diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 794ddf2..35a3589 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -53,3 +53,9 @@ repos:
         entry: bash -c 'cd "$(git rev-parse --show-toplevel)" && nix develop --command task check-ci-images'
         pass_filenames: false
         files: ^(ci/main\.go|\.fvmrc)$
+      - id: dagger-versions-aligned
+        name: verify Dagger version is consistent across dagger.json, flake.nix, Dockerfile and DAGGER.md
+        language: system
+        entry: bash -c 'cd "$(git rev-parse --show-toplevel)" && scripts/check_dagger_versions.sh'
+        pass_filenames: false
+        files: ^(ci/dagger\.json|flake\.nix|\.forgejo/Dockerfile|DAGGER\.md)$
diff --git a/Taskfile.yml b/Taskfile.yml
index 0cc1083..9279b97 100644
--- a/Taskfile.yml
+++ b/Taskfile.yml
@@ -712,6 +712,11 @@ tasks:
     cmds:
       - scripts/check_ci_images.sh
 
+  check-dagger-versions:
+    desc: Verify ci/dagger.json, flake.nix, .forgejo/Dockerfile and DAGGER.md pin the same Dagger version
+    cmds:
+      - scripts/check_dagger_versions.sh
+
   _integrations:
     internal: true
     run: once
diff --git a/flake.nix b/flake.nix
index 03c5ec3..b512860 100644
--- a/flake.nix
+++ b/flake.nix
@@ -49,14 +49,16 @@
           '';
         };
 
-        # The dagger/nix flake pins 0.20.8, whose Nix wrapper is a broken self-exec
-        # loop.  Fetch 0.21.4 directly so the pre-commit dart-check hook can run.
-        dagger021 = pkgs.stdenv.mkDerivation {
+        # The dagger/nix flake's Nix wrapper is a broken self-exec loop, so we
+        # fetch the CLI binary directly.  Keep this version in lockstep with
+        # ci/dagger.json (engineVersion) and .forgejo/Dockerfile (DAGGER_VERSION) —
+        # scripts/check_dagger_versions.sh enforces this.
+        daggerCli = pkgs.stdenv.mkDerivation {
           pname = "dagger";
-          version = "0.21.4";
+          version = "0.20.8";
           src = pkgs.fetchurl {
-            url = "https://dl.dagger.io/dagger/releases/0.21.4/dagger_v0.21.4_linux_amd64.tar.gz";
-            sha256 = "0wlnbr4g5069755131yjp2a6alacn64f1c8b27xn0cbynq3zicjd";
+            url = "https://dl.dagger.io/dagger/releases/0.20.8/dagger_v0.20.8_linux_amd64.tar.gz";
+            sha256 = "1ns6wq2z1skd2fq9lbrcali0s8kn24p3haamnjjgchg6zlv6b960";
           };
           sourceRoot = ".";
           installPhase = ''
@@ -69,7 +71,7 @@
         devShells.default = pkgs.mkShell {
           buildInputs = with pkgs; [
             # Dagger CLI
-            dagger021
+            daggerCli
 
             # Go compiler — for Dagger development
             go
diff --git a/scripts/check_dagger_versions.sh b/scripts/check_dagger_versions.sh
new file mode 100755
index 0000000..e479b77
--- /dev/null
+++ b/scripts/check_dagger_versions.sh
@@ -0,0 +1,49 @@
+#!/usr/bin/env bash
+# Verify that the Dagger version is consistent across the project.
+#
+# The Dagger CLI must speak the same protocol as the engine it talks to.  We
+# pin the version in four places (engine image in DAGGER.md, the CLI in
+# flake.nix, the CLI in the Forgejo runner Dockerfile, and the module
+# engineVersion in ci/dagger.json).  This script fails if any of them drift.
+set -euo pipefail
+
+ROOT=$(git rev-parse --show-toplevel)
+
+# ci/dagger.json — strip leading "v" for comparison.
+dagger_json=$(grep -oE '"engineVersion"[[:space:]]*:[[:space:]]*"[^"]+"' "$ROOT/ci/dagger.json" \
+  | sed -E 's/.*"v?([^"]+)"$/\1/')
+
+# flake.nix — the dagger021 derivation's CLI download URL.
+flake_nix=$(grep -oE 'dagger_v[0-9]+\.[0-9]+\.[0-9]+_linux' "$ROOT/flake.nix" \
+  | head -n1 \
+  | sed -E 's/dagger_v([0-9.]+)_linux/\1/')
+
+# .forgejo/Dockerfile — DAGGER_VERSION env on the install line.
+dockerfile=$(grep -oE 'DAGGER_VERSION=[0-9]+\.[0-9]+\.[0-9]+' "$ROOT/.forgejo/Dockerfile" \
+  | head -n1 \
+  | cut -d= -f2)
+
+# DAGGER.md — engine image tag in the example systemd unit.
+dagger_md=$(grep -oE 'dagger/nix/v[0-9]+\.[0-9]+\.[0-9]+' "$ROOT/DAGGER.md" \
+  | head -n1 \
+  | sed -E 's@.*/v@@')
+
+printf 'ci/dagger.json    engineVersion = v%s\n' "$dagger_json"
+printf 'flake.nix         dagger021     = %s\n'  "$flake_nix"
+printf '.forgejo/Dockerf. DAGGER_VERSION= %s\n'  "$dockerfile"
+printf 'DAGGER.md         engine tag    = v%s\n' "$dagger_md"
+
+for v in "$flake_nix" "$dockerfile" "$dagger_md"; do
+  if [ -z "$v" ]; then
+    echo "ERROR: failed to parse a Dagger version reference." >&2
+    exit 1
+  fi
+  if [ "$v" != "$dagger_json" ]; then
+    echo "" >&2
+    echo "ERROR: Dagger versions are out of sync." >&2
+    echo "  Align ci/dagger.json, flake.nix, .forgejo/Dockerfile and DAGGER.md to the same version." >&2
+    exit 1
+  fi
+done
+
+echo "Dagger versions aligned (v$dagger_json)."