# NewsletterFIT

> NewsletterFIT is a research tool for the newsletter ecosystem. It indexes public newsletter posts and connects them to the topics, companies, people and sponsors they mention, so marketers, agencies, PR teams, founders and researchers can find relevant newsletters, see who sponsors them, and monitor what changes.

It currently tracks 102,574 Substack and beehiiv newsletters across 5,009 topics. Public profiles show the subscriber figure the platform displays publicly (often rounded, not verified), publishing activity observed from indexed posts, topics and likely audience inferred from post content, and sponsors detected in posts that carry explicit sponsorship markers. See the methodology page for exact definitions.

This file is a short product map for assistants. It does not list individual newsletters, titles, or outbound links.

## Key pages
- [Newsletter directory](https://newsletterfit.com/newsletters): newsletters grouped by topic, and the largest by publicly shown subscribers
- [Rising newsletters](https://newsletterfit.com/newsletters/rising): Substack newsletters with sustained subscriber growth over the last 30 days, and those climbing Substack's category leaderboards
- [Newsletter sponsors](https://newsletterfit.com/sponsors): brands detected sponsoring newsletters in the last 12 months, with every newsletter and placement they appear in
- [All topics A–Z](https://newsletterfit.com/topics): every topic covered by at least three newsletters
- [Methodology](https://newsletterfit.com/methodology): data sources, freshness, and what "shown publicly", "observed", "inferred" and "detected" mean
- [Pricing](https://newsletterfit.com/pricing): Pro ($49/month), Intelligence ($99/month) and Enterprise ($199/month, $1,599/year); every account starts with a 7-day Pro trial, no card required
- [FAQ](https://newsletterfit.com/faq)
- [API reference (Scalar)](https://newsletterfit.com/docs/api): interactive REST docs
- [REST API reference as Markdown](https://newsletterfit.com/docs/api.md): auth, limits, errors, pagination, every endpoint with parameters and curl examples
- [OpenAPI 3.1](https://newsletterfit.com/api/v1/openapi.yaml): machine-readable REST contract
- [MCP server docs](https://newsletterfit.com/docs/mcp.md): setup for Claude.ai, ChatGPT, Claude Code, Cursor and Claude Desktop, limits, every tool with arguments, errors, agent guidance (Markdown)
- [MCP tool schemas](https://newsletterfit.com/api/v1/mcp-tools.json): MCP inputSchemas (same registry as tools/list)
- [MCP server](https://newsletterfit.com/mcp): streamable HTTP JSON-RPC for agents (OAuth 2.1 sign-in for connectors, or a Bearer API key; POST only)
- [Agent auth](https://newsletterfit.com/auth.md): how agents authenticate
- [Agent identity](https://newsletterfit.com/.well-known/agent.json): AID 1.0 — the honest MCP tool inventory (capabilities.actions)

## Profile URL patterns
- Newsletter profiles: https://newsletterfit.com/newsletters/{slug}
- Topic landscapes: https://newsletterfit.com/topics/{slug}

## Popular topics
- [mental health newsletters](https://newsletterfit.com/topics/mental-health): 1192 newsletters
- [mindfulness newsletters](https://newsletterfit.com/topics/mindfulness): 418 newsletters
- [personal growth newsletters](https://newsletterfit.com/topics/personal-growth): 385 newsletters
- [parenting newsletters](https://newsletterfit.com/topics/parenting): 316 newsletters
- [entrepreneurship newsletters](https://newsletterfit.com/topics/entrepreneurship): 294 newsletters
- [Philosophy newsletters](https://newsletterfit.com/topics/philosophy): 294 newsletters
- [Personal development newsletters](https://newsletterfit.com/topics/personal-development): 273 newsletters
- [venture capital newsletters](https://newsletterfit.com/topics/venture-capital): 264 newsletters
- [AI agents newsletters](https://newsletterfit.com/topics/ai-agents): 259 newsletters
- [Content creation newsletters](https://newsletterfit.com/topics/content-creation): 251 newsletters
- [AI safety newsletters](https://newsletterfit.com/topics/ai-safety): 246 newsletters
- [Personal branding newsletters](https://newsletterfit.com/topics/personal-branding): 245 newsletters
- [US Politics newsletters](https://newsletterfit.com/topics/us-politics): 245 newsletters
- [Travel newsletters](https://newsletterfit.com/topics/travel): 236 newsletters
- [Geopolitics newsletters](https://newsletterfit.com/topics/geopolitics): 234 newsletters
- [creative writing newsletters](https://newsletterfit.com/topics/creative-writing): 232 newsletters
- [psychology newsletters](https://newsletterfit.com/topics/psychology): 225 newsletters
- [self-reflection newsletters](https://newsletterfit.com/topics/self-reflection): 217 newsletters
- [AI ethics newsletters](https://newsletterfit.com/topics/ai-ethics): 214 newsletters
- [career development newsletters](https://newsletterfit.com/topics/career-development): 209 newsletters
- [Community building newsletters](https://newsletterfit.com/topics/community-building): 205 newsletters
- [Literature newsletters](https://newsletterfit.com/topics/literature): 203 newsletters
- [International relations newsletters](https://newsletterfit.com/topics/international-relations): 198 newsletters
- [productivity newsletters](https://newsletterfit.com/topics/productivity): 195 newsletters
- [AI infrastructure newsletters](https://newsletterfit.com/topics/ai-infrastructure): 192 newsletters