Nothing to try yetTier: how much it should matter to you
3Useful: my rating, 1 to 5
4Signal: worth watching, 1 to 5
Agent MessagingArea: what it touches
In DevelopmentKind: in v2.1.299,
What probably matters to youSection of the release
Unclear It is not clear what the pre-send check does with the MCP tool name once it receives it.
What
SendMessage is the tool Claude uses to send a message to another agent. One way it can address a message is through a bridge address. MCP servers are add-on programs that give Claude Code extra tools.
A new helper looks for a single connected MCP server that offers a tool named send_message.
The helper returns that tool's name only when the tengu_flickering_lagoon flag is on. The flag's built-in default is false. Nothing has been read about whether it is switched on for anyone.
When messages are sent to a bridge address, SendMessage now passes this helper to the check it runs before sending. When the flag is off, the helper returns nothing and the check behaves as it did before.
Why
This lays down a path for delivering messages between agents through an MCP server's own send_message tool. Until the flag is on, sending to a bridge address works as before.
Nothing here resolved what this flag was doing on this version, so nothing here should be read as on or off.
This account: no value returned · anonymous baseline: no value returned · compiled default in v2.1.299: off
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.
The entry above is what we published on the day. These lines were added later, as Anthropic's own pages caught up, and they sit beside the original rather than replacing it.
Confirmed sinceAnthropic's documentation has since written up SendMessage, on Agent SDK reference - TypeScript.A `peer` origin identifies which agent sent the message: an in-process [teammate](/docs/en/agent-teams) sending to `main` with `SendMessage`, or a [cross-session peer](/docs/en/cross-session-messaging), another of your Claude Code sessions…agent-sdk/typescriptsee the edit
Confirmed sinceAnthropic's documentation has since written up send_message, on Connect to external tools with MCP."mcp__slack__send_message" // Only send_message from slack serveragent-sdk/mcpsee the edit
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 doubtIt is not clear what the pre-send check does with the MCP tool name once it receives it.
Anthropic's documentation agreesAnthropic's documentation has since written up send_message, on Connect to external tools with MCP.
Anthropic's release notes agreeFixed SendMessage not finding a subagent by name after the CLI restarted and resumed the session (names given before the last compaction…
The name it cites is new in this buildNew in this build: tengu_flickering_lagoon
