diff --git a/__test__/git-auth-helper.test.ts b/__test__/git-auth-helper.test.ts
index ad3566a..590bd24 100644
--- a/__test__/git-auth-helper.test.ts
+++ b/__test__/git-auth-helper.test.ts
@@ -1103,6 +1103,7 @@ async function setup(testName: string): Promise<void> {
     ),
     tryDisableAutomaticGarbageCollection: jest.fn(),
     tryGetFetchUrl: jest.fn(),
+    tryGetObjectFormat: jest.fn(async () => ({format: '', succeeded: true})),
     tryGetConfigValues: jest.fn(
       async (
         key: string,
diff --git a/__test__/git-command-manager.test.ts b/__test__/git-command-manager.test.ts
index 8a97d82..8c4f503 100644
--- a/__test__/git-command-manager.test.ts
+++ b/__test__/git-command-manager.test.ts
@@ -378,6 +378,169 @@ describe('Test fetchDepth and fetchTags options', () => {
   })
 })
 
+describe('repository object format', () => {
+  beforeEach(async () => {
+    jest.spyOn(fshelper, 'fileExistsSync').mockImplementation(jest.fn())
+    jest.spyOn(fshelper, 'directoryExistsSync').mockImplementation(jest.fn())
+  })
+
+  afterEach(() => {
+    jest.restoreAllMocks()
+  })
+
+  it('detects SHA-256 from a 64-character HEAD oid', async () => {
+    mockExec.mockImplementation((path, args, options) => {
+      if (args.includes('version')) {
+        options.listeners.stdout(Buffer.from('git version 2.50.1'))
+      }
+
+      if (args.includes('ls-remote')) {
+        options.listeners.stdout(
+          Buffer.from(
+            'ref: refs/heads/main\tHEAD\n' +
+              '9422233ca7ee1b17f1e905d0e141faf0c401556c41cdc6acd71c6bd685da2e92\tHEAD\n'
+          )
+        )
+      }
+
+      return 0
+    })
+    jest.spyOn(exec, 'exec').mockImplementation(mockExec)
+
+    git = await commandManager.createCommandManager('test', false, false)
+
+    const objectFormat = await git.tryGetObjectFormat(
+      'https://github.com/example/repo'
+    )
+
+    expect(objectFormat).toEqual({format: 'sha256', succeeded: true})
+    expect(mockExec).toHaveBeenCalledWith(
+      expect.any(String),
+      [
+        '-c',
+        'protocol.version=2',
+        'ls-remote',
+        '--quiet',
+        '--exit-code',
+        '--symref',
+        'https://github.com/example/repo',
+        'HEAD'
+      ],
+      expect.objectContaining({
+        ignoreReturnCode: true,
+        silent: true
+      })
+    )
+  })
+
+  it('detects SHA-1 from a 40-character HEAD oid', async () => {
+    mockExec.mockImplementation((path, args, options) => {
+      if (args.includes('version')) {
+        options.listeners.stdout(Buffer.from('git version 2.50.1'))
+      }
+
+      if (args.includes('ls-remote')) {
+        options.listeners.stdout(
+          Buffer.from(
+            'ref: refs/heads/main\tHEAD\n' +
+              'c988866043f035e6a46509872215f91d879044c9\tHEAD\n'
+          )
+        )
+      }
+
+      return 0
+    })
+    jest.spyOn(exec, 'exec').mockImplementation(mockExec)
+
+    git = await commandManager.createCommandManager('test', false, false)
+
+    await expect(
+      git.tryGetObjectFormat('https://github.com/example/repo')
+    ).resolves.toEqual({format: 'sha1', succeeded: true})
+  })
+
+  it('returns unsuccessful when HEAD does not resolve to a recognized object id', async () => {
+    mockExec.mockImplementation((path, args, options) => {
+      if (args.includes('version')) {
+        options.listeners.stdout(Buffer.from('git version 2.50.1'))
+      }
+
+      if (args.includes('ls-remote')) {
+        options.listeners.stdout(Buffer.from('ref: refs/heads/main\tHEAD\n'))
+      }
+
+      return 0
+    })
+    jest.spyOn(exec, 'exec').mockImplementation(mockExec)
+
+    git = await commandManager.createCommandManager('test', false, false)
+
+    await expect(
+      git.tryGetObjectFormat('https://github.com/example/repo')
+    ).resolves.toEqual({format: '', succeeded: false})
+  })
+
+  it('returns unsuccessful when object format detection cannot reach the remote', async () => {
+    mockExec.mockImplementation((path, args, options) => {
+      if (args.includes('version')) {
+        options.listeners.stdout(Buffer.from('git version 2.50.1'))
+        return 0
+      }
+
+      return 128
+    })
+    jest.spyOn(exec, 'exec').mockImplementation(mockExec)
+
+    git = await commandManager.createCommandManager('test', false, false)
+
+    await expect(
+      git.tryGetObjectFormat('https://github.com/example/repo')
+    ).resolves.toEqual({format: '', succeeded: false})
+  })
+
+  it('initializes SHA-256 repositories with the matching object format', async () => {
+    mockExec.mockImplementation((path, args, options) => {
+      if (args.includes('version')) {
+        options.listeners.stdout(Buffer.from('git version 2.50.1'))
+      }
+
+      return 0
+    })
+    jest.spyOn(exec, 'exec').mockImplementation(mockExec)
+
+    git = await commandManager.createCommandManager('test', false, false)
+
+    await git.init('sha256')
+
+    expect(mockExec).toHaveBeenCalledWith(
+      expect.any(String),
+      ['init', '--object-format=sha256', 'test'],
+      expect.any(Object)
+    )
+  })
+
+  it('initializes SHA-1 repositories with existing default arguments', async () => {
+    mockExec.mockImplementation((path, args, options) => {
+      if (args.includes('version')) {
+        options.listeners.stdout(Buffer.from('git version 2.50.1'))
+      }
+
+      return 0
+    })
+    jest.spyOn(exec, 'exec').mockImplementation(mockExec)
+
+    git = await commandManager.createCommandManager('test', false, false)
+
+    await git.init('sha1')
+
+    expect(mockExec).toHaveBeenCalledWith(
+      expect.any(String),
+      ['init', 'test'],
+      expect.any(Object)
+    )
+  })
+})
+
 describe('git user-agent with orchestration ID', () => {
   beforeEach(async () => {
     jest.spyOn(fshelper, 'fileExistsSync').mockImplementation(jest.fn())
diff --git a/__test__/git-directory-helper.test.ts b/__test__/git-directory-helper.test.ts
index de79dc8..5cab554 100644
--- a/__test__/git-directory-helper.test.ts
+++ b/__test__/git-directory-helper.test.ts
@@ -501,6 +501,7 @@ async function setup(testName: string): Promise<void> {
       await fs.promises.stat(path.join(repositoryPath, '.git'))
       return repositoryUrl
     }),
+    tryGetObjectFormat: jest.fn(async () => ({format: '', succeeded: true})),
     tryGetConfigValues: jest.fn(),
     tryGetConfigKeys: jest.fn(),
     tryReset: jest.fn(async () => {
diff --git a/__test__/github-api-helper.test.ts b/__test__/github-api-helper.test.ts
new file mode 100644
index 0000000..46e0265
--- /dev/null
+++ b/__test__/github-api-helper.test.ts
@@ -0,0 +1,164 @@
+import * as core from '@actions/core'
+import * as github from '@actions/github'
+import * as githubApiHelper from '../lib/github-api-helper'
+
+describe('github-api-helper object format', () => {
+  let getOctokitSpy: jest.SpyInstance
+  let debugSpy: jest.SpyInstance
+  let repoGet: jest.Mock
+  let branchGet: jest.Mock
+
+  function mockObjectFormatApi(defaultBranch: string, commitSha: string): void {
+    repoGet = jest.fn(async () => ({
+      data: {
+        default_branch: defaultBranch
+      }
+    }))
+    branchGet = jest.fn(async () => ({
+      data: {
+        commit: {
+          sha: commitSha
+        }
+      }
+    }))
+    getOctokitSpy = jest.spyOn(github, 'getOctokit').mockReturnValue({
+      rest: {
+        repos: {
+          get: repoGet,
+          getBranch: branchGet
+        }
+      }
+    } as any)
+  }
+
+  beforeEach(() => {
+    debugSpy = jest.spyOn(core, 'debug').mockImplementation(jest.fn())
+  })
+
+  afterEach(() => {
+    jest.restoreAllMocks()
+  })
+
+  it('detects SHA-256 from the default branch commit SHA', async () => {
+    const commitSha =
+      '9422233ca7ee1b17f1e905d0e141faf0c401556c41cdc6acd71c6bd685da2e92'
+    mockObjectFormatApi('main', commitSha)
+
+    await expect(
+      githubApiHelper.tryGetRepositoryObjectFormat('token', 'owner', 'repo')
+    ).resolves.toEqual({
+      defaultBranch: 'main',
+      format: 'sha256',
+      succeeded: true
+    })
+
+    expect(getOctokitSpy).toHaveBeenCalledWith(
+      'token',
+      expect.objectContaining({baseUrl: 'https://api.github.com'})
+    )
+    expect(repoGet).toHaveBeenCalledWith({owner: 'owner', repo: 'repo'})
+    expect(branchGet).toHaveBeenCalledWith({
+      owner: 'owner',
+      repo: 'repo',
+      branch: 'main'
+    })
+  })
+
+  it('detects SHA-1 from the default branch commit SHA', async () => {
+    mockObjectFormatApi('main', 'c988866043f035e6a46509872215f91d879044c9')
+
+    await expect(
+      githubApiHelper.tryGetRepositoryObjectFormat('token', 'owner', 'repo')
+    ).resolves.toEqual({defaultBranch: 'main', format: 'sha1', succeeded: true})
+  })
+
+  it('detects object format from an existing commit without API calls', async () => {
+    const commitSha =
+      '9422233ca7ee1b17f1e905d0e141faf0c401556c41cdc6acd71c6bd685da2e92'
+    getOctokitSpy = jest.spyOn(github, 'getOctokit')
+
+    await expect(
+      githubApiHelper.tryGetRepositoryObjectFormat(
+        'token',
+        'owner',
+        'repo',
+        undefined,
+        undefined,
+        commitSha
+      )
+    ).resolves.toEqual({format: 'sha256', succeeded: true})
+
+    expect(getOctokitSpy).not.toHaveBeenCalled()
+  })
+
+  it('uses a branch ref directly without looking up the default branch', async () => {
+    const commitSha = 'c988866043f035e6a46509872215f91d879044c9'
+    repoGet = jest.fn()
+    branchGet = jest.fn(async () => ({
+      data: {
+        commit: {
+          sha: commitSha
+        }
+      }
+    }))
+    getOctokitSpy = jest.spyOn(github, 'getOctokit').mockReturnValue({
+      rest: {
+        repos: {
+          get: repoGet,
+          getBranch: branchGet
+        }
+      }
+    } as any)
+
+    await expect(
+      githubApiHelper.tryGetRepositoryObjectFormat(
+        'token',
+        'owner',
+        'repo',
+        undefined,
+        'refs/heads/feature'
+      )
+    ).resolves.toEqual({format: 'sha1', succeeded: true})
+
+    expect(repoGet).not.toHaveBeenCalled()
+    expect(branchGet).toHaveBeenCalledWith({
+      owner: 'owner',
+      repo: 'repo',
+      branch: 'feature'
+    })
+  })
+
+  it('returns unsuccessful when the default branch commit SHA is not recognized', async () => {
+    mockObjectFormatApi('main', 'not-a-sha')
+
+    await expect(
+      githubApiHelper.tryGetRepositoryObjectFormat('token', 'owner', 'repo')
+    ).resolves.toEqual({format: '', succeeded: false})
+    expect(debugSpy).toHaveBeenCalledWith(
+      'Unable to determine repository object format from commit SHA'
+    )
+  })
+
+  it('returns unsuccessful when the repository API lookup fails', async () => {
+    repoGet = jest.fn(async () => {
+      throw new Error('not found')
+    })
+    branchGet = jest.fn()
+    jest.spyOn(github, 'getOctokit').mockReturnValue({
+      rest: {
+        repos: {
+          get: repoGet,
+          getBranch: branchGet
+        }
+      }
+    } as any)
+
+    await expect(
+      githubApiHelper.tryGetRepositoryObjectFormat('token', 'owner', 'repo')
+    ).resolves.toEqual({format: '', succeeded: false})
+    expect(branchGet).not.toHaveBeenCalled()
+    expect(debugSpy).toHaveBeenCalledWith(
+      'Unable to determine repository object format: not found'
+    )
+  })
+})
diff --git a/dist/index.js b/dist/index.js
index 57729b2..2bfb9a1 100644
--- a/dist/index.js
+++ b/dist/index.js
@@ -896,9 +896,14 @@ class GitCommandManager {
     getWorkingDirectory() {
         return this.workingDirectory;
     }
-    init() {
+    init(objectFormat) {
         return __awaiter(this, void 0, void 0, function* () {
-            yield this.execGit(['init', this.workingDirectory]);
+            const args = ['init'];
+            if (objectFormat === 'sha256') {
+                args.push('--object-format=sha256');
+            }
+            args.push(this.workingDirectory);
+            yield this.execGit(args);
         });
     }
     isDetached() {
@@ -1056,6 +1061,45 @@ class GitCommandManager {
             return stdout;
         });
     }
+    tryGetObjectFormat(repositoryUrl) {
+        return __awaiter(this, void 0, void 0, function* () {
+            var _a;
+            try {
+                const output = yield this.execGit([
+                    '-c',
+                    'protocol.version=2',
+                    'ls-remote',
+                    '--quiet',
+                    '--exit-code',
+                    '--symref',
+                    repositoryUrl,
+                    'HEAD'
+                ], true, true);
+                if (output.exitCode !== 0) {
+                    core.debug(`Unable to determine repository object format: git ls-remote exited with ${output.exitCode}`);
+                    return { format: '', succeeded: false };
+                }
+                for (const line of output.stdout.trim().split('\n')) {
+                    const [oid, ref] = line.split('\t');
+                    if (ref !== 'HEAD') {
+                        continue;
+                    }
+                    if (/^[0-9a-fA-F]{64}$/.test(oid)) {
+                        return { format: 'sha256', succeeded: true };
+                    }
+                    if (/^[0-9a-fA-F]{40}$/.test(oid)) {
+                        return { format: 'sha1', succeeded: true };
+                    }
+                }
+                core.debug('Unable to determine repository object format from HEAD');
+                return { format: '', succeeded: false };
+            }
+            catch (err) {
+                core.debug(`Unable to determine repository object format: ${(_a = err === null || err === void 0 ? void 0 : err.message) !== null && _a !== void 0 ? _a : err}`);
+                return { format: '', succeeded: false };
+            }
+        });
+    }
     tryGetConfigValues(configKey, globalConfig, configFile) {
         return __awaiter(this, void 0, void 0, function* () {
             const args = ['config'];
@@ -1484,10 +1528,24 @@ function getSource(settings) {
             }
             // Save state for POST action
             stateHelper.setRepositoryPath(settings.repositoryPath);
+            let defaultBranch = '';
             // Initialize the repository
             if (!fsHelper.directoryExistsSync(path.join(settings.repositoryPath, '.git'))) {
+                core.startGroup('Determining repository object format');
+                let objectFormatResult = yield githubApiHelper.tryGetRepositoryObjectFormat(settings.authToken, settings.repositoryOwner, settings.repositoryName, settings.githubServerUrl, settings.ref, settings.commit);
+                if (!objectFormatResult.succeeded) {
+                    objectFormatResult = yield git.tryGetObjectFormat(repositoryUrl);
+                }
+                const objectFormat = objectFormatResult.succeeded
+                    ? objectFormatResult.format
+                    : '';
+                defaultBranch = objectFormatResult.defaultBranch || '';
+                if (objectFormat === 'sha256') {
+                    core.info('Detected SHA-256 repository object format');
+                }
+                core.endGroup();
                 core.startGroup('Initializing the repository');
-                yield git.init();
+                yield git.init(objectFormat);
                 yield git.remoteAdd('origin', repositoryUrl);
                 core.endGroup();
             }
@@ -1511,6 +1569,10 @@ function getSource(settings) {
                 if (settings.sshKey) {
                     settings.ref = yield git.getDefaultBranch(repositoryUrl);
                 }
+                else if (defaultBranch) {
+                    core.info(`Default branch '${defaultBranch}'`);
+                    settings.ref = `refs/heads/${defaultBranch}`;
+                }
                 else {
                     settings.ref = yield githubApiHelper.getDefaultBranch(settings.authToken, settings.repositoryOwner, settings.repositoryName, settings.githubServerUrl);
                 }
@@ -1810,6 +1872,7 @@ var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, ge
 Object.defineProperty(exports, "__esModule", ({ value: true }));
 exports.downloadRepository = downloadRepository;
 exports.getDefaultBranch = getDefaultBranch;
+exports.tryGetRepositoryObjectFormat = tryGetRepositoryObjectFormat;
 const assert = __importStar(__nccwpck_require__(9491));
 const core = __importStar(__nccwpck_require__(2186));
 const fs = __importStar(__nccwpck_require__(7147));
@@ -1911,6 +1974,69 @@ function getDefaultBranch(authToken, owner, repo, baseUrl) {
         }));
     });
 }
+function tryGetRepositoryObjectFormat(authToken, owner, repo, baseUrl, ref, commit) {
+    return __awaiter(this, void 0, void 0, function* () {
+        var _a;
+        try {
+            const commitFormat = getObjectFormat(commit);
+            if (commitFormat) {
+                return { format: commitFormat, succeeded: true };
+            }
+            const octokit = github.getOctokit(authToken, {
+                baseUrl: (0, url_helper_1.getServerApiUrl)(baseUrl)
+            });
+            let branchName = getBranchName(ref);
+            let defaultBranch = '';
+            if (!branchName) {
+                const repository = yield octokit.rest.repos.get({ owner, repo });
+                defaultBranch = repository.data.default_branch;
+                assert.ok(defaultBranch, 'default_branch cannot be empty');
+                branchName = defaultBranch;
+            }
+            const branch = yield octokit.rest.repos.getBranch({
+                owner,
+                repo,
+                branch: branchName
+            });
+            const branchFormat = getObjectFormat(branch.data.commit.sha);
+            if (branchFormat) {
+                return {
+                    defaultBranch: defaultBranch || undefined,
+                    format: branchFormat,
+                    succeeded: true
+                };
+            }
+            core.debug('Unable to determine repository object format from commit SHA');
+            return { format: '', succeeded: false };
+        }
+        catch (err) {
+            core.debug(`Unable to determine repository object format: ${(_a = err === null || err === void 0 ? void 0 : err.message) !== null && _a !== void 0 ? _a : err}`);
+            return { format: '', succeeded: false };
+        }
+    });
+}
+function getBranchName(ref) {
+    if (!ref) {
+        return '';
+    }
+    const headsPrefix = 'refs/heads/';
+    if (ref.startsWith(headsPrefix)) {
+        return ref.substring(headsPrefix.length);
+    }
+    if (!ref.startsWith('refs/') && !getObjectFormat(ref)) {
+        return ref;
+    }
+    return '';
+}
+function getObjectFormat(sha) {
+    if (/^[0-9a-fA-F]{64}$/.test(sha || '')) {
+        return 'sha256';
+    }
+    if (/^[0-9a-fA-F]{40}$/.test(sha || '')) {
+        return 'sha1';
+    }
+    return '';
+}
 function downloadArchive(authToken, owner, repo, ref, commit, baseUrl) {
     return __awaiter(this, void 0, void 0, function* () {
         const octokit = github.getOctokit(authToken, {
diff --git a/src/git-command-manager.ts b/src/git-command-manager.ts
index f5ba40e..9fde8b2 100644
--- a/src/git-command-manager.ts
+++ b/src/git-command-manager.ts
@@ -15,6 +15,11 @@ import {GitVersion} from './git-version'
 export const MinimumGitVersion = new GitVersion('2.18')
 export const MinimumGitSparseCheckoutVersion = new GitVersion('2.28')
 
+export interface GitObjectFormatResult {
+  format: string
+  succeeded: boolean
+}
+
 export interface IGitCommandManager {
   branchDelete(remote: boolean, branch: string): Promise<void>
   branchExists(remote: boolean, pattern: string): Promise<boolean>
@@ -43,7 +48,7 @@ export interface IGitCommandManager {
   getDefaultBranch(repositoryUrl: string): Promise<string>
   getSubmoduleConfigPaths(recursive: boolean): Promise<string[]>
   getWorkingDirectory(): string
-  init(): Promise<void>
+  init(objectFormat?: string): Promise<void>
   isDetached(): Promise<boolean>
   lfsFetch(ref: string): Promise<void>
   lfsInstall(): Promise<void>
@@ -68,6 +73,7 @@ export interface IGitCommandManager {
   ): Promise<boolean>
   tryDisableAutomaticGarbageCollection(): Promise<boolean>
   tryGetFetchUrl(): Promise<string>
+  tryGetObjectFormat(repositoryUrl: string): Promise<GitObjectFormatResult>
   tryGetConfigValues(
     configKey: string,
     globalConfig?: boolean,
@@ -364,8 +370,14 @@ class GitCommandManager {
     return this.workingDirectory
   }
 
-  async init(): Promise<void> {
-    await this.execGit(['init', this.workingDirectory])
+  async init(objectFormat?: string): Promise<void> {
+    const args = ['init']
+    if (objectFormat === 'sha256') {
+      args.push('--object-format=sha256')
+    }
+    args.push(this.workingDirectory)
+
+    await this.execGit(args)
   }
 
   async isDetached(): Promise<boolean> {
@@ -536,6 +548,55 @@ class GitCommandManager {
     return stdout
   }
 
+  async tryGetObjectFormat(
+    repositoryUrl: string
+  ): Promise<GitObjectFormatResult> {
+    try {
+      const output = await this.execGit(
+        [
+          '-c',
+          'protocol.version=2',
+          'ls-remote',
+          '--quiet',
+          '--exit-code',
+          '--symref',
+          repositoryUrl,
+          'HEAD'
+        ],
+        true,
+        true
+      )
+
+      if (output.exitCode !== 0) {
+        core.debug(
+          `Unable to determine repository object format: git ls-remote exited with ${output.exitCode}`
+        )
+        return {format: '', succeeded: false}
+      }
+
+      for (const line of output.stdout.trim().split('\n')) {
+        const [oid, ref] = line.split('\t')
+        if (ref !== 'HEAD') {
+          continue
+        }
+        if (/^[0-9a-fA-F]{64}$/.test(oid)) {
+          return {format: 'sha256', succeeded: true}
+        }
+        if (/^[0-9a-fA-F]{40}$/.test(oid)) {
+          return {format: 'sha1', succeeded: true}
+        }
+      }
+
+      core.debug('Unable to determine repository object format from HEAD')
+      return {format: '', succeeded: false}
+    } catch (err) {
+      core.debug(
+        `Unable to determine repository object format: ${(err as any)?.message ?? err}`
+      )
+      return {format: '', succeeded: false}
+    }
+  }
+
   async tryGetConfigValues(
     configKey: string,
     globalConfig?: boolean,
diff --git a/src/git-source-provider.ts b/src/git-source-provider.ts
index ec87178..f924f3b 100644
--- a/src/git-source-provider.ts
+++ b/src/git-source-provider.ts
@@ -105,12 +105,36 @@ export async function getSource(settings: IGitSourceSettings): Promise<void> {
     // Save state for POST action
     stateHelper.setRepositoryPath(settings.repositoryPath)
 
+    let defaultBranch = ''
+
     // Initialize the repository
     if (
       !fsHelper.directoryExistsSync(path.join(settings.repositoryPath, '.git'))
     ) {
+      core.startGroup('Determining repository object format')
+      let objectFormatResult =
+        await githubApiHelper.tryGetRepositoryObjectFormat(
+          settings.authToken,
+          settings.repositoryOwner,
+          settings.repositoryName,
+          settings.githubServerUrl,
+          settings.ref,
+          settings.commit
+        )
+      if (!objectFormatResult.succeeded) {
+        objectFormatResult = await git.tryGetObjectFormat(repositoryUrl)
+      }
+      const objectFormat = objectFormatResult.succeeded
+        ? objectFormatResult.format
+        : ''
+      defaultBranch = objectFormatResult.defaultBranch || ''
+      if (objectFormat === 'sha256') {
+        core.info('Detected SHA-256 repository object format')
+      }
+      core.endGroup()
+
       core.startGroup('Initializing the repository')
-      await git.init()
+      await git.init(objectFormat)
       await git.remoteAdd('origin', repositoryUrl)
       core.endGroup()
     }
@@ -138,6 +162,9 @@ export async function getSource(settings: IGitSourceSettings): Promise<void> {
       core.startGroup('Determining the default branch')
       if (settings.sshKey) {
         settings.ref = await git.getDefaultBranch(repositoryUrl)
+      } else if (defaultBranch) {
+        core.info(`Default branch '${defaultBranch}'`)
+        settings.ref = `refs/heads/${defaultBranch}`
       } else {
         settings.ref = await githubApiHelper.getDefaultBranch(
           settings.authToken,
diff --git a/src/github-api-helper.ts b/src/github-api-helper.ts
index 1ff27c2..1fffc97 100644
--- a/src/github-api-helper.ts
+++ b/src/github-api-helper.ts
@@ -11,6 +11,12 @@ import {getServerApiUrl} from './url-helper'
 
 const IS_WINDOWS = process.platform === 'win32'
 
+export interface RepositoryObjectFormatResult {
+  defaultBranch?: string
+  format: string
+  succeeded: boolean
+}
+
 export async function downloadRepository(
   authToken: string,
   owner: string,
@@ -122,6 +128,84 @@ export async function getDefaultBranch(
   })
 }
 
+export async function tryGetRepositoryObjectFormat(
+  authToken: string,
+  owner: string,
+  repo: string,
+  baseUrl?: string,
+  ref?: string,
+  commit?: string
+): Promise<RepositoryObjectFormatResult> {
+  try {
+    const commitFormat = getObjectFormat(commit)
+    if (commitFormat) {
+      return {format: commitFormat, succeeded: true}
+    }
+
+    const octokit = github.getOctokit(authToken, {
+      baseUrl: getServerApiUrl(baseUrl)
+    })
+
+    let branchName = getBranchName(ref)
+    let defaultBranch = ''
+    if (!branchName) {
+      const repository = await octokit.rest.repos.get({owner, repo})
+      defaultBranch = repository.data.default_branch
+      assert.ok(defaultBranch, 'default_branch cannot be empty')
+      branchName = defaultBranch
+    }
+
+    const branch = await octokit.rest.repos.getBranch({
+      owner,
+      repo,
+      branch: branchName
+    })
+    const branchFormat = getObjectFormat(branch.data.commit.sha)
+    if (branchFormat) {
+      return {
+        defaultBranch: defaultBranch || undefined,
+        format: branchFormat,
+        succeeded: true
+      }
+    }
+
+    core.debug('Unable to determine repository object format from commit SHA')
+    return {format: '', succeeded: false}
+  } catch (err) {
+    core.debug(
+      `Unable to determine repository object format: ${(err as any)?.message ?? err}`
+    )
+    return {format: '', succeeded: false}
+  }
+}
+
+function getBranchName(ref?: string): string {
+  if (!ref) {
+    return ''
+  }
+
+  const headsPrefix = 'refs/heads/'
+  if (ref.startsWith(headsPrefix)) {
+    return ref.substring(headsPrefix.length)
+  }
+
+  if (!ref.startsWith('refs/') && !getObjectFormat(ref)) {
+    return ref
+  }
+
+  return ''
+}
+
+function getObjectFormat(sha?: string): string {
+  if (/^[0-9a-fA-F]{64}$/.test(sha || '')) {
+    return 'sha256'
+  }
+  if (/^[0-9a-fA-F]{40}$/.test(sha || '')) {
+    return 'sha1'
+  }
+  return ''
+}
+
 async function downloadArchive(
   authToken: string,
   owner: string,