Quarkus MCP Server

"Model Context Protocol (MCP) is an open protocol that enables seamless integration between LLM applications and external data sources and tools."

This extension provides a declarative API that enables developers to implement the MCP server features easily.

Supported transports

MCP currently defines two standard transports for client-server communication. This extension supports both transports and defines a unified declarative API. In other words, the server features are defined with the same API but the selected transport determines the way the MCP server communicates with clients.

If you want to use the stdio transport you’ll need to add the io.quarkiverse.mcp:quarkus-mcp-server-stdio extension to your build file first. For instance, with Maven, add the following dependency to your POM file:

<dependency>
    <groupId>io.quarkiverse.mcp</groupId>
    <artifactId>quarkus-mcp-server-stdio</artifactId>
    <version>1.0.0.Alpha2</version>
</dependency>
If you use the stdio transport then your app should not write anything to the standard output. Logging in the console is automatically disabled. And the standard output stream is set to "null" when the app is started by default.

If you want to use the HTTP/SSE transport you’ll need to add the io.quarkiverse.mcp:quarkus-mcp-server-sse extension to your build file first. For instance, with Maven, add the following dependency to your POM file:

<dependency>
    <groupId>io.quarkiverse.mcp</groupId>
    <artifactId>quarkus-mcp-server-sse</artifactId>
    <version>1.0.0.Alpha2</version>
</dependency>

Supported server features

An MCP server provides some building blocks to enrich the context of language models in AI apps. In this extension, a server feature (prompt, resource, tool) is represented by an annotated business method of a CDI bean. The execution model and context handling follow the idiomatic approach used in fundamental Quarkus extensions (such as quarkus-rest and quarkus-scheduler). For example, when a server feature is executed the CDI request context is activated and a Vert.x duplicated context is created.

Execution model

A server feature method may use blocking or non-blocking logic. The execution model is determined by the method signature and additional annotations such as @Blocking and @NonBlocking.

  • Methods annotated with @RunOnVirtualThread, @Blocking or @Transactional are considered blocking.

  • Methods declared in a class annotated with @RunOnVirtualThread are considered blocking.

  • Methods annotated with @NonBlocking are considered non-blocking.

  • Methods declared in a class annotated with @Transactional are considered blocking unless annotated with @NonBlocking.

  • If the method does not declare any of the annotations listed above the execution model is derived from the return type:

    • Methods returning Uni and Multi are considered non-blocking.

    • Methods returning any other type are considered blocking.

  • Kotlin suspend functions are always considered non-blocking and may not be annotated with @Blocking, @NonBlocking or @RunOnVirtualThread and may not be in a class annotated with @RunOnVirtualThread.

  • Non-blocking methods must execute on the connection’s event loop thread.

  • Blocking methods must execute on a worker thread unless annotated with @RunOnVirtualThread or in a class annotated with @RunOnVirtualThread.

  • Methods annotated with @RunOnVirtualThread or declared in class annotated with @RunOnVirtualThread must execute on a virtual thread, each invocation spawns a new virtual thread.

Prompts

MCP provides a standardized way for servers to expose prompt templates to clients.

import io.quarkiverse.mcp.server.Prompt;
import io.quarkiverse.mcp.server.PromptArg;
import io.quarkiverse.mcp.server.PromptMessage;
import jakarta.inject.Inject;

// @Singleton (1)
public class MyPrompts {

    @Inject (2)
    FooService fooService;

    @Prompt(description = "Put you description here.") (3)
    PromptMessage foo(@PromptArg(description = "The name") String name) { (4)
        return PromptMessage.withUserRole(new TextContent(fooService.ping(name)));
    }

}
1 The @Singleton scope is added automatically, if needed.
2 MyPrompts is an ordinary CDI bean. It can inject other beans, use interceptors, etc.
3 @Prompt annotates a business method of a CDI bean that should be exposed as a prompt template. By default, the name of the prompt is derived from the method name.
4 The @PromptArg can be used to customize the description of an argument.

The result of a "prompt get" operation is always represented as a PromptResponse. However, the annotated method can also return other types that are converted according to the following rules.

  • If the method returns a PromptMessage then the reponse has no description and contains the single message object.

  • If the method returns a List of PromptMessages then the reponse has no description and contains the list of messages.

  • The method may return a Uni that wraps any of the type mentioned above.

In other words, the return type must be one of the following list:

  • PromptResponse

  • PromptMessage

  • List<PromptMessage>

  • Uni<PromptResponse>

  • Uni<PromptMessage>

  • Uni<List<PromptMessage>>

Resources

MCP provides a standardized way for servers to expose resources to clients.

import io.quarkiverse.mcp.server.Resource;
import jakarta.inject.Inject;
import java.nio.file.Files;

// @Singleton (1)
public class MyResources {

    @Inject (2)
    FooService fooService;

    @Resource(uri = "file:///project/alpha") (3)
    BlobResourceContents alpha(String uri) {
        return BlobResourceContents.create(uri, Files.readAllBytes(Paths.ALPHA));
    }

}
1 The @Singleton scope is added automatically, if needed.
2 MyResources is an ordinary CDI bean. It can inject other beans, use interceptors, etc.
3 @Resource annotates a business method of a CDI bean that should be exposed as a resource. By default, the name of the resource is derived from the method name.

The result of a "resource read" operation is always represented as a ResourceResponse. However, the annotated method can also return other types that are converted according to the following rules.

  • If the method returns an implementation of ResourceContents then the reponse contains the single contents object.

  • If the method returns a List of ResourceContents implementations then the reponse contains the list of contents objects.

  • The method may return a Uni that wraps any of the type mentioned above.

Tools

MCP provides a standardized way for servers to expose tools that can be invoked by clients.

import io.quarkiverse.mcp.server.Tool;
import io.quarkiverse.mcp.server.ToolArg;
import io.quarkiverse.mcp.server.ToolResponse;
import jakarta.inject.Inject;

// @Singleton (1)
public class MyTools {

    @Inject (2)
    FooService fooService;

    @Tool(description = "Put you description here.") (3)
    ToolResponse foo(@ToolArg(description = "The name") String name) {
        return ToolResponse.success(
                new TextContent(fooService.ping(name)));
    }

}
1 The @Singleton scope is added automatically, if needed.
2 MyTools is an ordinary CDI bean. It can inject other beans, use interceptors, etc.
3 @Tool annotates a business method of a CDI bean that should be exposed as a tool. By default, the name of the tool is derived from the method name.

A result of a "tool call" operation is always represented as a ToolResponse. However, the annotated method can also return other types that are converted according to the following rules.

  • If the method returns java.lang.String then the reponse is "success" and contains the single TextContent object.

  • If the method returns an implementation of io.quarkiverse.mcp.server.Content then the reponse is "success" and contains the single content object.

  • If the method returns a List of Content implementations or `String`s then the reponse is "success" and contains the list of relevant content objects.

  • The method may return a Uni that wraps any of the type mentioned above.

Traffic logging

The extension can log JSON messages sent and received for debugging purposes. To enable traffic logging, set the quarkus.mcp.server.traffic-logging.enabled configuration property to true. Note that the number of logged characters is limited. The default limit is 100, but you can change this limit with the quarkus.mcp.server.traffic-logging.text-limit configuration property.

Example server configuration
quarkus.mcp.server.traffic-logging.enabled=true (1)
quarkus.mcp.server.traffic-logging.text-limit=50 (2)
1 Enables traffic logging.
2 Set the number of characters of a JSON message which will be logged.

Extension configuration reference

Configuration property fixed at build time - All other configuration properties are overridable at runtime

Configuration property

Type

Default

quarkus.mcp.server.server-info.name

The name of the server is included in the response to an initialize request as defined by the spec. By default, the value of the quarkus.application.name config property is used.

Environment variable: QUARKUS_MCP_SERVER_SERVER_INFO_NAME

string

quarkus.mcp.server.server-info.version

The version of the server is included in the response to an initialize request as defined by the spec. By default, the value of the quarkus.application.version config property is used.

Environment variable: QUARKUS_MCP_SERVER_SERVER_INFO_VERSION

string

quarkus.mcp.server.traffic-logging.enabled

If set to true then JSON messages received/sent are logged.

Environment variable: QUARKUS_MCP_SERVER_TRAFFIC_LOGGING_ENABLED

boolean

false

quarkus.mcp.server.traffic-logging.text-limit

The number of characters of a text message which will be logged if traffic logging is enabled.

Environment variable: QUARKUS_MCP_SERVER_TRAFFIC_LOGGING_TEXT_LIMIT

int

100

Configuration property fixed at build time - All other configuration properties are overridable at runtime

Configuration property

Type

Default

quarkus.mcp.server.stdio.null-system-out

If set to true then the standard output stream is set to "null" when the app is started.

Environment variable: QUARKUS_MCP_SERVER_STDIO_NULL_SYSTEM_OUT

boolean

true

Configuration property fixed at build time - All other configuration properties are overridable at runtime

Configuration property

Type

Default

quarkus.mcp.server.sse.root-path

The root path.

Environment variable: QUARKUS_MCP_SERVER_SSE_ROOT_PATH

string

/mcp