MCP Core Concepts

MCP is useful because it turns context entry points into discoverable, testable, governable interfaces.

Tools

A tool is an executable action. It may compute, query, write, or trigger an external system.

Every tool should define:

• Input schema.
• Output schema.
• Side effects.
• Error representation.
• Permission or confirmation needs.

Resources

A resource is readable context. It is useful for files, knowledge records, configuration fragments, log excerpts, or business entities.

Good resources are:

• Paginated or size-limited.
• Stable by identifier.
• Clear about source and freshness.
• Small enough for context windows.

Prompts

Prompts are reusable task templates. Use them to package workflows instead of placing every behavior in a single system prompt.

Safety boundaries

• Default to read-only.
• Name write operations explicitly.
• Keep secrets outside model context.
• Avoid leaking internal paths or credentials.
• Cover permission denial with Harness scenarios.

Next: Build an MCP Server.