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

AI companion apps are the most demanding product in the character space: users message like it's a relationship (because to them it is), 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, relationship state — through the context field on every call, and keep the messages array for actual conversation. Editing a personality mid-relationship 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 she knows about him") 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 selfie at a relationship milestone (10 credits), a voice note when she misses him (3 credits per 100 characters) — scarce, earned, and priced as premium unlocks. Products that autoplay everything spend 10× and retain worse.

5. Persist media on NSFW-safe storage. Selfies 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 — mainstream buckets are a termination risk for exactly this media.

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+	~1,900	~$16.00
Obsessed	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. eroq's engines render adult scenes between adult characters by policy — the wall is a written AUP with a deterministic content_blocked code, not a filter's mood.
• 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 — she reacts to his day.
• Log persona breaks. Every turn where the character slipped, tag it and drop it from future windows. Drift compounds; curation is cheap.

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.