diff --git a/next/kernel/registry.erl b/next/kernel/registry.erl
index 5205325e..e762bce0 100644
--- a/next/kernel/registry.erl
+++ b/next/kernel/registry.erl
@@ -1,5 +1,8 @@
 -module(registry).
+-behaviour(gen_server).
 -export([new/0, kinds/0, register/4, lookup/3, list/2]).
+-export([start_link/0, register/3, lookup/2, list/1, stop/0]).
+-export([init/1, handle_call/3, handle_cast/2, handle_info/2]).
 
 %% Pure-functional registry for the seven bootstrap kinds.
 %%
@@ -67,3 +70,51 @@ put_pair(K, V, [P | Rest]) -> [P | put_pair(K, V, Rest)].
 find_pair(_, []) -> not_found;
 find_pair(K, [{K, V} | _]) -> {ok, V};
 find_pair(K, [_ | Rest]) -> find_pair(K, Rest).
+
+%% ── Step 5b: gen_server wrapper ─────────────────────────────────
+%%
+%% The named process owns the registry state; concurrent readers
+%% and writers serialize through gen_server:call. The pure /3 and
+%% /4 functions remain available for offline projection-replay and
+%% for tests that don't need a process at all.
+%%
+%% Port notes: gen_server:start_link returns the raw Pid (not
+%% `{ok, Pid}` as in OTP). `?MODULE` macro is unsupported here, so
+%% the registered name is the literal `registry` atom in every call.
+
+start_link() ->
+    Pid = gen_server:start_link(registry, []),
+    erlang:register(registry, Pid),
+    Pid.
+
+stop() ->
+    R = gen_server:call(registry, '$gen_stop'),
+    erlang:unregister(registry),
+    R.
+
+register(Kind, Name, Entry) ->
+    gen_server:call(registry, {register, Kind, Name, Entry}).
+
+lookup(Kind, Name) ->
+    gen_server:call(registry, {lookup, Kind, Name}).
+
+list(Kind) ->
+    gen_server:call(registry, {list, Kind}).
+
+%% gen_server callbacks
+
+init(_) -> {ok, new()}.
+
+handle_call({register, Kind, Name, Entry}, _From, State) ->
+    case register(Kind, Name, Entry, State) of
+        {ok, NewState} -> {reply, ok, NewState};
+        {error, R}     -> {reply, {error, R}, State}
+    end;
+handle_call({lookup, Kind, Name}, _From, State) ->
+    {reply, lookup(Kind, Name, State), State};
+handle_call({list, Kind}, _From, State) ->
+    {reply, list(Kind, State), State}.
+
+handle_cast(_, S) -> {noreply, S}.
+
+handle_info(_, S) -> {noreply, S}.
diff --git a/next/tests/registry_server.sh b/next/tests/registry_server.sh
new file mode 100755
index 00000000..fea294ad
--- /dev/null
+++ b/next/tests/registry_server.sh
@@ -0,0 +1,122 @@
+#!/usr/bin/env bash
+# next/tests/registry_server.sh — Step 5b acceptance test.
+#
+# Exercises the gen_server-wrapped registry. Each test combines
+# start_link + operations + assertion into a single
+# erlang-eval-ast expression because the Erlang-on-SX scheduler
+# does not preserve spawned processes across separate eval
+# invocations. 10 cases.
+
+set -uo pipefail
+cd "$(git rev-parse --show-toplevel)"
+
+SX_SERVER="${SX_SERVER:-hosts/ocaml/_build/default/bin/sx_server.exe}"
+if [ ! -x "$SX_SERVER" ]; then
+  SX_SERVER="/root/rose-ash/hosts/ocaml/_build/default/bin/sx_server.exe"
+fi
+if [ ! -x "$SX_SERVER" ]; then
+  echo "ERROR: sx_server.exe not found." >&2
+  exit 1
+fi
+
+VERBOSE="${1:-}"
+PASS=0; FAIL=0; ERRORS=""
+TMPFILE=$(mktemp); trap "rm -f $TMPFILE" EXIT
+
+cat > "$TMPFILE" <<'EPOCHS'
+(epoch 1)
+(load "lib/erlang/tokenizer.sx")
+(load "lib/erlang/parser.sx")
+(load "lib/erlang/parser-core.sx")
+(load "lib/erlang/parser-expr.sx")
+(load "lib/erlang/parser-module.sx")
+(load "lib/erlang/transpile.sx")
+(load "lib/erlang/runtime.sx")
+(load "lib/erlang/vm/dispatcher.sx")
+(epoch 2)
+(eval "(er-load-gen-server!)")
+(epoch 3)
+(eval "(get (erlang-load-module (file-read \"next/kernel/registry.erl\")) :name)")
+
+;; start_link returns a Pid
+(epoch 10)
+(eval "(get (erlang-eval-ast \"is_pid(registry:start_link())\") :name)")
+
+;; register + lookup round-trip
+(epoch 11)
+(eval "(get (erlang-eval-ast \"registry:start_link(), registry:register(activity_types, create, e1), registry:lookup(activity_types, create) =:= {ok, e1}\") :name)")
+
+;; lookup unknown name returns not_found
+(epoch 12)
+(eval "(get (erlang-eval-ast \"registry:start_link(), registry:lookup(activity_types, missing) =:= not_found\") :name)")
+
+;; register returns the atom 'ok'
+(epoch 13)
+(eval "(get (erlang-eval-ast \"registry:start_link(), registry:register(object_types, note, e_n) =:= ok\") :name)")
+
+;; list returns all pairs in a kind
+(epoch 14)
+(eval "(erlang-eval-ast \"registry:start_link(), registry:register(activity_types, a, 1), registry:register(activity_types, b, 2), registry:register(activity_types, c, 3), length(registry:list(activity_types))\")")
+
+;; Re-register overrides without growing the list
+(epoch 15)
+(eval "(erlang-eval-ast \"registry:start_link(), registry:register(activity_types, a, v1), registry:register(activity_types, a, v2), length(registry:list(activity_types))\")")
+(epoch 16)
+(eval "(get (erlang-eval-ast \"registry:start_link(), registry:register(activity_types, a, v1), registry:register(activity_types, a, v2), registry:lookup(activity_types, a) =:= {ok, v2}\") :name)")
+
+;; State persists across multiple calls in the same expression
+(epoch 17)
+(eval "(erlang-eval-ast \"registry:start_link(), registry:register(activity_types, x, 1), registry:register(object_types, x, 2), {registry:lookup(activity_types, x), registry:lookup(object_types, x)} =:= {{ok, 1}, {ok, 2}}\")")
+
+;; Unknown kind rejected via gen_server too
+(epoch 18)
+(eval "(get (erlang-eval-ast \"registry:start_link(), case registry:lookup(bogus_kind, foo) of {error, unknown_kind} -> ok; _ -> bad end\") :name)")
+
+;; Empty kind list returns []
+(epoch 19)
+(eval "(get (erlang-eval-ast \"registry:start_link(), registry:list(validators) =:= []\") :name)")
+EPOCHS
+
+OUTPUT=$(timeout 120 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
+
+check() {
+  local epoch="$1" desc="$2" expected="$3"
+  local actual
+  actual=$(echo "$OUTPUT" | awk -v e="$epoch" '
+    $0 ~ "^\\(ok-len " e " " { getline; print; exit }
+    $0 ~ "^\\(ok " e " "     { print; exit }
+    $0 ~ "^\\(error " e " "  { print; exit }
+  ')
+  [ -z "$actual" ] && actual="<no output for epoch $epoch>"
+  if echo "$actual" | grep -qF -- "$expected"; then
+    PASS=$((PASS+1))
+    [ "$VERBOSE" = "-v" ] && echo "  ok $desc"
+  else
+    FAIL=$((FAIL+1))
+    ERRORS+="  FAIL [$desc] (epoch $epoch) expected: $expected | actual: $actual
+"
+  fi
+}
+
+check  2  "gen_server loaded"                "gen_server"
+check  3  "registry module loaded"           "registry"
+check 10  "start_link returns Pid"           "true"
+check 11  "register + lookup round-trip"     "true"
+check 12  "lookup missing -> not_found"      "true"
+check 13  "register returns ok atom"         "true"
+check 14  "three registers, list = 3"        "3"
+check 15  "re-register doesn't grow list"    "1"
+check 16  "re-register overrides value"      "true"
+check 17  "different kinds independent"      "true"
+check 18  "lookup bogus kind"                "ok"
+check 19  "empty kind list = []"             "true"
+
+# 12 cases total (epoch 2 + 3 are setup, but counted for honesty)
+TOTAL=$((PASS+FAIL))
+if [ $FAIL -eq 0 ]; then
+  echo "ok $PASS/$TOTAL next/tests/registry_server.sh passed"
+else
+  echo "FAIL $PASS/$TOTAL passed, $FAIL failed:"
+  echo "$ERRORS"
+fi
+[ $FAIL -eq 0 ]
diff --git a/plans/fed-sx-milestone-1.md b/plans/fed-sx-milestone-1.md
index d40ee7dc..90b1f950 100644
--- a/plans/fed-sx-milestone-1.md
+++ b/plans/fed-sx-milestone-1.md
@@ -339,7 +339,7 @@ created with a known stable CID.
 
 **Sub-deliverables:**
 - [x] **5a** — Pure-functional `next/kernel/registry.erl`: `new/0`, `kinds/0`, `register/4`, `lookup/3`, `list/2`. State is a property list keyed by kind atom; per-kind storage is a property list of `{Name, Entry}`. Unknown kinds rejected with `{error, unknown_kind}`. `next/tests/registry_pure.sh` (14 cases).
-- [ ] **5b** — gen_server wrapper around the pure registry: `start_link/0`, registered name `registry`, the pure API delegated through `gen_server:call`.
+- [x] **5b** — gen_server wrapper around the pure registry: `start_link/0`, registered name `registry`, `register/3 lookup/2 list/1 stop/0` API delegating through `gen_server:call`. `next/tests/registry_server.sh` (12 cases). Port note: each test combines start_link + ops in a single expression because spawned processes don't survive across separate `erlang-eval-ast` invocations.
 - [ ] **5c** — `bootstrap:load_genesis/1` (Step 4e) populates the registry from `read_genesis` output. Dispatches by section atom → kind.
 - [ ] **5d** — define-registry projection fold integration: incoming `Create{Define*}` activities are routed through the projection scheduler (Step 7) and update the registry.
 
@@ -961,6 +961,7 @@ A few things still under-specified; resolve as work begins.
 Newest first. One line per sub-deliverable commit. Erlang conformance gate
 (`bash lib/erlang/conformance.sh`) must remain 729/729 on every entry.
 
+- **2026-05-28** — Step 5b: `registry.erl` extended with gen_server callbacks + named-process API. `start_link/0` spawns the worker, registers it under the literal `registry` atom, returns the Pid (port returns raw Pid not `{ok, Pid}` — diverges from OTP). 3-arity `register`, 2-arity `lookup`, 1-arity `list` delegate to the pure /4 and /3 functions inside handle_call. Port note documented: `?MODULE` macro unsupported; tests must inline start_link with operations since spawned processes don't persist across separate `erlang-eval-ast` calls. `next/tests/registry_server.sh` 12/12. Erlang conformance 729/729.
 - **2026-05-28** — Step 4e: `bootstrap:load_genesis/1` + `strip_sx_suffix/1` in `next/kernel/bootstrap.erl`. Walks `read_genesis` output and threads each entry through `registry:register/4`, using the section atom as the kind and the filename-minus-`.sx` as the entry name. Per-kind counts match the seven bootstrap sections exactly (3/10/7/3/3/2/3 = 31 entries total). `next/tests/bootstrap_load.sh` 15/15. Determinism verified by comparing `cid:to_string` of the loaded state across calls (faster than deep-equality on the nested-binary state). Step 4 is now complete end-to-end except for SX-source parsing of the loaded entries. Erlang conformance 729/729.
 - **2026-05-28** — Step 5a: `next/kernel/registry.erl` — pure-functional registry. State is `[{Kind, [{Name, Entry}, ...]}, ...]` keyed by the same seven section atoms as Step 4c (activity_types, object_types, projections, validators, codecs, sig_suites, audience). API: `new/0`, `kinds/0`, `register/4`, `lookup/3`, `list/2`. Unknown kinds rejected with `{error, unknown_kind}`; missing names return `not_found`; re-registering the same name overrides without growing the list. `next/tests/registry_pure.sh` 14/14. Step 5 broken into 5a–5d on the plan. Erlang conformance 729/729.
 - **2026-05-28** — Step 4d: `bootstrap:build_genesis/1` + `verify_genesis/2` + `.cidhash` helpers in `next/kernel/bootstrap.erl`. Bundle CID delegated to host `cid:to_string` over `{genesis_bundle, Sections}` — deterministic, ~59 byte CIDv1 binary. `verify_genesis/2` returns `ok` on match, `{error, {cid_mismatch, Got, Expected}}` on drift. `write_cidhash`/`read_cidhash` persist the CID to a `.cidhash` sibling file (path hand-spelled `<<...,47,46,99,...>>` per the string-literal-in-binary substrate quirk). `next/tests/bootstrap_build.sh` 12/12. Erlang conformance 729/729.