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HomeDocsClaude Code CLI 21 Aug 2026
One change · claude-code

Connect Claude Code to tools via MCP changedmcp

Nearest release: v2.1.239, published 2 hours before this site recorded the change. Shown because the two are within 24 hours of each other. Nothing here says the release caused the edit.

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302302### MCP client runtimes
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304Claude Code connects to MCP servers through one of two client runtimes. The v1 runtime is built on MCP TypeScript SDK 1.x. The v2 runtime is the same code on [MCP TypeScript SDK 2.0](https://ts.sdk.modelcontextprotocol.io/v2/), which adds MCP protocol revision 2026-07-28. The rest of this page applies to both.
304Claude Code connects to MCP servers through one of two client runtimes. The v1 runtime is built on MCP TypeScript SDK 1.x. The v2 runtime is the same code on [MCP TypeScript SDK 2.0](https://ts.sdk.modelcontextprotocol.io/v2/), which adds MCP protocol revision 2026-07-28. The rest of this page applies to both runtimes, except where a section names the v2 runtime.
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306306On Claude Code v2.1.232 or later, Claude Code uses the v2 runtime. It picks a runtime each time you start it and keeps it until you exit. It uses v1 when you run it:
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312312On v2, Claude Code also:
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314* Asks HTTP, claude.ai connector, and stdio servers whether they support the newer revision, and uses it with those that do. It connects to every other server as v1 does.
314* Asks HTTP and claude.ai connector servers whether they support the newer revision, and uses it with those that do. It asks stdio servers only if you set [`MCP_PROTOCOL_NEGOTIATION`](/docs/en/env-vars) to `auto`, and connects to every other server as v1 does.
315315* Receives `list_changed` notifications from servers on the newer revision over a [stream it holds open](#notification-streams-on-the-v2-runtime).
316316* Doesn't register a [channel](#push-messages-with-channels) server that connects on the newer revision, because that revision can't carry channel messages.
317317* Fails an [MCP OAuth sign-in](#authenticate-with-remote-mcp-servers) whose authorization response names an unexpected issuer.
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350350An MCP server can also push messages directly into your session so Claude can react to external events like CI results, monitoring alerts, or chat messages. To enable this, your server declares the `claude/channel` capability and you opt it in with the `--channels` flag at startup. See [Channels](/docs/en/channels) to use an officially supported channel, or [Channels reference](/docs/en/channels-reference) to build your own.
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352On the [v2 runtime](#mcp-client-runtimes), a channel server that negotiates MCP protocol revision 2026-07-28 can't deliver channel messages, so Claude Code doesn't register it as a channel. Setting [`MCP_PROTOCOL_NEGOTIATION`](/docs/en/env-vars) to `legacy` keeps it on the earlier handshake, along with every other server in the process.
352On the [v2 runtime](#mcp-client-runtimes), if you set [`MCP_PROTOCOL_NEGOTIATION`](/docs/en/env-vars) to `auto` and a channel server negotiates MCP protocol revision 2026-07-28, it can't deliver channel messages, so Claude Code doesn't register it as a channel. Leaving the variable unset, or setting it to `legacy`, keeps stdio servers on the earlier handshake.
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