diff --git a/__tests__/github-builder/runner-mapping.test.ts b/__tests__/github-builder/runner-mapping.test.ts
index abdce10e..3dd9473d 100644
--- a/__tests__/github-builder/runner-mapping.test.ts
+++ b/__tests__/github-builder/runner-mapping.test.ts
@@ -44,8 +44,8 @@ describe('parse', () => {
   });
 
   it.each([
-    ['amd64', 'ubuntu-24.04'],
-    ['arm64', 'ubuntu-24.04-arm']
+    ['amd64', 'ubuntu-26.04'],
+    ['arm64', 'ubuntu-26.04-arm']
   ])('preserves deprecated alias %s and its warning', (alias, runner) => {
     expect(RunnerMapping.parse([alias])).toEqual({defaultRunner: runner, rules: []});
     expect(core.warning).toHaveBeenCalledExactlyOnceWith(`The runner input value "${alias}" is deprecated; use runner=${runner} instead`);
@@ -53,13 +53,13 @@ describe('parse', () => {
 
   it('preserves auto mappings and its warning', () => {
     expect(RunnerMapping.parse(['auto'])).toEqual({
-      defaultRunner: 'ubuntu-24.04',
+      defaultRunner: 'ubuntu-26.04',
       rules: [
-        {pattern: 'linux/arm', runner: 'ubuntu-24.04-arm'},
-        {pattern: 'linux/arm64', runner: 'ubuntu-24.04-arm'}
+        {pattern: 'linux/arm', runner: 'ubuntu-26.04-arm'},
+        {pattern: 'linux/arm64', runner: 'ubuntu-26.04-arm'}
       ]
     });
-    expect(core.warning).toHaveBeenCalledExactlyOnceWith('The runner input value "auto" is deprecated; use a runner mapping with default=ubuntu-24.04, linux/arm=ubuntu-24.04-arm, and linux/arm64=ubuntu-24.04-arm instead');
+    expect(core.warning).toHaveBeenCalledExactlyOnceWith('The runner input value "auto" is deprecated; use a runner mapping with default=ubuntu-26.04, linux/arm=ubuntu-26.04-arm, and linux/arm64=ubuntu-26.04-arm instead');
   });
 
   it('does not expand aliases used as mapping values', () => {
diff --git a/src/github-builder/runner-mapping.ts b/src/github-builder/runner-mapping.ts
index dee70c3f..aeee533d 100644
--- a/src/github-builder/runner-mapping.ts
+++ b/src/github-builder/runner-mapping.ts
@@ -31,17 +31,17 @@ export class RunnerMapping {
     if (lines.length === 1 && !lines[0].includes('=')) {
       const label = lines[0];
       if (label === 'auto') {
-        core.warning('The runner input value "auto" is deprecated; use a runner mapping with default=ubuntu-24.04, linux/arm=ubuntu-24.04-arm, and linux/arm64=ubuntu-24.04-arm instead');
+        core.warning('The runner input value "auto" is deprecated; use a runner mapping with default=ubuntu-26.04, linux/arm=ubuntu-26.04-arm, and linux/arm64=ubuntu-26.04-arm instead');
         return {
-          defaultRunner: 'ubuntu-24.04',
+          defaultRunner: 'ubuntu-26.04',
           rules: [
-            {pattern: 'linux/arm', runner: 'ubuntu-24.04-arm'},
-            {pattern: 'linux/arm64', runner: 'ubuntu-24.04-arm'}
+            {pattern: 'linux/arm', runner: 'ubuntu-26.04-arm'},
+            {pattern: 'linux/arm64', runner: 'ubuntu-26.04-arm'}
           ]
         };
       }
       if (label === 'amd64' || label === 'arm64') {
-        const runner = label === 'amd64' ? 'ubuntu-24.04' : 'ubuntu-24.04-arm';
+        const runner = label === 'amd64' ? 'ubuntu-26.04' : 'ubuntu-26.04-arm';
         core.warning(`The runner input value "${label}" is deprecated; use runner=${runner} instead`);
         return {defaultRunner: runner, rules: []};
       }