diff --git a/.changeset/add-entity-id-relation-filter.md b/.changeset/add-entity-id-relation-filter.md
new file mode 100644
index 00000000..a0c2fe11
--- /dev/null
+++ b/.changeset/add-entity-id-relation-filter.md
@@ -0,0 +1,5 @@
+---
+"@graphprotocol/hypergraph": minor
+---
+
+add entityId filter for relation and backlink fields
diff --git a/.claude/settings.local.json b/.claude/settings.local.json
index 9358046e..5de25639 100644
--- a/.claude/settings.local.json
+++ b/.claude/settings.local.json
@@ -15,7 +15,12 @@
       "Bash(node -e:*)",
       "Bash(pnpm build:*)",
       "mcp__chrome-devtools__list_pages",
-      "Bash(pnpm lint:*)"
+      "Bash(pnpm lint:*)",
+      "Bash(npx @tanstack/router-cli generate:*)",
+      "mcp__chrome-devtools__navigate_page",
+      "mcp__chrome-devtools__list_network_requests",
+      "mcp__chrome-devtools__get_network_request",
+      "mcp__chrome-devtools__evaluate_script"
     ],
     "deny": [],
     "ask": []
diff --git a/apps/events/src/routeTree.gen.ts b/apps/events/src/routeTree.gen.ts
index 2e315526..b5ea8889 100644
--- a/apps/events/src/routeTree.gen.ts
+++ b/apps/events/src/routeTree.gen.ts
@@ -27,6 +27,7 @@ const PodcastsInfiniteLazyRouteImport = createFileRoute('/podcasts-infinite')()
 const PodcastsLazyRouteImport = createFileRoute('/podcasts')()
 const PlaygroundLazyRouteImport = createFileRoute('/playground')()
 const LoginLazyRouteImport = createFileRoute('/login')()
+const BountiesLazyRouteImport = createFileRoute('/bounties')()
 
 const PodcastsInfiniteLazyRoute = PodcastsInfiniteLazyRouteImport.update({
   id: '/podcasts-infinite',
@@ -50,6 +51,11 @@ const LoginLazyRoute = LoginLazyRouteImport.update({
   path: '/login',
   getParentRoute: () => rootRouteImport,
 } as any).lazy(() => import('./routes/login.lazy').then((d) => d.Route))
+const BountiesLazyRoute = BountiesLazyRouteImport.update({
+  id: '/bounties',
+  path: '/bounties',
+  getParentRoute: () => rootRouteImport,
+} as any).lazy(() => import('./routes/bounties.lazy').then((d) => d.Route))
 const AuthenticateSuccessRoute = AuthenticateSuccessRouteImport.update({
   id: '/authenticate-success',
   path: '/authenticate-success',
@@ -110,6 +116,7 @@ const SpaceSpaceIdChatRoute = SpaceSpaceIdChatRouteImport.update({
 export interface FileRoutesByFullPath {
   '/': typeof IndexRoute
   '/authenticate-success': typeof AuthenticateSuccessRoute
+  '/bounties': typeof BountiesLazyRoute
   '/login': typeof LoginLazyRoute
   '/playground': typeof PlaygroundLazyRoute
   '/podcasts': typeof PodcastsLazyRoute
@@ -127,6 +134,7 @@ export interface FileRoutesByFullPath {
 export interface FileRoutesByTo {
   '/': typeof IndexRoute
   '/authenticate-success': typeof AuthenticateSuccessRoute
+  '/bounties': typeof BountiesLazyRoute
   '/login': typeof LoginLazyRoute
   '/playground': typeof PlaygroundLazyRoute
   '/podcasts': typeof PodcastsLazyRoute
@@ -144,6 +152,7 @@ export interface FileRoutesById {
   __root__: typeof rootRouteImport
   '/': typeof IndexRoute
   '/authenticate-success': typeof AuthenticateSuccessRoute
+  '/bounties': typeof BountiesLazyRoute
   '/login': typeof LoginLazyRoute
   '/playground': typeof PlaygroundLazyRoute
   '/podcasts': typeof PodcastsLazyRoute
@@ -163,6 +172,7 @@ export interface FileRouteTypes {
   fullPaths:
     | '/'
     | '/authenticate-success'
+    | '/bounties'
     | '/login'
     | '/playground'
     | '/podcasts'
@@ -180,6 +190,7 @@ export interface FileRouteTypes {
   to:
     | '/'
     | '/authenticate-success'
+    | '/bounties'
     | '/login'
     | '/playground'
     | '/podcasts'
@@ -196,6 +207,7 @@ export interface FileRouteTypes {
     | '__root__'
     | '/'
     | '/authenticate-success'
+    | '/bounties'
     | '/login'
     | '/playground'
     | '/podcasts'
@@ -214,6 +226,7 @@ export interface FileRouteTypes {
 export interface RootRouteChildren {
   IndexRoute: typeof IndexRoute
   AuthenticateSuccessRoute: typeof AuthenticateSuccessRoute
+  BountiesLazyRoute: typeof BountiesLazyRoute
   LoginLazyRoute: typeof LoginLazyRoute
   PlaygroundLazyRoute: typeof PlaygroundLazyRoute
   PodcastsLazyRoute: typeof PodcastsLazyRoute
@@ -253,6 +266,13 @@ declare module '@tanstack/react-router' {
       preLoaderRoute: typeof LoginLazyRouteImport
       parentRoute: typeof rootRouteImport
     }
+    '/bounties': {
+      id: '/bounties'
+      path: '/bounties'
+      fullPath: '/bounties'
+      preLoaderRoute: typeof BountiesLazyRouteImport
+      parentRoute: typeof rootRouteImport
+    }
     '/authenticate-success': {
       id: '/authenticate-success'
       path: '/authenticate-success'
@@ -358,6 +378,7 @@ const SpaceSpaceIdRouteWithChildren = SpaceSpaceIdRoute._addFileChildren(
 const rootRouteChildren: RootRouteChildren = {
   IndexRoute: IndexRoute,
   AuthenticateSuccessRoute: AuthenticateSuccessRoute,
+  BountiesLazyRoute: BountiesLazyRoute,
   LoginLazyRoute: LoginLazyRoute,
   PlaygroundLazyRoute: PlaygroundLazyRoute,
   PodcastsLazyRoute: PodcastsLazyRoute,
diff --git a/apps/events/src/routes/__root.tsx b/apps/events/src/routes/__root.tsx
index 282f24fe..3f99c313 100644
--- a/apps/events/src/routes/__root.tsx
+++ b/apps/events/src/routes/__root.tsx
@@ -33,6 +33,9 @@ export const Route = createRootRoute({
           <nav className="ml-auto flex gap-4 sm:gap-6">
             {authenticated ? (
               <div className="flex items-center gap-4">
+                <Link className="text-xs" to="/bounties">
+                  Bounties
+                </Link>
                 <Link className="text-xs" to="/podcasts">
                   Podcasts
                 </Link>
diff --git a/apps/events/src/routes/bounties.lazy.tsx b/apps/events/src/routes/bounties.lazy.tsx
new file mode 100644
index 00000000..64e0def4
--- /dev/null
+++ b/apps/events/src/routes/bounties.lazy.tsx
@@ -0,0 +1,45 @@
+import { useEntities } from '@graphprotocol/hypergraph-react';
+import { createLazyFileRoute } from '@tanstack/react-router';
+import { Bounty } from '@/schema';
+
+export const Route = createLazyFileRoute('/bounties')({
+  component: RouteComponent,
+});
+
+const PERSON_ENTITY_ID = '7728d2458ae842d3a90a37e0bb8ee676';
+
+function RouteComponent() {
+  const {
+    data: bounties,
+    isLoading,
+    isError,
+  } = useEntities(Bounty, {
+    mode: 'public',
+    spaces: 'all',
+    filter: {
+      interestedIn: { entityId: PERSON_ENTITY_ID },
+    },
+  });
+
+  return (
+    <div className="flex flex-col gap-4 max-w-(--breakpoint-sm) mx-auto py-8">
+      <h1 className="text-2xl font-bold">Bounties</h1>
+      <p className="text-sm text-gray-500">Bounties where person {PERSON_ENTITY_ID} expressed interest</p>
+
+      {isLoading && <div>Loading...</div>}
+      {isError && <div>Error loading bounties</div>}
+
+      {!isLoading && bounties.length === 0 && <div className="text-gray-500">No bounties found</div>}
+
+      <ul className="flex flex-col gap-2">
+        {bounties.map((bounty) => (
+          <li key={bounty.id} className="border rounded p-4">
+            <h2 className="font-semibold">{bounty.name}</h2>
+            {bounty.description && <p className="text-sm text-gray-600">{bounty.description}</p>}
+            <p className="text-xs text-gray-400 mt-1">{bounty.id}</p>
+          </li>
+        ))}
+      </ul>
+    </div>
+  );
+}
diff --git a/apps/events/src/schema.ts b/apps/events/src/schema.ts
index ed1afdb1..56de14ff 100644
--- a/apps/events/src/schema.ts
+++ b/apps/events/src/schema.ts
@@ -360,6 +360,34 @@ export const PodcastHostTest = Entity.Schema(
   },
 );
 
+export const PersonBacklink = Entity.Schema(
+  {
+    name: Type.String,
+  },
+  {
+    types: [Id('7ed45f2bc48b419e8e4664d5ff680b0d')],
+    properties: {
+      name: Id(SystemIds.NAME_PROPERTY),
+    },
+  },
+);
+
+export const Bounty = Entity.Schema(
+  {
+    name: Type.String,
+    description: Type.optional(Type.String),
+    interestedIn: Type.Backlink(PersonBacklink),
+  },
+  {
+    types: [Id('808af0bad5884e3391f09dd4b25e18be')],
+    properties: {
+      name: Id(SystemIds.NAME_PROPERTY),
+      description: Id(SystemIds.DESCRIPTION_PROPERTY),
+      interestedIn: Id('ff7e1b4444a2419187324e6c222afe07'),
+    },
+  },
+);
+
 export const PersonHostTest = Entity.Schema(
   {
     name: Type.String,
diff --git a/docs/docs/filtering-query-results.md b/docs/docs/filtering-query-results.md
index 55d5f1d5..b52c6690 100644
--- a/docs/docs/filtering-query-results.md
+++ b/docs/docs/filtering-query-results.md
@@ -1,6 +1,6 @@
 # Filtering Query Results
 
-The filter API allows you to filter the results of a query by property values and in the future also by relations.
+The filter API allows you to filter the results of a query by property values and relations.
 
 ## Filtering by property values
 
@@ -145,6 +145,72 @@ const { data } = useEntities(Person, {
 
 ## Relation filtering
 
+### Filter by existence
+
+You can filter entities based on whether a relation or backlink exists:
+
+```tsx
+const { data } = useEntities(Todo, {
+  filter: {
+    assignees: { exists: true },
+  },
+});
+```
+
+### Filter by entity ID
+
+You can filter entities by which specific entity is on the other end of a relation or backlink. This is useful when you want to find all entities connected to a specific entity.
+
+```tsx
+// string shorthand
+const { data } = useEntities(Todo, {
+  filter: {
+    assignees: { entityId: 'user-id' },
+  },
+});
+
+// object form with `is`
+const { data } = useEntities(Todo, {
+  filter: {
+    assignees: { entityId: { is: 'user-id' } },
+  },
+});
+
+// object form with `in` to match multiple entities
+const { data } = useEntities(Todo, {
+  filter: {
+    assignees: { entityId: { in: ['user-1', 'user-2'] } },
+  },
+});
+```
+
+This works the same way for backlink fields:
+
+```tsx
+export const Bounty = Entity.Schema(
+  { name: Type.String, interestedIn: Type.Backlink(Person) },
+  {
+    types: [Id('bounty-type-id')],
+    properties: {
+      name: Id('name-property-id'),
+      interestedIn: Id('interested-in-property-id'),
+    },
+  },
+);
+
+// find all bounties where a specific person expressed interest
+const { data } = useEntities(Bounty, {
+  filter: {
+    interestedIn: { entityId: myPersonId },
+  },
+});
+```
+
+The framework automatically maps `entityId` to the correct GraphQL field:
+
+- **Forward relations** (e.g. `Type.Relation(...)`) use `toEntityId`
+- **Backlinks** (e.g. `Type.Backlink(...)`) use `fromEntityId`
+
 ### Filter on values of the to entity
 
 ```tsx
diff --git a/packages/hypergraph/src/entity/types.ts b/packages/hypergraph/src/entity/types.ts
index 159c9fb6..ea54c8c1 100644
--- a/packages/hypergraph/src/entity/types.ts
+++ b/packages/hypergraph/src/entity/types.ts
@@ -74,6 +74,16 @@ export type EntityIdFilter = {
   is?: string;
 };
 
+export type RelationEntityIdFilter =
+  | {
+      is: string;
+      in?: never;
+    }
+  | {
+      in: readonly string[];
+      is?: never;
+    };
+
 export type CrossFieldFilter<T, Extra extends object = Record<never, never>> = {
   [K in keyof T]?: EntityFieldFilter<T[K]>;
 } & Extra & {
@@ -84,6 +94,7 @@ export type CrossFieldFilter<T, Extra extends object = Record<never, never>> = {
 type RelationExistsFilter<T> = [T] extends [readonly unknown[] | undefined]
   ? {
       exists?: boolean;
+      entityId?: string | RelationEntityIdFilter;
     }
   : Record<never, never>;
 
diff --git a/packages/hypergraph/src/utils/translate-filter-to-graphql.ts b/packages/hypergraph/src/utils/translate-filter-to-graphql.ts
index b9004d6e..30debb58 100644
--- a/packages/hypergraph/src/utils/translate-filter-to-graphql.ts
+++ b/packages/hypergraph/src/utils/translate-filter-to-graphql.ts
@@ -3,6 +3,18 @@ import * as Option from 'effect/Option';
 import type * as Schema from 'effect/Schema';
 import * as SchemaAST from 'effect/SchemaAST';
 
+type EntityIdGraphqlFilter = { is: string } | { in: readonly string[] };
+
+type RelationSomeFilter = {
+  typeId: { is: string };
+  toEntityId?: EntityIdGraphqlFilter;
+};
+
+type BacklinkSomeFilter = {
+  typeId: { is: string };
+  fromEntityId?: EntityIdGraphqlFilter;
+};
+
 type GraphqlFilterEntry =
   | {
       values: {
@@ -32,9 +44,12 @@ type GraphqlFilterEntry =
     }
   | {
       relations: {
-        some: {
-          typeId: { is: string };
-        };
+        some: RelationSomeFilter;
+      };
+    }
+  | {
+      backlinks: {
+        some: BacklinkSomeFilter;
       };
     }
   | {
@@ -58,14 +73,6 @@ export function translateFilterToGraphql<S extends Schema.Schema.AnyNoContext>(
 
   const graphqlFilter: GraphqlFilterEntry[] = [];
 
-  const buildRelationExistsFilter = (propertyId: string): GraphqlFilterEntry => ({
-    relations: {
-      some: {
-        typeId: { is: propertyId },
-      },
-    },
-  });
-
   for (const [fieldName, fieldFilter] of Object.entries(filter)) {
     if (fieldName === 'or') {
       graphqlFilter.push({
@@ -107,20 +114,84 @@ export function translateFilterToGraphql<S extends Schema.Schema.AnyNoContext>(
     if (!Option.isSome(propertyId) || !Option.isSome(propertyType)) continue;
 
     if (propertyType.value === 'relation') {
-      const relationFilter = fieldFilter as { exists?: boolean };
+      const relationFilter = fieldFilter as {
+        exists?: boolean;
+        entityId?: string | { is?: string; in?: readonly string[] };
+      };
+
+      const isBacklink = SchemaAST.getAnnotation<boolean>(Constants.RelationBacklinkSymbol)(
+        propertySignature.type,
+      ).pipe(Option.getOrElse(() => false));
+
+      // Normalize entityId shorthand: string → { is: string }
+      const entityIdFilter =
+        typeof relationFilter.entityId === 'string' ? { is: relationFilter.entityId } : relationFilter.entityId;
+
+      if (entityIdFilter) {
+        const entityIdValue: EntityIdGraphqlFilter | undefined =
+          typeof entityIdFilter.is === 'string' && entityIdFilter.is
+            ? { is: entityIdFilter.is }
+            : Array.isArray(entityIdFilter.in) && entityIdFilter.in.length > 0
+              ? { in: entityIdFilter.in }
+              : undefined;
+
+        if (entityIdValue === undefined) {
+          continue;
+        }
+
+        if (isBacklink) {
+          graphqlFilter.push({
+            backlinks: {
+              some: {
+                typeId: { is: propertyId.value },
+                fromEntityId: entityIdValue,
+              },
+            },
+          });
+        } else {
+          graphqlFilter.push({
+            relations: {
+              some: {
+                typeId: { is: propertyId.value },
+                toEntityId: entityIdValue,
+              },
+            },
+          });
+        }
+      }
 
       if (relationFilter.exists === true) {
-        graphqlFilter.push(buildRelationExistsFilter(propertyId.value));
+        if (isBacklink) {
+          graphqlFilter.push({
+            backlinks: {
+              some: {
+                typeId: { is: propertyId.value },
+              },
+            },
+          });
+        } else {
+          graphqlFilter.push({
+            relations: {
+              some: {
+                typeId: { is: propertyId.value },
+              },
+            },
+          });
+        }
         continue;
       }
 
       if (relationFilter.exists === false) {
-        const existsFilter = buildRelationExistsFilter(propertyId.value);
+        const existsFilter: GraphqlFilterEntry = isBacklink
+          ? { backlinks: { some: { typeId: { is: propertyId.value } } } }
+          : { relations: { some: { typeId: { is: propertyId.value } } } };
         graphqlFilter.push({
           not: existsFilter,
         });
         continue;
       }
+
+      continue;
     }
 
     if (
diff --git a/packages/hypergraph/test/utils/translate-filter-to-graphql.test.ts b/packages/hypergraph/test/utils/translate-filter-to-graphql.test.ts
index 772e166b..b5722e31 100644
--- a/packages/hypergraph/test/utils/translate-filter-to-graphql.test.ts
+++ b/packages/hypergraph/test/utils/translate-filter-to-graphql.test.ts
@@ -34,7 +34,22 @@ export const Todo = Entity.Schema(
   },
 );
 
+export const Project = Entity.Schema(
+  {
+    title: Type.String,
+    todos: Type.Backlink(Todo),
+  },
+  {
+    types: [Id('b388444f06a340379ace66fe325864d1')],
+    properties: {
+      title: Id('b126ca530c8e48d5b88882c734c38936'),
+      todos: Id('b399677c2bf940c39622815be7b83345'),
+    },
+  },
+);
+
 type TodoFilter = Entity.EntityFilter<Schema.Schema.Type<typeof Todo>>;
+type ProjectFilter = Entity.EntityFilter<Schema.Schema.Type<typeof Project>>;
 
 describe('translateFilterToGraphql string filters', () => {
   it('should translate string `is` filter correctly', () => {
@@ -201,7 +216,6 @@ describe('translateFilterToGraphql id filters', () => {
 describe('translateFilterToGraphql relation filters', () => {
   it('should translate relation `exists` filter correctly', () => {
     const filter: TodoFilter = {
-      // @ts-expect-error - this is a test
       assignees: { exists: true },
     };
 
@@ -218,7 +232,6 @@ describe('translateFilterToGraphql relation filters', () => {
 
   it('should translate relation `exists: false` filter correctly', () => {
     const filter: TodoFilter = {
-      // @ts-expect-error - this is a test
       assignees: { exists: false },
     };
 
@@ -234,6 +247,172 @@ describe('translateFilterToGraphql relation filters', () => {
       },
     });
   });
+
+  it('should translate relation `entityId` string shorthand filter correctly', () => {
+    const filter: TodoFilter = {
+      assignees: { entityId: 'user-123' },
+    };
+
+    const result = translateFilterToGraphql(filter, Todo);
+
+    expect(result).toEqual({
+      relations: {
+        some: {
+          typeId: { is: 'f399677c2bf940c39622815be7b83344' },
+          toEntityId: { is: 'user-123' },
+        },
+      },
+    });
+  });
+
+  it('should translate relation `entityId: { is }` filter correctly', () => {
+    const filter: TodoFilter = {
+      assignees: { entityId: { is: 'user-123' } },
+    };
+
+    const result = translateFilterToGraphql(filter, Todo);
+
+    expect(result).toEqual({
+      relations: {
+        some: {
+          typeId: { is: 'f399677c2bf940c39622815be7b83344' },
+          toEntityId: { is: 'user-123' },
+        },
+      },
+    });
+  });
+
+  it('should translate relation `entityId: { in }` filter correctly', () => {
+    const filter: TodoFilter = {
+      assignees: { entityId: { in: ['user-1', 'user-2'] } },
+    };
+
+    const result = translateFilterToGraphql(filter, Todo);
+
+    expect(result).toEqual({
+      relations: {
+        some: {
+          typeId: { is: 'f399677c2bf940c39622815be7b83344' },
+          toEntityId: { in: ['user-1', 'user-2'] },
+        },
+      },
+    });
+  });
+
+  it('should combine entityId with exists filter on a relation', () => {
+    const filter: TodoFilter = {
+      assignees: { exists: true, entityId: 'user-123' },
+    };
+
+    const result = translateFilterToGraphql(filter, Todo);
+
+    expect(result).toEqual({
+      and: [
+        {
+          relations: {
+            some: {
+              typeId: { is: 'f399677c2bf940c39622815be7b83344' },
+              toEntityId: { is: 'user-123' },
+            },
+          },
+        },
+        {
+          relations: {
+            some: {
+              typeId: { is: 'f399677c2bf940c39622815be7b83344' },
+            },
+          },
+        },
+      ],
+    });
+  });
+});
+
+describe('translateFilterToGraphql backlink filters', () => {
+  it('should translate backlink `exists` filter using `backlinks` key', () => {
+    const filter: ProjectFilter = {
+      todos: { exists: true },
+    };
+
+    const result = translateFilterToGraphql(filter, Project);
+
+    expect(result).toEqual({
+      backlinks: {
+        some: {
+          typeId: { is: 'b399677c2bf940c39622815be7b83345' },
+        },
+      },
+    });
+  });
+
+  it('should translate backlink `exists: false` filter using `backlinks` key', () => {
+    const filter: ProjectFilter = {
+      todos: { exists: false },
+    };
+
+    const result = translateFilterToGraphql(filter, Project);
+
+    expect(result).toEqual({
+      not: {
+        backlinks: {
+          some: {
+            typeId: { is: 'b399677c2bf940c39622815be7b83345' },
+          },
+        },
+      },
+    });
+  });
+
+  it('should translate backlink `entityId` string shorthand using `fromEntityId`', () => {
+    const filter: ProjectFilter = {
+      todos: { entityId: 'todo-123' },
+    };
+
+    const result = translateFilterToGraphql(filter, Project);
+
+    expect(result).toEqual({
+      backlinks: {
+        some: {
+          typeId: { is: 'b399677c2bf940c39622815be7b83345' },
+          fromEntityId: { is: 'todo-123' },
+        },
+      },
+    });
+  });
+
+  it('should translate backlink `entityId: { is }` using `fromEntityId`', () => {
+    const filter: ProjectFilter = {
+      todos: { entityId: { is: 'todo-123' } },
+    };
+
+    const result = translateFilterToGraphql(filter, Project);
+
+    expect(result).toEqual({
+      backlinks: {
+        some: {
+          typeId: { is: 'b399677c2bf940c39622815be7b83345' },
+          fromEntityId: { is: 'todo-123' },
+        },
+      },
+    });
+  });
+
+  it('should translate backlink `entityId: { in }` using `fromEntityId`', () => {
+    const filter: ProjectFilter = {
+      todos: { entityId: { in: ['todo-1', 'todo-2'] } },
+    };
+
+    const result = translateFilterToGraphql(filter, Project);
+
+    expect(result).toEqual({
+      backlinks: {
+        some: {
+          typeId: { is: 'b399677c2bf940c39622815be7b83345' },
+          fromEntityId: { in: ['todo-1', 'todo-2'] },
+        },
+      },
+    });
+  });
 });
 
 describe('translateFilterToGraphql multiple filters', () => {