Building a FastAPI replacement is a years-long engineering project, not an agent task.

Every phase of framework development — API design, performance tuning, ecosystem compatibility, breaking change management — requires unique judgment. There is no repeatable template; each decision shapes all future ones.

Success criteria are deeply undefined: what does 'replacement' mean, what features are in scope, what performance targets matter, and who are the users? An agent cannot determine when this work is done.

An agent can access codebases, documentation, and testing tools, but it lacks access to user feedback loops, community signals, real-world production failure data, and the long-term context needed to make sound architectural tradeoffs.

Architectural mistakes in a framework are extremely costly and often irreversible — they propagate into every downstream user's codebase and can require breaking changes to fix. Maintenance errors can silently break production systems at scale.

Framework design requires deep taste in API ergonomics, community consensus-building, long-term vision, and tradeoff reasoning that current AI cannot reliably sustain across months or years of development.