diff --git a/packages/preact-query-devtools/package.json b/packages/preact-query-devtools/package.json
index cbcbe3f9493..f9a03c29dee 100644
--- a/packages/preact-query-devtools/package.json
+++ b/packages/preact-query-devtools/package.json
@@ -27,6 +27,8 @@
     "test:types:ts59": "node ../../node_modules/typescript59/lib/tsc.js --build tsconfig.legacy.json",
     "test:types:tscurrent": "tsc --build",
     "test:types:ts60": "node ../../node_modules/typescript60/lib/tsc.js --build tsconfig.legacy.json",
+    "test:lib": "vitest",
+    "test:lib:dev": "pnpm run test:lib --watch",
     "test:build": "publint --strict && attw --pack",
     "build": "tsup --tsconfig tsconfig.prod.json",
     "build:dev": "tsup --watch"
diff --git a/packages/preact-query-devtools/src/__tests__/PreactQueryDevtools.test.tsx b/packages/preact-query-devtools/src/__tests__/PreactQueryDevtools.test.tsx
new file mode 100644
index 00000000000..25fd5c41698
--- /dev/null
+++ b/packages/preact-query-devtools/src/__tests__/PreactQueryDevtools.test.tsx
@@ -0,0 +1,77 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+import { render } from '@testing-library/preact'
+import { QueryClient, QueryClientProvider } from '@tanstack/preact-query'
+import type { TanstackQueryDevtools } from '@tanstack/query-devtools'
+
+const mountMock = vi.fn()
+const unmountMock = vi.fn()
+const setClientMock = vi.fn()
+const setButtonPositionMock = vi.fn()
+const setPositionMock = vi.fn()
+const setInitialIsOpenMock = vi.fn()
+const setErrorTypesMock = vi.fn()
+const setThemeMock = vi.fn()
+
+vi.mock('@tanstack/query-devtools', () => ({
+  TanstackQueryDevtools: vi.fn(function (this: TanstackQueryDevtools) {
+    this.mount = mountMock
+    this.unmount = unmountMock
+    this.setClient = setClientMock
+    this.setButtonPosition = setButtonPositionMock
+    this.setPosition = setPositionMock
+    this.setInitialIsOpen = setInitialIsOpenMock
+    this.setErrorTypes = setErrorTypesMock
+    this.setTheme = setThemeMock
+  }),
+}))
+
+describe('PreactQueryDevtools', () => {
+  beforeEach(() => {
+    vi.clearAllMocks()
+  })
+
+  it('should throw an error if no query client has been set', async () => {
+    const { PreactQueryDevtools } = await import('../PreactQueryDevtools')
+
+    expect(() => render(<PreactQueryDevtools />)).toThrow(
+      'No QueryClient set, use QueryClientProvider to set one',
+    )
+  })
+
+  it('should not throw an error if query client is provided via context', async () => {
+    const { PreactQueryDevtools } = await import('../PreactQueryDevtools')
+    const queryClient = new QueryClient()
+
+    expect(() =>
+      render(
+        <QueryClientProvider client={queryClient}>
+          <PreactQueryDevtools />
+        </QueryClientProvider>,
+      ),
+    ).not.toThrow()
+    expect(mountMock).toHaveBeenCalled()
+  })
+
+  it('should not throw an error if query client is provided via props', async () => {
+    const { PreactQueryDevtools } = await import('../PreactQueryDevtools')
+    const queryClient = new QueryClient()
+
+    expect(() =>
+      render(<PreactQueryDevtools client={queryClient} />),
+    ).not.toThrow()
+    expect(mountMock).toHaveBeenCalled()
+  })
+
+  it('should return null in non-development environments', async () => {
+    vi.stubEnv('NODE_ENV', 'production')
+    vi.resetModules()
+
+    try {
+      const { PreactQueryDevtools } = await import('..')
+      expect(PreactQueryDevtools({})).toBeNull()
+    } finally {
+      vi.unstubAllEnvs()
+      vi.resetModules()
+    }
+  })
+})
diff --git a/packages/preact-query-devtools/src/__tests__/PreactQueryDevtoolsPanel.test.tsx b/packages/preact-query-devtools/src/__tests__/PreactQueryDevtoolsPanel.test.tsx
new file mode 100644
index 00000000000..cc03560a367
--- /dev/null
+++ b/packages/preact-query-devtools/src/__tests__/PreactQueryDevtoolsPanel.test.tsx
@@ -0,0 +1,78 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+import { render } from '@testing-library/preact'
+import { QueryClient, QueryClientProvider } from '@tanstack/preact-query'
+import type { TanstackQueryDevtoolsPanel } from '@tanstack/query-devtools'
+
+const mountMock = vi.fn()
+const unmountMock = vi.fn()
+const setClientMock = vi.fn()
+const setOnCloseMock = vi.fn()
+const setErrorTypesMock = vi.fn()
+const setThemeMock = vi.fn()
+
+vi.mock('@tanstack/query-devtools', () => ({
+  TanstackQueryDevtoolsPanel: vi.fn(function (
+    this: TanstackQueryDevtoolsPanel,
+  ) {
+    this.mount = mountMock
+    this.unmount = unmountMock
+    this.setClient = setClientMock
+    this.setOnClose = setOnCloseMock
+    this.setErrorTypes = setErrorTypesMock
+    this.setTheme = setThemeMock
+  }),
+}))
+
+describe('PreactQueryDevtoolsPanel', () => {
+  beforeEach(() => {
+    vi.clearAllMocks()
+  })
+
+  it('should throw an error if no query client has been set', async () => {
+    const { PreactQueryDevtoolsPanel } =
+      await import('../PreactQueryDevtoolsPanel')
+
+    expect(() => render(<PreactQueryDevtoolsPanel />)).toThrow(
+      'No QueryClient set, use QueryClientProvider to set one',
+    )
+  })
+
+  it('should not throw an error if query client is provided via context', async () => {
+    const { PreactQueryDevtoolsPanel } =
+      await import('../PreactQueryDevtoolsPanel')
+    const queryClient = new QueryClient()
+
+    expect(() =>
+      render(
+        <QueryClientProvider client={queryClient}>
+          <PreactQueryDevtoolsPanel />
+        </QueryClientProvider>,
+      ),
+    ).not.toThrow()
+    expect(mountMock).toHaveBeenCalled()
+  })
+
+  it('should not throw an error if query client is provided via props', async () => {
+    const { PreactQueryDevtoolsPanel } =
+      await import('../PreactQueryDevtoolsPanel')
+    const queryClient = new QueryClient()
+
+    expect(() =>
+      render(<PreactQueryDevtoolsPanel client={queryClient} />),
+    ).not.toThrow()
+    expect(mountMock).toHaveBeenCalled()
+  })
+
+  it('should return null in non-development environments', async () => {
+    vi.stubEnv('NODE_ENV', 'production')
+    vi.resetModules()
+
+    try {
+      const { PreactQueryDevtoolsPanel } = await import('..')
+      expect(PreactQueryDevtoolsPanel({})).toBeNull()
+    } finally {
+      vi.unstubAllEnvs()
+      vi.resetModules()
+    }
+  })
+})
diff --git a/packages/react-query-devtools/src/__tests__/ReactQueryDevtools.test.tsx b/packages/react-query-devtools/src/__tests__/ReactQueryDevtools.test.tsx
new file mode 100644
index 00000000000..0d48c3d173d
--- /dev/null
+++ b/packages/react-query-devtools/src/__tests__/ReactQueryDevtools.test.tsx
@@ -0,0 +1,77 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+import { render } from '@testing-library/react'
+import { QueryClient, QueryClientProvider } from '@tanstack/react-query'
+import type { TanstackQueryDevtools } from '@tanstack/query-devtools'
+
+const mountMock = vi.fn()
+const unmountMock = vi.fn()
+const setClientMock = vi.fn()
+const setButtonPositionMock = vi.fn()
+const setPositionMock = vi.fn()
+const setInitialIsOpenMock = vi.fn()
+const setErrorTypesMock = vi.fn()
+const setThemeMock = vi.fn()
+
+vi.mock('@tanstack/query-devtools', () => ({
+  TanstackQueryDevtools: vi.fn(function (this: TanstackQueryDevtools) {
+    this.mount = mountMock
+    this.unmount = unmountMock
+    this.setClient = setClientMock
+    this.setButtonPosition = setButtonPositionMock
+    this.setPosition = setPositionMock
+    this.setInitialIsOpen = setInitialIsOpenMock
+    this.setErrorTypes = setErrorTypesMock
+    this.setTheme = setThemeMock
+  }),
+}))
+
+describe('ReactQueryDevtools', () => {
+  beforeEach(() => {
+    vi.clearAllMocks()
+  })
+
+  it('should throw an error if no query client has been set', async () => {
+    const { ReactQueryDevtools } = await import('../ReactQueryDevtools')
+
+    expect(() => render(<ReactQueryDevtools />)).toThrow(
+      'No QueryClient set, use QueryClientProvider to set one',
+    )
+  })
+
+  it('should not throw an error if query client is provided via context', async () => {
+    const { ReactQueryDevtools } = await import('../ReactQueryDevtools')
+    const queryClient = new QueryClient()
+
+    expect(() =>
+      render(
+        <QueryClientProvider client={queryClient}>
+          <ReactQueryDevtools />
+        </QueryClientProvider>,
+      ),
+    ).not.toThrow()
+    expect(mountMock).toHaveBeenCalled()
+  })
+
+  it('should not throw an error if query client is provided via props', async () => {
+    const { ReactQueryDevtools } = await import('../ReactQueryDevtools')
+    const queryClient = new QueryClient()
+
+    expect(() =>
+      render(<ReactQueryDevtools client={queryClient} />),
+    ).not.toThrow()
+    expect(mountMock).toHaveBeenCalled()
+  })
+
+  it('should return null in non-development environments', async () => {
+    vi.stubEnv('NODE_ENV', 'production')
+    vi.resetModules()
+
+    try {
+      const { ReactQueryDevtools } = await import('..')
+      expect(ReactQueryDevtools({})).toBeNull()
+    } finally {
+      vi.unstubAllEnvs()
+      vi.resetModules()
+    }
+  })
+})
diff --git a/packages/react-query-devtools/src/__tests__/ReactQueryDevtoolsPanel.test.tsx b/packages/react-query-devtools/src/__tests__/ReactQueryDevtoolsPanel.test.tsx
new file mode 100644
index 00000000000..4beb7d99c02
--- /dev/null
+++ b/packages/react-query-devtools/src/__tests__/ReactQueryDevtoolsPanel.test.tsx
@@ -0,0 +1,78 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+import { render } from '@testing-library/react'
+import { QueryClient, QueryClientProvider } from '@tanstack/react-query'
+import type { TanstackQueryDevtoolsPanel } from '@tanstack/query-devtools'
+
+const mountMock = vi.fn()
+const unmountMock = vi.fn()
+const setClientMock = vi.fn()
+const setOnCloseMock = vi.fn()
+const setErrorTypesMock = vi.fn()
+const setThemeMock = vi.fn()
+
+vi.mock('@tanstack/query-devtools', () => ({
+  TanstackQueryDevtoolsPanel: vi.fn(function (
+    this: TanstackQueryDevtoolsPanel,
+  ) {
+    this.mount = mountMock
+    this.unmount = unmountMock
+    this.setClient = setClientMock
+    this.setOnClose = setOnCloseMock
+    this.setErrorTypes = setErrorTypesMock
+    this.setTheme = setThemeMock
+  }),
+}))
+
+describe('ReactQueryDevtoolsPanel', () => {
+  beforeEach(() => {
+    vi.clearAllMocks()
+  })
+
+  it('should throw an error if no query client has been set', async () => {
+    const { ReactQueryDevtoolsPanel } =
+      await import('../ReactQueryDevtoolsPanel')
+
+    expect(() => render(<ReactQueryDevtoolsPanel />)).toThrow(
+      'No QueryClient set, use QueryClientProvider to set one',
+    )
+  })
+
+  it('should not throw an error if query client is provided via context', async () => {
+    const { ReactQueryDevtoolsPanel } =
+      await import('../ReactQueryDevtoolsPanel')
+    const queryClient = new QueryClient()
+
+    expect(() =>
+      render(
+        <QueryClientProvider client={queryClient}>
+          <ReactQueryDevtoolsPanel />
+        </QueryClientProvider>,
+      ),
+    ).not.toThrow()
+    expect(mountMock).toHaveBeenCalled()
+  })
+
+  it('should not throw an error if query client is provided via props', async () => {
+    const { ReactQueryDevtoolsPanel } =
+      await import('../ReactQueryDevtoolsPanel')
+    const queryClient = new QueryClient()
+
+    expect(() =>
+      render(<ReactQueryDevtoolsPanel client={queryClient} />),
+    ).not.toThrow()
+    expect(mountMock).toHaveBeenCalled()
+  })
+
+  it('should return null in non-development environments', async () => {
+    vi.stubEnv('NODE_ENV', 'production')
+    vi.resetModules()
+
+    try {
+      const { ReactQueryDevtoolsPanel } = await import('..')
+      expect(ReactQueryDevtoolsPanel({})).toBeNull()
+    } finally {
+      vi.unstubAllEnvs()
+      vi.resetModules()
+    }
+  })
+})
diff --git a/packages/react-query-devtools/src/__tests__/devtools.test.tsx b/packages/react-query-devtools/src/__tests__/devtools.test.tsx
deleted file mode 100644
index 0e44d74db67..00000000000
--- a/packages/react-query-devtools/src/__tests__/devtools.test.tsx
+++ /dev/null
@@ -1,7 +0,0 @@
-import { describe, expect, it } from 'vitest'
-
-describe('ReactQueryDevtools', () => {
-  it('should be able to open and close devtools', () => {
-    expect(1).toBe(1)
-  })
-})
diff --git a/packages/react-query-devtools/src/__tests__/not-development.test.tsx b/packages/react-query-devtools/src/__tests__/not-development.test.tsx
deleted file mode 100644
index 6353147ed4d..00000000000
--- a/packages/react-query-devtools/src/__tests__/not-development.test.tsx
+++ /dev/null
@@ -1,9 +0,0 @@
-import { describe, expect, it } from 'vitest'
-import { ReactQueryDevtools } from '..'
-
-describe('ReactQueryDevtools not in process.env.NODE_ENV=development', () => {
-  it('should return null', () => {
-    expect(process.env.NODE_ENV).not.toBe('development')
-    expect(ReactQueryDevtools({})).toBeNull()
-  })
-})
diff --git a/packages/svelte-query-devtools/package.json b/packages/svelte-query-devtools/package.json
index 79cca89edbe..c6b58ea9a25 100644
--- a/packages/svelte-query-devtools/package.json
+++ b/packages/svelte-query-devtools/package.json
@@ -24,6 +24,8 @@
     "compile": "tsc --build",
     "test:types": "svelte-check --tsconfig ./tsconfig.json",
     "test:eslint": "eslint --concurrency=auto ./src",
+    "test:lib": "vitest",
+    "test:lib:dev": "pnpm run test:lib --watch",
     "test:build": "publint --strict && attw --pack",
     "build": "svelte-package --input ./src --output ./dist"
   },
@@ -53,6 +55,7 @@
     "@sveltejs/package": "^2.4.0",
     "@sveltejs/vite-plugin-svelte": "^5.1.1",
     "@tanstack/svelte-query": "workspace:*",
+    "@testing-library/svelte": "^5.2.8",
     "@typescript-eslint/parser": "^8.48.0",
     "eslint-plugin-svelte": "^3.11.0",
     "svelte": "^5.39.3",
diff --git a/packages/svelte-query-devtools/tests/Devtools.svelte.test.ts b/packages/svelte-query-devtools/tests/Devtools.svelte.test.ts
new file mode 100644
index 00000000000..dc773f54309
--- /dev/null
+++ b/packages/svelte-query-devtools/tests/Devtools.svelte.test.ts
@@ -0,0 +1,39 @@
+import { describe, expect, it } from 'vitest'
+import { render } from '@testing-library/svelte'
+import { QueryClient } from '@tanstack/svelte-query'
+import SvelteQueryDevtools from '../src/Devtools.svelte'
+import Wrapper from './Wrapper.svelte'
+
+describe('SvelteQueryDevtools', () => {
+  it('should render the parent container without throwing in non-development environments', () => {
+    const queryClient = new QueryClient()
+
+    const { container } = render(SvelteQueryDevtools, {
+      props: { client: queryClient },
+    })
+
+    expect(
+      container.querySelector('.tsqd-parent-container'),
+    ).toBeInTheDocument()
+  })
+
+  it('should throw an error if no query client has been set', () => {
+    expect(() => render(SvelteQueryDevtools)).toThrow(
+      'No QueryClient was found in Svelte context. Did you forget to wrap your component with QueryClientProvider?',
+    )
+  })
+
+  it('should not throw an error if query client is provided via context', () => {
+    const queryClient = new QueryClient()
+
+    expect(() => render(Wrapper, { props: { queryClient } })).not.toThrow()
+  })
+
+  it('should not throw an error if query client is provided via props', () => {
+    const queryClient = new QueryClient()
+
+    expect(() =>
+      render(SvelteQueryDevtools, { props: { client: queryClient } }),
+    ).not.toThrow()
+  })
+})
diff --git a/packages/svelte-query-devtools/tests/Wrapper.svelte b/packages/svelte-query-devtools/tests/Wrapper.svelte
new file mode 100644
index 00000000000..35c5cf15176
--- /dev/null
+++ b/packages/svelte-query-devtools/tests/Wrapper.svelte
@@ -0,0 +1,15 @@
+<script lang="ts">
+  import { QueryClientProvider } from '@tanstack/svelte-query'
+  import SvelteQueryDevtools from '../src/Devtools.svelte'
+  import type { QueryClient } from '@tanstack/svelte-query'
+
+  type Props = {
+    queryClient: QueryClient
+  }
+
+  let { queryClient }: Props = $props()
+</script>
+
+<QueryClientProvider client={queryClient}>
+  <SvelteQueryDevtools />
+</QueryClientProvider>
diff --git a/packages/svelte-query-devtools/tests/test-setup.ts b/packages/svelte-query-devtools/tests/test-setup.ts
new file mode 100644
index 00000000000..a9d0dd31aa6
--- /dev/null
+++ b/packages/svelte-query-devtools/tests/test-setup.ts
@@ -0,0 +1 @@
+import '@testing-library/jest-dom/vitest'
diff --git a/packages/svelte-query-devtools/tsconfig.json b/packages/svelte-query-devtools/tsconfig.json
index 773fd3d525d..cce7cfa8c38 100644
--- a/packages/svelte-query-devtools/tsconfig.json
+++ b/packages/svelte-query-devtools/tsconfig.json
@@ -4,6 +4,6 @@
     "outDir": "./dist-ts",
     "rootDir": "."
   },
-  "include": ["src", "*.config.*", "package.json"],
+  "include": ["src", "tests", "*.config.*", "package.json"],
   "references": [{ "path": "../query-devtools" }, { "path": "../svelte-query" }]
 }
diff --git a/packages/svelte-query-devtools/vite.config.ts b/packages/svelte-query-devtools/vite.config.ts
index 01ebc554bd3..5b2d97f30f2 100644
--- a/packages/svelte-query-devtools/vite.config.ts
+++ b/packages/svelte-query-devtools/vite.config.ts
@@ -1,8 +1,11 @@
 import { svelte } from '@sveltejs/vite-plugin-svelte'
-import { defineConfig } from 'vite'
+import { defineConfig } from 'vitest/config'
+import { svelteTesting } from '@testing-library/svelte/vite'
+
+import packageJson from './package.json'
 
 export default defineConfig({
-  plugins: [svelte()],
+  plugins: [svelte(), svelteTesting()],
   // fix from https://github.com/vitest-dev/vitest/issues/6992#issuecomment-2509408660
   resolve: {
     conditions: ['@tanstack/custom-condition'],
@@ -14,4 +17,17 @@ export default defineConfig({
       },
     },
   },
+  test: {
+    name: packageJson.name,
+    dir: './tests',
+    watch: false,
+    environment: 'jsdom',
+    setupFiles: ['./tests/test-setup.ts'],
+    coverage: {
+      enabled: true,
+      provider: 'istanbul',
+      include: ['src/**/*'],
+    },
+    typecheck: { enabled: true },
+  },
 })
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 258ab23246e..82cb34c9228 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -2936,6 +2936,9 @@ importers:
       '@tanstack/svelte-query':
         specifier: workspace:*
         version: link:../svelte-query
+      '@testing-library/svelte':
+        specifier: ^5.2.8
+        version: 5.3.1(svelte@5.55.1)(vite@6.4.1(@types/node@22.19.15)(jiti@2.6.1)(lightningcss@1.32.0)(sass@1.90.0)(terser@5.46.1)(yaml@2.8.3))(vitest@4.1.2(@types/node@22.19.15)(jsdom@27.4.0)(msw@2.12.14(@types/node@22.19.15)(typescript@5.9.3))(vite@6.4.1(@types/node@22.19.15)(jiti@2.6.1)(lightningcss@1.32.0)(sass@1.90.0)(terser@5.46.1)(yaml@2.8.3)))
       '@typescript-eslint/parser':
         specifier: 8.58.1
         version: 8.58.1(eslint@9.39.4(jiti@2.6.1))(typescript@5.9.3)