diff --git a/src/tools/platform/files.ts b/src/tools/platform/files.ts
index 944f754..209d157 100644
--- a/src/tools/platform/files.ts
+++ b/src/tools/platform/files.ts
@@ -202,11 +202,27 @@ interface WriteInput {
   description?: string;
 }
 
-function handleWrite(registryPath: string, filesDir: string, args: WriteInput): object {
+function handleWrite(
+  registryPath: string,
+  filesDir: string,
+  args: WriteInput,
+  maxFileSize: number,
+): object {
   ensureDir(filesDir);
 
   const id = generateId();
   const decoded = Buffer.from(args.base64_data, "base64");
+
+  // Defense-in-depth: enforce the configured per-file size limit at the point
+  // of write. The API body-limit middleware caps HTTP uploads, but
+  // agent-initiated tool calls reach this function without traversing that
+  // middleware — so the limit must hold here independently.
+  if (decoded.length > maxFileSize) {
+    throw new Error(
+      `File "${args.filename}" (${decoded.length} bytes) exceeds limit of ${maxFileSize} bytes`,
+    );
+  }
+
   const filePath = join(filesDir, `${id}_${args.filename}`);
 
   writeFileSync(filePath, decoded);
@@ -450,7 +466,10 @@ export function createFilesSource(runtime: Runtime): InlineSource {
       handler: async (input: Record<string, unknown>): Promise<ToolResult> => {
         try {
           const { filesDir, registryPath } = getFilePaths();
-          return ok(handleWrite(registryPath, filesDir, input as unknown as WriteInput));
+          const { maxFileSize } = runtime.getFilesConfig();
+          return ok(
+            handleWrite(registryPath, filesDir, input as unknown as WriteInput, maxFileSize),
+          );
         } catch (err) {
           return fail(err instanceof Error ? err.message : String(err));
         }
diff --git a/test/unit/tools/platform-files.test.ts b/test/unit/tools/platform-files.test.ts
new file mode 100644
index 0000000..4ee09c0
--- /dev/null
+++ b/test/unit/tools/platform-files.test.ts
@@ -0,0 +1,83 @@
+import { afterEach, beforeEach, describe, expect, test } from "bun:test";
+import { mkdirSync, mkdtempSync, rmSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import type { FileConfig } from "../../../src/files/types.ts";
+import { DEFAULT_FILE_CONFIG } from "../../../src/files/types.ts";
+import type { Runtime } from "../../../src/runtime/runtime.ts";
+import { createFilesSource } from "../../../src/tools/platform/files.ts";
+
+function makeStubRuntime(workDir: string, config: Partial<FileConfig> = {}): Runtime {
+  const merged = { ...DEFAULT_FILE_CONFIG, ...config };
+  return {
+    getWorkspaceScopedDir: () => workDir,
+    getFilesConfig: () => merged,
+  } as unknown as Runtime;
+}
+
+function extractText(result: { content: { type: string; text?: string }[] }): string {
+  return result.content
+    .filter((c): c is { type: "text"; text: string } => c.type === "text")
+    .map((c) => c.text)
+    .join("");
+}
+
+describe("platform files source — write size enforcement", () => {
+  let workDir: string;
+
+  beforeEach(() => {
+    workDir = mkdtempSync(join(tmpdir(), "nb-files-test-"));
+    mkdirSync(join(workDir, "files"), { recursive: true });
+  });
+
+  afterEach(() => {
+    rmSync(workDir, { recursive: true, force: true });
+  });
+
+  test("write succeeds when decoded size is within maxFileSize", async () => {
+    const runtime = makeStubRuntime(workDir, { maxFileSize: 1024 });
+    const source = createFilesSource(runtime);
+
+    const payload = Buffer.from("hello, world").toString("base64");
+    const result = await source.execute("write", {
+      filename: "hello.txt",
+      base64_data: payload,
+      mime_type: "text/plain",
+    });
+
+    expect(result.isError).toBe(false);
+  });
+
+  test("write rejects when decoded size exceeds maxFileSize", async () => {
+    const runtime = makeStubRuntime(workDir, { maxFileSize: 8 });
+    const source = createFilesSource(runtime);
+
+    // 16 bytes decoded — over the 8-byte limit
+    const payload = Buffer.from("0123456789abcdef").toString("base64");
+    const result = await source.execute("write", {
+      filename: "too-big.bin",
+      base64_data: payload,
+      mime_type: "application/octet-stream",
+    });
+
+    expect(result.isError).toBe(true);
+    expect(extractText(result)).toContain("exceeds limit");
+    expect(extractText(result)).toContain("too-big.bin");
+  });
+
+  test("write uses runtime config override (not just defaults)", async () => {
+    const runtime = makeStubRuntime(workDir, { maxFileSize: 100 });
+    const source = createFilesSource(runtime);
+
+    // 50-byte decoded — under the 100 limit. Pins the runtime config as the
+    // source of truth (not a hard-coded default).
+    const payload = Buffer.from("x".repeat(50)).toString("base64");
+    const result = await source.execute("write", {
+      filename: "ok.txt",
+      base64_data: payload,
+      mime_type: "text/plain",
+    });
+
+    expect(result.isError).toBe(false);
+  });
+});