Substack API
Substack — newsletter and subscription platform with an unofficial/limited API for post creation and subscriber management, primarily designed for human writers rather than programmatic agent publishing workflows.
Score Breakdown
⚙ Agent Friendliness
🔒 Security
GDPR compliant for the platform. TLS enforced. But session cookie auth for unofficial API access is a security anti-pattern — cookies can be stolen, have broad account access, and are not designed for programmatic use. US data processing.
⚡ Reliability
Best When
You already have an established Substack audience and need limited automation for draft creation or data export — not for building new agent-driven publishing pipelines.
Avoid When
You need a proper REST API for newsletter automation — Beehiiv and Ghost provide far better developer-friendly APIs for programmatic newsletter management.
Use Cases
- • Agents drafting newsletter content for human review before sending to Substack subscriber lists
- • Content cross-posting — agents using Substack as one newsletter distribution channel alongside Beehiiv or ConvertKit
- • Subscriber data queries — agents reading subscriber counts and growth metrics for publication analytics
- • Post scheduling — agents creating posts at optimal times based on subscriber engagement patterns
- • Migration automation — agents bulk-importing content archives from other platforms into Substack
Not For
- • Fully automated publishing pipelines — Substack has no official API and limits programmatic access
- • Developers needing a proper REST API — use Beehiiv, Ghost, or ConvertKit which have proper developer APIs
- • High-volume programmatic publishing — Substack is designed for individual human writers, not agent-scale automation
Interface
Authentication
No official API. Unofficial access requires session cookies from web login. Substack has no public API key system or OAuth. Limited unofficial endpoints exist but are subject to change without notice.
Pricing
Free to publish. Substack takes 10% + Stripe processing fees (~2.9% + 30¢) from paid subscriptions. No API pricing since there's no official API.
Agent Metadata
Known Gotchas
- ⚠ Substack has NO official public API — any programmatic access uses unofficial reverse-engineered endpoints
- ⚠ Session cookie auth breaks on password changes, logouts, and Substack-side session invalidation
- ⚠ Unofficial endpoints change without notice and have broken existing automation tools repeatedly
- ⚠ Substack TOS prohibits automated posting without authorization — risk of account suspension
- ⚠ No webhook support — agents cannot receive real-time events for subscriber actions or post events
Alternatives
Full Evaluation Report
Detailed scoring breakdown, competitive positioning, security analysis, and improvement recommendations for Substack API.
Scores are editorial opinions as of 2026-03-06.