McpClient should allow passing _meta in InitializeRequest

[nquinquenel]

The MCP specification defines _meta as a valid field on every request, including initialize, via the RequestParams base type:

// schema.ts (2025-11-25)
interface RequestParams {
  _meta?: {
    progressToken?: ProgressToken;
    [key: string]: unknown;
  };
}

interface InitializeRequestParams extends RequestParams {
  protocolVersion: string;
  capabilities: ClientCapabilities;
  clientInfo: Implementation;
}

McpSchema.InitializeRequest in the Java SDK correctly models this (the _meta field was added in #344). However, McpSyncClient#initialize() / LifecycleInitializer#doInitialize always constructs the request with the 3-arg convenience constructor, hardwiring _meta to null. There is no API to populate it.

Impact

Servers that rely on _meta of the initialize request will always receive null metadata when called from a Java client, even though the spec explicitly allows it. This is inconsistent with tools/call, where CallToolRequest._meta is fully caller-controlled.

Workaround (current)

The only workaround is to wrap the McpClientTransport and intercept the outgoing initialize JSONRPCRequest in sendMessage, rebuilding it with _meta populated. This works but couples application code to internal wire-format details (JSONRPCRequest, InitializeRequest record structure) that should be opaque to callers.

Proposed solution

Expose _meta on the McpClient builder, consistent with the pattern used in PR #906 for paginated list methods:

// Option A — builder-level (static meta per client lifetime)
McpClient.sync(transport)
    .initializeMeta(Map.of("server_id", "…", "invocation_id", "…"))
    .build();

// Option B — overloaded initialize() (dynamic meta per call)
client.initialize(Map<String, Object> meta);

Related

• feat: Add support for _meta across fields defined in MCPSchema #344 — added _meta field to McpSchema classes (schema layer, no client API)
• feat: Add support for meta parameter to list tools call( paginated request already support meta field). #907 / PR feat: add support for meta parameter in client paginated list queries #906 — added _meta to paginated list client methods (listTools, etc.)
• MCP spec — General fields: _meta
• MCP spec schema (2025-11-25)