René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"McpClient should allow passing _meta in InitializeRequest","articleBody":"The MCP specification defines `_meta` as a valid field on every request, including `initialize`, via the `RequestParams` base type:\n\n```typescript\n// schema.ts (2025-11-25)\ninterface RequestParams {\n  _meta?: {\n    progressToken?: ProgressToken;\n    [key: string]: unknown;\n  };\n}\n\ninterface InitializeRequestParams extends RequestParams {\n  protocolVersion: string;\n  capabilities: ClientCapabilities;\n  clientInfo: Implementation;\n}\n```\n\n`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.\n\n### Impact\n\nServers 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.\n\n### Workaround (current)\n\nThe 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.\n\n### Proposed solution\n\nExpose `_meta` on the `McpClient` builder, consistent with the pattern used in PR #906 for paginated list methods:\n\n```java\n// Option A — builder-level (static meta per client lifetime)\nMcpClient.sync(transport)\n    .initializeMeta(Map.of(\"server_id\", \"…\", \"invocation_id\", \"…\"))\n    .build();\n\n// Option B — overloaded initialize() (dynamic meta per call)\nclient.initialize(Map\u003cString, Object\u003e meta);\n```\n\n### Related\n\n- #344 — added `_meta` field to `McpSchema` classes (schema layer, no client API)\n- #907 / PR #906 — added `_meta` to paginated list client methods (`listTools`, etc.)\n- [MCP spec — General fields: `_meta`](https://modelcontextprotocol.io/specification/2025-11-25/basic#meta)\n- [MCP spec schema (2025-11-25)](https://github.com/modelcontextprotocol/specification/blob/main/schema/2025-11-25/schema.ts)","author":{"url":"https://github.com/nquinquenel","@type":"Person","name":"nquinquenel"},"datePublished":"2026-04-28T13:52:08.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":8},"url":"https://github.com/940/java-sdk/issues/940"}