Easy piecy in the end

src/build.jl

@@ -367,7 +367,7 @@ end
 
 function _add_extra_preamble!(session::ServerSession)
     current = session.options.evaluation.workspace_custom_startup_expr
-    config = CONFIG_PLUTORUNNER
+    config = string(CONFIG_PLUTORUNNER)::String
     # Avoids warning message to show up multiple every time
     # https://github.com/rikhuijzer/PlutoStaticHTML.jl/pull/172.
     if current !== nothing

src/mimeoverride.jl

@@ -1,7 +1,8 @@
-const CONFIG_PLUTORUNNER = string(quote
+# Keep this a quote because it's used in `eval` below.
+const CONFIG_PLUTORUNNER = quote
         PlutoRunner.is_mime_enabled(::MIME"application/vnd.pluto.tree+object") = false
         PlutoRunner.PRETTY_STACKTRACES[] = false
-    end)::String
+    end
 
 # These overrides are used when `use_distributed=false`.
 eval(CONFIG_PLUTORUNNER)