diff --git a/.changeset/tfy-sandbox-upload-api.md b/.changeset/tfy-sandbox-upload-api.md
new file mode 100644
index 000000000..be742d5d3
--- /dev/null
+++ b/.changeset/tfy-sandbox-upload-api.md
@@ -0,0 +1,5 @@
+---
+"@truefoundry/trueforge-core": patch
+---
+
+Upload TFY sandbox files via POST /files/upload so large files are not stuffed onto exec argv.
diff --git a/packages/trueforge-core/src/core/sandbox/provider/TFYSandboxProvider.ts b/packages/trueforge-core/src/core/sandbox/provider/TFYSandboxProvider.ts
index 7544eba18..950a96d71 100644
--- a/packages/trueforge-core/src/core/sandbox/provider/TFYSandboxProvider.ts
+++ b/packages/trueforge-core/src/core/sandbox/provider/TFYSandboxProvider.ts
@@ -16,7 +16,6 @@ import {
 } from '../SandboxErrors';
 import { absolutizeRelativeExecEnv } from './execEnv';
 import {
-  ensureExecSuccess,
   shellEscape,
   type ExecResult,
   type SandboxBuild,
@@ -29,6 +28,9 @@ const DEFAULT_TIMEOUT_SECONDS = 60;
 // Buffer for network latency + response processing on top of the server-side timeout.
 const CLIENT_TIMEOUT_BUFFER_SECONDS = 5;
 
+// TFY sandbox file upload timeout (same as Daytona SDK uploadFile default timeout).
+const FILE_UPLOAD_TIMEOUT_MS = 30 * 60 * 1000;
+
 const TFY_MCP_CLIENT_BIN = 'mcp-client/bin';
 
 /** Put the layout CLI first on PATH (cwd-relative; {@link absolutizeRelativeExecEnv} makes it absolute). */
@@ -228,14 +230,23 @@ export class TFYSandboxProvider implements SandboxProvider {
   }
 
   async uploadFile(params: { sandboxId: string; remotePath: string; content: Buffer }): Promise<void> {
-    const encoded = params.content.toString('base64');
-    const escapedPath = shellEscape(params.remotePath);
+    validateSandboxOwnedByTenant({ sandboxId: params.sandboxId, tenantName: this.tenantName });
 
-    const result = await this.exec({
-      sandboxId: params.sandboxId,
-      command: `echo ${shellEscape(encoded)} | base64 -d > ${escapedPath}`,
+    const query = new URLSearchParams({ sandbox_id: params.sandboxId, path: params.remotePath });
+    const response = await fetch(`${this.serverUrl}/files/upload?${query.toString()}`, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/octet-stream' },
+      body: params.content,
+      signal: AbortSignal.timeout(FILE_UPLOAD_TIMEOUT_MS),
     });
-    ensureExecSuccess(result);
+    if (!response.ok) {
+      throw new Error(`Sandbox server returned ${String(response.status)}: ${await response.text()}`);
+    }
+
+    const result = (await response.json()) as { success: true } | { success: false; error: string };
+    if (!result.success) {
+      throw new Error(result.error);
+    }
   }
 
   // The TFY sandbox exposes a static, cluster-internal NATS WebSocket URL (no signed URLs).
diff --git a/packages/trueforge-core/tests/core/sandbox/provider/tfyUpload.test.ts b/packages/trueforge-core/tests/core/sandbox/provider/tfyUpload.test.ts
new file mode 100644
index 000000000..26844f495
--- /dev/null
+++ b/packages/trueforge-core/tests/core/sandbox/provider/tfyUpload.test.ts
@@ -0,0 +1,73 @@
+import { TFYSandboxProvider } from '../../../../src/core/sandbox/provider/TFYSandboxProvider';
+import { makeSilentLogger } from '../../harnessMocks';
+
+const SERVER_URL = 'http://sandbox.example';
+const SANDBOX_ID = 'acme.00000000-0000-0000-0000-000000000001';
+
+function makeProvider(): TFYSandboxProvider {
+  return new TFYSandboxProvider({
+    serverUrl: SERVER_URL,
+    natsBridgeUrl: 'ws://nats.example',
+    tenantName: 'acme',
+    fileMaxBytesForDownload: 1024,
+    logger: makeSilentLogger(),
+  });
+}
+
+function mockFetch({ status, body }: { status: number; body: unknown }): jest.SpiedFunction<typeof globalThis.fetch> {
+  return jest
+    .spyOn(globalThis, 'fetch')
+    .mockResolvedValue(new Response(JSON.stringify(body), { status, headers: { 'Content-Type': 'application/json' } }));
+}
+
+afterEach(() => {
+  jest.restoreAllMocks();
+});
+
+describe('TFYSandboxProvider.uploadFile', () => {
+  it('POSTs the raw bytes to /files/upload', async () => {
+    const timeout = jest.spyOn(AbortSignal, 'timeout');
+    const fetchMock = mockFetch({ status: 200, body: { success: true } });
+    const content = Buffer.from([0x00, 0xff, 0x0a]);
+
+    await makeProvider().uploadFile({
+      sandboxId: SANDBOX_ID,
+      remotePath: 'uploads/report.docx',
+      content,
+    });
+
+    expect(timeout).toHaveBeenCalledWith(30 * 60 * 1000);
+    expect(fetchMock).toHaveBeenCalledTimes(1);
+    const [url, init] = fetchMock.mock.calls[0] ?? [];
+    expect(String(url)).toBe(`${SERVER_URL}/files/upload?sandbox_id=${SANDBOX_ID}&path=uploads%2Freport.docx`);
+    expect(init?.method).toBe('POST');
+    expect(init?.headers).toEqual({ 'Content-Type': 'application/octet-stream' });
+    expect(init?.body).toEqual(content);
+    expect(init?.signal).toBe(timeout.mock.results[0]?.value);
+  });
+
+  it('throws the server error body when success is false', async () => {
+    mockFetch({ status: 200, body: { success: false, error: 'File exceeds 20971520 bytes' } });
+
+    await expect(
+      makeProvider().uploadFile({
+        sandboxId: SANDBOX_ID,
+        remotePath: 'uploads/big.bin',
+        content: Buffer.from('ok'),
+      }),
+    ).rejects.toThrow('File exceeds 20971520 bytes');
+  });
+
+  it('rejects a sandbox that is not owned by the tenant', async () => {
+    const fetchMock = mockFetch({ status: 200, body: { success: true } });
+
+    await expect(
+      makeProvider().uploadFile({
+        sandboxId: 'other.00000000-0000-0000-0000-000000000001',
+        remotePath: 'uploads/a.txt',
+        content: Buffer.from('x'),
+      }),
+    ).rejects.toMatchObject({ name: 'SandboxTenantMismatchError' });
+    expect(fetchMock).not.toHaveBeenCalled();
+  });
+});