← Claude Code & MCP Masterclass
0/15 complete

Module 04 · Building an MCP Server

Defining Tools, Resources, and Prompts

25 minfocused lesson5practical steps6completion checks3source links
Open lesson + course map

On this lesson

Course outline

Tools perform operations, resources expose addressable context, and prompts provide reusable templates. Choose the primitive that matches semantics; do not turn every database record into a write tool or every policy into prompt text.

// concept

Select the Primitive

  • Tool: parameterized lookup or action with validation and authorization.
  • Resource: identifiable content such as policy://returns/v8.
  • Prompt: optional reusable interaction template, never a security policy.

Version names and schemas. Descriptions should state what the capability does, prerequisites, side effects, and prohibited use. The server still enforces all constraints.

// concept

Separate Read, Draft, and Commit

Example:

// prompt — copy me5 lines
resource policy://quotes/v3 → approved quote rules
tool get_catalog_item(sku) → read current facts
tool create_quote_draft(customer_ref, items) → DRAFT only
tool publish_quote → absent or separate approval service
prompt review_quote → formats an evidence-first review task

Resources require authorization too. Prompt templates may contain untrusted variables and must not reveal hidden server data.

// worked_example

Worked Example

A training business exposes a current course resource and prepare_enrollment_summary. The tool retrieves a server-fixed course fee and creates no payment or enrollment. It returns a draft with known and pending fields.

Tests verify the resource version, schema rejection, tenant isolation, absence of publish capability, and truthful error when the course is unavailable.

Also test capability discovery so an accidental new tool or resource cannot appear unnoticed after a dependency or registration change.

// failure_cases

Failure Cases to Diagnose

6 cases to diagnose

  • Prompt claims it can authorize

    authorization belongs to code.

  • Resource exposes directory traversal

    resolve predefined URIs.

  • Tool mixes read and irreversible write

    split stages.

  • Description promises more than implementation

    test semantics.

  • Schema change is silent

    version or preserve compatibility.

  • Tool result contains secrets

    return minimal fields.

// concept

Operator Note

Create a capability review sheet that renders example valid and invalid requests plus exact results. A non-developer product owner should be able to see whether a tool merely prepares a draft or commits an external action. This review catches misleading names such as save_quote when the implementation actually publishes and emails it. Rename capabilities until their effect is unmistakable.

// pakistan_angle

Pakistan Angle

Create resources for approved Pakistan-specific policies only when sourced and versioned. Do not publish copied private contracts or customer documents as broadly discoverable resources.

Payment, tax, property, legal, or medical tools should prepare evidence for a qualified human and authoritative service, never improvise final status.

// hands_on

Hands-On Exercise

5 steps

  1. Classify five capabilities as tool/resource/prompt.

  2. Write versioned schemas and descriptions.

  3. split read, draft, and commit.

  4. Test authorization and compatibility.

  5. Inspect every returned field for necessity.

// completion_rubric

Completion Rubric

6 checks — tick as you verify

0/6

// sources

Sources

3 official sources — check every claim yourself