Letter IRL

Set the arrival date

set_arrival_date

Idempotent

Set, change or clear the date a previewed letter or postcard should arrive by, without previewing it again. Give the draftId from the preview and arriveBy as YYYY-MM-DD; leave arriveBy out to clear the date, so the mail goes to the printer as soon as it is sent. With a date set, once it is sent, Letter IRL holds the mail and sends it to the printer in time; USPS does not guarantee First-Class dates.

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "draftId": "<draftId>"
    },
    "name": "set_arrival_date"
  }
}

Input Schema

Name	Required	Description	Default
`draftId`	Yes	The draftId from a letter or postcard preview
`arriveBy`	No	The date the mail should arrive by, YYYY-MM-DD. Leave it out to clear the date, so the mail goes to the printer as soon as it is sent.

Output Schema

Name	Required	Description
`draftId`	Yes
`message`	Yes
`schedule`	No	The draft's dates now; absent when it mails as soon as it is sent
`contentVersion`	No	The version of this mail as it is now: send it with the send, so what is mailed is what the person saw
`deliveryEstimate`	Yes
`contentVersionBefore`	No	The version of this mail the change was made to: for the preview card, which takes contentVersion only when this is the version it shows

Schema Changelog

Changes observed during successful MCP inspections.

First observedNov 7, 2026

TDQS

A4.3/5.0

Behavior4/5

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

The annotations already declare non-read-only, idempotent, non-destructive behavior. The description adds important operational details beyond the annotations: clearing the date causes immediate printer routing once sent, while setting a date makes Letter IRL hold the mail appropriately, and USPS does not guarantee First-Class dates.

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 description is compact and front-loaded, moving from the action and required draftId to the optional arriveBy clearing behavior and then to the downstream mailing implications. Every sentence contributes operational guidance without wasted wording.

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?

Given the annotations and output schema, the description is complete enough for correct invocation: it covers the required draftId, optional arriveBy, clearing behavior, and what happens after sending when a date is or is not set.

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

Parameters3/5

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

Schema coverage is 100%, and the schema already explains draftId and arriveBy, including the YYYY-MM-DD format and clearing behavior. The description reinforces these semantics but adds little parameter-level meaning beyond the schema itself.

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 description states a specific verb and resource: set, change, or clear the arrival date for a previewed letter or postcard. It also distinguishes this from previewing again, so an agent can tell it is a post-preview draft mutation rather than a new quote/preview.

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?

The description clearly explains when to use it: with a draftId from a preview, and optionally arriveBy to set or omit the date to clear it. It does not explicitly compare against sibling tools such as cancel_scheduled_mail, but the operational context is clear.

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

All Letter IRL tools Inspect

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