Anthropic draws a clean line between two tools that look similar but solve different problems. MCP connects Claude to data—credentials, live systems, writes that persist. Skills teach Claude what to do with that data—the recipes, not the kitchen. I tested this distinction against the reference server repo and found the clearest case study: sequential-thinking should have been a Skill, not an MCP server. Here's why. Sequential-thinking provides a tool for structured problem-solving through dynamic reasoning—it manages thoughts, revisions, branches, and step counts. But it fetches nothing. No external system, no auth, no side effects, no network calls. The server hands back your own reasoning with bookkeeping fields attached. As one way to frame it: it's a prompt pattern wearing a protocol costume. That means it pays MCP's costs for none of MCP's benefits: Install: a Node/npx or Docker process, versus dropping a markdown folder in. Context cost: the tool schema lives in context on every message, versus name and description until the skill triggers. What you get: your reasoning round-tripped through a subprocess, versus the same instructions with no subprocess at all. The context point is the real one—MCP tool definitions compete for context space, which is exactly why Skills use a different loading approach. And honestly, extended and interleaved thinking now do natively what sequential-thinking bolts on from outside. The one genuinely distinct feature left is the branching and revision bookkeeping—an explicit audit trail of a plan that gets revised. But a Skill that tells Claude to write its plan to a file and amend it produces the same artifact. Linear's MCP server is the contrast case—the archetype of what MCP is actually for. It's remote, centrally hosted, behind OAuth 2.1, with live data and writes that persist. When you call a tool, the client sends JSON-RPC over HTTPS with your bearer token, Linear translates that into GraphQL against its own API scoped to what you can see, and the result comes back. That's state you don't have, behind auth you had to grant, with side effects that outlive the conversation. Sequential-thinking has none of that. The working rule: if it has no credentials, no live data, no writes, and no execution outside the sandbox, it's procedural knowledge and belongs in a Skill. The composition pattern that actually works: a Skill describing your procedure, calling MCP servers for the data.