YNAB MCP Server

Assign money

assign_money

Destructive

Assign an amount to a YNAB category from Ready to Assign, adding to its current assigned total; use a negative amount to return money to Ready to Assign.

Instructions

Assign more money to a category from Ready to Assign, or take some back.

This adds to what is already assigned: assigning 100 to a category with 250 assigned leaves 350 assigned. A negative amount returns money to Ready to Assign. Calling this twice assigns twice, so do not retry it blindly after an error; check the category with get_month_category first.

For categories with NEED targets that refill each month, the full target is usually assigned every month, whatever is available.

Input Schema

Name	Required	Description	Default
`month`	No	Which plan month (default: "current"): • "current": This calendar month • "last": The previous calendar month • "next": The next calendar month • A date: The month containing that date, such as 2024-03-01	current
`amount`	Yes	Amount in currency units to add to the assigned amount. Negative takes money back to Ready to Assign (required)
`category_id`	Yes	Category to assign to (required)

Output Schema

Name	Required	Description	Default
`changes`	Yes	Each category that changed, in the order they changed

Schema Changelog

Changes observed during successful MCP inspections.

First observedv0.1.0Oct 26, 2026

TDQS

A4.3/5.0

Behavior4/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Corroborates and enriches the annotations (idempotentHint=false, destructiveHint=true) with a worked arithmetic example (100 onto 250 leaves 350) and a clear statement that repeated calls compound. It does not discuss permissions, reversibility of a mistaken assignment, or currency/precision handling, so it stops short of full disclosure.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The action and direction are front-loaded in sentence one, and the retry warning is well placed. The closing sentence about NEED targets is the weakest link – it is vague ('usually assigned every month, whatever is available') and adds bulk without a crisp rule.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness4/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

With annotations carrying the safety profile and an output schema present, the description covers the mutation's additive nature and its retry hazard, which is what an agent needs before calling it. It omits any mention of how it relates to set_assigned/move_money or what error conditions look like beyond the retry warning.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters4/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100%, so the baseline is 3; the description earns an extra point by clarifying delta semantics with a concrete stacked-assignment example and by pairing negative amounts with the Ready to Assign return path. The month parameter is left entirely to the schema.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The first sentence names a specific verb (assign) and resource (money to a category from Ready to Assign) and states both directions of the operation. 'This adds to what is already assigned' implicitly separates it from the set-based sibling set_assigned without needing the schema.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

Gives concrete operational guidance: negative amounts reverse the flow, and it explicitly warns against blind retries after an error, pointing to get_month_category as the verification step. It does not explicitly compare against set_assigned or move_money, so the agent must infer the choice from the additive wording.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.