@@ -32,10 +32,12 @@ defmodule Trinity.MCP.Server.Plug do
       catalog: Catalog,
       dispatch: nil,
       # Slice 062: the profile's authorization runs in `call/2` before the transport, which
-      # then finds the principal it left; the transport's own hook is that second look.
-      authorize: fn _conn ->
-        if Process.get(@principal_key), do: :ok, else: {:error, :unauthorized}
-      end,
+      # then finds the principal it left; the transport's own hook is that second look. A
+      # remote capture and never an anonymous function: `Plug.Builder` calls `init/1` at
+      # compile time in `:prod` and escapes what it returns into the endpoint, and a closure
+      # cannot be escaped (`fix(s062)`: the release build was broken and the gate, which runs
+      # in `:test` where `init/1` is called at runtime, could not see it).
+      authorize: &__MODULE__.authorized/1,
       allowed_origins: Keyword.get(config, :allowed_origins, loopback_origins()),
       server_name: Keyword.get(config, :server_name, "trinity"),
       tools_ttl_ms: Keyword.get(config, :tools_ttl_ms, 60_000),
@@ -66,6 +68,16 @@ defmodule Trinity.MCP.Server.Plug do
 
   def call(conn, _opts), do: conn
 
+  @doc """
+  The transport's `:authorize` hook: the second look at what `call/2` decided. `call/2`
+  authorizes the request and leaves the principal in this process before the transport runs, so
+  a request that reaches the transport without one never passed the profile's check.
+  """
+  @spec authorized(Plug.Conn.t()) :: :ok | {:error, :unauthorized}
+  def authorized(_conn) do
+    if Process.get(@principal_key), do: :ok, else: {:error, :unauthorized}
+  end
+
   @doc "The principal the plug left for the wrapper, in this process."
   @spec principal() :: Trinity.MCP.Auth.Principal.t() | nil
   def principal, do: Process.get(@principal_key)
 
@@ -241,3 +241,48 @@ name, so the older could stay the signer (`Keys.rotate!/1` now makes the new key
 - The `TrustedHeaders` profile, only if a deployment asks (deviation 2).
 - The `[::1]` origin in `Server.Plug.loopback_origins/0` is a browser `Origin` header (bracketed there,
   rightly); F2 is about `URI.host`, not that list.
+
+## Correction, 2026-09-22, after the tag: the release build was broken
+
+**F5, found by the `package` workflow at the `slice/062` tag** (run 35786247006, the first run of
+that workflow over this slice: its push trigger is tags only). The linux and windows legs failed to
+compile:
+
+```
+== Compilation error in file lib/trinity_web/endpoint.ex ==
+** (ArgumentError) cannot escape #Function<0.132933281/1 in Trinity.MCP.Server.Plug.init/1>.
+   The supported values are: lists, tuples, maps, atoms, numbers, bitstrings, PIDs and remote
+   functions in the format &Mod.fun/arity
+```
+
+`Plug.Builder` calls a plug's `init/1` **at compile time** when `MIX_ENV=prod` and escapes what it
+returns into the compiled endpoint. Slice 062 replaced 061's `authorize: &Auth.Local.authorize/1`
+(a remote capture, which escapes) with an anonymous function closing over the process-dictionary
+key (which cannot be escaped). `mix gate` runs in `MIX_ENV=test`, where `init/1` is called per
+request, so the local gate and all three CI legs were green over a tree whose release could not
+compile. Reproduced on `main` at `9859fbe` in one command, before any fix:
+
+```
+$ MIX_ENV=prod mix compile
+== Compilation error in file lib/trinity_web/endpoint.ex ==
+** (ArgumentError) cannot escape #Function<0.132933281/1 in Trinity.MCP.Server.Plug.init/1>. …
+exit 1
+```
+
+Fixed in `fix(s062)` on `fix/s062-prod-compile`: the hook is `&__MODULE__.authorized/1`, a public
+function with the same body, as 061's was a remote capture. The regression test is
+`test/trinity/mcp/server_test.exs`, "every option the plug's init returns can be escaped, as a
+compile-time init must be": it runs `Macro.escape/1` over `Plug.init([])`, which is the compile-time
+step that failed. Demonstrated red against the closure first, with the same message CI printed.
+
+The slice is not rewritten and the tag is not moved: this is a fix commit referencing the original,
+as CLAUDE.md section 4 and docs/04 require.
+
+**Follow-up (new, and the reason this was missed): the gate never compiles `:prod`.** Nothing in
+`mix gate` or in the push-triggered CI builds a release, and `package.yml` only triggers on tags,
+so a compile-time defect in a plug's options, a release-only config error, or anything else that
+differs between `:test` and `:prod` is invisible until a tag exists, which is after approval. Two
+candidate fixes, for the owner: add a `MIX_ENV=prod mix compile` step to the gate (about a minute
+per run, catches this whole class), or give `package.yml` a `branches:` push trigger beside its
+tags one (slower, but proves the bundle too). Owner's call; not done here, since the gate is every
+slice's contract and this slice does not own it.
@@ -337,3 +337,18 @@ One of them is scoped `s061`, correctly: it amends 061's own test for the `401`
 **Not done, and deliberately.** No pull request is open, nothing is merged, and no tag exists: the
 slice's status is `done`, which is the agent's half, and `approved` is yours (G4). The `slice/062`
 tag and the three-OS `package` run that the tag triggers both come after that.
+
+## Correction, 2026-09-22, after the tag: the release build, and what this proof did not cover
+
+Appended, not rewritten. Everything above stands as measured, and it did not measure one thing: the
+`package` workflow, which this file's "Gate" section correctly said would run only at the
+`slice/062` tag. It ran (35786247006) and its linux and windows legs failed to compile the release,
+because `Plug.Builder` escapes a plug's `init/1` result at compile time under `MIX_ENV=prod` and
+this slice had put an anonymous function in those options. NOTES.md records it as **F5** with the
+reproduction on `main` at `9859fbe`, the fix (`authorize: &__MODULE__.authorized/1`) and the
+regression test, which was demonstrated red against the closure before the fix.
+
+What this says about the proof above: the AC evidence is unaffected (every criterion was measured
+through the running server in `:test`, and the fix changes no behaviour, only how the same function
+is passed). What it says about the gate is in NOTES.md as a follow-up: `mix gate` never compiles
+`:prod`, so this class of defect cannot be caught before a tag exists under the current CI.
@@ -313,4 +313,19 @@ defmodule Trinity.MCP.ServerTest do
     assert %{"result" => %{"supportedVersions" => ["2026-07-28"], "resultType" => "complete"}} =
              Jason.decode!(answer)
   end
+
+  # fix(s062). `Plug.Builder` calls a plug's `init/1` at compile time in `:prod` and escapes what
+  # it returns into the compiled endpoint, so an option that cannot be escaped breaks the release
+  # build and nothing else: the gate runs in `:test`, where `init/1` is called per request, and
+  # was green over it for the whole of slice 062. This is that compile-time step, in a test.
+  test "every option the plug's init returns can be escaped, as a compile-time init must be" do
+    opts = Trinity.MCP.Server.Plug.init([])
+    assert is_list(opts) or is_map(opts)
+    assert Macro.escape(opts)
+
+    # The authorize hook is the option that broke it: a remote capture escapes, a closure does not.
+    assert_raise ArgumentError, ~r/cannot escape/, fn ->
+      Macro.escape(authorize: fn _conn -> :ok end)
+    end
+  end
 end