diff --git a/pandoc-cli/pandoc-cli.cabal b/pandoc-cli/pandoc-cli.cabal
index ecfa91185..29549b952 100644
--- a/pandoc-cli/pandoc-cli.cabal
+++ b/pandoc-cli/pandoc-cli.cabal
@@ -71,21 +71,20 @@ common common-executable
 
 executable pandoc
   import:          common-executable
-  hs-source-dirs:  src
+  main-is:         pandoc.hs
   buildable:       True
   -- Note: we always link to an exact version of pandoc, with the
   -- same version as this package:
-  build-depends:   pandoc == 3.8.3
+  build-depends:   pandoc == 3.8.3, text
   other-modules:   PandocCLI.Lua
                  , PandocCLI.Server
 
   if arch(wasm32)
-    main-is:       pandoc-wasm.hs
+    hs-source-dirs:  wasm
     build-depends: aeson, containers, bytestring
-    ghc-options:    -optl-Wl,--export=__wasm_call_ctors,--export=hs_init_with_rtsopts,--export=malloc,--export=convert,--export=get_extensions_for_format
+    ghc-options:    -optl-Wl,--export=__wasm_call_ctors,--export=hs_init_with_rtsopts,--export=malloc,--export=convert,--export=query
   else
-    main-is:       pandoc.hs
-    build-depends: text
+    hs-source-dirs:  src
 
   if flag(nightly)
     cpp-options:    -DNIGHTLY
diff --git a/pandoc-cli/src/pandoc-wasm.hs b/pandoc-cli/wasm/pandoc.hs
similarity index 61%
rename from pandoc-cli/src/pandoc-wasm.hs
rename to pandoc-cli/wasm/pandoc.hs
index 2fd3a00f9..0372a9197 100644
--- a/pandoc-cli/src/pandoc-wasm.hs
+++ b/pandoc-cli/wasm/pandoc.hs
@@ -1,4 +1,5 @@
 {-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE OverloadedStrings #-}
 
 {- |
    Module      : Main
@@ -25,13 +26,14 @@ import PandocCLI.Lua
 import Control.Exception
 import Foreign
 import Foreign.C
-import qualified Data.Aeson as Aeson
+import Data.Aeson as Aeson
 import qualified Text.Pandoc.UTF8 as UTF8
 import qualified Data.ByteString.Lazy as BL
+import qualified Data.Text as T
 
 foreign export ccall "convert" convert :: Ptr CChar -> Int -> IO ()
 
--- | The parameters are a pointer and length for a C string
+  -- | The parameters are a pointer and length for a C string
 -- containing JSON-encoded pandoc options (isomorphic to a defaults
 -- file).  The calling program should set up a virtual file system
 -- containing @/stdin@ (input), @/stdout@ (output), and @/warnings@ (output).
@@ -59,22 +61,41 @@ convert ptr len =
                           }
           convertWithOpts engine opts'
 
-foreign export ccall "get_extensions_for_format" get_extensions_for_format
-   :: Ptr CChar -> Int -> IO ()
+foreign export ccall "query" query :: Ptr CChar -> Int -> IO ()
 
--- | Given a C string with the format name, returnsa  JSON object
--- mapping extensions relevant for this format to true or false,
--- depending on whether they are enabled by default.
-get_extensions_for_format :: Ptr CChar -> Int -> IO ()
-get_extensions_for_format ptr len = do
-  formatName <- readMaybe <$> getCString ptr len
-  case formatName of
-    Just fmt -> do
-       let allExts = getAllExtensions fmt
-       let defExts = getDefaultExtensions fmt
-       let addExt x = M.insert (drop 4 (show x)) (extensionEnabled x defExts)
-       BL.writeFile "/stdout" $ Aeson.encode $ foldr addExt mempty (extensionsToList allExts)
-    Nothing -> writeFile "/stdout" "{}"
+-- | The parameters are a pointer and length for a C string
+-- containing a JSON-encoded query, e.g.,
+-- @{ "query": "extensions-for-format", "argument": "markdown" }@.
+-- The calling program should set up a virtual file system
+-- containing @/stdout@, which will be used to hold the output,
+-- and @/stderr@, which will be used to hold any error messages.
+query :: Ptr CChar -> Int -> IO ()
+query ptr len =
+  E.catch act (\(err :: SomeException) ->
+                 writeFile "/stderr" ("ERROR: " <> displayException err))
+  where
+    act = do
+      args <- getCString ptr len
+      case Aeson.eitherDecode (UTF8.fromStringLazy args) of
+        Left e -> error e
+        Right (ExtensionsForFormat fmt) -> do
+          let allExts = getAllExtensions fmt
+          let defExts = getDefaultExtensions fmt
+          let addExt x = M.insert (drop 4 (show x))
+                              (extensionEnabled x defExts)
+          BL.writeFile "/stdout" $ Aeson.encode $
+                 foldr addExt mempty (extensionsToList allExts)
+
+data Query =
+  ExtensionsForFormat T.Text
+  deriving (Show)
+
+instance FromJSON Query where
+  parseJSON = withObject "Query" $ \o -> do
+    queryType <- o .: "query"
+    case queryType of
+      "extensions-for-format" -> ExtensionsForFormat <$> o .: "format"
+      _ -> fail $ "Unknown query type " <> queryType
 
 getCString :: Ptr CChar -> Int -> IO String
 getCString ptr len = peekCStringLen (ptr, len) <* free ptr
diff --git a/wasm/index.js b/wasm/index.js
index 26e43ca99..4de040f95 100644
--- a/wasm/index.js
+++ b/wasm/index.js
@@ -1,11 +1,11 @@
 import { zipSync, unzipSync, strToU8, strFromU8 } from 'https://esm.sh/fflate@0.8.2';
-import { convert, getExtensionsForFormat } from "./pandoc.js?sha1=SHA1_PANDOC_JS";
+import { convert, query } from "./pandoc.js?sha1=SHA1_PANDOC_JS";
 
 // Make fflate available globally
 window.fflate = { zipSync, unzipSync, strToU8, strFromU8 };
 
 // Make pandoc available globally for the app
-window.pandocModule = { convert, getExtensionsForFormat };
+window.pandocModule = { convert, query };
 
 // Lazy-load typst library only when needed
 let typstLoaded = false;
@@ -1014,7 +1014,7 @@ window.pandocApp = function() {
 
       if (this.inputFormat !== 'auto') {
         try {
-          const extData = await window.pandocModule.getExtensionsForFormat(this.inputFormat);
+          const extData = await window.pandocModule.query({ query: "extensions-for-format", format: this.inputFormat });
           this.inputExtensionsList = Object.entries(extData)
             .map(([name, defaultOn]) => ({ name, defaultOn }))
             .sort((a, b) => a.name.localeCompare(b.name));
@@ -1024,7 +1024,7 @@ window.pandocApp = function() {
       const outFmt = this.outputFormat === 'pdf-typst' ? 'typst' : this.outputFormat;
       if (outFmt !== 'auto') {
         try {
-          const extData = await window.pandocModule.getExtensionsForFormat(outFmt);
+          const extData = await window.pandocModule.query({ query: "extensions-for-format", format: outFmt});
           this.outputExtensionsList = Object.entries(extData)
             .map(([name, defaultOn]) => ({ name, defaultOn }))
             .sort((a, b) => a.name.localeCompare(b.name));
diff --git a/wasm/pandoc.js b/wasm/pandoc.js
index 0d602c076..99bdc2ac8 100644
--- a/wasm/pandoc.js
+++ b/wasm/pandoc.js
@@ -70,7 +70,7 @@ memory_data_view().setUint32(argv_ptr, argv, true);
 
 instance.exports.hs_init_with_rtsopts(argc_ptr, argv_ptr);
 
-export async function getExtensionsForFormat(options) {
+export async function query(options) {
   const opts_str = JSON.stringify(options);
   const opts_ptr = instance.exports.malloc(opts_str.length);
   new TextEncoder().encodeInto(
@@ -83,11 +83,15 @@ export async function getExtensionsForFormat(options) {
   const err_file = new File(new Uint8Array(), { readonly: false });
   fileSystem.set("stdout", out_file);
   fileSystem.set("stderr", err_file);
-  instance.exports.get_extensions_for_format(opts_ptr, opts_str.length);
+  instance.exports.query(opts_ptr, opts_str.length);
 
-  return JSON.parse(new TextDecoder("utf-8", { fatal: true }).decode(out_file.data));
+  const err_text = new TextDecoder("utf-8", { fatal: true }).decode(err_file.data);
+  if (err_text) console.log(err_text);
+  const out_text = new TextDecoder("utf-8", { fatal: true }).decode(out_file.data);
+  return JSON.parse(out_text);
 }
 
+
 export async function convert(options, stdin, files) {
   const opts_str = JSON.stringify(options);
   const opts_ptr = instance.exports.malloc(opts_str.length);