Mighty Accounting API

The Mighty Accounting API lets you read and create accounting data in your Mighty organisation from your own code — or from an AI assistant such as Claude, using the built-in MCP server.

Everything is authenticated with an organisation API key that you create yourself in the Mighty app. A key works for one organisation only, and takes about a minute to set up.

What you can do

Connect an AI assistant and it gains twenty-six tools:

So you can ask things like "what is my VAT liability?", "which invoices are still unpaid?", "raise a draft invoice to Acme Ltd for 3 days of consultancy at £600 a day", "go through my uncategorised bank transactions, read the receipts and draft the categories for me to approve", "match the receipts in my Post Box to their bank payments", "claim mileage for my return trip from Leeds to York yesterday" or "show me this year's profit and loss against last year".

The bank tools never approve anything: the assistant can only save drafts, and VAT can only be entered on transactions dated while you were VAT-registered.

The Post Box tools follow the same rule. An expense the assistant creates from a document is left for you to check and finalise, and matching a document to a bank payment only attaches the receipt.

Mileage is different: Mighty has no draft mileage claim, so a claim the assistant records is posted to your accounts straight away, exactly as if you had added it on the Mileage Claims screen. The rate, the 10,000-mile threshold and any fuel VAT are worked out by Mighty. To remove a claim, delete it in the app.

The report tools are read-only, and return the same figures as the reports screens in the app. Ask for comparatives and you get the prior period alongside every line.

You can also call the REST endpoints directly — they are documented below this guide.

Step 1 — Get your API key

Click any screenshot below to open it full size.

1. Open your organisation settings

In Mighty, go to Company Settings, then the Other tab, and choose API Keys.

2. Create a key

Click Create Key.

This page also shows your Organisation ID. You do not need it for an AI assistant, but you do need it for the REST API — there is a Copy button next to it.

3. Give the key a name

Describe what the key is for, so you can tell your keys apart later and know what stops working if you revoke one. Something like Claude on my laptop is ideal.

4. Copy the key straight away

Your key is shown once. Copy it now and store it somewhere safe, such as a password manager.

If you lose it, you cannot look it up again — revoke it and create a new one.

Treat the key like a password. Anyone who has it can read your accounting data, create invoices and post mileage claims in your organisation. Do not email it, paste it into a chat message, or commit it to a code repository.

5. That is it

Your key is now listed as Active. Only the first 8 characters are ever shown again, which is enough to identify it. The list also shows when it was last used, so you can spot a key that nothing is using any more.

Step 2 — Connect your AI assistant

The MCP server lives at:

https://api.mightyaccounting.com/mcp

Send your key in an X-Api-Key header. If a tool can only send an Authorization header, that works too — Authorization: Bearer YOUR_KEY is accepted.

Not every AI product can attach an API key to a connection. Those that cannot sign in to Mighty with OAuth instead: the user signs in, chooses a company and selects Allow, and no key is involved. Each route is set out underneath.

Claude.ai and Claude Desktop

Add Mighty as a custom connector. It belongs to your Claude account, so the steps are the same on the website and in the desktop app.

1. Go to Customize → Connectors, click + Add, then Add custom connector.

2. Enter a name, such as Mighty, and this server URL, then click Continue:

   https://api.mightyaccounting.com/mcp
   

3. Under Authentication, choose No sign in.

4. Under Request headers, add a header named X-Api-Key with your key as the value.

5. Click Add.

In a chat, click + under the message box, then Connectors, and switch the connector on.

On Team and Enterprise plans an owner adds the connector under Organization settings → Connectors, and members then click Connect. Everyone who connects shares that one key, and so the one Mighty organisation.

Anthropic is still rolling Request headers out. If your dialog does not show it, your Claude account does not have it yet — use Claude Code below in the meantime.

There is a plain-language version of these steps, written for people who are not developers, in the Mighty Knowledge Base: How to connect Claude to Mighty.

Claude Code

One command, then restart Claude Code:

claude mcp add --transport http --scope user mighty https://api.mightyaccounting.com/mcp --header "X-Api-Key: YOUR_KEY"

--scope user makes the connection available in every project and keeps the key in your own settings. Do not use --scope project — that writes the key into a .mcp.json file in the project folder, where it can be committed and shared.

Check it worked by running /mcp inside Claude Code — you should see mighty connected, with twenty-six tools.

Claude Desktop, using a config file

Use the custom connector above where you can. If you would rather keep the connection in Claude Desktop's config file, a small helper called mcp-remote reaches the remote server and attaches your key. You will need Node.js installed.

Open Settings → Developer → Edit Config, and add a mighty entry:

{
  "mcpServers": {
    "mighty": {
      "command": "npx",
      "args": [
        "-y",
        "mcp-remote",
        "https://api.mightyaccounting.com/mcp",
        "--header",
        "X-Api-Key:${MIGHTY_API_KEY}"
      ],
      "env": {
        "MIGHTY_API_KEY": "YOUR_KEY"
      }
    }
  }
}

Save the file and restart Claude Desktop.

Write X-Api-Key:${MIGHTY_API_KEY} with no space after the colon, exactly as above. Claude Desktop on Windows mangles arguments that contain spaces, which silently breaks the header.

Cursor, VS Code, Windsurf and other MCP clients

Most other MCP clients take a server URL and a set of headers. Where the file lives differs between them — check your client's documentation — but the entry is:

{
  "mcpServers": {
    "mighty": {
      "url": "https://api.mightyaccounting.com/mcp",
      "headers": {
        "X-Api-Key": "YOUR_KEY"
      }
    }
  }
}

Your own code, using the OpenAI API

The Responses API can call a remote MCP server directly:

{
  "model": "gpt-5",
  "input": "What is my current VAT liability?",
  "tools": [
    {
      "type": "mcp",
      "server_label": "mighty",
      "server_description": "Mighty Accounting — dashboard figures, reports, sales invoices, bank categorisation and mileage claims.",
      "server_url": "https://api.mightyaccounting.com/mcp",
      "authorization": "YOUR_KEY",
      "require_approval": "never"
    }
  ]
}

OpenAI does not store the authorization value, so send it on every request.

ChatGPT and Gemini (OAuth sign-in)

The ChatGPT and Gemini apps cannot send an API key, so they sign in instead. Add a custom MCP server with the address https://api.mightyaccounting.com/mcp and no credentials. The app discovers the rest:

1. An unauthenticated request to /mcp returns 401 with WWW-Authenticate: Bearer resource_metadata="https://api.mightyaccounting.com/.well-known/oauth-protected-resource/mcp".
2. That document names this API as the authorization server; its metadata is at https://api.mightyaccounting.com/.well-known/oauth-authorization-server.
3. The app registers itself at https://api.mightyaccounting.com/connect/register (OAuth Dynamic Client Registration). Registration is open only for redirect addresses belonging to supported AI assistants.
4. The user signs in to Mighty, chooses one company and selects Allow. The app then exchanges the authorization code (PKCE with S256 is required) for an access token and a refresh token.

A connection is for one person and one company, like an API key. It appears on the API Keys page under Connected apps, where anyone who manages the company can revoke it; revoking takes effect on the app's next request. Step-by-step guides: How to connect ChatGPT to Mighty and How to connect Gemini to Mighty.

Using the REST API directly

Every REST route includes your organisation ID, which you will find on the API Keys page:

curl "https://api.mightyaccounting.com/api/v1/YOUR_ORGANISATION_ID/sales-invoices?page=1&pageSize=20" \
  -H "X-Api-Key: YOUR_KEY"

The organisation ID in the URL must be the one the key belongs to. Pointing a key at another organisation returns 403 — a key can never reach data outside its own organisation.

List endpoints are paged (page, pageSize, default 20, maximum 100) and return the rows alongside totalCount, totalPages and hasNextPage. Full details for every endpoint are below.

Reports

The three financial statements come back as flat rows, ready to drop into a spreadsheet:

curl "https://api.mightyaccounting.com/api/v1/YOUR_ORGANISATION_ID/reports/trial-balance?comparative=true" \
  -H "X-Api-Key: YOUR_KEY"

Swap trial-balance for profit-and-loss or balance-sheet. Each line carries accountName, accountCode, debit, credit and balance, plus comparativeDebit, comparativeCredit and comparativeBalance when you pass comparative=true.

Report on a whole accounting period with ?accountingPeriodId=... — list them at /api/v1/YOUR_ORGANISATION_ID/accounting-periods — or on any window with ?from=2025-04-01&to=2026-03-31. Pass neither and you get the most recent accounting period. Comparatives are the prior accounting period, or the same window a year earlier when you asked for a date range.

Reports are limited to 20 requests a minute rather than 100, because each one may refresh the figures first. Pass rebuildCache=false for a faster read of the figures as they were last calculated, at the risk of them trailing the app.

Limits and troubleshooting

Requests are limited to 100 per minute per key.

A few specifics worth knowing:

• list_sales_invoices takes fromDate and toDate as ordinary 2026-04-30 dates, as do the report endpoints and tools.
• On a report, balance is signed with debits positive, so income, liabilities and reserves come back negative. debit and credit are both positive, and only one of them is filled per line.
• If a trial balance comes back with anything in warnings, the accounts do not balance — treat the figures as unreliable until that is resolved in the app.
• Invoices created through the API are drafts unless you explicitly ask for them to be raised, and bank categorisations are always drafts. Mileage claims are the exception: they have no draft state and are posted as soon as they are recorded.
• Records created this way are tagged with the assistant that created them, and are listed under AI Created Records in the Mighty admin area.
• Some tools on this server are internal to Mighty and need a staff-issued key; they are not listed to organisation keys and are not part of the public API.

Revoking a key

On the API Keys page, click Revoke next to the key. It stops working immediately, and anything using it loses access straight away. Revoking cannot be undone — create a new key instead.

Revoke a key as soon as you stop using it, or if it has been shared or exposed anywhere.