Pre-trip budget by category (lodging/food/transport/activities); during-trip log expenses in any currency, app converts via free ExchangeRate-API to home currency; real-time "you're at 80% of food budget" push notifications; post-trip spend report with AI insights ("you spent 40% more on food than budget — 70% was lunches in tourist areas"). All data user-owned, exportable.

What we ship

**Core features (MVP)**

• Pre-trip budget setup: 4 categories (lodging/food/transport/activities) + custom + total daily allowance.
• One-tap expense logging: amount + category + currency (Flutter UI optimized for 3-tap entry).
• Real-time multi-currency conversion via ExchangeRate-API (auto-refresh daily).
• Push alerts: "You're at 80% of your food budget for this trip" on threshold breach.
• Post-trip AI insights report: Claude Sonnet 4.6 analyzes expense entries → narrative breakdown by category, spending patterns, recommendations for next trip.

**Primary user flow**

1. User finds us via r/solotravel post / ProductHunt.

2. Downloads Flutter app, signs up, creates "Trip: Bali Aug 2026" + pre-trip budget $1500 ($600 lodging / $400 food / $300 transport / $200 activities).

3. During trip logs each expense in one tap (amount + category + currency).

4. Gets push "You're at 80% of your food budget — 4 days remaining" — wake-up moment.

5. After trip — generates AI insights report → upgrade $9.99/yr for unlimited trips + comparison + cloud sync.

**What it looks like**

• **Landing**: split hero. Left: "Trail Wallet died in 2022. Your $1,500 Bali budget didn't." Right: animated mockup — user logs an expense → progress bars update → push alert pops.
• **Main screen** (Flutter mobile): top — total spend / total budget progress bar; mid — category cards with per-category progress; bottom — "+ Add Expense" FAB.
• **Output format**: real-time push notifications + post-trip PDF report (1-2 pages, beautiful chart + AI narrative).

**MVP build plan (9 days)**

• Day 1-2: FastAPI + Postgres on Railway, Stripe Subscriptions, JWT auth.
• Day 3-4: Flutter mobile (iOS + Android) — trip + budget setup screens, expense entry FAB.
• Day 5: ExchangeRate-API integration + multi-currency rollups in Postgres (daily FX refresh job).
• Day 6: Push notification engine (Expo Push) — threshold breach detection (80% / 100% / 120%).
• Day 7: Claude Sonnet 4.6 prompt engineering for trip-insights JSON output + Markdown narrative.
• Day 8: Landing + pricing page, paywall logic (free 1 trip / $9.99/yr).
• Day 9: ProductHunt assets, smoke tests, Stripe webhook production, ship.

Source signals

• `SIG-20260504-global-micro-saas-utility-trip-budget-tracker` — IndieHackers solo-SaaS validation + FlowJam SaaS Ideas 2025 + Trail Wallet 200K+ downloads with top review "WHERE I'm overspending?" — literal validated wedge for AI insights

Reasoning

The wedge is explicit: Trail Wallet has 200K+ downloads, 4.6-star, top review reads "why doesn't this app show me WHERE I'm overspending?". That tells us (a) market exists and pays, (b) leaders are abandoned, (c) users are literally asking for AI insights — which is what we build.

Why now: Claude Sonnet 4.6 can generate a spend narrative from structured expense rows ($0.05/report in LLM cost) — what previously required custom data viz. Plus ExchangeRate-API.com free tier provides full FX without an enterprise contract. Combined: real-time multi-currency tracking + AI insights = a feature set the abandoned competitors don't have.

Differentiation: (a) pre-trip budget-by-category — competitors require manual setup without templates; (b) real-time push "you're at 80% of food budget" — nobody does this; (c) AI post-trip insights with category breakdown ("70% of food spend was lunches in tourist areas"); (d) Flutter cross-platform from launch (Trail Wallet — iOS-only).

Gross margin profile: $9.99/yr × $0.50/yr LLM cost = 95% gross margin. Solo-build in 9 days — fastest of the five trip-utilities in this round. Regulatory-clean — no PCI (manual entry), no Plaid.

Distribution angle: r/solotravel + r/backpacking + r/digitalnomad — exactly the audience that complains about budget overrun publicly. Cold-DM these communities with an MVP video → high conversion.

Quick competitive read

Direct competitors — low density (3 named, each with a specific weakness):

• **TrabeePocket** (free, 14 yrs old): travel-expense tracker, abandoned since 2022; no AI insights; clunky multi-currency entry; no real-time budget alerts; no cloud sync.
• **Trail Wallet** ($4.99 one-time, 13 yrs old): one-time-purchase iOS-only app; single-currency at a time; dated UI from 2014 era; no AI; no Android parity; not actively updated.
• **Splitwise** (free + premium $4/mo, 15 yrs old): group-cost-splitting primary use case; not solo-budgeting; no pre-trip budget categories; no AI spend insights; not travel-aware.

White-space: pre-trip budget-by-category + real-time alerts + AI post-trip insights + cross-platform (iOS + Android) — empty.

Initial pricing hypothesis

$0 free for 1 trip (including basic AI insights). $9.99/yr unlimited trips + cloud sync + cross-device + full AI insights + historical comparison. Annual-only (no monthly — anchored to hobbyist-utility low-price pattern). Anchored to developed-tier hobbyist-utility $5-10/yr taxonomy.yaml — we are 5× cheaper than Splitwise Pro ($48/yr).

Distribution hypothesis

r/solotravel + r/backpacking + r/digitalnomad cold-DM with MVP video ("I built X because Trail Wallet died") — high conversion. ProductHunt launch + SEO long-tail on "trip budget tracker app", "how much does Bali cost backpacking". TikTok backpacker creator outreach. After 1000 paying — App Store / Google Play submission, partnership with travel-blogger affiliates.