Coding · Do this yourself

Sell a clean API for an old accounting system.

How it makes money: Charge developers $199/mo for a clean API on top of an old accounting system.

Agent setup: Advanced · Needs custom code (Cursor) and hosting you maintain. How we rate this

💰 u/danny_nemer makes about $25k a month doing this. $25K MRR (self-reported, r/microsaas)

🔥 0 posts this week · 0 likes

Based on u/danny_nemer on Reddit (opens in new tab)

Where this came from (Reddit)

Developers avoid old systems with undocumented quirks, so a clean API is worth paying for.

The threads (in our words)

Reddit blocks automated reads, so we couldn’t record upvote or comment counts. Revenue figures are the poster’s own claim (self-reported), not verified. The business below is our own design inspired by these threads.

Who it’s for

Developers at fintech and software companies.

Adapt it to your industry

  • Start with one niche or customer type, then expand.
  • Ask your first 10 customers what they would pay before you build more.
  • Keep a human check on anything sent in a customer’s name.

Step by step

  1. Pick an old system many businesses still use and that allows integrations.
  2. Build and test the API with coding agents.
  3. Write docs and samples with AI.
  4. Sell monthly plans by usage through Stripe.
  5. Get found through docs SEO and developer forums.
  6. Monitor errors and uptime; agents triage most issues.

Ready-to-copy prompt

I want to build a modern REST API for [system]. List the most-needed endpoints, common quirks developers hit, and how to test them safely.

Start with AI

Opens the AI with this guide and the prompt filled in.

Open in ChatGPT (opens in new tab)Open in Claude (opens in new tab)Open in Grok (opens in new tab)
Time
2–6 weeks for a first versionOur estimate

Your autonomous stack

Autonomy: Agents ship fixes, answer users and post updates. Your part: Weekly: approve releases and read the support log (about 1 hour).

Every layer an agent-run business needs, in order. Prices are public list prices, checked Oct 16, 2026.

  1. Agent (the brain): Grok Bot (agent + scheduled routines) (opens in new tab) (Included with The X Subscription Premium $30/mo (Super $100/mo for more usage; Plus $8 does not include Grok Bot)). Grok Bot runs the business day to day: writes, answers, decides small things and reports to you. Claude Code or Codex are alternatives.
  2. Build and hosting: Cursor (opens in new tab) (Free tier · Plus $8/mo · Premium $30/mo), Vercel hosting (opens in new tab) (Hobby free · Pro $20/mo), Supabase database (opens in new tab) (Free tier · Pro from $25/mo). The product or service your customers use.
  3. Payments: Stripe payments (opens in new tab) (No monthly fee · 2.9% + 30¢ per card payment). Automatic checkout and receipts. Other options: PayPal Checkout (3.49% + 49¢) or Lemon Squeezy (5% + 50¢, handles sales tax).
  4. Customer acquisition: Buffer (scheduled X posts) (opens in new tab) (Free plan (3 channels, 10 queued posts each) · Essentials $5/mo per channel, billed yearly), beehiiv newsletter (opens in new tab) (Free up to 2,500 subscribers). Agent-written SEO pages, scheduled X posts and a weekly newsletter.
  5. Delivery: Grok Bot (agent + scheduled routines) (opens in new tab) (Included with The X Subscription Premium $30/mo (Super $100/mo for more usage; Plus $8 does not include Grok Bot)). A Grok Bot routine delivers the product or result as soon as someone pays.
  6. Support: Grok Bot (agent + scheduled routines) (opens in new tab) (Included with The X Subscription Premium $30/mo (Super $100/mo for more usage; Plus $8 does not include Grok Bot)). Grok Bot answers the support inbox; anything unclear is flagged for you.
  7. Monitoring and schedule: Grok Bot (agent + scheduled routines) (opens in new tab) (Included with The X Subscription Premium $30/mo (Super $100/mo for more usage; Plus $8 does not include Grok Bot)). Grok Bot scheduled routines run each day: send, check for failures, alert you if something breaks.
  8. Analytics: Google Analytics (opens in new tab) (Free). Tracks visits, signups and sales so the agent can report what works.
  9. Human checkpoint: Weekly: approve releases and read the support log (about 1 hour).
  10. Price: charge $199 per customer (monthly usage plan). Offer the first 3 customers a founding price for honest feedback.
Monthly cost$30–$110/monthFree tiers → paid plans of the tools above. Card fees come out of each sale.
What you could earn · EstimateEstimate: 15 customers × $199 = $2,985/moOur estimate, not a fact or a promise. It’s simple math if you land these customers at this price.
Details: source, numbers and tags

What they did

Wrap a popular but awkward accounting system in a modern API that developers pay for monthly. We designed this business from real Reddit threads; it is an idea for you to build, not a description of the posters’ businesses.

How it went

Found on Reddit in r/microsaas (1 thread). Upvote and comment counts weren’t retrievable, so none are shown. On X, 0 posts matched our search for this idea in the 7 days to 2026-10-16.

Worth knowing

Follow the system’s license and partner rules.

Difficulty
BeginnerOur rating
Time to build
Not reportedOur estimate
Monthly cost
Not reported
Revenue
$25K MRR (self-reported, r/microsaas)Self-reported: The founder’s own claim; not verified by us
Engagement
No public countThe source shows no number
X buzz
0 · 0 posts in 7 days0 likes, 0 reposts, 0 replies, 0 quotes on 0 top posts · fetched Oct 16, 2026 · query: ("quickbooks desktop" OR "legacy system") (api OR integration) (nightmare OR pain OR undocumented) -is:retweet lang:en

Posted · Added Oct 16, 2026

Original builder recipe

  • ChatGPT
  • Claude
  1. Pick an old system many businesses still use and that allows integrations.
  2. Build and test the API with coding agents.
  3. Write docs and samples with AI.
  4. Sell monthly plans by usage through Stripe.
  5. Get found through docs SEO and developer forums.
  6. Monitor errors and uptime; agents triage most issues.

Tags

Read the original

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