diff --git a/.env.example b/.env.example
index ec9fad6..6f3154b 100644
--- a/.env.example
+++ b/.env.example
@@ -16,8 +16,8 @@ RATE_LIMIT_MAX_REQUESTS=30
 # S3_VHOST_DOMAINS=upload.asepharyana.my.id,upload.asepharyana.web.id
 
 # Telegram-safe internal chunking for large stored files
-# Telegram max upload ~49MB, using 48MB for safety
-# TELEGRAM_CHUNK_SIZE_BYTES=50331648
+# Telegram getFile download limit is 20 MB; guard rejects > 19922944 (19 MB)
+# TELEGRAM_CHUNK_SIZE_BYTES=19922944
 # COMPRESS_CHUNKED_UPLOADS=true
 # CHUNK_COMPRESSION_MIN_SIZE_BYTES=4096
 
diff --git a/CLAUDE.md b/CLAUDE.md
index fe9fd8b..3137be2 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -25,6 +25,12 @@ Default to using Bun instead of Node.js.
 - Pengiriman berkas ke Telegram dieksekusi secara responsif dan paralel penuh tanpa batas konkurensi/antrian.
 - Berkas API upload ditulis secara sementara ke disk `/tmp/teleuploader-*` dan di-stream ke Telegram menggunakan `fs.createReadStream` (RAM-optimized) lalu dihapus otomatis setelah 50ms (timeout aman).
 
+## Chunk size (TELEGRAM_CHUNK_SIZE_BYTES)
+
+- Batas keras: Telegram Bot API `getFile` hanya bisa resolve file ≤ 20 MB — di atas itu error `Bad Request: file is too big` dan part tidak bisa di-download.
+- Guard fail-fast di `src/env.ts`: service MENOLAK start (exit non-zero) jika `TELEGRAM_CHUNK_SIZE_BYTES` > 19922944 (19 MB, margin aman dari limit 20 MB). Konstanta: `TELEGRAM_CHUNK_SIZE_MAX_BYTES` di `src/shared/utils/validation.ts`, juga dipakai `asSafeChunkSize()` di runtime.
+- Default 19 MB; berlaku untuk chunked storage DAN S3 multipart parts (sama-sama disimpan ke Telegram lalu di-resolve via getFile).
+
 ## Testing
 
 Use `bun test` to run tests. Jalankan tes secara spesifik (misal `bun test test/rateLimit.test.ts`) untuk menghindari polusi mock antar berkas tes ketika dijalankan bersamaan.
diff --git a/src/env.ts b/src/env.ts
index eff0a3d..396db34 100644
--- a/src/env.ts
+++ b/src/env.ts
@@ -1,4 +1,5 @@
 import logger from './shared/logger/index';
+import { TELEGRAM_CHUNK_SIZE_MAX_BYTES } from './shared/utils/validation';
 
 interface AppConfig {
   /** All bot tokens merged from BOT_TOKENS (or BOT_TOKEN + ADDITIONAL_BOT_TOKENS fallback) */
@@ -107,6 +108,29 @@ const maskSecret = (value: string): string => {
 const maskDatabaseUrl = (value: string): string =>
   value.replace(/:\/\/([^:]+):([^@]+)@/, '://$1:***@');
 
+// Fail-fast guard for TELEGRAM_CHUNK_SIZE_BYTES: chunked uploads store every
+// part as a Telegram document and later resolve it via getFile, which only
+// supports files up to 20 MB ("Bad Request: file is too big" above that).
+// A chunk above the limit makes every part undownloadable — refuse to start
+// instead of failing on the first large-file download.
+const telegramChunkSizeBytes = parseNumber(
+  process.env.TELEGRAM_CHUNK_SIZE_BYTES,
+  TELEGRAM_CHUNK_SIZE_MAX_BYTES,
+);
+if (telegramChunkSizeBytes > TELEGRAM_CHUNK_SIZE_MAX_BYTES) {
+  logger.error(
+    `TELEGRAM_CHUNK_SIZE_BYTES=${telegramChunkSizeBytes} exceeds the maximum allowed chunk size ` +
+      `${TELEGRAM_CHUNK_SIZE_MAX_BYTES} bytes (${TELEGRAM_CHUNK_SIZE_MAX_BYTES / (1024 * 1024)} MB). ` +
+      'Telegram Bot API getFile cannot download files larger than 20 MB, so every stored part would ' +
+      'be undownloadable ("Bad Request: file is too big"). ' +
+      `Set TELEGRAM_CHUNK_SIZE_BYTES to ${TELEGRAM_CHUNK_SIZE_MAX_BYTES} or lower.`,
+  );
+  throw new Error(
+    `TELEGRAM_CHUNK_SIZE_BYTES=${telegramChunkSizeBytes} exceeds the maximum allowed chunk size ` +
+      `${TELEGRAM_CHUNK_SIZE_MAX_BYTES} bytes (${TELEGRAM_CHUNK_SIZE_MAX_BYTES / (1024 * 1024)} MB)`,
+  );
+}
+
 export const config: AppConfig = {
   botTokens: parseTokens(botTokensRaw),
   telegramBotConcurrency: parseNumber(process.env.TELEGRAM_BOT_CONCURRENCY, 1),
@@ -122,7 +146,7 @@ export const config: AppConfig = {
   batchMaxItems: parseNumber(process.env.BATCH_MAX_ITEMS, 20),
   batchMaxSizeBytes: parseNumber(process.env.BATCH_MAX_SIZE_BYTES, 500 * 1024 * 1024),
   maxRequestBodyBytes: parseNumber(process.env.MAX_REQUEST_BODY_BYTES, 2 * 1024 * 1024 * 1024),
-  telegramChunkSizeBytes: parseNumber(process.env.TELEGRAM_CHUNK_SIZE_BYTES, 20 * 1024 * 1024),
+  telegramChunkSizeBytes,
   compressChunkedUploads: process.env.COMPRESS_CHUNKED_UPLOADS !== 'false',
   chunkCompressionMinSizeBytes: parseNumber(process.env.CHUNK_COMPRESSION_MIN_SIZE_BYTES, 4096),
   adminApiToken: process.env.ADMIN_API_TOKEN || '',
diff --git a/src/shared/utils/validation.ts b/src/shared/utils/validation.ts
index 6c20475..5a0bf19 100644
--- a/src/shared/utils/validation.ts
+++ b/src/shared/utils/validation.ts
@@ -1,13 +1,33 @@
+/**
+ * Maximum allowed chunk/part size in bytes for Telegram storage.
+ *
+ * Telegram Bot API `getFile` can only resolve files up to 20 MB; anything
+ * larger fails with "Bad Request: file is too big". Chunked uploads store
+ * each part as a Telegram document and later resolve it via `getFile`, so a
+ * part must never reach that limit. 19 MB (19922944 bytes) leaves a safety
+ * margin and is the value used in production (/etc/teleuploader/env).
+ */
+export const TELEGRAM_CHUNK_SIZE_MAX_BYTES = 19 * 1024 * 1024;
+
 /**
  * Validates a chunk size value and returns it as a safe integer.
  *
  * @param chunkSizeBytes - The desired chunk size in bytes.
- * @returns The same value if it is a positive safe integer.
- * @throws {Error} If the chunk size is not a safe positive integer.
+ * @returns The same value if it is a positive safe integer at or below
+ *   {@link TELEGRAM_CHUNK_SIZE_MAX_BYTES}.
+ * @throws {Error} If the chunk size is not a safe positive integer or exceeds
+ *   the Telegram `getFile` limit (with margin).
  */
 export const asSafeChunkSize = (chunkSizeBytes: number): number => {
   if (!Number.isSafeInteger(chunkSizeBytes) || chunkSizeBytes <= 0) {
     throw new Error('Invalid Telegram chunk size');
   }
+  if (chunkSizeBytes > TELEGRAM_CHUNK_SIZE_MAX_BYTES) {
+    throw new Error(
+      `Telegram chunk size ${chunkSizeBytes} exceeds the maximum allowed part size ` +
+        `${TELEGRAM_CHUNK_SIZE_MAX_BYTES} bytes (${TELEGRAM_CHUNK_SIZE_MAX_BYTES / (1024 * 1024)} MB). ` +
+        'Telegram getFile cannot download files larger than 20 MB, so such parts would be undownloadable.',
+    );
+  }
   return chunkSizeBytes;
 };
diff --git a/test/env.test.ts b/test/env.test.ts
index ba222cb..d46b7fd 100644
--- a/test/env.test.ts
+++ b/test/env.test.ts
@@ -1,5 +1,6 @@
 import { describe, expect, it } from 'bun:test';
 import { config } from '../src/env';
+import { asSafeChunkSize, TELEGRAM_CHUNK_SIZE_MAX_BYTES } from '../src/shared/utils/validation';
 
 describe('Environment Variables Validation', () => {
   it('config should have all required fields', () => {
@@ -67,3 +68,77 @@ describe('Environment Variables Validation', () => {
     expect(config.s3SecretKey).toBeDefined();
   });
 });
+
+describe('Telegram chunk size validation', () => {
+  it('config.telegramChunkSizeBytes should default to the safe 19 MB value when unset', () => {
+    // setup-env.ts does not set TELEGRAM_CHUNK_SIZE_BYTES, so the default must
+    // be the safe 19 MB value — never the raw 20 MB getFile limit.
+    expect(config.telegramChunkSizeBytes).toBe(TELEGRAM_CHUNK_SIZE_MAX_BYTES);
+    expect(config.telegramChunkSizeBytes).toBeLessThan(20 * 1024 * 1024);
+  });
+
+  it('asSafeChunkSize accepts sizes at or below the maximum (19 MB)', () => {
+    expect(asSafeChunkSize(TELEGRAM_CHUNK_SIZE_MAX_BYTES)).toBe(TELEGRAM_CHUNK_SIZE_MAX_BYTES);
+    expect(asSafeChunkSize(1024)).toBe(1024);
+    expect(asSafeChunkSize(19 * 1024 * 1024)).toBe(19 * 1024 * 1024);
+  });
+
+  it('asSafeChunkSize rejects sizes above the Telegram getFile limit (incl. the 48 MB production bug)', () => {
+    // The production bug value (48 MB / 50331648) must be rejected.
+    expect(() => asSafeChunkSize(48 * 1024 * 1024)).toThrow(/exceeds/);
+    // Even exactly 20 MB is at the raw Telegram limit — rejected by the margin.
+    expect(() => asSafeChunkSize(20 * 1024 * 1024)).toThrow(/exceeds/);
+    expect(() => asSafeChunkSize(TELEGRAM_CHUNK_SIZE_MAX_BYTES + 1)).toThrow(/exceeds/);
+  });
+
+  it('asSafeChunkSize rejects non-positive or non-integer sizes', () => {
+    expect(() => asSafeChunkSize(0)).toThrow('Invalid Telegram chunk size');
+    expect(() => asSafeChunkSize(-1)).toThrow('Invalid Telegram chunk size');
+    expect(() => asSafeChunkSize(1.5)).toThrow('Invalid Telegram chunk size');
+  });
+
+  it('startup fails fast when TELEGRAM_CHUNK_SIZE_BYTES exceeds the limit', async () => {
+    // Spawn a real process that imports src/env with an oversized chunk size;
+    // it must exit non-zero with a clear error instead of starting silently.
+    const proc = Bun.spawn({
+      cmd: ['bun', '-e', "import('./src/env')"],
+      cwd: `${import.meta.dir}/..`,
+      env: {
+        ...process.env,
+        BOT_TOKENS: '123456:ABC-DEF',
+        STORAGE_CHANNEL_ID: '-1001234567890',
+        BASE_URL: 'https://example.com',
+        DATABASE_URL: 'postgresql://user:***@localhost:5432/test',
+        PORT: '3000',
+        TELEGRAM_CHUNK_SIZE_BYTES: String(48 * 1024 * 1024),
+      },
+      stdout: 'pipe',
+      stderr: 'pipe',
+    });
+    const exitCode = await proc.exited;
+    const stderr = await new Response(proc.stderr).text();
+    expect(exitCode).not.toBe(0);
+    expect(stderr).toContain('TELEGRAM_CHUNK_SIZE_BYTES');
+    expect(stderr).toContain('exceeds');
+  });
+
+  it('startup succeeds when TELEGRAM_CHUNK_SIZE_BYTES is at the safe limit', async () => {
+    const proc = Bun.spawn({
+      cmd: ['bun', '-e', "import('./src/env')"],
+      cwd: `${import.meta.dir}/..`,
+      env: {
+        ...process.env,
+        BOT_TOKENS: '123456:ABC-DEF',
+        STORAGE_CHANNEL_ID: '-1001234567890',
+        BASE_URL: 'https://example.com',
+        DATABASE_URL: 'postgresql://user:***@localhost:5432/test',
+        PORT: '3000',
+        TELEGRAM_CHUNK_SIZE_BYTES: String(TELEGRAM_CHUNK_SIZE_MAX_BYTES),
+      },
+      stdout: 'pipe',
+      stderr: 'pipe',
+    });
+    const exitCode = await proc.exited;
+    expect(exitCode).toBe(0);
+  });
+});
diff --git a/test/helpers/setup-env.ts b/test/helpers/setup-env.ts
index 3066afc..0382783 100644
--- a/test/helpers/setup-env.ts
+++ b/test/helpers/setup-env.ts
@@ -17,4 +17,10 @@ process.env.DATABASE_URL ||= 'postgresql://user:pass@localhost:5432/test';
 process.env.PORT ||= '3000';
 process.env.NODE_ENV = 'test';
 
+// Pin the chunk size to the safe 19 MB value UNCONDITIONALLY. Bun auto-loads
+// the repo .env before preloads run, and a stale oversized value there would
+// trip the fail-fast guard in src/env.ts and break every test file's import.
+// Tests that need a different value set it explicitly in their own process.
+process.env.TELEGRAM_CHUNK_SIZE_BYTES = String(19 * 1024 * 1024);
+
 // Keep old env names for backward compat with tests that reference them directly