postalref

USPS codes and terms

An unofficial reference for USPS package barcodes, service type codes and mailing terms.

Service type code 055

Priority Mail: USPS Tracking

Mail class
PMPriority Mail
Banner text
USPS TRACKING #
Extra service
920USPS Tracking
In a tracking number
9205591234567812345674The Mailer ID and serial number are examples.
3 sources
  1. 1Service Type Code List, Appendix I, Effective June 24, 2026 · Sheet STC List, row 23USPS original
  2. 2Publication 199, Version 35, July 2, 2026 · Table 8, page 461USPS original
  3. 3Publication 199, Version 35, July 2, 2026 · Table 7, page 460USPS original

History Unchanged in all 8 editions since July 9, 2023, the oldest on file.

Use it from your agent

claude mcp add --transport http postalref https://postalref.com/mcp

Prompts to try

  • What does service type code 055 mean?

  • What does 92055 at the start of a tracking number mean?

  • Which service type codes did the June 24, 2026 list add?

  • What is a Mailer ID?

Questions

Is this an official USPS site?

No. It is an unofficial reference, not affiliated with or endorsed by the United States Postal Service. Every page links the USPS documents it cites, and those documents are the authority.

What can I search?

You can search the 342 service type codes in the USPS list effective June 24, 2026, by number or by name. You can also search 2835 terms for the package barcode and its label.

How current is it?

Every page shows the revision and effective date of its sources. The service type codes come from the list effective June 24, 2026, and the barcode rules from Publication 199, Version 35, July 2, 2026.

Where do the facts come from?

From USPS documents on postalpro.usps.com: the IMpb specification, Publication 199 and the Service Type Code List, Appendix I. Each fact names its document, revision and page, and a citation that doesn’t resolve fails the build.

What happens when USPS changes a document?

Each new revision is fetched and compared with the last, and the changes page lists every value that moved. The revision itself publishes on its own. A change to what a code, rule or term means waits for review, because a wrong meaning does more harm than a stale one.

Can my agent use it?

Yes. Copy the prompt into a chat, or add https://postalref.com/mcp to your agent as an MCP server. Its tools only read. Add .md or .json to any page’s address for that page as Markdown or JSON.

Does it cost anything?

No. The first version has no accounts, keys or paid plans, so pages, files and the MCP server are open to anyone.

Does a valid check digit mean the label is genuine?

No. The check digit only catches mistyped digits. Anyone can compute it with the method in the IMpb specification, so it says nothing about whether postage was paid.

Which numbers can it decode?

Package barcodes of 22, 26, 30 or 34 digits whose tracking number starts with 92, 93, 94 or 95. Letters and flats carry a different barcode, which this decoder doesn’t read.

Why is the barcode longer than the printed number?

The barcode starts with 420 and the destination ZIP Code, and the label doesn’t print either one. The tracking number under the bars is everything after the ZIP Code.