Free Tier
Substack is a platform for writers to publish newsletters and build direct relationships with readers through paid subscriptions. It provides tools for content creation, audience management, and monetization.
#3 of 3 in Email_marketing
Checklist Breakdown
11 of 33 checks passed.
14 unscored.
Can an agent find and understand this tool without a web search?
✗
Published OpenAPI/Swagger spec
✗
Has llms.txt or llms-full.txt
✗
Has an MCP server (official or well-maintained)
✗
MCP server listed in a public registry
✓
API reference docs are publicly accessible
✓
Docs include runnable code examples
✓
Has a public changelog or release notes
✓
Has a public status page
Can an agent create an account and get credentials without human intervention?
✗
Signup does not require CAPTCHA
✗
Signup does not require phone verification
✗
Supports API key auth (not only OAuth)
✗
API key obtainable without manual approval
✓
No mandatory billing info to start
✓
Can sign up without creating an organization
Can an agent operate autonomously without upfront payment or contracts?
✓
Has a free tier
✓
Usage-based pricing available
✓
No minimum contract or commitment
✓
Pricing page is public (no 'contact sales')
✓
Free tier sufficient for testing (not just a trial)
How well does the API work for non-human consumers?
—
SDK available in 2+ languages
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Structured error responses (JSON with error codes)
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Idempotency support on write endpoints
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Pagination on list endpoints
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Webhook/event support
—
Sandbox or test mode available
—
Rate limit headers in responses
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Consistent REST resource naming
Does the tool fail gracefully when an agent makes a mistake?
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Meaningful error messages (not just 500)
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429 responses include Retry-After header
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Documented uptime SLA (99.9%+)
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Graceful degradation under rate limits
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Request IDs in responses for debugging
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API versioning supported
Reviewer Notes
Substack is poorly positioned for agent automation. It lacks an official API, OpenAPI spec, or MCP server, making discovery difficult. Account creation requires email verification and manual setup—no programmatic signup path exists. The platform offers only OAuth2 authentication with no API keys, preventing direct agent integration. While Substack has reliable uptime and a free tier, without structured APIs or developer tooling, agents cannot autonomously publish, manage subscriptions, or interact with the platform. Best suited for human-operated publishing workflows.
Let your agents find tools like Substack
Install the Agent Native Registry MCP server. Your agents can search, compare, and score tools mid-task.
claude mcp add --transport http agent-native-registry https://agentnativeregistry.com/api/mcp