diff --git a/.changeset/require-complete-store-seeds.md b/.changeset/require-complete-store-seeds.md
new file mode 100644
index 000000000..72915d07b
--- /dev/null
+++ b/.changeset/require-complete-store-seeds.md
@@ -0,0 +1,6 @@
+---
+"@solidjs/signals": patch
+"solid-js": patch
+---
+
+Require complete values for projection store seeds. `createProjection`, derived `createStore`, and derived `createOptimisticStore` no longer accept `Partial<T>`, preventing a store typed as `T` from being created without all required properties.
diff --git a/packages/signals/src/store/index.ts b/packages/signals/src/store/index.ts
index 0c7efc3ac..80ed97c0c 100644
--- a/packages/signals/src/store/index.ts
+++ b/packages/signals/src/store/index.ts
@@ -36,7 +36,7 @@ export function createStore<T extends object = {}>(
 ): [get: Store<T>, set: StoreSetter<T>];
 export function createStore<T extends object = {}>(
   fn: (store: T) => void | T | Promise<void | T> | AsyncIterable<void | T>,
-  store: Partial<T> | Store<NoFn<T>>,
+  store: NoFn<T> | Store<NoFn<T>>,
   options?: ProjectionOptions
 ): [get: Refreshable<Store<T>>, set: StoreSetter<T>];
 export function createStore(first: any, second?: any, third?: any): any {
diff --git a/packages/signals/src/store/next/optimistic.ts b/packages/signals/src/store/next/optimistic.ts
index 6e924adaf..9d4936e26 100644
--- a/packages/signals/src/store/next/optimistic.ts
+++ b/packages/signals/src/store/next/optimistic.ts
@@ -175,6 +175,14 @@ function familyHasLiveOverrides(fam: { overlaid?: Set<any> }): boolean {
   return false;
 }
 
+export function createOptimisticStoreNext<T extends object = {}>(
+  store: NoFn<T> | Store<NoFn<T>>
+): [get: Store<T>, set: StoreSetter<T>];
+export function createOptimisticStoreNext<T extends object = {}>(
+  fn: (store: T) => void | T | Promise<void | T> | AsyncIterable<void | T>,
+  store: NoFn<T> | Store<NoFn<T>>,
+  options?: ProjectionOptions
+): [get: Store<T>, set: StoreSetter<T>];
 export function createOptimisticStoreNext<T extends object = {}>(
   first: T | ((store: T) => void | T | Promise<void | T> | AsyncIterable<void | T>),
   second?: NoFn<T> | Store<NoFn<T>>,
diff --git a/packages/signals/src/store/next/projection.ts b/packages/signals/src/store/next/projection.ts
index 7ff2afbeb..b46a7f9d1 100644
--- a/packages/signals/src/store/next/projection.ts
+++ b/packages/signals/src/store/next/projection.ts
@@ -177,7 +177,7 @@ function wrapDraft(
 
 function createProjectionNextInternal<T extends object = {}>(
   fn: (draft: T) => void | T | Promise<void | T> | AsyncIterable<void | T>,
-  seed: Partial<T>,
+  seed: T,
   options?: ProjectionOptions
 ) {
   const fam: StoreNextFamily = {
@@ -211,7 +211,7 @@ function createProjectionNextInternal<T extends object = {}>(
 
 export function createProjectionNext<T extends object = {}>(
   fn: (draft: T) => void | T | Promise<void | T> | AsyncIterable<void | T>,
-  seed: Partial<T> | Store<NoFn<T>>,
+  seed: NoFn<T> | Store<NoFn<T>>,
   options?: ProjectionOptions
 ): Refreshable<Store<T>> {
   return createProjectionNextInternal(fn, seed, options).store;
@@ -222,7 +222,7 @@ export function createProjectionNext<T extends object = {}>(
  * same-flush dependency change). */
 export function createStoreDerivedNext<T extends object = {}>(
   fn: (draft: T) => void | T | Promise<void | T> | AsyncIterable<void | T>,
-  seed: Partial<T> | Store<NoFn<T>>,
+  seed: NoFn<T> | Store<NoFn<T>>,
   options?: ProjectionOptions
 ): [Refreshable<Store<T>>, (f: (draft: T) => T | void) => void] {
   const { store, node } = createProjectionNextInternal(fn, seed, options);
diff --git a/packages/signals/tests/store/createProjection.test.ts b/packages/signals/tests/store/createProjection.test.ts
index e0038a498..4b787ba87 100644
--- a/packages/signals/tests/store/createProjection.test.ts
+++ b/packages/signals/tests/store/createProjection.test.ts
@@ -518,7 +518,7 @@ describe("projection over a store — chained backing (#2941)", () => {
       proj = createProjection(() => {
         derive();
         return store;
-      }, {}) as { a: number };
+      }, {} as any) as { a: number };
       createEffect(
         () => proj.a,
         v => {
@@ -547,7 +547,7 @@ describe("projection over a store — chained backing (#2941)", () => {
     const dispose = createRoot(dispose => {
       const [store, set] = createStore({ top: "t1", nest: { deep: "d1" } });
       setStore = set;
-      const proj = createProjection(() => store, {}) as typeof store;
+      const proj = createProjection(() => store, {} as any) as typeof store;
       createEffect(
         () => proj.top,
         v => {
@@ -587,7 +587,7 @@ describe("projection over a store — chained backing (#2941)", () => {
       const proj = createProjection(() => {
         const m = mode();
         return m === "a" ? a : m === "b" ? b : { v: "plain" };
-      }, {}) as { v: string };
+      }, {} as any) as { v: string };
       createEffect(
         () => proj.v,
         v => {
@@ -676,7 +676,7 @@ describe("projection over a store — chained backing (#2941)", () => {
     const dispose = createRoot(dispose => {
       const [store, set] = createStore({ a: 1, nest: { b: 2 } });
       setStore = set;
-      proj = createProjection(() => store, {});
+      proj = createProjection(() => store, {} as any);
       return dispose;
     });
     flush();
@@ -697,7 +697,7 @@ describe("projection over a store — chained backing (#2941)", () => {
     const dispose = createRoot(dispose => {
       const [store, set] = createStore<Record<string, number>>({ a: 1 });
       setStore = set;
-      const proj = createProjection(() => store, {}) as Record<string, number>;
+      const proj = createProjection(() => store, {} as any) as Record<string, number>;
       createEffect(
         () => Object.keys(proj).join(","),
         v => {
diff --git a/packages/signals/tests/store/store.type-tests.ts b/packages/signals/tests/store/store.type-tests.ts
index c1438ac00..bef3823b9 100644
--- a/packages/signals/tests/store/store.type-tests.ts
+++ b/packages/signals/tests/store/store.type-tests.ts
@@ -32,24 +32,29 @@ import {
   store.count satisfies number;
 }
 
-// ── createStore (projection) — partial seed ───────────────────────────
+// ── projection seeds must be complete ────────────────────────────────
 
-{
-  const [store] = createStore(() => ({ foo: true }), {});
-  store.foo satisfies boolean;
-}
+type UserState = { user: { name: string }; ready: boolean };
 
-{
-  const [store] = createStore(() => ({ a: 1, b: "hello" }), {});
-  store.a satisfies number;
-  store.b satisfies string;
-}
+// @ts-expect-error An inferred seed cannot omit required properties.
+createStore(() => ({ user: { name: "Ada" }, ready: true }), { ready: false });
 
-{
-  const [store] = createStore(() => ({ a: 1, b: "hello" }), { a: 0 });
-  store.a satisfies number;
-  store.b satisfies string;
-}
+// @ts-expect-error An explicit type argument cannot opt back into a partial seed.
+createStore<UserState>(() => ({ user: { name: "Ada" }, ready: true }), {});
+
+// ── callable store roots are rejected ────────────────────────────────
+
+type CallableState = (() => void) & { count: number };
+const callableState = Object.assign(() => {}, { count: 0 });
+
+// @ts-expect-error A projection draft cannot represent a callable root.
+createStore<CallableState>(() => callableState, callableState);
+
+// @ts-expect-error A projection draft cannot represent a callable root.
+createProjection<CallableState>(() => callableState, callableState);
+
+// @ts-expect-error A projection draft cannot represent a callable root.
+createOptimisticStore<CallableState>(() => callableState, callableState);
 
 // ── createProjection — mutation only (void return, T from seed) ───────
 
@@ -75,17 +80,8 @@ import {
   store.active satisfies boolean;
 }
 
-// ── createProjection — partial seed ───────────────────────────────────
-
-{
-  const store = createProjection(() => ({ foo: true }), {});
-  store.foo satisfies boolean;
-}
-
-{
-  const store = createProjection(() => ({ nested: { x: 1 } }), {});
-  store.nested.x satisfies number;
-}
+// @ts-expect-error createProjection also requires a complete seed.
+createProjection(() => ({ user: { name: "Ada" }, ready: true }), { ready: false });
 
 // ── createProjection — empty array seed ───────────────────────────────
 
@@ -113,12 +109,8 @@ import {
   proj.count satisfies number;
 }
 
-// ── createOptimisticStore (projection) — partial seed ─────────────────
-
-{
-  const [store] = createOptimisticStore(() => ({ foo: true }), {});
-  store.foo satisfies boolean;
-}
+// @ts-expect-error createOptimisticStore also requires a complete seed.
+createOptimisticStore(() => ({ user: { name: "Ada" }, ready: true }), { ready: false });
 
 // ── createOptimisticStore (projection) — seed matches return type ─────
 
diff --git a/packages/solid/src/client/hydration.ts b/packages/solid/src/client/hydration.ts
index 5733581e7..4e84e2bb4 100644
--- a/packages/solid/src/client/hydration.ts
+++ b/packages/solid/src/client/hydration.ts
@@ -1592,7 +1592,7 @@ export const createOptimistic: {
  */
 export const createProjection: <T extends object = {}>(
   fn: (draft: T) => void | T | Promise<void | T> | AsyncIterable<void | T>,
-  initialValue: Partial<T> | Store<NoFn<T>>,
+  initialValue: NoFn<T> | Store<NoFn<T>>,
   options?: HydrationProjectionOptions
 ) => Refreshable<Store<T>> = ((...args: any[]) => {
   // `hydrating` can only be true once enableHydration() installed the
diff --git a/packages/solid/src/server/signals.ts b/packages/solid/src/server/signals.ts
index a5d41ea45..4a08a48e3 100644
--- a/packages/solid/src/server/signals.ts
+++ b/packages/solid/src/server/signals.ts
@@ -495,6 +495,7 @@ type ServerMemoOptions<T> = MemoOptions<T> & {
 };
 type ServerSignalOptions<T> = SignalOptions<T>;
 type ServerStoreOptions = ServerSsrOptions;
+type NoFn<T> = T extends Function ? never : T;
 
 /**
  * The pending source for BARE `ssrSource: "client"` (no declared commit #0):
@@ -1778,12 +1779,12 @@ function setProperty(state: any, property: PropertyKey, value: any) {
 }
 
 export function createStore<T extends object>(
-  store: T | Store<T>,
+  store: NoFn<T> | Store<NoFn<T>>,
   options?: { name?: string; shallow?: boolean }
 ): [get: Store<T>, set: StoreSetter<T>];
 export function createStore<T extends object>(
   fn: (store: T) => void | T | Promise<void | T>,
-  store: Partial<T> | Store<T>,
+  store: NoFn<T> | Store<NoFn<T>>,
   options?: ServerStoreOptions & { name?: string; shallow?: boolean }
 ): [get: Store<T>, set: StoreSetter<T>];
 export function createStore<T extends object>(
@@ -1798,7 +1799,7 @@ export function createStore<T extends object>(
     // The impl signature stays loose; the public overload above enforces the
     // client/seedLoadingValue pairing, and createProjection re-checks at
     // runtime.
-    const store = createProjection(first as any, second as T, options as any);
+    const store = createProjection(first as any, second as NoFn<T>, options as any);
     return [store as Store<T>, storeSetter(store as T)];
   }
   const state = first as T;
@@ -1823,12 +1824,12 @@ function storeSetter<T extends object>(state: T): StoreSetter<T> {
 }
 
 export function createOptimisticStore<T extends object>(
-  store: T | Store<T>,
+  store: NoFn<T> | Store<NoFn<T>>,
   options?: { name?: string; shallow?: boolean }
 ): [get: Store<T>, set: StoreSetter<T>];
 export function createOptimisticStore<T extends object>(
   fn: (store: T) => void | T | Promise<void | T>,
-  store: Partial<T> | Store<T>,
+  store: NoFn<T> | Store<NoFn<T>>,
   options?: ServerStoreOptions & { name?: string; shallow?: boolean }
 ): [get: Store<T>, set: StoreSetter<T>];
 export function createOptimisticStore<T extends object>(
@@ -1969,7 +1970,7 @@ function replaceState<T extends object>(target: T, next: T): void {
 
 export function createProjection<T extends object>(
   fn: (draft: T) => void | T | Promise<void | T> | AsyncIterable<void | T>,
-  initialValue: Partial<T> | Store<T>,
+  initialValue: NoFn<T> | Store<NoFn<T>>,
   options?: ServerStoreOptions
 ): Store<T> {
   const ctx = sharedConfig.context;
@@ -1999,7 +2000,7 @@ export function createProjection<T extends object>(
     if (slots) slots[slotId!] = proxy;
     return proxy;
   };
-  const [state] = createStore(initialValue as T);
+  const [state] = createStore(initialValue as NoFn<T>);
 
   if (options?.ssrSource === "client") {
     // seedLoadingValue = declared commit #0: the seed renders. Bare = the
diff --git a/packages/solid/test/server/ssr-async.spec.ts b/packages/solid/test/server/ssr-async.spec.ts
index 9b19e38a4..840c78089 100644
--- a/packages/solid/test/server/ssr-async.spec.ts
+++ b/packages/solid/test/server/ssr-async.spec.ts
@@ -1593,7 +1593,7 @@ describe("Stream Blocking / deferStream", () => {
 
     createRoot(
       () => {
-        createProjection(() => d.promise, {} as { name?: string });
+        createProjection(() => d.promise, {} as any);
       },
       { id: "t" }
     );