diff --git a/abogen/webui/app.py b/abogen/webui/app.py
index 2a336b9..c79aac1 100644
--- a/abogen/webui/app.py
+++ b/abogen/webui/app.py
@@ -84,6 +84,12 @@ def create_app(config: Optional[dict[str, Any]] = None) -> Flask:
         "UPLOAD_FOLDER": str(uploads_dir),
         "OUTPUT_FOLDER": str(outputs_dir),
         "MAX_CONTENT_LENGTH": 1024 * 1024 * 400,  # 400 MB uploads
+        # Large books can submit four form fields per chapter. Werkzeug's
+        # defaults reject those requests before the wizard route can process
+        # them, even though the encoded payload is much smaller than the upload
+        # limit above.
+        "MAX_FORM_MEMORY_SIZE": 10 * 1024 * 1024,
+        "MAX_FORM_PARTS": 10_000,
     }
     if config:
         base_config.update(config)
diff --git a/pyproject.toml b/pyproject.toml
index 3722fb5..884cd98 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -44,7 +44,7 @@ dependencies = [
   "python-dotenv>=1.0.1",
   "static_ffmpeg>=2.13",
   "Markdown>=3.9",
-  "Flask>=3.0.3",
+  "Flask>=3.1.0",
   "numpy>=1.24.0",
   "gpustat>=1.1.1",
   "num2words>=0.5.13",
diff --git a/tests/test_webui_form_limits.py b/tests/test_webui_form_limits.py
new file mode 100644
index 0000000..d917455
--- /dev/null
+++ b/tests/test_webui_form_limits.py
@@ -0,0 +1,53 @@
+from abogen.webui.app import create_app
+
+
+def _large_chapter_form() -> dict[str, str]:
+    data = {"step": "chapters"}
+    for index in range(370):
+        prefix = f"chapter-{index}"
+        data[f"{prefix}-enabled"] = "on"
+        data[f"{prefix}-title"] = f"Chapter {index} " + ("x" * 1400)
+        data[f"{prefix}-voice"] = "af_heart"
+        data[f"{prefix}-formula"] = "default"
+    return data
+
+
+def test_large_chapter_form_reaches_wizard_route(tmp_path):
+    app = create_app(
+        {
+            "TESTING": True,
+            "SECRET_KEY": "test",
+            "OUTPUT_FOLDER": str(tmp_path / "output"),
+            "UPLOAD_FOLDER": str(tmp_path / "uploads"),
+        }
+    )
+
+    with app.test_client() as client:
+        response = client.post(
+            "/wizard/update?format=json",
+            data=_large_chapter_form(),
+        )
+
+    assert response.status_code == 400
+    assert response.get_json()["error"] == "Missing job ID"
+
+
+def test_large_multipart_chapter_form_reaches_wizard_route(tmp_path):
+    app = create_app(
+        {
+            "TESTING": True,
+            "SECRET_KEY": "test",
+            "OUTPUT_FOLDER": str(tmp_path / "output"),
+            "UPLOAD_FOLDER": str(tmp_path / "uploads"),
+        }
+    )
+
+    with app.test_client() as client:
+        response = client.post(
+            "/wizard/update?format=json",
+            data=_large_chapter_form(),
+            content_type="multipart/form-data",
+        )
+
+    assert response.status_code == 400
+    assert response.get_json()["error"] == "Missing job ID"