Server-Modified Values and Defaults

Services often provide default values for resource fields, and occasionally normalize the user input before returning it in the response. The guidance herein describes how services document such behavior for the benefit of consumers.


Single Owner Fields

Fields must have a single owner, whether that is the client or the server. Server owned fields must be indicated with the OUTPUT_ONLY field_behavior. All other types of fields must be considered to be owned by the client. The server must respect the value (or lack thereof) for all client owned fields and not modify them.

Effective Values

There are instances where a service will allocate, generate, or calculate a value if the client chooses not to specify one. For example: a client creates a virtual machine without specifying a static IP address for the virtual machine to be available on. Such a scenario is opting into dynamic IP address allocation.

Some examples of these types of fields are ones that are:

  • generated (UUID)
  • allocated (dynamic IP address)
  • assigned (most recent software package version)

An attribute with an effective value must be expressed as two fields in the API:

  • a mutable field that can be optionally set by the user and must not be modified by the service
  • an OUTPUT_ONLY field that records the effective value decided on by the service


message VirtualMachine {
  string ip_address = 4;
  string effective_ip_address = 5 [
    (google.api.field_behavior) = OUTPUT_ONLY


Effective values must be named by prefixing effective_ to the mutable field's name.

User-Specified Fields

For user-specified fields, the value in response from the service must be the same as provided by the create or update request. For string fields this means returning the value unchanged, with one exception:

  • When a string field has a data type annotation, a normalized string that represents the given value may be returned.


A field that is normalized by the service must be annotated with the google.api.field_info extension. See AIP-202 for guidance on using this extension The allowed set of normalizations includes the following formats:

  • uuid
  • ipv4
  • ipv6
  • email

Normalizations on fields must be described using the google.api.field_info annotation.


Server-modified and default values often make it harder to implement declarative clients. These clients are often unable to tell when their desired state matches the current state for these fields, as the rules by which a server may modify and return values are complex, not public, and not repeatable.

Rationale for Single Owner Fields

When fields do not have a single owner they can cause issues for declarative clients. These clients may attempt to set values for fields that are overwritten by server set values, leading to the client entering an infinite loop to correct the change.

Rationale for Naming

Consistent naming is important for identifying standard behavior across APIs and fields. Programmatic association between user-specified and effective values depends on consistent naming.

Rationale for Normalizations

Normalizations are important to allow services to store and return values in a standard way while communicating to clients what changes are semantically identical. Normalizing a value on the service side allows the service to accept a wider range of semantically identical inputs without needing to maintain every value as a raw string. Surfacing the normalization that is being applied to clients allows for client side comparison of sent and retrieved values to check for differences.

For example, in a resource that accepts an email address on a particular field a client may specify a given email address in a variety of ways. For the email a client may choose to specify,, or These are semantically identical and should all be accepted by the service. The service then may choose to normalize the email address for storage and retrieval through downcasing or canonicalization. Importantly, the information surfaced to clients on the normalization of a field will not describe the normalization algorithm itself, but instead the comparison method used to accurately compute if two values should be considered equal.

Rationale for Field Value Handling

For fields not using an allowed normalization, Declarative clients will not be able to identify which changes are semantically meaningful. When a Declarative client sends a particular value it will ensure that the value is being returned by the service to validate it was set correctly.


  • 2023-10-31: Update to approved.