diff --git a/mcp_server.md b/mcp_server.md
index f5c3df8..d4cfeca 100644
--- a/mcp_server.md
+++ b/mcp_server.md
@@ -178,9 +178,33 @@ judged by where it points.
 
 Two arguments are deliberately **not** confined: `binary` (the
 `qelectrotech` executable) and `elements_dir` (the element collection).
-Those are configuration, chosen once by whoever runs the server, and both
-normally live in `/usr` or a build tree — outside any sensible workspace.
-Confining them would reject the ordinary case while stopping nothing.
+**This changes with the PR below.**
+
+#### The program the server runs
+
+> **Status: pending.** This section describes
+> [PR #1129](https://github.com/qelectrotech/qelectrotech-source-mirror/pull/1129),
+> not yet merged. Nothing here works until that lands — check the PR
+> before trying any of this against your own build. This section will drop
+> this notice once it does.
+
+Before this change, `binary` was taken from each tool call and whatever
+executable file it named was run, with the call's own paths as arguments.
+Text inside a project from someone else could therefore steer an assistant
+into starting another program. Now the server finds QElectroTech itself
+and a call cannot choose:
+
+| Variable | Effect |
+|---|---|
+| `QET_BINARY` | the QElectroTech the tools launch. Without it: `qelectrotech` on `PATH`, else the installation the server sits in (`<prefix>/share/qelectrotech/mcp/qet_mcp.py`) |
+| `QET_MCP_BINARIES` | other executables a call may name as `binary`, separated like `QET_MCP_WORKSPACE` — for comparing two builds |
+| `QET_MCP_ALLOW_ANY_BINARY=1` | turns the check off: a call can then run any program |
+| `QET_MCP_ELEMENTS` | element collections a call may name as `elements_dir`, besides the workspace and the installed one |
+
+`binary` can be left out of every call. When given, it must be the same file
+(after following symlinks) or one listed in `QET_MCP_BINARIES`; anything else
+is refused, even a file inside the workspace. A setup that relied on passing
+a program that is not on `PATH` needs `QET_BINARY` in its `env` block.
 
 **Nothing is overwritten unasked.** `qet_export`, `qet_edit`,
 `qet_project_new` and `qet_element_build` refuse an `output` that already