diff --git a/core/workspace/BUILD.bazel b/core/workspace/BUILD.bazel
index 8a48972b..af29035b 100644
--- a/core/workspace/BUILD.bazel
+++ b/core/workspace/BUILD.bazel
@@ -19,6 +19,7 @@ go_library(
 go_test(
     name = "workspace_test",
     srcs = [
+        "gitrequest_realgit_test.go",
         "gitrequest_test.go",
         "request_test.go",
         "workspace_test.go",
diff --git a/core/workspace/gitrequest.go b/core/workspace/gitrequest.go
index 304492a0..f8d18659 100644
--- a/core/workspace/gitrequest.go
+++ b/core/workspace/gitrequest.go
@@ -58,7 +58,9 @@ func (r *gitRequest) Apply(ctx context.Context) error {
 	if !isAncestor {
 		return fmt.Errorf("head SHA %q is not an ancestor of PR %s", r.headSHA, r.requestID)
 	}
-	patch, err := r.git.Diff(ctx, r.baseRef, fmt.Sprintf("pull/%s/head", r.requestID), "--binary", "--merge-base")
+	// Diff against the pinned head SHA so the materialized tree is
+	// deterministic for a given change URI even as the PR advances.
+	patch, err := r.git.Diff(ctx, r.baseRef, r.headSHA, "--binary", "--merge-base")
 	if err != nil {
 		return fmt.Errorf("compute diff for PR %s: %w", r.requestID, err)
 	}
diff --git a/core/workspace/gitrequest_realgit_test.go b/core/workspace/gitrequest_realgit_test.go
new file mode 100644
index 00000000..bcfbb976
--- /dev/null
+++ b/core/workspace/gitrequest_realgit_test.go
@@ -0,0 +1,161 @@
+// Copyright (c) 2026 Uber Technologies, Inc.
+//
+// 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
+//
+//     http://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.
+
+package workspace_test
+
+import (
+	"context"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"github.com/uber/tango/core/git"
+	"github.com/uber/tango/core/workspace"
+	"go.uber.org/zap"
+)
+
+func runGit(t *testing.T, dir string, args ...string) string {
+	t.Helper()
+	cmd := exec.Command("git", args...)
+	cmd.Dir = dir
+	cmd.Env = append(os.Environ(),
+		"GIT_AUTHOR_NAME=test",
+		"GIT_AUTHOR_EMAIL=test@test.com",
+		"GIT_COMMITTER_NAME=test",
+		"GIT_COMMITTER_EMAIL=test@test.com",
+	)
+	out, err := cmd.CombinedOutput()
+	require.NoError(t, err, "git %v in %s failed: %s", args, dir, string(out))
+	return strings.TrimSpace(string(out))
+}
+
+func setupBareRepoWithPR(t *testing.T, prContent string) (bareDir, baseSHA, prSHA string) {
+	t.Helper()
+
+	workDir := filepath.Join(t.TempDir(), "work")
+	bareDir = filepath.Join(t.TempDir(), "bare.git")
+
+	require.NoError(t, os.MkdirAll(workDir, 0o755))
+	runGit(t, workDir, "init", "-b", "main")
+
+	require.NoError(t, os.WriteFile(filepath.Join(workDir, "base.txt"), []byte("base"), 0o644))
+	runGit(t, workDir, "add", "base.txt")
+	runGit(t, workDir, "commit", "-m", "base commit")
+	baseSHA = runGit(t, workDir, "rev-parse", "HEAD")
+
+	runGit(t, workDir, "checkout", "-b", "pr-branch")
+	require.NoError(t, os.WriteFile(filepath.Join(workDir, "pr.txt"), []byte(prContent), 0o644))
+	runGit(t, workDir, "add", "pr.txt")
+	runGit(t, workDir, "commit", "-m", "PR commit: "+prContent)
+	prSHA = runGit(t, workDir, "rev-parse", "HEAD")
+
+	runGit(t, t.TempDir(), "clone", "--bare", workDir, bareDir)
+	runGit(t, workDir, "push", bareDir, "HEAD:refs/pull/1/head")
+	runGit(t, bareDir, "branch", "-D", "pr-branch")
+
+	return bareDir, baseSHA, prSHA
+}
+
+func cloneWorker(t *testing.T, bareDir, baseSHA string) string {
+	t.Helper()
+	workerDir := filepath.Join(t.TempDir(), "worker")
+	runGit(t, t.TempDir(), "clone", bareDir, workerDir)
+	runGit(t, workerDir, "checkout", baseSHA)
+	// git.Interface commits need a repo-local identity: the Bazel test
+	// sandbox has no global git config.
+	runGit(t, workerDir, "config", "user.name", "test")
+	runGit(t, workerDir, "config", "user.email", "test@test.com")
+	return workerDir
+}
+
+func advancePR(t *testing.T, bareDir, newContent string) {
+	t.Helper()
+	advanceDir := filepath.Join(t.TempDir(), "advance")
+	runGit(t, t.TempDir(), "clone", bareDir, advanceDir)
+	runGit(t, advanceDir, "fetch", "origin", "refs/pull/1/head:refs/pull/1/head")
+	runGit(t, advanceDir, "checkout", "refs/pull/1/head")
+	require.NoError(t, os.WriteFile(filepath.Join(advanceDir, "pr.txt"), []byte(newContent), 0o644))
+	runGit(t, advanceDir, "add", "pr.txt")
+	runGit(t, advanceDir, "commit", "-m", "advance PR")
+	runGit(t, advanceDir, "push", bareDir, "HEAD:refs/pull/1/head")
+}
+
+func TestGitRequest_RealGit_AppliesPinnedContent(t *testing.T) {
+	t.Parallel()
+	ctx := context.Background()
+	logger := zap.NewNop().Sugar()
+
+	bareDir, baseSHA, prSHA := setupBareRepoWithPR(t, "pr-content")
+	workerDir := cloneWorker(t, bareDir, baseSHA)
+
+	req := workspace.NewGitRequest(git.New(workerDir, logger), "1", baseSHA, prSHA, logger)
+	require.NoError(t, req.Apply(ctx))
+
+	content, err := os.ReadFile(filepath.Join(workerDir, "pr.txt"))
+	require.NoError(t, err)
+	assert.Equal(t, "pr-content", string(content))
+}
+
+func TestGitRequest_RealGit_StableTreeAcrossPRAdvance(t *testing.T) {
+	t.Parallel()
+	ctx := context.Background()
+	logger := zap.NewNop().Sugar()
+
+	bareDir, baseSHA, prSHA1 := setupBareRepoWithPR(t, "version-1")
+
+	worker1 := cloneWorker(t, bareDir, baseSHA)
+	g1 := git.New(worker1, logger)
+	require.NoError(t, workspace.NewGitRequest(g1, "1", baseSHA, prSHA1, logger).Apply(ctx))
+	tree1, err := g1.RevParse(ctx, "HEAD^{tree}")
+	require.NoError(t, err)
+
+	advancePR(t, bareDir, "version-2")
+
+	worker2 := cloneWorker(t, bareDir, baseSHA)
+	g2 := git.New(worker2, logger)
+	require.NoError(t, workspace.NewGitRequest(g2, "1", baseSHA, prSHA1, logger).Apply(ctx))
+	tree2, err := g2.RevParse(ctx, "HEAD^{tree}")
+	require.NoError(t, err)
+
+	assert.Equal(t, tree1, tree2, "the pinned head SHA must yield the same tree regardless of PR head advancement")
+
+	content, err := os.ReadFile(filepath.Join(worker2, "pr.txt"))
+	require.NoError(t, err)
+	assert.Equal(t, "version-1", string(content), "the applied content must be the pinned version, not the advanced head")
+}
+
+func TestGitRequest_RealGit_RejectsNonAncestorSHA(t *testing.T) {
+	t.Parallel()
+	ctx := context.Background()
+	logger := zap.NewNop().Sugar()
+
+	bareDir, baseSHA, _ := setupBareRepoWithPR(t, "original")
+
+	sideDir := filepath.Join(t.TempDir(), "side")
+	runGit(t, t.TempDir(), "clone", bareDir, sideDir)
+	runGit(t, sideDir, "checkout", "-b", "side", baseSHA)
+	require.NoError(t, os.WriteFile(filepath.Join(sideDir, "other.txt"), []byte("different"), 0o644))
+	runGit(t, sideDir, "add", "other.txt")
+	runGit(t, sideDir, "commit", "-m", "side commit")
+	sideSHA := runGit(t, sideDir, "rev-parse", "HEAD")
+	runGit(t, sideDir, "push", bareDir, "HEAD:refs/heads/side")
+
+	workerDir := cloneWorker(t, bareDir, baseSHA)
+	req := workspace.NewGitRequest(git.New(workerDir, logger), "1", baseSHA, sideSHA, logger)
+	require.Error(t, req.Apply(ctx))
+}
diff --git a/core/workspace/gitrequest_test.go b/core/workspace/gitrequest_test.go
index efcdbda0..b06e6333 100644
--- a/core/workspace/gitrequest_test.go
+++ b/core/workspace/gitrequest_test.go
@@ -70,3 +70,17 @@ func TestGitRequest_Apply_IsAncestorFails_ReturnsError(t *testing.T) {
 	require.Error(t, err)
 	assert.Contains(t, err.Error(), "failed to read PR commit history")
 }
+
+func TestGitRequest_Apply_DiffsAgainstPinnedHeadSHA(t *testing.T) {
+	ctrl := gomock.NewController(t)
+	git := gitmock.NewMockInterface(ctrl)
+	git.EXPECT().Fetch(gomock.Any(), "origin", gomock.Any(), gomock.Any()).Return(nil)
+	git.EXPECT().IsAncestor(gomock.Any(), "pinnedsha", "pull/10/head").Return(true, nil)
+	git.EXPECT().Diff(gomock.Any(), "baseRef", "pinnedsha", "--binary", "--merge-base").Return([]byte("patch"), nil)
+	git.EXPECT().ApplyPatch(gomock.Any(), []byte("patch")).Return(nil)
+	git.EXPECT().Commit(gomock.Any(), gomock.Any(), gomock.Any()).Return(nil)
+	git.EXPECT().SubmoduleUpdate(gomock.Any()).Return(nil)
+	req := NewGitRequest(git, "10", "baseRef", "pinnedsha", zap.NewNop().Sugar())
+	err := req.Apply(context.Background())
+	require.NoError(t, err)
+}