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/migrating-the-shape-of-state/https://aigirlfriend.dev/blogs/migrating-the-shape-of-state/A definition can be rewritten because it was authored. Accumulated state cannot — it is the only copy, so changing its shape is a migration.Fri, 31 Jul 2026 00:00:00 GMTstatememorycosthttps://aigirlfriend.dev/blogs/state-that-did-not-arrive/https://aigirlfriend.dev/blogs/state-that-did-not-arrive/One reply in a thousand is amnesiac because a piece of state was missing at assembly. Partial state fails silently and looks like a successful turn.Fri, 31 Jul 2026 00:00:00 GMTstatememoryturnshttps://aigirlfriend.dev/blogs/two-sessions-one-record/https://aigirlfriend.dev/blogs/two-sessions-one-record/When two conversations write the same relationship state, a blob-shaped record loses one of them. Per-fact writes narrow the conflict but do not remove it.Thu, 30 Jul 2026 00:00:00 GMTstatememoryconsistencyhttps://aigirlfriend.dev/blogs/which-definition-produced-a-reply/https://aigirlfriend.dev/blogs/which-definition-produced-a-reply/A transcript records the output and none of the inputs. Without a per-turn state fingerprint, every drift report is unattributable and irreproducible.Thu, 30 Jul 2026 00:00:00 GMTconsistencydriftstatehttps://aigirlfriend.dev/blogs/first-turn-has-no-history/https://aigirlfriend.dev/blogs/first-turn-has-no-history/At turn one the definition is the entire input, so the character is at maximum expression. The demo is the ceiling, and cold start has its own failures.Wed, 29 Jul 2026 00:00:00 GMTstateconsistencyturnshttps://aigirlfriend.dev/blogs/model-change-established-character/https://aigirlfriend.dev/blogs/model-change-established-character/The definition is not a specification, it is text interpreted by one model. Change the model and the same words produce a different character, unevenly.Tue, 28 Jul 2026 00:00:00 GMTdriftconsistencyturnshttps://aigirlfriend.dev/blogs/detecting-drift-without-transcripts/https://aigirlfriend.dev/blogs/detecting-drift-without-transcripts/Production has no ground truth, so drift must be measured against each conversation's own early behaviour rather than against a corpus or a golden sample.Mon, 27 Jul 2026 00:00:00 GMTdriftconsistencycosthttps://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 GMTstateconsistencyturnshttps://aigirlfriend.dev/blogs/testing-character-before-shipping/https://aigirlfriend.dev/blogs/testing-character-before-shipping/A change that passes a fresh-session check fails in production: consistency is a distribution over samples at depth, not a property you can spot-check.Tue, 21 Jul 2026 00:00:00 GMTconsistencystatedrifthttps://aigirlfriend.dev/blogs/staying-in-character-costs-usefulness/https://aigirlfriend.dev/blogs/staying-in-character-costs-usefulness/A persona will answer questions it has no information about, in register and with confidence. Some categories have to leave the character entirely.Mon, 20 Jul 2026 00:00:00 GMTconsistencymoderationstatehttps://aigirlfriend.dev/blogs/refusals-break-character/https://aigirlfriend.dev/blogs/refusals-break-character/Moderation sits outside the persona, so its output arrives in a different register — and then enters the history as an example of how the character speaks.Sun, 19 Jul 2026 00:00:00 GMTmoderationconsistencystatehttps://aigirlfriend.dev/blogs/small-facts-users-check/https://aigirlfriend.dev/blogs/small-facts-users-check/A wrong name discredits an otherwise perfect memory. Continuity is judged on a handful of details, so state budget should follow salience, not volume.Sat, 18 Jul 2026 00:00:00 GMTmemoryconsistencystatehttps://aigirlfriend.dev/blogs/one-definition-many-relationships/https://aigirlfriend.dev/blogs/one-definition-many-relationships/One authored character becomes as many characters as you have long conversations, and per-user customisation turns that divergence into forks you maintain.Fri, 17 Jul 2026 00:00:00 GMTstateconsistencycosthttps://aigirlfriend.dev/blogs/character-has-no-clock/https://aigirlfriend.dev/blogs/character-has-no-clock/Elapsed time is not in the state unless you put it there. Turns are a sequence, so three weeks of absence and three seconds look identical in the input.Thu, 16 Jul 2026 00:00:00 GMTstateturnsmemoryhttps://aigirlfriend.dev/blogs/deleting-history-summary-absorbed/https://aigirlfriend.dev/blogs/deleting-history-summary-absorbed/Removing turns does not remove what was derived from them. A deleted message survives in the summary, the fact store and every response written under it.Wed, 15 Jul 2026 00:00:00 GMTmemorysummarisationstatehttps://aigirlfriend.dev/blogs/rebuilding-summary-cadence/https://aigirlfriend.dev/blogs/rebuilding-summary-cadence/Rebuild cadence decides how much a summary degrades. Frequent rebuilds compound generational loss; rare ones leave a stale summary against fresh history.Tue, 14 Jul 2026 00:00:00 GMTsummarisationmemorycosthttps://aigirlfriend.dev/blogs/wrong-fact-outlives-missing-fact/https://aigirlfriend.dev/blogs/wrong-fact-outlives-missing-fact/Extraction failures are not symmetrical. A missed fact is a gap the user can fill; a wrong one is included every turn and reinforces itself.Mon, 13 Jul 2026 00:00:00 GMTmemorystateconsistencyhttps://aigirlfriend.dev/blogs/user-contradicts-stored-facts/https://aigirlfriend.dev/blogs/user-contradicts-stored-facts/Stored facts go stale and users change their minds. Overwriting loses the history; keeping both puts two true-looking answers in the same input.Sun, 12 Jul 2026 00:00:00 GMTmemorystateconsistencyhttps://aigirlfriend.dev/blogs/constraints-erode-first/https://aigirlfriend.dev/blogs/constraints-erode-first/The voice survives a long conversation better than the rules do. Constraints are the part of a definition that decays first, and the failure is invisible.Sat, 11 Jul 2026 00:00:00 GMTdriftconsistencystatehttps://aigirlfriend.dev/blogs/changing-persona-definition-live/https://aigirlfriend.dev/blogs/changing-persona-definition-live/A definition edit is a migration. Existing conversations get the new text with the old history, so the seam lands mid-relationship.Fri, 10 Jul 2026 00:00:00 GMTstateconsistencydrifthttps://aigirlfriend.dev/blogs/persona-definition-bloat/https://aigirlfriend.dev/blogs/persona-definition-bloat/A character definition that started as a paragraph is now two pages, every line justified by a real complaint, and each line binds less than before.Thu, 09 Jul 2026 00:00:00 GMTstateconsistencycost