MCP vs A2A: Open Standards That Connect Your Accounting

Why do open standards matter for a small business's books?

A small business's money lives in many systems: the bank, the card processor, the till, suppliers' portals, the accountant's software, the tax portal, and now AI assistants. For thirty years the connection between them has been a person retyping or importing a file.

Open standards are publicly documented, freely implementable ways for systems to exchange data. A business whose data moves in a standard format can change providers, give its accountant access without sharing passwords, and connect new tools without waiting for a custom integration. A business locked into one vendor's private formats cannot.

What are the four layers of interoperability?

1. Documents: e-invoice standards

The most mature layer is the invoice itself. In Europe, the semantic standard EN 16931 defines what an electronic invoice contains, and the EU's ViDA directive builds on it for cross-border e-invoicing from July 2030 (see e-invoicing mandates by country). Networks such as Peppol carry these invoices between businesses, with each business reachable through an access point rather than an email address. Other regions use national formats and platforms.

What this gives a small business: a supplier's invoice that arrives as data, readable by any compliant system.

2. Money: open banking

Open banking rules require banks to let authorised third parties access account data, and sometimes initiate payments, with the customer's consent. In the EU, a provisional agreement on a new payments package, reached in November 2025, aims to reduce obstacles to that access and give customers a dashboard to view and withdraw the permissions they have granted. Other countries have their own regimes, at very different stages.

What this gives a small business: bank transactions that reach the books through a regulated, revocable permission, not a shared password.

3. Systems: documented APIs

An API is the set of requests a system accepts from other software. A documented API, often described in the OpenAPI format, shows exactly what can be asked, what comes back and what each error means. For accounting, three properties matter most:

• Scoped, revocable access. A tool gets a token for specific permissions, not your password.
• Clear errors. A refusal says why, with a stable code that software can act on.
• Idempotency. A request sent twice because of a network glitch is recognised as a repeat, so a sale or payment is not posted twice.

4. AI assistants: MCP and A2A

The newest layer connects AI agents to business systems and to each other.

What is the difference between MCP and A2A?

Model Context Protocol (MCP) defines how an AI application connects to tools and data: for example, how an assistant asks an accounting system for a list of overdue invoices or prepares a draft purchase. Anthropic introduced it in 2024 and in December 2025 donated it to the Agentic AI Foundation, a fund under the Linux Foundation co-founded with Block and OpenAI and supported by other large technology companies. Its specification is published in dated revisions; the 2025-11-25 revision extended its OAuth 2.1 authorisation for remote connections, including incremental consent for additional permissions.

Agent2Agent (A2A), introduced by Google in 2025 and contributed to the Linux Foundation, defines how independent agents talk to each other: discover what another agent can do, send it a task, and follow the task as it progresses. Its specification has reached version 1.0, which includes signed "Agent Cards", the machine-readable description of what an agent offers, so that the card's contents can be checked for tampering. In August 2026 A2A was reported to be moving under the same Agentic AI Foundation as MCP.

In plain terms: MCP is how your assistant uses your systems; A2A is how your agent deals with someone else's.

Where are the limits of open standards?

Open standards solve the plumbing. They do not decide who is allowed to do what.

• Maturity varies. E-invoice standards and open banking are regulated and well established in many countries. MCP and A2A are young, revised frequently and implemented unevenly. Use early integrations, but expect change.
• A signature is not an identity. A signed Agent Card proves the card has not been altered since signing. It does not, by itself, prove the signer is the business it claims to be.
• Connections are doors. Each MCP or A2A connection is a new way in. The safeguards for AI agents in the books apply: narrow permissions, drafts rather than posting, a visible identity in the audit trail, and one-step revocation.
• Standards do not fix data. A wrong tax number on an e-invoice is still wrong, just faster.

A worked example: the accountant's assistant and the supplier's agent

A phone and electronics shop works with an external accountant who uses her own AI assistant across a dozen clients.

Without open standards, every month the owner emails a trial balance spreadsheet that is already out of date, and the accountant emails back questions.

With MCP, the owner lets the accountant's assistant connect to the shop's accounting system (in systems that tie agents to people, by giving the accountant's role the right to connect one), with read permission on the ledger and reports, and permission to prepare journal drafts, never to post them. The assistant pulls the trial balance when needed and drafts the month's electricity accrual for the owner to approve. When the engagement ends, the owner revokes the connection in one step and the audit trail shows everything the assistant did.

With A2A, the shop's distributor runs an agent that sends price updates and order confirmations. The shop's agent receives a confirmation for 20 handsets at 410 each and prepares a draft purchase. The task stays "waiting for approval" until the owner decides; when she approves, the distributor's agent is told. Neither agent moves money on its own.

And if the shop changes accounting systems? With standard invoices, regulated bank access and a documented API, the move is a migration project, not a re-typing project.

How this looks in GrowMore

GrowMore's MCP server offers 44 documented tools to Claude, ChatGPT (on its business plans), Gemini or any MCP agent: find, read, draft nine kinds of document, and preview a posting before anything is written. Since October 2026 GrowMore also answers other agents over A2A version 1.0, with a signed Agent Card: each skill is the MCP tool of the same name, under the same permissions, and a drafting task stays open until a person approves or rejects it; free-text tasks come in a later phase. An agent connects through a person whose role allows it: the owner by default, or a custom role the owner gives that right. A request that creates something carries an idempotency key, and a refusal carries a stable code. GrowMore does not connect to e-invoicing networks or import bank feeds. The AI page has the details.

What should you ask before you commit your books to a system?

1. Can I export all my data, including history and attachments, in a documented format?
2. Is there a documented API, and can I give a tool scoped, revocable access without sharing a password?
3. Which e-invoice formats and networks do you support in my country?
4. Do you support MCP or A2A, and if so, can an agent be limited to reading and drafting?
5. Do agent actions appear in the audit trail under their own identity?
6. Are repeated requests recognised so nothing is posted twice?

What to do this month

• Make a list of every system that holds your financial data and how data moves between them today.
• Identify one manual export-and-import you do every month and ask whether a standard connection exists.
• Ask your accounting provider questions 1, 2 and 5 above.
• Review the permissions of every connected app, bank link and AI assistant; revoke what you do not use.
• Agree with your accountant how they will access your books: a scoped connection, not your password.

Frequently asked questions

What is the difference between MCP and A2A?

MCP defines how an AI application connects to tools and data, such as your assistant reading your accounting system. A2A defines how independent agents find each other, send each other tasks and follow those tasks, such as your agent dealing with a supplier's agent.

What is an MCP server?

A program that offers a system's tools to AI assistants over the Model Context Protocol: each tool with a name, a description and the inputs it accepts. A remote MCP server that asks for sign-in uses OAuth 2.1, so a person grants access without sharing a password.

Sources

1. Anthropic, 'Donating the Model Context Protocol and establishing the Agentic AI Foundation' (9 December 2025). https://www.anthropic.com/news/donating-the-model-context-protocol-and-establishing-of-the-agentic-ai-foundation
2. Model Context Protocol specification, revision 2025-11-25, Key Changes (authorization discovery, incremental scope consent, client registration). https://modelcontextprotocol.io/specification/2025-11-25/changelog
3. Model Context Protocol specification, Authorization (OAuth 2.1 for HTTP transports). https://modelcontextprotocol.io/specification/2025-06-18/basic/authorization
4. A2A Protocol specification (version 1.0.0; Agent Card signing in section 8.4). https://a2a-protocol.org/latest/specification/
5. Axios, 'Google-backed agentic A2A protocol gets a new home' (17 August 2026). https://axios.com/2026/08/17/a2a-agentic-ai-foundation-open-ai-standards
6. SD Times, 'Google's Agent2Agent protocol finds new home at the Linux Foundation'. https://sdtimes.com/ai/googles-agent2agent-protocol-finds-new-home-at-the-linux-foundation/
7. Council Directive (EU) 2025/516 (ViDA), e-invoicing based on the European standard EN 16931. https://eur-lex.europa.eu/eli/dir/2025/516/oj/eng
8. OpenAPI Initiative, OpenAPI Specification. https://spec.openapis.org/oas/latest.html
9. OpenPeppol, the Peppol network. https://peppol.org/
10. Taylor Wessing, PSD3 and PSR provisional agreement (open banking access and permission dashboard). https://www.taylorwessing.com/en/insights-and-events/insights/2025/11/eu-lawmakers-strike-a-deal-on-payments-reforms