MCP Tool
An MCP tool is a single named, narrowly scoped action — such as post_journal_entry or reconcile_account — that an MCP client can call and an MCP server executes.
In short
The atomic unit of the whole connection. Each of BalanceMCP's nineteen tools does exactly one thing its name describes — categorize_transactions categorizes, reverse_entry reverses — with no hidden broader capability sitting behind any of them.
Also called: tool call
An MCP tool is the smallest unit of capability in the whole protocol — one specifically named action, with its own defined inputs and its own defined behavior, that an AI client can call and an MCP server actually executes. BalanceMCP exposes nineteen of them: list_books, create_book, manage_accounts, import_statement, list_transactions, categorize_transactions, create_rule, post_journal_entry, reverse_entry, reconcile_account, lock_period, trial_balance, profit_and_loss, balance_sheet, general_ledger, cash_position, find_anomalies, spending_summary, and export_book.
A tool does exactly what its name says and nothing more. Reconcile_account reconciles one account against a statement; it doesn't also post entries or lock periods. Post_journal_entry posts a manual entry; it deliberately has no parameter for creating a reversal, since that's reverse_entry's job specifically and no one else's.
Most tools that write anything follow the same preview-then-confirm shape: called without confirming, a tool shows exactly what it would do; called with confirmation, it actually does it. That consistency is deliberate — a model calling nineteen different tools benefits from all of them behaving the same way at the seams, rather than each inventing its own convention. The one exception is create_book: there is no preceding ledger state a preview could check, so it creates and seeds the book on the first call.
Tools are also where BalanceMCP's honesty commitments live in practice — a tool's description and its error messages are read by a model that acts on them, so a tool is never allowed to claim more than the underlying code can actually prove, the same standard the rest of the ledger is held to.
What people get wrong
- Assuming a tool does more than its specific, named job — each of the nineteen tools is scoped narrowly, with no hidden broader capability.
- Expecting post_journal_entry to also handle reversals — that's deliberately reverse_entry's job alone, with no reversal parameter on the posting tool.
- Assuming tools behave inconsistently with each other — the preview-then-confirm pattern is deliberately uniform across every tool that writes to the ledger, with the sole exception of create_book, which has no preceding ledger state to preview.
Common questions
- How many tools does BalanceMCP expose?
- Nineteen, each a narrowly scoped, specifically named action — from importing a statement to running a balance sheet.
- Do all the tools that write to the ledger behave the same way?
- Yes — the ones that change anything follow the same preview-then-confirm pattern, so a model calling any of them can rely on consistent behavior at the seams.
Machine-readable: /api/knowledge/concept:mcp-tool