The engineering behind persona chatbots: character state in a system prompt, conversation memory, summarisation, drift across long sessions, and cost per turn.https://aigirlfriend.dev/https://aigirlfriend.dev/blogs/the-cost-of-a-turn/https://aigirlfriend.dev/blogs/the-cost-of-a-turn/Costs scale with usage and with history length, so your heaviest users get more expensive the longer they stay. That shapes every state decision you make.Sun, 26 Jul 2026 00:00:00 GMTcostturnsstatehttps://aigirlfriend.dev/blogs/why-characters-drift/https://aigirlfriend.dev/blogs/why-characters-drift/A fixed definition against growing history, and the history includes the model's own output. Drift is a proportion problem that reinforces itself.Sat, 25 Jul 2026 00:00:00 GMTdriftconsistencystatehttps://aigirlfriend.dev/blogs/summarisation-and-what-it-loses/https://aigirlfriend.dev/blogs/summarisation-and-what-it-loses/Compression is not neutral. It destroys specifics first and decays generationally, so the oldest history degrades fastest — what a relationship rests on.Fri, 24 Jul 2026 00:00:00 GMTsummarisationmemorystatehttps://aigirlfriend.dev/blogs/what-the-model-remembers-between-turns/https://aigirlfriend.dev/blogs/what-the-model-remembers-between-turns/Nothing. Continuity is entirely something you build, and the four ways of building it fail in four different directions.Thu, 23 Jul 2026 00:00:00 GMTmemorystateturnshttps://aigirlfriend.dev/blogs/a-persona-is-state-not-personality/https://aigirlfriend.dev/blogs/a-persona-is-state-not-personality/Four categories of state get stored as one blob, and that is why a character cannot be versioned, corrected, or debugged when it goes wrong.Wed, 22 Jul 2026 00:00:00 GMTstateconsistencyturns