diff --git a/.github/workflows/nix-build.yml b/.github/workflows/nix-build.yml
index d96fb1e3b..901c8ffb4 100644
--- a/.github/workflows/nix-build.yml
+++ b/.github/workflows/nix-build.yml
@@ -23,6 +23,7 @@ jobs:
           - 5_2
           - 5_3
           - 5_4
+          - 5_5
 
     runs-on: ubuntu-latest
     steps:
@@ -43,10 +44,10 @@ jobs:
     strategy:
       matrix:
         setup:
-          - {ocaml-version: '5_3', os: macos-15-intel}
           - {ocaml-version: '4_14', os: macos-latest}
           - {ocaml-version: '5_3', os: macos-latest}
           - {ocaml-version: '5_4', os: macos-latest}
+          - {ocaml-version: '5_5', os: macos-latest}
     runs-on: ${{ matrix.setup.os }}
     steps:
     - uses: actions/checkout@v6
diff --git a/.github/workflows/opam-ci.yml b/.github/workflows/opam-ci.yml
index ab307aadc..434273fbe 100644
--- a/.github/workflows/opam-ci.yml
+++ b/.github/workflows/opam-ci.yml
@@ -19,22 +19,22 @@ jobs:
       fail-fast: false
       matrix:
         setup:
-          - {ocaml-compiler: '4.08.x', os: ubuntu-latest}
-          - {ocaml-compiler: '4.10.x', os: ubuntu-latest}
+          - {ocaml-compiler: 'ocaml-base-compiler.4.08.1', os: ubuntu-latest, opam-repositories: 'default: git+https://github.com/ocaml/opam-repository.git#622e2cd03db68197f7299596fa614b2f91aa9260'}
+          - {ocaml-compiler: 'ocaml-base-compiler.4.10.2', os: ubuntu-latest, opam-repositories: 'default: git+https://github.com/ocaml/opam-repository.git#622e2cd03db68197f7299596fa614b2f91aa9260'}
           - {ocaml-compiler: '4.12.x', os: ubuntu-latest}
           - {ocaml-compiler: '4.13.x', os: ubuntu-latest}
           - {ocaml-compiler: '4.14.x', os: ubuntu-latest}
           - {ocaml-compiler: 'ocaml-base-compiler.5.3.0', os: ubuntu-latest}
           - {ocaml-compiler: 'ocaml-base-compiler.5.4.0', os: ubuntu-latest}
-          - {ocaml-compiler: 'ocaml-base-compiler.5.5.0~alpha1', os: ubuntu-latest}
+          - {ocaml-compiler: 'ocaml-base-compiler.5.5.0', os: ubuntu-latest}
           - {ocaml-compiler: 'ocaml-base-compiler.5.4.0', os: macos-15-intel}
           - {ocaml-compiler: 'ocaml-base-compiler.5.4.0', os: macos-latest}
-          - {ocaml-compiler: 'ocaml-base-compiler.5.5.0~alpha1', os: macos-latest}
+          - {ocaml-compiler: 'ocaml-base-compiler.5.5.0', os: macos-latest}
             # looks like setup-ocaml@v3 can only run actions on windows for
             # OCaml >= 4.13
             # https://github.com/ocaml/setup-ocaml/issues/822#issuecomment-2215525942
           - {ocaml-compiler: '4.14.x', os: windows-latest}
-          - {ocaml-compiler: 'ocaml-base-compiler.5.4.0', os: windows-latest}
+          - {ocaml-compiler: 'ocaml-base-compiler.5.5.0', os: windows-latest}
 
     runs-on: ${{ matrix.setup.os }}
 
@@ -46,6 +46,7 @@ jobs:
         uses: ocaml/setup-ocaml@v3
         with:
           ocaml-compiler: ${{ matrix.setup.ocaml-compiler }}
+          opam-repositories: "${{ matrix.setup.opam-repositories || 'default: git+https://github.com/ocaml/opam-repository.git' }}"
           opam-pin: true
 
       - name: Load opam cache when not Windows
@@ -64,10 +65,6 @@ jobs:
           path: _opam
           key: v0.0.1-opam-${{ matrix.setup.os }}-${{ matrix.setup.ocaml-compiler }}-${{ hashFiles('**.opam') }}
 
-      - name: Pin utop for OCaml 5.5
-        if: ${{ matrix.setup.ocaml-compiler == 'ocaml-base-compiler.5.5.0~alpha1' }}
-        run: opam pin add utop --dev-repo
-
       - name: Install dependencies
         run: opam install . --deps-only
 
@@ -102,4 +99,3 @@ jobs:
         with:
           path: _opam
           key: v0.0.1-opam-${{ matrix.setup.os }}-${{ matrix.setup.ocaml-compiler }}-${{ hashFiles('**.opam') }}
-
diff --git a/flake.lock b/flake.lock
index 73a4c0186..ea93fdef5 100644
--- a/flake.lock
+++ b/flake.lock
@@ -5,11 +5,11 @@
         "nixpkgs": "nixpkgs_2"
       },
       "locked": {
-        "lastModified": 1772923463,
-        "narHash": "sha256-doF5P6h72PgGgZosq1CV6XGPg6J+gRQCOHN/GmRwsu0=",
+        "lastModified": 1782090703,
+        "narHash": "sha256-2+ZNFE9G4QJz3W+cwRIHp3ckDvIhgLZNchAGgwemUSA=",
         "owner": "nix-ocaml",
         "repo": "nix-overlays",
-        "rev": "27d83e36fd6d0ac659ac6736451444ffbcef3f67",
+        "rev": "e50f291c63f3e992cc726e9adc5a775bbaf7bb79",
         "type": "github"
       },
       "original": {
@@ -20,17 +20,17 @@
     },
     "nixpkgs_2": {
       "locked": {
-        "lastModified": 1772906963,
-        "narHash": "sha256-jT3m0eiRH9TLqMsMVblze5/DPupInp8Qc8Gop8Zxfho=",
+        "lastModified": 1782058286,
+        "narHash": "sha256-U7bO7gEY2KvrJ+Ha8tKQKJATdPi9Wf1ygrJLdVV6F+M=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "5a60c0dff24e7109c0e87fd53e5bbc0032fa0cbd",
+        "rev": "1745eb556759f12578e02d8a45e782a7b5924e2b",
         "type": "github"
       },
       "original": {
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "5a60c0dff24e7109c0e87fd53e5bbc0032fa0cbd",
+        "rev": "1745eb556759f12578e02d8a45e782a7b5924e2b",
         "type": "github"
       }
     },
diff --git a/flake.nix b/flake.nix
index f2dcd45de..1ffdb66be 100644
--- a/flake.nix
+++ b/flake.nix
@@ -13,7 +13,7 @@
           let
             pkgs = nixpkgs.legacyPackages.${system}.extend (
               self: super: {
-                ocamlPackages = super.ocaml-ng.ocamlPackages_5_4;
+                ocamlPackages = super.ocaml-ng.ocamlPackages_5_5;
               }
             );
           in