diff --git a/packages/source-control-gitlab/src/server/gitLabMergeRequestJson.test.ts b/packages/source-control-gitlab/src/server/gitLabMergeRequestJson.test.ts
index c9bc12743756c2593c55348a13c4b3c9edd8c918..1cb81087e0c8ce767a9bdfcbadc5fa0d9cf68956 100644
--- a/packages/source-control-gitlab/src/server/gitLabMergeRequestJson.test.ts
+++ b/packages/source-control-gitlab/src/server/gitLabMergeRequestJson.test.ts
@@ -181,6 +181,20 @@ describe("decodeMergeRequestDetailJson", () => {
     expect(detail.changedFiles).toBe(0);
   });
 
+  it("reads the head commit, and says nothing where GitLab omitted it", () => {
+    const headSha = (entry: Record<string, unknown>) =>
+      expectSuccess(decodeMergeRequestDetailJson(detailJson(entry))).headSha;
+    const sha = "afce4da4d5fe7d2fd0647b5b864526ea0a2827cb";
+
+    expect(headSha({ sha })).toBe(sha);
+    // A new merge request can report its head before GitLab has computed `diff_refs`.
+    expect(headSha({ sha, diff_refs: null })).toBe(sha);
+    expect(headSha({ sha, merge_commit_sha: "0".repeat(40) })).toBe(sha);
+    expect(headSha({})).toBeUndefined();
+    expect(headSha({ sha: null })).toBeUndefined();
+    expect(headSha({ sha: "  " })).toBeUndefined();
+  });
+
   it("reads either auto-merge field, and says nothing where GitLab named neither", () => {
     const armed = (entry: Record<string, unknown>) =>
       expectSuccess(decodeMergeRequestDetailJson(detailJson(entry))).autoMergeEnabled;
diff --git a/packages/source-control-gitlab/src/server/gitLabMergeRequestJson.ts b/packages/source-control-gitlab/src/server/gitLabMergeRequestJson.ts
index b9991920d0d3a880bcf1edb12b0bb777f87d012a..d61816b516b032477069125e14fddbd09971d08e 100644
--- a/packages/source-control-gitlab/src/server/gitLabMergeRequestJson.ts
+++ b/packages/source-control-gitlab/src/server/gitLabMergeRequestJson.ts
@@ -51,6 +51,8 @@ const RawMergeRequestSchema = Schema.Struct({
   author: Schema.optional(Schema.NullOr(RawUserSchema)),
   source_branch: Schema.String,
   target_branch: Schema.String,
+  /** The source branch's head commit. */
+  sha: Schema.optional(Schema.NullOr(Schema.String)),
   state: Schema.optional(Schema.NullOr(Schema.String)),
   draft: Schema.optional(Schema.Boolean),
   work_in_progress: Schema.optional(Schema.Boolean),
@@ -240,6 +242,8 @@ export interface GitLabMergeRequestDetail extends GitLabMergeRequestListItem {
    * it: an install too old to answer must not be read as saying the branch is current.
    */
   readonly divergedCommits?: number;
+  /** Absent where GitLab did not report the head commit. */
+  readonly headSha?: string;
 }
 
 function trimmed(value: string | null | undefined): string | null {
@@ -365,6 +369,7 @@ function toListItem(
 
 function toDetail(raw: Schema.Schema.Type<typeof RawMergeRequestSchema>): GitLabMergeRequestDetail {
   const listItem = toListItem(raw);
+  const headSha = trimmed(raw.sha);
   const autoMerge =
     raw.merge_when_pipeline_succeeds == null && raw.auto_merge_enabled == null
       ? undefined
@@ -390,6 +395,7 @@ function toDetail(raw: Schema.Schema.Type<typeof RawMergeRequestSchema>): GitLab
       ? { autoMergeMethod: "squash" as const }
       : {}),
     ...(raw.diverged_commits_count == null ? {} : { divergedCommits: raw.diverged_commits_count }),
+    ...(headSha === null ? {} : { headSha }),
   };
 }