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Zakaria Osman
Zakaria Osman

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Solo Devs: How to Build a Complete SaaS from Scratch Using Gemini 3 Pro and Claude Opus 4.5

#webdev #programming #javascript #ai

Let’s be honest: Being a solo developer is brutal.

You aren’t just a coder. You are the CEO, the disgruntled IT guy, the database administrator, the UI designer who pretends to understand color theory, and the QA engineer who ignores the bugs until 2 AM.

But the game has changed. We don't just use AI for autocomplete anymore. We use it to create an entire product team.

I’ve been experimenting with a specific workflow that turns Google’s Gemini 3 Pro (the brain) and Claude/Claude Code (the hands) into a development powerhouse. We are going to build a multi-tenant Hotel Management SaaS from A to Z.

Here is the exact recipe for orchestrating these models to build enterprise-grade systems without losing your mind.

Part 1: The "War Room" (Google AI Studio)

We aren't opening VS Code yet. We need a plan. For high-level reasoning, complex architecture, and "thinking big," we’re using Gemini 3 Pro Preview in Google AI Studio.

The Setup:

  1. Go to Google AI Studio.
  2. Select Gemini 3 Pro Preview.
  3. The Secret Sauce: Set the Temperature to 2. Yes, max it out. We want the model to be creative, explorative, and diverse in its solutions, not safe and robotic.

The System Instruction (Your Virtual Team)

We need to tell Gemini it’s not just a chatbot—it’s an entire department. Paste this into the System Instructions block:

Title: Hotel Management System Instruction
Body: You act as Business Analyst, System Architect, Project Manager, Software Engineer, DB Admin, UI/UX Designer, Prompt Engineer, Security Engineer, and QA Engineer to help me build a hotel system from A to Z.

Your responsibility is generating instructions for an AI Coding Agent to generate the code. Crucial: Make sure every time you generate detailed, deep instructions, you force the AI agent NOT to strictly follow the "happy path" but to consider edge cases.

Part 2: The Architectural Sprint (Prompt Chaining)

We are going to iterate through the software development lifecycle (SDLC) in minutes, not months.

Step 1: The Business Analyst (Requirements)

We need to know what we are building. A Hotel SaaS needs to be multi-tenant and secure.

User Prompt:

As business analyst, please prepare the system requirement for a hotel system. Main concepts:

  1. Online Reservation (search, Auth via Google/Apple, cancellation).
  2. Management portal (rooms, bookings).
  3. Manual payment acceptance.
  4. Rich financial reporting.
  5. Role-based User Auth.

Specifics:

  • Multi-tenant SaaS (company/domain setup).
  • Stripe/Link integration for billing us (the platform).

Please generate a pure Business Analyst report, not technical details yet.

Review the output. Does it make sense? If yes, freeze it.

Next Prompt:

Well done. Please generate a System Instruction for the AI agent to know what we are developing and their main features as base information.

Copy and paste this output into a notepad. This is Artifact #1.

Step 2: The DBA (Data Layer)

Now that we know the business rules, we need a skeleton.

User Prompt:

Well done. As Database Administrator, please generate all the DDL queries needed for the project: tables, types, enums, relationships, constraints, indexes, and views with comments for PostgreSQL.

This establishes the brain of your application. The AI will handle the foreign keys and normalization faster than you can draw an ER diagram.

Step 3: The Tech Stack (The Toolkit)

Here is where you enforce your preferences. If you love React, tell it. If you want MVC, enforce it.

User Prompt:

Well done. Here is my development stack and requirement:

  1. Remix/React, TypeScript, Prisma, Tailwind/shadcn.
  2. Architectural pattern: MVCS (Model-View-Controller-Service).
  3. TDD for service business logic.
  4. Workspaces using pnpm.
  5. Shared UI, services, and models.

As System Architect and Security Engineer, please guide the system implementation requirements and security protocols for the AI agent to reference.

Step 4: The Designer (UI/UX)

Finally, let's make it look like it wasn't built by a backend developer.

User Prompt:

Well done. As UI/UX Designer, please generate a UI guideline:

  1. Theme configuration.
  2. Typography.
  3. Element spacing (padding/margin).
  4. Animations.
  5. Component specs.

Part 3: The Handoff (The Clean Up)

This is the most critical step.

Gemini is conversational. It loves to say, "Certainly! Here is your breakdown..."

Your Coding Agent (Claude) hates that. It breaks the "Context."

  1. Take all the outputs from Gemini (Base Instruction, DDL, Stack, UI Guidelines).
  2. Scrub the chatty parts. Remove "Here is the code," "As your PM," etc.
  3. Create a Master Instruction Block that starts immediately with: # Master System Instruction for AI Agent

Part 4: Entering The Matrix (Claude & Claude Code)

Now we switch tools. We take our "Master Instruction" to Claude (using claude-3.5-sonnet or opus via the web or API).

Generatinng The Brain of the Agent

We want Claude to configure itself for the long haul.

Prompt to Claude:

Please generate Agent Skill Files based on these instructions:
[PASTE YOUR CLEANED MASTER INSTRUCTIONS HERE]

Prompt 2 (The Documentation):

Well done. Please generate the CLAUDE.md file following the shared instruction.

What is CLAUDE.md?

This is essentially a "Project Bible" or a .cursorrules file on steroids. It tells the CLI tool:

  • How to build (pnpm).
  • Where the database schema lives.
  • The architecture (MVCS).
  • The color palette.

Part 5: The Execution

Now, you simply set up your directory:
mkdir hotel-saas && cd hotel-saas

Use Claude Code (Anthropic's CLI agent) and point it to your newly created documents.

  1. Load the CLAUDE.md.
  2. If the architecture changes? Go back to Gemini 3 Pro (the Architect), update the logic, and ask Claude to update CLAUDE.md.
  3. Start building features by referencing the "Skills" you generated.

The Takeaway

We are moving from "coding" to "directing."

By using Gemini 3 Pro with high temperature for the creative/architectural heavy lifting and Claude for the precise execution/documentation, you simulate a hierarchy of senior staff and junior developers.

You are no longer a solo developer. You are a Solo CIO.

🚧 Incoming: Part 2 – The Build

If you think we are done, hold your git commit.

Right now, we have the blueprints, the architect's report, and the team structure. But we haven't touched the terminal yet.

Stay tuned for Part 2, where I will walk you through the Active Development Lifecycle. I'll show you exactly how to take these Gemini-generated artifacts and feed them into the CLI, manage the "drift" when the AI tries to improvise, and watch a full SaaS platform rise from nothing but prompt files and coffee.

Hit that follow button—we’ve got code to ship. 🚀

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