diff --git a/.agents/skills/data-client-react-testing/references/Fixtures.md b/.agents/skills/data-client-react-testing/references/Fixtures.md
index 1ca3e8c00c37..210c61804467 100644
--- a/.agents/skills/data-client-react-testing/references/Fixtures.md
+++ b/.agents/skills/data-client-react-testing/references/Fixtures.md
@@ -27,7 +27,7 @@ export interface SuccessFixture<
   E extends EndpointInterface = EndpointInterface,
 > {
   readonly endpoint: E;
-  readonly args: Parameters<E>;
+  readonly args: Readonly<Parameters<E>>;
   readonly response:
     | ResolveType<E>
     | ((...args: Parameters<E>) => ResolveType<E>);
@@ -62,7 +62,7 @@ export interface ErrorFixture {
 ```ts
 export interface ErrorFixture<E extends EndpointInterface = EndpointInterface> {
   readonly endpoint: E;
-  readonly args: Parameters<E>;
+  readonly args: Readonly<Parameters<E>>;
   readonly response: any;
   readonly error: true;
   /** Number of milliseconds to wait before resolving */
diff --git a/.agents/skills/data-client-react-testing/references/MockResolver.md b/.agents/skills/data-client-react-testing/references/MockResolver.md
index 0afbf3bf78a7..bc214e98d30f 100644
--- a/.agents/skills/data-client-react-testing/references/MockResolver.md
+++ b/.agents/skills/data-client-react-testing/references/MockResolver.md
@@ -64,7 +64,7 @@ const results = [
   // fixture
   {
     endpoint: ArticleResource.getList,
-    args: [{ maxResults: 10 }],
+    args: [{ maxResults: 10 }] as const,
     response: [
       {
         id: 5,
diff --git a/.agents/skills/data-client-react-testing/references/unit-testing-components.md b/.agents/skills/data-client-react-testing/references/unit-testing-components.md
index 580361af24ca..3f757a67d895 100644
--- a/.agents/skills/data-client-react-testing/references/unit-testing-components.md
+++ b/.agents/skills/data-client-react-testing/references/unit-testing-components.md
@@ -28,7 +28,7 @@ export default {
   full: [
     {
       endpoint: ArticleResource.getList,
-      args: [{ maxResults: 10 }],
+      args: [{ maxResults: 10 }] as const,
       response: [
         {
           id: 5,
@@ -46,7 +46,7 @@ export default {
     },
     {
       endpoint: ArticleResource.update,
-      args: [{ id: 532 }],
+      args: [{ id: 532 }] as const,
       response({ id }, body) {
         return {
           id,
@@ -58,14 +58,14 @@ export default {
   empty: [
     {
       endpoint: ArticleResource.getList,
-      args: [{ maxResults: 10 }],
+      args: [{ maxResults: 10 }] as const,
       response: [],
     },
   ],
   error: [
     {
       endpoint: ArticleResource.getList,
-      args: [{ maxResults: 10 }],
+      args: [{ maxResults: 10 }] as const,
       response: { message: 'Bad request', status: 400, name: 'Not Found' },
       error: true,
     },
diff --git a/.agents/skills/data-client-rest/references/Fixtures.md b/.agents/skills/data-client-rest/references/Fixtures.md
index 0b72fffd8011..196d7b0068ad 100644
--- a/.agents/skills/data-client-rest/references/Fixtures.md
+++ b/.agents/skills/data-client-rest/references/Fixtures.md
@@ -27,7 +27,7 @@ export interface SuccessFixture<
   E extends EndpointInterface = EndpointInterface,
 > {
   readonly endpoint: E;
-  readonly args: Parameters<E>;
+  readonly args: Readonly<Parameters<E>>;
   readonly response:
     | ResolveType<E>
     | ((...args: Parameters<E>) => ResolveType<E>);
@@ -62,7 +62,7 @@ export interface ErrorFixture {
 ```ts
 export interface ErrorFixture<E extends EndpointInterface = EndpointInterface> {
   readonly endpoint: E;
-  readonly args: Parameters<E>;
+  readonly args: Readonly<Parameters<E>>;
   readonly response: any;
   readonly error: true;
   /** Number of milliseconds to wait before resolving */
diff --git a/.agents/skills/data-client-rest/references/Fixtures.vue.md b/.agents/skills/data-client-rest/references/Fixtures.vue.md
index cefb6692aa34..dbbe20761348 100644
--- a/.agents/skills/data-client-rest/references/Fixtures.vue.md
+++ b/.agents/skills/data-client-rest/references/Fixtures.vue.md
@@ -27,7 +27,7 @@ export interface SuccessFixture<
   E extends EndpointInterface = EndpointInterface,
 > {
   readonly endpoint: E;
-  readonly args: Parameters<E>;
+  readonly args: Readonly<Parameters<E>>;
   readonly response:
     | ResolveType<E>
     | ((...args: Parameters<E>) => ResolveType<E>);
@@ -62,7 +62,7 @@ export interface ErrorFixture {
 ```ts
 export interface ErrorFixture<E extends EndpointInterface = EndpointInterface> {
   readonly endpoint: E;
-  readonly args: Parameters<E>;
+  readonly args: Readonly<Parameters<E>>;
   readonly response: any;
   readonly error: true;
   /** Number of milliseconds to wait before resolving */
diff --git a/.agents/skills/data-client-vue-testing/references/Fixtures.md b/.agents/skills/data-client-vue-testing/references/Fixtures.md
index ed542d8b7502..a52c67a6ee41 100644
--- a/.agents/skills/data-client-vue-testing/references/Fixtures.md
+++ b/.agents/skills/data-client-vue-testing/references/Fixtures.md
@@ -27,7 +27,7 @@ export interface SuccessFixture<
   E extends EndpointInterface = EndpointInterface,
 > {
   readonly endpoint: E;
-  readonly args: Parameters<E>;
+  readonly args: Readonly<Parameters<E>>;
   readonly response:
     | ResolveType<E>
     | ((...args: Parameters<E>) => ResolveType<E>);
@@ -62,7 +62,7 @@ export interface ErrorFixture {
 ```ts
 export interface ErrorFixture<E extends EndpointInterface = EndpointInterface> {
   readonly endpoint: E;
-  readonly args: Parameters<E>;
+  readonly args: Readonly<Parameters<E>>;
   readonly response: any;
   readonly error: true;
   /** Number of milliseconds to wait before resolving */
diff --git a/.changeset/fixture-args-readonly.md b/.changeset/fixture-args-readonly.md
new file mode 100644
index 000000000000..e95a5a622954
--- /dev/null
+++ b/.changeset/fixture-args-readonly.md
@@ -0,0 +1,25 @@
+---
+'@data-client/core': patch
+'@data-client/test': patch
+'@data-client/react': patch
+'@data-client/vue': patch
+---
+
+Fixture `args` accept readonly tuples
+
+Fixtures written with `args: [...] as const` now type-check when passed to [MockResolver](https://dataclient.io/docs/api/MockResolver), `renderDataHook()`, Vue's `renderDataCompose()` or `mockInitialState()`. Before, TypeScript rejected them with "The type 'readonly [...]' is 'readonly' and cannot be assigned to the mutable type", so you had to drop `as const` or cast.
+
+```ts
+const fixtures = [
+  {
+    endpoint: TodoResource.getList,
+    args: [{ userId: 1 }] as const,
+    response: [{ id: 1, title: 'Write tests', userId: 1 }],
+  },
+];
+
+// Before: type error on `fixtures`. After: works as written
+<MockResolver fixtures={fixtures}>
+  <TodoList />
+</MockResolver>;
+```
diff --git a/docs/core/api/Fixtures.md b/docs/core/api/Fixtures.md
index 3e8c0d986819..7d725a493485 100644
--- a/docs/core/api/Fixtures.md
+++ b/docs/core/api/Fixtures.md
@@ -35,7 +35,7 @@ export interface SuccessFixture<
   E extends EndpointInterface = EndpointInterface,
 > {
   readonly endpoint: E;
-  readonly args: Parameters<E>;
+  readonly args: Readonly<Parameters<E>>;
   readonly response:
     | ResolveType<E>
     | ((...args: Parameters<E>) => ResolveType<E>);
@@ -74,7 +74,7 @@ export interface ErrorFixture {
 ```ts
 export interface ErrorFixture<E extends EndpointInterface = EndpointInterface> {
   readonly endpoint: E;
-  readonly args: Parameters<E>;
+  readonly args: Readonly<Parameters<E>>;
   readonly response: any;
   readonly error: true;
   /** Number of milliseconds to wait before resolving */
diff --git a/docs/core/api/MockResolver.md b/docs/core/api/MockResolver.md
index 170bfc2b22dc..887f88286cc5 100644
--- a/docs/core/api/MockResolver.md
+++ b/docs/core/api/MockResolver.md
@@ -70,7 +70,7 @@ const results = [
   // fixture
   {
     endpoint: ArticleResource.getList,
-    args: [{ maxResults: 10 }],
+    args: [{ maxResults: 10 }] as const,
     response: [
       {
         id: 5,
diff --git a/docs/core/guides/unit-testing-components.md b/docs/core/guides/unit-testing-components.md
index 9060eca55a75..af2676aadf14 100644
--- a/docs/core/guides/unit-testing-components.md
+++ b/docs/core/guides/unit-testing-components.md
@@ -31,7 +31,7 @@ export default {
   full: [
     {
       endpoint: ArticleResource.getList,
-      args: [{ maxResults: 10 }],
+      args: [{ maxResults: 10 }] as const,
       response: [
         {
           id: 5,
@@ -49,7 +49,7 @@ export default {
     },
     {
       endpoint: ArticleResource.update,
-      args: [{ id: 532 }],
+      args: [{ id: 532 }] as const,
       response({ id }, body) {
         return {
           id,
@@ -61,14 +61,14 @@ export default {
   empty: [
     {
       endpoint: ArticleResource.getList,
-      args: [{ maxResults: 10 }],
+      args: [{ maxResults: 10 }] as const,
       response: [],
     },
   ],
   error: [
     {
       endpoint: ArticleResource.getList,
-      args: [{ maxResults: 10 }],
+      args: [{ maxResults: 10 }] as const,
       response: { message: 'Bad request', status: 400, name: 'Not Found' },
       error: true,
     },
diff --git a/packages/core/src/mock/fixtureTypes.ts b/packages/core/src/mock/fixtureTypes.ts
index 40e6e2b7f292..7f408cc192e0 100644
--- a/packages/core/src/mock/fixtureTypes.ts
+++ b/packages/core/src/mock/fixtureTypes.ts
@@ -9,7 +9,7 @@ export interface SuccessFixtureEndpoint<
   E extends EndpointInterface & { update?: Updater } = EndpointInterface,
 > {
   readonly endpoint: E;
-  readonly args: Parameters<E>;
+  readonly args: Readonly<Parameters<E>>;
   readonly response:
     ResolveType<E> | ((...args: Parameters<E>) => ResolveType<E>);
   readonly error?: false;
@@ -84,7 +84,7 @@ export interface ErrorFixtureEndpoint<
   E extends EndpointInterface & { update?: Updater } = EndpointInterface,
 > {
   readonly endpoint: E;
-  readonly args: Parameters<E>;
+  readonly args: Readonly<Parameters<E>>;
   readonly response: any;
   readonly error: true;
   /** Number of miliseconds to wait before resolving */
diff --git a/packages/core/src/mock/mockState.ts b/packages/core/src/mock/mockState.ts
index f7ce5d706b15..b7e995f6c815 100644
--- a/packages/core/src/mock/mockState.ts
+++ b/packages/core/src/mock/mockState.ts
@@ -32,7 +32,7 @@ export default function mockInitialState(
 
 function dispatchFixture(
   fixture: Fixture,
-  args: any[],
+  args: readonly any[],
   controller: Controller,
   fetchedAt?: number,
 ) {
diff --git a/packages/test/src/fixtureTypes.ts b/packages/test/src/fixtureTypes.ts
index a7bb6d91c075..ca96e571e557 100644
--- a/packages/test/src/fixtureTypes.ts
+++ b/packages/test/src/fixtureTypes.ts
@@ -9,7 +9,7 @@ export interface SuccessFixtureEndpoint<
   E extends EndpointInterface & { update?: Updater } = EndpointInterface,
 > {
   readonly endpoint: E;
-  readonly args: Parameters<E>;
+  readonly args: Readonly<Parameters<E>>;
   readonly response:
     ResolveType<E> | ((...args: Parameters<E>) => ResolveType<E>);
   readonly error?: false;
@@ -84,7 +84,7 @@ export interface ErrorFixtureEndpoint<
   E extends EndpointInterface & { update?: Updater } = EndpointInterface,
 > {
   readonly endpoint: E;
-  readonly args: Parameters<E>;
+  readonly args: Readonly<Parameters<E>>;
   readonly response: any;
   readonly error: true;
   /** Number of miliseconds to wait before resolving */
diff --git a/scripts/typeperf/README.md b/scripts/typeperf/README.md
index f11b529d85f3..5a493a2ff659 100644
--- a/scripts/typeperf/README.md
+++ b/scripts/typeperf/README.md
@@ -42,6 +42,7 @@ When a count drops more than 10% below its budget, the check says so; re-record
 | setValues | `ctrl.set()` values on a Union, Collection and big Entity |
 | setUpdaters | `ctrl.set()` updaters spreading `prev` on a 30-member Union |
 | redux | `prepareStore()`'s store passed to react-redux's `Provider` and `ExternalDataProvider` |
+| fixtures | 100 sets of `@data-client/test` fixtures (mutable and `as const` args) and interceptors passed to `MockResolver` and `mockInitialState()` |
 | setInvalidate | `ctrl.set()` single and batch Invalidate rows on a 30-member Union and big Entity |
 | patheq | path types against [patheq/orig.ts](./patheq/orig.ts) on fixed-seed fuzzed paths |
 
diff --git a/scripts/typeperf/budget.json b/scripts/typeperf/budget.json
index 82ff9d31fac0..bdc1e058b102 100644
--- a/scripts/typeperf/budget.json
+++ b/scripts/typeperf/budget.json
@@ -12,6 +12,7 @@
     "setUpdaters": 66031,
     "redux": 12986,
     "setInvalidate": 7325,
+    "fixtures": 76837,
     "patheq": 934395
   },
   "typescript": "7.0.2"
diff --git a/scripts/typeperf/gen.mjs b/scripts/typeperf/gen.mjs
index 3a3913a9742c..85c3598b1ee9 100644
--- a/scripts/typeperf/gen.mjs
+++ b/scripts/typeperf/gen.mjs
@@ -267,7 +267,40 @@ declare function provide<A extends { type: string }, S>(props: ProviderProps<A,
   return s;
 };
 
-// 11. path types vs their frozen pre-#4173 implementation (a type error is a mismatch)
+// 11. @data-client/test fixtures and interceptors (mutable and `as const` args) passed to MockResolver
+scen.fixtures = () => {
+  let s = `import { Entity, resource } from '@data-client/rest';
+import { MockResolver, mockInitialState } from '@data-client/test';
+import type { Fixture, Interceptor } from '@data-client/test';
+`;
+  // a few resources, so the count tracks Fixture/Interceptor/MockResolver typing rather than resource()
+  const R = 4;
+  for (let r = 0; r < R; r++)
+    s += `export class E${r} extends Entity {\n  id = '';\n${fields(20)}\n  static key = 'E${r}';\n}
+export const R${r} = resource({ path: '/e${r}/:id', schema: E${r}, searchParams: {} as { q?: string } | undefined });
+`;
+  for (let i = 0; i < 100 * N; i++) {
+    const r = `R${i % R}`;
+    s += `export const fixtures${i} = [
+  { endpoint: ${r}.get, args: [{ id: '${i}' }] as const, response: { id: '${i}', f0: 'a' } },
+  { endpoint: ${r}.getList, args: [{ q: 'x${i}' }], response: [{ id: '${i}' }] },
+  { endpoint: ${r}.update, args: [{ id: '${i}' }, { f0: 'b' }] as const, response: { id: '${i}', f0: 'b' } },
+  { endpoint: ${r}.delete, args: [{ id: '${i}' }] as const, response: { message: 'nope' }, error: true },
+];
+export const typed${i}: Fixture<typeof ${r}.get> = { endpoint: ${r}.get, args: [{ id: '${i}' }] as const, response: { id: '${i}' } };
+export const interceptor${i}: Interceptor<{ n: number }> = {
+  endpoint: ${r}.get,
+  response({ id }) { return { id, f0: \`\${this.n++}\` }; },
+  delay: () => ${i},
+};
+MockResolver({ fixtures: [...fixtures${i}, typed${i}, interceptor${i}], getInitialInterceptorData: () => ({ n: 0 }), children: null });
+mockInitialState([...fixtures${i}, typed${i}]);
+`;
+  }
+  return s;
+};
+
+// 12. path types vs their frozen pre-#4173 implementation (a type error is a mismatch)
 scen.patheq = patheq;
 
 /** Writes scenarios/<name>/ for the named fixtures (all when empty); returns the names */
diff --git a/website/src/components/Playground/editor-types/@data-client/core/mock.d.ts b/website/src/components/Playground/editor-types/@data-client/core/mock.d.ts
index 2c366daa543c..2e1bd81d4e11 100644
--- a/website/src/components/Playground/editor-types/@data-client/core/mock.d.ts
+++ b/website/src/components/Playground/editor-types/@data-client/core/mock.d.ts
@@ -1193,7 +1193,7 @@ interface SuccessFixtureEndpoint<E extends EndpointInterface & {
     update?: Updater;
 } = EndpointInterface> {
     readonly endpoint: E;
-    readonly args: Parameters<E>;
+    readonly args: Readonly<Parameters<E>>;
     readonly response: ResolveType<E> | ((...args: Parameters<E>) => ResolveType<E>);
     readonly error?: false;
     /** Number of miliseconds to wait before resolving */
@@ -1252,7 +1252,7 @@ interface ErrorFixtureEndpoint<E extends EndpointInterface & {
     update?: Updater;
 } = EndpointInterface> {
     readonly endpoint: E;
-    readonly args: Parameters<E>;
+    readonly args: Readonly<Parameters<E>>;
     readonly response: any;
     readonly error: true;
     /** Number of miliseconds to wait before resolving */
diff --git a/website/src/components/Playground/editor-types/@data-client/test.d.ts b/website/src/components/Playground/editor-types/@data-client/test.d.ts
index bba4e6f3a4ae..878c1a916d16 100644
--- a/website/src/components/Playground/editor-types/@data-client/test.d.ts
+++ b/website/src/components/Playground/editor-types/@data-client/test.d.ts
@@ -11,7 +11,7 @@ interface SuccessFixtureEndpoint<E extends EndpointInterface & {
     update?: Updater;
 } = EndpointInterface> {
     readonly endpoint: E;
-    readonly args: Parameters<E>;
+    readonly args: Readonly<Parameters<E>>;
     readonly response: ResolveType<E> | ((...args: Parameters<E>) => ResolveType<E>);
     readonly error?: false;
     /** Number of miliseconds to wait before resolving */
@@ -70,7 +70,7 @@ interface ErrorFixtureEndpoint<E extends EndpointInterface & {
     update?: Updater;
 } = EndpointInterface> {
     readonly endpoint: E;
-    readonly args: Parameters<E>;
+    readonly args: Readonly<Parameters<E>>;
     readonly response: any;
     readonly error: true;
     /** Number of miliseconds to wait before resolving */