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[Bug]: a2a-java client sends message with empty task ID and context ID #770
Description
What happened?
While investigating errors reported by https://darrelmiller.github.io/agentbin/index.html, I found out that messages sent by the A2A Java client contains empty taskId and contextId.
They look like (verbatim):
curl -sN -X POST https://agentbin.greensmoke-1163cb63.eastus.azurecontainerapps.io/spec \
-H "Content-Type: application/json" \
-H "Accept: text/event-stream" \
-d '{
"jsonrpc": "2.0",
"id": "839ffa13-d966-462c-88a2-136805189343",
"method": "SendStreamingMessage",
"params": {
"tenant": "",
"message": {
"messageId": "'$(uuidgen | tr '[:upper:]' '[:lower:]')'",
"taskId": "",
"contextId": "",
"role": "ROLE_USER",
"parts": [
{
"text": "task-lifecycle process this",
"metadata": {},
"filename": "",
"mediaType": ""
}
],
"metadata": {},
"extensions": [],
"referenceTaskIds": []
},
"configuration": {
"acceptedOutputModes": [],
"returnImmediately": false
},
"metadata": {}
}
}'
I think the .Net remote agent should reject these messages (or ignore the empty taskId) but it replies with responses such as
data: {"jsonrpc":"2.0","id":"839ffa13-d966-462c-88a2-136805189343","result":{"task":{"id":"","contextId":"","status":{"state":"TASK_STATE_SUBMITTED","timestamp":"2026-03-26T12:06:00.4665426+00:00"}}}}
which are not deserialised by the a2a-java SDK due to the empty task.id.
The a2a-java client should omit optional fields that are not set when sending messages.
If I remove the taskId: "" field from the reproducer, the .Net agent replies with a generated taskId as expected.
Relevant log output
Code of Conduct
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Activity
fix: omit optional fields with default values from client JSON serial…
fix: omit optional fields with default values from client JSON serial…
fix: omit optional fields with default values from client JSON serial…
fix: omit optional fields with default values from client JSON serial…
fix: omit optional fields with default values from client JSON serial…
fix: omit optional fields with default values from client JSON serial…
ehsavoie commentedon Mar 31, 2026
@jmesnil I remember that there were some case where not having the parameter was different than having it but empty. Not sure if those are still valid
rohansiddam commentedon Oct 1, 2026
I have hit the blank identifier version of this in my own setup, and it is annoying because everything downstream looks almost normal. I built Synthe as a standalone packet checker, with a GitHub Action, that runs before the receiver accepts the work. I encoded this client case with an empty task ID on the handoff. It gets stopped as REJECT invalid because the handoff ID is missing. Repo link: https://github.com/rohansiddam/Synthe. If you run it on one real handoff and tell me where it breaks, that would be super helpful.
To try it: paste the repo link into your coding agent and tell it to follow AGENTS.md. You can keep working with your agents and giving them instructions as normal; the tool checks each handoff and flags it if something is off. If you would rather run it by hand:
python3 src/handoff_check.py packet.json --registry registry.json --workspace . --ledger ledger.json.