diff --git a/backend/src/infrastructure/auth/oauth.py b/backend/src/infrastructure/auth/oauth.py
index f13ae981..7c09fb6b 100644
--- a/backend/src/infrastructure/auth/oauth.py
+++ b/backend/src/infrastructure/auth/oauth.py
@@ -41,5 +41,6 @@ def _build_provider(name: str, client_id: str, client_secret: str):
 
 oauth_account_service = OAuthAccountService(
     repo=auth.repo,
-    new_user_fields=lambda ctx: {"name": ctx.suggested_name},
+    # suggested_name is the provider's full name, unbounded; User.name is String(30).
+    new_user_fields=lambda ctx: {"name": ctx.suggested_name[:30]},
 )
diff --git a/backend/src/modules/user/models.py b/backend/src/modules/user/models.py
index 8c0d7eec..4b7a36c0 100644
--- a/backend/src/modules/user/models.py
+++ b/backend/src/modules/user/models.py
@@ -26,7 +26,10 @@ class User(Base, TimestampMixin, SoftDeleteMixin):
     )
 
     name: Mapped[str] = mapped_column(String(30))
-    username: Mapped[str] = mapped_column(String(20), unique=True, index=True)
+    # 32 = crudauth's OAuth username generator cap (USERNAME_MAX_LENGTH); a narrower
+    # column rejects OAuth signups whose sanitized username exceeds it (e.g. OIDC
+    # preferred_username values shaped like user@org.domain).
+    username: Mapped[str] = mapped_column(String(32), unique=True, index=True)
     email: Mapped[str] = mapped_column(String(50), unique=True, index=True)
     hashed_password: Mapped[str] = mapped_column(String(100))
 
diff --git a/backend/src/modules/user/schemas.py b/backend/src/modules/user/schemas.py
index 4473cee6..2486000e 100644
--- a/backend/src/modules/user/schemas.py
+++ b/backend/src/modules/user/schemas.py
@@ -5,12 +5,20 @@
 
 from ..common.schemas import PersistentDeletion, TimestampSchema
 
+# Kept in one place because every user schema has to agree with the ``username``
+# column (``String(32)``) *and* with what crudauth's OAuth username generator
+# emits: it caps at 32 and sanitizes to lowercase alphanumerics plus underscores.
+# A narrower rule here does not reject the signup - crudauth writes the row
+# directly - it makes the resulting user unreadable through ``UserRead``.
+USERNAME_MAX_LENGTH = 32
+USERNAME_PATTERN = r"^[a-z0-9_]+$"
+
 
 class UserBase(BaseModel):
     name: Annotated[str, Field(min_length=2, max_length=30, examples=["User Userson"])]
     username: Annotated[
         str,
-        Field(min_length=2, max_length=20, pattern=r"^[a-z0-9]+$", examples=["userson"]),
+        Field(min_length=2, max_length=USERNAME_MAX_LENGTH, pattern=USERNAME_PATTERN, examples=["userson"]),
     ]
     email: Annotated[EmailStr, Field(examples=["user.userson@example.com"])]
 
@@ -44,7 +52,7 @@ class UserRead(BaseModel):
     name: Annotated[str, Field(min_length=2, max_length=30, examples=["User Userson"])]
     username: Annotated[
         str,
-        Field(min_length=2, max_length=20, pattern=r"^[a-z0-9]+$", examples=["userson"]),
+        Field(min_length=2, max_length=USERNAME_MAX_LENGTH, pattern=USERNAME_PATTERN, examples=["userson"]),
     ]
     email: Annotated[EmailStr, Field(examples=["user.userson@example.com"])]
     profile_image_url: str
@@ -105,8 +113,8 @@ class UserUpdate(BaseModel):
         str | None,
         Field(
             min_length=2,
-            max_length=20,
-            pattern=r"^[a-z0-9]+$",
+            max_length=USERNAME_MAX_LENGTH,
+            pattern=USERNAME_PATTERN,
             examples=["userberg"],
             default=None,
         ),
diff --git a/backend/tests/integration/auth/test_endpoints.py b/backend/tests/integration/auth/test_endpoints.py
index f423047c..8a229386 100644
--- a/backend/tests/integration/auth/test_endpoints.py
+++ b/backend/tests/integration/auth/test_endpoints.py
@@ -325,6 +325,55 @@ async def test_oauth_callback_success_creates_user(client: AsyncClient):
     mock_storage.delete.assert_awaited_once()
 
 
+@pytest.mark.asyncio
+async def test_oauth_callback_handles_long_provider_usernames(client: AsyncClient):
+    """OAuth signup survives providers whose usernames sanitize to >20 chars.
+
+    crudauth's provisioning caps generated usernames at 32 chars
+    (USERNAME_MAX_LENGTH), and OIDC ``preferred_username`` values shaped like
+    ``user@org.domain`` routinely exceed 20 after sanitizing, so the column
+    must fit the generator's cap. The provider's display name is unbounded
+    and gets truncated to ``User.name``'s width by ``new_user_fields``.
+    """
+    valid_state = OAuthState(
+        state="long-name-state",
+        provider="google",
+        redirect_to="/",
+        code_verifier="test-code-verifier",
+    )
+    mock_storage = MagicMock()
+    mock_storage.get = AsyncMock(return_value=valid_state)
+    mock_storage.delete = AsyncMock(return_value=None)
+
+    mock_provider = MagicMock()
+    mock_provider.exchange_code = AsyncMock(return_value={"access_token": "tok"})
+    mock_provider.get_user_info = AsyncMock(return_value={})
+    mock_provider.process_user_info = AsyncMock(
+        return_value=OAuthUserInfo(
+            provider="google",
+            provider_user_id="google-uid-long",
+            email="long_username@example.com",
+            email_verified=True,
+            name="A" * 50,  # unbounded provider display name
+            username="verylongusername@subdomain.example.com",  # sanitizes to >20 chars
+        )
+    )
+
+    with (
+        patch(f"{ROUTES}.oauth_state_storage", mock_storage),
+        patch(f"{ROUTES}.oauth_providers", {"google": mock_provider}),
+    ):
+        response = await client.get(
+            "/api/v1/auth/oauth/callback/google",
+            params={"code": "test-code", "state": "long-name-state", "response_format": "json"},
+        )
+
+    assert response.status_code == 200
+    body = response.json()
+    assert body["success"] is True
+    assert 20 < len(body["user"]["username"]) <= 32
+
+
 @pytest.mark.asyncio
 async def test_check_auth_user_not_found(client: AsyncClient):
     """A resolved principal whose user row is missing reports authenticated=false."""
diff --git a/docs/user-guide/authentication/user-management.md b/docs/user-guide/authentication/user-management.md
index 6fb34400..b69c4b12 100644
--- a/docs/user-guide/authentication/user-management.md
+++ b/docs/user-guide/authentication/user-management.md
@@ -160,13 +160,15 @@ If the body changes `username` or `email`, the service also re-checks uniqueness
 The `UserUpdate` schema makes every field optional so clients can send partial updates:
 
 ```python
+# USERNAME_MAX_LENGTH (32) and USERNAME_PATTERN are defined at the top of
+# modules/user/schemas.py and shared by every schema carrying a username
 class UserUpdate(BaseModel):
     model_config = ConfigDict(extra="forbid")
 
     name: Annotated[str | None, Field(min_length=2, max_length=30, default=None)]
     username: Annotated[
         str | None,
-        Field(min_length=2, max_length=20, pattern=r"^[a-z0-9]+$", default=None),
+        Field(min_length=2, max_length=USERNAME_MAX_LENGTH, pattern=USERNAME_PATTERN, default=None),
     ]
     email: Annotated[EmailStr | None, Field(default=None)]
     profile_image_url: Annotated[
diff --git a/docs/user-guide/database/index.md b/docs/user-guide/database/index.md
index 649528a9..5a4eb98d 100644
--- a/docs/user-guide/database/index.md
+++ b/docs/user-guide/database/index.md
@@ -89,7 +89,7 @@ from pydantic import BaseModel, EmailStr, Field
 
 class UserCreate(BaseModel):
     name: str = Field(min_length=2, max_length=30)
-    username: str = Field(min_length=2, max_length=20)
+    username: str = Field(min_length=2, max_length=32)
     email: EmailStr
     password: str = Field(min_length=8)
 
diff --git a/docs/user-guide/database/schemas.md b/docs/user-guide/database/schemas.md
index ee1b32cb..bf9342a7 100644
--- a/docs/user-guide/database/schemas.md
+++ b/docs/user-guide/database/schemas.md
@@ -47,12 +47,18 @@ from pydantic import BaseModel, ConfigDict, EmailStr, Field
 from ..common.schemas import PersistentDeletion, TimestampSchema
 
 
+# Declared once, reused by every schema that carries a username: the rule has to
+# match the column (String(32)) and whatever OAuth provisioning generates
+USERNAME_MAX_LENGTH = 32
+USERNAME_PATTERN = r"^[a-z0-9_]+$"
+
+
 # Common fields shared by create/update/full-record
 class UserBase(BaseModel):
     name: Annotated[str, Field(min_length=2, max_length=30, examples=["User Userson"])]
     username: Annotated[
         str,
-        Field(min_length=2, max_length=20, pattern=r"^[a-z0-9]+$", examples=["userson"]),
+        Field(min_length=2, max_length=USERNAME_MAX_LENGTH, pattern=USERNAME_PATTERN, examples=["userson"]),
     ]
     email: Annotated[EmailStr, Field(examples=["user.userson@example.com"])]
 
@@ -75,7 +81,7 @@ class User(TimestampSchema, UserBase, PersistentDeletion):
 class UserRead(BaseModel):
     id: int
     name: Annotated[str, Field(min_length=2, max_length=30)]
-    username: Annotated[str, Field(min_length=2, max_length=20, pattern=r"^[a-z0-9]+$")]
+    username: Annotated[str, Field(min_length=2, max_length=USERNAME_MAX_LENGTH, pattern=USERNAME_PATTERN)]
     email: EmailStr
     profile_image_url: str
     is_deleted: bool = False
@@ -123,7 +129,7 @@ class UserUpdate(BaseModel):
     name: Annotated[str | None, Field(min_length=2, max_length=30, default=None)]
     username: Annotated[
         str | None,
-        Field(min_length=2, max_length=20, pattern=r"^[a-z0-9]+$", default=None),
+        Field(min_length=2, max_length=USERNAME_MAX_LENGTH, pattern=USERNAME_PATTERN, default=None),
     ]
     email: Annotated[EmailStr | None, Field(default=None)]
     profile_image_url: Annotated[