diff --git a/packages/react-native/scripts/spm/__doc__/spm-scripts.md b/packages/react-native/scripts/spm/__doc__/spm-scripts.md
index d2db90c4f59..2fd6cb601bf 100644
--- a/packages/react-native/scripts/spm/__doc__/spm-scripts.md
+++ b/packages/react-native/scripts/spm/__doc__/spm-scripts.md
@@ -209,6 +209,14 @@ configuration selects Release. Selection uses only generated build settings and
 standard macOS tools: builds do not run Node, mutate symlinks, regenerate the
 package graph, or require a second build.
 
+Those same debug-flavored configurations also get
+`SWIFT_ACTIVE_COMPILATION_CONDITIONS = "$(inherited) DEBUG"` — the only thing
+that makes Swift's `#if DEBUG` true (`GCC_PREPROCESSOR_DEFINITIONS` reaches
+C/ObjC/C++ only), and what `AppDelegate.swift`'s `bundleURL()` branches on to
+load from Metro instead of a bundled `main.jsbundle`. CocoaPods injects it at
+`pod install` time, so this keeps SwiftPM apps at parity. An existing value is
+left alone.
+
 ## What to commit
 
 | Path | Commit? | Why |
diff --git a/packages/react-native/scripts/spm/__tests__/inject-spm-xcodeproj-test.js b/packages/react-native/scripts/spm/__tests__/inject-spm-xcodeproj-test.js
index bc77f7e1eec..fce82bf8ad6 100644
--- a/packages/react-native/scripts/spm/__tests__/inject-spm-xcodeproj-test.js
+++ b/packages/react-native/scripts/spm/__tests__/inject-spm-xcodeproj-test.js
@@ -31,6 +31,32 @@ const PODS = PLAIN.replace(
   'AA0000000000000000000901 /* Debug */ = {\n\t\t\tisa = XCBuildConfiguration;\n\t\t\tbaseConfigurationReference = BB0000000000000000000001 /* Pods-MyApp.debug.xcconfig */;\n\t\t\tbuildSettings = {',
 );
 
+// The app target's two XCBuildConfiguration UUIDs in the fixture.
+const APP_DEBUG_CONFIG = 'AA0000000000000000000901';
+const APP_RELEASE_CONFIG = 'AA00000000000000000000A2';
+
+const DEBUG_CONFIG_HEAD =
+  'AA0000000000000000000901 /* Debug */ = {\n\t\t\tisa = XCBuildConfiguration;\n\t\t\tbuildSettings = {';
+
+// Seed the app target's Debug config with a SWIFT_ACTIVE_COMPILATION_CONDITIONS
+// the user already had, in the scalar form Xcode and the app template write.
+function withDebugCondition(text, value) {
+  return text.replace(
+    DEBUG_CONFIG_HEAD,
+    `${DEBUG_CONFIG_HEAD}\n\t\t\t\tSWIFT_ACTIVE_COMPILATION_CONDITIONS = ${value};`,
+  );
+}
+
+// One XCBuildConfiguration's buildSettings dict, by config UUID. Build settings
+// hold only scalars and `( … )` arrays, so the first `};` closes the dict.
+function buildSettingsOf(text, configUuid) {
+  const open = text.indexOf(
+    'buildSettings = {',
+    text.indexOf(`${configUuid} /*`),
+  );
+  return text.slice(open, text.indexOf('};', open));
+}
+
 const RN_PATH = '../node_modules/react-native';
 
 // Absolute, mirroring resolveHermesCliPathSetting (a `..`-relative path through
@@ -218,6 +244,40 @@ describe('injectSpmIntoPbxproj — Tier 2 (build settings + phase)', () => {
     expect(text).not.toContain('HERMES_CLI_PATH');
   });
 
+  // Swift's `#if DEBUG` — which AppDelegate.swift's bundleURL() uses to pick the
+  // Metro URL — is gated by this setting alone. CocoaPods injects it at `pod
+  // install`; an SPM app has to get it here or a Debug build looks for a
+  // main.jsbundle it never built.
+  it('sets SWIFT_ACTIVE_COMPILATION_CONDITIONS = DEBUG on the debug config only', () => {
+    const {text} = inject(PLAIN);
+    const debugSettings = buildSettingsOf(text, APP_DEBUG_CONFIG);
+    expect(debugSettings).toMatch(
+      /SWIFT_ACTIVE_COMPILATION_CONDITIONS = \(\s*"\$\(inherited\)",\s*DEBUG,\s*\)/,
+    );
+    expect(buildSettingsOf(text, APP_RELEASE_CONFIG)).not.toContain(
+      'SWIFT_ACTIVE_COMPILATION_CONDITIONS',
+    );
+  });
+
+  it('leaves a config that already sets DEBUG (scalar form) untouched', () => {
+    const {text} = inject(withDebugCondition(PLAIN, '"$(inherited) DEBUG"'));
+    // Not promoted to an array, not re-appended — DEBUG is already there.
+    expect(buildSettingsOf(text, APP_DEBUG_CONFIG)).toContain(
+      'SWIFT_ACTIVE_COMPILATION_CONDITIONS = "$(inherited) DEBUG";',
+    );
+    expect(text.match(/\bDEBUG\b/g)).toHaveLength(1);
+  });
+
+  it("adds DEBUG alongside the user's own compilation conditions", () => {
+    const {text} = inject(
+      withDebugCondition(PLAIN, '"$(inherited) MY_DEBUG_UI"'),
+    );
+    // MY_DEBUG_UI must not be mistaken for DEBUG by a substring check.
+    const debugSettings = buildSettingsOf(text, APP_DEBUG_CONFIG);
+    expect(debugSettings).toContain('"$(inherited) MY_DEBUG_UI"');
+    expect(debugSettings).toMatch(/^\s*DEBUG,$/m);
+  });
+
   it('prepends the Sync SPM Autolinking build phase', () => {
     const {text} = inject(PLAIN);
     expect(text).toContain('Sync SPM Autolinking');
diff --git a/packages/react-native/scripts/spm/__tests__/remove-spm-injection-test.js b/packages/react-native/scripts/spm/__tests__/remove-spm-injection-test.js
index d023fa9a2d8..3625dd79e83 100644
--- a/packages/react-native/scripts/spm/__tests__/remove-spm-injection-test.js
+++ b/packages/react-native/scripts/spm/__tests__/remove-spm-injection-test.js
@@ -190,6 +190,38 @@ describe('removeSpmInjection — the surgical inverse of add', () => {
     expect(fs.existsSync(schemePath)).toBe(false);
   });
 
+  // A Debug config that already carries DEBUG gets no edit at all, so there is
+  // nothing for the marker to record — and nothing left behind. Injecting into
+  // the scalar form regardless (addArrayStringValues dedupes by exact array
+  // member, which the scalar never matches) would promote it to an array the
+  // marker has no record of, and deinit would strand it.
+  it('leaves a Debug config that already sets DEBUG alone, add through deinit', () => {
+    const {appRoot, xcodeprojPath, rnRoot} = scaffoldApp();
+    const head =
+      'AA0000000000000000000901 /* Debug */ = {\n\t\t\tisa = XCBuildConfiguration;\n\t\t\tbuildSettings = {';
+    fs.writeFileSync(
+      path.join(xcodeprojPath, 'project.pbxproj'),
+      PLAIN.replace(
+        head,
+        `${head}\n\t\t\t\tSWIFT_ACTIVE_COMPILATION_CONDITIONS = "$(inherited) DEBUG";`,
+      ),
+      'utf8',
+    );
+    const before = pbxprojOf(xcodeprojPath);
+
+    injectSpmIntoExistingXcodeproj({
+      appRoot,
+      reactNativeRoot: rnRoot,
+      xcodeprojPath,
+    });
+    expect(pbxprojOf(xcodeprojPath)).toContain(
+      'SWIFT_ACTIVE_COMPILATION_CONDITIONS = "$(inherited) DEBUG";',
+    );
+
+    expect(removeSpmInjection({appRoot, xcodeprojPath}).status).toBe('removed');
+    expect(pbxprojOf(xcodeprojPath)).toBe(before);
+  });
+
   it('preserves an unrelated edit made to the pbxproj after add', () => {
     const {appRoot, xcodeprojPath, rnRoot} = scaffoldApp();
 
diff --git a/packages/react-native/scripts/spm/generate-spm-xcodeproj.js b/packages/react-native/scripts/spm/generate-spm-xcodeproj.js
index 3126df74d0c..def64b4e34a 100644
--- a/packages/react-native/scripts/spm/generate-spm-xcodeproj.js
+++ b/packages/react-native/scripts/spm/generate-spm-xcodeproj.js
@@ -1093,6 +1093,25 @@ const INJECTED_ARRAY_SETTINGS = [
   },
 ];
 
+// Array build settings injected only into debug-flavored configurations.
+//
+// Swift's `#if DEBUG` is gated by SWIFT_ACTIVE_COMPILATION_CONDITIONS, NOT by
+// GCC_PREPROCESSOR_DEFINITIONS (which only reaches C/ObjC/C++). The app
+// template does not commit the setting: CocoaPods injects it at `pod install`
+// time (react_native_post_install → set_build_setting
+// SWIFT_ACTIVE_COMPILATION_CONDITIONS = ["$(inherited)", "DEBUG"] on Debug).
+// An SPM app never runs CocoaPods, so without this `#if DEBUG` is false even
+// in a Debug build — AppDelegate.swift's `bundleURL()` skips the Metro URL,
+// falls back to a main.jsbundle that a Debug build never produced, and the app
+// dies at launch with "No script url provided … unsanitizedScriptURLString =
+// (null)" while Metro is running right there.
+//
+// Paired with RN_SPM_FLAVOR via flavorForBuildConfiguration, so a config that
+// links the debug xcframeworks also compiles its Swift with DEBUG.
+const DEBUG_ARRAY_SETTINGS = [
+  {key: 'SWIFT_ACTIVE_COMPILATION_CONDITIONS', values: ['DEBUG']},
+];
+
 /** The XCBuildConfiguration UUIDs of a target (via its buildConfigurationList). */
 function targetBuildConfigUuids(
   text /*: string */,
@@ -1687,6 +1706,28 @@ function resolveHermesCliPathSetting(
   }
 }
 
+/** Strip the surrounding plist quotes from a build-setting token, if any. */
+function unquotePlist(s /*: string */) /*: string */ {
+  return s.replace(/^"/, '').replace(/"$/, '');
+}
+
+/**
+ * The individual values a build setting already carries, unquoted — for both
+ * shapes a pbxproj uses: the array form Xcode writes for a multi-value setting
+ * (`("$(inherited)", DEBUG)`) and the scalar form the app template and
+ * hand-edits use (`"$(inherited) DEBUG"`). Membership, not substring: the
+ * latter would read `MY_DEBUG_FLAG` as `DEBUG` already being set and silently
+ * skip the injection.
+ */
+function buildSettingValueTokens(value /*: string */) /*: Set<string> */ {
+  return new Set(
+    value
+      .split(/[\s,()]+/)
+      .filter(Boolean)
+      .map(unquotePlist),
+  );
+}
+
 function mergeReactBuildSettings(
   input /*: string */,
   configUuid /*: string */,
@@ -1732,6 +1773,9 @@ function mergeReactBuildSettings(
   const createdScalars /*: Array<string> */ = [];
   const arraySettings = [
     ...INJECTED_ARRAY_SETTINGS,
+    ...(flavorForBuildConfiguration(configurationName) === 'debug'
+      ? DEBUG_ARRAY_SETTINGS
+      : []),
     ...frameworkArrayBuildSettings(flavoredFrameworks),
   ];
   for (const {key, values} of arraySettings) {
@@ -1743,10 +1787,17 @@ function mergeReactBuildSettings(
     if (existing == null) {
       createdArrayKeys.push(key);
     } else {
-      const fresh = values.filter(v => !existing.value.includes(v));
-      if (fresh.length > 0) {
-        appendedArrayValues[key] = fresh;
+      const present = buildSettingValueTokens(existing.value);
+      const fresh = values.filter(v => !present.has(unquotePlist(v)));
+      if (fresh.length === 0) {
+        // Nothing to add. Skip addArrayStringValues entirely: its dedupe is by
+        // EXACT array member, so a value the user carries in the scalar form
+        // (`SWIFT_ACTIVE_COMPILATION_CONDITIONS = "$(inherited) DEBUG"`) would
+        // otherwise be promoted to an array and re-appended — an edit `deinit`
+        // has no record of and so could never reverse.
+        continue;
       }
+      appendedArrayValues[key] = fresh;
     }
     text = addArrayStringValues(text, d, key, values);
   }