diff --git a/packages/runtime-playground/src/mount-materialization.ts b/packages/runtime-playground/src/mount-materialization.ts
index ddc5bd6c..154626ad 100644
--- a/packages/runtime-playground/src/mount-materialization.ts
+++ b/packages/runtime-playground/src/mount-materialization.ts
@@ -369,24 +369,43 @@ export async function materializePlaygroundStagedFiles(server: PlaygroundCliServ
   for (const mount of mounts) {
     if (mount.type !== "file") continue
 
-    const handle = await open(mount.source, "r")
+    // The target can be a writable NodeFS mount of mount.source. The first
+    // truncating write would otherwise shorten the file being streamed.
+    const snapshotDirectory = await mkdtemp(join(tmpdir(), "wp-codebox-staged-file-"))
+    const snapshot = join(snapshotDirectory, basename(mount.source) || "staged-file")
     try {
-      const buffer = Buffer.allocUnsafe(STAGED_FILE_CHUNK_SIZE)
-      let position = 0
-      let append = false
-      do {
-        const { bytesRead } = await handle.read(buffer, 0, buffer.length, position)
-        const response = await server.playground.run({ code: stagedFileWritePhp(mount.target, buffer.subarray(0, bytesRead).toString("base64"), append) })
-        const result = JSON.parse(response.text || "{}") as { schema?: string; written?: number }
-        if (result.schema !== "wp-codebox/staged-file-write/v1" || result.written !== bytesRead) {
-          throw new Error(`Could not materialize staged file at ${mount.target}`)
+      await cp(mount.source, snapshot)
+      const handle = await open(snapshot, "r")
+      try {
+        const buffer = Buffer.allocUnsafe(STAGED_FILE_CHUNK_SIZE)
+        const hash = createHash("sha256")
+        let position = 0
+        let append = false
+        do {
+          const { bytesRead } = await handle.read(buffer, 0, buffer.length, position)
+          const contents = buffer.subarray(0, bytesRead)
+          hash.update(contents)
+          const response = await server.playground.run({ code: stagedFileWritePhp(mount.target, contents.toString("base64"), append) })
+          const result = JSON.parse(response.text || "{}") as { schema?: string; written?: number }
+          if (result.schema !== "wp-codebox/staged-file-write/v1" || result.written !== bytesRead) {
+            throw new Error(`Could not materialize staged file at ${mount.target}`)
+          }
+          position += bytesRead
+          append = true
+          if (bytesRead === 0 || bytesRead < buffer.length) break
+        } while (true)
+
+        const response = await server.playground.run({ code: stagedFileVerificationPhp(mount.target) })
+        const result = JSON.parse(response.text || "{}") as { schema?: string; bytes?: number; sha256?: string }
+        const expectedSha256 = hash.digest("hex")
+        if (result.schema !== "wp-codebox/staged-file-verification/v1" || result.bytes !== position || result.sha256 !== expectedSha256) {
+          throw new Error(`Staged file verification failed at ${mount.target}: expected ${position} bytes sha256 ${expectedSha256}, received ${typeof result.bytes === "number" ? result.bytes : "unknown"} bytes sha256 ${typeof result.sha256 === "string" ? result.sha256 : "unknown"}`)
         }
-        position += bytesRead
-        append = true
-        if (bytesRead === 0 || bytesRead < buffer.length) break
-      } while (true)
+      } finally {
+        await handle.close()
+      }
     } finally {
-      await handle.close()
+      await rm(snapshotDirectory, { recursive: true, force: true })
     }
     materialized++
   }
@@ -410,6 +429,25 @@ echo json_encode(array('schema' => 'wp-codebox/staged-file-write/v1', 'written'
 `
 }
 
+function stagedFileVerificationPhp(target: string): string {
+  const payload = JSON.stringify(JSON.stringify({ target }))
+  return `<?php
+$payload = json_decode(${payload}, true);
+$target = (string) ($payload['target'] ?? '');
+$bytes = -1;
+$sha256 = '';
+if ('' !== $target && !str_contains($target, "\0") && is_file($target)) {
+    $size = filesize($target);
+    $hash = hash_file('sha256', $target);
+    if (false !== $size && false !== $hash) {
+        $bytes = $size;
+        $sha256 = $hash;
+    }
+}
+echo json_encode(array('schema' => 'wp-codebox/staged-file-verification/v1', 'bytes' => $bytes, 'sha256' => $sha256), JSON_UNESCAPED_SLASHES);
+`
+}
+
 function nestedMountPaths(mounts: MountSpec[], mountIndex: number, parentTarget: string): string[] {
   const normalizedParent = parentTarget.replace(/\/+$/, "")
   return mounts
diff --git a/tests/staged-file-materialization.test.ts b/tests/staged-file-materialization.test.ts
new file mode 100644
index 00000000..de42e57c
--- /dev/null
+++ b/tests/staged-file-materialization.test.ts
@@ -0,0 +1,63 @@
+import assert from "node:assert/strict"
+import { createHash } from "node:crypto"
+import { mkdtemp, readFile, rm, writeFile } from "node:fs/promises"
+import { tmpdir } from "node:os"
+import { join } from "node:path"
+
+import { materializePlaygroundStagedFiles } from "../packages/runtime-playground/src/mount-materialization.js"
+
+const root = await mkdtemp(join(tmpdir(), "wp-codebox-staged-file-materialization-"))
+
+try {
+  const text = Buffer.from(`{"fixture":"${"text-".repeat(60_000)}end"}\n`)
+  const binary = Buffer.allocUnsafe(256 * 1024 + 137)
+  for (let index = 0; index < binary.length; index++) binary[index] = (index * 31 + 255) % 256
+
+  for (const [name, contents] of [["artifact.json", text], ["artifact.bin", binary]] as const) {
+    const source = join(root, name)
+    const target = `/workspace/${name}`
+    await writeFile(source, contents)
+    const sandboxFiles = new Map<string, Buffer>()
+
+    const materialized = await materializePlaygroundStagedFiles({
+      playground: {
+        async run({ code }) {
+          const payload = JSON.parse(JSON.parse(code.match(/\$payload = json_decode\((.*), true\);/)?.[1] ?? "\"{}\"")) as { target: string; contentsBase64?: string; append?: boolean }
+          if (code.includes("wp-codebox/staged-file-write/v1")) {
+            const chunk = Buffer.from(payload.contentsBase64 ?? "", "base64")
+            const existing = payload.append ? sandboxFiles.get(payload.target) ?? Buffer.alloc(0) : Buffer.alloc(0)
+            const next = Buffer.concat([existing, chunk])
+            sandboxFiles.set(payload.target, next)
+            // This simulates the mounted target overwriting its host source.
+            await writeFile(source, next)
+            return { text: JSON.stringify({ schema: "wp-codebox/staged-file-write/v1", written: chunk.length }) }
+          }
+          const actual = sandboxFiles.get(payload.target) ?? Buffer.alloc(0)
+          return { text: JSON.stringify({ schema: "wp-codebox/staged-file-verification/v1", bytes: actual.length, sha256: createHash("sha256").update(actual).digest("hex") }) }
+        },
+      },
+    } as never, [{ type: "file", source, target, mode: "readwrite" }])
+
+    assert.equal(materialized, 1)
+    assert.deepEqual(sandboxFiles.get(target), contents, `${name} is written across all chunks`)
+    assert.deepEqual(await readFile(source), contents, `${name} remains byte-identical after mounted writes`)
+  }
+
+  const truncatedSource = join(root, "truncated.json")
+  await writeFile(truncatedSource, text)
+  await assert.rejects(materializePlaygroundStagedFiles({
+    playground: {
+      async run({ code }) {
+        const payload = JSON.parse(JSON.parse(code.match(/\$payload = json_decode\((.*), true\);/)?.[1] ?? "\"{}\"")) as { contentsBase64?: string }
+        if (code.includes("wp-codebox/staged-file-write/v1")) {
+          return { text: JSON.stringify({ schema: "wp-codebox/staged-file-write/v1", written: Buffer.from(payload.contentsBase64 ?? "", "base64").length }) }
+        }
+        return { text: JSON.stringify({ schema: "wp-codebox/staged-file-verification/v1", bytes: 262144, sha256: "0".repeat(64) }) }
+      },
+    },
+  } as never, [{ type: "file", source: truncatedSource, target: "/workspace/truncated.json", mode: "readwrite" }]), /expected \d+ bytes sha256 [a-f0-9]{64}, received 262144 bytes sha256 0{64}/)
+} finally {
+  await rm(root, { recursive: true, force: true })
+}
+
+console.log("staged file materialization preserves large text and binary files")