Algolia has actually a very decent synonyms support:

  * mono & multi-words synonyms,

  * typo-tolerance is compliant with synonyms,

  * matching synonyms are highlighted,

  * prefix search works also on synonyms (even on multi words).

A v2 of the synonyms API will be released in the next few days including:

  * dedicated API endpoints (leveraging existing synonyms but also Algolia's "alternative corrections" and "placeholders")

  * new edition UI (with search & filtering capabilities)

  * one/bi direction synonyms