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1Scan for outdated or missing drivers - takes under a minute2Clear out junk files and repair common Windows errors3Fix the driver behind crashes, sound loss and screen glitchesAn MCP server is a program that implements the Model Context Protocol (MCP) and exposes external capabilities to an AI application through an MCP client. It can publish tools (functions the model may call), resources (data the application can attach as context), and prompts (reusable instruction templates). MCP messages use JSON-RPC 2.0, while a separate transport layer carries those messages between the host and server.
The result is a standard connection between an AI host—such as an assistant, editor, or agent—and services such as APIs, databases, files, or internal systems. The exact behavior depends on the protocol revision and transport you deploy; this article distinguishes the dated 2025-11-25 specification from changes described in the project’s July 28, 2026 release.
What an MCP server actually is
MCP is a protocol, not a hosting product or a particular programming language. An MCP server is the software on the service side of an MCP connection. It translates protocol requests into operations on an API, database, file system, browser, or another system, then returns structured results.
The server does not normally talk directly to the language model. An AI host creates an MCP client for each server connection. The client handles the protocol conversation and passes discovered capabilities and results to the host and model. This separation lets one host connect to many servers without every integration using a different custom API.
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All client-server messages must follow JSON-RPC 2.0, according to the MCP Basic Protocol Overview. MCP then defines the methods, capability negotiation, notifications, and data structures used over that JSON-RPC channel.
Host, client, and server: the three roles
| Component | What it does | Who controls it |
|---|---|---|
| Host | The AI application, editor, or agent that the user operates. It presents context and results and decides which connected servers are available. | The application vendor or operator |
| MCP client | A protocol connection created by the host for one server. It initializes the session, negotiates capabilities, discovers features, sends requests, and receives responses. | The host manages its client |
| MCP server | A program that advertises tools, resources, and prompts, validates requests, performs the underlying operation, and returns a result or error. | The server developer or service operator |
This one-client-per-server relationship is important. If a host connects to three servers, it generally maintains three independent MCP client sessions, each with its own negotiated capabilities and transport connection.
How an MCP request works, step by step
- Connection: The host starts a local server process or connects to a remote server endpoint using a supported transport.
- Initialization: Client and server exchange protocol-version information and capability declarations. This establishes the feature set that both sides understand.
- Discovery: The client asks what tools, resources, and prompts are available. Servers can also send notifications when relevant lists or state change.
- Selection: The host or model chooses a tool, the application attaches a resource, or a user selects a prompt. The control split is deliberate: tools are model-controlled, resources are application-controlled, and prompts are user-controlled, as summarized by the server overview.
- Request: The client sends a JSON-RPC request containing a method name, an identifier, and arguments (for example, a tool name and its input object).
- Execution: The server validates the arguments, checks authorization, performs the operation against its API, database, files, or other dependency, and creates a structured result.
- Response: The server returns a JSON-RPC result or a JSON-RPC error. The client gives that result to the host, which decides how to show it or provide it to the model.
A simplified exchange looks like this; method names and fields must match the protocol revision and server implementation you use:
{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-11-25","capabilities":{},"clientInfo":{"name":"ExampleHost","version":"1.0"}}}
{"jsonrpc":"2.0","id":1,"result":{"protocolVersion":"2025-11-25","capabilities":{"tools":{}},"serverInfo":{"name":"ExampleServer","version":"1.0"}}}
{"jsonrpc":"2.0","id":2,"method":"tools/list","params":{}}
{"jsonrpc":"2.0","id":3,"method":"tools/call","params":{"name":"lookup_record","arguments":{"id":"A-17"}}}The snippet is an illustration of message shape, not a drop-in server: the server must implement the advertised methods, schemas, authorization, and transport framing.
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Tools, resources, and prompts
Tools: actions the model can request
A tool is a callable function. Examples include querying an API, reading a permitted file, writing a ticket, or running a calculation. Because a model may choose a tool, treat its name, description, input schema, and output as security-sensitive. A tool should validate every argument and enforce authorization independently of the model’s instructions.
Resources: context the application attaches
A resource supplies structured data or content, such as file contents, a database schema, or Git history. The application—not the model alone—controls when a resource is selected and attached, which helps the host apply its own context and access policy.
Prompts: reusable user-facing templates
Prompts are templates selected by a user through a menu, command, or similar interface. They standardize recurring workflows without granting the model an unreviewed action by themselves.
Transport choices: stdio or Streamable HTTP?
| Characteristic | stdio | Streamable HTTP |
|---|---|---|
| Deployment | The host launches the server as a subprocess. | The server exposes an HTTP endpoint that supports POST and GET. |
| Message path | JSON-RPC travels over the process’s standard input and output. | JSON-RPC travels over HTTP; Server-Sent Events may stream messages. |
| Best fit | Local tools, desktop applications, and single-user development. | Remote services, shared infrastructure, and multiple client connections. |
| Operational requirement | Standard output must contain only valid MCP messages; logs belong elsewhere. | Validate Origin, authenticate clients, and apply network access controls. |
When stdio is the practical choice
Use stdio when the host can install or launch your server locally and the data should stay on that machine. The parent process owns the lifecycle, so upgrades, environment variables, file permissions, and crash handling are local concerns. Any diagnostic text accidentally written to stdout can corrupt the protocol stream; send logs to stderr or a file instead.
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When Streamable HTTP is appropriate
Use Streamable HTTP when a server must be reachable as a service or support more than one client connection. One endpoint accepts POST and GET, and SSE can stream events. The transport specification requires Origin validation to prevent DNS-rebinding attacks, recommends binding local deployments to 127.0.0.1, and calls for authentication. Do not expose an unauthenticated endpoint merely because the tools seem harmless.
State, lifecycle, and protocol versions
The architecture separates a JSON-RPC data layer from a transport layer. The data layer covers initialization, capability negotiation, discovery, tools, resources, prompts, and notifications. The transport layer covers connection establishment, message framing, and authorization. Keeping these concerns separate means a server can change how it is reached without redesigning every tool.
Pin and document the protocol revision your host and server expect. The dated specification linked above is the 2025-11-25 revision. The July 2026 release announcement describes a stateless protocol core, multi-round-trip requests, header-based routing, cacheable list results, authorization hardening, an extensions framework, and updated Tier 1 SDKs. Those changes make version negotiation and compatibility testing essential rather than optional.
Designing an MCP server safely
- Minimize capabilities: publish only the tools, resources, and prompts a workflow needs.
- Validate inputs: enforce type, length, format, and allowed-value checks on the server, even when the client supplies a schema.
- Separate read and write actions: make destructive operations distinct tools and require explicit authorization or confirmation in the host.
- Protect secrets: keep API keys and database credentials in the server environment or secret manager; never place them in tool descriptions or returned content.
- Control returned data: redact credentials and unrelated records before sending results back through the model context.
- Secure HTTP: validate every Origin header, bind local listeners to localhost, authenticate remote clients, and authorize each tool.
- Audit operations: record request identity, tool name, outcome, and latency without logging sensitive arguments or secrets.
- Plan failure behavior: return structured errors, use timeouts for dependencies, and avoid retrying non-idempotent actions blindly.
An MCP connection is not a trust boundary that makes an unsafe API safe. The server inherits the risks of every system it can reach, and prompt injection or a misleading tool description can cause a model to request an operation the user did not intend.
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Building and operating a server: a practical checklist
- Define the smallest useful tool, resource, or prompt surface and write precise descriptions and input schemas.
- Choose stdio for a host-launched local process or Streamable HTTP for a network service.
- Implement initialization and capability negotiation for the protocol revision your clients support.
- Implement discovery and return stable names and schemas; handle list changes with the notification mechanisms supported by your revision.
- Validate authorization before touching the underlying API, database, or file system.
- Return structured success and error results and set bounded timeouts around downstream calls.
- Test malformed JSON-RPC, unknown methods, invalid arguments, expired credentials, dependency timeouts, and partial failures.
- Monitor process exits, request latency, error rates, and authentication failures. For stdio, verify that stdout remains protocol-only.
Troubleshooting common MCP failures
The host cannot start a stdio server
Check the executable path, working directory, permissions, and environment variables first. Run the command outside the host to confirm it starts, then inspect stderr. Remove banners, debug prints, and logging from stdout; one stray character can make initialization unparsable.
Initialization reports an unsupported version
The client and server have not agreed on a protocol revision. Upgrade one side, configure a compatible revision, or implement the negotiation behavior required by the revision you target. Do not assume a feature described in a newer release exists in an older client.
Tools do not appear
Confirm that initialization completed successfully, the server advertised the tools capability, and the host actually called the tools-list method. A server may also be returning an empty list because authorization or configuration filtered the tools.
HTTP connections are rejected
Inspect the URL, TLS certificate, authentication header, and Origin handling. A server that correctly rejects an unexpected Origin is protecting itself; configure the host’s allowed origin rather than disabling the check.
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Compare the arguments with the server’s current input schema, then inspect downstream credentials, timeouts, rate limits, and resource permissions. Return a useful JSON-RPC error while avoiding secrets in the message.
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Frequently asked questions
Is an MCP server the same as an API?
No. An API exposes application-specific endpoints. An MCP server is an adapter that presents capabilities through MCP’s discovery, lifecycle, and JSON-RPC conventions, often calling one or more APIs underneath.
Can one MCP server expose both tools and resources?
Yes. A server can advertise any combination of the primitives its implementation supports, and capability negotiation tells the client which features are available.
Does MCP require a particular AI model?
No. MCP standardizes the host-client-server connection. The host determines which model, interface, and approval workflow use the discovered capabilities.
Should a remote MCP server be public on the internet?
Only when its threat model, authentication, authorization, Origin checks, logging, and dependency protections are designed for that exposure. A private network or localhost binding is safer when remote access is unnecessary.
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