diff --git a/nix/templates/harness-base.nix b/nix/templates/harness-base.nix
index a1948ac..9127ac8 100644
--- a/nix/templates/harness-base.nix
+++ b/nix/templates/harness-base.nix
@@ -505,6 +505,7 @@
       path = [
         pkgs.curl
         pkgs.coreutils
+        pkgs.jq
         pkgs.librsvg
       ];
       script = ''
@@ -535,12 +536,15 @@
         fi
         IMAGE=$(base64 -w 0 < "$PNG")
         rm -f "$PNG"
-        DATA_URI="data:image/png;base64,$IMAGE"
+        # Forgejo POST /user/avatar expects {"image":"<base64>"} — just the
+        # raw base64 string, NOT a data URI (data:image/png;base64,...).
+        # Use jq to build the payload so the large base64 value is safely quoted.
+        PAYLOAD=$(jq -n --arg img "$IMAGE" '{image:$img}')
         RESP=$(curl -sf --max-time 10 \
           -X POST "$FORGE_URL/api/v1/user/avatar" \
           -H "Authorization: token $TOKEN" \
           -H "Content-Type: application/json" \
-          -d "{\"image\":\"$DATA_URI\"}" \
+          -d "$PAYLOAD" \
           -w "\n%{http_code}" 2>/dev/null || true)
         CODE=$(printf '%s' "$RESP" | tail -1)
         if [ "$CODE" = "204" ] || [ "$CODE" = "200" ]; then