@@ -350,10 +350,11 @@ somewhere outside the host, on any schedule, makes backdated forgery detectable
 without needing a transparency service. This is not built yet and is named here as the gap it is.
 
 **An external authority plane proves the decision was never the machine's to make.** Point
-`TRINITY_AUTHORITY` at an adapter and Trinity keeps no executor for the effects that adapter
-governs, which is asserted by a census over the tree rather than by intent. The decision is made
-elsewhere by something with its own key, so relying on it does not require trusting this host at
-all.
+`TRINITY_AUTHORITY` at an adapter and the decision is made elsewhere, by something with its own
+key, so relying on it does not require trusting this host to have decided honestly. There is no
+model-side executor: after the adapter allows, the tool runs in this VM, behind the same membrane
+and the same admission receipt as always, and a census over the tree asserts that there is exactly
+one place it can run.
 
 The honest one-line version: **standalone proves what happened to someone who trusts the machine, a
 witness proves it to someone who does not, and an external authority proves the decision was never
@@ -373,10 +374,12 @@ the three fields that have no equivalent, is in
 Trinity runs standalone, and it is also one component of **Sanction OS**, the platform formed by Requisition and
 Ultraviolet.
 
-**Requisition** is the authority layer. Point `TRINITY_AUTHORITY` at the adapter module and Trinity delegates every
-catalogued effect to it: Trinity proposes, Requisition decides, and Trinity keeps no executor for those effects.
-Trinity refuses to start if the module is absent or does not implement `Trinity.Authority`. See
-`docs/adr/0008-authority-is-an-adapter.md` and `docs/adr/0010-authority-selection-at-boot.md`.
+**Requisition** is the authority layer. Point `TRINITY_AUTHORITY` at the adapter module and every catalogued effect
+is put to it for a decision: anything may propose, the authority layer authorizes, and after an allow this node's
+Elixir runs the tool. The adapter does not run it and may not change the arguments it was asked about. Trinity
+refuses to start if the module is absent or does not implement `Trinity.Authority`. See
+`docs/authority-adapter.md` for the contract, and `docs/adr/0008-authority-is-an-adapter.md` and
+`docs/adr/0010-authority-selection-at-boot.md` for the decisions behind it.
 
 **Ultraviolet** is the purple-team tool. Add it on the `/mcp` page like any other MCP server (since slice 060 a server is a row, not a config entry). Its
 read tools return query-receipted results, its proposal tools return proposal ids rather than effects, and content
 
@@ -20,8 +20,10 @@ whether the thing you asked for is allowed to happen.
    behind `Trinity.MCP.Auth`. That answers who is calling and with what scopes. Whether an effect happens is
    answered by the gate, or by whichever `Trinity.Authority` implementation is in force. The auth layer is never
    described as an authority layer.
-5. When an external adapter is in force, Trinity keeps **no executor** for the effects that adapter governs. The
-   absence is asserted by a census over the tree, not by intent.
+5. **Amended 2026-09-30, see below.** ~~When an external adapter is in force, Trinity keeps **no executor** for the
+   effects that adapter governs. The absence is asserted by a census over the tree, not by intent.~~
+   There is **no model-side executor**, and after a `decide/3` allow **this VM runs the tool**. The census over the
+   tree still asserts the shape, and now asserts one executor rather than none.
 
 ## Consequences
 - Adapters can be written against a published behaviour by anyone, including for authority layers this project
@@ -64,3 +66,39 @@ record. That is deliberate and belongs in this ADR because it is a statement abo
 can and cannot deliver: after a partition, two devices hold two true accounts of what each did, and nothing
 available afterwards turns them into one account of what happened. A system that claimed otherwise would be
 inventing the part it could not know.
+
+## Amended 2026-09-30 — point 5: one executor, here, not none
+
+**Source: the owner, 2026-09-30.** Point 5 as accepted said that an external adapter meant Trinity kept no executor
+for the effects that adapter governed, and the census test asserted the absence by finding exactly one file able to
+call a tool's `execute/2` and noting that the effect never reached it under an adapter.
+
+That is no longer the design. The split is:
+
+> Any actor proposes. The authority in force authorizes. This node's Elixir executes Trinity tools after an allow.
+
+The adapter **does not run the tool** and **does not change the arguments**. It answers `:allow` or `:deny` with a
+basis, and stops.
+
+### Why the earlier shape was worse
+
+It put the effect in the adapter's hands, in another repository, on the far side of the membrane. Every check the
+membrane makes, the effect class, the catalogue, the fingerprint, the idempotency key, the admission receipt, applied
+to something that then happened somewhere else. A second executor is a second place for the rules to be different.
+
+It also made `stage/2` load-bearing in a way nothing checked: the membrane verified the fingerprint, handed the
+`Staged` to the authority, and used whatever came back. An adapter could change `args` after the fingerprint had been
+verified, or keep `args` and change `module`, which no fingerprint covers at all.
+
+### What is true now, with where it is enforced
+
+| claim | enforced at |
+| --- | --- |
+| the tool runs in this VM, after `decide/3` allow and after the admission receipt is in the chain | `lib/trinity/effects.ex`, `run_tool/2` |
+| exactly one file calls a tool's `execute/2` on the effect path | `test/trinity/effects/census_test.exs` |
+| `stage/2` may set `staged_at` and `basis` and nothing else | `lib/trinity/effects.ex`, `check_stage_kept_the_subject/2` |
+| the fingerprint is re-derived after `stage/2` against the one the gate bound | `lib/trinity/effects.ex`, `check_fingerprint_against/2` |
+| an adapter's `execute/3` is on no path and must not run a tool | `lib/trinity/authority.ex`, `lib/trinity/authority/local.ex` |
+
+Decisions 1 to 4 are unchanged. The behaviour still has four callbacks, because removing `execute/3` would silently
+accept an adapter written against the older contract, where returning a result from it was how the effect happened.
@@ -0,0 +1,156 @@
+<!--
+SPDX-FileCopyrightText: Sudo Apt Holdings LLC
+SPDX-License-Identifier: Apache-2.0
+-->
+# Writing an authority adapter
+
+How to make an external authority layer the one that decides for this node.
+
+**Every claim here is a path in this repository, or says NOT IN TREE.** A sentence you cannot follow
+to a file is a sentence you should not trust.
+
+## The split
+
+> Anything may propose. The authority in force authorizes. After an allow, this node's Elixir runs
+> the tool.
+
+The adapter **decides**. It does not run the tool, and it does not change what it was asked about.
+
+## One module, chosen once, at boot
+
+`TRINITY_AUTHORITY` names one module. It is read once, by `Trinity.Authority.Selection`
+(`lib/trinity/authority/selection.ex`), as the first child of the application after the data
+directory lock, and the selection is kept in `:persistent_term`. No runtime path changes it.
+
+- unset, `""`, or `local` selects `Trinity.Authority.Local`.
+- any other value is an Elixir module name, with or without the `Elixir.` prefix.
+- the module must be **loaded** and must export every callback, or the boot stops and the refusal
+  names which: `{:not_loaded, name}` or `{:missing_callback, module, {name, arity}}`.
+- a name that is not an existing atom is reported as not loaded, and no atom is created from the
+  environment's text.
+
+Selection is **synchronous**: `Selection.start/0` completes before any later child starts, so there
+is no window in which the node is running with the selection still being made. Before that,
+`Trinity.Authority.impl/0` (`lib/trinity/authority.ex`) answers `Local` under `:default` and
+**raises** under `:regulated`, because a brief fallback to the local authority is still a fallback.
+
+`Trinity.Authority.Local` (`lib/trinity/authority/local.ex`) is **the only implementation in this
+tree**. Everything else is NOT IN TREE: this repository holds no client, no envelope, no schema and
+no transport for any particular authority layer, and a test asserts that under `local` no adapter
+module is loaded (`test/trinity/authority/selection_test.exs`).
+
+## The regulated profile refuses the local authority
+
+Under `TRINITY_PROFILE=regulated` the node refuses to boot on `Trinity.Authority.Local`:
+
+- the rule is `Trinity.Profile.check_authority/2` in `lib/trinity/profile.ex`, which answers
+  `{:error, :regulated_refuses_local_authority}`.
+- it is asked at boot, before any child exists, by `verify_regulated_configuration!/0` in
+  `lib/trinity/application.ex`.
+- an unset `TRINITY_AUTHORITY` resolves to `Local` and is therefore refused too.
+- a name that cannot be resolved is refused as `{:regulated_authority_unselectable, reason}`,
+  carrying the selection's own reason, and `check_authority/2` is total, so a caller that loses the
+  reason still gets `{:regulated_authority_unresolved, value}` rather than a crash.
+
+The refusals are tested in `test/trinity/profile_test.exs`, and at an actual boot, in spawned OS
+processes, in `test/trinity/regulated_boot_node_test.exs`.
+
+## The callbacks
+
+Four are required. `Trinity.Authority.callbacks/0` is the list the selection checks against, in
+`lib/trinity/authority.ex`.
+
+| callback | answer | what it is for |
+| --- | --- | --- |
+| `stage/2` | `{:ok, Staged.t()}` or `{:error, term()}` | record that an effect is about to happen |
+| `decide/3` | `{:ok, :allow \| :deny, basis}` or `{:error, term()}` | **the decision**, with a basis saying why |
+| `execute/3` | `{:error, :not_the_effect_path}` | reserved; see below |
+| `receipt/2` | `{:ok, term()}` or `{:error, term()}` | record a receipt of a kind |
+
+And one optional:
+
+| callback | answer | what it is for |
+| --- | --- | --- |
+| `forward_receipt/2` | `:ok` or `{:error, term()}` | acknowledge a queued receipt envelope |
+
+### `decide/3` returns allow or deny, and never falls back
+
+`:allow` or `:deny`. An adapter that cannot reach its far side returns `{:error, reason}` or
+`:deny`; it must **not** decide locally, and it must not answer as though the local gate had
+decided. The membrane treats `{:ok, :deny, basis}` as a denial with a receipt, and `{:error, _}` as
+a denial with a receipt. Both are safe. A fallback to `Local` is the one answer that is not.
+
+### `execute/3` must not run a tool
+
+It is **not** the place the effect happens, and nothing on the effect path calls it. After a
+`decide/3` allow and after the admission receipt is in the chain, `Trinity.Effects.run_tool/2`
+(`lib/trinity/effects.ex`) calls the tool's own `execute/2`, in this VM.
+
+It stays in the behaviour so that an adapter written against the older contract, where returning a
+result from it was how the effect happened, is not silently accepted. Answer
+`{:error, :not_the_effect_path}`, which is what `Trinity.Authority.Local` answers.
+
+### `stage/2` may not change the subject
+
+Set `staged_at` and `basis`. Nothing else. Every other field of the `Trinity.Authority.Staged`
+(`lib/trinity/authority/staged.ex`) you return must be the one you were given.
+
+After `stage/2` the membrane re-derives the fingerprint over the arguments you returned and compares
+it to the one the gate bound, not to the one your struct now carries, so rewriting `args` and
+`fingerprint` to agree with each other does not pass. It then compares every remaining field,
+because the fingerprint covers the session, the tool name, the arguments and the working directory,
+and covers `module` not at all: an adapter could otherwise leave every fingerprinted field alone and
+still change which code runs.
+
+Both refusals are tested against adapters that actually attempt them, in
+`test/trinity/effects/membrane_test.exs` with `Trinity.TestAuthority.MutatesArgs` and
+`Trinity.TestAuthority.SwapsModule` (`test/support/authority/adapters.ex`), alongside a control
+adapter that changes nothing and whose effect runs.
+
+## Receipts: Trinity writes them, the adapter acknowledges them
+
+Trinity signs and appends its own receipts, locally, around the effect: an `admit` before the tool
+runs and a `done` after it, or a `denied` instead. That is `Trinity.Effects` and
+`Trinity.Receipts`. **The adapter does not write Trinity's receipts.**
+
+What an adapter may do is receive them. `forward_receipt/2` is offered the exported envelope
+**byte for byte, exactly as it was signed**. An implementation must not re-sign it, rebuild it or
+alter a leaf: the point is that a verifier checks the signature over those bytes offline, which is
+what lets a queue wrap, delay and re-deliver an envelope without the far side knowing a queue
+exists. `:ok` acknowledges; any error leaves the entry pending to be offered again, oldest first.
+An implementation that does not export it is one this machine never forwards to, and the queue for
+its scopes simply does not drain. See `lib/trinity/receipts/queue.ex` and
+`test/trinity/receipts/queue_test.exs`.
+
+## `act_hash` is opaque
+
+An authority layer may bind its decision to an act identifier. **Trinity treats it as opaque.** It
+does not parse it, does not recompute it and does not depend on its shape; it carries it and stores
+it beside the decision.
+
+Where an adapter records one, it should record the algorithm beside it, as `act_hash` and
+`act_hash_alg`, so a reader is never guessing which construction produced the digest. The value
+this contract expects is `sha256+rfc8785-v1`: SHA-256 over the [RFC 8785](https://www.rfc-editor.org/rfc/rfc8785)
+canonical JSON form. Trinity already uses RFC 8785 for its own approval fingerprints and holds a
+conformance vector for it (`test/support/rfc8785_input.json`, `test/support/rfc8785_expected.txt`),
+so the canonicalisation is not a new dependency here.
+
+**`act_hash` is NOT IN TREE.** No field, column or function of that name exists in this repository.
+It is named here so an adapter and this node agree on the word, not because Trinity reads it.
+
+## Packaging an adapter
+
+The adapter lives **outside this repository**, and the release that contains both is built outside
+it too. This tree builds a Trinity that can be pointed at one; it does not build, vendor or depend
+on any. A wrapper release adds the adapter module to the code path and sets `TRINITY_AUTHORITY`.
+Anything more than that is NOT IN TREE.
+
+## Checklist
+
+- [ ] the module is loaded in the release, and `TRINITY_AUTHORITY` names it
+- [ ] `stage/2`, `decide/3`, `execute/3`, `receipt/2` are exported
+- [ ] `stage/2` changes nothing but `staged_at` and `basis`
+- [ ] `decide/3` answers `:allow` or `:deny`, and never falls back to a local decision
+- [ ] `execute/3` runs no tool
+- [ ] `forward_receipt/2` acknowledges bytes it did not alter, or is absent on purpose
+- [ ] under `TRINITY_PROFILE=regulated`, the node boots, which means it did not select `Local`
@@ -120,12 +120,11 @@ defmodule Trinity.Application do
     if profile == :regulated do
       mcp_auth = Application.get_env(:trinity, :mcp_auth, []) |> Keyword.get(:profile, :local)
       llm = Application.get_env(:trinity, :llm, [])
-      authority = authority_module()
 
       [
         Trinity.Profile.check_mcp_auth(profile, mcp_auth),
         Trinity.Profile.check_embedded_as(profile, mcp_auth),
-        Trinity.Profile.check_authority(profile, authority),
+        check_authority(profile),
         Trinity.Profile.check_llm_endpoints(
           profile,
           Trinity.Profile.raw_endpoints(),
@@ -143,10 +142,19 @@ defmodule Trinity.Application do
 
   # AC5's input. `Trinity.Authority.Selection.select/1` is the same pure rule the selection child
   # uses, asked here before that child exists rather than duplicated.
-  defp authority_module do
+  #
+  # Owner ruling 2026-09-30: a refusal is reported with the selection's own reason. This used to
+  # answer `nil` for every refusal, and `nil` reached `check_authority/2` as an argument no clause
+  # matched, so `TRINITY_AUTHORITY=SomethingNotLoaded` under `:regulated` stopped the boot with a
+  # FunctionClauseError in `Trinity.Profile` rather than with the name of the module that could
+  # not be loaded.
+  defp check_authority(profile) do
     case Trinity.Authority.Selection.select(System.get_env("TRINITY_AUTHORITY")) do
-      {:ok, module} -> module
-      _ -> nil
+      {:ok, module} ->
+        Trinity.Profile.check_authority(profile, module)
+
+      {:error, reason} ->
+        {:error, {:regulated_authority_unselectable, reason}}
     end
   end