diff --git a/src/fastmcp/apps/file_upload.py b/src/fastmcp/apps/file_upload.py
index 8cc91e039..b3acc90e2 100644
--- a/src/fastmcp/apps/file_upload.py
+++ b/src/fastmcp/apps/file_upload.py
@@ -71,6 +71,15 @@ _TEXT_EXTENSIONS = frozenset(
 )
 
 
+def _b64_decoded_size(b64: str) -> int:
+    """Return the exact decoded byte-length of a base64 string without decoding it."""
+    n = len(b64)
+    if n == 0:
+        return 0
+    padding = b64.count("=", max(0, n - 2))
+    return n * 3 // 4 - padding
+
+
 def _format_size(size: int) -> str:
     if size < 1024:
         return f"{size} B"
@@ -274,10 +283,13 @@ class FileUpload(FastMCPApp):
         def store_files(files: list[dict], ctx: Context) -> list[dict]:
             """Store uploaded files. Receives file objects with name, size, type, data (base64)."""
             for f in files:
-                if f.get("size", 0) > provider._max_file_size:
+                # Compute actual data size from the base64 payload rather
+                # than trusting the client-reported ``size`` field.
+                actual_size = _b64_decoded_size(f.get("data", ""))
+                if actual_size > provider._max_file_size:
                     raise ValueError(
                         f"File {f.get('name', '?')!r} exceeds max size "
-                        f"({_format_size(f['size'])} > "
+                        f"({_format_size(actual_size)} > "
                         f"{_format_size(provider._max_file_size)})"
                     )
             return provider.on_store(files, ctx)
diff --git a/tests/apps/test_file_upload.py b/tests/apps/test_file_upload.py
index c8bd7470e..0ef107a3d 100644
--- a/tests/apps/test_file_upload.py
+++ b/tests/apps/test_file_upload.py
@@ -5,7 +5,32 @@ import base64
 import pytest
 
 from fastmcp import FastMCP
-from fastmcp.apps.file_upload import FileUpload
+from fastmcp.apps.file_upload import FileUpload, _b64_decoded_size
+
+
+class TestB64DecodedSize:
+    """Unit tests for the _b64_decoded_size helper."""
+
+    @pytest.mark.parametrize("size", [0, 1, 2, 3, 4, 50, 99, 100, 101, 1000])
+    def test_matches_actual_decode(self, size: int):
+        data = b"x" * size
+        b64 = base64.b64encode(data).decode()
+        assert _b64_decoded_size(b64) == size
+
+    def test_empty_string(self):
+        assert _b64_decoded_size("") == 0
+
+    def test_no_padding(self):
+        # 3 bytes → 4 base64 chars, no padding
+        assert _b64_decoded_size(base64.b64encode(b"abc").decode()) == 3
+
+    def test_one_pad(self):
+        # 2 bytes → 4 base64 chars with 1 '='
+        assert _b64_decoded_size(base64.b64encode(b"ab").decode()) == 2
+
+    def test_two_pads(self):
+        # 1 byte → 4 base64 chars with 2 '='
+        assert _b64_decoded_size(base64.b64encode(b"a").decode()) == 1
 
 
 def _make_file(
@@ -123,6 +148,23 @@ class TestFileUploadProvider:
         with pytest.raises(Exception, match="exceeds max size"):
             await server.call_tool("Files___store_files", {"files": [big_file]})
 
+    async def test_max_file_size_checks_actual_data_not_reported_size(self):
+        """Size limit should be enforced on actual base64 payload, not the
+        client-reported ``size`` field which can be spoofed."""
+        server = FastMCP("test", providers=[FileUpload(max_file_size=100)])
+
+        big_content = "x" * 200
+        big_b64 = base64.b64encode(big_content.encode()).decode()
+        spoofed_file = {
+            "name": "spoofed.bin",
+            "size": 1,  # lies about size
+            "type": "application/octet-stream",
+            "data": big_b64,
+        }
+
+        with pytest.raises(Exception, match="exceeds max size"):
+            await server.call_tool("Files___store_files", {"files": [spoofed_file]})
+
 
 class TestFileUploadSubclass:
     async def test_custom_storage(self):