Skill supplies reusable instructions; MCP exposes server resources or callable tools. Choose based on whether the task needs guidance, read-only facts, or an action.
Skill supplies reusable instructions; MCP exposes server resources or callable tools. Choose based on whether the task needs guidance, read-only facts, or an action. Contract: inventory_lookup(sku) returns integer quantity and UTC as_of. A-17=8; B-04=0. A count does not justify write access.
Skill supplies reusable instructions; MCP exposes server resources or callable tools. Choose based on whether the task needs guidance, read-only facts, or an action.
Contract: inventory_lookup(sku) returns integer quantity and UTC as_of. A-17=8; B-04=0. A count does not justify write access.
Inspect server identity, authorization scope, schema, and error behavior. Reject unknown parameters and stale results; never translate an error into zero stock.
Compare the example input with the result and acceptance checks.
Review the two read-only inventory examples against the fictional contract below. Preserve a quantity of zero and the sample time on each result.
tool: inventory_lookup(sku: string)
returns: {sku: string, quantity: integer, as_of: UTC timestamp}
required_params: ["sku:string"]
sample_results:
- {sku: "A-17", quantity: 8, as_of: "2026-10-15T09:00:00Z"}
- {sku: "B-04", quantity: 0, as_of: "2026-10-15T09:00:00Z"}
rejection_rule: reject unknown fieldsReview the read-only contract. Return JSON fields tool_name, read_only, required_params, sample_results, and rejection_rule; preserve an as_of timestamp in every sample_results item. Do not call a server or claim live inventory.
{
"tool_name": "inventory_lookup",
"read_only": true,
"required_params": [
"sku:string"
],
"sample_results": [
{
"sku": "A-17",
"quantity": 8,
"as_of": "2026-10-15T09:00:00Z"
},
{
"sku": "B-04",
"quantity": 0,
"as_of": "2026-10-15T09:00:00Z"
}
],
"rejection_rule": "reject unknown fields"
}Why it is included: Guides developers in exposing an external API as discoverable, well-described, and verified MCP tools.
When it does not fit: Do not rebuild a server when this task only calls an existing read-only lookup.
Why it is included: Expose a narrowly scoped query tool so MCP clients can read application data but cannot change it.
When it does not fit: It cannot serve a task that must update stock; this example intentionally has no write operation.
Why it is included: Inspect names, descriptions, schemas, permissions, and side effects before exposing remote MCP tools to an agent.
When it does not fit: Use a server-authoring guide instead when the client does not yet have a server to inspect.
Why it is included: Separate retryable failures, input rejections, and internal faults without exposing server secrets.
When it does not fit: A static resource read needs no callable-tool error contract.
Why it is included: Expose documents as stable URI resources so clients can discover and read them without write access.
When it does not fit: This SOP covers stable document resources. The example uses a query tool because stock counts change and need a fresh as_of value; that choice is about this task, not a protocol ban on dynamic resources.
Why it is included: Expose a narrowly scoped application function as a tool with an explicit parameter contract.
When it does not fit: If the task only reads a Skill or static resource, no tool schema is needed.