I just found it: https://missiveapp.com/help/api-documentation
Is planned any further development of API system?
Bravo, @Etienne Lemay. This would provide some fairly straightforward integration points with Zapier and the like. Are there some plans for this? Would you like some assistance creating the basic actions for Zapier? Although there might not be any "outbound" integrations at first, at the very least, you could open up the api for "consumption" of "Zaps" by triggering emails to be created, etc.
You can now get an API token in the API section of the preferences.
The API is to push new content into Missive. Using Missive to do actions on third-party services will be part of what will most likely be called plugins. Plugins development has not been started yet, but it is on our roadmap.
Any API suggestions are more than welcome, feel free to create specific posts about it.
How about a way to add an attachment to a draft? That could be a powerful way to add generated and/or standard documents to e-mails (although if you only need the latter a Response could make do).