diff --git a/src/ObservableObject.ts b/src/ObservableObject.ts
index f09bdb7a..24911aed 100644
--- a/src/ObservableObject.ts
+++ b/src/ObservableObject.ts
@@ -412,6 +412,11 @@ const proxyHandler: ProxyHandler<any> = {
             }
         }
 
+        if (p === 'constructor') {
+            const ctor = peekInternal(node)?.constructor;
+            return typeof ctor === 'function' ? ctor : Object;
+        }
+
         let value = peekInternal(node, /*activateRecursive*/ p === 'get' || p === 'peek');
 
         // Trying to get an iterator if the raw value is a primitive should return undefined.
@@ -639,7 +644,20 @@ const proxyHandler: ProxyHandler<any> = {
     },
     has(node: NodeInfo, prop: string) {
         const value = getNodeValue(node);
-        return Reflect.has(value, prop);
+
+        // Short-circuit for the inputs that make Reflect.has error.
+        if (value === undefined || value === null) {
+            return false;
+        }
+
+        // Functions behave like objects here, so let them flow through.
+        if (typeof value === 'object' || typeof value === 'function') {
+            return Reflect.has(value, prop);
+        }
+
+        // For primitives (number, string, boolean, bigint, symbol) report “no key”.
+        // That keeps inspection code happy without pretending properties exist.
+        return false;
     },
     apply(target, thisArg, argArray) {
         // If it's a function call it as a function
diff --git a/src/observableTypes.ts b/src/observableTypes.ts
index 76e52501..99c8fc48 100644
--- a/src/observableTypes.ts
+++ b/src/observableTypes.ts
@@ -43,7 +43,8 @@ export type RemoveObservables<T> =
                     : T;
 
 interface ObservableArray<T, U>
-    extends ObservablePrimitive<T>,
+    extends
+        ObservablePrimitive<T>,
         Pick<Array<Observable<U>>, ArrayOverrideFnNames>,
         Omit<RemoveIndex<Array<U>>, ArrayOverrideFnNames> {}
 
diff --git a/src/sync-plugins/crud.ts b/src/sync-plugins/crud.ts
index 38cd8a53..dee705bd 100644
--- a/src/sync-plugins/crud.ts
+++ b/src/sync-plugins/crud.ts
@@ -69,8 +69,10 @@ export interface CrudErrorParams extends Omit<SyncedErrorParams, 'source'> {
 
 export type CrudOnErrorFn = (error: Error, params: CrudErrorParams) => void;
 
-export interface SyncedCrudPropsBase<TRemote extends object, TLocal = TRemote>
-    extends Omit<SyncedOptions<TRemote, TLocal>, 'get' | 'set' | 'initial' | 'subscribe' | 'waitForSet' | 'onError'> {
+export interface SyncedCrudPropsBase<TRemote extends object, TLocal = TRemote> extends Omit<
+    SyncedOptions<TRemote, TLocal>,
+    'get' | 'set' | 'initial' | 'subscribe' | 'waitForSet' | 'onError'
+> {
     create?(input: TRemote, params: SyncedSetParams<TRemote>): Promise<CrudResult<TRemote> | null | undefined | void>;
     update?(
         input: Partial<TRemote>,
diff --git a/src/sync-plugins/fetch.ts b/src/sync-plugins/fetch.ts
index 0205ae6d..2456960f 100644
--- a/src/sync-plugins/fetch.ts
+++ b/src/sync-plugins/fetch.ts
@@ -8,8 +8,10 @@ export interface SyncedFetchOnSavedParams<TRemote, TLocal = TRemote> {
     props: SyncedFetchProps<TRemote, TLocal>;
 }
 
-export interface SyncedFetchProps<TRemote, TLocal = TRemote>
-    extends Omit<SyncedOptions<TRemote, TLocal>, 'get' | 'set'> {
+export interface SyncedFetchProps<TRemote, TLocal = TRemote> extends Omit<
+    SyncedOptions<TRemote, TLocal>,
+    'get' | 'set'
+> {
     get: Selector<string>;
     set?: Selector<string>;
     getInit?: RequestInit;
diff --git a/src/sync-plugins/firebase.ts b/src/sync-plugins/firebase.ts
index bdaa9a8e..ee3afe94 100644
--- a/src/sync-plugins/firebase.ts
+++ b/src/sync-plugins/firebase.ts
@@ -50,7 +50,8 @@ import { clone } from '../globals';
 // Should it have mode merge by default?
 
 export interface SyncedFirebaseProps<TRemote extends object, TLocal, TAs extends CrudAsOption = 'value'>
-    extends Omit<SyncedCrudPropsMany<TRemote, TLocal, TAs>, 'list' | 'retry'>,
+    extends
+        Omit<SyncedCrudPropsMany<TRemote, TLocal, TAs>, 'list' | 'retry'>,
         Omit<SyncedCrudPropsBase<TRemote, TLocal>, 'onError'> {
     refPath: (uid: string | undefined) => string;
     query?: (ref: DatabaseReference) => DatabaseReference | Query;
diff --git a/src/sync-plugins/keel.ts b/src/sync-plugins/keel.ts
index 1d8d40e7..e2e75ddf 100644
--- a/src/sync-plugins/keel.ts
+++ b/src/sync-plugins/keel.ts
@@ -82,8 +82,10 @@ interface PageInfo {
     totalCount: number;
 }
 
-interface SyncedKeelPropsManyBase<TRemote extends { id: string }, TLocal, AOption extends CrudAsOption>
-    extends Omit<SyncedCrudPropsMany<TRemote, TLocal, AOption>, 'list'> {
+interface SyncedKeelPropsManyBase<TRemote extends { id: string }, TLocal, AOption extends CrudAsOption> extends Omit<
+    SyncedCrudPropsMany<TRemote, TLocal, AOption>,
+    'list'
+> {
     first?: number;
     get?: never;
 }
@@ -103,8 +105,11 @@ interface SyncedKeelPropsManyWhere<
     >;
     where?: Where | (() => Where);
 }
-interface SyncedKeelPropsManyNoWhere<TRemote extends { id: string }, TLocal, AOption extends CrudAsOption>
-    extends SyncedKeelPropsManyBase<TRemote, TLocal, AOption> {
+interface SyncedKeelPropsManyNoWhere<
+    TRemote extends { id: string },
+    TLocal,
+    AOption extends CrudAsOption,
+> extends SyncedKeelPropsManyBase<TRemote, TLocal, AOption> {
     list?: (params: KeelListParams<{}>) => Promise<
         CrudResult<
             APIResult<{
@@ -127,8 +132,10 @@ type SyncedKeelPropsMany<
         ? SyncedKeelPropsManyWhere<TRemote, TLocal, AOption, Where>
         : SyncedKeelPropsManyNoWhere<TRemote, TLocal, AOption>;
 
-interface SyncedKeelPropsSingle<TRemote extends { id: string }, TLocal>
-    extends Omit<SyncedCrudPropsSingle<TRemote, TLocal>, 'get'> {
+interface SyncedKeelPropsSingle<TRemote extends { id: string }, TLocal> extends Omit<
+    SyncedCrudPropsSingle<TRemote, TLocal>,
+    'get'
+> {
     get?: (params: KeelGetParams) => Promise<APIResult<TRemote>>;
 
     first?: never;
@@ -141,11 +148,10 @@ export interface KeelErrorParams extends CrudErrorParams {
     action: string;
 }
 
-export interface SyncedKeelPropsBase<TRemote extends { id: string }, TLocal = TRemote>
-    extends Omit<
-        SyncedCrudPropsBase<TRemote, TLocal>,
-        'create' | 'update' | 'delete' | 'updatePartial' | 'fieldUpdatedAt' | 'fieldCreatedAt' | 'onError'
-    > {
+export interface SyncedKeelPropsBase<TRemote extends { id: string }, TLocal = TRemote> extends Omit<
+    SyncedCrudPropsBase<TRemote, TLocal>,
+    'create' | 'update' | 'delete' | 'updatePartial' | 'fieldUpdatedAt' | 'fieldCreatedAt' | 'onError'
+> {
     client?: KeelClient;
     create?: (i: NoInfer<Partial<TRemote>>) => Promise<APIResult<NoInfer<TRemote>>>;
     update?: (params: { where: any; values?: Partial<NoInfer<TRemote>> }) => Promise<APIResult<TRemote>>;
diff --git a/src/sync-plugins/supabase.ts b/src/sync-plugins/supabase.ts
index 8734e629..005aa650 100644
--- a/src/sync-plugins/supabase.ts
+++ b/src/sync-plugins/supabase.ts
@@ -70,11 +70,10 @@ export type SyncedSupabaseConfig<TRemote extends { id: string | number }, TLocal
     'create' | 'update' | 'delete'
 >;
 
-export interface SyncedSupabaseConfiguration
-    extends Omit<
-        SyncedSupabaseConfig<{ id: string | number }, { id: string | number }>,
-        'persist' | keyof SyncedOptions
-    > {
+export interface SyncedSupabaseConfiguration extends Omit<
+    SyncedSupabaseConfig<{ id: string | number }, { id: string | number }>,
+    'persist' | keyof SyncedOptions
+> {
     persist?: SyncedOptionsGlobal;
     enabled?: Observable<boolean>;
     as?: Exclude<CrudAsOption, 'value'>;
@@ -87,8 +86,8 @@ interface SyncedSupabaseProps<
     TOption extends CrudAsOption = 'object',
     TRemote extends SupabaseRowOf<Client, Collection, SchemaName> = SupabaseRowOf<Client, Collection, SchemaName>,
     TLocal = TRemote,
-> extends SyncedSupabaseConfig<TRemote, TLocal>,
-        Omit<SyncedCrudPropsMany<TRemote, TRemote, TOption>, 'list'> {
+>
+    extends SyncedSupabaseConfig<TRemote, TLocal>, Omit<SyncedCrudPropsMany<TRemote, TRemote, TOption>, 'list'> {
     supabase?: Client;
     collection: Collection;
     schema?: SchemaName;
diff --git a/src/sync-plugins/tanstack-query.ts b/src/sync-plugins/tanstack-query.ts
index f5d8acff..4b5afed0 100644
--- a/src/sync-plugins/tanstack-query.ts
+++ b/src/sync-plugins/tanstack-query.ts
@@ -14,13 +14,17 @@ import {
 
 let nextMutationKey = 0;
 
-export interface ObservableQueryOptions<TQueryFnData, TError, TData, TQueryKey extends QueryKey>
-    extends Omit<QueryObserverOptions<TQueryFnData, TError, TData, TData, TQueryKey>, 'queryKey'> {
+export interface ObservableQueryOptions<TQueryFnData, TError, TData, TQueryKey extends QueryKey> extends Omit<
+    QueryObserverOptions<TQueryFnData, TError, TData, TData, TQueryKey>,
+    'queryKey'
+> {
     queryKey?: TQueryKey | (() => TQueryKey);
 }
 
-export interface SyncedQueryParams<TQueryFnData, TError, TData, TQueryKey extends QueryKey>
-    extends Omit<SyncedOptions<TData>, 'get' | 'set' | 'retry'> {
+export interface SyncedQueryParams<TQueryFnData, TError, TData, TQueryKey extends QueryKey> extends Omit<
+    SyncedOptions<TData>,
+    'get' | 'set' | 'retry'
+> {
     queryClient: QueryClient;
     query: ObservableQueryOptions<TQueryFnData, TError, TData, TQueryKey>;
     mutation?: MutationObserverOptions<TQueryFnData, TError, TData>;
diff --git a/src/sync/syncTypes.ts b/src/sync/syncTypes.ts
index 6206dda8..af82c327 100644
--- a/src/sync/syncTypes.ts
+++ b/src/sync/syncTypes.ts
@@ -104,11 +104,10 @@ export interface SyncedOptions<TRemote = any, TLocal = TRemote> extends Omit<Lin
     // log?: (message?: any, ...optionalParams: any[]) => void;
 }
 
-export interface SyncedOptionsGlobal<T = any>
-    extends Omit<
-        SyncedOptions<T>,
-        'get' | 'set' | 'persist' | 'initial' | 'waitForSet' | 'waitFor' | 'transform' | 'subscribe'
-    > {
+export interface SyncedOptionsGlobal<T = any> extends Omit<
+    SyncedOptions<T>,
+    'get' | 'set' | 'persist' | 'initial' | 'waitForSet' | 'waitFor' | 'transform' | 'subscribe'
+> {
     persist?: ObservablePersistPluginOptions & Omit<PersistOptions, 'name' | 'transform' | 'options'>;
 }
 
diff --git a/tests/tests.test.ts b/tests/tests.test.ts
index 944e22fa..f8fda2f9 100644
--- a/tests/tests.test.ts
+++ b/tests/tests.test.ts
@@ -3881,4 +3881,16 @@ describe('Misc', () => {
         expect(descriptProxy).toEqual(undefined);
         expect(descriptValue).toEqual(undefined);
     });
+    test('Observable is compatible with vitest equality checks and error printing', () => {
+        const obs = observable({ foo: 'bar' });
+        const obj = obs.foo;
+        const ctor = obj.constructor;
+
+        // https://github.com/vitest-dev/vitest/blob/v3.2.4/packages/expect/src/jest-utils.ts#L42-L49
+        expect(!!obj && typeof obj === 'object' && 'asymmetricMatch' in obj).toBe(false);
+
+        // https://github.com/vitest-dev/vitest/blob/v3.2.4/packages/pretty-format/src/index.ts#L288
+        expect(typeof ctor).toBe('function');
+        expect(obj.constructor).toBe(ctor);
+    });
 });