diff --git a/packages/server/src/utils/change-request-checkout.test.ts b/packages/server/src/utils/change-request-checkout.test.ts
index b630ee15af..dbd9f55a0d 100644
--- a/packages/server/src/utils/change-request-checkout.test.ts
+++ b/packages/server/src/utils/change-request-checkout.test.ts
@@ -61,4 +61,29 @@ describe("buildForkLocalBranchName", () => {
   it("falls back to the pr number when the owner is not a safe ref segment", () => {
     expect(buildForkLocalBranchName({ ...fork, headOwnerLogin: "-zaphod" })).toBe("pr-42/patch-1");
   });
+
+  // pull-ref-only heads (agit PRs, deleted branches) report headRef as the
+  // literal pull ref, e.g. refs/pull/42/head, not a real branch name
+  it("collapses a same-repository pull-ref-only head to pr-<number>", () => {
+    expect(
+      buildForkLocalBranchName({
+        ...fork,
+        isCrossRepository: false,
+        headOwnerLogin: "arthur",
+        hasHeadBranch: false,
+      }),
+    ).toBe("pr-42");
+  });
+
+  it("prefixes pr-<number> with the owner for a fork pull-ref-only head", () => {
+    expect(
+      buildForkLocalBranchName({ ...fork, headOwnerLogin: "Arthur", hasHeadBranch: false }),
+    ).toBe("arthur/pr-42");
+  });
+
+  it("does not double the pr number when the fork owner is unknown and there is no head branch", () => {
+    expect(buildForkLocalBranchName({ ...fork, headOwnerLogin: null, hasHeadBranch: false })).toBe(
+      "pr-42",
+    );
+  });
 });
diff --git a/packages/server/src/utils/change-request-checkout.ts b/packages/server/src/utils/change-request-checkout.ts
index f3f0b3feeb..7fc6e2190b 100644
--- a/packages/server/src/utils/change-request-checkout.ts
+++ b/packages/server/src/utils/change-request-checkout.ts
@@ -26,12 +26,22 @@ export interface ForkLocalBranchNameInput {
   number: number;
   isCrossRepository: boolean;
   headOwnerLogin: string | null;
+  /** false when headRef is a synthetic pull ref, not a real branch name (agit PRs, deleted branches) */
+  hasHeadBranch?: boolean;
 }
 
 export function buildForkLocalBranchName(params: ForkLocalBranchNameInput): string {
+  const pullRefFallback = `pr-${params.number}`;
+  // headRef is the literal pull ref (e.g. refs/pull/42/head) when there is no
+  // real branch to name after, so use the pr number instead
+  const headSegment = params.hasHeadBranch === false ? pullRefFallback : params.headRef;
   if (!params.isCrossRepository) {
-    return params.headRef;
+    return headSegment;
   }
   const owner = normalizeForgeOwnerForBranch(params.headOwnerLogin);
-  return `${owner ?? `pr-${params.number}`}/${params.headRef}`;
+  if (!owner) {
+    // don't double up pr-<n>/pr-<n> when the owner is also unknown
+    return headSegment === pullRefFallback ? headSegment : `${pullRefFallback}/${headSegment}`;
+  }
+  return `${owner}/${headSegment}`;
 }