diff --git a/ext/bg/js/backend.js b/ext/bg/js/backend.js
index 1fa7ede1..ed4a24ed 100644
--- a/ext/bg/js/backend.js
+++ b/ext/bg/js/backend.js
@@ -77,33 +77,33 @@ class Backend {
         this.messageToken = yomichan.generateId(16);
 
         this._messageHandlers = new Map([
-            ['yomichanCoreReady', this._onApiYomichanCoreReady.bind(this)],
-            ['optionsSchemaGet', this._onApiOptionsSchemaGet.bind(this)],
-            ['optionsGet', this._onApiOptionsGet.bind(this)],
-            ['optionsGetFull', this._onApiOptionsGetFull.bind(this)],
-            ['optionsSet', this._onApiOptionsSet.bind(this)],
-            ['optionsSave', this._onApiOptionsSave.bind(this)],
-            ['kanjiFind', this._onApiKanjiFind.bind(this)],
-            ['termsFind', this._onApiTermsFind.bind(this)],
-            ['textParse', this._onApiTextParse.bind(this)],
-            ['textParseMecab', this._onApiTextParseMecab.bind(this)],
-            ['definitionAdd', this._onApiDefinitionAdd.bind(this)],
-            ['definitionsAddable', this._onApiDefinitionsAddable.bind(this)],
-            ['noteView', this._onApiNoteView.bind(this)],
-            ['templateRender', this._onApiTemplateRender.bind(this)],
-            ['commandExec', this._onApiCommandExec.bind(this)],
-            ['audioGetUri', this._onApiAudioGetUri.bind(this)],
-            ['screenshotGet', this._onApiScreenshotGet.bind(this)],
-            ['forward', this._onApiForward.bind(this)],
-            ['frameInformationGet', this._onApiFrameInformationGet.bind(this)],
-            ['injectStylesheet', this._onApiInjectStylesheet.bind(this)],
-            ['getEnvironmentInfo', this._onApiGetEnvironmentInfo.bind(this)],
-            ['clipboardGet', this._onApiClipboardGet.bind(this)],
-            ['getDisplayTemplatesHtml', this._onApiGetDisplayTemplatesHtml.bind(this)],
-            ['getQueryParserTemplatesHtml', this._onApiGetQueryParserTemplatesHtml.bind(this)],
-            ['getZoom', this._onApiGetZoom.bind(this)],
-            ['getMessageToken', this._onApiGetMessageToken.bind(this)],
-            ['getDefaultAnkiFieldTemplates', this._onApiGetDefaultAnkiFieldTemplates.bind(this)]
+            ['yomichanCoreReady', {handler: this._onApiYomichanCoreReady.bind(this), async: true}],
+            ['optionsSchemaGet', {handler: this._onApiOptionsSchemaGet.bind(this), async: true}],
+            ['optionsGet', {handler: this._onApiOptionsGet.bind(this), async: true}],
+            ['optionsGetFull', {handler: this._onApiOptionsGetFull.bind(this), async: true}],
+            ['optionsSet', {handler: this._onApiOptionsSet.bind(this), async: true}],
+            ['optionsSave', {handler: this._onApiOptionsSave.bind(this), async: true}],
+            ['kanjiFind', {handler: this._onApiKanjiFind.bind(this), async: true}],
+            ['termsFind', {handler: this._onApiTermsFind.bind(this), async: true}],
+            ['textParse', {handler: this._onApiTextParse.bind(this), async: true}],
+            ['textParseMecab', {handler: this._onApiTextParseMecab.bind(this), async: true}],
+            ['definitionAdd', {handler: this._onApiDefinitionAdd.bind(this), async: true}],
+            ['definitionsAddable', {handler: this._onApiDefinitionsAddable.bind(this), async: true}],
+            ['noteView', {handler: this._onApiNoteView.bind(this), async: true}],
+            ['templateRender', {handler: this._onApiTemplateRender.bind(this), async: true}],
+            ['commandExec', {handler: this._onApiCommandExec.bind(this), async: true}],
+            ['audioGetUri', {handler: this._onApiAudioGetUri.bind(this), async: true}],
+            ['screenshotGet', {handler: this._onApiScreenshotGet.bind(this), async: true}],
+            ['forward', {handler: this._onApiForward.bind(this), async: true}],
+            ['frameInformationGet', {handler: this._onApiFrameInformationGet.bind(this), async: true}],
+            ['injectStylesheet', {handler: this._onApiInjectStylesheet.bind(this), async: true}],
+            ['getEnvironmentInfo', {handler: this._onApiGetEnvironmentInfo.bind(this), async: true}],
+            ['clipboardGet', {handler: this._onApiClipboardGet.bind(this), async: true}],
+            ['getDisplayTemplatesHtml', {handler: this._onApiGetDisplayTemplatesHtml.bind(this), async: true}],
+            ['getQueryParserTemplatesHtml', {handler: this._onApiGetQueryParserTemplatesHtml.bind(this), async: true}],
+            ['getZoom', {handler: this._onApiGetZoom.bind(this), async: true}],
+            ['getMessageToken', {handler: this._onApiGetMessageToken.bind(this), async: true}],
+            ['getDefaultAnkiFieldTemplates', {handler: this._onApiGetDefaultAnkiFieldTemplates.bind(this), async: true}]
         ]);
 
         this._commandHandlers = new Map([
@@ -167,16 +167,23 @@ class Backend {
     }
 
     onMessage({action, params}, sender, callback) {
-        const handler = this._messageHandlers.get(action);
-        if (typeof handler !== 'function') { return false; }
+        const messageHandler = this._messageHandlers.get(action);
+        if (typeof messageHandler === 'undefined') { return false; }
+
+        const {handler, async} = messageHandler;
 
         try {
-            const promise = handler(params, sender);
-            promise.then(
-                (result) => callback({result}),
-                (error) => callback({error: errorToJson(error)})
-            );
-            return true;
+            const promiseOrResult = handler(params, sender);
+            if (async) {
+                promiseOrResult.then(
+                    (result) => callback({result}),
+                    (error) => callback({error: errorToJson(error)})
+                );
+                return true;
+            } else {
+                callback({result: promiseOrResult});
+                return false;
+            }
         } catch (error) {
             callback({error: errorToJson(error)});
             return false;