Fusion MCP ServerDocumentation

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Configuration

All the options are read through the standard Fusion configuration, so each key can be set with a system property, an environment variable - the uppercased name in the second column - or any ConfigurationSource bean.

MCP options

The following table is generated from the MCPConfiguration record at build time - by the ConfigurationDocumentationGenerator pre-action of the minisite plugin - it can therefore not drift from the implementation.

Name Env Variable Description Default

fusion.mcp.cacheScope

FUSION_MCP_CACHESCOPE

The scope a stateless client may share a cached result at (the cacheScope field).

private

fusion.mcp.cacheTtlMs

FUSION_MCP_CACHETTLMS

How long, in milliseconds, a stateless client may cache a list result (the ttlMs field).

30000

fusion.mcp.clientRequestTimeout

FUSION_MCP_CLIENTREQUESTTIMEOUT

Timeout, in seconds, of the requests sent to the client (sampling, elicitation, roots).

30

fusion.mcp.instructions

FUSION_MCP_INSTRUCTIONS

Instructions sent to the client/model.

Use the exposed tools to answer the user.

fusion.mcp.name

FUSION_MCP_NAME

Programmatic server name.

fusion-mcp-server

fusion.mcp.rejectNonLocalHosts

FUSION_MCP_REJECTNONLOCALHOSTS

Reject the requests whose Host/Origin headers are not loopback addresses, the DNS rebinding protection.

true

fusion.mcp.requestStateSecret

FUSION_MCP_REQUESTSTATESECRET

Secret signing the requestState tokens with HMAC-SHA256, empty to use the opaque codec.

fusion.mcp.requireSession

FUSION_MCP_REQUIRESESSION

Should a valid Mcp-Session-Id header be mandatory once the session was created.

false

fusion.mcp.sessionTimeout

FUSION_MCP_SESSIONTIMEOUT

Session idle timeout in seconds, 0 to disable the expiration.

1800

fusion.mcp.title

FUSION_MCP_TITLE

Human readable server name.

Fusion MCP Server

fusion.mcp.version

FUSION_MCP_VERSION

Server version.

1.0.0

Example: 

java -Dfusion.mcp.name=my-server \
     -Dfusion.mcp.instructions='Use my/search before answering anything about our catalog.' \
     -Dfusion.mcp.requireSession=true \
     ... io.yupiik.fusion.framework.api.main.Launcher

or, in a container: 

FUSION_MCP_NAME=my-server
FUSION_MCP_SESSIONTIMEOUT=600

instructions

fusion.mcp.instructions is sent in the initialize result and prepended to the model context by most clients. It is the one option worth writing carefully: it is where you tell the model when to use this server.

Use the catalog tools to answer any question about products, prices and stock.
Never guess a price, always call my/price.

Session strictness

fusion.mcp.requireSession (default false) trades interoperability for strictness - see Stateless clients . Keep the default unless every client you target is known to send the Mcp-Session-Id header back.

Inherited Fusion options

A MCP server is a plain Fusion HTTP application, so the whole Fusion configuration applies as is - the server port and host, the access log, observability, tracing, JWT. Those keys are documented - and generated - in the Fusion documentation , they are deliberately not duplicated here:

java -Dfusion.http-server.port=8081 ... io.yupiik.fusion.framework.api.main.Launcher

The only inherited key this project changes the default of is fusion.jsonrpc.binding: it is bound to /mcp instead of /jsonrpc so the MCP endpoint is the JSON-RPC one. Setting it back to /jsonrpc exposes the very same methods as a plain JSON-RPC API next to MCP - see Transport .

Overriding a default programmatically

A ConfigurationSource bean wins over the built-in defaults, which is how this project itself binds the JSON-RPC endpoint on /mcp:

@DefaultScoped
public class MyDefaults implements ConfigurationSource {
    @Override
    public String get(final String key) {
        return "fusion.mcp.name".equals(key) ? "my-server" : null;
    }
}

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