How to build an AI companion app in 2026 — architecture, unit economics, pitfalls

AI companion apps are the most demanding product in the character space: users come back to the same character every day and expect it to remember them, retention hinges on continuity, and your heaviest users are your best customers and your biggest cost. This is the blueprint we'd use, with real numbers.

The architecture, in five decisions

1. Persona lives in context, not in the transcript. Send the character sheet — identity, voice, boundaries, shared history — through the context field on every call, and keep the messages array for actual conversation. Editing a personality six months in becomes a data change, not transcript surgery.

2. You own memory. The API is stateless by design. Window the last 20–40 turns, and maintain a rolling summary ("what the character knows about this user") that you fold into context. Companion products die of amnesia; a summary refreshed every 30 turns is the cheapest fix that works.

3. Stream everything. Set stream: true and relay SSE chunks to the client. A companion that "types" feels present; a spinner feels like a form. Messaging mode (mode: "messaging") keeps replies texting-length and snappy — the right register for DMs, and it caps your token ceiling.

4. Media are moments, not wallpaper. A snapshot from the character's day at a story milestone (10 credits), a voice note when the user comes back after a week away (3 credits per 100 characters) — scarce, earned, and priced as premium unlocks. Products that autoplay everything spend 10× and retain worse.

5. Persist the media users come back to. Pictures your users unlock are assets they revisit. Add store: true to the generation call and the render lands on the eroq Store CDN with a durable URL — object storage built into generation, under the same written policy as the models, instead of a second vendor to wire (how render, storage and serving fit together).

The unit economics (the part that kills apps)

Flat credits make this math checkable — a completion is 1 credit on RP mini, 3 on RP+, full table here. The number that matters: an engaged companion user sends 30–60 messages a day.

User profile	Messages/day	Model	Credits/month	Cost/month
Casual	8	RP mini	~240	~$2.00
Engaged	40	RP mini	~1,200	~$10.00
Engaged, premium	40	30% RP+	~1,920	~$16.00
Power user	100	RP mini	~3,000	~$25.00

Three conclusions fall out immediately:

• Uncapped free tiers are suicide. An energy/message-cap system isn't a growth hack, it's solvency.
• Price tiers around message volume, not features. $9.99 casual / $24.99 unlimited maps to the actual cost curve.
• RP+ is the premium lever: route paying users' emotional beats to the flagship, keep small talk on mini. The model field switches per call.

Don't take the table's word for it — the prices are flat and public, so your own spreadsheet reproduces any cell above in a minute.

The pitfalls, from production

• Refusal mid-scene is churn. It's the moment companions break: the story takes a dark turn — a betrayal, a death, a fight — and the character answers with a disclaimer. eroq's engines are uncensored by policy — the wall is a written AUP enforced at the account level, not a filter's mood deciding mid-conversation.
• Treat HTTP 402 as an upsell, not an error. Balance-empty is a product event: catch it, route to your top-up flow, never show a stack trace.
• Let users send photos. Vision-enabled chat (+2 credits per image) is the highest-retention feature per credit in the category — the character reacts to the user's day.
• Log persona breaks. Every turn where the character slipped, tag it and drop it from future windows. Drift compounds; curation is cheap.
• Say what you keep. People confide in companions, so your privacy page is part of the product — write it from facts. eroq's own models (RP+, RP mini, Voice One, Voice Turbo) retain nothing upstream; prompts are kept in eroq's usage ledger for billing and abuse handling; and on image and video calls, private: true keeps no file and no library entry and replaces the prompt with stars in the ledger.

FAQ

How much does an AI companion app cost to run per user?

On flat credits, a casual user at 8 messages a day is around $2 a month and an engaged one at 40 a day around $10 — 1 credit per reply on RP mini, 3 on RP+. Route 30% of turns to the flagship and the engaged user lands near $16. The numbers above are just those rates multiplied out, so your own spreadsheet reproduces them.

Which model should an AI companion app use?

Default to RP mini at 1 credit and escalate to RP+ at 3 for the turns that carry weight — reveals, reunions, the end of an arc. mode: "messaging" caps replies at 160 tokens and keeps the texting register, which is the right shape for DM-style products and the cheapest to serve.

Can the character send pictures and voice notes?

Yes, as separate calls: an image is 10 credits on Image One, Image Anime or Image Art, and a voice line is 3 credits per 100 characters on Voice One (2 on Voice Turbo), in a roster voice or one you cloned. What does not exist is lip-sync or a talking avatar — there is no feature that animates a face to match the audio, so design the moment around a still image plus a voice note.

Is an AI companion app allowed on eroq?

Yes — character fiction is the core use case, and it renders in character, dark turns included. The limits are written and absolute: fiction only; nothing involving minors, no real identifiable people without consent, nothing illegal — enforced at the account level. Rate your product for the audience it serves, pass through an acceptable-use policy at least as strict as ours, and handle content_blocked as a product event.

Ship the walking skeleton first

Day one is a key, a system context, a windowed history and a streamed reply — the character chat tutorial has the code. The 50 free credits cover an evening of testing the exact scenes your product will live on. Everything above is iteration.