diff --git a/.drone.yml b/.drone.yml
deleted file mode 100644
index 5e5ef38..0000000
--- a/.drone.yml
+++ /dev/null
@@ -1,18 +0,0 @@
-kind: pipeline
-name: amd
-
-platform:
-  os: linux
-  arch: amd64
-
-steps:
-  - name: build
-    image: ocaml/opam:debian-ocaml-4.12-afl
-    commands:
-      - sudo chown -R opam .
-      - git -C /home/opam/opam-repository pull origin 1ce065bc0cbef7bc06effcd3865af0d430c6273b && opam update
-      - opam pin add -n .
-      - opam depext -u cstruct-async cstruct-lwt cstruct-unix cstruct ppx_cstruct
-      - opam install -y .
-      - opam install -y dune crowbar fmt 'bun>=0.3.4'
-      - opam exec -- dune build @fuzz --no-buffer
diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index b7d0667..535647f 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -1,57 +1,42 @@
 name: Cstruct
 on: [push, pull_request]
 jobs:
-  latest:
-    name: Latest
-    runs-on: ${{ matrix.operating-system }}
+  # cstruct and its runtime siblings build on modern OCaml, including 5.x.
+  build:
+    name: ${{ matrix.ocaml-compiler }} / ${{ matrix.os }}
+    runs-on: ${{ matrix.os }}
     strategy:
       fail-fast: false
       matrix:
-        ocaml-version: [ '4.13.1', '4.10.0', '4.08.1' ]
-        operating-system: [macos-latest, ubuntu-latest, windows-latest]
+        os: [ubuntu-latest, macos-latest]
+        ocaml-compiler:
+          - "5.3"
+          - "5.1"
+          - "4.14"
     steps:
-    - uses: actions/checkout@v2
-    - uses: avsm/setup-ocaml@v1
-      with:
-        ocaml-version: ${{ matrix.ocaml-version }}
-    - run: opam pin add -n .
-    - name: Packages
-      run: opam depext -yt cstruct cstruct-sexp cstruct-unix cstruct-lwt
-    - name: Build
-      run: opam install -t cstruct cstruct-sexp cstruct-unix cstruct-lwt
-  ppx:
-    name: PPX
-    runs-on: ${{ matrix.operating-system }}
+      - uses: actions/checkout@v4
+      - uses: ocaml/setup-ocaml@v3
+        with:
+          ocaml-compiler: ${{ matrix.ocaml-compiler }}
+      - run: >-
+          opam install ./cstruct.opam ./cstruct-sexp.opam ./cstruct-unix.opam
+          ./cstruct-lwt.opam --deps-only
+      - run: opam exec -- dune build -p cstruct,cstruct-sexp,cstruct-unix,cstruct-lwt
+
+  # ppx_cstruct (ocaml-migrate-parsetree) and cstruct-async (async) do not build
+  # on OCaml 5.x, so run the full build and test on 4.14.
+  test:
+    name: full build + test (4.14) / ${{ matrix.os }}
+    runs-on: ${{ matrix.os }}
     strategy:
       fail-fast: false
       matrix:
-        ocaml-version: [ '4.13.1', '4.10.0', '4.08.1' ]
-        operating-system: [macos-latest, ubuntu-latest, windows-latest]
+        os: [ubuntu-latest, macos-latest]
     steps:
-    - uses: actions/checkout@v2
-    - uses: avsm/setup-ocaml@v1
-      with:
-        ocaml-version: ${{ matrix.ocaml-version }}
-    - run: opam pin add -n .
-    - name: Packages
-      run: opam depext -yt ppx_cstruct
-    - name: Build
-      run: opam install -t ppx_cstruct
-  async:
-    name: Async
-    runs-on: ${{ matrix.operating-system }}
-    strategy:
-      fail-fast: false
-      matrix:
-        ocaml-version: [ '4.13.1', '4.10.0', '4.08.1' ]
-        operating-system: [macos-latest, ubuntu-latest]
-    steps:
-    - uses: actions/checkout@v2
-    - uses: avsm/setup-ocaml@v1
-      with:
-        ocaml-version: ${{ matrix.ocaml-version }}
-    - run: opam pin add -n .
-    - name: Packages
-      run: opam depext -yt cstruct-async
-    - name: Dependencies
-      run: opam install -t cstruct-async
+      - uses: actions/checkout@v4
+      - uses: ocaml/setup-ocaml@v3
+        with:
+          ocaml-compiler: "4.14"
+      - run: opam install . --deps-only --with-test
+      - run: opam exec -- dune build
+      - run: opam exec -- dune runtest
diff --git a/cstruct-async.opam b/cstruct-async.opam
index 9504af7..340b953 100644
--- a/cstruct-async.opam
+++ b/cstruct-async.opam
@@ -16,9 +16,9 @@ build: [
 depends: [
   "ocaml" {>= "4.08.0"}
   "dune" {>= "2.0.0"}
-  "async" {>= "v0.9.0"}
-  "async_unix" {>= "v0.9.0"}
-  "core" {>= "v0.9.0"}
+  "async" {>= "v0.9.0" & < "v0.17.0"}
+  "async_unix" {>= "v0.9.0" & < "v0.17.0"}
+  "core" {>= "v0.9.0" & < "v0.17.0"}
   "cstruct" {=version}
 ]
 synopsis: "Access C-like structures directly from OCaml"
diff --git a/cstruct.opam b/cstruct.opam
index 19ba9c5..6830762 100644
--- a/cstruct.opam
+++ b/cstruct.opam
@@ -20,7 +20,7 @@ depends: [
   "dune" {>= "2.0.0"}
   "alcotest" {with-test}
   "fmt" {>= "0.8.9"}
-  "crowbar" {with-test}
+  "crowbar" {with-test & >= "0.2"}
 ]
 conflicts: [ "js_of_ocaml" {<"3.5.0"} ]
 synopsis: "Access C-like structures directly from OCaml"
diff --git a/fuzz/dune b/fuzz/dune
index 171f5f2..6228f94 100644
--- a/fuzz/dune
+++ b/fuzz/dune
@@ -2,11 +2,11 @@
  (name fuzz)
  (libraries cstruct bigarray cstruct-sexp crowbar fmt))
 
+; The fuzzer is afl-only (see fuzz.ml): its generators are unbounded, so
+; quickcheck mode hangs. Build it under runtest as a compile check; run the
+; campaign with an afl-instrumented switch and afl-fuzz on fuzz.exe.
+
 (rule
- (alias fuzz)
- (deps
-  fuzz.exe
-  (source_tree input))
- (action
-  (run timeout --preserve-status 30m bun -v --input=input --output=output --
-    ./fuzz.exe)))
+ (alias runtest)
+ (deps fuzz.exe)
+ (action (progn)))