diff --git a/.npmignore b/.npmignore
new file mode 100644
index 00000000..451a5a6b
--- /dev/null
+++ b/.npmignore
@@ -0,0 +1 @@
+src/*.type-test.ts
diff --git a/docs/reference/functions/checkIdField.md b/docs/reference/functions/checkIdField.md
index 82d7c5a9..36a6563b 100644
--- a/docs/reference/functions/checkIdField.md
+++ b/docs/reference/functions/checkIdField.md
@@ -6,9 +6,9 @@
 
 # Function: checkIdField()
 
-> **checkIdField**(`options`): `any`
+> **checkIdField**(`options`): `string` \| `undefined`
 
-Defined in: [src/index.ts:47](https://github.com/FirebaseExtended/reactfire/blob/main/src/index.ts#L47)
+Defined in: [src/index.ts:49](https://github.com/FirebaseExtended/reactfire/blob/main/src/index.ts#L49)
 
 ## Parameters
 
@@ -18,4 +18,4 @@ Defined in: [src/index.ts:47](https://github.com/FirebaseExtended/reactfire/blob
 
 ## Returns
 
-`any`
+`string` \| `undefined`
diff --git a/docs/reference/functions/checkOptions.md b/docs/reference/functions/checkOptions.md
index 4f4fdaf0..ce0923b0 100644
--- a/docs/reference/functions/checkOptions.md
+++ b/docs/reference/functions/checkOptions.md
@@ -6,9 +6,9 @@
 
 # Function: checkOptions()
 
-> **checkOptions**(`options`, `field`): `any`
+> **checkOptions**(`options`, `field`): `unknown`
 
-Defined in: [src/index.ts:34](https://github.com/FirebaseExtended/reactfire/blob/main/src/index.ts#L34)
+Defined in: [src/index.ts:36](https://github.com/FirebaseExtended/reactfire/blob/main/src/index.ts#L36)
 
 ## Parameters
 
@@ -22,4 +22,4 @@ Defined in: [src/index.ts:34](https://github.com/FirebaseExtended/reactfire/blob
 
 ## Returns
 
-`any`
+`unknown`
diff --git a/docs/reference/functions/checkinitialData.md b/docs/reference/functions/checkinitialData.md
index f2c78405..96792dcb 100644
--- a/docs/reference/functions/checkinitialData.md
+++ b/docs/reference/functions/checkinitialData.md
@@ -6,9 +6,9 @@
 
 # Function: checkinitialData()
 
-> **checkinitialData**(`options`): `any`
+> **checkinitialData**(`options`): `unknown`
 
-Defined in: [src/index.ts:43](https://github.com/FirebaseExtended/reactfire/blob/main/src/index.ts#L43)
+Defined in: [src/index.ts:45](https://github.com/FirebaseExtended/reactfire/blob/main/src/index.ts#L45)
 
 ## Parameters
 
@@ -18,4 +18,4 @@ Defined in: [src/index.ts:43](https://github.com/FirebaseExtended/reactfire/blob
 
 ## Returns
 
-`any`
+`unknown`
diff --git a/docs/reference/interfaces/ReactFireOptions.md b/docs/reference/interfaces/ReactFireOptions.md
index 7ca63c5e..0503ebb7 100644
--- a/docs/reference/interfaces/ReactFireOptions.md
+++ b/docs/reference/interfaces/ReactFireOptions.md
@@ -30,7 +30,7 @@ Defined in: [src/index.ts:25](https://github.com/FirebaseExtended/reactfire/blob
 
 ### initialData?
 
-> `optional` **initialData?**: `any`
+> `optional` **initialData?**: `T`
 
 Defined in: [src/index.ts:26](https://github.com/FirebaseExtended/reactfire/blob/main/src/index.ts#L26)
 
@@ -38,7 +38,7 @@ Defined in: [src/index.ts:26](https://github.com/FirebaseExtended/reactfire/blob
 
 ### ~~startWithValue?~~
 
-> `optional` **startWithValue?**: `any`
+> `optional` **startWithValue?**: `T`
 
 Defined in: [src/index.ts:30](https://github.com/FirebaseExtended/reactfire/blob/main/src/index.ts#L30)
 
diff --git a/docs/reference/interfaces/SignInCheckOptionsBasic.md b/docs/reference/interfaces/SignInCheckOptionsBasic.md
index 6c9f2d66..425e0439 100644
--- a/docs/reference/interfaces/SignInCheckOptionsBasic.md
+++ b/docs/reference/interfaces/SignInCheckOptionsBasic.md
@@ -41,7 +41,7 @@ Defined in: [src/index.ts:25](https://github.com/FirebaseExtended/reactfire/blob
 
 ### initialData?
 
-> `optional` **initialData?**: `any`
+> `optional` **initialData?**: [`SigninCheckResult`](../type-aliases/SigninCheckResult.md)
 
 Defined in: [src/index.ts:26](https://github.com/FirebaseExtended/reactfire/blob/main/src/index.ts#L26)
 
@@ -53,7 +53,7 @@ Defined in: [src/index.ts:26](https://github.com/FirebaseExtended/reactfire/blob
 
 ### ~~startWithValue?~~
 
-> `optional` **startWithValue?**: `any`
+> `optional` **startWithValue?**: [`SigninCheckResult`](../type-aliases/SigninCheckResult.md)
 
 Defined in: [src/index.ts:30](https://github.com/FirebaseExtended/reactfire/blob/main/src/index.ts#L30)
 
diff --git a/docs/reference/interfaces/SignInCheckOptionsClaimsObject.md b/docs/reference/interfaces/SignInCheckOptionsClaimsObject.md
index 092cde82..8407e95a 100644
--- a/docs/reference/interfaces/SignInCheckOptionsClaimsObject.md
+++ b/docs/reference/interfaces/SignInCheckOptionsClaimsObject.md
@@ -40,7 +40,7 @@ Defined in: [src/index.ts:25](https://github.com/FirebaseExtended/reactfire/blob
 
 ### initialData?
 
-> `optional` **initialData?**: `any`
+> `optional` **initialData?**: [`SigninCheckResult`](../type-aliases/SigninCheckResult.md)
 
 Defined in: [src/index.ts:26](https://github.com/FirebaseExtended/reactfire/blob/main/src/index.ts#L26)
 
@@ -60,7 +60,7 @@ Defined in: [src/auth.tsx:88](https://github.com/FirebaseExtended/reactfire/blob
 
 ### ~~startWithValue?~~
 
-> `optional` **startWithValue?**: `any`
+> `optional` **startWithValue?**: [`SigninCheckResult`](../type-aliases/SigninCheckResult.md)
 
 Defined in: [src/index.ts:30](https://github.com/FirebaseExtended/reactfire/blob/main/src/index.ts#L30)
 
diff --git a/docs/reference/interfaces/SignInCheckOptionsClaimsValidator.md b/docs/reference/interfaces/SignInCheckOptionsClaimsValidator.md
index 83517d7a..914cb6ac 100644
--- a/docs/reference/interfaces/SignInCheckOptionsClaimsValidator.md
+++ b/docs/reference/interfaces/SignInCheckOptionsClaimsValidator.md
@@ -40,7 +40,7 @@ Defined in: [src/index.ts:25](https://github.com/FirebaseExtended/reactfire/blob
 
 ### initialData?
 
-> `optional` **initialData?**: `any`
+> `optional` **initialData?**: [`SigninCheckResult`](../type-aliases/SigninCheckResult.md)
 
 Defined in: [src/index.ts:26](https://github.com/FirebaseExtended/reactfire/blob/main/src/index.ts#L26)
 
@@ -52,7 +52,7 @@ Defined in: [src/index.ts:26](https://github.com/FirebaseExtended/reactfire/blob
 
 ### ~~startWithValue?~~
 
-> `optional` **startWithValue?**: `any`
+> `optional` **startWithValue?**: [`SigninCheckResult`](../type-aliases/SigninCheckResult.md)
 
 Defined in: [src/index.ts:30](https://github.com/FirebaseExtended/reactfire/blob/main/src/index.ts#L30)
 
diff --git a/src/firestore.tsx b/src/firestore.tsx
index d5efa8bd..7fbb1c79 100644
--- a/src/firestore.tsx
+++ b/src/firestore.tsx
@@ -63,7 +63,7 @@ export function useFirestoreDocData<T = unknown>(ref: DocumentReference<T>, opti
   const idField = options ? checkIdField(options) : undefined;
 
   const observableId = `firestore:docData:${ref.firestore.app.name}:${ref.path}:idField=${JSON.stringify(idField)}`;
-  const observable = docData(ref, { idField });
+  const observable = docData(ref, { idField: idField as keyof T });
 
   return useObservable(observableId, observable, options) as ObservableStatus<T>;
 }
@@ -75,7 +75,7 @@ export function useFirestoreDocDataOnce<T = unknown>(ref: DocumentReference<T>,
   const idField = options ? checkIdField(options) : undefined;
 
   const observableId = `firestore:docDataOnce:${ref.firestore.app.name}:${ref.path}:idField=${JSON.stringify(idField)}`;
-  const observable$ = docData(ref, { idField }).pipe(first());
+  const observable$ = docData(ref, { idField: idField as keyof T }).pipe(first());
 
   return useObservable(observableId, observable$, options) as ObservableStatus<T>;
 }
@@ -96,7 +96,7 @@ export function useFirestoreCollection<T = DocumentData>(query: FirestoreQuery<T
 export function useFirestoreCollectionData<T = DocumentData>(query: FirestoreQuery<T>, options?: ReactFireOptions<T[]>): ObservableStatus<T[]> {
   const idField = options ? checkIdField(options) : undefined;
   const observableId = `firestore:collectionData:${getUniqueIdForFirestoreQuery(query)}:idField=${JSON.stringify(idField)}`;
-  const observable$ = collectionData(query, { idField });
+  const observable$ = collectionData(query, { idField: idField as (string & keyof T) });
 
   return useObservable(observableId, observable$, options);
 }
diff --git a/src/index.ts b/src/index.ts
index ab60a1fe..5a4f2b36 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -23,14 +23,16 @@ export class ReactFireError extends Error {
 
 export interface ReactFireOptions<T = unknown> {
   idField?: string;
-  initialData?: T | any;
+  initialData?: T;
   /**
    * @deprecated use initialData instead
    */
-  startWithValue?: T | any;
+  startWithValue?: T;
   suspense?: boolean;
 }
 
+// Deprecated: unused internally as of the ReactFireOptions generic tightening.
+// Kept as exports to avoid a breaking removal; slated for removal in v5.
 export function checkOptions(options: ReactFireOptions, field: string) {
   // make sure the field passed in is a valid key of ReactFire Options
   if (field === 'idField' || field === 'initialData' || field === 'suspense') {
@@ -44,8 +46,8 @@ export function checkinitialData(options: ReactFireOptions) {
   return checkOptions(options, 'initialData');
 }
 
-export function checkIdField(options: ReactFireOptions) {
-  return checkOptions(options, 'idField');
+export function checkIdField(options: ReactFireOptions): string | undefined {
+  return options?.idField;
 }
 
 export * from './auth';
diff --git a/src/reactfire-options.type-test.ts b/src/reactfire-options.type-test.ts
new file mode 100644
index 00000000..739b141a
--- /dev/null
+++ b/src/reactfire-options.type-test.ts
@@ -0,0 +1,18 @@
+/**
+ * Type-level regression tests for ReactFireOptions<T> generic constraints.
+ * Checked by `tsc --noEmit` in CI. No runtime behavior — not bundled.
+ */
+import type { ReactFireOptions } from './index';
+
+// ---- initialData must match T ----
+
+void ((): ReactFireOptions<string> => ({ initialData: 'hello' }))();
+void ((): ReactFireOptions<number> => ({ initialData: 42 }))();
+
+// @ts-expect-error initialData must be T, not a different type
+const _wrongInitialData: ReactFireOptions<string> = { initialData: 123 };
+void _wrongInitialData;
+
+// @ts-expect-error startWithValue must be T, not a different type
+const _wrongStartWithValue: ReactFireOptions<string> = { startWithValue: 123 };
+void _wrongStartWithValue;