fix: preserve sandbox write payload stdin (#43876)

Merged via squash.

Prepared head SHA: a10fd4b21c78ec57411e6a4f387f16b1441660c2
Co-authored-by: glitch418x <189487110+glitch418x@users.noreply.github.com>
Co-authored-by: altaywtf <9790196+altaywtf@users.noreply.github.com>
Reviewed-by: @altaywtf

CHANGELOG.md

@@ -27,6 +27,7 @@ Docs: https://docs.openclaw.ai
 - Mattermost/block streaming: fix duplicate message delivery (one threaded, one top-level) when block streaming is active by excluding `replyToId` from the block reply dedup key and adding an explicit `threading` dock to the Mattermost plugin. (#41362) Thanks @mathiasnagler and @vincentkoc.
 - BlueBubbles/self-chat echo dedupe: drop reflected duplicate webhook copies only when a matching `fromMe` event was just seen for the same chat, body, and timestamp, preventing self-chat loops without broad webhook suppression. Related to #32166. (#38442) Thanks @vincentkoc.
 - Models/Kimi Coding: send `anthropic-messages` tools in native Anthropic format again so `kimi-coding` stops degrading tool calls into XML/plain-text pseudo invocations instead of real `tool_use` blocks. (#38669, #39907, #40552) Thanks @opriz.
+- Sandbox/write: preserve pinned mutation-helper payload stdin so sandboxed `write` no longer reports success while creating empty files. (#43876) Thanks @glitch418x.
 
 ## 2026.3.11
 

src/agents/sandbox/fs-bridge-mutation-helper.test.ts

@@ -3,7 +3,10 @@ import fs from "node:fs/promises";
 import os from "node:os";
 import path from "node:path";
 import { describe, expect, it } from "vitest";
-import { SANDBOX_PINNED_MUTATION_PYTHON } from "./fs-bridge-mutation-helper.js";
+import {
+  buildPinnedWritePlan,
+  SANDBOX_PINNED_MUTATION_PYTHON,
+} from "./fs-bridge-mutation-helper.js";
 
 async function withTempRoot<T>(prefix: string, run: (root: string) => Promise<T>): Promise<T> {
   const root = await fs.mkdtemp(path.join(os.tmpdir(), prefix));
@@ -22,6 +25,35 @@ function runMutation(args: string[], input?: string) {
   });
 }
 
+function runWritePlan(args: string[], input?: string) {
+  const plan = buildPinnedWritePlan({
+    check: {
+      target: {
+        hostPath: args[1] ?? "",
+        containerPath: args[1] ?? "",
+        relativePath: path.posix.join(args[2] ?? "", args[3] ?? ""),
+        writable: true,
+      },
+      options: {
+        action: "write files",
+        requireWritable: true,
+      },
+    },
+    pinned: {
+      mountRootPath: args[1] ?? "",
+      relativeParentPath: args[2] ?? "",
+      basename: args[3] ?? "",
+    },
+    mkdir: args[4] === "1",
+  });
+
+  return spawnSync("sh", ["-c", plan.script, "moltbot-sandbox-fs", ...(plan.args ?? [])], {
+    input,
+    encoding: "utf8",
+    stdio: ["pipe", "pipe", "pipe"],
+  });
+}
+
 describe("sandbox pinned mutation helper", () => {
   it("writes through a pinned directory fd", async () => {
     await withTempRoot("openclaw-mutation-helper-", async (root) => {
@@ -37,6 +69,26 @@ describe("sandbox pinned mutation helper", () => {
     });
   });
 
+  it.runIf(process.platform !== "win32")(
+    "preserves stdin payload bytes when the pinned write plan runs through sh",
+    async () => {
+      await withTempRoot("openclaw-mutation-helper-", async (root) => {
+        const workspace = path.join(root, "workspace");
+        await fs.mkdir(workspace, { recursive: true });
+
+        const result = runWritePlan(
+          ["write", workspace, "nested/deeper", "note.txt", "1"],
+          "hello",
+        );
+
+        expect(result.status).toBe(0);
+        await expect(
+          fs.readFile(path.join(workspace, "nested", "deeper", "note.txt"), "utf8"),
+        ).resolves.toBe("hello");
+      });
+    },
+  );
+
   it.runIf(process.platform !== "win32")(
     "rejects symlink-parent writes instead of materializing a temp file outside the mount",
     async () => {

src/agents/sandbox/fs-bridge-mutation-helper.ts

@@ -257,7 +257,13 @@ function buildPinnedMutationPlan(params: {
   return {
     checks: params.checks,
     recheckBeforeCommand: true,
-    script: ["set -eu", "python3 - \"$@\" <<'PY'", SANDBOX_PINNED_MUTATION_PYTHON, "PY"].join("\n"),
+    // Feed the helper source over fd 3 so stdin stays available for write payload bytes.
+    script: [
+      "set -eu",
+      "python3 /dev/fd/3 \"$@\" 3<<'PY'",
+      SANDBOX_PINNED_MUTATION_PYTHON,
+      "PY",
+    ].join("\n"),
     args: params.args,
   };
 }

src/agents/sandbox/fs-bridge.anchored-ops.test.ts

@@ -120,7 +120,7 @@ describe("sandbox fs bridge anchored ops", () => {
       const opCall = mockedExecDockerRaw.mock.calls.find(
         ([args]) =>
           typeof args[5] === "string" &&
-          args[5].includes("python3 - \"$@\" <<'PY'") &&
+          args[5].includes("python3 /dev/fd/3 \"$@\" 3<<'PY'") &&
           getDockerArg(args, 1) === testCase.expectedArgs[0],
       );
       expect(opCall).toBeDefined();

src/agents/sandbox/fs-bridge.shell.test.ts

@@ -129,6 +129,10 @@ describe("sandbox fs bridge shell compatibility", () => {
     await bridge.writeFile({ filePath: "b.txt", data: "hello" });
 
     const scripts = getScriptsFromCalls();
+    expect(scripts.some((script) => script.includes("python3 - \"$@\" <<'PY'"))).toBe(false);
+    expect(scripts.some((script) => script.includes("python3 /dev/fd/3 \"$@\" 3<<'PY'"))).toBe(
+      true,
+    );
     expect(scripts.some((script) => script.includes('cat >"$1"'))).toBe(false);
     expect(scripts.some((script) => script.includes('cat >"$tmp"'))).toBe(false);
     expect(scripts.some((script) => script.includes("os.replace("))).toBe(true);