diff --git a/packages/adapter-kit/src/interfaces.ts b/packages/adapter-kit/src/interfaces.ts
index c432310..3968d14 100644
--- a/packages/adapter-kit/src/interfaces.ts
+++ b/packages/adapter-kit/src/interfaces.ts
@@ -51,6 +51,7 @@ import type {
   PageBrowserCommand,
   PageBrowserResult,
   PortableFile,
+  PortableFileStream,
   ProcessEvent,
   SandboxCapabilities,
   ScreenRequest,
@@ -145,6 +146,21 @@ export interface SandboxProvider {
     options?: { maxBytes?: number },
   ): Promise<Uint8Array>;
   writeFile(computer: ComputerRef, file: PortableFile, context: AdapterContext): Promise<void>;
+  /**
+   * Bounded-memory variants of readFile and writeFile for large media on computers without a
+   * transfer cap. Same path rules as their byte versions; a failed or aborted write leaves no file.
+   */
+  readFileStream?(
+    computer: ComputerRef,
+    path: string,
+    context: AdapterContext,
+    options?: { maxBytes?: number },
+  ): Promise<ReadableStream<Uint8Array>>;
+  writeFileStream?(
+    computer: ComputerRef,
+    file: PortableFileStream,
+    context: AdapterContext,
+  ): Promise<void>;
   exportWorkspace(computer: ComputerRef, context: AdapterContext): AsyncIterable<PortableFile>;
   importWorkspace(
     computer: ComputerRef,
diff --git a/packages/adapter-kit/src/types.ts b/packages/adapter-kit/src/types.ts
index 67669e5..3b29a16 100644
--- a/packages/adapter-kit/src/types.ts
+++ b/packages/adapter-kit/src/types.ts
@@ -52,6 +52,13 @@ export interface PortableFile {
   executable?: boolean;
 }
 
+/** A workspace file whose bytes arrive as a stream instead of one buffer. */
+export interface PortableFileStream {
+  path: string;
+  stream: ReadableStream<Uint8Array> | AsyncIterable<Uint8Array>;
+  executable?: boolean;
+}
+
 export interface ComputerRef {
   id: string;
   botId: string;
diff --git a/packages/adapters/src/artifacts.ts b/packages/adapters/src/artifacts.ts
index 6143df4..ee8d59c 100644
--- a/packages/adapters/src/artifacts.ts
+++ b/packages/adapters/src/artifacts.ts
@@ -1,6 +1,5 @@
 import { createHash, randomUUID } from "node:crypto";
 import { constants } from "node:fs";
-import type { FileHandle } from "node:fs/promises";
 import { lstat, mkdir, open, rm } from "node:fs/promises";
 import path from "node:path";
 import type {
@@ -14,9 +13,9 @@ import type {
   StoredArtifactStream,
 } from "@rakazo/adapter-kit";
 import { ArtifactTooLargeError } from "@rakazo/adapter-kit";
+import { byteChunks, fileByteStream, writeFully } from "./byte-streams.js";
 
 const O_NOFOLLOW = constants.O_NOFOLLOW ?? 0;
-const READ_CHUNK_BYTES = 256 * 1024;
 /** Space ids and artifact keys are generated ids; anything else could address another path. */
 const SAFE_SEGMENT = /^[A-Za-z0-9][A-Za-z0-9_-]{0,127}$/;
 
@@ -155,72 +154,6 @@ export class LocalArtifactStore implements ArtifactStore {
   }
 }
 
-async function* byteChunks(
-  source: ReadableStream<Uint8Array> | AsyncIterable<Uint8Array>,
-): AsyncIterable<Uint8Array> {
-  if (!("getReader" in source)) {
-    yield* source;
-    return;
-  }
-  const reader = source.getReader();
-  try {
-    for (;;) {
-      const { done, value } = await reader.read();
-      if (done) return;
-      yield value;
-    }
-  } finally {
-    // Stop the producer (e.g. an upload body) when the store gives up early, without waiting on
-    // a producer that may never confirm.
-    void reader.cancel().catch(() => undefined);
-    reader.releaseLock();
-  }
-}
-
-async function writeFully(handle: FileHandle, chunk: Uint8Array) {
-  let offset = 0;
-  while (offset < chunk.byteLength) {
-    const { bytesWritten } = await handle.write(chunk, offset, chunk.byteLength - offset);
-    offset += bytesWritten;
-  }
-}
-
-/** Streams [start, end] of an open file and closes it when done, failed or cancelled. */
-function fileByteStream(handle: FileHandle, start: number, end: number) {
-  let position = start;
-  let closed = false;
-  const close = async () => {
-    if (closed) return;
-    closed = true;
-    await handle.close().catch(() => undefined);
-  };
-  return new ReadableStream<Uint8Array>({
-    async pull(controller) {
-      try {
-        const length = Math.min(end - position + 1, READ_CHUNK_BYTES);
-        if (length <= 0) {
-          await close();
-          controller.close();
-          return;
-        }
-        const buffer = new Uint8Array(length);
-        const { bytesRead } = await handle.read(buffer, 0, length, position);
-        if (bytesRead === 0) throw new Error("Artifact ended before the requested range");
-        position += bytesRead;
-        controller.enqueue(bytesRead === length ? buffer : buffer.subarray(0, bytesRead));
-        if (position > end) {
-          await close();
-          controller.close();
-        }
-      } catch (error) {
-        await close();
-        controller.error(error);
-      }
-    },
-    cancel: close,
-  });
-}
-
 export class CapturingNotificationProvider implements NotificationProvider {
   readonly sent: NotificationMessage[] = [];
 
diff --git a/packages/adapters/src/byte-streams.ts b/packages/adapters/src/byte-streams.ts
new file mode 100644
index 0000000..b7e2da5
--- /dev/null
+++ b/packages/adapters/src/byte-streams.ts
@@ -0,0 +1,91 @@
+import type { FileHandle } from "node:fs/promises";
+
+type ByteSource = ReadableStream<Uint8Array> | AsyncIterable<Uint8Array>;
+
+const READ_CHUNK_BYTES = 256 * 1024;
+
+export async function* byteChunks(source: ByteSource): AsyncIterable<Uint8Array> {
+  if (!("getReader" in source)) {
+    yield* source;
+    return;
+  }
+  const reader = source.getReader();
+  try {
+    for (;;) {
+      const { done, value } = await reader.read();
+      if (done) return;
+      yield value;
+    }
+  } finally {
+    // Stop the producer (e.g. an upload body) when the consumer gives up early, without waiting
+    // on a producer that may never confirm.
+    void reader.cancel().catch(() => undefined);
+    reader.releaseLock();
+  }
+}
+
+/** Releases a stream nobody will read, so its producer (e.g. an open file) is closed. */
+export function discardByteStream(source: ByteSource): void {
+  if ("cancel" in source) void source.cancel().catch(() => undefined);
+}
+
+/** Collects a stream into one buffer, for destinations that only accept bytes. */
+export async function collectBytes(source: ByteSource): Promise<Uint8Array> {
+  const chunks: Uint8Array[] = [];
+  let size = 0;
+  for await (const chunk of byteChunks(source)) {
+    chunks.push(chunk);
+    size += chunk.byteLength;
+  }
+  const bytes = new Uint8Array(size);
+  let offset = 0;
+  for (const chunk of chunks) {
+    bytes.set(chunk, offset);
+    offset += chunk.byteLength;
+  }
+  return bytes;
+}
+
+export async function writeFully(handle: FileHandle, chunk: Uint8Array) {
+  let offset = 0;
+  while (offset < chunk.byteLength) {
+    const { bytesWritten } = await handle.write(chunk, offset, chunk.byteLength - offset);
+    offset += bytesWritten;
+  }
+}
+
+/** Streams [start, end] of an open file and closes it when done, failed or cancelled. */
+export function fileByteStream(handle: FileHandle, start: number, end: number) {
+  let position = start;
+  let closed = false;
+  const close = async () => {
+    if (closed) return;
+    closed = true;
+    await handle.close().catch(() => undefined);
+  };
+  return new ReadableStream<Uint8Array>({
+    async pull(controller) {
+      try {
+        const length = Math.min(end - position + 1, READ_CHUNK_BYTES);
+        if (length <= 0) {
+          await close();
+          controller.close();
+          return;
+        }
+        const buffer = new Uint8Array(length);
+        const { bytesRead } = await handle.read(buffer, 0, length, position);
+        if (bytesRead === 0) throw new Error("File ended before the requested range");
+        position += bytesRead;
+        controller.enqueue(bytesRead === length ? buffer : buffer.subarray(0, bytesRead));
+        if (position > end) {
+          await close();
+          controller.close();
+        }
+      } catch (error) {
+        await close();
+        controller.error(error);
+      }
+    },
+    cancel: close,
+  });
+}
diff --git a/packages/adapters/src/computer-support.ts b/packages/adapters/src/computer-support.ts
index a6e6519..00948e0 100644
--- a/packages/adapters/src/computer-support.ts
+++ b/packages/adapters/src/computer-support.ts
@@ -1,7 +1,15 @@
 import { createHash } from "node:crypto";
 import path from "node:path";
-import type { ComputerAction, ComputerObservation, ComputerRef } from "@rakazo/adapter-kit";
+import type {
+  AdapterContext,
+  ComputerAction,
+  ComputerObservation,
+  ComputerRef,
+  PortableFileStream,
+  SandboxProvider,
+} from "@rakazo/adapter-kit";
 import type { ComputerMode } from "@rakazo/contracts";
+import { collectBytes, discardByteStream } from "./byte-streams.js";
 
 export function toComputerRef(computer: {
   homeKey: string;
@@ -29,6 +37,48 @@ export function computerFileTransferMaxBytes(computer: Pick<ComputerRef, "kind">
     : REMOTE_COMPUTER_FILE_TRANSFER_MAX_BYTES;
 }
 
+/** Streams a file into the workspace; providers without streaming writes receive its bytes. */
+export async function writeComputerFileStream(
+  sandbox: SandboxProvider,
+  computer: ComputerRef,
+  file: PortableFileStream,
+  context: AdapterContext,
+): Promise<void> {
+  try {
+    if (sandbox.writeFileStream) {
+      await sandbox.writeFileStream(computer, file, context);
+      return;
+    }
+    const content = await collectBytes(file.stream);
+    await sandbox.writeFile(
+      computer,
+      { path: file.path, content, ...(file.executable ? { executable: true } : {}) },
+      context,
+    );
+  } catch (error) {
+    discardByteStream(file.stream);
+    throw error;
+  }
+}
+
+/** Opens a workspace file as a stream; providers without streaming reads return it in one chunk. */
+export async function readComputerFileStream(
+  sandbox: SandboxProvider,
+  computer: ComputerRef,
+  filePath: string,
+  context: AdapterContext,
+  options?: { maxBytes?: number },
+): Promise<ReadableStream<Uint8Array>> {
+  if (sandbox.readFileStream) return sandbox.readFileStream(computer, filePath, context, options);
+  const bytes = await sandbox.readFile(computer, filePath, context, options);
+  return new ReadableStream({
+    start(controller) {
+      controller.enqueue(bytes);
+      controller.close();
+    },
+  });
+}
+
 const EMPTY_PNG = Uint8Array.from(
   Buffer.from(
     "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR42mNk+A8AAQUBAScY42YAAAAASUVORK5CYII=",
diff --git a/packages/adapters/src/desktop-sandbox-write-containment.test.ts b/packages/adapters/src/desktop-sandbox-write-containment.test.ts
index 313a3d7..46b2dcb 100644
--- a/packages/adapters/src/desktop-sandbox-write-containment.test.ts
+++ b/packages/adapters/src/desktop-sandbox-write-containment.test.ts
@@ -265,7 +265,7 @@ describe("desktop sandbox write containment without O_NOFOLLOW", () => {
       lstatRace.afterRealpath = async (inspected) => {
         if (path.basename(inspected) !== "notes") return;
         notesRealpaths += 1;
-        // After openContainedWorkspaceFile's parent realpath, point the pathname at an
+        // After writeContainedWorkspaceFile's parent realpath, point the pathname at an
         // outside directory. Without binding the opened parent inode to a fresh
         // contained realpath, later checks could validate a restored inside path
         // while the handle still referred outside.
@@ -313,6 +313,74 @@ describe("desktop sandbox write containment without O_NOFOLLOW", () => {
     expect(await readFile(outside, "utf8")).toBe("outside-before");
   });
 
+  it("applies the same containment to streamed writes", async () => {
+    const { root, desktop, computer } = await fixture("streamed-escapes");
+    const outsideFile = path.join(root, "outside.txt");
+    const outsideDirectory = path.join(root, "outside-directory");
+    await writeFile(outsideFile, "outside-before");
+    await mkdir(outsideDirectory);
+    await symlink(outsideFile, path.join(computer.providerRef, "final-link.txt"));
+    await link(outsideFile, path.join(computer.providerRef, "hard-link.txt"));
+    await symlink(
+      outsideDirectory,
+      path.join(computer.providerRef, "escape-directory"),
+      "junction",
+    );
+    const streamed = (filePath: string) =>
+      desktop.writeFileStream(
+        computer,
+        {
+          path: filePath,
+          stream: (async function* () {
+            yield new TextEncoder().encode("after");
+          })(),
+        },
+        ctx,
+      );
+
+    for (const filePath of [
+      "final-link.txt",
+      "hard-link.txt",
+      "escape-directory/outside.txt",
+      "escape-directory/nested/outside.txt",
+      "../outside.txt",
+    ]) {
+      await expect(streamed(filePath)).rejects.toThrow();
+    }
+    expect(await readFile(outsideFile, "utf8")).toBe("outside-before");
+    await expect(readFile(path.join(outsideDirectory, "outside.txt"))).rejects.toMatchObject({
+      code: "ENOENT",
+    });
+    await expect(readFile(path.join(outsideDirectory, "nested"))).rejects.toMatchObject({
+      code: "ENOENT",
+    });
+
+    await streamed("notes/result.txt");
+    expect(await readFile(path.join(computer.providerRef, "notes/result.txt"), "utf8")).toBe(
+      "after",
+    );
+  });
+
+  it("removes a streamed file whose source fails part-way", async () => {
+    const { desktop, computer } = await fixture("streamed-failure");
+    const target = path.join(computer.providerRef, "clip.mp4");
+
+    await expect(
+      desktop.writeFileStream(
+        computer,
+        {
+          path: "clip.mp4",
+          stream: (async function* () {
+            yield new Uint8Array(1024);
+            throw new Error("store read failed");
+          })(),
+        },
+        ctx,
+      ),
+    ).rejects.toThrow("store read failed");
+    expect(await pathExists(target)).toBe(false);
+  });
+
   it("still creates and replaces ordinary workspace files", async () => {
     const { desktop, computer } = await fixture("ordinary-files");
 
diff --git a/packages/adapters/src/desktop-sandbox.ts b/packages/adapters/src/desktop-sandbox.ts
index f4c1638..8559e48 100644
--- a/packages/adapters/src/desktop-sandbox.ts
+++ b/packages/adapters/src/desktop-sandbox.ts
@@ -23,12 +23,14 @@ import type {
   ComputerRef,
   ControlLeaseRef,
   PortableFile,
+  PortableFileStream,
   ProcessEvent,
   SandboxProvider,
   ScreenRequest,
   ScreenSession,
 } from "@rakazo/adapter-kit";
 import { boundedSandboxCommandTimeoutMs } from "@rakazo/core";
+import { byteChunks, fileByteStream } from "./byte-streams.js";
 import {
   applyPlaceholderAction,
   boundedComputerActions,
@@ -232,21 +234,51 @@ export class DesktopSandboxProvider implements SandboxProvider {
     return new Uint8Array(await readFile(target));
   }
 
+  async readFileStream(
+    computer: ComputerRef,
+    filePath: string,
+    _context?: AdapterContext,
+    options?: { maxBytes?: number },
+  ) {
+    const box = this.requiredBox(computer);
+    const target = await localWorkspaceTarget(box.home, filePath, true);
+    const handle = await open(target, constants.O_RDONLY | O_NOFOLLOW);
+    try {
+      const info = await handle.stat();
+      if (!info.isFile()) throw new Error("computer file not found");
+      if (options?.maxBytes !== undefined && info.size > options.maxBytes) {
+        throw new Error(`computer file exceeds ${options.maxBytes} bytes`);
+      }
+      return fileByteStream(handle, 0, info.size - 1);
+    } catch (error) {
+      await handle.close();
+      throw error;
+    }
+  }
+
   async writeFile(computer: ComputerRef, file: PortableFile) {
     const box = this.requiredBox(computer);
     const target = await localWorkspaceTarget(box.home, file.path, false);
-    const handle = await openContainedWorkspaceFile(
+    await writeContainedWorkspaceFile(box.home, target, Boolean(file.executable), (handle) =>
+      handle.writeFile(file.content),
+    );
+  }
+
+  async writeFileStream(computer: ComputerRef, file: PortableFileStream, context: AdapterContext) {
+    const box = this.requiredBox(computer);
+    const target = await localWorkspaceTarget(box.home, file.path, false);
+    await writeContainedWorkspaceFile(
       box.home,
       target,
-      file.executable ? 0o700 : 0o600,
+      Boolean(file.executable),
+      async (handle) => {
+        // Each write continues at the file position, so memory stays at one chunk.
+        for await (const chunk of byteChunks(file.stream)) {
+          context.signal.throwIfAborted();
+          await handle.writeFile(chunk);
+        }
+      },
     );
-    try {
-      await handle.truncate(0);
-      await handle.writeFile(file.content);
-      if (file.executable) await handle.chmod(0o700);
-    } finally {
-      await handle.close();
-    }
   }
 
   async *exportWorkspace(computer: ComputerRef): AsyncIterable<PortableFile> {
@@ -416,7 +448,17 @@ async function localWorkspaceTarget(home: string, relative: string, mustExist: b
   return resolved;
 }
 
-async function openContainedWorkspaceFile(home: string, target: string, mode: number) {
+/**
+ * Opens `target` without following links outside the workspace, truncates it and runs `write`.
+ * A file that fails to write is removed again, so no partial file stays behind.
+ */
+async function writeContainedWorkspaceFile(
+  home: string,
+  target: string,
+  executable: boolean,
+  write: (handle: ContainedHandle) => Promise<void>,
+) {
+  const mode = executable ? 0o700 : 0o600;
   const resolvedHome = await realpath(home);
   const parentPath = path.dirname(target);
   const name = path.basename(target);
@@ -434,6 +476,7 @@ async function openContainedWorkspaceFile(home: string, target: string, mode: nu
 
   let handle: ContainedHandle | undefined;
   let created = false;
+  let writing = false;
   let openedPath = path.join(parentReal, name);
   try {
     // Bind the parent handle to a still-contained realpath. Otherwise a swap
@@ -496,9 +539,15 @@ async function openContainedWorkspaceFile(home: string, target: string, mode: nu
       throw new Error("Path escapes the computer workspace");
     }
     await assertContainedFileHandle(handle, resolvedHome, parentHandle, containedParent, name);
-    return handle;
+    writing = true;
+    await handle.truncate(0);
+    await write(handle);
+    if (executable) await handle.chmod(0o700);
+    const written = handle;
+    handle = undefined;
+    await written.close();
   } catch (error) {
-    if (created && handle) {
+    if ((created || writing) && handle) {
       const createdStat = await handle.stat({ bigint: true }).catch(() => undefined);
       let cleanupPath = openedPath;
       if (useWin32Relative) {
@@ -510,7 +559,7 @@ async function openContainedWorkspaceFile(home: string, target: string, mode: nu
       }
       await handle.close().catch(() => undefined);
       handle = undefined;
-      // Only unlink if the path still names the inode we created.
+      // Only unlink if the path still names the inode we created or started writing.
       if (createdStat) {
         const present = await lstat(cleanupPath, { bigint: true }).catch(() => undefined);
         if (present && present.dev === createdStat.dev && present.ino === createdStat.ino) {
diff --git a/packages/adapters/src/executor.ts b/packages/adapters/src/executor.ts
index 5953f06..80ed17f 100644
--- a/packages/adapters/src/executor.ts
+++ b/packages/adapters/src/executor.ts
@@ -208,6 +208,7 @@ import {
   computerFileTransferMaxBytes,
   displayBotWorkspacePath,
   filterTeamWorkspaceEntries,
+  readComputerFileStream,
   resolveBotWorkspaceCwd,
   resolveBotWorkspacePath,
   teamBotWorkspaceDirectory,
@@ -2410,7 +2411,7 @@ export function createRunExecutor(deps: ExecutorDeps) {
                     groupId: thread.groupId ?? undefined,
                     runId: run.id,
                     filePath: outPath,
-                    bytes: png,
+                    content: png,
                     operationId: executionId,
                   },
                 );
@@ -2441,9 +2442,12 @@ export function createRunExecutor(deps: ExecutorDeps) {
               attachableFileMaxBytes(deps.artifacts, mimeType),
               computerFileTransferMaxBytes(computer),
             );
-            let bytes: Uint8Array;
+            let content: ReadableStream<Uint8Array>;
             try {
-              bytes = await deps.sandbox.readFile(computer, storedPath, context, { maxBytes });
+              // Streamed, so a large video from a local computer is never held in memory whole.
+              content = await readComputerFileStream(deps.sandbox, computer, storedPath, context, {
+                maxBytes,
+              });
             } catch {
               return finish({
                 error: `file not found, unreadable or larger than ${formatMiB(maxBytes)}`,
@@ -2460,8 +2464,9 @@ export function createRunExecutor(deps: ExecutorDeps) {
                   groupId: thread.groupId ?? undefined,
                   runId: run.id,
                   filePath,
-                  bytes,
+                  content,
                   operationId: executionId,
+                  signal: context.signal,
                   present: args.present === true,
                 },
               );
diff --git a/packages/adapters/src/host-aware-sandbox.ts b/packages/adapters/src/host-aware-sandbox.ts
index 08574e1..d5533ef 100644
--- a/packages/adapters/src/host-aware-sandbox.ts
+++ b/packages/adapters/src/host-aware-sandbox.ts
@@ -6,11 +6,13 @@ import type {
   ComputerRef,
   ControlLeaseRef,
   PortableFile,
+  PortableFileStream,
   ProcessEvent,
   SandboxProvider,
   ScreenRequest,
 } from "@rakazo/adapter-kit";
 import type { PrismaClient } from "@rakazo/db";
+import { readComputerFileStream, writeComputerFileStream } from "./computer-support.js";
 import { DesktopSandboxProvider } from "./desktop-sandbox.js";
 import { createSandboxProvider, type SandboxProviderOptions } from "./sandbox-factory.js";
 
@@ -141,6 +143,19 @@ export class HostAwareSandbox implements SandboxProvider {
     return this.route(computer).writeFile(computer, file, context);
   }
 
+  readFileStream(
+    computer: ComputerRef,
+    path: string,
+    context: AdapterContext,
+    options?: { maxBytes?: number },
+  ) {
+    return readComputerFileStream(this.route(computer), computer, path, context, options);
+  }
+
+  writeFileStream(computer: ComputerRef, file: PortableFileStream, context: AdapterContext) {
+    return writeComputerFileStream(this.route(computer), computer, file, context);
+  }
+
   exportWorkspace(computer: ComputerRef, context: AdapterContext) {
     return this.route(computer).exportWorkspace(computer, context);
   }
diff --git a/packages/adapters/src/media-runtime.ts b/packages/adapters/src/media-runtime.ts
index 5e26917..d699a40 100644
--- a/packages/adapters/src/media-runtime.ts
+++ b/packages/adapters/src/media-runtime.ts
@@ -598,7 +598,12 @@ export function createMediaRuntime(
   };
 
   /** Copies a stored artifact to `file`: a clone of the local file when possible, else a stream. */
-  const copyArtifact = async (job: MediaJob, storageKey: string, file: string) => {
+  const copyArtifact = async (
+    job: MediaJob,
+    storageKey: string,
+    file: string,
+    signal: AbortSignal,
+  ) => {
     const context = artifactContext(job, `media-input:${job.id}`);
     const local = await deps.artifacts.localPath?.(storageKey, context).catch(() => undefined);
     if (local) {
@@ -609,6 +614,8 @@ export function createMediaRuntime(
     await pipeline(
       Readable.fromWeb(stream as NodeReadableStream<Uint8Array>),
       createWriteStream(file, { flags: "wx", mode: 0o600 }),
+      // A cancelled job stops copying; the job's temp dir (and the partial file) is removed after.
+      { signal },
     );
   };
 
@@ -722,7 +729,7 @@ export function createMediaRuntime(
           attachmentExtensionForMimeType(row.mimeType) ||
           (/^\.[a-z0-9]{1,8}$/i.test(path.extname(row.name)) ? path.extname(row.name) : "");
         const file = path.join(tempDir, `input-${row.id}${extension.toLowerCase()}`);
-        await copyArtifact(job, row.storageKey, file);
+        await copyArtifact(job, row.storageKey, file, signal);
         const input: MediaJobInputFile = {
           artifactId: row.id,
           path: file,
diff --git a/packages/adapters/src/thread-artifacts.test.ts b/packages/adapters/src/thread-artifacts.test.ts
index d3d355e..075d14b 100644
--- a/packages/adapters/src/thread-artifacts.test.ts
+++ b/packages/adapters/src/thread-artifacts.test.ts
@@ -1,13 +1,33 @@
 import { readFileSync } from "node:fs";
+import {
+  mkdir,
+  mkdtemp,
+  readdir,
+  readFile,
+  realpath,
+  rm,
+  stat,
+  symlink,
+  writeFile,
+} from "node:fs/promises";
+import { tmpdir } from "node:os";
+import path from "node:path";
 import type {
   AdapterContext,
   ArtifactStore,
+  ArtifactStreamPut,
   ComputerRef,
   SandboxProvider,
 } from "@rakazo/adapter-kit";
+import { ArtifactTooLargeError } from "@rakazo/adapter-kit";
 import type { MessageBlock } from "@rakazo/contracts";
 import type { PrismaClient } from "@rakazo/db";
-import { describe, expect, it, vi } from "vitest";
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { byteChunks } from "./byte-streams.js";
+import { readComputerFileStream } from "./computer-support.js";
+import { DesktopSandboxProvider } from "./desktop-sandbox.js";
+import { FakeSandboxProvider } from "./fake-sandbox.js";
+import { HostAwareSandbox } from "./host-aware-sandbox.js";
 import {
   attachableFileMaxBytes,
   attachWorkspaceFileToThread,
@@ -15,6 +35,15 @@ import {
   materializeCurrentTurnFiles,
 } from "./thread-artifacts.js";
 
+function byteStream(chunks: Uint8Array[]) {
+  return new ReadableStream<Uint8Array>({
+    start(controller) {
+      for (const chunk of chunks) controller.enqueue(chunk);
+      controller.close();
+    },
+  });
+}
+
 describe("current-turn thread files", () => {
   it("removes stored bytes when artifact metadata cannot be created", async () => {
     const remove = vi.fn().mockResolvedValue(undefined);
@@ -38,7 +67,7 @@ describe("current-turn thread files", () => {
           botId: "bot-1",
           runId: "run-1",
           filePath: "report.pdf",
-          bytes: new Uint8Array([1, 2, 3]),
+          content: new Uint8Array([1, 2, 3]),
           operationId: "attach-1",
         },
       ),
@@ -73,7 +102,7 @@ describe("current-turn thread files", () => {
       botId: "bot-1",
       runId: "run-1",
       filePath: "out/report.md",
-      bytes: new TextEncoder().encode("# Report\n"),
+      content: new TextEncoder().encode("# Report\n"),
       operationId: "attach-1",
     };
 
@@ -95,7 +124,9 @@ describe("current-turn thread files", () => {
         storageKey: "stored-1",
       },
     ]);
-    const get = vi.fn().mockResolvedValue(new Uint8Array([1, 2, 3, 4]));
+    const createReadStream = vi.fn(async () =>
+      byteStream([new Uint8Array([1, 2]), new Uint8Array([3, 4])]),
+    );
     const writeFile = vi.fn().mockResolvedValue(undefined);
     const markWorkspaceDirty = vi.fn();
     const context: AdapterContext & { botId: string } = {
@@ -126,14 +157,15 @@ describe("current-turn thread files", () => {
     const files = await materializeCurrentTurnFiles(
       {
         prisma: { artifact: { findMany } } as unknown as PrismaClient,
-        artifacts: { get } as unknown as ArtifactStore,
+        artifacts: { createReadStream } as unknown as ArtifactStore,
         sandbox: { writeFile } as unknown as SandboxProvider,
       },
       blocks,
       { context, computer, computerMode: "team", markWorkspaceDirty },
     );
 
-    expect(get).toHaveBeenCalledWith("stored-1", context);
+    // A provider without streaming writes still receives the whole file as bytes.
+    expect(createReadStream).toHaveBeenCalledWith("stored-1", context);
     expect(writeFile).toHaveBeenCalledWith(
       computer,
       {
@@ -165,7 +197,7 @@ describe("current-turn thread files", () => {
       { id: "aud-1", name: "memo.mp3", mimeType: "audio/mpeg", size: 3, storageKey: "k-aud" },
       { id: "vid-1", name: "clip.mp4", mimeType: "video/mp4", size: 40 * mb, storageKey: "k-vid" },
     ]);
-    const get = vi.fn().mockResolvedValue(new Uint8Array([1, 2, 3]));
+    const createReadStream = vi.fn(async () => byteStream([new Uint8Array([1, 2, 3])]));
     const writeFile = vi.fn().mockResolvedValue(undefined);
     const context: AdapterContext & { botId: string } = {
       operationId: "run-1",
@@ -189,7 +221,7 @@ describe("current-turn thread files", () => {
     ];
     const deps = {
       prisma: { artifact: { findMany } } as unknown as PrismaClient,
-      artifacts: { get } as unknown as ArtifactStore,
+      artifacts: { createReadStream } as unknown as ArtifactStore,
       sandbox: { writeFile } as unknown as SandboxProvider,
     };
 
@@ -203,7 +235,7 @@ describe("current-turn thread files", () => {
       ["aud-1", "attachments/aud-1.mp3"],
       ["vid-1", null],
     ]);
-    expect(get).not.toHaveBeenCalledWith("k-vid", context);
+    expect(createReadStream).not.toHaveBeenCalledWith("k-vid", context);
     const instruction = currentTurnFilesInstruction(remote);
     expect(instruction).toContain('"attachments/img-1.png"');
     expect(instruction).toContain("too large to copy to your computer");
@@ -265,6 +297,254 @@ describe("current-turn thread files", () => {
   });
 });
 
+describe("streamed media on a local computer", () => {
+  const CHUNK = 64 * 1024;
+  /** 8 MiB, far above the one chunk the copy may hold at a time. */
+  const CHUNKS = 128;
+  const roots: string[] = [];
+  const context: AdapterContext & { botId: string } = {
+    operationId: "run-1",
+    traceId: "run-1",
+    spaceId: "workspace-1",
+    userId: "user-1",
+    botId: "bot-1",
+    signal: new AbortController().signal,
+  };
+  const videoBlock: MessageBlock = {
+    kind: "video",
+    artifactId: "vid-1",
+    name: "clip.mp4",
+    mimeType: "video/mp4",
+    size: CHUNK * CHUNKS,
+  };
+
+  afterEach(async () => {
+    vi.restoreAllMocks();
+    await Promise.all(roots.splice(0).map((root) => rm(root, { recursive: true, force: true })));
+  });
+
+  async function localComputer() {
+    const root = await realpath(await mkdtemp(path.join(tmpdir(), "rakazo-media-stream-")));
+    roots.push(root);
+    const desktop = new DesktopSandboxProvider({ root });
+    // The production wrapper: desktop computers route to the host provider.
+    const sandbox = new HostAwareSandbox(new FakeSandboxProvider(), desktop, async () => true);
+    const computer = await desktop.provision({ botId: "bot-1", homePath: "/unused" }, context);
+    return { root, sandbox, computer, home: computer.providerRef! };
+  }
+
+  /** A stored video served one chunk per read, like the local store's file stream. */
+  function storedVideo(onPull: (index: number) => Promise<void> = async () => {}) {
+    let index = 0;
+    const cancel = vi.fn();
+    const stream = new ReadableStream<Uint8Array>(
+      {
+        async pull(controller) {
+          if (index === CHUNKS) {
+            controller.close();
+            return;
+          }
+          await onPull(index);
+          controller.enqueue(new Uint8Array(CHUNK).fill(index % 251));
+          index += 1;
+        },
+        cancel,
+      },
+      { highWaterMark: 0 },
+    );
+    const get = vi.fn();
+    const deps = (sandbox: SandboxProvider) => ({
+      prisma: {
+        artifact: {
+          findMany: vi.fn().mockResolvedValue([
+            {
+              id: "vid-1",
+              name: "clip.mp4",
+              mimeType: "video/mp4",
+              size: CHUNK * CHUNKS,
+              storageKey: "k-vid",
+            },
+          ]),
+        },
+      } as unknown as PrismaClient,
+      artifacts: { get, createReadStream: vi.fn(async () => stream) } as unknown as ArtifactStore,
+      sandbox,
+    });
+    return { deps, get, cancel };
+  }
+
+  it("copies a large video to disk chunk by chunk instead of reading it whole", async () => {
+    const { sandbox, computer, home } = await localComputer();
+    const target = path.join(home, "attachments/vid-1.mp4");
+    let mostAhead = 0;
+    const video = storedVideo(async (index) => {
+      const written = await stat(target).then(
+        (info) => info.size,
+        () => 0,
+      );
+      mostAhead = Math.max(mostAhead, index * CHUNK - written);
+    });
+
+    const files = await materializeCurrentTurnFiles(video.deps(sandbox), [videoBlock], {
+      context,
+      computer,
+      computerMode: "dedicated",
+    });
+
+    expect(files.map((file) => file.path)).toEqual(["attachments/vid-1.mp4"]);
+    expect(video.get).not.toHaveBeenCalled();
+    // Every chunk reached the disk before the next one was read from the store.
+    expect(mostAhead).toBe(0);
+    const copied = await readFile(target);
+    expect(copied.byteLength).toBe(CHUNK * CHUNKS);
+    expect(copied[100 * CHUNK]).toBe(100);
+  });
+
+  it("removes a partly copied video, even one replacing an earlier copy, when the run aborts", async () => {
+    const { sandbox, computer, home } = await localComputer();
+    const target = path.join(home, "attachments/vid-1.mp4");
+    await mkdir(path.dirname(target));
+    await writeFile(target, "earlier copy");
+    const controller = new AbortController();
+    const video = storedVideo(async (index) => {
+      if (index === 3) controller.abort(new Error("run cancelled"));
+    });
+
+    await expect(
+      materializeCurrentTurnFiles(video.deps(sandbox), [videoBlock], {
+        context: { ...context, signal: controller.signal },
+        computer,
+        computerMode: "dedicated",
+      }),
+    ).rejects.toThrow("run cancelled");
+    await expect(stat(target)).rejects.toMatchObject({ code: "ENOENT" });
+    expect(video.cancel).toHaveBeenCalled();
+  });
+
+  it("keeps streamed copies inside the workspace and releases the stored file", async () => {
+    const { root, sandbox, computer, home } = await localComputer();
+    const outside = path.join(root, "outside");
+    await mkdir(outside);
+    await symlink(outside, path.join(home, "attachments"), "junction");
+    const video = storedVideo();
+
+    await expect(
+      materializeCurrentTurnFiles(video.deps(sandbox), [videoBlock], {
+        context,
+        computer,
+        computerMode: "dedicated",
+      }),
+    ).rejects.toThrow("Path escapes the computer workspace");
+    expect(await readdir(outside)).toEqual([]);
+    expect(video.cancel).toHaveBeenCalled();
+  });
+
+  it("attaches a large local file by streaming it into the store", async () => {
+    const { sandbox, computer, home } = await localComputer();
+    await mkdir(path.join(home, "out"));
+    await writeFile(path.join(home, "out/render.mp4"), new Uint8Array(CHUNK * CHUNKS + 5).fill(7));
+    const readWhole = vi.spyOn(DesktopSandboxProvider.prototype, "readFile");
+    let largestChunk = 0;
+    let stored = 0;
+    const putStream = vi.fn(async (artifact: ArtifactStreamPut) => {
+      for await (const chunk of byteChunks(artifact.stream)) {
+        largestChunk = Math.max(largestChunk, chunk.byteLength);
+        stored += chunk.byteLength;
+      }
+      return { key: "stored-1", size: stored, sha256: "hash" };
+    });
+
+    const content = await readComputerFileStream(sandbox, computer, "out/render.mp4", context, {
+      maxBytes: 500 * 1024 * 1024,
+    });
+    const attached = await attachWorkspaceFileToThread(
+      {
+        prisma: {
+          artifact: {
+            create: vi.fn(async ({ data }: { data: Record<string, unknown> }) => ({
+              id: "artifact-1",
+              ...data,
+            })),
+          },
+        } as unknown as PrismaClient,
+        artifacts: {
+          describe: () => ({ capabilities: { stream: true } }),
+          putStream,
+          remove: vi.fn(),
+        } as unknown as ArtifactStore,
+      },
+      {
+        spaceId: "workspace-1",
+        userId: "user-1",
+        botId: "bot-1",
+        runId: "run-1",
+        filePath: "out/render.mp4",
+        content,
+        operationId: "attach-1",
+      },
+    );
+
+    expect(attached.block).toMatchObject({ kind: "video", name: "render.mp4" });
+    expect(stored).toBe(CHUNK * CHUNKS + 5);
+    expect(largestChunk).toBeLessThanOrEqual(256 * 1024);
+    expect(readWhole).not.toHaveBeenCalled();
+  });
+
+  it("refuses streamed reads above the cap or outside the workspace", async () => {
+    const { root, sandbox, computer, home } = await localComputer();
+    await writeFile(path.join(home, "clip.mp4"), new Uint8Array(10));
+    await writeFile(path.join(root, "secret.mp4"), new Uint8Array(10));
+    await symlink(path.join(root, "secret.mp4"), path.join(home, "escape.mp4"));
+
+    await expect(
+      readComputerFileStream(sandbox, computer, "clip.mp4", context, { maxBytes: 9 }),
+    ).rejects.toThrow("computer file exceeds 9 bytes");
+    await expect(readComputerFileStream(sandbox, computer, "escape.mp4", context)).rejects.toThrow(
+      "Path escapes the computer workspace",
+    );
+    await expect(
+      readComputerFileStream(sandbox, computer, "../secret.mp4", context),
+    ).rejects.toThrow();
+  });
+
+  it("releases an attached stream that is not stored and keeps the limit message", async () => {
+    const deps = (putStream: ArtifactStore["putStream"]) => ({
+      prisma: { artifact: { create: vi.fn() } } as unknown as PrismaClient,
+      artifacts: {
+        describe: () => ({ capabilities: { stream: true } }),
+        putStream,
+        remove: vi.fn(),
+      } as unknown as ArtifactStore,
+    });
+    const input = {
+      spaceId: "workspace-1",
+      userId: "user-1",
+      botId: "bot-1",
+      runId: "run-1",
+      operationId: "attach-1",
+    };
+    const unread = vi.fn();
+    const putStream = vi.fn();
+
+    await expect(
+      attachWorkspaceFileToThread(deps(putStream), {
+        ...input,
+        filePath: "notes.unknown-type",
+        content: new ReadableStream({ cancel: unread }),
+      }),
+    ).rejects.toThrow("Unsupported attachment type");
+    expect(unread).toHaveBeenCalled();
+    expect(putStream).not.toHaveBeenCalled();
+
+    await expect(
+      attachWorkspaceFileToThread(
+        deps(vi.fn().mockRejectedValue(new ArtifactTooLargeError(500 * 1024 * 1024))),
+        { ...input, filePath: "render.mp4", content: byteStream([new Uint8Array(1)]) },
+      ),
+    ).rejects.toThrow("file exceeds the 500 MiB attachment limit");
+  });
+});
+
 describe("executor attachments", () => {
   it("records the thread's group on every file the executor attaches", () => {
     // Driving render_plot's PNG fallback needs a full run; this guards the wiring that once
diff --git a/packages/adapters/src/thread-artifacts.ts b/packages/adapters/src/thread-artifacts.ts
index ef976d3..0903f7b 100644
--- a/packages/adapters/src/thread-artifacts.ts
+++ b/packages/adapters/src/thread-artifacts.ts
@@ -5,6 +5,7 @@ import type {
   ComputerRef,
   SandboxProvider,
 } from "@rakazo/adapter-kit";
+import { ArtifactTooLargeError } from "@rakazo/adapter-kit";
 import type { ComputerMode, MessageBlock } from "@rakazo/contracts";
 import { ATTACHMENT_MAX_BYTES } from "@rakazo/contracts";
 import {
@@ -14,10 +15,12 @@ import {
 } from "@rakazo/core";
 import type { PrismaClient } from "@rakazo/db";
 import { artifactMaxBytes, storeArtifactStream } from "./artifact-files.js";
+import { discardByteStream } from "./byte-streams.js";
 import {
   assertTeamWorkspacePathAllowed,
   computerFileTransferMaxBytes,
   resolveBotWorkspacePath,
+  writeComputerFileStream,
 } from "./computer-support.js";
 
 export type MaterializedThreadFile = {
@@ -56,35 +59,47 @@ export async function attachWorkspaceFileToThread(
     groupId?: string;
     runId: string;
     filePath: string;
-    bytes: Uint8Array;
+    /** A stream (e.g. from readComputerFileStream) is stored without buffering the file. */
+    content: Uint8Array | ReadableStream<Uint8Array>;
     operationId: string;
+    signal?: AbortSignal;
     present?: boolean;
   },
 ): Promise<{ artifactId: string; block: AttachmentBlock }> {
+  const content = input.content;
+  const stream =
+    content instanceof Uint8Array
+      ? (async function* () {
+          yield content;
+        })()
+      : content;
   const name = path.basename(input.filePath) || input.filePath;
-  const mimeType = inferAttachmentMimeType(name);
-  if (!mimeType) {
-    throw new Error(`Unsupported attachment type for ${name}`);
+  let row: Awaited<ReturnType<typeof storeArtifactStream>>;
+  try {
+    const mimeType = inferAttachmentMimeType(name);
+    if (!mimeType) {
+      throw new Error(`Unsupported attachment type for ${name}`);
+    }
+    row = await storeArtifactStream(deps, {
+      spaceId: input.spaceId,
+      userId: input.userId,
+      botId: input.botId,
+      groupId: input.groupId ?? null,
+      runId: input.runId,
+      name,
+      mimeType,
+      stream,
+      maxBytes: attachableFileMaxBytes(deps.artifacts, mimeType),
+      operationId: input.operationId,
+      signal: input.signal,
+    });
+  } catch (error) {
+    discardByteStream(stream);
+    if (error instanceof ArtifactTooLargeError) {
+      throw new Error(`file exceeds the ${formatMiB(error.maxBytes)} attachment limit`);
+    }
+    throw error;
   }
-  const maxBytes = attachableFileMaxBytes(deps.artifacts, mimeType);
-  if (input.bytes.byteLength > maxBytes) {
-    throw new Error(`file exceeds the ${formatMiB(maxBytes)} attachment limit`);
-  }
-  const bytes = input.bytes;
-  const row = await storeArtifactStream(deps, {
-    spaceId: input.spaceId,
-    userId: input.userId,
-    botId: input.botId,
-    groupId: input.groupId ?? null,
-    runId: input.runId,
-    name,
-    mimeType,
-    stream: (async function* () {
-      yield bytes;
-    })(),
-    maxBytes,
-    operationId: input.operationId,
-  });
   const block = messageBlockForArtifact(row);
   const presentable = block.kind === "image" || block.kind === "file";
   return {
@@ -137,7 +152,6 @@ export async function materializeCurrentTurnFiles(
       materialized.push({ ...entry, path: null });
       continue;
     }
-    const bytes = await deps.artifacts.get(row.storageKey, input.context);
     const relativePath = `attachments/${row.id}${attachmentExtensionForMimeType(row.mimeType)}`;
     const storedPath = resolveBotWorkspacePath(
       input.computerMode,
@@ -145,13 +159,13 @@ export async function materializeCurrentTurnFiles(
       relativePath,
     );
     if (input.teamBotIds) assertTeamWorkspacePathAllowed(storedPath, input.teamBotIds);
+    // Streamed, so a large video copied to a local computer never sits in memory whole.
+    const stream = await deps.artifacts.createReadStream(row.storageKey, input.context);
     input.markWorkspaceDirty?.();
-    await deps.sandbox.writeFile(
+    await writeComputerFileStream(
+      deps.sandbox,
       input.computer,
-      {
-        path: storedPath,
-        content: bytes,
-      },
+      { path: storedPath, stream },
       input.context,
     );
     materialized.push({ ...entry, path: relativePath });