[Feat]: Add `min_items = 1` constraint to `Artifact` in protobuf schema

[punkyoon]

Is your feature request related to a problem? Please describe.

The A2A Protocol Specification (Section 4.1.7) states that an Artifact's parts array "Must contain at least one part."

However, this constraint is not reflected in the protobuf schema definition at specification/a2a.proto.

Describe the solution you'd like

Current Schema

specification/a2a.proto

message Artifact {
  // Unique identifier (e.g. UUID) for the artifact. It must be at least unique
  // within a task.
  string artifact_id = 1 [(google.api.field_behavior) = REQUIRED];
  // A human readable name for the artifact.
  string name = 3;
  // A human readable description of the artifact, optional.
  string description = 4;
  // The content of the artifact. Must contain at least one part.
  repeated Part parts = 5 [(google.api.field_behavior) = REQUIRED];
  // Optional metadata included with the artifact.
  google.protobuf.Struct metadata = 6;
  // The URIs of extensions that are present or contributed to this Artifact.
  repeated string extensions = 7;
}

specification/buf.yaml

version: v2
deps:
  # Common Protobuf types.
  - buf.build/googleapis/googleapis

Proposed Change

specification/a2a.proto

+ import "buf/validate/validate.proto"; 

message Artifact {
  // Unique identifier (e.g. UUID) for the artifact. It must be at least unique
  // within a task.
  string artifact_id = 1 [(google.api.field_behavior) = REQUIRED];
  // A human readable name for the artifact.
  string name = 3;
  // A human readable description of the artifact, optional.
  string description = 4;
  // The content of the artifact. Must contain at least one part.
- repeated Part parts = 5 [(google.api.field_behavior) = REQUIRED];
+ repeated Part parts = 5 [
+   (google.api.field_behavior) = REQUIRED,
+   (buf.validate.field).repeated.min_items = 1
+ ];
  // Optional metadata included with the artifact.
  google.protobuf.Struct metadata = 6;
  // The URIs of extensions that are present or contributed to this Artifact.
  repeated string extensions = 7;
}

specification/buf.yaml

version: v2
deps:
  # Common Protobuf types.
  - buf.build/googleapis/googleapis
+ - buf.build/bufbuild/protovalidate

Why This Matters

SDKs that generate models from predefined schema in A2A repository (e.g., https://github.com/a2aproject/a2a-python using datamodel-codegen) do not automatically enforce the non-empty constraint because the schema lacks min_items. This means invalid Artifact objects with empty parts can be created without any validation error.

• The Java SDK already enforces this at the code level but this should be driven by the schema itself.
  • https://github.com/a2aproject/a2a-java/blob/v0.3.3.Final/spec/src/main/java/io/a2a/spec/Artifact.java
• The Python SDK applied a manual fix in below but since types.py is auto-generated from this schema, the fix will be overwritten on the next regeneration.
  • [Bug]: Enforce non-empty parts in Artifact model to match A2A Specification a2a-python#670
  • fix(types): check min_length for parts in Artifact a2a-python#671

Describe alternatives you've considered

No response

Additional context

No response

Code of Conduct

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