diff --git a/.github/workflows/test-kmp.yml b/.github/workflows/test-kmp.yml
new file mode 100644
index 000000000000..62a192367d93
--- /dev/null
+++ b/.github/workflows/test-kmp.yml
@@ -0,0 +1,89 @@
+name: Test Kotlin Multiplatform foundation
+
+on:
+  workflow_dispatch:
+  pull_request:
+    paths:
+      - '.github/workflows/test-kmp.yml'
+      - '.github/actions/setup-gradle/**'
+      - '.github/actions/setup-xcode/**'
+      - 'packages/react-native/ReactShared/**'
+  push:
+    branches:
+      - main
+      - '*-stable'
+    paths:
+      - '.github/workflows/test-kmp.yml'
+      - '.github/actions/setup-gradle/**'
+      - '.github/actions/setup-xcode/**'
+      - 'packages/react-native/ReactShared/**'
+
+permissions:
+  contents: read
+
+concurrency:
+  group: kmp-${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  smoke:
+    runs-on: macos-26
+    timeout-minutes: 45
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v6
+      - name: Set up JDK 17
+        uses: actions/setup-java@v5
+        with:
+          java-version: '17'
+          distribution: 'zulu'
+      - name: Set up Xcode
+        uses: ./.github/actions/setup-xcode
+        with:
+          xcode-version: '26.4.1'
+      - name: Set up Gradle
+        uses: ./.github/actions/setup-gradle
+      - name: Cache Kotlin Native toolchain
+        uses: actions/cache@v5
+        with:
+          path: ~/.konan
+          key: kmp-foundation-${{ runner.os }}-${{ runner.arch }}-${{ hashFiles('packages/react-native/ReactShared/*.gradle.kts', 'packages/react-native/ReactShared/gradle.properties', 'packages/react-native/ReactShared/gradle/wrapper/gradle-wrapper.properties') }}
+      - name: Test common code and build Apple frameworks
+        working-directory: packages/react-native/ReactShared
+        run: >-
+          ./gradlew -PreactNativeSharedSmoke=true
+          jvmTest iosSimulatorArm64Test
+          linkDebugFrameworkIosArm64 linkReleaseFrameworkIosArm64
+          linkDebugFrameworkIosSimulatorArm64 linkReleaseFrameworkIosSimulatorArm64
+          linkDebugFrameworkIosX64 linkReleaseFrameworkIosX64
+          --max-workers=2 --stacktrace --console=plain
+      - name: Test Objective-C consumption
+        working-directory: packages/react-native/ReactShared
+        run: ./scripts/test-apple-smoke.sh
+      - name: Verify nonempty tests and fixture isolation
+        working-directory: packages/react-native/ReactShared
+        run: |
+          ./gradlew jvmJar --max-workers=2 --console=plain
+          python3 - <<'PY'
+          from pathlib import Path
+          import xml.etree.ElementTree as ET
+          import zipfile
+          for target in ('jvmTest', 'iosSimulatorArm64Test'):
+              suites = [ET.parse(p).getroot() for p in Path('build/smoke/test-results', target).glob('TEST-*.xml')]
+              assert suites and sum(int(s.get('tests', 0)) for s in suites) > 0, target
+              assert all(not int(s.get(k, 0)) for s in suites for k in ('failures', 'errors', 'skipped')), target
+          jars = list(Path('build/libs').glob('*.jar'))
+          assert len(jars) == 1, jars
+          with zipfile.ZipFile(jars[0]) as jar:
+              assert not any('/smoke/' in name for name in jar.namelist()), 'Test fixture leaked into default output'
+          PY
+      - name: Upload smoke reports
+        if: always()
+        uses: actions/upload-artifact@v6
+        with:
+          name: kotlin-multiplatform-smoke-results
+          path: |
+            packages/react-native/ReactShared/build/smoke/reports/tests
+            packages/react-native/ReactShared/build/smoke/test-results
+            packages/react-native/ReactShared/build/smoke/apple-interop/*.log
+          if-no-files-found: warn
diff --git a/packages/react-native/ReactShared/.gitignore b/packages/react-native/ReactShared/.gitignore
new file mode 100644
index 000000000000..5d8581f71fc8
--- /dev/null
+++ b/packages/react-native/ReactShared/.gitignore
@@ -0,0 +1,3 @@
+/build/
+/.gradle/
+/.kotlin/
diff --git a/packages/react-native/ReactShared/README.md b/packages/react-native/ReactShared/README.md
new file mode 100644
index 000000000000..3bf5e0b0ca78
--- /dev/null
+++ b/packages/react-native/ReactShared/README.md
@@ -0,0 +1,43 @@
+# Kotlin Multiplatform foundation
+
+This standalone build provides a place to assess sharing library code between
+the JVM and iOS with Kotlin Multiplatform. It has no Compose or other UI dependency.
+No React Native application, Android artifact, CocoaPods target, or npm package
+consumes this module. Normal React Native builds and runtime behavior are unchanged.
+
+The build has no production sources yet. Its small arithmetic fixture is enabled
+only by `-PreactNativeSharedSmoke=true`, and its outputs go under `build/smoke`.
+The fixture checks the compiler, common tests and Objective-C integer interop;
+it is not a proposed React Native API.
+
+## Validation
+
+Use JDK 17. Apple validation also requires macOS, Xcode and an installed iOS
+simulator. The separate Gradle wrapper isolates the Kotlin/Native plugin from the
+main Android build. CI selects Xcode 26.4.1 and uses an Apple Silicon runner.
+
+From this directory:
+
+```sh
+./gradlew -PreactNativeSharedSmoke=true jvmTest iosSimulatorArm64Test \
+  linkDebugFrameworkIosArm64 linkReleaseFrameworkIosArm64 \
+  linkDebugFrameworkIosSimulatorArm64 linkReleaseFrameworkIosSimulatorArm64 \
+  linkDebugFrameworkIosX64 linkReleaseFrameworkIosX64 \
+  --max-workers=2 --console=plain
+./scripts/test-apple-smoke.sh
+```
+
+The native runner checks Debug and Release frameworks through Objective-C on the
+host simulator. Set `RCT_KMP_SIMULATOR_UDID` to select an existing simulator.
+Intel simulator frameworks are built; Intel execution and physical-device tests
+require their respective hosts and are not part of this CI job.
+
+## Adoption is separate
+
+Each use case needs its own behavior, dependency, packaging and performance
+review before any application consumes shared code. Earlier gradient experiments
+measured additional adapter latency, allocation and Kotlin/Native runtime cost;
+this foundation does not establish that those costs are acceptable. Android AAR
+embedding, Apple runtime ownership, CocoaPods, SwiftPM and prebuilt distribution
+belong to those follow-ups. No Apple consumer or framework is designated as the
+runtime owner here.
diff --git a/packages/react-native/ReactShared/build.gradle.kts b/packages/react-native/ReactShared/build.gradle.kts
new file mode 100644
index 000000000000..34ae7aff89ac
--- /dev/null
+++ b/packages/react-native/ReactShared/build.gradle.kts
@@ -0,0 +1,55 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import org.jetbrains.kotlin.gradle.dsl.JvmTarget
+import org.jetbrains.kotlin.gradle.dsl.KotlinVersion
+import org.jetbrains.kotlin.gradle.plugin.mpp.KotlinNativeTarget
+
+plugins { kotlin("multiplatform") version "2.4.20" }
+
+group = "com.facebook.react"
+
+version = "0.0.0-local"
+
+val smokeEnabled =
+    providers.gradleProperty("reactNativeSharedSmoke").map { it.toBooleanStrict() }.getOrElse(false)
+
+if (smokeEnabled) {
+  // Keep test-only exports separate from any future production artifacts.
+  layout.buildDirectory.set(layout.projectDirectory.dir("build/smoke"))
+}
+
+kotlin {
+  explicitApi()
+  jvmToolchain(17)
+  compilerOptions {
+    // Preserve compatibility with the repository's existing Kotlin consumers.
+    languageVersion.set(KotlinVersion.KOTLIN_2_2)
+    apiVersion.set(KotlinVersion.KOTLIN_2_2)
+  }
+  jvm { compilerOptions { jvmTarget.set(JvmTarget.JVM_17) } }
+  iosArm64()
+  iosSimulatorArm64()
+  iosX64()
+
+  targets.withType<KotlinNativeTarget>().configureEach {
+    binaries.framework {
+      baseName = "ReactNativeShared"
+      isStatic = true
+      binaryOption("bundleId", "com.facebook.react.shared")
+    }
+  }
+
+  sourceSets {
+    commonMain.dependencies { implementation(kotlin("stdlib", "2.2.0")) }
+    commonTest.dependencies { implementation(kotlin("test")) }
+    if (smokeEnabled) {
+      commonMain { kotlin.srcDir("tests/smoke/kotlin") }
+      commonTest { kotlin.srcDir("tests/smoke/kotlinTest") }
+    }
+  }
+}
diff --git a/packages/react-native/ReactShared/gradle.properties b/packages/react-native/ReactShared/gradle.properties
new file mode 100644
index 000000000000..e8b989142732
--- /dev/null
+++ b/packages/react-native/ReactShared/gradle.properties
@@ -0,0 +1,3 @@
+org.gradle.jvmargs=-Xmx2g -XX:MaxMetaspaceSize=768m -Dfile.encoding=UTF-8
+org.gradle.caching=true
+kotlin.stdlib.default.dependency=false
diff --git a/packages/react-native/ReactShared/gradle/wrapper/gradle-wrapper.jar b/packages/react-native/ReactShared/gradle/wrapper/gradle-wrapper.jar
new file mode 100644
index 000000000000..d997cfc60f4c
Binary files /dev/null and b/packages/react-native/ReactShared/gradle/wrapper/gradle-wrapper.jar differ
diff --git a/packages/react-native/ReactShared/gradle/wrapper/gradle-wrapper.properties b/packages/react-native/ReactShared/gradle/wrapper/gradle-wrapper.properties
new file mode 100644
index 000000000000..80d82296e83e
--- /dev/null
+++ b/packages/react-native/ReactShared/gradle/wrapper/gradle-wrapper.properties
@@ -0,0 +1,8 @@
+distributionBase=GRADLE_USER_HOME
+distributionPath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-9.4.1-bin.zip
+distributionSha256Sum=2ab2958f2a1e51120c326cad6f385153bb11ee93b3c216c5fccebfdfbb7ec6cb
+networkTimeout=10000
+validateDistributionUrl=true
+zipStoreBase=GRADLE_USER_HOME
+zipStorePath=wrapper/dists
diff --git a/packages/react-native/ReactShared/gradlew b/packages/react-native/ReactShared/gradlew
new file mode 100755
index 000000000000..739907dfd159
--- /dev/null
+++ b/packages/react-native/ReactShared/gradlew
@@ -0,0 +1,248 @@
+#!/bin/sh
+
+#
+# Copyright © 2015 the original authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+# SPDX-License-Identifier: Apache-2.0
+#
+
+##############################################################################
+#
+#   Gradle start up script for POSIX generated by Gradle.
+#
+#   Important for running:
+#
+#   (1) You need a POSIX-compliant shell to run this script. If your /bin/sh is
+#       noncompliant, but you have some other compliant shell such as ksh or
+#       bash, then to run this script, type that shell name before the whole
+#       command line, like:
+#
+#           ksh Gradle
+#
+#       Busybox and similar reduced shells will NOT work, because this script
+#       requires all of these POSIX shell features:
+#         * functions;
+#         * expansions «$var», «${var}», «${var:-default}», «${var+SET}»,
+#           «${var#prefix}», «${var%suffix}», and «$( cmd )»;
+#         * compound commands having a testable exit status, especially «case»;
+#         * various built-in commands including «command», «set», and «ulimit».
+#
+#   Important for patching:
+#
+#   (2) This script targets any POSIX shell, so it avoids extensions provided
+#       by Bash, Ksh, etc; in particular arrays are avoided.
+#
+#       The "traditional" practice of packing multiple parameters into a
+#       space-separated string is a well documented source of bugs and security
+#       problems, so this is (mostly) avoided, by progressively accumulating
+#       options in "$@", and eventually passing that to Java.
+#
+#       Where the inherited environment variables (DEFAULT_JVM_OPTS, JAVA_OPTS,
+#       and GRADLE_OPTS) rely on word-splitting, this is performed explicitly;
+#       see the in-line comments for details.
+#
+#       There are tweaks for specific operating systems such as AIX, CygWin,
+#       Darwin, MinGW, and NonStop.
+#
+#   (3) This script is generated from the Groovy template
+#       https://github.com/gradle/gradle/blob/2d6327017519d23b96af35865dc997fcb544fb40/platforms/jvm/plugins-application/src/main/resources/org/gradle/api/internal/plugins/unixStartScript.txt
+#       within the Gradle project.
+#
+#       You can find Gradle at https://github.com/gradle/gradle/.
+#
+##############################################################################
+
+# Attempt to set APP_HOME
+
+# Resolve links: $0 may be a link
+app_path=$0
+
+# Need this for daisy-chained symlinks.
+while
+    APP_HOME=${app_path%"${app_path##*/}"}  # leaves a trailing /; empty if no leading path
+    [ -h "$app_path" ]
+do
+    ls=$( ls -ld "$app_path" )
+    link=${ls#*' -> '}
+    case $link in             #(
+      /*)   app_path=$link ;; #(
+      *)    app_path=$APP_HOME$link ;;
+    esac
+done
+
+# This is normally unused
+# shellcheck disable=SC2034
+APP_BASE_NAME=${0##*/}
+# Discard cd standard output in case $CDPATH is set (https://github.com/gradle/gradle/issues/25036)
+APP_HOME=$( cd -P "${APP_HOME:-./}" > /dev/null && printf '%s\n' "$PWD" ) || exit
+
+# Use the maximum available, or set MAX_FD != -1 to use that value.
+MAX_FD=maximum
+
+warn () {
+    echo "$*"
+} >&2
+
+die () {
+    echo
+    echo "$*"
+    echo
+    exit 1
+} >&2
+
+# OS specific support (must be 'true' or 'false').
+cygwin=false
+msys=false
+darwin=false
+nonstop=false
+case "$( uname )" in                #(
+  CYGWIN* )         cygwin=true  ;; #(
+  Darwin* )         darwin=true  ;; #(
+  MSYS* | MINGW* )  msys=true    ;; #(
+  NONSTOP* )        nonstop=true ;;
+esac
+
+
+
+# Determine the Java command to use to start the JVM.
+if [ -n "$JAVA_HOME" ] ; then
+    if [ -x "$JAVA_HOME/jre/sh/java" ] ; then
+        # IBM's JDK on AIX uses strange locations for the executables
+        JAVACMD=$JAVA_HOME/jre/sh/java
+    else
+        JAVACMD=$JAVA_HOME/bin/java
+    fi
+    if [ ! -x "$JAVACMD" ] ; then
+        die "ERROR: JAVA_HOME is set to an invalid directory: $JAVA_HOME
+
+Please set the JAVA_HOME variable in your environment to match the
+location of your Java installation."
+    fi
+else
+    JAVACMD=java
+    if ! command -v java >/dev/null 2>&1
+    then
+        die "ERROR: JAVA_HOME is not set and no 'java' command could be found in your PATH.
+
+Please set the JAVA_HOME variable in your environment to match the
+location of your Java installation."
+    fi
+fi
+
+# Increase the maximum file descriptors if we can.
+if ! "$cygwin" && ! "$darwin" && ! "$nonstop" ; then
+    case $MAX_FD in #(
+      max*)
+        # In POSIX sh, ulimit -H is undefined. That's why the result is checked to see if it worked.
+        # shellcheck disable=SC2039,SC3045
+        MAX_FD=$( ulimit -H -n ) ||
+            warn "Could not query maximum file descriptor limit"
+    esac
+    case $MAX_FD in  #(
+      '' | soft) :;; #(
+      *)
+        # In POSIX sh, ulimit -n is undefined. That's why the result is checked to see if it worked.
+        # shellcheck disable=SC2039,SC3045
+        ulimit -n "$MAX_FD" ||
+            warn "Could not set maximum file descriptor limit to $MAX_FD"
+    esac
+fi
+
+# Collect all arguments for the java command, stacking in reverse order:
+#   * args from the command line
+#   * the main class name
+#   * -classpath
+#   * -D...appname settings
+#   * --module-path (only if needed)
+#   * DEFAULT_JVM_OPTS, JAVA_OPTS, and GRADLE_OPTS environment variables.
+
+# For Cygwin or MSYS, switch paths to Windows format before running java
+if "$cygwin" || "$msys" ; then
+    APP_HOME=$( cygpath --path --mixed "$APP_HOME" )
+
+    JAVACMD=$( cygpath --unix "$JAVACMD" )
+
+    # Now convert the arguments - kludge to limit ourselves to /bin/sh
+    for arg do
+        if
+            case $arg in                                #(
+              -*)   false ;;                            # don't mess with options #(
+              /?*)  t=${arg#/} t=/${t%%/*}              # looks like a POSIX filepath
+                    [ -e "$t" ] ;;                      #(
+              *)    false ;;
+            esac
+        then
+            arg=$( cygpath --path --ignore --mixed "$arg" )
+        fi
+        # Roll the args list around exactly as many times as the number of
+        # args, so each arg winds up back in the position where it started, but
+        # possibly modified.
+        #
+        # NB: a `for` loop captures its iteration list before it begins, so
+        # changing the positional parameters here affects neither the number of
+        # iterations, nor the values presented in `arg`.
+        shift                   # remove old arg
+        set -- "$@" "$arg"      # push replacement arg
+    done
+fi
+
+
+# Add default JVM options here. You can also use JAVA_OPTS and GRADLE_OPTS to pass JVM options to this script.
+DEFAULT_JVM_OPTS='"-Xmx64m" "-Xms64m"'
+
+# Collect all arguments for the java command:
+#   * DEFAULT_JVM_OPTS, JAVA_OPTS, and optsEnvironmentVar are not allowed to contain shell fragments,
+#     and any embedded shellness will be escaped.
+#   * For example: A user cannot expect ${Hostname} to be expanded, as it is an environment variable and will be
+#     treated as '${Hostname}' itself on the command line.
+
+set -- \
+        "-Dorg.gradle.appname=$APP_BASE_NAME" \
+        -jar "$APP_HOME/gradle/wrapper/gradle-wrapper.jar" \
+        "$@"
+
+# Stop when "xargs" is not available.
+if ! command -v xargs >/dev/null 2>&1
+then
+    die "xargs is not available"
+fi
+
+# Use "xargs" to parse quoted args.
+#
+# With -n1 it outputs one arg per line, with the quotes and backslashes removed.
+#
+# In Bash we could simply go:
+#
+#   readarray ARGS < <( xargs -n1 <<<"$var" ) &&
+#   set -- "${ARGS[@]}" "$@"
+#
+# but POSIX shell has neither arrays nor command substitution, so instead we
+# post-process each arg (as a line of input to sed) to backslash-escape any
+# character that might be a shell metacharacter, then use eval to reverse
+# that process (while maintaining the separation between arguments), and wrap
+# the whole thing up as a single "set" statement.
+#
+# This will of course break if any of these variables contains a newline or
+# an unmatched quote.
+#
+
+eval "set -- $(
+        printf '%s\n' "$DEFAULT_JVM_OPTS $JAVA_OPTS $GRADLE_OPTS" |
+        xargs -n1 |
+        sed ' s~[^-[:alnum:]+,./:=@_]~\\&~g; ' |
+        tr '\n' ' '
+    )" '"$@"'
+
+exec "$JAVACMD" "$@"
diff --git a/packages/react-native/ReactShared/gradlew.bat b/packages/react-native/ReactShared/gradlew.bat
new file mode 100644
index 000000000000..c4bdd3ab8e3c
--- /dev/null
+++ b/packages/react-native/ReactShared/gradlew.bat
@@ -0,0 +1,93 @@
+@rem
+@rem Copyright 2015 the original author or authors.
+@rem
+@rem Licensed under the Apache License, Version 2.0 (the "License");
+@rem you may not use this file except in compliance with the License.
+@rem You may obtain a copy of the License at
+@rem
+@rem      https://www.apache.org/licenses/LICENSE-2.0
+@rem
+@rem Unless required by applicable law or agreed to in writing, software
+@rem distributed under the License is distributed on an "AS IS" BASIS,
+@rem WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+@rem See the License for the specific language governing permissions and
+@rem limitations under the License.
+@rem
+@rem SPDX-License-Identifier: Apache-2.0
+@rem
+
+@if "%DEBUG%"=="" @echo off
+@rem ##########################################################################
+@rem
+@rem  Gradle startup script for Windows
+@rem
+@rem ##########################################################################
+
+@rem Set local scope for the variables with windows NT shell
+if "%OS%"=="Windows_NT" setlocal
+
+set DIRNAME=%~dp0
+if "%DIRNAME%"=="" set DIRNAME=.
+@rem This is normally unused
+set APP_BASE_NAME=%~n0
+set APP_HOME=%DIRNAME%
+
+@rem Resolve any "." and ".." in APP_HOME to make it shorter.
+for %%i in ("%APP_HOME%") do set APP_HOME=%%~fi
+
+@rem Add default JVM options here. You can also use JAVA_OPTS and GRADLE_OPTS to pass JVM options to this script.
+set DEFAULT_JVM_OPTS="-Xmx64m" "-Xms64m"
+
+@rem Find java.exe
+if defined JAVA_HOME goto findJavaFromJavaHome
+
+set JAVA_EXE=java.exe
+%JAVA_EXE% -version >NUL 2>&1
+if %ERRORLEVEL% equ 0 goto execute
+
+echo. 1>&2
+echo ERROR: JAVA_HOME is not set and no 'java' command could be found in your PATH. 1>&2
+echo. 1>&2
+echo Please set the JAVA_HOME variable in your environment to match the 1>&2
+echo location of your Java installation. 1>&2
+
+goto fail
+
+:findJavaFromJavaHome
+set JAVA_HOME=%JAVA_HOME:"=%
+set JAVA_EXE=%JAVA_HOME%/bin/java.exe
+
+if exist "%JAVA_EXE%" goto execute
+
+echo. 1>&2
+echo ERROR: JAVA_HOME is set to an invalid directory: %JAVA_HOME% 1>&2
+echo. 1>&2
+echo Please set the JAVA_HOME variable in your environment to match the 1>&2
+echo location of your Java installation. 1>&2
+
+goto fail
+
+:execute
+@rem Setup the command line
+
+
+
+@rem Execute Gradle
+"%JAVA_EXE%" %DEFAULT_JVM_OPTS% %JAVA_OPTS% %GRADLE_OPTS% "-Dorg.gradle.appname=%APP_BASE_NAME%" -jar "%APP_HOME%\gradle\wrapper\gradle-wrapper.jar" %*
+
+:end
+@rem End local scope for the variables with windows NT shell
+if %ERRORLEVEL% equ 0 goto mainEnd
+
+:fail
+rem Set variable GRADLE_EXIT_CONSOLE if you need the _script_ return code instead of
+rem the _cmd.exe /c_ return code!
+set EXIT_CODE=%ERRORLEVEL%
+if %EXIT_CODE% equ 0 set EXIT_CODE=1
+if not ""=="%GRADLE_EXIT_CONSOLE%" exit %EXIT_CODE%
+exit /b %EXIT_CODE%
+
+:mainEnd
+if "%OS%"=="Windows_NT" endlocal
+
+:omega
diff --git a/packages/react-native/ReactShared/scripts/test-apple-smoke.sh b/packages/react-native/ReactShared/scripts/test-apple-smoke.sh
new file mode 100755
index 000000000000..81ed4afe4c4e
--- /dev/null
+++ b/packages/react-native/ReactShared/scripts/test-apple-smoke.sh
@@ -0,0 +1,51 @@
+#!/bin/bash
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# This source code is licensed under the MIT license found in the
+# LICENSE file in the root directory of this source tree.
+
+set -euo pipefail
+
+shared_root="$(cd "$(dirname "$0")/.." && pwd)"
+architecture="$(uname -m)"
+case "$architecture" in
+  arm64) target=IosSimulatorArm64; kotlin_target=iosSimulatorArm64 ;;
+  x86_64) target=IosX64; kotlin_target=iosX64 ;;
+  *) echo "error: Unsupported simulator architecture: $architecture" >&2; exit 1 ;;
+esac
+
+"$shared_root/gradlew" -p "$shared_root" -PreactNativeSharedSmoke=true \
+  "linkDebugFramework$target" "linkReleaseFramework$target" --max-workers=2 --console=plain
+
+output="$shared_root/build/smoke/apple-interop"
+mkdir -p "$output"
+sdk="$(xcrun --sdk iphonesimulator --show-sdk-path)"
+simulator="${RCT_KMP_SIMULATOR_UDID:-}"
+if [[ -z "$simulator" ]]; then
+  simulator="$(xcrun simctl list -j | python3 -c '
+import json, sys
+state = json.load(sys.stdin)
+runtimes = sorted((r for r in state["runtimes"] if r.get("isAvailable") and ".iOS-" in r["identifier"]),
+                  key=lambda r: tuple(map(int, r["version"].split("."))), reverse=True)
+for runtime in runtimes:
+    devices = [d for d in state["devices"].get(runtime["identifier"], []) if d.get("isAvailable")]
+    if devices:
+        print(devices[0]["udid"])
+        break
+')"
+fi
+if [[ -z "$simulator" ]]; then
+  echo 'error: Install an iOS simulator runtime and create a device in Xcode.' >&2
+  exit 1
+fi
+
+for configuration in debug release; do
+  frameworks="$shared_root/build/smoke/bin/$kotlin_target/${configuration}Framework"
+  executable="$output/AppleKmpSmoke-$configuration"
+  xcrun clang++ -std=c++20 -fobjc-arc -target "$architecture-apple-ios15.1-simulator" \
+    -isysroot "$sdk" -F "$frameworks" "$shared_root/tests/smoke/AppleKmpSmoke.mm" \
+    -framework ReactNativeShared -framework Foundation -o "$executable"
+  # Standalone execution does not boot or alter the selected simulator.
+  xcrun simctl spawn --standalone "$simulator" "$executable" 2>&1 | tee "$output/$configuration.log"
+  grep -q '^Kotlin Objective-C smoke passed: 5 cases$' "$output/$configuration.log"
+done
diff --git a/packages/react-native/ReactShared/settings.gradle.kts b/packages/react-native/ReactShared/settings.gradle.kts
new file mode 100644
index 000000000000..4d578820419a
--- /dev/null
+++ b/packages/react-native/ReactShared/settings.gradle.kts
@@ -0,0 +1,18 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+pluginManagement {
+  repositories {
+    mavenCentral()
+    gradlePluginPortal()
+  }
+}
+
+dependencyResolutionManagement { repositories { mavenCentral() } }
+
+// A separate build isolates the Native compiler from AGP's built-in Kotlin plugin.
+rootProject.name = "react-native-shared"
diff --git a/packages/react-native/ReactShared/tests/smoke/AppleKmpSmoke.mm b/packages/react-native/ReactShared/tests/smoke/AppleKmpSmoke.mm
new file mode 100644
index 000000000000..9227fd266193
--- /dev/null
+++ b/packages/react-native/ReactShared/tests/smoke/AppleKmpSmoke.mm
@@ -0,0 +1,31 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#import <ReactNativeShared/ReactNativeShared.h>
+
+#include <cstdint>
+#include <cstdio>
+
+int main()
+{
+  @autoreleasepool {
+    struct Case {
+      int32_t left;
+      int32_t right;
+      int32_t expected;
+    };
+    const Case cases[] = {{19, 23, 42}, {-7, 4, -3}, {0, 0, 0}, {INT32_MAX, 0, INT32_MAX}, {INT32_MIN, 0, INT32_MIN}};
+    for (const auto &test : cases) {
+      if ([RNSKmpSmoke.shared addLeft:test.left right:test.right] != test.expected) {
+        fprintf(stderr, "Kotlin Objective-C integer interop failed\n");
+        return 1;
+      }
+    }
+    puts("Kotlin Objective-C smoke passed: 5 cases");
+  }
+  return 0;
+}
diff --git a/packages/react-native/ReactShared/tests/smoke/kotlin/KmpSmoke.kt b/packages/react-native/ReactShared/tests/smoke/kotlin/KmpSmoke.kt
new file mode 100644
index 000000000000..f8e52f414e2a
--- /dev/null
+++ b/packages/react-native/ReactShared/tests/smoke/kotlin/KmpSmoke.kt
@@ -0,0 +1,13 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+package com.facebook.react.shared.smoke
+
+// Unpublished compiler/interop fixture, included only with reactNativeSharedSmoke=true.
+public object KmpSmoke {
+  public fun add(left: Int, right: Int): Int = left + right
+}
diff --git a/packages/react-native/ReactShared/tests/smoke/kotlinTest/KmpSmokeTest.kt b/packages/react-native/ReactShared/tests/smoke/kotlinTest/KmpSmokeTest.kt
new file mode 100644
index 000000000000..73325f799788
--- /dev/null
+++ b/packages/react-native/ReactShared/tests/smoke/kotlinTest/KmpSmokeTest.kt
@@ -0,0 +1,34 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+package com.facebook.react.shared.smoke
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class KmpSmokeTest {
+  @Test
+  fun positiveIntegers() {
+    assertEquals(42, KmpSmoke.add(19, 23))
+  }
+
+  @Test
+  fun signedIntegers() {
+    assertEquals(-3, KmpSmoke.add(-7, 4))
+  }
+
+  @Test
+  fun zero() {
+    assertEquals(0, KmpSmoke.add(0, 0))
+  }
+
+  @Test
+  fun integerBoundaries() {
+    assertEquals(Int.MAX_VALUE, KmpSmoke.add(Int.MAX_VALUE, 0))
+    assertEquals(Int.MIN_VALUE, KmpSmoke.add(Int.MIN_VALUE, 0))
+  }
+}