A tool gives an agent a defined way to read information or perform an action. You can implement it as an application function, an API adapter or a capability exposed by another system. The interface should make inputs, outputs and consequences clear.
This guide uses a fictional case lookup and draft-saving service. Its designs are suggestions, not a protocol requirement.
Design the tool around one action
A case lookup should require a case identifier and return structured status, relevant records and source timestamps. It should distinguish a missing case from access denied or a temporary outage.
Saving a draft should be a separate action. Specify the case ID, proposed content and an operation identifier used to recognize retries. Sending the draft should have its own permission and approval policy.
A broad tool that accepts arbitrary commands or unrestricted database queries can expose much more than the task requires. Narrow interfaces make the action easier to test and constrain.
What MCP provides
Model Context Protocol defines communication between an application host, clients and servers exposing capabilities. Servers can supply tools, resources and prompts. Clients discover capabilities and call the ones used by the application. MCP architecture.
A tool performs an operation. A resource supplies context. A prompt supplies a reusable interaction template. An application may support only some capabilities, so verify compatibility rather than assuming every MCP connection behaves identically.
MCP does not select the model, define your business workflow or decide whether a particular user may read a case. Those decisions belong to the surrounding application and services.
Follow a lookup across boundaries
- The user requests case SC-1042 in your application.
- The model proposes a lookup.
- The application checks the requested tool and task scope.
- The tool service authenticates the caller and checks record access.
- It returns the permitted record or a defined error.
- The application provides the result for the next decision.
This path works as a reasoning aid whether the integration uses MCP or an ordinary API. The exact transport and authentication depend on your implementation.
Do not accept a model-supplied user ID as proof of identity. Derive identity from the authenticated context and enforce authorization where data is accessed.
Discovery is not approval
A server can describe a tool that sends email. The application discovering that tool does not mean every task may send email. Likewise, a tool description claiming an operation is harmless should not override your action policy.
Review configured servers, their operators, versions and destinations. For tools exposing external URLs or file paths, constrain where requests can go. Treat server results as untrusted data.
The MCP security documentation covers risks such as token passthrough, confused-deputy attacks and unintended network access. Its guidance requires concrete controls in clients and servers. MCP security best practices.
Keep failures understandable
Return structured errors with enough detail to choose the next step without disclosing secrets. A generic empty result can encourage an agent to treat an outage as “no records found.”
Define which failures can be retried. Read timeouts and uncertain write outcomes need different handling. Confirm whether the destination supports duplicate detection before allowing automatic write retries.
Record operation IDs and outcomes for investigation. Avoid logging full records by default when identifiers and statuses suffice.
Integration checklist
- One clearly described action per tool.
- Typed inputs and distinguishable failure results.
- Authentication and record-level authorization.
- Approved servers and constrained destinations.
- Explicit policies for writes, approval and retries.
- Evidence connecting the request to the actual destination result.
Test the integration directly as well as through the agent. A model should not be required to compensate for an ambiguous or unreliable tool interface.
Choosing frameworks, SDKs and runtimes ↗
Separate models, development tools and hosted products, then compare them against your task.
Prepared with AI assistance and checked against the linked documentation. Examples and numerical limits are illustrative unless stated otherwise. These guides do not report independent product testing. Check current documentation before choosing a tool.