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An MCP server is the protocol-side application that exposes capabilities to an MCP client. Its core primitives are tools (operations a model can invoke), resources (contextual data an application can provide), and prompts (reusable interaction templates a user can select). Around those primitives, a working server needs identity metadata, capability declarations, registrations and handlers, a message transport, and protocol lifecycle behavior.
The exact classes, function names and handshake rules depend on the SDK and protocol revision. Treat the architecture below as the stable mental model, then verify the version-specific API in the SDK you deploy.
The core components at a glance
| Component | What it provides | Typical controller | Context or operation? |
|---|---|---|---|
| Tools | Callable functions that retrieve information or perform an action | Model-controlled | Operation |
| Resources | Structured content such as files, records or Git history | Application-controlled | Context |
| Prompts | Named templates or instructions for starting an interaction | User-controlled | Instructions |
| Metadata and capabilities | Server identity and the features it advertises | Negotiated with the client | Configuration |
| Registrations and handlers | The definitions and code that implement each feature | Server implementation | Behavior |
| Transport and lifecycle | Message delivery, initialization and session rules | Client and server together | Connectivity |
These pieces are related but not interchangeable. A tool definition without a handler is only a description; a handler without an advertised capability may never be exposed to a client.
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Tools: model-invoked operations
A tool is an executable function. The model can discover its name, description and input schema, decide when it is useful, and request an invocation through the client. The server validates the arguments, runs the implementation and returns a structured result or an error.
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What belongs in a tool definition
- Name: stable and specific, such as
search_issuesrather thanrun. - Description: explain the action, data source and important limits in language a model can use for selection.
- Input schema: required and optional fields, types, allowed values and constraints.
- Result shape: text, structured JSON, links or other content blocks supported by the SDK.
What a tool is not
Tools are not automatically safe merely because they are exposed through MCP. A tool that writes files, sends email or changes a cloud resource still needs authorization, input validation, logging and an explicit policy for destructive operations. The protocol overview uses API requests and file writing as examples of possible tools, not requirements for every server.
Resources: application-selected context
Resources expose information for the model or user to inspect, such as a document, configuration file, database record or Git history. The official overview characterizes resources as application-controlled: the host decides when and how available content is selected and presented, rather than asking the model to execute an arbitrary function for every read.
Resource identity and content
A resource normally has an identifier (often a URI), a human-readable name and metadata describing its media type or purpose. Reading it should return the content in a predictable encoding. Large or sensitive data can be summarized, paged or filtered by the host instead of being dumped into every model context.
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Use a resource when the main job is supplying stable context. Use a tool when obtaining the result requires an operation, parameters, side effects or an external call. A server might expose a repository file as a resource and also provide a run_tests tool; the first is context, while the second performs work.
Prompts: reusable, user-controlled instructions
Prompts are predefined templates for model interactions. A client can present them as choices such as “summarize this incident” or “review this pull request.” The user selects the template, and the host supplies any arguments before sending the resulting messages to the model.
Why prompts are separate from tools
A prompt does not execute a server-side operation. It standardizes wording, role instructions and expected context. Keeping it separate lets users choose the interaction while tools remain available for the model to call during that interaction.
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Identity and capability declarations
Before using features, a client needs to know which server it connected to and what the server supports. Server metadata commonly includes a name and version. Capability declarations indicate whether the implementation offers tools, resources, prompts and optional features such as completions or logging.
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The Java SDK guide demonstrates this pattern by configuring server information and capability flags, then registering the available features. The names and builder methods differ between SDKs, so copy the concept rather than assuming a Java-specific API exists in TypeScript, Python or Go.
Advertise only what is implemented
- Declare a capability only when the corresponding operation is registered and tested.
- Keep server version metadata aligned with the deployed build.
- Do not infer that a client supports every capability your server advertises; capability negotiation is two-sided.
Registrations and handlers: connecting declarations to code
Registration binds a public feature definition to executable behavior. A typical server startup sequence creates the server object, registers tool/resource/prompt definitions, attaches handlers, and then connects a transport.
Handler responsibilities
- Validate arguments against the declared schema.
- Apply authentication and authorization before accessing a system.
- Call the underlying API, filesystem or database with timeouts and cancellation.
- Normalize successful results into protocol content blocks.
- Return actionable errors without leaking secrets or internal stack traces.
Some SDKs combine registration and implementation in one call; others separate a catalogue from handler functions. Both are valid implementation patterns.
Transport: how protocol messages travel
Transport carries protocol messages between the client and server. The Java guide describes STDIO, SSE and Streamable HTTP implementations, each suited to different deployment models.
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STDIO runs the server as a child process and exchanges messages over standard input and output. It is convenient for local desktop hosts because process startup and credentials stay on the same machine. Keep stdout reserved for protocol traffic; send diagnostics to stderr.
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SSE
Server-Sent Events provide a streaming HTTP connection for server-to-client messages, with a complementary path for client requests. This model can fit a separately hosted service, but requires careful handling of reconnects, authentication and proxy timeouts.
Streamable HTTP
Streamable HTTP uses HTTP as the connection model while allowing streamed protocol messages. It is suitable for services behind normal HTTP infrastructure, provided the deployment preserves request identity, authorization and any required streaming behavior.
The evidence establishes that these transports coexist; it does not establish a universal performance or security ranking. Choose based on where the server runs, how clients connect and what your hosting environment supports.
Lifecycle and protocol version
Lifecycle rules are version-dependent. Older protocol revisions use an initialize handshake in which client and server exchange protocol versions and capabilities before normal requests. The Go SDK documentation describes this legacy flow through versions up to 2025-11-25.
The same guide describes a stateless model introduced with the 2026-07-28 protocol revision. The TypeScript SDK v2 documentation identifies v2 as its stable release line implementing that 2026-07-28 specification. Do not mix examples from those models: a client expecting initialization state and a stateless endpoint can disagree about sessions, capabilities or request ordering.
Practical version checklist
- Identify the protocol revision implemented by your client and server.
- Confirm the SDK release line and its lifecycle documentation.
- Use the matching initialization or stateless connection pattern.
- Test reconnects, cancellation and capability negotiation with the same revision used in production.
What happens during a request
- Connect: the client opens STDIO, SSE or HTTP transport.
- Negotiate: the parties exchange protocol/version and capability information when required by that revision.
- Discover: the client obtains the server’s available tools, resources and prompts.
- Select: the host presents resources or prompts, while the model may choose a tool.
- Dispatch: the server routes the request to the registered handler.
- Respond: the handler returns content or a protocol error through the transport.
- Close or continue: the connection follows the lifecycle rules of the negotiated model.
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Start with the contract
Write down the capabilities clients need before choosing an SDK. For each tool, specify inputs, side effects and failure behavior. For each resource, define identifiers, access rules and size limits. For each prompt, document arguments and the user outcome it is intended to produce.
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Separate policy from implementation
Keep authorization, rate limits and audit logging outside individual business calls where possible. A shared policy layer prevents one handler from accidentally bypassing checks applied to another.
Make failures explicit
- Reject malformed arguments before making an external request.
- Use bounded timeouts for network and database calls.
- Return a clear “not found” result rather than an empty success when a resource is missing.
- Distinguish authentication failures, permission failures and upstream outages.
- Log correlation IDs, but never log access tokens or private resource contents.
Security, reliability and operations
An MCP server inherits the risks of every system behind its handlers. Restrict filesystem paths, use least-privilege credentials, validate URLs and shell arguments, and require confirmation for destructive tools. Treat resource content as untrusted input: documents can contain instructions that should not override the host’s policy.
For reliability, make handlers idempotent when practical, support cancellation, and avoid blocking the transport thread during long work. For remote transports, configure proxy and load-balancer timeouts longer than the expected request duration, and test reconnect behavior. Capability discovery should be deterministic so a client does not see a different interface on each connection.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Common implementation problems
The client shows no tools
Check that the server advertised the tools capability, registered at least one tool, and completed the version-appropriate lifecycle. In STDIO mode, remove diagnostic prints from stdout.
A resource reads successfully but has no useful content
Verify the resource identifier, media type and encoding returned by the handler. Add pagination or summarization for large content instead of silently truncating it.
Initialization or connection errors
Confirm that both sides use compatible protocol revisions and transport implementations. A legacy initialize client cannot be assumed to work with a stateless 2026-07-28 endpoint without an adapter or matching SDK configuration.
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Tool calls fail intermittently
Inspect upstream timeouts, cancellation handling and connection reuse. Return structured errors, retry only operations that are safe to retry, and include a request ID in server logs and client-visible diagnostics.
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Choosing the right mental model
When debugging an MCP integration, ask three questions in order: Is the capability advertised? Is its definition registered? Can its handler complete over the negotiated transport and lifecycle? That sequence separates catalogue problems from business-logic failures and from connection-version mismatches.
Frequently Asked Questions
Does every MCP server need tools, resources and prompts?
No. They are the central primitives, but a server can expose only the subset its use case requires. Its capability declaration should match what is actually implemented.
Are MCP resources the same as files on disk?
No. A file is one possible source. A resource is protocol-visible contextual content and may come from a database, API, Git history or another system.
Which transport should a production server use?
Choose according to deployment: STDIO for a local child process, SSE or Streamable HTTP for hosted connections. The protocol material does not establish a universal performance winner.
Why must protocol revision be checked before copying an example?
Lifecycle behavior changed between the legacy initialize model and the stateless model introduced in the 2026-07-28 revision. SDK examples are therefore version-specific.
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