Mix and Match: One Agent, Three Clouds, One Protocol

This article provides a step by step comparison of the same research agent built three times, on Google ADK, AWS Strands and Microsoft Agent Framework, all three speaking A2A to one coordinator.

The code is here:

github.com/xbill9/multicloud-a2a-subagent

What is this project trying to Do?

This project aims to find out what actually differs between the three hyperscalers' agent frameworks once the protocol between them already works.

All three now ship an agent framework, and all three speak A2A. The protocol page will tell you that is the interoperability story finished:

In a world where agents are built using diverse frameworks and by different
vendors, A2A provides the definitive common language for agent
interoperability.

— a2a-protocol.org

That is true on the wire, and the wire is not the whole job.

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

The Three Stacks

One research agent, one instruction, one search tool, one word budget — built three times and hosted on each vendor's own runtime:

One coordinator fans the same brief out to all three and scores what comes back.

Nothing below is about A2A being broken. A2A worked. This is about the nine other things that differ once it does — and about the two questions worth separating, which almost nobody separates. What differs because of the platform, and what differs because of the model.

What Actually Has to be the Same

The first version of this was a demo: three agents, three SDKs, three green ticks. It told me nothing. When three columns differ in nine ways, you cannot attribute any result to any of them.

So the rule became one line. Share everything that is not the variable under test:

The right column is the article. The left column is what makes it evidence instead of an anecdote.

The one people argue with is the search tool. I gave all three clouds the same search function rather than each vendor's own, and it is the decision I would defend hardest.

Only Google ships a ready search tool. 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 none at all.

"Native search everywhere" would have meant Gemini grounded against Google's index, a Foundry model against Bing, and Bedrock against nothing. Three retrieval products, and a comparison that reports the gap between them as a gap between models.

What is still native is the part worth seeing anyway — how each framework binds and drives a tool. That part is now the only part that varies.

Three Frameworks, Three Shapes

Here is the entire model-side construction on each cloud. Not excerpts — this is all of it.

Google, ADK:

from google.adk.agents import LlmAgent

LlmAgent(
    model="gemini-2.5-flash",          # a model id string
    name=..., description=...,
    instruction=INSTRUCTION,           # `instruction`
    tools=[web_search],                # a plain callable
)

AWS, Strands:

from strands import Agent, tool
from strands.models import BedrockModel

Agent(
    model=BedrockModel(model_id="us.amazon.nova-micro-v1:0"),
    system_prompt=INSTRUCTION,         # `system_prompt`
    tools=[tool(web_search)],          # explicitly decorated
)

Azure, Agent Framework:

from agent_framework import Agent
from agent_framework.foundry import FoundryChatClient
from azure.identity import DefaultAzureCredential

Agent(
    client=FoundryChatClient(          # a *chat client*, not a model
        project_endpoint=os.environ["FOUNDRY_PROJECT_ENDPOINT"],
        model=model_id(),
        credential=DefaultAzureCredential(),
    ),
    instructions=INSTRUCTION,          # `instructions`, plural
    tools=[web_search],
    default_options={"store": False},
)

Three names for the system prompt. Three levels at which the model is named: a string, a model object, a client holding an endpoint and a credential. Three tool conventions.

None of that is hard. All of it is untranslatable.

There is no adapter that turns these into one object, and every hour I have seen spent trying to build one produced a fourth thing to maintain that then became what was actually under test. Share the prompt, the tool and the wire format. Do not try to share the agent.

Two of the Three Give You Nowhere to Stand

Strands hands you a function. Everything else can wrap it from outside:

async def respond(prompt: str) -> str:
    return str(await agent.invoke_async(prompt))

ADK and Agent Framework do not. to_a2a() takes an agent and serialises its event stream, and Agent Framework's A2AExecutor calls the agent too. Neither gives you a (prompt) -> reply boundary, so anything you need to do between the model and the wire has to be done inside that framework's own object model.

This is not a style complaint. It decides where a fact can be recorded. Every draft in this system carries one line:

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

That line is written by the server, never by the model. It carries the two things the coordinator cannot reconstruct from its own side of the wire — which model actually answered, and whether a model answered at all. Ask the model 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.

Note- on ADK that wrapper became load-bearing the moment I added a tool. The first version concatenated the text of every event in the stream, which was correct while the stream held exactly one event. With web_search attached the stream also carries the model's commentary around each tool call, and concatenating those produces a draft that opens with the model narrating its own research. Keep only event.is_final_response().

A Completed Task is Not The Answer

ADK and Agent Framework both return a Task in TASK_STATE_COMPLETED. Both are spec-conformant. They disagree about where the reply goes.

ADK attaches it as an artifact — and also leaves a copy in history. Agent Framework's A2AExecutor drives the full lifecycle and leaves the reply as a ROLE_AGENT message in history, with artifacts empty. The a2a-sdk reference executor, which is what the AWS agent sits on, enqueues a single Message and runs no task lifecycle at all.

So the obvious client — read task.artifacts — works perfectly against Google and returns an empty string against Microsoft. Not an error. Not a timeout. A successful call with no content, which then fails somewhere downstream as a parse error pointing at the wrong layer.

Read every carrier the spec allows and you get the mirror-image bug: ADK's reply arrives twice, once per envelope.

That one is worth dwelling on, because of how it stayed hidden. In the predecessor version of this project the agents returned an exchange rate, and the parser indexed quotes by target currency — so a duplicate object quietly overwrote its twin and the answer was correct. Change the domain to a written draft and the body doubles, the word count doubles, and the scorer 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, and the suite was green throughout. There is now a live test 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.

"The call succeeded" and "you received the answer" are different claims in A2A.

The Agent Card Advertises an Address You Cannot Dial

to_a2a(agent, host, port) writes the bind address straight into the 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. The AWS and Azure agents take a PUBLIC_URL and advertise that — the behaviour ADK is missing, not anything clever.

Note- it cannot reproduce locally, because on a laptop the bind address and the dial address are the same string. It needs a deployment, which is exactly how it survives into one.

Which clients survive it is the opposite of what the ergonomics would predict:

ADK's own client cannot reach ADK's own server once hosted. Both halves ship green in Google's own tests, because locally the two addresses are identical. And the stack that has no seam to patch a resolved card is the one that never needed it, because it dials the URL you constructed it with.

Then the failure is reported at the wrong layer:

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 — lands on a separate log line. Two defects compounding: the first sends the client to an unroutable address, the second deletes the evidence of where it went.

The Platform Edits Your Request

The runtime is not a deployment detail either. Each imposes a contract on the container, and they do not agree:

Three of those rows cost real time.

AgentCore does not forward the A2A-Version header. a2a-sdk reads the protocol version from that header and, when it is absent, assumes 0.3 — then rejects the request its own handler cannot serve:

A2A version '0.3' is not supported by this handler. Expected version '1.0'.

Cloud Run and Container Apps pass it through untouched. So the same client, the same a2a-sdk on both ends, the same server code, and the third cloud fails with an error that blames the protocol version and names nothing about the platform that removed it.

The fix is to assume the current version when the header is missing, and only when it is missing. A header that says 0.3 is a real client statement and should still be rejected. Absent is not evidence of an old client. It is no evidence at all.

An AgentCore session gets its own microVM. I was minting a fresh session id per call, so every call paid for a microVM start. It presented as a fixed per-client cost until the slow cell moved between clients — and a fixed per-client cost cannot move. Something per-call can:

Pin the session id unless you actually want per-call isolation. There is no equivalent knob on the other two clouds, and this cost is invisible in any per-leg average.

Container Apps splits "who may get a token" from "who must present one." One deploy step creates the federated credential; a separate step enforces identity on the ingress. Ship only the first and the leg reports its auth mode happily while answering anybody who asks.

That is not hypothetical. On 2026-08-13 the negative control for that leg answered without a credential, and a direct check confirmed /health, the agent card and the JSON-RPC invoke endpoint all returned 200 to an anonymous caller — on an agent that invokes a billable model. Every other signal in the project was green at the time, which is the entire argument for having negative controls at all.

Calling Them is Three Different Jobs

The framework difference exists on the client side too, and it decides what you are able to fix.

agent-framework A2AAgent — build from a URL, await .run(prompt), read .text. Two lines. Card resolution and transport are internal, which is ergonomic right up to the moment a server advertises a bad card.

a2a-sdk — resolve the card, mutate it, build a client, iterate typed chunks, close it. Verbose, and the only one low-level enough to work around the card defect above.

google-adk RemoteA2aAgent — a BaseAgent meant to live inside an agent tree. Using it as a plain client means standing up a Runner, a session service and a session, per request. The ADK stack assumes A2A is something an agent does, not something a program does.

Every client against every server, local and with no model in the path:

$ 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

Read that as an ordering and nothing more — single runs on loopback. And read it with the honest dependency in front of you: all three client stacks resolve to the same a2a-sdk wire implementation underneath, and two of the three servers share serving scaffolding.

Note- nine cells is a presentation, not nine independent experiments. Which is what makes the failures above interesting. Shared implementation on both ends should have made all of this trivial.

The Models Differ Where a Rubric Cannot See

Now hold the frameworks still and look at the other axis. Three models, chosen to be unmatched — the heterogeneity is the point, not a confound:

That last cell is my favourite example of a model choice that is not a preference. FoundryChatClient speaks the OpenAI Responses API. Passing store=False to keep anything from being stored server-side makes the framework request reasoning.encrypted_content, and gpt-4.1-mini rejects that outright — only a reasoning model accepts it.

The region is forced too: the Container App lives in westus2, which offers no Azure OpenAI models, so the call crosses to westus3. Two constraints that have nothing to do with writing quality decide both the model and the latency on that leg.

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

Four things fall out of that, and only one of them is about writing.

Availability moved more than eloquence did. Gemini answered 58% of the briefs it was invited to — the lowest of the three, on the one leg that never leaves its own cloud. The failure recorded against it is a Vertex 429, so quota is the documented cause rather than a proven one. Either way, a rate limit is a vendor difference no essay-scoring rubric will ever capture, and on this corpus it dominates.

The scorer changes which model looks good, and by a lot. The rubric puts Nova 1.32 points behind the panel's best; the model judge puts it 9.38 behind. Under the rubric no model dominates; under the model judge gpt-5-mini takes 87% and Nova takes none. So best-of-breed is currently a property of the scorer, not of the models — and the model judge here is Gemini, ranking the Gemini participant at 43% while putting Azure at 87%, which weakens the obvious vendor-bias objection without removing it.

Latency is a runtime fact before it is a model fact. The slowest leg is a reasoning model called across regions because of a storage flag. The fastest is a tiny model on the platform that also charges you a microVM start when you forget to pin a session.

What no scorer can move is whether a draft existed at all. The availability column is identical under both judges, which makes it the only column that does not wait on calibrating the rubric against human review.

Tool Parity in Availability is Not Tool Parity in Use

All three got the same tool, at the same time, with the same six-call budget. Use of it split by model and by prompt version:

Two ends of one finding. Nova had to be told, twice, and still skips a run in seven. Gemini sits on the ceiling in every single v3 run, which means the budget is now shaping the drafts I am comparing.

And the first model-backed run gave me the sharpest version of it:

azure  searches=2   evidence 0.0
gcp    searches=0   evidence 5.0
aws    searches=0   evidence 0.0

The model that scored full marks on evidence never searched. Five points of citation-shaped text with nothing behind it. The rubric counts the gesture, which I had written down as a known weakness before search existed and now had as a measured one.

The cause was upstream of the models: the shared instruction never told anyone to search. Fixing it took three versions, and v2 is a warning in the other direction — it said "one search for each specific figure", Gemini read that literally and spent 24 searches on a 300-word brief, which is enough to exhaust the project's Vertex quota on its own. v3 names the budget the tool enforces, so the model plans against the bound instead of being cut off by it.

Note- version the instruction like you version the rubric. Runs either side of a prompt change are answering different questions, and an audit that averages across one reports a prompt edit as a change in the models.

Almost Nothing on This Path Fails Loudly

The recurring shape, across all three clouds:

• An agent served llm mode with zero tools registered, because the tool connection failed as a WARNING, and answered /health with 200 the whole time.
• A provider quota error came back as a 31-word body, cleared the minimum word count, was stamped as a draft, scored 7.97 of 25, and sent back for a rewrite — and the run reported 3/3 clouds answering.
• An agent served the public internet for days behind a correctly configured federated credential, because enforcement was a second step.
• The control harness that eventually found that had five defects of its own, four of which produced false passes.
• The first deployed run had both remote legs return 200 and then ten words of refusal, because they were still running the previous project's agents from a week earlier.

Two habits came out of that and I would carry both to any mesh like this.

Type your failures. transport, protocol, timeout, authentication, provider. The one that earns its keep here is provider: a model that declines the topic is a provider outcome, and filing it as protocol turns "Bedrock refused" into "AgentCore broke A2A."

Let the agent report its own facts. Brain, model, degraded flag and search count are served by the agent, because only the agent knows them. My matrix used to print the mode from its own process — a different container once deployed — and duly reported brain=direct for a mesh of three model-backed agents.

So Why? Just Why? Why Build The Same Agent Three Times?

Because one agent tells you nothing about a framework.

Every finding in this article is a difference, and a difference needs two things to compare. The card defect needed a client that was not ADK. The empty reply needed a client written against a different vendor's server. The dropped header needed the same code running on a platform that does not drop it. The model results needed the prompt, the tool and the rubric held identical while the cloud moved.

Build it once and every one of those is either invisible or unattributable.

Troubleshooting Quick Reference

HTTP 200, task COMPLETED, and the reply is an empty string. The reply is in task.history, not artifacts — that is where Agent Framework's executor leaves it. Read every carrier the spec allows.

The draft arrives twice and the word count doubles. ADK returns it as an artifact and in history. Deduplicate: one reply carried in two envelopes is one reply.

A2A version '0.3' is not supported by this handler. AgentCore dropped the A2A-Version header. Assume the current version when the header is absent, and only when it is absent.

AttributeError: 'A2AClientError' object has no attribute 'status_code'. The ADK client dialled the card's bind address and could not connect. Advertise a PUBLIC_URL, or rewrite the interfaces after card resolution.

One leg costs about six seconds, and the slow leg moves between clients. A fresh AgentCore session id per call, each getting its own microVM. Pin the session id.

The leg reports federated auth and answers anonymous callers. Ingress enforcement is a separate deploy step from creating the credential. Run it, then probe that leg with no credential and confirm it is denied.

403 from inference although the role assignment looks right. The container is holding a managed-identity token minted before the grant. Restart the revision.

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

A model scores full marks for evidence with zero searches. Your scorer is counting citation-shaped text. Record the search count per draft and read it next to the score.

A quota error is recorded in the audit as a score. A provider error is not short, so a minimum word count will not catch it. Detect provider signatures before stamping a draft.

Discovery 403s while the invocation would have worked. The agent card sits behind the same authorization as the agent. Attach the credential to the client, not to the request.

Summary

The three frameworks do not reduce to one object, so share the prompt, the tool and the wire format and never try to share the agent. Assume the platform edits your request, treat the agent card as configuration rather than truth, and expect the biggest model difference to be availability rather than eloquence.

Twenty-four briefs is enough to compute a rate and not enough to trust one, and all of mine were technology surveys. I would not quote these numbers as a model comparison and I do not. What I would claim is the shape: the platform differences are structural and repeatable, the model differences are mostly about whether you get an answer at all, and every framework will hide a different one from you.

A2A did the thing it promised. Everything above is what is left over — and it will be different again for whoever wires the fourth cloud in, which is rather the point of writing it down.

The repo carries the three agents, the shared instruction and tool, the coordinator and judge, the 3×3 interop matrix, the negative controls and the deploy scripts. docs/INTEROP.md carries every finding above with the date it was measured, and docs/RUNBOOK.md lists which claims are measured and which are still open:

github.com/xbill9/multicloud-a2a-subagent