A client that starts the MCP server from a checkout with no executable to run, or none it can build, is served by the installed release and told so, instead of being turned away

A client that spawns the MCP server is waiting on a handshake, and one that does not get it in time reports the server as failed and carries on without it, so the session is silently the only worker of a repository that has many. When this checkout has no executable to run, or cannot make one, mcp is served by the installed release instead, and stderr names the release and the reason. Nothing else changes: every other command still exits 12, and a stale executable that can be rebuilt is rebuilt.

declared statusdeclared status: guaranteed Declared deterministic and blocking, implemented, and naming the behavioural test that is to prove it. Whether a recorded run supports it is the evidence verdict, shown beside it.

recorded evidencerecorded evidence: not run Declared guaranteed, and not supported by a recorded run: the test is named, and no passing run of the current inputs is recorded. how evidence is recorded

Only `mcp` falls back; every other command still exits 12. The fallback runs after a build that could succeed was tried, or without one when there is no cargo or less free space than the floor, and it names the release and the reason on stderr. A file inside the repository, and one that does not answer --version, is never taken for the installed release. MAJORDOMUS_MCP_BUILD=1 restores the build for a contributor. The 2026-10-16 session that began as the only worker of a repository (ADR 0156) began here, with exit 12 on 4.5GB free against a 5120MB floor.

What it means

A client that spawns the MCP server is waiting on a handshake, and one that does not get it in time reports the server as failed and carries on without it, so the session is silently the only worker of a repository that has many. When this checkout has no executable to run, or cannot make one, mcp is served by the installed release instead, and stderr names the release and the reason. Nothing else changes: every other command still exits 12, and a stale executable that can be rebuilt is rebuilt.

How it works

bin/majordomus-cli mcp decides before it builds. If the crate's executable is missing it runs the installed release (MAJORDOMUS_INSTALLED_BIN, by default majordomus in ~/.local/bin, where install.sh puts it). If the executable is stale it tries the build as before, and falls back when there is no cargo, less free space than MAJORDOMUS_MIN_FREE_MB (5120 by default) or the build fails. A file inside the repository is never taken for the installed release, and neither is one that does not answer --version. MAJORDOMUS_MCP_BUILD=1 makes mcp build like every other command, for a contributor working on the server itself.

How to see it

MAJORDOMUS_NO_BUILD=1 bin/majordomus-mcp --inspect      # with no built executable: serves from the installed release, stderr says why
bash test/run.sh 3177_a_client_always_gets_a_server      # the cases, over stubs of cargo, the free-space floor and the installed release

What it does not cover

It does not install a release: with none installed the launcher fails as it always did, exit 12 and the command to run. It does not make the installed release the checkout's build, so a worker may be served by an older release than the sources; the line on stderr names which. It does not start a turn: a session that is merely opened still waits for a first message (session-start-prompt).

Why it exists

On 2026-10-16 a session opened in a checkout whose executable had never been built, on a volume with 4.5GB free against a floor of 5120MB. The launcher refused the build, exited 12, and the client reported CONNECTION_CLOSED while a release that speaks the same protocol served the same checkout on its port. The session worked without majordomus_peers and majordomus_announce, which is the failure ADR 0156 describes. ADR 0172 decided the fallback; project.shared-server-resilience carries it.

Detail rendered from docs/claims/mcp-client-always-served.md.

From the statement to the proof

1 · the public statement

A client that starts the MCP server from a checkout with no executable to run, or none it can build, is served by the installed release and told so, instead of being turned away

No feature or README responsibility names it; this page is where it is stated.
2 · the canonical claim

mcp-client-always-serveddeclared status: guaranteed in docs/CLAIMS.yaml

Defined in read it on this site · docs/MCP.md
3 · the implementation

bin/majordomus-cli
4 · the test and the scenarios
Test: test/cases/3177_a_client_always_gets_a_server.sh
5 · the recorded runs

recorded evidence: not run how evidence is recorded

Verify it yourself

The test runs in a disposable temporary repository and asserts the behaviour, not a string in the source.

from a clone of the repository

bash test/run.sh 3177_a_client_always_gets_a_server