diff --git a/eslint_temporary_suppressions.js b/eslint_temporary_suppressions.js
index 2ed0c0edec..935d9de79c 100644
--- a/eslint_temporary_suppressions.js
+++ b/eslint_temporary_suppressions.js
@@ -707,6 +707,12 @@ export default [
       'n/no-missing-import': 'off',
     },
   },
+  {
+    files: ['packages/build/src/log/messages/lambda_compat_deprecation.ts'],
+    rules: {
+      'n/no-missing-import': 'off',
+    },
+  },
   {
     files: ['packages/build/src/log/messages/mutations.js'],
     rules: {
diff --git a/packages/build/src/core/build.ts b/packages/build/src/core/build.ts
index 2a43602e87..95f98474d2 100644
--- a/packages/build/src/core/build.ts
+++ b/packages/build/src/core/build.ts
@@ -704,6 +704,7 @@ const runBuild = async function ({
     explicitSecretKeys,
     enhancedSecretScan,
     edgeFunctionsBootstrapURL,
+    pluginsOptions,
   })
 
   return {
diff --git a/packages/build/src/log/messages/lambda_compat_deprecation.ts b/packages/build/src/log/messages/lambda_compat_deprecation.ts
new file mode 100644
index 0000000000..62acda3f78
--- /dev/null
+++ b/packages/build/src/log/messages/lambda_compat_deprecation.ts
@@ -0,0 +1,87 @@
+import path from 'path'
+
+import type { FunctionResult } from '@netlify/zip-it-and-ship-it'
+import semver from 'semver'
+
+import type { PluginsOptions } from '../../plugins/node_version.js'
+import { isRuntime } from '../../utils/runtime.js'
+import { log, logArray, logWarningSubHeader, type Logs } from '../logger.js'
+
+const LEGACY_NEXTJS_RUNTIME_RANGE = '4.x'
+
+interface LegacyNextjsRuntime {
+  name: string
+  version: string
+}
+
+const detectLegacyNextjsRuntime = (pluginsOptions: PluginsOptions[] | undefined): LegacyNextjsRuntime | null => {
+  const runtime = pluginsOptions?.find(isRuntime)
+  const version: unknown = runtime?.pluginPackageJson?.version
+
+  if (runtime && typeof version === 'string' && semver.satisfies(version, LEGACY_NEXTJS_RUNTIME_RANGE)) {
+    return { name: runtime.packageName, version }
+  }
+
+  return null
+}
+
+interface LogLambdaCompatibilityDeprecationOptions {
+  logs: Logs | undefined
+  results: FunctionResult[]
+  functionsSrc: string | undefined
+  pluginsOptions: PluginsOptions[] | undefined
+}
+
+/**
+ * Warns about JavaScript/TypeScript functions still using Lambda compatibility
+ * mode (V1) after bundling.
+ */
+export const logLambdaCompatibilityDeprecation = ({
+  logs,
+  results,
+  functionsSrc,
+  pluginsOptions,
+}: LogLambdaCompatibilityDeprecationOptions): void => {
+  const v1JsFunctions = results.filter((result) => result.runtime === 'js' && result.runtimeAPIVersion !== 2)
+
+  if (v1JsFunctions.length === 0) {
+    return
+  }
+
+  const userFns: string[] = []
+  const generatedFns: string[] = []
+
+  for (const fn of v1JsFunctions) {
+    if (functionsSrc && fn.mainFile.startsWith(`${functionsSrc}${path.sep}`)) {
+      userFns.push(fn.name)
+    } else {
+      generatedFns.push(fn.name)
+    }
+  }
+
+  logWarningSubHeader(
+    logs,
+    'Lambda compatibility mode is deprecated and will be removed on June 1, 2027. Refer to https://ntl.fyi/lambda-deprecate for details.',
+  )
+
+  if (userFns.length > 0) {
+    log(logs, '\n  These functions in your project use Lambda compatibility mode:')
+    logArray(logs, userFns)
+    log(logs, '\n  Migrate them to the modern Netlify Functions API. Refer to https://ntl.fyi/functions-migrate.')
+  }
+
+  if (generatedFns.length > 0) {
+    log(logs, '\n  These functions were generated by a framework adapter and use Lambda compatibility mode:')
+    logArray(logs, generatedFns)
+
+    const legacyNextjsRuntime = detectLegacyNextjsRuntime(pluginsOptions)
+    if (legacyNextjsRuntime) {
+      log(
+        logs,
+        `\n  Your project is using ${legacyNextjsRuntime.name} v${legacyNextjsRuntime.version}. Upgrade to the latest Netlify Next.js Adapter to emit modern Netlify Functions. Refer to https://ntl.fyi/nextjs-adapter-upgrade for upgrade guidance.`,
+      )
+    } else {
+      log(logs, '\n  Update the framework adapter to a version that emits modern Netlify Functions.')
+    }
+  }
+}
diff --git a/packages/build/src/plugins_core/functions/index.ts b/packages/build/src/plugins_core/functions/index.ts
index 34f80ebe6f..3dd7414ac4 100644
--- a/packages/build/src/plugins_core/functions/index.ts
+++ b/packages/build/src/plugins_core/functions/index.ts
@@ -7,6 +7,7 @@ import { addErrorInfo } from '../../error/info.js'
 import { log } from '../../log/logger.js'
 import { type GeneratedFunction, getGeneratedFunctions } from '../../steps/return_values.js'
 import { logBundleResults, logFunctionsNonExistingDir, logFunctionsToBundle } from '../../log/messages/core_steps.js'
+import { logLambdaCompatibilityDeprecation } from '../../log/messages/lambda_compat_deprecation.js'
 import { FRAMEWORKS_API_FUNCTIONS_PATH } from '../../utils/frameworks_api.js'
 
 import { getZipError } from './error.js'
@@ -77,6 +78,7 @@ const zipFunctionsAndLogResults = async ({
   repositoryRoot,
   userNodeVersion,
   systemLog,
+  pluginsOptions,
 }) => {
   const zisiParameters = getZisiParameters({
     branch,
@@ -115,6 +117,12 @@ const zipFunctionsAndLogResults = async ({
     const bundlers = Array.from(getBundlers(results))
 
     logBundleResults({ logs, results })
+    logLambdaCompatibilityDeprecation({
+      logs,
+      results,
+      functionsSrc,
+      pluginsOptions,
+    })
 
     return { bundlers }
   } catch (error) {
@@ -142,6 +150,7 @@ const coreStep = async function ({
   userNodeVersion,
   systemLog,
   returnValues,
+  pluginsOptions,
 }) {
   const functionsSrc = relativeFunctionsSrc === undefined ? undefined : resolve(buildDir, relativeFunctionsSrc)
   const functionsDist = resolve(buildDir, relativeFunctionsDist)
@@ -206,6 +215,7 @@ const coreStep = async function ({
     userNodeVersion,
     systemLog,
     generatedFunctions: generatedFunctions.map((func) => func.path),
+    pluginsOptions,
   })
 
   const metrics = getMetrics(internalFunctions, userFunctions)
diff --git a/packages/build/src/steps/core_step.ts b/packages/build/src/steps/core_step.ts
index e037783dd5..e29c7f089e 100644
--- a/packages/build/src/steps/core_step.ts
+++ b/packages/build/src/steps/core_step.ts
@@ -44,6 +44,7 @@ export const fireCoreStep = async function ({
   outputFlusher,
   api,
   returnValues,
+  pluginsOptions,
 }) {
   const logsA = outputFlusher ? addOutputFlusher(logs, outputFlusher) : logs
 
@@ -87,6 +88,7 @@ export const fireCoreStep = async function ({
       edgeFunctionsBootstrapURL,
       deployId,
       returnValues,
+      pluginsOptions,
     })
     const {
       netlifyConfig: netlifyConfigA,
diff --git a/packages/build/src/steps/run_step.ts b/packages/build/src/steps/run_step.ts
index 53c6c8c5b4..57a7fffd2e 100644
--- a/packages/build/src/steps/run_step.ts
+++ b/packages/build/src/steps/run_step.ts
@@ -73,6 +73,7 @@ export const runStep = async function ({
   edgeFunctionsBootstrapURL,
   extensionMetadata,
   returnValues,
+  pluginsOptions,
 }) {
   // Add relevant attributes to the upcoming span context
   const attributes: StepExecutionAttributes = {
@@ -195,6 +196,7 @@ export const runStep = async function ({
       api,
       returnValues,
       deployEnvVars,
+      pluginsOptions,
     })
 
     const newValues = await getStepReturn({
@@ -367,6 +369,7 @@ const tFireStep = function ({
   extensionMetadata,
   api,
   returnValues,
+  pluginsOptions,
 }) {
   if (coreStep !== undefined) {
     return fireCoreStep({
@@ -408,6 +411,7 @@ const tFireStep = function ({
       deployId,
       api,
       returnValues,
+      pluginsOptions,
     })
   }
 
diff --git a/packages/build/src/steps/run_steps.js b/packages/build/src/steps/run_steps.js
index 50a012dcc8..aa77b8c2c2 100644
--- a/packages/build/src/steps/run_steps.js
+++ b/packages/build/src/steps/run_steps.js
@@ -47,6 +47,7 @@ export const runSteps = async function ({
   explicitSecretKeys,
   enhancedSecretScan,
   edgeFunctionsBootstrapURL,
+  pluginsOptions,
 }) {
   const {
     index: stepsCount,
@@ -164,6 +165,7 @@ export const runSteps = async function ({
         explicitSecretKeys,
         enhancedSecretScan,
         edgeFunctionsBootstrapURL,
+        pluginsOptions,
       })
 
       const statusesA = addStatus({ newStatus, statuses, event, packageName, pluginPackageJson })
diff --git a/packages/build/tests/core/snapshots/tests.js.md b/packages/build/tests/core/snapshots/tests.js.md
index b960cebcb0..bfc6ebffd8 100644
--- a/packages/build/tests/core/snapshots/tests.js.md
+++ b/packages/build/tests/core/snapshots/tests.js.md
@@ -1292,6 +1292,13 @@ Generated by [AVA](https://avajs.dev).
      - test.js␊
     ␊
     ␊
+    > Lambda compatibility mode is deprecated and will be removed on June 1, 2027. Refer to https://ntl.fyi/lambda-deprecate for details.␊
+    ␊
+      These functions in your project use Lambda compatibility mode:␊
+       - test␊
+    ␊
+      Migrate them to the modern Netlify Functions API. Refer to https://ntl.fyi/functions-migrate.␊
+    ␊
     (Functions bundling completed in 1ms)␊
     Build step duration: Functions bundling completed in 1ms␊
     ␊
@@ -1761,6 +1768,14 @@ Generated by [AVA](https://avajs.dev).
      - function_two.js␊
     ␊
     ␊
+    > Lambda compatibility mode is deprecated and will be removed on June 1, 2027. Refer to https://ntl.fyi/lambda-deprecate for details.␊
+    ␊
+      These functions in your project use Lambda compatibility mode:␊
+       - function_one␊
+       - function_two␊
+    ␊
+      Migrate them to the modern Netlify Functions API. Refer to https://ntl.fyi/functions-migrate.␊
+    ␊
     (Functions bundling completed in 1ms)␊
     Build step duration: Functions bundling completed in 1ms␊
     ␊
@@ -1827,6 +1842,13 @@ Generated by [AVA](https://avajs.dev).
      - function_one.js␊
     ␊
     ␊
+    > Lambda compatibility mode is deprecated and will be removed on June 1, 2027. Refer to https://ntl.fyi/lambda-deprecate for details.␊
+    ␊
+      These functions in your project use Lambda compatibility mode:␊
+       - function_one␊
+    ␊
+      Migrate them to the modern Netlify Functions API. Refer to https://ntl.fyi/functions-migrate.␊
+    ␊
     (Functions bundling completed in 1ms)␊
     Build step duration: Functions bundling completed in 1ms␊
     ␊
@@ -1966,6 +1988,15 @@ Generated by [AVA](https://avajs.dev).
        - function_with_dynamic_require.zip␊
        - function_with_native.zip␊
     ␊
+    > Lambda compatibility mode is deprecated and will be removed on June 1, 2027. Refer to https://ntl.fyi/lambda-deprecate for details.␊
+    ␊
+      These functions in your project use Lambda compatibility mode:␊
+       - function_simple␊
+       - function_with_dynamic_require␊
+       - function_with_native␊
+    ␊
+      Migrate them to the modern Netlify Functions API. Refer to https://ntl.fyi/functions-migrate.␊
+    ␊
     (Functions bundling completed in 1ms)␊
     Build step duration: Functions bundling completed in 1ms␊
     ␊
@@ -2021,6 +2052,13 @@ Generated by [AVA](https://avajs.dev).
     > Failed to bundle functions with selected bundler (fallback used):␊
        - function_with_module.zip␊
     ␊
+    > Lambda compatibility mode is deprecated and will be removed on June 1, 2027. Refer to https://ntl.fyi/lambda-deprecate for details.␊
+    ␊
+      These functions in your project use Lambda compatibility mode:␊
+       - function_with_module␊
+    ␊
+      Migrate them to the modern Netlify Functions API. Refer to https://ntl.fyi/functions-migrate.␊
+    ␊
     (Functions bundling completed in 1ms)␊
     Build step duration: Functions bundling completed in 1ms␊
     ␊
diff --git a/packages/build/tests/core/snapshots/tests.js.snap b/packages/build/tests/core/snapshots/tests.js.snap
index 82c05655b4..8e7fbc8e27 100644
Binary files a/packages/build/tests/core/snapshots/tests.js.snap and b/packages/build/tests/core/snapshots/tests.js.snap differ
diff --git a/packages/build/tests/functions/snapshots/tests.js.md b/packages/build/tests/functions/snapshots/tests.js.md
index c3d44ca742..89546cc897 100644
--- a/packages/build/tests/functions/snapshots/tests.js.md
+++ b/packages/build/tests/functions/snapshots/tests.js.md
@@ -136,6 +136,13 @@ Generated by [AVA](https://avajs.dev).
      - test.js␊
     ␊
     ␊
+    > Lambda compatibility mode is deprecated and will be removed on June 1, 2027. Refer to https://ntl.fyi/lambda-deprecate for details.␊
+    ␊
+      These functions in your project use Lambda compatibility mode:␊
+       - test␊
+    ␊
+      Migrate them to the modern Netlify Functions API. Refer to https://ntl.fyi/functions-migrate.␊
+    ␊
     (Functions bundling completed in 1ms)␊
     Build step duration: Functions bundling completed in 1ms␊
     ␊
@@ -223,6 +230,13 @@ Generated by [AVA](https://avajs.dev).
      - test.js␊
     ␊
     ␊
+    > Lambda compatibility mode is deprecated and will be removed on June 1, 2027. Refer to https://ntl.fyi/lambda-deprecate for details.␊
+    ␊
+      These functions in your project use Lambda compatibility mode:␊
+       - test␊
+    ␊
+      Migrate them to the modern Netlify Functions API. Refer to https://ntl.fyi/functions-migrate.␊
+    ␊
     (Functions bundling completed in 1ms)␊
     Build step duration: Functions bundling completed in 1ms␊
     ␊
@@ -321,6 +335,13 @@ Generated by [AVA](https://avajs.dev).
      - test.js␊
     ␊
     ␊
+    > Lambda compatibility mode is deprecated and will be removed on June 1, 2027. Refer to https://ntl.fyi/lambda-deprecate for details.␊
+    ␊
+      These functions in your project use Lambda compatibility mode:␊
+       - test␊
+    ␊
+      Migrate them to the modern Netlify Functions API. Refer to https://ntl.fyi/functions-migrate.␊
+    ␊
     (Functions bundling completed in 1ms)␊
     Build step duration: Functions bundling completed in 1ms␊
     ␊
diff --git a/packages/build/tests/functions/snapshots/tests.js.snap b/packages/build/tests/functions/snapshots/tests.js.snap
index d0bfc0a82f..655f268556 100644
Binary files a/packages/build/tests/functions/snapshots/tests.js.snap and b/packages/build/tests/functions/snapshots/tests.js.snap differ
diff --git a/packages/build/tests/install/snapshots/tests.js.md b/packages/build/tests/install/snapshots/tests.js.md
index ceb36508bc..561cbc656f 100644
--- a/packages/build/tests/install/snapshots/tests.js.md
+++ b/packages/build/tests/install/snapshots/tests.js.md
@@ -56,6 +56,13 @@ Generated by [AVA](https://avajs.dev).
      - function/index.js␊
     ␊
     ␊
+    > Lambda compatibility mode is deprecated and will be removed on June 1, 2027. Refer to https://ntl.fyi/lambda-deprecate for details.␊
+    ␊
+      These functions in your project use Lambda compatibility mode:␊
+       - function␊
+    ␊
+      Migrate them to the modern Netlify Functions API. Refer to https://ntl.fyi/functions-migrate.␊
+    ␊
     (Functions bundling completed in 1ms)␊
     Build step duration: Functions bundling completed in 1ms␊
     ␊
@@ -110,6 +117,13 @@ Generated by [AVA](https://avajs.dev).
      - test.js␊
     ␊
     ␊
+    > Lambda compatibility mode is deprecated and will be removed on June 1, 2027. Refer to https://ntl.fyi/lambda-deprecate for details.␊
+    ␊
+      These functions in your project use Lambda compatibility mode:␊
+       - test␊
+    ␊
+      Migrate them to the modern Netlify Functions API. Refer to https://ntl.fyi/functions-migrate.␊
+    ␊
     (Functions bundling completed in 1ms)␊
     Build step duration: Functions bundling completed in 1ms␊
     ␊
@@ -171,6 +185,13 @@ Generated by [AVA](https://avajs.dev).
      - index.js␊
     ␊
     ␊
+    > Lambda compatibility mode is deprecated and will be removed on June 1, 2027. Refer to https://ntl.fyi/lambda-deprecate for details.␊
+    ␊
+      These functions in your project use Lambda compatibility mode:␊
+       - index␊
+    ␊
+      Migrate them to the modern Netlify Functions API. Refer to https://ntl.fyi/functions-migrate.␊
+    ␊
     (Functions bundling completed in 1ms)␊
     Build step duration: Functions bundling completed in 1ms␊
     ␊
@@ -232,6 +253,13 @@ Generated by [AVA](https://avajs.dev).
      - index.js␊
     ␊
     ␊
+    > Lambda compatibility mode is deprecated and will be removed on June 1, 2027. Refer to https://ntl.fyi/lambda-deprecate for details.␊
+    ␊
+      These functions in your project use Lambda compatibility mode:␊
+       - index␊
+    ␊
+      Migrate them to the modern Netlify Functions API. Refer to https://ntl.fyi/functions-migrate.␊
+    ␊
     (Functions bundling completed in 1ms)␊
     Build step duration: Functions bundling completed in 1ms␊
     ␊
@@ -295,6 +323,13 @@ Generated by [AVA](https://avajs.dev).
      - index.js␊
     ␊
     ␊
+    > Lambda compatibility mode is deprecated and will be removed on June 1, 2027. Refer to https://ntl.fyi/lambda-deprecate for details.␊
+    ␊
+      These functions in your project use Lambda compatibility mode:␊
+       - index␊
+    ␊
+      Migrate them to the modern Netlify Functions API. Refer to https://ntl.fyi/functions-migrate.␊
+    ␊
     (Functions bundling completed in 1ms)␊
     Build step duration: Functions bundling completed in 1ms␊
     ␊
@@ -344,6 +379,13 @@ Generated by [AVA](https://avajs.dev).
      - index.js␊
     ␊
     ␊
+    > Lambda compatibility mode is deprecated and will be removed on June 1, 2027. Refer to https://ntl.fyi/lambda-deprecate for details.␊
+    ␊
+      These functions in your project use Lambda compatibility mode:␊
+       - index␊
+    ␊
+      Migrate them to the modern Netlify Functions API. Refer to https://ntl.fyi/functions-migrate.␊
+    ␊
     (Functions bundling completed in 1ms)␊
     Build step duration: Functions bundling completed in 1ms␊
     ␊
@@ -534,6 +576,13 @@ Generated by [AVA](https://avajs.dev).
      - index.js␊
     ␊
     ␊
+    > Lambda compatibility mode is deprecated and will be removed on June 1, 2027. Refer to https://ntl.fyi/lambda-deprecate for details.␊
+    ␊
+      These functions in your project use Lambda compatibility mode:␊
+       - index␊
+    ␊
+      Migrate them to the modern Netlify Functions API. Refer to https://ntl.fyi/functions-migrate.␊
+    ␊
     (Functions bundling completed in 1ms)␊
     Build step duration: Functions bundling completed in 1ms␊
     ␊
@@ -583,6 +632,13 @@ Generated by [AVA](https://avajs.dev).
      - index.js␊
     ␊
     ␊
+    > Lambda compatibility mode is deprecated and will be removed on June 1, 2027. Refer to https://ntl.fyi/lambda-deprecate for details.␊
+    ␊
+      These functions in your project use Lambda compatibility mode:␊
+       - index␊
+    ␊
+      Migrate them to the modern Netlify Functions API. Refer to https://ntl.fyi/functions-migrate.␊
+    ␊
     (Functions bundling completed in 1ms)␊
     Build step duration: Functions bundling completed in 1ms␊
     ␊
diff --git a/packages/build/tests/install/snapshots/tests.js.snap b/packages/build/tests/install/snapshots/tests.js.snap
index f205564e3d..53ba03010a 100644
Binary files a/packages/build/tests/install/snapshots/tests.js.snap and b/packages/build/tests/install/snapshots/tests.js.snap differ
diff --git a/packages/build/tests/plugins/snapshots/tests.js.md b/packages/build/tests/plugins/snapshots/tests.js.md
index bf32664143..8e97b511bc 100644
--- a/packages/build/tests/plugins/snapshots/tests.js.md
+++ b/packages/build/tests/plugins/snapshots/tests.js.md
@@ -1809,6 +1809,13 @@ Generated by [AVA](https://avajs.dev).
      - test/test.js␊
     ␊
     ␊
+    > Lambda compatibility mode is deprecated and will be removed on June 1, 2027. Refer to https://ntl.fyi/lambda-deprecate for details.␊
+    ␊
+      These functions were generated by a framework adapter and use Lambda compatibility mode:␊
+       - test␊
+    ␊
+      Update the framework adapter to a version that emits modern Netlify Functions.␊
+    ␊
     (Functions bundling completed in 1ms)␊
     Build step duration: Functions bundling completed in 1ms␊
     ␊
@@ -1955,6 +1962,18 @@ Generated by [AVA](https://avajs.dev).
      - user_1.js␊
     ␊
     ␊
+    > Lambda compatibility mode is deprecated and will be removed on June 1, 2027. Refer to https://ntl.fyi/lambda-deprecate for details.␊
+    ␊
+      These functions in your project use Lambda compatibility mode:␊
+       - user_1␊
+    ␊
+      Migrate them to the modern Netlify Functions API. Refer to https://ntl.fyi/functions-migrate.␊
+    ␊
+      These functions were generated by a framework adapter and use Lambda compatibility mode:␊
+       - internal_1␊
+    ␊
+      Update the framework adapter to a version that emits modern Netlify Functions.␊
+    ␊
     (Functions bundling completed in 1ms)␊
     Build step duration: Functions bundling completed in 1ms␊
     ␊
diff --git a/packages/build/tests/plugins/snapshots/tests.js.snap b/packages/build/tests/plugins/snapshots/tests.js.snap
index 27ccf55412..b9dc6880ce 100644
Binary files a/packages/build/tests/plugins/snapshots/tests.js.snap and b/packages/build/tests/plugins/snapshots/tests.js.snap differ