00-META/how-we-build.md

@@ -268,6 +268,10 @@ Data access, business logic and the interface layer are separate.
 
 ### Types
 
+*Scope: the mesh's services and surfaces. Tier 0 is a statically linked binary that must depend
+on nothing installed first, and is written in Go —
+[ADR 0041](../02-DECISIONS/0041-the-host-depends-on-nothing.md).*
+
 - TypeScript throughout; no new untyped JavaScript.
 - Strict, with no implicit `any` and no unchecked index access.
 - Public functions state their return type. Prefer `unknown` with a guard over `any`. Errors

00-META/repos.md

@@ -21,13 +21,13 @@ and a forge address is an operational detail (see [`README`](../README.md)).
 ## What the mesh becomes
 
 [ADR 0030](../02-DECISIONS/0030-the-repository-structure.md) records the repositories the
-monorepo decomposes into. **Only `mesh-lab` exists so far** — it is built first
+monorepo decomposes into. **`mesh-lab` and `mesh-host` exist so far** — the lab is built first
 ([ADR 0029](../02-DECISIONS/0029-the-labs-first-scenario-has-no-pipeline.md)); the rest are the
 target, not the present.
 
 | Repository | Tier | Holds |
 |---|---|---|
-| `mesh-host` | 0 | the node host — the one binary installed by hand |
+| `mesh-host` | 0 | **exists.** The node host — one statically linked binary, requiring nothing present ([ADR 0041](../02-DECISIONS/0041-the-host-depends-on-nothing.md)) |
 | `mesh-substrate` | 1 | the four pinned services, as declarations |
 | `mesh-control` | 2 | the control plane and its contexts |
 | `mesh-surfaces` | 3 | tools, web, cli |

02-DECISIONS/0041-the-host-depends-on-nothing.md

@@ -0,0 +1,78 @@
+---
+status: accepted
+date: 2026-08-26
+deciders: jochen
+reconstructed: false
+extends: 0037-the-host-applies-it-does-not-decide.md
+---
+
+# 41. The host depends on nothing that must be installed first
+
+## Context
+
+[ADR 0030](0030-the-repository-structure.md) calls tier 0 *"the one binary installed by hand"*,
+and [research 006](../01-RESEARCH/006-mesh-from-scratch/00-overview.md) states the property the
+whole tier rests on: *"a binary whose whole argument is that it has no dependencies"*.
+
+Building it forced the question that phrase had been carrying unexamined. Everything else in
+the mesh is TypeScript, and `how-we-build` §8 says so. A TypeScript host needs a runtime present
+before it can run — so the thing installed by hand becomes **two** things, and the second must
+be installed by the means the host exists to replace.
+
+## Considered options
+
+1. **TypeScript, with a runtime installed first.** Simplest, and matches every other
+   repository. Rejected: it breaks the property the tier is built on. A host that cannot run
+   until something else has been installed by hand is not the bottom of the stack.
+2. **TypeScript, bundled as a single executable.** Preserves the language and produces one
+   file. Rejected on two grounds: it carries roughly ninety megabytes of runtime to preserve a
+   language choice, and single-executable bundling is a young feature — tier 0 is the worst
+   place in the system to discover its edges.
+3. **A statically linked binary in a language built for it.** Chosen; Go.
+
+## Decision
+
+**The host is a single statically linked binary that requires nothing to be present.** Copy it
+onto a machine and run it. That is the whole installation.
+
+**It is written in Go.** The job is system-level — run commands, write files, speak to the
+firewall, the overlay, the service manager and the package manager — which is what Go's
+ecosystem is for, and it cross-compiles to every architecture the mesh might reach, including
+the lighter devices requirement 6 anticipates.
+
+**The second language costs less here than anywhere else it could appear**, and the reason is
+architectural rather than convenient. [ADR 0037](0037-the-host-applies-it-does-not-decide.md)
+means the host never queries the mesh database.
+[ADR 0039](0039-the-link-is-the-security-boundary.md) means it only ever receives declarations.
+So the host shares **no code** with any other tier — not a client, not a schema, not the SDK.
+It is joined to the mesh by a message contract and nothing else.
+
+The language boundary therefore falls exactly on an architectural boundary that already exists.
+A second language usually costs duplicated logic; here there is none to duplicate.
+
+## Consequences
+
+- **`how-we-build` §8 needs a scope.** It reads *"TypeScript throughout"*, which was true when
+  everything was a service or a surface. It is now scoped to those, with tier 0 named as the
+  exception and this record as the reason. That is a constitution change, and the sync it owes
+  is part of it.
+- **Agents must write Go to work on the host.** A real cost, and the one genuine argument
+  against this. It is bounded by the host being the only thing in tier 0 — nothing else in the
+  mesh acquires a second language because of this.
+- **The dependency-direction lint the design calls for gets easier, not harder.** A Go module
+  cannot accidentally import a TypeScript control-plane client; the boundary is enforced by
+  there being no path across it.
+- **Cross-compilation replaces per-node builds.** The host is built once per architecture and
+  copied, rather than built on the machine it runs on — which is what makes *"copy it and run
+  it"* true rather than nearly true.
+- **Two toolchains in the lab.** Scenarios that place a host need a Go build available, and the
+  lab is TypeScript. The binary is built before the scenario runs, not inside it.
+- **This is reversible at a cost that will only grow.** It is being taken at the moment the
+  first line is written, which is the cheapest point it will ever be taken.
+
+## References
+
+- [ADR 0030](0030-the-repository-structure.md) — *the one binary installed by hand*.
+- [ADR 0037](0037-the-host-applies-it-does-not-decide.md) — why the host shares no code.
+- [ADR 0039](0039-the-link-is-the-security-boundary.md) — why it receives declarations only.
+- [`05-the-node-host.md`](../03-DESIGN/01-to-be/05-the-node-host.md) — the design this serves.

03-DESIGN/01-to-be/05-the-node-host.md

@@ -1,7 +1,7 @@
 ---
 layer: to-be
-status: designed
+status: in-progress
-code: []
+code: [mesh-host]
 updated: 2026-08-26
 decisions:
   - 02-DECISIONS/0030-the-repository-structure.md
@@ -10,6 +10,7 @@ decisions:
   - 02-DECISIONS/0038-a-node-joins-by-linking-first.md
   - 02-DECISIONS/0039-the-link-is-the-security-boundary.md
   - 02-DECISIONS/0008-a-failed-step-fails-the-job.md
+  - 02-DECISIONS/0041-the-host-depends-on-nothing.md
 ---
 
 # The node host
@@ -18,6 +19,11 @@ Tier 0. The one thing ever installed by hand, and the only thing that changes a
 
 ## What it is
 
+A **statically linked binary that requires nothing to be present** — copy it onto a machine and
+run it, and that is the whole installation
+([ADR 0041](../../02-DECISIONS/0041-the-host-depends-on-nothing.md)). Written in Go, because the
+job is system-level and because the host shares no code with any other tier.
+
 A single binary with one job: **apply declared state on this machine**
 ([ADR 0037](../../02-DECISIONS/0037-the-host-applies-it-does-not-decide.md)). Overlay
 membership, packet filtering, packages, services, containers and filesystems are not six