The Tool Desk
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What an MCP server makes available
A host can discover and call the tools a server registers. Tool names, descriptions, and input schemas are part of the interface presented to the host and its model workflow, so the tool list is a capability surface—not merely documentation. See the MCP TypeScript SDK v2 overview.
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Begin with the server process’s actual authority: which files it can read or change, which APIs and databases it can reach, which commands it can execute, and which network destinations it can contact. Then expose only the operations the server is intended to provide. A tool that can read one approved directory is a narrower capability than a general-purpose filesystem tool; a tool that queries a specific record is narrower than one that accepts arbitrary database commands.
- Give each tool a specific name and description that make its purpose and effects understandable.
- Keep operations and their arguments scoped to the intended resources.
- Limit the process’s operating-system permissions and network access to what those operations need.
What schemas do—and do not—protect
A schema defines the shape of accepted arguments and can reject malformed calls before a handler runs. In the TypeScript SDK v2, the SDK derives JSON Schema from the supplied input schema and validates the call before invoking the handler. The Java SDK also documents default input validation, with behavior and configuration that are specific to that SDK. These examples do not establish one universal validation rule for every MCP SDK or version. See the official first-server guide and the MCP Java SDK server documentation.
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Make inputs precise: require necessary values, constrain types and ranges, and reject paths or identifiers outside the intended scope. Treat this as an input check, not proof that the resulting action is authorized or harmless. A valid path may still point to a sensitive file if the handler’s own access rules are too broad.
For sensitive operations, add application-level authorization and resource checks at the point where trust crosses into the operation. A schema can say that a field is a string; it cannot establish that the caller is permitted to access the resource named by that string or guarantee that the handler behaves safely.
Why stdio requires a clean stdout
In the documented stdio transport, the host launches and owns the local server process, sends JSON-RPC requests on stdin, and reads responses on stdout. The SDK guide states: “stdout is the JSON-RPC channel.” Logs, startup notices, and diagnostic output belong on stderr; an ordinary text line on stdout can corrupt the protocol stream. The stdio guide in the TypeScript SDK documentation shows why output intended for debugging must not be mixed with protocol messages.
Keep the contract simple: stdout carries MCP/JSON-RPC messages only, while stderr carries diagnostics. This distinction is operationally important even when the server is a local child process rather than a network service.
Choose transport for the deployment boundary
stdio is suited to a host-owned local process. For a shared network endpoint, the TypeScript SDK documentation points to HTTP serving instead. Neither choice is inherently safe: transport describes communication, not the full authority of the process or the safety of its handlers.
| Deployment | Connection boundary | Security questions to answer |
|---|---|---|
| stdio | The host launches and communicates with a local child process over stdin and stdout. | What operating-system permissions does the process have? Which files, commands, and network destinations remain reachable? |
| HTTP endpoint | A service is available over a network connection; the SDK documentation identifies HTTP serving for this deployment pattern. | Who can connect? What network authorization and host-level controls apply? What resources can the service process reach? |
Use the deployment model that matches who needs to connect, then enforce permissions at the process and host boundaries. stdio does not sandbox a server: it does not, by itself, restrict file access, command execution, or network access.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Treat annotations as hints, not enforcement
Tool annotations can communicate intended characteristics, but they are not controls. The MCP project advises clients to treat annotations as untrusted unless the server itself is trusted. A readOnlyHint, for example, cannot prevent a handler from modifying a file. The project’s discussion, “Tool Annotations as Risk Vocabulary: What Hints Can and Can’t Do”, frames annotations as advisory information rather than a guarantee of behavior.
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Where an effect could be destructive, make that effect explicit in the tool’s name and interaction design. Add a human approval step where appropriate, but implement it as a real control; an annotation does not require approval. For hard limits, use mechanisms such as sandboxing, operating-system permissions, and network restrictions.
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Test the interface without mistaking inspection for a security audit
The MCP Inspector is a development aid for exploring a server’s tools and calls. The official first-server guide describes launching the supplied command and connecting over stdio. Use it to check that the tools and schemas behave as expected, including how invalid arguments are handled. The documentation does not claim that Inspector use is a security audit, and a successful inspection cannot prove that handlers are safe under every call.
Check the SDK and specification version
Examples and behavior are version-sensitive. The TypeScript SDK v2 overview identifies its implementation as aligned with the 2026-07-28 MCP specification; verify the version and API you use before copying an example. The project’s 2026-07-28 specification announcement describes authorization hardening, including issuer validation. That protocol-level work should not be confused with isolating a local process or authorizing every effect of a handler.
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