Developing the Sourcegraph GraphQL API

Internal comments vs external documentation

Comments starting with #! in schema.graphql will be stripped out before the GraphQL documentation is generated. #! is useful for implementation details and security notes that shouldn’t be displayed to API users.

GraphQL schema evolution

When changing the GraphQL schema, try to make changes that cause the API to “grow”, such as:

  • Providing more: adding a new field to a return type, marking a field in a return type as non-nullable (e.g. String!)
  • Requiring less: marking a parameter (or field in an input object) as nullable

Avoid “shrinking” the API with changes such as:

  • Removing fields in a return type
  • Marking fields in a return type as nullable
  • Marking a parameter as non-nullable

Changing the return type of a field

Because most queries do not mention return types directly, it’s often possible to change the return type of a field as long as the new return type is a superclass of the old one. This breaks when clients use __typename, for example to determine the union variant of a value, so use caution when changing this.

As an example of changing a return type, imagine this query that deletes a user by ID:

mutation {
  deleteUser(id: 1) {

Let’s say we want to return the User that was deleted by deleteUser, but it’s currently returning EmptyResponse. We could change the return type to User and temporarily add the alwaysNil field to User while clients migrate:

 type Mutation {
-  deleteUser(id: ID!): EmptyResponse
+  deleteUser(id: ID!): User!

 type EmptyResponse {
   alwaysNil: String

 type User {
   id: ID!
+  # For compatibility with old clients only.
+  alwaysNil: String

When the new type is not a superclass of the old one (e.g. String -> User), a breaking change is required.

Making breaking changes

Try to avoid making breaking changes, but if you have to, be sure to give clients time to migrate. This usually involves splitting the change into 2 parts. For example, if you want to rename a field in the return type of a query:

  • Add a new field with a different name, deprecate the old field, and communicate the deprecation in the changelog
  • Give clients time to migrate (2 release cycles is a common guideline), remove the old deprecated field, and communicate the removal in the changelog