diff --git a/__tests__/buildx/build.test.ts b/__tests__/buildx/build.test.ts
index 2857ef3..5344b94 100644
--- a/__tests__/buildx/build.test.ts
+++ b/__tests__/buildx/build.test.ts
@@ -18,6 +18,7 @@ import {afterEach, beforeEach, describe, expect, it, vi, test} from 'vitest';
 import fs from 'fs';
 import os from 'os';
 import path from 'path';
+import * as github from '@actions/github';
 import * as rimraf from 'rimraf';
 
 import {Context} from '../../src/context.js';
@@ -46,6 +47,8 @@ afterEach(() => {
 
 describe('gitContext', () => {
   const originalEnv = process.env;
+  const githubContextSha = '860c1904a1ce19322e91ac35af1ab07466440c37';
+  const pullRequestHeadSha = 'f11797113e5a9b86bd976329c5dbb8a8bfdfadfa';
   beforeEach(() => {
     vi.resetModules();
     process.env = {
@@ -53,9 +56,17 @@ describe('gitContext', () => {
       DOCKER_DEFAULT_GIT_CONTEXT_PR_HEAD_REF: '',
       BUILDX_SEND_GIT_QUERY_AS_INPUT: ''
     };
+    github.context.sha = githubContextSha;
+    github.context.payload.pull_request = {
+      number: 15,
+      head: {
+        sha: pullRequestHeadSha
+      }
+    };
   });
   afterEach(() => {
     process.env = originalEnv;
+    delete github.context.payload.pull_request;
   });
 
   type GitContextTestCase = {
@@ -75,13 +86,13 @@ describe('gitContext', () => {
     // no format set (defaults to fragment)
     [{ref: 'refs/heads/master', format: undefined, prHeadRef: false, sendGitQueryAsInput: false, buildxQuerySupport: true}, 'https://github.com/docker/actions-toolkit.git#860c1904a1ce19322e91ac35af1ab07466440c37'],
     [{ref: 'master', format: undefined, prHeadRef: false, sendGitQueryAsInput: false, buildxQuerySupport: true}, 'https://github.com/docker/actions-toolkit.git#860c1904a1ce19322e91ac35af1ab07466440c37'],
-    [{ref: 'refs/pull/15/merge', format: undefined, prHeadRef: false, sendGitQueryAsInput: false, buildxQuerySupport: true}, 'https://github.com/docker/actions-toolkit.git#refs/pull/15/merge'],
+    [{ref: 'refs/pull/15/merge', checksum: undefined, format: undefined, prHeadRef: false, sendGitQueryAsInput: false, buildxQuerySupport: true}, 'https://github.com/docker/actions-toolkit.git#860c1904a1ce19322e91ac35af1ab07466440c37'],
     [{ref: 'refs/tags/v1.0.0', format: undefined, prHeadRef: false, sendGitQueryAsInput: false, buildxQuerySupport: true}, 'https://github.com/docker/actions-toolkit.git#860c1904a1ce19322e91ac35af1ab07466440c37'],
-    [{ref: 'refs/pull/15/merge', format: undefined, prHeadRef: true, sendGitQueryAsInput: false, buildxQuerySupport: true}, 'https://github.com/docker/actions-toolkit.git#refs/pull/15/head'],
+    [{ref: 'refs/pull/15/merge', checksum: undefined, format: undefined, prHeadRef: true, sendGitQueryAsInput: false, buildxQuerySupport: true}, 'https://github.com/docker/actions-toolkit.git#f11797113e5a9b86bd976329c5dbb8a8bfdfadfa'],
     // no format set (defaults to query only when client-side query resolution is enabled and supported)
     [{ref: 'refs/heads/master', format: undefined, prHeadRef: false, sendGitQueryAsInput: true, buildxQuerySupport: true}, 'https://github.com/docker/actions-toolkit.git?ref=refs/heads/master&checksum=860c1904a1ce19322e91ac35af1ab07466440c37'],
-    [{ref: 'refs/pull/15/merge', format: undefined, prHeadRef: false, sendGitQueryAsInput: true, buildxQuerySupport: true}, 'https://github.com/docker/actions-toolkit.git?ref=refs/pull/15/merge&checksum=860c1904a1ce19322e91ac35af1ab07466440c37'],
-    [{ref: 'refs/pull/15/merge', format: undefined, prHeadRef: true, sendGitQueryAsInput: true, buildxQuerySupport: true}, 'https://github.com/docker/actions-toolkit.git?ref=refs/pull/15/head&checksum=860c1904a1ce19322e91ac35af1ab07466440c37'],
+    [{ref: 'refs/pull/15/merge', checksum: undefined, format: undefined, prHeadRef: false, sendGitQueryAsInput: true, buildxQuerySupport: true}, 'https://github.com/docker/actions-toolkit.git?ref=860c1904a1ce19322e91ac35af1ab07466440c37'],
+    [{ref: 'refs/pull/15/merge', checksum: undefined, format: undefined, prHeadRef: true, sendGitQueryAsInput: true, buildxQuerySupport: true}, 'https://github.com/docker/actions-toolkit.git?ref=f11797113e5a9b86bd976329c5dbb8a8bfdfadfa'],
     [{ref: 'refs/heads/master', format: undefined, prHeadRef: false, sendGitQueryAsInput: true, buildxQuerySupport: false}, 'https://github.com/docker/actions-toolkit.git#860c1904a1ce19322e91ac35af1ab07466440c37'],
     [{ref: 'refs/heads/master', format: undefined, prHeadRef: false, sendGitQueryAsInput: false, buildxQuerySupport: true, attrs: {}}, 'https://github.com/docker/actions-toolkit.git#860c1904a1ce19322e91ac35af1ab07466440c37'],
     [{ref: 'refs/heads/master', checksum: undefined, format: undefined, prHeadRef: false, sendGitQueryAsInput: false, buildxQuerySupport: true, attrs: {checksum: 'cafebabe'}}, 'https://github.com/docker/actions-toolkit.git#cafebabe'],
@@ -95,9 +106,10 @@ describe('gitContext', () => {
     // query format
     [{ref: 'refs/heads/master', format: 'query', prHeadRef: false, sendGitQueryAsInput: false, buildxQuerySupport: true}, 'https://github.com/docker/actions-toolkit.git?ref=refs/heads/master&checksum=860c1904a1ce19322e91ac35af1ab07466440c37'],
     [{ref: 'master', format: 'query', prHeadRef: false, sendGitQueryAsInput: false, buildxQuerySupport: true}, 'https://github.com/docker/actions-toolkit.git?ref=refs/heads/master&checksum=860c1904a1ce19322e91ac35af1ab07466440c37'],
-    [{ref: 'refs/pull/15/merge', format: 'query', prHeadRef: false, sendGitQueryAsInput: false, buildxQuerySupport: true}, 'https://github.com/docker/actions-toolkit.git?ref=refs/pull/15/merge&checksum=860c1904a1ce19322e91ac35af1ab07466440c37'],
+    [{ref: 'refs/pull/15/merge', checksum: undefined, format: 'query', prHeadRef: false, sendGitQueryAsInput: false, buildxQuerySupport: true}, 'https://github.com/docker/actions-toolkit.git?ref=860c1904a1ce19322e91ac35af1ab07466440c37'],
     [{ref: 'refs/tags/v1.0.0', format: 'query', prHeadRef: false, sendGitQueryAsInput: false, buildxQuerySupport: true}, 'https://github.com/docker/actions-toolkit.git?ref=refs/tags/v1.0.0&checksum=860c1904a1ce19322e91ac35af1ab07466440c37'],
-    [{ref: 'refs/pull/15/merge', format: 'query', prHeadRef: true, sendGitQueryAsInput: false, buildxQuerySupport: true}, 'https://github.com/docker/actions-toolkit.git?ref=refs/pull/15/head&checksum=860c1904a1ce19322e91ac35af1ab07466440c37'],
+    [{ref: 'refs/pull/15/merge', checksum: undefined, format: 'query', prHeadRef: true, sendGitQueryAsInput: false, buildxQuerySupport: true}, 'https://github.com/docker/actions-toolkit.git?ref=f11797113e5a9b86bd976329c5dbb8a8bfdfadfa'],
+    [{ref: 'refs/pull/15/merge', checksum: undefined, format: 'query', prHeadRef: false, sendGitQueryAsInput: false, buildxQuerySupport: true, attrs: {checksum: 'cafebabe'}}, 'https://github.com/docker/actions-toolkit.git?ref=refs/pull/15/merge&checksum=cafebabe'],
     [{ref: 'refs/heads/master', format: 'query', prHeadRef: false, sendGitQueryAsInput: false, buildxQuerySupport: true, subdir: 'subdir'}, 'https://github.com/docker/actions-toolkit.git?ref=refs/heads/master&checksum=860c1904a1ce19322e91ac35af1ab07466440c37&subdir=subdir'],
     [{ref: 'refs/heads/master', format: 'query', prHeadRef: false, sendGitQueryAsInput: false, buildxQuerySupport: true, subdir: '.'}, 'https://github.com/docker/actions-toolkit.git?ref=refs/heads/master&checksum=860c1904a1ce19322e91ac35af1ab07466440c37'],
     [{ref: 'refs/heads/master', checksum: undefined, format: 'query', prHeadRef: false, sendGitQueryAsInput: false, buildxQuerySupport: true, attrs: {ref: 'refs/tags/v1.0.0', checksum: 'cafebabe', subdir: 'subdir', submodules: 'false'}}, 'https://github.com/docker/actions-toolkit.git?ref=refs/heads/master&checksum=cafebabe&subdir=subdir&submodules=false'],
@@ -108,14 +120,15 @@ describe('gitContext', () => {
     // fragment format
     [{ref: 'refs/heads/master', format: 'fragment', prHeadRef: false, sendGitQueryAsInput: false, buildxQuerySupport: true}, 'https://github.com/docker/actions-toolkit.git#860c1904a1ce19322e91ac35af1ab07466440c37'],
     [{ref: 'master', format: 'fragment', prHeadRef: false, sendGitQueryAsInput: false, buildxQuerySupport: true}, 'https://github.com/docker/actions-toolkit.git#860c1904a1ce19322e91ac35af1ab07466440c37'],
-    [{ref: 'refs/pull/15/merge', format: 'fragment', prHeadRef: false, sendGitQueryAsInput: false, buildxQuerySupport: true}, 'https://github.com/docker/actions-toolkit.git#refs/pull/15/merge'],
+    [{ref: 'refs/pull/15/merge', checksum: undefined, format: 'fragment', prHeadRef: false, sendGitQueryAsInput: false, buildxQuerySupport: true}, 'https://github.com/docker/actions-toolkit.git#860c1904a1ce19322e91ac35af1ab07466440c37'],
     [{ref: 'refs/tags/v1.0.0', format: 'fragment', prHeadRef: false, sendGitQueryAsInput: false, buildxQuerySupport: true}, 'https://github.com/docker/actions-toolkit.git#860c1904a1ce19322e91ac35af1ab07466440c37'],
-    [{ref: 'refs/pull/15/merge', format: 'fragment', prHeadRef: true, sendGitQueryAsInput: false, buildxQuerySupport: true}, 'https://github.com/docker/actions-toolkit.git#refs/pull/15/head'],
+    [{ref: 'refs/pull/15/merge', checksum: undefined, format: 'fragment', prHeadRef: true, sendGitQueryAsInput: false, buildxQuerySupport: true}, 'https://github.com/docker/actions-toolkit.git#f11797113e5a9b86bd976329c5dbb8a8bfdfadfa'],
+    [{ref: 'refs/pull/15/merge', checksum: undefined, format: 'fragment', prHeadRef: false, sendGitQueryAsInput: false, buildxQuerySupport: true, attrs: {checksum: 'cafebabe'}}, 'https://github.com/docker/actions-toolkit.git#refs/pull/15/merge'],
     [{ref: 'refs/heads/master', checksum: undefined, format: 'fragment', prHeadRef: false, sendGitQueryAsInput: false, buildxQuerySupport: true, attrs: {checksum: 'cafebabe', subdir: 'subdir', ref: 'refs/tags/v1.0.0'}}, 'https://github.com/docker/actions-toolkit.git#cafebabe:subdir'],
     [{ref: 'refs/heads/master', format: 'fragment', prHeadRef: false, sendGitQueryAsInput: false, buildxQuerySupport: true, attrs: {'keep-git-dir': 'true'}}, 'https://github.com/docker/actions-toolkit.git#860c1904a1ce19322e91ac35af1ab07466440c37'],
     [{ref: 'refs/heads/master', format: 'fragment', prHeadRef: false, sendGitQueryAsInput: false, buildxQuerySupport: true, subdir: 'subdir'}, 'https://github.com/docker/actions-toolkit.git#860c1904a1ce19322e91ac35af1ab07466440c37:subdir'],
     [{ref: 'refs/heads/master', format: 'fragment', prHeadRef: false, sendGitQueryAsInput: false, buildxQuerySupport: true, subdir: '.'}, 'https://github.com/docker/actions-toolkit.git#860c1904a1ce19322e91ac35af1ab07466440c37'],
-    [{ref: 'refs/pull/15/merge', format: 'fragment', prHeadRef: true, sendGitQueryAsInput: false, buildxQuerySupport: true, subdir: 'subdir'}, 'https://github.com/docker/actions-toolkit.git#refs/pull/15/head:subdir'],
+    [{ref: 'refs/pull/15/merge', checksum: undefined, format: 'fragment', prHeadRef: true, sendGitQueryAsInput: false, buildxQuerySupport: true, subdir: 'subdir'}, 'https://github.com/docker/actions-toolkit.git#f11797113e5a9b86bd976329c5dbb8a8bfdfadfa:subdir'],
   ];
 
   test.each(gitContextCases)('given %o should return %o', async (input: GitContextTestCase, expected: string) => {
diff --git a/src/buildx/build.ts b/src/buildx/build.ts
index 4cf2d95..8c65c50 100644
--- a/src/buildx/build.ts
+++ b/src/buildx/build.ts
@@ -59,28 +59,37 @@ export class Build {
 
   public async gitContext(opts?: GitContextOpts): Promise<string> {
     const gitContextCommonAttrs = new Set(['ref', 'checksum', 'subdir']);
-    const setPullRequestHeadRef = Util.parseBoolOrDefault(process.env.DOCKER_DEFAULT_GIT_CONTEXT_PR_HEAD_REF);
-    const commonAttrs = {
-      ref: opts?.attrs?.ref,
-      checksum: opts?.attrs?.checksum,
-      subdir: opts?.attrs?.subdir
-    };
+    const commonAttrs = opts?.attrs || {};
+    const extraAttrs = Object.entries(commonAttrs).filter(([name]) => !gitContextCommonAttrs.has(name));
 
-    const gitChecksum = opts?.checksum || commonAttrs.checksum || github.context.sha;
     let ref = opts?.ref || commonAttrs.ref || github.context.ref;
-    const subdir = opts?.subdir || commonAttrs.subdir;
-    const attrs = Object.entries(opts?.attrs || {}).filter(([name]) => !gitContextCommonAttrs.has(name));
     if (!ref.startsWith('refs/')) {
       ref = `refs/heads/${ref}`;
-    } else if (ref.startsWith(`refs/pull/`) && setPullRequestHeadRef) {
+    } else if (ref.startsWith(`refs/pull/`) && Util.parseBoolOrDefault(process.env.DOCKER_DEFAULT_GIT_CONTEXT_PR_HEAD_REF)) {
       ref = ref.replace(/\/merge$/g, '/head');
     }
 
+    const inputChecksum = opts?.checksum || commonAttrs.checksum;
+    const inputSubdir = opts?.subdir || commonAttrs.subdir;
+    const checksum = inputChecksum || (ref.startsWith(`refs/pull/`) ? undefined : github.context.sha);
+
+    // BuildKit resolves PR refs remotely at build time, so mutable refs like
+    // refs/pull/*/{merge,head} can drift away from the event SHA. actions/checkout
+    // avoids that by fetching the exact commit into a local PR ref; here we do the
+    // equivalent for implicit PR contexts by rewriting them to the event's commit SHA.
+    if (!inputChecksum && ref.startsWith(`refs/pull/`)) {
+      if (ref.endsWith('/merge')) {
+        ref = github.context.sha;
+      } else if (ref.endsWith('/head') && typeof github.context.payload.pull_request?.head?.sha === 'string') {
+        ref = github.context.payload.pull_request.head.sha;
+      }
+    }
+
     const baseURL = `${GitHub.serverURL}/${github.context.repo.owner}/${github.context.repo.repo}.git`;
     let format = opts?.format;
     if (!format) {
       format = 'fragment';
-      if (attrs.length > 0) {
+      if (extraAttrs.length > 0) {
         format = 'query';
       } else if (Util.parseBoolOrDefault(process.env.BUILDX_SEND_GIT_QUERY_AS_INPUT)) {
         try {
@@ -92,21 +101,23 @@ export class Build {
         }
       }
     }
+
     if (format === 'query') {
       const query = [`ref=${ref}`];
-      if (gitChecksum) {
-        query.push(`checksum=${gitChecksum}`);
+      if (checksum) {
+        query.push(`checksum=${checksum}`);
       }
-      if (subdir && subdir !== '.') {
-        query.push(`subdir=${subdir}`);
+      if (inputSubdir && inputSubdir !== '.') {
+        query.push(`subdir=${inputSubdir}`);
       }
-      for (const [name, value] of attrs) {
+      for (const [name, value] of extraAttrs) {
         query.push(`${name}=${value}`);
       }
       return `${baseURL}?${query.join('&')}`;
     }
-    const fragmentRef = gitChecksum && !ref.startsWith(`refs/pull/`) ? gitChecksum : ref;
-    return `${baseURL}#${fragmentRef}${subdir && subdir !== '.' ? `:${subdir}` : ''}`;
+
+    const fragmentRef = inputChecksum && ref.startsWith(`refs/pull/`) ? ref : (checksum ?? ref);
+    return `${baseURL}#${fragmentRef}${inputSubdir && inputSubdir !== '.' ? `:${inputSubdir}` : ''}`;
   }
 
   public getImageIDFilePath(): string {