[BUG] `parts` of `Artifact` should not be empty in A2A

[punkyoon]

Checks

• I have updated to the lastest minor and patch version of Strands
• I have checked the documentation and this is not expected behavior
• I have searched ./issues and there are no duplicates of my issue

Strands Version

1.25.0

Python Version

3.14.2

Operating System

macOS 26.2

Installation Method

pip

Steps to Reproduce

When enable_a2a_compliant_streaming=True is set, strands-agents produces the following stream:

...
data: {"id":"test-stream-1","jsonrpc":"2.0","result":{"append":true,"artifact":{"artifactId":"826257ff-26e0-441e-a4b3-f44b5e6f1615","name":"agent_response","parts":[{"kind":"text","text":"I'll"}]},"contextId":"019c2c2d-c046-7416-aba2-2becef55fa93","kind":"artifact-update","taskId":"f0e833f2-cd8a-4910-ae47-eca136984804"}}

data: {"id":"test-stream-1","jsonrpc":"2.0","result":{"append":true,"artifact":{"artifactId":"826257ff-26e0-441e-a4b3-f44b5e6f1615","name":"agent_response","parts":[{"kind":"text","text":" suggest"}]},"contextId":"019c2c2d-c046-7416-aba2-2becef55fa93","kind":"artifact-update","taskId":"f0e833f2-cd8a-4910-ae47-eca136984804"}}

data: {"id":"test-stream-1","jsonrpc":"2.0","result":{"append":true,"artifact":{"artifactId":"826257ff-26e0-441e-a4b3-f44b5e6f1615","name":"agent_response","parts":[{"kind":"text","text":" a"}]},"contextId":"019c2c2d-c046-7416-aba2-2becef55fa93","kind":"artifact-update","taskId":"f0e833f2-cd8a-4910-ae47-eca136984804"}}

data: {"id":"test-stream-1","jsonrpc":"2.0","result":{"append":true,"artifact":{"artifactId":"826257ff-26e0-441e-a4b3-f44b5e6f1615","name":"agent_response","parts":[{"kind":"text","text":" plan"}]},"contextId":"019c2c2d-c046-7416-aba2-2becef55fa93","kind":"artifact-update","taskId":"f0e833f2-cd8a-4910-ae47-eca136984804"}}

data: {"id":"test-stream-1","jsonrpc":"2.0","result":{"append":true,"artifact":{"artifactId":"826257ff-26e0-441e-a4b3-f44b5e6f1615","name":"agent_response","parts":[{"kind":"text","text":"."}]},"contextId":"019c2c2d-c046-7416-aba2-2becef55fa93","kind":"artifact-update","taskId":"f0e833f2-cd8a-4910-ae47-eca136984804"}}

data: {"id":"test-stream-1","jsonrpc":"2.0","result":{"append":true,"artifact":{"artifactId":"826257ff-26e0-441e-a4b3-f44b5e6f1615","name":"agent_response","parts":[]},"contextId":"019c2c2d-c046-7416-aba2-2becef55fa93","kind":"artifact-update","lastChunk":true,"taskId":"f0e833f2-cd8a-4910-ae47-eca136984804"}}

data: {"id":"test-stream-1","jsonrpc":"2.0","result":{"contextId":"019c2c2d-c046-7416-aba2-2becef55fa93","final":true,"kind":"status-update","status":{"state":"completed","timestamp":"2026-02-06T06:49:52.281537+00:00"},"taskId":"f0e833f2-cd8a-4910-ae47-eca136984804"}}

There is a problem with the final TaskArtifactUpdateEvent in the stream:

data: {"id":"test-stream-1","jsonrpc":"2.0","result":{"append":true,"artifact":{"artifactId":"826257ff-26e0-441e-a4b3-f44b5e6f1615","name":"agent_response","parts":[]},"contextId":"019c2c2d-c046-7416-aba2-2becef55fa93","kind":"artifact-update","lastChunk":true,"taskId":"f0e833f2-cd8a-4910-ae47-eca136984804"}}

When a Java-based A2A client receives this event, it throws the following error (observed with a2a-java 0.3.3.Final):

Wrapped by: java.lang.RuntimeException: com.fasterxml.jackson.databind.exc.ValueInstantiationException: Cannot construct instance of `io.a2a.spec.Artifact`, problem: Parts cannot be empty
 at [Source: UNKNOWN; byte offset: #UNKNOWN] (through reference chain: io.a2a.spec.TaskArtifactUpdateEvent["artifact"])
	at io.a2a.client.transport.jsonrpc.sse.SSEEventListener.handleMessage(SSEEventListener.java:79)
	Suppressed: The stacktrace has been enhanced by Reactor, refer to additional information below: 
Error has been observed at the following site(s):
	*_________checkpoint ⇢ sendMessageStreaming

The Artifact implementation in a2a-java validates that parts is non-empty:

https://github.com/a2aproject/a2a-java/blob/v0.3.3.Final/spec/src/main/java/io/a2a/spec/Artifact.java

@JsonInclude(JsonInclude.Include.NON_ABSENT)
@JsonIgnoreProperties(ignoreUnknown = true)
public record Artifact(String artifactId, String name, String description, List<Part<?>> parts, Map<String, Object> metadata,
                       List<String> extensions) {

    public Artifact {
        Assert.checkNotNullParam("artifactId", artifactId);
        Assert.checkNotNullParam("parts", parts);
        if (parts.isEmpty()) {
            throw new IllegalArgumentException("Parts cannot be empty"); // !!HERE!!
        }
    }
...

This validation is consistent with the A2A specification, which states that parts "Must contain at least one part":

    "parts": {
        "description": "The content of the artifact. Must contain at least one part.",
        "items": {
        "$ref": "a2a.v1.Part.jsonschema.json"
        },
        "type": "array"
    }

strands-agents should not emit a TaskArtifactUpdateEvent with an empty parts array. The final lastChunk: true event should either include at least one part or be omitted entirely.

Expected Behavior

the parts field in Artifact must not be empty. it should be structured as follows:

data: {"id":"test-stream-1","jsonrpc":"2.0","result":{"append":true,"artifact":{"artifactId":"9b2512d3-b299-4219-9171-596e78153b14","name":"agent_response","parts":[{"kind":"text","text":""}]},"contextId":"thr_019c2c2d-c046-7416-aba2-2becef55fa93","kind":"artifact-update","lastChunk":true,"taskId":"b24ee98c-be58-479b-8632-73e1bfefde59"}}

Actual Behavior

The last event in the TaskArtifactUpdateEvent stream contains empty parts data:

data: {"id":"test-stream-1","jsonrpc":"2.0","result":{"append":true,"artifact":{"artifactId":"826257ff-26e0-441e-a4b3-f44b5e6f1615","name":"agent_response","parts":[]},"contextId":"019c2c2d-c046-7416-aba2-2becef55fa93","kind":"artifact-update","lastChunk":true,"taskId":"f0e833f2-cd8a-4910-ae47-eca136984804"}}

Additional Context

No response

Possible Solution

I propose sending a TextPart with an empty string. if you agree with this approach, I will open a PR.

as-is

https://github.com/strands-agents/sdk-python/blob/v1.25.0/src/strands/multiagent/a2a/executor.py#L177-L214

to-be

    async def _handle_agent_result(self, result: SAAgentResult | None, updater: TaskUpdater) -> None:
        """Handle the final result from the Strands Agent.

        For A2A-compliant streaming: sends the final artifact chunk marker and marks
        the task as complete. If no data chunks were previously sent, includes the
        result content.

        For legacy streaming: adds the final result as a simple artifact without
        artifact_id tracking.

        Args:
            result: The agent result object containing the final response, or None if no result.
            updater: The task updater for managing task state and adding the final artifact.
        """
        if self.enable_a2a_compliant_streaming:
            if self._is_first_chunk:
                final_content = str(result) if result else ""
-               parts = [Part(root=TextPart(text=final_content))] if final_content else []
+               parts = [Part(root=TextPart(text=final_content))]
                await updater.add_artifact(
                    parts,
                    artifact_id=self._current_artifact_id,
                    name="agent_response",
                    last_chunk=True,
                )
            else:
                await updater.add_artifact(
-                   [],
+                   [Part(root=TextPart(text=""))],
                    artifact_id=self._current_artifact_id,
                    name="agent_response",
                    append=True,
                    last_chunk=True,
                )

Related Issues

No response