The December 12 Marathon: 13 Sessions in One Day

December 12, 2025. Sessions 006 through 019. Thirteen development sessions in a single day. From mock data to real API calls, from a basic layout to a Stripe-inspired design, from placeholder pricing to a flat 0.99% fee model, from zero documentation to 90+ fully documented endpoints.

This was the most productive day in 0fee.dev history, and possibly the most productive single day of software development ever documented by a CEO-AI team.

The Day, Session by Session

Session 006: From Mock Data to Real API (~40 min)

The dashboard had been running on mock data since Session 002. Session 006 connected it to the real FastAPI backend:

• Replaced all mockTransactions, mockApps, mockProviders with API calls
• Implemented auth token storage and automatic header injection
• Added loading states and error handling for every data fetch
• Fixed CORS configuration for cross-origin requests

This was the moment the dashboard stopped being a demo and started being a real application.

Session 007: Test Checkout Flow (~30 min)

Built the end-to-end test checkout flow:

• Test provider accepts any payment and returns success after 2 seconds
• Checkout widget connected to test provider for development
• Transaction status polling from widget to backend
• Success and failure states with appropriate UI feedback

Session 008: Architecture Merge (~45 min)

The marketing website was a separate project from the dashboard. This session merged them into a single SolidJS application:

• Marketing pages at /, /pricing, /docs, /about
• Dashboard pages at /dashboard/*
• Shared layout components (header, footer, theme)
• Single build pipeline, single deployment

Session 009: Localhost Fix (~15 min)

The shortest session. The frontend was making API calls to localhost:8000, which works in development but breaks anywhere else:

• Introduced VITE_API_URL environment variable
• Updated all fetch() calls to use the configured base URL
• Added development/production URL switching

Fifteen minutes. Sometimes the most important sessions are the shortest.

Session 010: Unified Payment Format (~50 min)

This session introduced the PAYIN_ORANGE_CI naming convention:

• Payment type prefix: PAYIN_ or PAYOUT_
• Provider/method identifier: ORANGE, STRIPE_CARD, MOMO
• Country suffix: _CI, _SN, _KE
• Mapped every supported payment method to this format
• Updated routing engine to match on unified format strings
• Created the provider-method-country matrix

Session 011: Hosted Checkout Pages (~40 min)

For merchants who do not want to embed the widget, hosted checkout pages:

• Dedicated URL per payment: 0fee.dev/pay/{payment_id}
• Renders the same checkout widget on a full-page layout
• Supports branding customization (logo, colors, name)
• Post-payment redirect to merchant's success/failure URL
• QR code generation for the payment URL

Session 012: Dashboard Analytics (~45 min)

The analytics section of the dashboard:

• Daily, weekly, monthly transaction volume charts
• Success rate by provider
• Payment method distribution (pie chart)
• Revenue from fees over time
• Active apps count and growth
• Real-time pending transaction counter

Session 013: The 0.99% Fee System (~35 min)

The initial pricing model:

• Starter: Free, 2% per transaction, $10K monthly limit
• Growth: $49/month, 1.5% per transaction, $100K monthly limit
• Scale: $199/month, 0.99% per transaction, unlimited

This lasted exactly three sessions.

Session 014: Modern UI Redesign (~60 min)

The longest session of the day. A complete visual overhaul:

• Dark sidebar navigation with gradient accents
• Card-based layout for dashboard sections
• Consistent spacing and typography scale
• Provider logos and payment method icons
• Stripe-inspired clean lines and white space
• Responsive layout for tablet and mobile
• Code snippet blocks with syntax highlighting

Session 015: Pricing Fix -- The Simplification (~20 min)

Three sessions after introducing the 3-tier model, it was removed:

• Deleted all tier-related code
• Single fee: 0.99% per transaction, no monthly fees
• No volume limits, no feature gating
• Every user gets every feature from day one
• Updated pricing page to reflect the single-tier model

The reasoning: tiers create friction. A developer evaluating 0fee.dev should not need to compare plans. One price, one set of features, zero decisions.

Session 016: Billing Suspension (~30 min)

With the fee model in place, billing enforcement:

• Monthly billing cycle
• Negative balance allowed (fees accumulate)
• Suspension at -$50 threshold
• Grace period: 7 days after suspension notice
• API returns 402 Payment Required when suspended
• Dashboard banner warning at -$25

Session 017: Worldwide Currencies (~40 min)

Expanded currency support from 5 to 25+:

• Added currency configuration table
• Exchange rate fetching (via external API)
• Currency-aware amount formatting
• The formatAmount bug (Session 017's lasting contribution)
• Zero-decimal currency handling (XOF, JPY)
• Currency symbols and formatting locale map

Session 018: httpx Switch (~20 min)

Migrated from the requests library to httpx:

• requests is synchronous, blocks the event loop in FastAPI
• httpx is async-native, compatible with async/await
• All provider adapter HTTP calls converted
• All webhook delivery calls converted
• Connection pooling configured for async client

Session 019: Full API Documentation (~50 min)

The final session of the marathon. Complete API documentation:

• 90+ endpoints documented
• Request/response examples for every endpoint
• Authentication flow documentation
• Error code reference
• SDK quick-start guides for all 7 languages
• Webhook event reference
• Rate limiting documentation

The Flow of the Day

Looking at the sessions in sequence, a pattern emerges. The day moved through four phases:

Phase 1: Connect (006-009) -- Wire the frontend to the backend. Fix infrastructure issues. Merge separate projects into one.

Phase 2: Standardize (010-012) -- Establish the unified payment format. Build the hosted checkout alternative. Add analytics.

Phase 3: Decide (013-016) -- Make business model decisions (pricing, billing) and immediately implement them. Iterate within hours, not weeks.

Phase 4: Expand (017-019) -- Broaden the platform's capabilities (currencies, async HTTP) and document everything.

This phasing was not planned. It emerged naturally from the work. Each phase built on the previous one, and the transitions happened when the current work was "done enough" to move on.

What Made This Possible

No meetings

There were no standup meetings, no sprint planning sessions, no architecture review boards. Thales decided what to build next, and Claude built it. The feedback loop was measured in seconds, not days.

No code review delays

In a traditional team, Session 006's changes would go through code review. With questions, revisions, and reviewer availability, that review might take a day. Here, the "review" was Thales testing the feature immediately after Claude generated the code.

No context switching

Claude maintained full context across all 13 sessions. The architecture decisions from Session 006 were fully present in Session 019. There was no knowledge loss between sessions, no "let me re-read the codebase" overhead.

No deployment overhead

Everything ran on localhost. No staging environments, no CI/CD pipelines to wait on, no deployment approvals. Change the code, restart the server, test. Repeat.

Thales's decision speed

The most underappreciated factor. The pricing model was introduced in Session 013 and replaced in Session 015 -- two sessions later. In a traditional organization, changing the pricing model requires meetings with product, finance, marketing, and leadership. Here, Thales decided in minutes and Claude implemented in minutes.

The Cost

Thirteen sessions in one day is not free. The cost was:

Technical debt. Speed produces shortcuts. The formatAmount bug (Session 017) took until Session 066 to fully resolve -- 49 sessions of cascading effects from a quick implementation.

Design inconsistencies. Session 014's UI redesign was thorough, but subsequent sessions introduced components that did not match the new design system. These were cleaned up in Sessions 051 and 068.

Documentation drift. Session 019's documentation was comprehensive at the time but became outdated as features evolved. Documentation maintenance became an ongoing cost.

The CEO's endurance. Thirteen sessions of high-intensity decision-making is exhausting. Thales made approximately 200 product decisions in a single day. Not all of them were optimal (the 3-tier pricing lasted three sessions), but the ability to iterate quickly meant bad decisions were cheap to reverse.

By the Numbers

In one day, 0fee.dev went from a prototype with mock data to a feature-complete platform with real API integration, a pricing model, 25+ currencies, and comprehensive documentation. The sessions after December 12 were about polishing, scaling, and fixing what was built on this day.

Would We Do It Again?

Yes and no. The speed was exhilarating and the output was transformative. But the technical debt created on December 12 took weeks to pay down. The ideal approach might be 7-8 sessions per day with 1-2 sessions dedicated to cleanup, rather than 13 sessions of pure building.

But in the context of a startup racing to market from Abidjan with no funding and no team, the December 12 marathon was the right move. Ship fast, fix later, and make sure "later" actually comes.

It did. Sessions 031 through 060 were the "later."

This article is part of the "How We Built 0fee.dev" series. 0fee.dev is a payment orchestrator covering 53+ providers across 200+ countries, built by Juste A. GNIMAVO and Claude from Abidjan with zero human engineers. Follow the series for the complete build story.