# What changed in @langchain/mcp-adapters

> 35 changes in @langchain/mcp-adapters 2 from its official release notes, upgrade guides and advisories: 20 breaking, 11 new APIs and options. Each one cited. Sources checked 2026-10-19. HTML: https://unstale.sh/npm/@langchain/mcp-adapters/

To see which of these apply to a repository, run `npx @unstale/cli init` in it.

## @langchain/mcp-adapters 2

### Breaking changes

- MCP server and tool names are joined with a single underscore instead of two; the additionalToolNamePrefix separator remains unchanged. (2.0.1) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.1)
- Zod 4 validation rejects unknown or inapplicable options, conflicting transports, empty server maps, and setting both servers and mcpServers; loadMcpTools options are validated too. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)
- Read server configuration from adapter.config.servers; the returned snapshot no longer has mcpServers, and changing it does not reconfigure the adapter. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)
- This legacy connection option now requires mode legacy; automatic HTTP-to-SSE fallback in auto mode is limited to HTTP 404 and 405. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)
- This setting no longer emits logs; use onConnectionError and per-server notification callbacks for diagnostics. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)
- Duplicate names exposed across servers or repeated by one server now throw MCPClientError when server-name prefixing is disabled. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)
- Modern MCP elicitation is enabled by default and pauses a run with a LangGraph interrupt when a tool requests user input; set elicitation false to opt out. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)
- Now returns an SDK 2 client; SDK clients negotiate only the legacy protocol by default unless configured for automatic version negotiation. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)
- A server-reported tool failure now returns a ToolMessage with status error for agent calls; direct invocation still throws ToolException. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)
- Now surfaces server errors instead of returning an empty list and logging only under DEBUG. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)
- Read structuredContent from this artifact entry; protocol metadata and structured output no longer appear in model-visible single-text-block content. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)
- Use the new client package import when creating an SDK client yourself for loadMcpTools. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)
- Images and audio now expose data and mimeType, while resource links become file blocks; update consumers of the former content fields. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)
- Configure this notification callback on each server rather than at the adapter level. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)
- Resource conversion no longer fetches URIs automatically; call readResource explicitly when you need to fetch a resource. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)
- Throws on servers using the modern protocol; set the server's logLevel instead. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)
- The type now describes SSE only; use StreamableHTTPConnection for HTTP or Connection for any transport. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)
- Schemas now reach the model as declared by the server instead of being simplified by inlining or flattening schema constructs. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)
- Tool names now include the server name by default, such as docs__search; update references or disable prefixing. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)
- Remove this option; it is no longer supported. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)

### New APIs and options

- Accepts token providers as well as OAuth providers; a provider token takes precedence over a configured Authorization header once available. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)
- Use refresh to refresh discovery or bypass to skip the cache; a failed refresh leaves previously returned tools usable. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)
- Resume an interrupted LangGraph run with elicitation responses supplied inside a LangGraph Command. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)
- Replaces getTools as the recommended API and returns a flat list of executable LangChain tools. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)
- Replaces initializeConnections as the recommended API and groups tools by server. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)
- Use MCPAdapter for new code; it replaces MultiServerMCPClient as the recommended adapter API. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)
- Replaces ClientConfig as the recommended adapter configuration type. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)
- Each server can negotiate independently; mode defaults to auto, or can require modern protocol or skip probing a known legacy server. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)
- Use this server option to list URIs to watch on modern servers, which reject resources/subscribe. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)
- Now exported so callers can identify authentication failures by walking an error's cause chain. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)
- Set mode to auto when constructing an SDK 2 Client to negotiate with modern servers; its default negotiates only the legacy protocol. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)

### Behavior and default changes

- Await close to stop active discovery and pending reconnects and clear connections and caches; the adapter can be reused by discovering tools again. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)
- Set to false to retain unprefixed tool names; duplicate names then cause listTools and getTools to throw MCPClientError. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)
- Hooks now preserve ToolMessage and LangGraph Command results instead of flattening or rejecting them; hook state is typed unknown. (2.0.0) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.0)

### Fixes and other changes

- After an HTTP-to-SSE fallback fails, the thrown MCPClientError now exposes the SSE attempt's SDK error as cause, allowing checks such as UnauthorizedError.isInstance(error.cause) to work. (2.0.1) — [release notes](https://github.com/langchain-ai/langchainjs/releases/tag/%40langchain/mcp-adapters%402.0.1)