Choose the Right Extension

Lesson 7 · Skills, plugins, and MCP · ~12 minutes

Codex has several extension mechanisms. They are complementary, but using the wrong one creates unnecessary configuration.

NeedUse
Durable behaviorAGENTS.md
Reusable procedureSkill
Installable bundlePlugin
External data or actionsMCP or connector
Specialized parallel workerSubagent

Build in This Order

  1. Check whether Codex or an installed plugin already provides the capability.
  2. Write a skill when the workflow itself is the missing piece.
  3. Add MCP only when the workflow needs a system outside the repository.
  4. Package the skill as a plugin when installation and distribution matter.
Progressive disclosure

Codex initially loads skill metadata, then reads full instructions and references only when the skill is selected. A precise description matters more than a large always-loaded prompt.

---
name: release-check
description: Verify a release candidate. Use before tagging a production release.
---

1. Run the repository release checks.
2. Summarize failures with file references.
3. Do not publish or tag anything.

Apply the Smallest-Surface Test

Before creating an extension, ask what capability is actually missing:

  1. Can a five-line repository instruction solve it?
  2. Does the task repeat often enough to justify a skill?
  3. Does it need executable helpers, or are instructions sufficient?
  4. Does it need live data or actions outside the repository?
  5. Does anyone else need to install it as a unit?
ExampleSmallest fit
Always run one project-specific test commandAGENTS.md
Prepare a release using a repeatable checklistSkill
Distribute release and incident skills togetherPlugin
Read current incidents from SentryMCP or connector
Investigate logs while another agent traces codeExplicit subagents

Review the Trust Boundary

A skill is instructions until it invokes scripts or tools. An MCP server may expose read operations, destructive actions, or both. Plugins can bundle several capabilities. Review tool lists, authentication, approval modes, and update sources before installation.

Avoid extension accumulation

Every installed description competes for discovery and every active tool expands the decision surface. Disable unused skills and MCP servers instead of keeping them “just in case.”

Check Your Understanding

A repeatable release procedure needs repository instructions and helper scripts. What fits?
Correct. A skill packages the procedure, references, and optional scripts.
MCP supplies external capabilities; a skill defines the repeatable workflow.

Do This Now

Inventory three existing Kiro or Claude customizations. Classify each as guidance, workflow, distribution, external capability, or parallel role. For each, explain why the next larger mechanism would be unnecessary.

Primary Source

Codex customization explains how AGENTS.md, skills, plugins, MCP, and subagents fit together.

Questions? Describe a customization you rely on and I will challenge whether it needs to exist in Codex.
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