A2A where it earns it. MCP everywhere else.

I build Blossom, a system that reads the documents involved in a French flat sale: several agents extract them, cross-check them against each other, and flag what is missing or contradictory. Today they coordinate through a shared database.

Deciding whether to adopt A2A, I couldn't answer the question in the abstract. So I went through every exchange and asked, one at a time, what that one actually needs. Eight exchanges. Five are plain MCP tool calls. Two need A2A. One is an email, because there's a person at the other end.

This is the walkthrough of that exercise.

→ Open it

🇫🇷 La page est bilingue et suit la langue du navigateur. Un sélecteur FR/EN est en haut à droite.

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The argument

Most writing about the Agent2Agent protocol argues for adopting it. This argues for bounding it.

Five agents do the work: Karl turns a listing into a draft, Extract turns PDFs into data, Coherence confronts the sources with each other, Iris carries the questions to the estate agent, and Arsène handles the mail in and out. Here is every exchange between them, and the protocol each one needs.

#	Exchange	Protocol	Why this one
1	Karl → file	MCP	Three seconds, one answer, nothing to wait for. Karl is a tool, not an agent — no goal, no memory
2	Extract → file	MCP	A long call, but one that comes back. It can report progress; what it cannot do is stop and ask a question
3	file → Coherence	MCP	A data cross-check: no counterpart, no waiting, no decision
4	Coherence → Iris → human	A2A	This is where MCP breaks. See below
5	human → Iris	A2A	The answer lands on the same task, open since earlier
6	Iris → Arsène → third party	A2A + email	Arsène is going to wait for days. An MCP call cannot stay open that long
7	third party → Arsène → Extract	email + MCP	An ordinary tool call — carrying a task id that is three days old
8	Extract → file → Coherence	MCP + A2A	Back to normal. The last A2A message is the one that closes the task

Where MCP breaks

At step 4 the file contradicts itself: the listing says 71 m², the surveyor's certificate says 68.4 m². And a document that should be there is missing.

A tool call answers or it fails. It cannot say "I'm stopping, I need you, come back to me." The A2A task moves to input-required and stays alive while the human thinks. That is the whole reason A2A is in this system.

The moment worth the page

Step 7. The third party replies with the missing PDF three days later. Arsène pushes it into the pipeline through the same door as the first twelve documents:

{
  "method": "tools/call",
  "params": {
    "name": "extraction_documents",
    "arguments": { "files": ["pv-ag-2023.pdf"] },
    "_meta": {
      "a2a/taskId": "task-9d02",     // open for 3 days
      "a2a/contextId": "ctx-4e77"
    }
  }
}

Same tool name, same arguments as step 2. Pure MCP. What changed is what it carries: a task id that survived three days, a round trip to a third party, and a human decision.

MCP carries. A2A remembers. Each does exactly its job, and nothing is replayed.

Two rules written into the engine

"resolution": "human"     // no agent picks the source of truth
"authorised_by": "user"   // without this field, Arsène does not send

Neither is a technical limit. When two documents disagree, the professional carries the liability — so the arbitration is theirs, and it is recorded in the task with who decided and when. And no message leaves the system on an agent's own initiative.

Status — what this is, and what it is not

This is a design study, not a deployed system. The agents are real and the roles are real; today they coordinate through a shared Supabase database. This page shows what the same choreography would look like split between MCP and A2A, and argues where the split should fall.

The file, the parties, the addresses and the correspondence are fictional. Every domain used is under .example, reserved by RFC 2606 for exactly this.

Context

Loi Alur is the French statute requiring a seller to hand over a defined set of co-ownership documents before a flat can be sold. The documents disagree with each other more often than not — which is what makes the cross-check, and the arbitration, the interesting part of the problem.

The two protocols: Model Context Protocol · Agent2Agent

Running it locally

open index.html

One file, no build, no dependencies, no network calls. Keyboard: ← → to step, space to play or pause. Honours prefers-reduced-motion and the system colour scheme.

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© 2026 Garance Husson. All rights reserved.