Mix and Match: Serving an ADK Agent to AWS and Azure

This article provides a step by step look at running a Google ADK agent on Cloud Run, and serving it over the A2A protocol to callers that are not ADK.

The code is here:

github.com/xbill9/multicloud-a2a-subagent

What is this project trying to Do?

This project aims to answer one research brief with three agents on three clouds, over one protocol, with no stored credentials between them.

Google runs an ADK agent on Cloud Run. AWS runs a Strands agent on Bedrock AgentCore. Azure runs an Agent Framework agent on Container Apps. A coordinator asks all three the same question and scores what comes back.

Everything below is the Google side of it, and none of it is visible from a laptop.

Aren't Those Clients on the Wrong Clouds?

Mix and Match. That is the entire point of the exercise.

An ADK agent that only ever answers ADK clients is not interoperating with anything. The findings below all needed two things at once: a deployment, and a caller that was not Google's.

Google ADK

The Agent Development Kit is Google's open source framework for building and deploying AI agents. It is model agnostic and deployment agnostic, it runs Gemini through Vertex AI or an API key, and to_a2a() turns an agent into an A2A server in one line.

More information is available here:

Agent Development Kit

What is A2A

A2A (Agent2Agent) is an open protocol for agents built by different teams, on different frameworks, to call each other. An agent publishes a card at /.well-known/agent-card.json describing what it does and how to reach it, and speaks JSON-RPC over HTTP. This project runs A2A v1.0.

More details are available here:

Agent2Agent (A2A) Protocol

Serving ADK Over A2A

This is the whole server:

from google.adk.agents import LlmAgent
from google.adk.a2a.utils.agent_to_a2a import to_a2a

agent = LlmAgent(
    model="gemini-2.5-flash",
    name=..., description=...,
    instruction=INSTRUCTION,
    tools=[web_search],
)

app = to_a2a(agent, host=HOST, port=PORT)

That is genuinely the shortest path from an LlmAgent to something another vendor's agent can call, and it is the reason I started here.

Then deploy it to Cloud Run from source, no Dockerfile:

PUBLIC=1 MODEL_MODE=llm ./infra/deploy_gcp.sh deploy

Verify The Agent Card

First step after any deploy — fetch your own card:

$ curl -s https://<the-adk-agent>.run.app/.well-known/agent-card.json

{"url": null,
 "additionalInterfaces": [
   {"url": "http://0.0.0.0:8080", "protocolBinding": "JSONRPC"}
 ]}

A public HTTPS endpoint advertising unroutable plaintext.

to_a2a(agent, host, port) writes host:port straight into the card's interface URL. On Cloud Run the process binds 0.0.0.0:8080, so that is what the card says.

Note- it cannot reproduce locally. On a laptop the bind address and the dial address are the same string, which is exactly how it survives into a deployment. The other two agents take a PUBLIC_URL and advertise that — the behaviour ADK is missing, not anything clever.

Which clients survive it is the part worth knowing:

ADK's own client cannot reach ADK's own server once hosted. Both halves pass Google's own tests, because locally the two addresses are identical. The one pairing that is entirely first-party code is the one that cannot complete a hop.

And the failure is reported at the wrong layer. Having dialled 0.0.0.0:8080 and failed, RemoteA2aAgent raises:

AttributeError: 'A2AClientError' object has no attribute 'status_code'

The error handler assumes any A2AClientError carries a status code, which a transport failure does not. The real cause — All connection attempts failed — goes to a separate log line. Two defects compounding: the first sends the client somewhere unroutable, the second deletes the evidence of where it went.

So if you serve with to_a2a() today: fetch your own card after deploying, and if you cannot fix the card, make sure your callers rewrite the interface URL after resolution rather than routing by it.

The Same Reply, Delivered Twice

ADK's executor attaches the reply as a task artifact — and also leaves a copy in task history.

That only matters when you talk to someone else. Microsoft's A2AExecutor drives the full task lifecycle and leaves the reply only in history, with artifacts empty. So a client that reads artifacts alone — the obvious implementation, and the one that works perfectly against ADK — returns an empty string against Agent Framework. Not an error, not a timeout: a successful call with no content.

Fix that by reading every carrier the spec allows, and ADK's reply now arrives twice.

In an earlier version of this project the agents returned an exchange rate, and the duplicate was invisible: the parser indexed quotes by target currency, so the second copy overwrote the first and the answer was correct. Change the payload to a written draft and the body doubles, the word count doubles, and the scorer downstream marks a compliant draft as a 100% length overrun.

I found it by reading output — one cloud returned 202 words of text the other two returned in 98.

Note- no test caught it. The suite was green throughout. There is a live test now asserting all three serving stacks return the same canned text at the same length, which is the cheapest detector I know for the whole class.

There is No Function Seam

Strands hands you async (prompt) -> reply. You can wrap that from outside and be done.

to_a2a() takes an agent and serialises its event stream, so anything you need to do between the model and the wire has to happen inside ADK's object model — in my case a BaseAgent that consumes inner.run_async(ctx) and yields one final event.

Why bother: every draft in this system carries one line, written by the server and never by the model.

<!-- a2a-research agent=gcp model=gemini-2.5-flash brain=llm -->

It carries the two things the coordinator cannot reconstruct from its side of the wire — which model actually answered, and whether a model answered at all. Ask Gemini to emit its own metadata and a model that gets it wrong misattributes a draft in the audit, which is the one error an audit cannot detect from the inside.

That wrapper became load-bearing the moment I attached a tool. The first version concatenated the text of every event in the stream, which is correct while the stream holds exactly one event. With a web_search tool bound, the stream also carries the model's commentary around each tool call — "Let me look that up", a summary of what it found — and concatenating those produces a draft that opens with Gemini narrating its own research. The rubric then scores the narration.

async for event in inner.run_async(ctx):
    if not event.is_final_response():
        continue          # <- the whole fix
    ...

Function calls and their results have no text and were already skipped by a part.text filter. What has to be excluded explicitly is the model text that accompanies them.

The Tool is Deliberately Not google_search

ADK gives you google_search as native hosted grounding, and I did not use it.

The other two clouds cannot match it. Microsoft's Agent Framework exports SupportsWebSearchTool, which is a protocol a chat client may declare rather than a tool you can hand an agent, and Foundry's own grounding wants a Bing resource connection created out of band. Strands bundles no search at all.

Using each vendor's native option would have meant Gemini grounded against Google's index and Bedrock grounded against nothing — three retrieval products, and a comparison that reports the difference between them as a difference between models.

So all three get the same plain function against the same backend. What stays native is the part worth measuring: ADK wraps the plain callable itself and runs its own tool-calling loop, which is a genuinely different implementation from Strands' @tool decorator and Agent Framework's function machinery.

Calling ADK From a Program

As a client, ADK is the heaviest of the three stacks by a wide margin. RemoteA2aAgent is a BaseAgent meant to sit inside an agent tree, so using it as a plain client means standing up a Runner, an InMemorySessionService and a session — per request.

Local, direct-brain, no model in the path, every client against every server:

$ python3 -m matrix.runner

client \ server  gcp          aws          azure
-------------------------------------------------
a2a-sdk          ok 134ms     ok 8ms       ok 8ms
agent-framework  ok 129ms     ok 7ms       ok 8ms
google-adk       ok 920ms     ok 9ms       ok 10ms

9/9 attempted cells succeeded

Single loopback runs, so read it as an ordering and nothing more. The shape holds anyway: against the ADK server the ADK client costs about seven times what the other two do, and it still emits [EXPERIMENTAL] warnings on every call.

If you are writing a program that calls an agent rather than an agent that calls an agent, the reference a2a-sdk client is the lighter object.

What Cloud Run Brings

Two Cloud Run properties shaped this whole project.

It mints workload OIDC tokens for an audience you choose. That is what makes the mesh keyless: the coordinator takes a Google-minted token to AWS STS for AssumeRoleWithWebIdentity and to Entra as a client assertion, so there is no stored secret on any leg.

It is also why the coordinator lives on Google rather than anywhere else. The other two runtimes' minting ability was never confirmed, so hosting it there would have meant storing a credential. Where your coordinator runs decides how many secrets your system has.

And One Thing It Costs

A cold start is visible in the wrong place. On a scale-to-zero service the first thing a caller touches is not your agent, it is your agent card:

 +792ms  gcp  D research-gcp-...run.app/.well-known/  200   5849ms
+6643ms  gcp  I research-gcp-...run.app/              200     55ms

5849ms on discovery, 55ms on the actual invocation once warm. A single per-leg number would have recorded that run as "Gemini is slow."

The Buildpack Entrypoint Trap

This one cost a deploy cycle. The GCP side is built once from source and deployed several times, with the entrypoint overridden to run a different process out of the same image. The documented override fails:

--command python --args="-m,agents.gcp.server"

failed to resolve binary path: error finding executable "python" in PATH

A buildpack image keeps its interpreter in CNB layers, and /cnb/lifecycle/launcher is what puts them on PATH before exec'ing your process. Overriding the entrypoint replaces the launcher, so the override runs where Python does not exist.

Run it through the launcher instead:

--command /cnb/lifecycle/launcher --args="python,-m,agents.gcp.server"

Note- Cloud Run reports the failure as "the user-provided container failed to start and listen on the port defined by PORT=8080", which is the symptom of any startup crash and names the one subsystem that was fine.

How Did Gemini Do?

Twenty-four briefs, each answered by all three clouds, scored twice. Once by a deterministic rubric, once by re-ranking the same stored drafts with a model judge:

Three things about the Gemini row.

It answered 58% of the briefs it was invited to — the lowest of the three, on the one leg that never leaves its own cloud and is otherwise the most reliable path in the mesh. The failure recorded against it is a Vertex 429. Quota is the documented cause rather than a proven one; I have not attributed the ten missing drafts individually. Either way, the largest difference between these three models on this corpus is not prose quality, it is whether a draft came back.

It is the only one that spends its entire search budget, every run. All three agents get six searches. Under the current instruction Gemini uses all six in every single run, where the Bedrock model still skips searching entirely in one run out of seven. That is a real behavioural difference — and it also means the ceiling is now shaping the drafts I am comparing. A model that always spends its last search would spend more if it had it.

The middle column is where it gets uncomfortable. Under a model judge, Gemini wins 43% while gpt-5-mini takes 87%. And the judge is itself Gemini 2.5 Pro.

Note- my judge shares a vendor with one of the participants. That is a real bias risk, it is recorded on every verdict, and I am not arguing it away — though it is notable that the Gemini judge ranks the Gemini participant below Azure rather than above it, which is the opposite of the failure mode you would expect.

Twenty-four briefs is enough to compute a rate and not enough to trust one. All of mine were technology surveys, and the instruction changed twice inside them. I would not quote these as a model comparison.

So Why? Just Why? Why Serve ADK to Two Other Clouds?

Because every finding above needed a caller that was not Google's.

The card defect needs a client that routes by card. The doubled reply needs a client that reads every carrier because some other vendor's server made it. The event-stream narration needs a scorer outside the framework looking at the text. The cold-start-on-discovery finding needs a timeline that separates the card fetch from the invocation.

Run ADK against ADK on a laptop and every one of those is green.

Quick Reference, Google Side

Your deployed card advertises 0.0.0.0:8080. to_a2a() writes the bind address. Rewrite interfaces client-side after resolution, or serve a card you control.

AttributeError: 'A2AClientError' object has no attribute 'status_code'. RemoteA2aAgent dialled a card address it could not reach. The real cause is on another log line.

A remote client gets your reply twice. ADK sends it as artifact and history. The client should deduplicate; one reply in two envelopes is one reply.

Your draft opens with the model narrating its research. The event stream carries commentary around each tool call. Keep only is_final_response().

error finding executable "python" in PATH on a buildpack image. The entrypoint override replaced /cnb/lifecycle/launcher. Run the command through the launcher.

A leg looks slow but the model is fine. Check whether the time is in the agent-card fetch — on scale-to-zero that is where the cold start lands.

No API key was provided, inside a task body, with HTTP 200. ADK does not use application-default credentials for Gemini. Set GOOGLE_GENAI_USE_VERTEXAI=true with project and location, or an API key.

Summary

Cloud Run and to_a2a() give you the shortest path there is from an LlmAgent to something another vendor's agent can call. Fetch your own card after every deploy, assume your reply is read by something that is not ADK, and decide where you stand between the model and the wire before you attach a tool.

A2A did what it promised. Everything above is what sits just outside the protocol, and every one of them needed a deployment and a caller that was not Google's to show up at all.

The repo carries the three agents, the shared instruction and tool, the coordinator and judge, the 3×3 interop matrix and the deploy scripts. docs/INTEROP.md carries each finding with the date it was measured:

github.com/xbill9/multicloud-a2a-subagent