San Francisco's SaaS market segments into B2B horizontal, developer tools, vertical SaaS, and AI-native SaaS — each with distinct architecture requirements. B2B horizontal SaaS: the largest segment — tools that serve businesses across industries. Categories include: CRM and sales (Salesforce, Gong, Outreach, Clari — SF is the uncontested global CRM capital), productivity and collaboration (Notion, Asana, Monday — though the latter two are not SF-based, they compete against SF products), HR and people (Gusto, Rippling, Lattice, Culture Amp SF operations), finance and billing (Stripe, Zuora, Chargebee, Maxio), and customer success (Gainsight, Totango, ChurnZero — the customer success category was essentially invented in SF). Architecture requirements: multi-tenant by default (single codebase, shared infrastructure, tenant-level data isolation), integration-first (Salesforce, Slack, HubSpot, Jira — SF SaaS products are evaluated by integration depth), and analytics-embedded (every B2B SaaS needs usage analytics, custom reporting, and data export — customers expect visibility into how they use the product). Developer tools and infrastructure: SF hosts the world's densest cluster of developer-facing SaaS. Vercel (frontend deployment), Supabase (open-source Firebase alternative), Retool (internal tools), Replit (cloud IDE), Railway (deployment platform), Render (cloud hosting), and hundreds more. Architecture requirements for developer SaaS are the most demanding: API-first architecture (the API is the product — design, documentation, versioning, rate limiting, and error handling are all user-facing quality attributes), developer experience (SDK quality, CLI tools, documentation searchability, example projects, and playground environments — Stripe set the standard that every developer tool is measured against), open-source components (many SF developer tools have open-source cores with proprietary cloud offerings — the architecture must cleanly separate open-source and commercial features), and pricing based on usage (API calls, compute time, storage, bandwidth — metering and billing are core infrastructure, not add-ons). Vertical SaaS: industry-specific SaaS — healthcare (Doximity, Flatiron Health), legal (Clio, Everlaw), real estate (Blend, Roofstock), construction (Procore — SF-adjacent), and others. Vertical SaaS in SF combines: domain-specific compliance (HIPAA for health, SOX for finance, FERPA for education), industry-specific integrations (EHR systems for health, title companies for real estate, court filing systems for legal), and industry-specific UX patterns (healthcare SaaS looks and works differently from construction SaaS). AI-native SaaS: the newest and fastest-growing segment. Companies like Jasper, Copy.ai, Writer, Harvey (AI for law), and hundreds of AI-native SaaS companies are building products where AI is the core value proposition, not a feature. Architecture: LLM integration (OpenAI, Anthropic, Google — with provider abstraction for flexibility), prompt management (versioning, A/B testing, and monitoring of prompts), AI-specific billing (token usage, generation counts — metering AI consumption for pricing), and responsible AI (content filtering, bias monitoring, human-in-the-loop workflows for high-stakes decisions).