Bridge-launched processes hand down a one-use marker and verify the prompt they are piped / Claude Code v2.1.239

Under the hood

Useful1 Signal3

Bridge not in their notes

Processes launched by the bridge mark their marker as spent, so grandchildren are told apart.

CLAUDE_CODE_BRIDGE_MCP_CARRIERCLAUDE_CODE_BRIDGE_PROMPT_SHA256

What

A process started by the bridge is identified by an environment variable it consumes: it rewrites CLAUDE_CODE_BRIDGE_MCP_CARRIER from "1" to "spent" for anything it spawns, so a grandchild is recognisable as further removed rather than as the bridge child itself.

Details

Three variables are new: CLAUDE_CODE_BRIDGE_MCP_CARRIER, CLAUDE_CODE_BRIDGE_PROMPT_SHA256 and CLAUDE_CODE_BRIDGE_SESSION_ID.
A prompt piped to such a process is accepted only if its sha256 matches CLAUDE_CODE_BRIDGE_PROMPT_SHA256, and that variable is cleared immediately after reading.
Under CLAUDE_CODE_ENVIRONMENT_KIND of "byoc" with CLAUDE_CODE_REMOTE set, the carrier variable is deleted for children rather than marked spent.

Evidence

CLAUDE_CODE_BRIDGE_MCP_CARRIER

Strings lifted out of the shipped bundle, so the claim above can be checked against them.