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Claude Code v2.1.299 ·

MCP protocol negotiation gate for ccr-proxy now defaults to true, with new fallback handling

The default for tengu_mcp_protocol_negotiation_ccr moved from false to true, alongside a new error code and a primary-then-fallback MCP handler

Group of 2 Under the hood Internal Changes
JSON All of v2.1.299
Under the hoodTier: how much it should matter to you
1Useful: my rating, 1 to 5
2Signal: worth watching, 1 to 5
MCPArea: what it touches
Internal ChangesKind: in v2.1.299,
Internal ChangesSection of the release

Unclear Whether this new fallback step is tied to the protocol negotiation switch is not shown.

What

MCP is the system Claude Code uses to talk to outside programs that add tools. The gate tengu_mcp_protocol_negotiation_ccr controls whether the ccr-proxy connection negotiates which protocol version to use. Its default is listed as moving from false to true. Two code changes appear in the same area, though neither finding confirmed that they are tied to this gate:

  • Error code: a check that used to match error codes -32601 and -32003 now also matches -32021.
  • Fallback handler: a new wrapper tries a primary handler and, if that returns nothing, runs a fallback. It marks the result mcpWire as answered or stood_aside.

The flag server returned on for this gate for this site's account and for the anonymous baseline, and no reading has been taken under this release yet.

Why

With a default of true, protocol negotiation over ccr-proxy no longer depends on a remote value to be switched on. In practice, MCP servers reached that way may negotiate their protocol version instead of using the older fixed method.

Read from
Feature flag
tengu_mcp_protocol_negotiation_ccr On for this account, and not off by default

The flag server returned on for the one account this site reads, and nothing in this release compiles it off by default. The compiled default is shown below, and says which it is when we cannot read one: a fifth of gates compile in a string or a number rather than on or off, and most published releases have no gate table behind them at all. No client can see what the server returns for your account.

This account: on · anonymous baseline: on · compiled default in v2.1.299: on

These values were read against a different version of Claude Code, so treat them as the nearest reading available instead of one taken on this release.

Read once, for one account on one subscription tier, against v2.1.299. It isn't a statement about your account. What a flag value here can and cannot tell you

How sure we are
Something disagreesSomething we can check disagrees with this entry, or the writer said they could not settle it.
The writer flagged doubtWhether this new fallback step is tied to the protocol negotiation switch is not shown.

See this entry in the whole of v2.1.299 →

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