diff --git a/packages/adapter-utils/src/command-managed-runtime.test.ts b/packages/adapter-utils/src/command-managed-runtime.test.ts
index 7b79daccf05..a910fb804a9 100644
--- a/packages/adapter-utils/src/command-managed-runtime.test.ts
+++ b/packages/adapter-utils/src/command-managed-runtime.test.ts
@@ -22,9 +22,10 @@ interface SpawnRunnerHandle {
 // A runner that actually executes the shell scripts (piping stdin through a real
 // pipe so multi-MB payloads work) and replays stdout through onLog in several
 // chunks so the streaming readFile byte-counter is exercised.
-function makeSpawnRunner(): SpawnRunnerHandle {
+function makeSpawnRunner(options: { supportsSingleStreamStdinProgress?: boolean } = {}): SpawnRunnerHandle {
   const calls: Array<{ command: string; args?: string[]; cwd?: string; stdin?: string }> = [];
   const runner: CommandManagedRuntimeRunner = {
+    supportsSingleStreamStdinProgress: options.supportsSingleStreamStdinProgress,
     execute: async (input) =>
       await new Promise<RunProcessResult>((resolve) => {
         calls.push({ command: input.command, args: input.args, cwd: input.cwd, stdin: input.stdin });
@@ -238,7 +239,7 @@ describe("command managed runtime", () => {
     const payload = Buffer.alloc(3 * 1024 * 1024);
     for (let i = 0; i < payload.length; i++) payload[i] = i % 256;
 
-    const { runner, calls } = makeSpawnRunner();
+    const { runner, calls } = makeSpawnRunner({ supportsSingleStreamStdinProgress: true });
     const client = createCommandManagedRuntimeClient({ runner, commandCwd: "/", timeoutMs: 30_000 });
 
     const progress: Array<{ done: number; total: number | null }> = [];
@@ -263,6 +264,37 @@ describe("command managed runtime", () => {
     expect(progress.at(-1)).toEqual({ done: payload.length, total: payload.length });
   });
 
+  it("falls back to chunked upload progress when the runner cannot report mid-stream stdin progress", async () => {
+    const rootDir = await mkdtemp(path.join(os.tmpdir(), "paperclip-command-write-fallback-"));
+    cleanupDirs.push(rootDir);
+    const remotePath = path.join(rootDir, "nested", "payload.bin");
+
+    const payload = Buffer.alloc(12 * 1024 * 1024);
+    for (let i = 0; i < payload.length; i++) payload[i] = i % 256;
+
+    const { runner, calls } = makeSpawnRunner({ supportsSingleStreamStdinProgress: false });
+    const client = createCommandManagedRuntimeClient({ runner, commandCwd: "/", timeoutMs: 30_000 });
+
+    const progress: Array<{ done: number; total: number | null }> = [];
+    await client.writeFile(remotePath, toArrayBuffer(payload), {
+      onProgress: (done, total) => {
+        progress.push({ done, total });
+      },
+    });
+
+    const written = await readFile(remotePath);
+    expect(written.equals(payload)).toBe(true);
+
+    // Provider-backed sandbox runners cannot surface mid-flight progress for a
+    // single stdin RPC, so we intentionally use several large append commands.
+    expect(calls.length).toBeGreaterThan(2);
+    expect(progress.length).toBeGreaterThan(2);
+    for (let i = 1; i < progress.length; i++) {
+      expect(progress[i].done).toBeGreaterThanOrEqual(progress[i - 1].done);
+    }
+    expect(progress.at(-1)).toEqual({ done: payload.length, total: payload.length });
+  });
+
   it("streams a download through onLog and reports monotonic byte progress to the total", async () => {
     const rootDir = await mkdtemp(path.join(os.tmpdir(), "paperclip-command-read-"));
     cleanupDirs.push(rootDir);
diff --git a/packages/adapter-utils/src/command-managed-runtime.ts b/packages/adapter-utils/src/command-managed-runtime.ts
index 30ace78de78..a74789cc2f6 100644
--- a/packages/adapter-utils/src/command-managed-runtime.ts
+++ b/packages/adapter-utils/src/command-managed-runtime.ts
@@ -11,6 +11,13 @@ import type { RunProcessResult } from "./server-utils.js";
 import type { RuntimeProgressSink } from "./runtime-progress.js";
 
 export interface CommandManagedRuntimeRunner {
+  /**
+   * True only when `execute({ stdin })` can surface useful in-flight progress
+   * for a single stdin-backed command. Provider-backed sandbox runners usually
+   * complete the entire RPC before returning, so they should leave this false
+   * and let the caller choose a chunked upload path when progress is requested.
+   */
+  supportsSingleStreamStdinProgress?: boolean;
   execute(input: {
     command: string;
     args?: string[];
@@ -100,13 +107,18 @@ export function createCommandManagedRuntimeClient(input: {
       const body = buffer.toString("base64");
       const remoteDir = path.posix.dirname(remotePath);
       const remoteTempPath = `${remotePath}.paperclip-upload`;
+      const canUseSingleStreamProgressPath =
+        !options?.onProgress || input.runner.supportsSingleStreamStdinProgress === true;
 
       // Primary path: a single round-trip. Stream the entire base64 body to one
       // `base64 -d` process via stdin, decode straight into a temp file, then
       // atomically rename into place. This replaces the previous loop that did
       // one `printf >> tmpfile` shell round-trip per 32 KB — thousands of serial
       // processes for a large workspace — with exactly one process.
-      if (body.length <= REMOTE_WRITE_SINGLE_STREAM_MAX_BASE64_BYTES) {
+      if (
+        body.length <= REMOTE_WRITE_SINGLE_STREAM_MAX_BASE64_BYTES &&
+        canUseSingleStreamProgressPath
+      ) {
         await options?.onProgress?.(0, total);
         await runShell(
           `mkdir -p ${shellQuote(remoteDir)} && ` +
diff --git a/server/src/services/environment-execution-target.ts b/server/src/services/environment-execution-target.ts
index 33ea70155c8..0e50bd7b6a8 100644
--- a/server/src/services/environment-execution-target.ts
+++ b/server/src/services/environment-execution-target.ts
@@ -75,6 +75,7 @@ export async function resolveEnvironmentExecutionTarget(input: {
       timeoutMs,
       runner: input.environmentRuntime && input.lease
         ? {
+            supportsSingleStreamStdinProgress: false,
             execute: async (commandInput) => {
               const startedAt = new Date().toISOString();
               const result = await input.environmentRuntime!.execute({