 +161,7 @@ export {
   Run,
   type WorkflowReadableStream,
   type WorkflowReadableStreamOptions,
+  type WorkflowRunWritableStreamOptions,
 } from './runtime/run.js';
 export {
   type CancelRunOptions,