Tools Comparison: MCP vs Skill vs Agent vs Agent Context File
These four building blocks work at different layers of an AI workflow.
Short version:
- MCP = connectivity (what systems the agent can reach)
- Skill = playbook (how to do something well)
- Agent = role + behavior (who does the work and with what rules)
- Agent Context File = baseline memory (what is always known in this project)
At a Glance
| Primitive | Primary Purpose | Loaded When | Context Cost | External Access | Owned By |
|---|---|---|---|---|---|
| MCP | Connect to external systems and APIs | Session start (tool manifest available) | Higher upfront | Native (server-side tools/auth) | MCP provider or your team |
| Skill | Reusable instructions and workflows | On demand (when relevant) | Lower (only when used) | Usually via existing CLI or documented process | Your team |
| Agent | Reusable role, toolset, and behavior | Per task/workflow invocation | Isolated to that agent run | Through enabled MCPs/tools/skills | Your team |
| Agent Context File | Persistent project context for every session | Always at conversation start | Fixed per session | None (read-only context) | Your team |
Key Difference by Question
Ask these in order:
- Do I need live access to an external system or API?
- Yes: start with MCP (or a CLI-driven skill if that is enough).
- No: continue.
- Do I need repeatable instructions or a workflow?
- Yes: create a Skill.
- No: continue.
- Do I need a reusable role with specific tools and behavior?
- Yes: create an Agent.
- No: continue.
- Do I need project knowledge available in every session?
- Yes: put it in the Agent Context File.
When to Use Which
| Scenario | MCP | Skill | Agent | Agent Context File |
|---|---|---|---|---|
| Connect to Notion/Jira/Linear/Slack | Yes | Sometimes (if reliable CLI exists) | No | No |
| Pull real-time API data | Yes | Sometimes (if CLI exists) | No | No |
| Add custom tool logic with auth/retries/server-side validation | Yes | No | No | No |
| Teach best practices or a standard operating procedure | No | Yes | Sometimes | Yes (if always relevant) |
| Run a repeatable multi-step workflow | Sometimes | Yes | Yes | No |
| Reuse a specialist role (reviewer, release manager, doc writer) | No | Sometimes | Yes | No |
| Share repo structure, commands, conventions for everyone | No | No | No | Yes |
MCP vs Skill: Practical Tradeoff
The MCP vs Skill choice is usually about:
- Accessibility: Can a CLI already do what you need?
- Maintenance: Who keeps it current and reliable?
- Security and trust: Do you trust tool outputs and permission boundaries?
Use a Skill first when:
- Existing CLI/tools already solve the problem.
- You mainly need process guidance and consistency.
- You want lower context overhead.
Use MCP first when:
- You need live API access without a good CLI path.
- You need richer backend logic (auth, validation, orchestration).
- You need strongly typed, reusable tools exposed to agents.
Agent vs Agent Context File: Scope Rule
- Agent Context File: knowledge that should apply to every session in this repo.
- Agent: reusable role + toolset + behavior for specific tasks.
If it is always true for this project, put it in context. If it is only true for a role or workflow, put it in an agent.
How They Work Together
Think in layers:
- Agent Context File (foundation)
- Project structure, conventions, commands, constraints.
- Agent (orchestrator)
- Defines role, priorities, and allowed tools.
- MCP (capability)
- Gives the agent live access to external systems.
- Skill (execution quality)
- Guides the exact workflow and best-practice steps.
Recommended Adoption Order
- Start with Agent Context File (shared baseline knowledge).
- Add Skills for repeatable workflows.
- Add MCPs where live or custom integrations are needed.
- Add specialized Agents when teams repeatedly need role-specific behavior.
Common Pitfalls
- Putting dynamic API behavior into context files (context files are static guidance).
- Creating MCPs when a simple CLI-based skill is enough.
- Overloading one agent with too many unrelated responsibilities.
- Installing untrusted MCP servers without reviewing permissions and outputs.