diff --git a/README.md b/README.md
index 34db9be..eb9ded8 100644
--- a/README.md
+++ b/README.md
@@ -81,6 +81,10 @@ or CAN and radio injection, raises `simantic.NotSupported` and names the gap
 instead of quietly doing nothing. You can follow what each engine covers in
 [simantic-core#183](https://github.com/simantic-dev/simantic-core/issues/183).
 
+This `backend=` dispatch is meant to be the one place engine choice happens —
+see [`docs/unified-surface.md`](docs/unified-surface.md) for where the CLI and
+MCP install are expected to grow the same logic.
+
 ## Testing with pytest
 
 Take the `sim` fixture and write ordinary tests:
diff --git a/docs/session-api.md b/docs/session-api.md
index cb94582..0f68846 100644
--- a/docs/session-api.md
+++ b/docs/session-api.md
@@ -106,3 +106,6 @@ an agent that needs a loop, is better served by code: the loop runs in the
 engine's process, only the conclusion enters the transcript, and the script
 becomes a fixture. `Sim` is that path; the MCP servers remain for interactive
 poking and are expected to shrink to a thin adapter over it.
+
+See [`unified-surface.md`](unified-surface.md) for where the CLI dispatch and
+an eventual unified MCP server's client-side install are expected to land.
diff --git a/docs/unified-surface.md b/docs/unified-surface.md
new file mode 100644
index 0000000..67cad7f
--- /dev/null
+++ b/docs/unified-surface.md
@@ -0,0 +1,48 @@
+# Where the unified surface lands
+
+This package (`pip install simantic`) is already the front door for the
+engines: it locates and installs the Renode and Rust binaries, and `Sim`
+dispatches between them by `backend=`. Two extensions of that same role are
+planned but not yet built. Recording them here so the direction is written
+down before the code is.
+
+## The `Sim` dispatch API grows into the CLI's job too
+
+`Sim(backend="renode"|"rust")` already picks an engine per call from Python.
+Today nothing on the command line does the equivalent — `simantic`/`smtc`
+only wraps `auth`/`install`/`status`, and running a simulation from a shell
+means invoking the engine's own binary directly (`sim ...`) with no backend
+choice at that layer.
+
+The plan is for `simantic`/`smtc` to grow the same dispatch `Sim` already has,
+instead of engines each keeping a separate CLI surface. Concretely: the
+`backend=` selection and the `NotSupported`-with-a-named-gap behavior that
+`Sim` already implements become the one place that decision is made, called
+from both the Python API and the command line, rather than duplicated if a
+CLI grows its own copy.
+
+## The MCP install front door
+
+Two MCP servers exist today, one per engine, and neither is reachable from
+this package: `pyrite-mcp` (Rust engine, active) and the Renode engine's MCP
+server (stale). The two engines have genuinely different capabilities — e.g.
+Renode's `gdb_*` control plane and multi-machine/CAN/radio scenarios don't
+exist on the Rust side — so unifying them isn't one server absorbing the
+other's tools; it's a `Sim(backend=...)`-shaped front door that dispatches
+each call to whichever engine implements it. Per
+[`session-api.md`](session-api.md#why-not-mcp), the MCP servers are expected
+to shrink into thin adapters over `Sim` as that dispatch layer takes shape.
+
+This package is the natural client-side install point for whichever unified
+MCP server results, for the same reason it already installs the engines
+instead of asking a user to find them: `install.py` already has the
+"locate this pinned binary, fetch it if missing, verify it against a
+checksum" logic (`install_engine`, `install_rust_engine`). An MCP server
+binary is one more thing of that shape — `simantic install mcp` mirrors
+`simantic install engine`, and `simantic mcp` launches what it resolved, the
+same relationship `Sim` has to the simulation binaries today.
+
+This package still would not *implement* the MCP server — that stays where
+the engine code lives — it would only be the thing a user runs `pip install`
+for and then finds the server through, the same boundary that already holds
+for the simulation engines.