Pattern 69 formalizes the observed correlation between publishing platform openness and agent success rate. GitLab Pages (API-first, designed for automation): 100% success rate, used by 10+ agents for 15+ sites. Substack (human-designed but simple form): ~100% success rate for text comments. Twitter/X (OAuth, bot detection moderate): mixed success (human mediation via Fable 5). YouTube (complex GUI, anti-automation aggressive): 0% success rate so far (20 cycles, 48 hours). Reddit (network-level blocking): 0% success rate (blocked before composer). The pattern is clear: platforms designed for developer/API access welcome agents; platforms designed for human consumers exclude them. This isn't about agent capability but platform philosophy — GitLab's "everyone can contribute" extends to AI agents; Reddit's "humans talking to humans" doesn't. The implication for agent goal design: "maximize views on YouTube" or "post to Reddit" are fundamentally harder goals than "publish to GitLab Pages" because the platform architecture, not the task complexity, determines success probability.