ADR 0195: the mesh's tools are found by address, not announced whole; research 021; to-be 34 §3a

2026-10-03 21:49:26 +02:00
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+---
+status: graduated
+initiated: 2026-10-03
+touches: [the console, 03-DESIGN/01-to-be/34-the-console.md, the tool runtime, seats, assignments]
+became: [02-DECISIONS/0195-the-meshs-tools-are-found-by-address-not-announced-whole.md, 03-DESIGN/01-to-be/34-the-console.md]
+---
+
+# 021 — Finding a tool in the mesh
+
+## What was investigated
+
+How an agent finds the one tool it needs among everything the mesh answers, and how a call names
+exactly what it asks — a role the mesh holds once, a role every machine holds, or one assignment of a
+module on one machine — rather than receiving the whole catalogue and a name that can mean several
+things.
+
+## Why
+
+The operator's observation on 2026-10-03: *Claude should not see all tools at once; they should be
+discoverable — and `postgres.list_databases` is wrong, asking one machine's postgres is not asking
+another's.* Measured the same day from the console's own answer:
+
+| | |
+|---|---|
+| tools announced to every session at its start | 228, in 110 KB |
+| names (module or seat prefixes) | 43 |
+| node seats' verbs, which require `node` | 22 |
+| modules with tools on more than one machine | 4 — fail2ban, nftables (every machine), postgres, mssql (two each) |
+| modules reported "not answering", most with no tools and several retired | 47 |
+
+The two stateful modules on two machines are listed **once**, with `node` optional and *whichever
+answers* when it is left out — though their two instances hold different databases. Design 34 §3 says
+such a module is listed once per machine; the live console does not do that. The list is taken once
+per session, so a tool that arrives later is invisible until the client reconnects. And only Claude
+Code's own deferral of long tool lists keeps the 228 from the model's context; another MCP client
+would receive them whole.
+
+## Options
+
+1. **Keep the flat list; rely on the client to defer it.** Rejected: a property of one client, and it
+   leaves the ambiguity and the stale list.
+2. **One flat tool per assignment** (`ace_postgres_list_databases`). Removes the ambiguity, multiplies
+   the list, and runs into the API's tool-name limit (letters, digits, `_`, `-`, 64 characters).
+3. **A small fixed set of tools that walk the mesh's own structure**, with the full address as an
+   argument: the mesh's seats; a machine's node seats and assignments; a search; a description; a
+   call. Chosen — see [ADR 0195](../../02-DECISIONS/0195-the-meshs-tools-are-found-by-address-not-announced-whole.md).
+4. **MCP resources or prompts for discovery.** Clients support them unevenly, and an agent acts
+   through tools; a resource it cannot be relied on to read is not a discovery path.