A single-roundtrip server-based spell checking service is provided. A client provides a composition to the server to be spell checked. The server performs a spell check on the composition and flags the words that are determined to be errors. The server may provide suggested corrections for the flagged words in the composition depending on the error. The suggested corrections are compressed before they are sent to the client. While a word may be misspelled several times throughout the composition, the suggested corrections for the word are only sent one time. Spell check options may also be set to help control the spell check operation.