SendLettersOnline UK MCP

Create and pay for a letter order

create_letter_order

Creates a letter order and immediately debits the customer's balance to pay for it - this spends real money and cannot be undone. Call it WITHOUT confirmed:true first: it will only return the price, without charging or creating anything. Only after the price has been shown to the human and they've agreed to it, call it again with confirmed: true and expectedTotalPence set to the exact totalPence you were quoted. If the recomputed price no longer matches expectedTotalPence, the order is rejected so you cannot accidentally confirm a stale price.

Input Schema

TableJSON Schema

Name	Required	Description	Default
`pages`	No	Number of content pages (excludes the address page).
`colour`	No	Print in colour instead of black & white.
`content`	No	Plain-text letter body. Required unless contentUploadPath is supplied.
`confirmed`	No	Set to true only after the customer has explicitly agreed to the previewed price.
`senderName`	No
`doubleSided`	No	Print double-sided (duplex).
`senderLine1`	No
`serviceClass`	Yes	Royal Mail service to send with. Domestic: SECOND_CLASS, FIRST_CLASS, SECOND_CLASS_SIGNED, SIGNED_FOR, SPECIAL_DELIVERY, TRACKED_48, TRACKED_24. International: INTERNATIONAL_STANDARD, INTERNATIONAL_TRACKED, INTERNATIONAL_TRACKED_SIGNED.
`recipientCity`	Yes
`recipientName`	Yes
`recipientLine1`	Yes
`recipientLine2`	No
`senderPostcode`	No
`recipientCountry`	No	ISO 3166-1 alpha-2 country code, e.g. GB, US, FR.	GB
`contentUploadPath`	No	Path returned by the customer's own upload flow (POST /api/v1/uploads/request-url) for a pre-uploaded PDF/DOCX. The MCP server cannot upload files itself.
`recipientPostcode`	Yes
`expectedTotalPence`	No	The totalPence you were quoted when confirmed is true. Required to actually place the order.

Schema Changelog

Changes observed during successful MCP inspections.

First observedOct 4, 2026

TDQS

A4.9/5.0

Behavior5/5

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

With no annotations provided, the description carries the full burden and does so: it warns this spends real money, cannot be undone, that the preview call charges and creates nothing, and that a mismatched expectedTotalPence causes rejection. This is exactly the safety context an agent needs for an irreversible financial mutation.

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

Conciseness5/5

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

The irreversible-charge warning is front-loaded, followed by the two-step call sequence in logical order. Every sentence adds a distinct operational constraint with no filler.

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

Completeness5/5

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

For a 17-param irreversible mutation tool with no annotations and no output schema, the description supplies the critical missing context: the danger profile, the preview-then-confirm workflow, and the anti-stale-price guard. Nothing essential for correct invocation is absent.

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 description coverage is only 53% across 17 parameters, so the description must add value, and it does for the two highest-stakes params: confirmed (only after human agreement) and expectedTotalPence (set to the exact quoted total, required to place the order). It does not explain the recipient/sender/serviceClass fields, but the enum and per-field schema descriptions cover much of that.

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?

States a specific verb and resource (creates a letter order) plus the consequential side effect (immediately debits the customer's balance). It also distinguishes itself from the sibling preview_letter_price by explaining that calling without confirmed:true acts as a price-only preview.

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

Usage Guidelines5/5

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

Gives an explicit two-call protocol: first without confirmed:true to fetch the price, then with confirmed:true and expectedTotalPence after the human agrees. It states the precondition (human agreement) and the rejection condition (stale price), leaving nothing to inference.

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

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