diff --git a/packages/next/src/server/config-shared.ts b/packages/next/src/server/config-shared.ts
index b5774cd7db9b..8d61008625f4 100644
--- a/packages/next/src/server/config-shared.ts
+++ b/packages/next/src/server/config-shared.ts
@@ -36,7 +36,7 @@ export type NextConfigComplete = Required<Omit<NextConfig, 'configFile'>> & {
     prefetchInlining?: PrefetchInliningConfig
     // Normalized by config.ts: defaulted to 90% of staticPageGenerationTimeout
     useCacheTimeout: number
-    // Normalized by config.ts `finalizeConfig`: defaulted to `'manual-warning'`
+    // Normalized by config.ts `finalizeConfig`: defaulted to `'warning'`
     instantInsights: { validationLevel: ValidationLevel }
   }
   // The root directory of the distDir. In development mode, this is the parent directory of `distDir`
@@ -1086,10 +1086,10 @@ export interface ExperimentalConfig {
     /**
      * Controls the validation behavior of Instant Insights
      *
-     * - `'warning'`: Validates all navigations for Instant UI in development
-     * - `'manual-warning'`: Validates navigations for Instant UI in development when configured with `unstable_instant` in Pages and Layouts
+     * - `'warning'` (default): Validates all navigations for Instant UI in development
+     * - `'manual-warning'`: Validates navigations for Instant UI in development only when configured with `unstable_instant` in Pages and Layouts
      * - `'experimental-error'`: Validates all navigations for Instant in development and build. Use with caution.
-     * - `'experimental-manual-error'`: Validates navigations for Instant UI in developement and build when configured with `unstable_instant` in Pages and Layouts. Use with caution.
+     * - `'experimental-manual-error'`: Validates navigations for Instant UI in development and build when configured with `unstable_instant` in Pages and Layouts. Use with caution.
      */
     validationLevel?: ValidationLevel
   }
diff --git a/packages/next/src/server/config.ts b/packages/next/src/server/config.ts
index c61f17984c65..bb4e2d59bdf0 100644
--- a/packages/next/src/server/config.ts
+++ b/packages/next/src/server/config.ts
@@ -1618,7 +1618,7 @@ function assignDefaultsAndValidate(
 function finalizeConfig(config: NextConfigComplete): NextConfigComplete {
   config.experimental.instantInsights = {
     validationLevel:
-      config.experimental.instantInsights?.validationLevel ?? 'manual-warning',
+      config.experimental.instantInsights?.validationLevel ?? 'warning',
   }
   return config
 }
diff --git a/test/development/app-dir/cache-components-dev-errors/next.config.js b/test/development/app-dir/cache-components-dev-errors/next.config.js
index e64bae22d658..94fcd455df28 100644
--- a/test/development/app-dir/cache-components-dev-errors/next.config.js
+++ b/test/development/app-dir/cache-components-dev-errors/next.config.js
@@ -3,6 +3,11 @@
  */
 const nextConfig = {
   cacheComponents: true,
+  experimental: {
+    instantInsights: {
+      validationLevel: 'manual-warning',
+    },
+  },
 }
 
 module.exports = nextConfig
diff --git a/test/development/app-dir/error-overlay/error-ignored-frames/next.config.js b/test/development/app-dir/error-overlay/error-ignored-frames/next.config.js
index 807126e4cf0b..3f1322dc956a 100644
--- a/test/development/app-dir/error-overlay/error-ignored-frames/next.config.js
+++ b/test/development/app-dir/error-overlay/error-ignored-frames/next.config.js
@@ -1,6 +1,12 @@
 /**
  * @type {import('next').NextConfig}
  */
-const nextConfig = {}
+const nextConfig = {
+  experimental: {
+    instantInsights: {
+      validationLevel: 'manual-warning',
+    },
+  },
+}
 
 module.exports = nextConfig
diff --git a/test/development/app-dir/hmr-iframe/next.config.js b/test/development/app-dir/hmr-iframe/next.config.js
new file mode 100644
index 000000000000..3f1322dc956a
--- /dev/null
+++ b/test/development/app-dir/hmr-iframe/next.config.js
@@ -0,0 +1,12 @@
+/**
+ * @type {import('next').NextConfig}
+ */
+const nextConfig = {
+  experimental: {
+    instantInsights: {
+      validationLevel: 'manual-warning',
+    },
+  },
+}
+
+module.exports = nextConfig
diff --git a/test/development/app-dir/owner-stack-invalid-element-type/next.config.js b/test/development/app-dir/owner-stack-invalid-element-type/next.config.js
index 807126e4cf0b..3f1322dc956a 100644
--- a/test/development/app-dir/owner-stack-invalid-element-type/next.config.js
+++ b/test/development/app-dir/owner-stack-invalid-element-type/next.config.js
@@ -1,6 +1,12 @@
 /**
  * @type {import('next').NextConfig}
  */
-const nextConfig = {}
+const nextConfig = {
+  experimental: {
+    instantInsights: {
+      validationLevel: 'manual-warning',
+    },
+  },
+}
 
 module.exports = nextConfig
diff --git a/test/development/app-dir/owner-stack/next.config.js b/test/development/app-dir/owner-stack/next.config.js
index 807126e4cf0b..3f1322dc956a 100644
--- a/test/development/app-dir/owner-stack/next.config.js
+++ b/test/development/app-dir/owner-stack/next.config.js
@@ -1,6 +1,12 @@
 /**
  * @type {import('next').NextConfig}
  */
-const nextConfig = {}
+const nextConfig = {
+  experimental: {
+    instantInsights: {
+      validationLevel: 'manual-warning',
+    },
+  },
+}
 
 module.exports = nextConfig
diff --git a/test/e2e/app-dir/cache-components-errors/fixtures/default/next.config.js b/test/e2e/app-dir/cache-components-errors/fixtures/default/next.config.js
index e64bae22d658..94fcd455df28 100644
--- a/test/e2e/app-dir/cache-components-errors/fixtures/default/next.config.js
+++ b/test/e2e/app-dir/cache-components-errors/fixtures/default/next.config.js
@@ -3,6 +3,11 @@
  */
 const nextConfig = {
   cacheComponents: true,
+  experimental: {
+    instantInsights: {
+      validationLevel: 'manual-warning',
+    },
+  },
 }
 
 module.exports = nextConfig
diff --git a/test/e2e/app-dir/cache-components-errors/fixtures/http-access-fallback-prerender/next.config.js b/test/e2e/app-dir/cache-components-errors/fixtures/http-access-fallback-prerender/next.config.js
index 34aa2b5aed26..e561e049a1eb 100644
--- a/test/e2e/app-dir/cache-components-errors/fixtures/http-access-fallback-prerender/next.config.js
+++ b/test/e2e/app-dir/cache-components-errors/fixtures/http-access-fallback-prerender/next.config.js
@@ -5,6 +5,9 @@ const nextConfig = {
   cacheComponents: true,
   experimental: {
     authInterrupts: true,
+    instantInsights: {
+      validationLevel: 'manual-warning',
+    },
   },
 }
 
diff --git a/test/e2e/app-dir/cache-components/next.config.js b/test/e2e/app-dir/cache-components/next.config.js
index 86f364bdd422..e83f1cf6305a 100644
--- a/test/e2e/app-dir/cache-components/next.config.js
+++ b/test/e2e/app-dir/cache-components/next.config.js
@@ -5,6 +5,11 @@ const nextConfig = {
   cacheComponents: true,
   adapterPath:
     process.env.NEXT_ADAPTER_PATH ?? require.resolve('./my-adapter.mjs'),
+  experimental: {
+    instantInsights: {
+      validationLevel: 'manual-warning',
+    },
+  },
 }
 
 module.exports = nextConfig
diff --git a/test/e2e/app-dir/instant-validation-level-default/app/bare/page.tsx b/test/e2e/app-dir/instant-validation-level-default/app/bare/page.tsx
new file mode 100644
index 000000000000..307c49df3a08
--- /dev/null
+++ b/test/e2e/app-dir/instant-validation-level-default/app/bare/page.tsx
@@ -0,0 +1,16 @@
+// Bare page (no `unstable_instant` config). Under the framework default
+// (`'warning'`), implicit validation fires on this page in dev. The runtime
+// data accessed at the top of the page is the "Suspense too high for instant
+// navigation" violation that instant validation specifically flags. The root
+// layout's Suspense satisfies static-shell validation, so the only error in
+// dev is the instant one.
+import { connection } from 'next/server'
+
+export default async function Page() {
+  await connection()
+  return (
+    <main>
+      <p>bare page (no unstable_instant), runtime data at the top.</p>
+    </main>
+  )
+}
diff --git a/test/e2e/app-dir/instant-validation-level-default/app/explicit-false/page.tsx b/test/e2e/app-dir/instant-validation-level-default/app/explicit-false/page.tsx
new file mode 100644
index 000000000000..27e70e2c2631
--- /dev/null
+++ b/test/e2e/app-dir/instant-validation-level-default/app/explicit-false/page.tsx
@@ -0,0 +1,16 @@
+// Page explicitly opts out of instant validation. Under the framework
+// default (`'warning'`), this segment-level override suppresses the
+// implicit validation that would otherwise fire on a bare page, so no
+// redbox appears.
+import { connection } from 'next/server'
+
+export const unstable_instant = false
+
+export default async function Page() {
+  await connection()
+  return (
+    <main>
+      <p>explicit-false page (segment opts out of validation).</p>
+    </main>
+  )
+}
diff --git a/test/e2e/app-dir/instant-validation-level-default/app/layout.tsx b/test/e2e/app-dir/instant-validation-level-default/app/layout.tsx
new file mode 100644
index 000000000000..a32d71748e1c
--- /dev/null
+++ b/test/e2e/app-dir/instant-validation-level-default/app/layout.tsx
@@ -0,0 +1,23 @@
+import { Suspense, type ReactNode } from 'react'
+
+// Validation level is not set in next.config.ts, so the framework default
+// applies. The default is 'warning' — implicit validation fires on bare
+// page/default segments in dev only (build is unaffected unless a segment
+// explicitly escalates with `level: 'experimental-error'`).
+//
+// Children are wrapped in Suspense so that pages with runtime data
+// accessed at the top of the page don't fail static-shell validation
+// (the Suspense fallback renders into the static shell). Instant
+// validation flags "Suspense too high for instant navigation" as an
+// instant-specific violation when it runs.
+export const unstable_instant = false
+
+export default function RootLayout({ children }: { children: ReactNode }) {
+  return (
+    <html>
+      <body>
+        <Suspense fallback={<p>loading…</p>}>{children}</Suspense>
+      </body>
+    </html>
+  )
+}
diff --git a/test/e2e/app-dir/instant-validation-level-default/instant-validation-level-default.test.ts b/test/e2e/app-dir/instant-validation-level-default/instant-validation-level-default.test.ts
new file mode 100644
index 000000000000..461bf9067e0d
--- /dev/null
+++ b/test/e2e/app-dir/instant-validation-level-default/instant-validation-level-default.test.ts
@@ -0,0 +1,93 @@
+import { nextTestSetup } from 'e2e-utils'
+import { expectBuildValidationSkipped } from 'e2e-utils/instant-validation'
+import { waitForNoErrorToast } from '../../../lib/next-test-utils'
+
+// This fixture intentionally omits `experimental.instantInsights` from
+// next.config.ts. It pins the framework default for `validationLevel` —
+// the framework should resolve the default to `'warning'`, which means
+// implicit validation fires on bare pages in dev. If the framework default
+// ever changes, this test should fail, alerting whoever changes it.
+//
+// For exhaustive coverage of explicit levels and per-segment overrides,
+// see the sibling `instant-validation-level-{warning,manual-warning,error,
+// manual-error}` fixtures.
+describe('instant validation - default level', () => {
+  const { next, skipped, isNextDev, isNextStart, isTurbopack } = nextTestSetup({
+    files: __dirname,
+    skipStart: true,
+    skipDeployment: true,
+    env: {
+      NEXT_TEST_LOG_VALIDATION: '1',
+    },
+  })
+  if (skipped) return
+
+  if (isNextStart && !isTurbopack) {
+    it.skip('TODO: snapshot tests for webpack', () => {})
+    return
+  }
+
+  if (isNextStart) {
+    beforeAll(async () => {
+      await next.build({ args: ['--experimental-build-mode', 'compile'] })
+    })
+    afterEach(async () => {
+      await next.stop()
+    })
+  } else {
+    beforeAll(async () => {
+      await next.start()
+    })
+  }
+
+  const prerender = async (pathname: string) => {
+    return await next.build({
+      args: [
+        '--experimental-build-mode',
+        'generate',
+        '--debug-build-paths',
+        `app${pathname}/page.tsx`,
+      ],
+    })
+  }
+
+  if (isNextDev) {
+    describe('dev', () => {
+      it('bare page: framework default matches `warning`, implicit validation fires', async () => {
+        const browser = await next.browser('/bare')
+        await expect(browser).toDisplayCollapsedRedbox(`
+         {
+           "code": "E1264",
+           "description": "Next.js encountered uncached data during a navigation.",
+           "environmentLabel": "Server",
+           "label": "Instant",
+           "source": "app/bare/page.tsx (10:19) @ Page
+         > 10 |   await connection()
+              |                   ^",
+           "stack": [
+             "Page app/bare/page.tsx (10:19)",
+           ],
+         }
+        `)
+      })
+
+      it('explicit-false page: per-segment opt-out still works under default', async () => {
+        const browser = await next.browser('/explicit-false')
+        await browser.elementByCss('main')
+        await waitForNoErrorToast(browser, { waitInMs: 500 })
+      })
+    })
+  } else {
+    describe('build', () => {
+      it('bare page: framework default is dev-only, build skips validation', async () => {
+        const result = await prerender('/bare')
+        expectBuildValidationSkipped(result)
+      })
+
+      it('explicit-false page: per-segment opt-out keeps build clean', async () => {
+        const result = await prerender('/explicit-false')
+        expectBuildValidationSkipped(result)
+      })
+    })
+  }
+})
diff --git a/test/e2e/app-dir/instant-validation-level-default/next.config.ts b/test/e2e/app-dir/instant-validation-level-default/next.config.ts
new file mode 100644
index 000000000000..9fd0c7a083d4
--- /dev/null
+++ b/test/e2e/app-dir/instant-validation-level-default/next.config.ts
@@ -0,0 +1,10 @@
+import type { NextConfig } from 'next'
+
+const nextConfig: NextConfig = {
+  cacheComponents: true,
+  typescript: {
+    ignoreBuildErrors: true,
+  },
+}
+
+export default nextConfig
diff --git a/test/e2e/app-dir/loader-file-named-export-custom-loader-error/next.config.js b/test/e2e/app-dir/loader-file-named-export-custom-loader-error/next.config.js
index 77961243627b..b625c77b5dca 100644
--- a/test/e2e/app-dir/loader-file-named-export-custom-loader-error/next.config.js
+++ b/test/e2e/app-dir/loader-file-named-export-custom-loader-error/next.config.js
@@ -5,6 +5,11 @@ const nextConfig = {
   images: {
     loaderFile: '/dummy-loader.ts',
   },
+  experimental: {
+    instantInsights: {
+      validationLevel: 'manual-warning',
+    },
+  },
 }
 
 module.exports = nextConfig
diff --git a/test/e2e/app-dir/non-root-project-monorepo/apps/web/next.config.js b/test/e2e/app-dir/non-root-project-monorepo/apps/web/next.config.js
index 807126e4cf0b..3f1322dc956a 100644
--- a/test/e2e/app-dir/non-root-project-monorepo/apps/web/next.config.js
+++ b/test/e2e/app-dir/non-root-project-monorepo/apps/web/next.config.js
@@ -1,6 +1,12 @@
 /**
  * @type {import('next').NextConfig}
  */
-const nextConfig = {}
+const nextConfig = {
+  experimental: {
+    instantInsights: {
+      validationLevel: 'manual-warning',
+    },
+  },
+}
 
 module.exports = nextConfig
diff --git a/test/e2e/app-dir/router-autoscroll/next.config.js b/test/e2e/app-dir/router-autoscroll/next.config.js
index 36e8d82ea1af..ad60a12885cd 100644
--- a/test/e2e/app-dir/router-autoscroll/next.config.js
+++ b/test/e2e/app-dir/router-autoscroll/next.config.js
@@ -1,6 +1,12 @@
 /**
  * @type {import('next').NextConfig}
  */
-const config = {}
+const config = {
+  experimental: {
+    instantInsights: {
+      validationLevel: 'manual-warning',
+    },
+  },
+}
 
 module.exports = config
diff --git a/test/e2e/app-dir/server-source-maps/fixtures/default/next.config.js b/test/e2e/app-dir/server-source-maps/fixtures/default/next.config.js
index 8a61ee02d1f4..19c0893004fa 100644
--- a/test/e2e/app-dir/server-source-maps/fixtures/default/next.config.js
+++ b/test/e2e/app-dir/server-source-maps/fixtures/default/next.config.js
@@ -6,6 +6,9 @@ const nextConfig = {
   experimental: {
     cpus: 1,
     serverSourceMaps: true,
+    instantInsights: {
+      validationLevel: 'manual-warning',
+    },
   },
   serverExternalPackages: ['external-pkg'],
 }
diff --git a/test/e2e/app-dir/server-source-maps/fixtures/edge/next.config.js b/test/e2e/app-dir/server-source-maps/fixtures/edge/next.config.js
index 86773e2ae9a6..0341e872150f 100644
--- a/test/e2e/app-dir/server-source-maps/fixtures/edge/next.config.js
+++ b/test/e2e/app-dir/server-source-maps/fixtures/edge/next.config.js
@@ -5,6 +5,9 @@ const nextConfig = {
   experimental: {
     cpus: 1,
     serverSourceMaps: true,
+    instantInsights: {
+      validationLevel: 'manual-warning',
+    },
   },
 }
 
diff --git a/test/e2e/legacy-link-behavior/next.config.js b/test/e2e/legacy-link-behavior/next.config.js
index e64bae22d658..94fcd455df28 100644
--- a/test/e2e/legacy-link-behavior/next.config.js
+++ b/test/e2e/legacy-link-behavior/next.config.js
@@ -3,6 +3,11 @@
  */
 const nextConfig = {
   cacheComponents: true,
+  experimental: {
+    instantInsights: {
+      validationLevel: 'manual-warning',
+    },
+  },
 }
 
 module.exports = nextConfig
diff --git a/test/e2e/next-image-new/app-dir-localpatterns/next.config.js b/test/e2e/next-image-new/app-dir-localpatterns/next.config.js
index 10c28b1a185c..635050bf29c1 100644
--- a/test/e2e/next-image-new/app-dir-localpatterns/next.config.js
+++ b/test/e2e/next-image-new/app-dir-localpatterns/next.config.js
@@ -7,4 +7,9 @@ module.exports = {
       },
     ],
   },
+  experimental: {
+    instantInsights: {
+      validationLevel: 'manual-warning',
+    },
+  },
 }
diff --git a/test/unit/instant-config-normalization.test.ts b/test/unit/instant-config-normalization.test.ts
index e33a8c7811db..670a6d0bbe12 100644
--- a/test/unit/instant-config-normalization.test.ts
+++ b/test/unit/instant-config-normalization.test.ts
@@ -14,7 +14,7 @@ function uniqueDir(tag: string) {
 // `experimental.instantInsights.validationLevel` to a concrete value in
 // one place so consumers don't each need to know the current framework default.
 describe('experimental.instantInsights validationLevel normalization', () => {
-  it('defaults to manual-warning when the instantInsights config is absent', async () => {
+  it('defaults to warning when the instantInsights config is absent', async () => {
     const config = await loadConfig(
       PHASE_PRODUCTION_SERVER,
       uniqueDir('absent'),
@@ -23,11 +23,11 @@ describe('experimental.instantInsights validationLevel normalization', () => {
       }
     )
     expect(config.experimental.instantInsights).toEqual({
-      validationLevel: 'manual-warning',
+      validationLevel: 'warning',
     })
   })
 
-  it('defaults to manual-warning when experimental.instantInsights is an empty object', async () => {
+  it('defaults to warning when experimental.instantInsights is an empty object', async () => {
     const config = await loadConfig(
       PHASE_PRODUCTION_SERVER,
       uniqueDir('empty'),
@@ -36,7 +36,7 @@ describe('experimental.instantInsights validationLevel normalization', () => {
       }
     )
     expect(config.experimental.instantInsights).toEqual({
-      validationLevel: 'manual-warning',
+      validationLevel: 'warning',
     })
   })