EvoSkill is a CLI tool and Python library for automatically discovering and improving skills for coding agents. It implements a self-improvement loop: a base agent is evaluated on a task, failures are analyzed by a proposer agent which suggests skill/prompt mutations, a generator creates new skills/prompts, and the evolved agent is re-evaluated. The best programs are tracked via git branches, enabling frontier management and rollback. The system supports multiple agent SDKs (Claude Code, Codex CLI, OpenCode, OpenHands, Goose, Harbor) through a unified harness abstraction, allowing the same evolution loop to work across different agent platforms. The architecture is modular: CLI commands (init, run, eval, skills, diff, logs, reset, remote) provide the user interface; an API layer (EvoSkill, EvalRunner) offers programmatic access; the loop engine (SelfImprovingLoop) orchestrates the evolution; the registry (ProgramManager) handles git-based program versioning; the harness layer abstracts SDK-specific execution with retry, timeout, and structured output parsing; agent profiles define role-specific configurations (base agent, proposer, generator, evaluator); and evaluation modules support multiple scoring methods (exact match, tolerance, LLM-as-judge, script-based, Harbor). External dependencies include various agent SDKs (claude-agent-sdk, opencode-ai, openhands-ai, etc.), LLM providers (OpenRouter, Fireworks), and infrastructure tools (Docker, Daytona for remote execution). The repository also includes example benchmarks (OfficeQA, SEAL-QA, Dabstep, LiveCodeBench) with datasets and evaluation scripts, demonstrating the system's application to real-world tasks. The codebase exhibits strong engineering practices with clean abstractions, reusable components, and comprehensive testing, though there are some areas of duplication (e.g., skill-creator scripts) and global mutable state in the SDK configuration.