diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7cb88e16813..a1f200fe811 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -34,6 +34,7 @@ Docs: https://docs.openclaw.ai
 
 - Security/Voice Call: harden media stream WebSocket handling against pre-auth idle-connection DoS by adding strict pre-start timeouts, pending/per-IP connection limits, and total connection caps for streaming endpoints. This ships in the next npm release. Thanks @jiseoung for reporting.
 - Agents/Exec: honor explicit agent context when resolving `tools.exec` defaults for runs with opaque/non-agent session keys, so per-agent `host/security/ask` policies are applied consistently. (#11832)
+- Sandbox/Docker: default sandbox container user to the workspace owner `uid:gid` when `agents.*.sandbox.docker.user` is unset, fixing non-root gateway file-tool permissions under capability-dropped containers. (#20979)
 - Telegram/Discord extensions: propagate trusted `mediaLocalRoots` through extension outbound `sendMedia` options so extension direct-send media paths honor agent-scoped local-media allowlists. (#20029, #21903, #23227)
 - Exec/Background: stop applying the default exec timeout to background sessions (`background: true` or explicit `yieldMs`) when no explicit timeout is set, so long-running background jobs are no longer terminated at the default timeout boundary. (#23303)
 - Plugins/Media sandbox: propagate trusted `mediaLocalRoots` through plugin action dispatch (including Discord/Telegram action adapters) so plugin send paths enforce the same agent-scoped local-media sandbox roots as core outbound sends. (#20258, #22718)
diff --git a/src/agents/sandbox/context.ts b/src/agents/sandbox/context.ts
index 34bc45846b9..8468dd2c556 100644
--- a/src/agents/sandbox/context.ts
+++ b/src/agents/sandbox/context.ts
@@ -14,7 +14,7 @@ import { createSandboxFsBridge } from "./fs-bridge.js";
 import { maybePruneSandboxes } from "./prune.js";
 import { resolveSandboxRuntimeStatus } from "./runtime-status.js";
 import { resolveSandboxScopeKey, resolveSandboxWorkspaceDir } from "./shared.js";
-import type { SandboxContext, SandboxWorkspaceInfo } from "./types.js";
+import type { SandboxContext, SandboxDockerConfig, SandboxWorkspaceInfo } from "./types.js";
 import { ensureSandboxWorkspace } from "./workspace.js";
 
 async function ensureSandboxWorkspaceLayout(params: {
@@ -64,6 +64,29 @@ async function ensureSandboxWorkspaceLayout(params: {
   return { agentWorkspaceDir, scopeKey, sandboxWorkspaceDir, workspaceDir };
 }
 
+export async function resolveSandboxDockerUser(params: {
+  docker: SandboxDockerConfig;
+  workspaceDir: string;
+  stat?: (workspaceDir: string) => Promise<{ uid: number; gid: number }>;
+}): Promise<SandboxDockerConfig> {
+  const configuredUser = params.docker.user?.trim();
+  if (configuredUser) {
+    return params.docker;
+  }
+  const stat = params.stat ?? ((workspaceDir: string) => fs.stat(workspaceDir));
+  try {
+    const workspaceStat = await stat(params.workspaceDir);
+    const uid = Number.isInteger(workspaceStat.uid) ? workspaceStat.uid : null;
+    const gid = Number.isInteger(workspaceStat.gid) ? workspaceStat.gid : null;
+    if (uid === null || gid === null || uid < 0 || gid < 0) {
+      return params.docker;
+    }
+    return { ...params.docker, user: `${uid}:${gid}` };
+  } catch {
+    return params.docker;
+  }
+}
+
 function resolveSandboxSession(params: { config?: OpenClawConfig; sessionKey?: string }) {
   const rawSessionKey = params.sessionKey?.trim();
   if (!rawSessionKey) {
@@ -102,11 +125,17 @@ export async function resolveSandboxContext(params: {
     workspaceDir: params.workspaceDir,
   });
 
+  const docker = await resolveSandboxDockerUser({
+    docker: cfg.docker,
+    workspaceDir,
+  });
+  const resolvedCfg = docker === cfg.docker ? cfg : { ...cfg, docker };
+
   const containerName = await ensureSandboxContainer({
     sessionKey: rawSessionKey,
     workspaceDir,
     agentWorkspaceDir,
-    cfg,
+    cfg: resolvedCfg,
   });
 
   const evaluateEnabled =
@@ -132,7 +161,7 @@ export async function resolveSandboxContext(params: {
     scopeKey,
     workspaceDir,
     agentWorkspaceDir,
-    cfg,
+    cfg: resolvedCfg,
     evaluateEnabled,
     bridgeAuth,
   });
@@ -142,12 +171,12 @@ export async function resolveSandboxContext(params: {
     sessionKey: rawSessionKey,
     workspaceDir,
     agentWorkspaceDir,
-    workspaceAccess: cfg.workspaceAccess,
+    workspaceAccess: resolvedCfg.workspaceAccess,
     containerName,
-    containerWorkdir: cfg.docker.workdir,
-    docker: cfg.docker,
-    tools: cfg.tools,
-    browserAllowHostControl: cfg.browser.allowHostControl,
+    containerWorkdir: resolvedCfg.docker.workdir,
+    docker: resolvedCfg.docker,
+    tools: resolvedCfg.tools,
+    browserAllowHostControl: resolvedCfg.browser.allowHostControl,
     browser: browser ?? undefined,
   };
 
diff --git a/src/agents/sandbox/context.user-fallback.test.ts b/src/agents/sandbox/context.user-fallback.test.ts
new file mode 100644
index 00000000000..11751918009
--- /dev/null
+++ b/src/agents/sandbox/context.user-fallback.test.ts
@@ -0,0 +1,44 @@
+import { describe, expect, it } from "vitest";
+import { resolveSandboxDockerUser } from "./context.js";
+import type { SandboxDockerConfig } from "./types.js";
+
+const baseDocker: SandboxDockerConfig = {
+  image: "ghcr.io/example/sandbox:latest",
+  containerPrefix: "openclaw-sandbox-",
+  workdir: "/workspace",
+  readOnlyRoot: true,
+  tmpfs: ["/tmp"],
+  network: "none",
+  capDrop: ["ALL"],
+};
+
+describe("resolveSandboxDockerUser", () => {
+  it("keeps configured docker.user", async () => {
+    const resolved = await resolveSandboxDockerUser({
+      docker: { ...baseDocker, user: "2000:2000" },
+      workspaceDir: "/tmp/unused",
+      stat: async () => ({ uid: 1000, gid: 1000 }),
+    });
+    expect(resolved.user).toBe("2000:2000");
+  });
+
+  it("falls back to workspace ownership when docker.user is unset", async () => {
+    const resolved = await resolveSandboxDockerUser({
+      docker: baseDocker,
+      workspaceDir: "/tmp/workspace",
+      stat: async () => ({ uid: 1001, gid: 1002 }),
+    });
+    expect(resolved.user).toBe("1001:1002");
+  });
+
+  it("leaves docker.user unset when workspace stat fails", async () => {
+    const resolved = await resolveSandboxDockerUser({
+      docker: baseDocker,
+      workspaceDir: "/tmp/workspace",
+      stat: async () => {
+        throw new Error("ENOENT");
+      },
+    });
+    expect(resolved.user).toBeUndefined();
+  });
+});