diff --git a/actions/setup/js/update_pull_request.cjs b/actions/setup/js/update_pull_request.cjs
index 8d22e06f8ff..59e72cf3e85 100644
--- a/actions/setup/js/update_pull_request.cjs
+++ b/actions/setup/js/update_pull_request.cjs
@@ -18,6 +18,30 @@ const { generateHistoryUrl } = require("./generate_history_link.cjs");
 const { getErrorMessage } = require("./error_helpers.cjs");
 const { withRetry, isTransientError } = require("./error_recovery.cjs");
 
+/**
+ * @param {unknown} error
+ * @returns {boolean}
+ */
+function isNonFatalUpdateBranchError(error) {
+  /** @type {number | undefined} */
+  let status;
+  if (typeof error === "object" && error !== null && "status" in error) {
+    status = /** @type {{status?: number}} */ (error).status;
+  }
+  if (status !== undefined && status !== 422) {
+    return false;
+  }
+
+  // GitHub update-branch API can return these 422 messages for benign conditions:
+  // - already up to date ("There are no new commits on the base branch")
+  // - cannot auto-update due to conflict ("merge conflict between base and head")
+  // These should not fail safe output processing.
+  const message = getErrorMessage(error).toLowerCase();
+  return (
+    message.includes("there are no new commits on the base branch") || message.includes("merge conflict between base and head")
+  );
+}
+
 /**
  * Execute the pull request update API call
  * @param {any} github - GitHub API client
@@ -55,8 +79,13 @@ async function executePRUpdate(github, context, prNumber, updateData) {
         `update pull request #${prNumber} branch from base`
       );
     } catch (error) {
-      core.warning(`Failed to update pull request #${prNumber} branch from base: ${getErrorMessage(error)}`);
-      throw error;
+      const errorMessage = getErrorMessage(error);
+      if (isNonFatalUpdateBranchError(error)) {
+        core.warning(`Failed to update pull request #${prNumber} branch from base (non-fatal): ${errorMessage}`);
+      } else {
+        core.warning(`Failed to update pull request #${prNumber} branch from base: ${errorMessage}`);
+        throw error;
+      }
     }
   }
 
diff --git a/actions/setup/js/update_pull_request.test.cjs b/actions/setup/js/update_pull_request.test.cjs
index e4a34d88f67..b33d7361847 100644
--- a/actions/setup/js/update_pull_request.test.cjs
+++ b/actions/setup/js/update_pull_request.test.cjs
@@ -847,4 +847,36 @@ describe("update_pull_request.cjs - update_branch behavior", () => {
     expect(mockGithub.rest.pulls.updateBranch).toHaveBeenCalledTimes(1);
     expect(mockCore.warning).toHaveBeenCalledWith(expect.stringContaining("Failed to update pull request #100 branch from base"));
   });
+
+  it("should treat no-new-commits updateBranch response as a non-fatal no-op", async () => {
+    mockGithub.rest.pulls.updateBranch.mockRejectedValueOnce(new Error("There are no new commits on the base branch."));
+
+    const handler = await updatePRModule.main({ update_branch: true });
+    const result = await handler({ pull_request_number: 100 });
+
+    expect(result.success).toBe(true);
+    expect(mockGithub.rest.pulls.updateBranch).toHaveBeenCalledTimes(1);
+    expect(mockGithub.rest.pulls.update).not.toHaveBeenCalled();
+    expect(mockCore.warning).toHaveBeenCalledWith(expect.stringContaining("branch from base (non-fatal)"));
+  });
+
+  it("should continue title/body updates when updateBranch reports merge conflict", async () => {
+    mockGithub.rest.pulls.updateBranch.mockRejectedValueOnce(new Error("merge conflict between base and head"));
+
+    const handler = await updatePRModule.main({ update_branch: true });
+    const result = await handler({
+      pull_request_number: 100,
+      title: "Updated PR",
+    });
+
+    expect(result.success).toBe(true);
+    expect(mockGithub.rest.pulls.updateBranch).toHaveBeenCalledTimes(1);
+    expect(mockGithub.rest.pulls.update).toHaveBeenCalledWith({
+      owner: "testowner",
+      repo: "testrepo",
+      pull_number: 100,
+      title: "Updated PR",
+    });
+    expect(mockCore.warning).toHaveBeenCalledWith(expect.stringContaining("branch from base (non-fatal)"));
+  });
 });
diff --git a/docs/src/content/docs/reference/safe-outputs-pull-requests.md b/docs/src/content/docs/reference/safe-outputs-pull-requests.md
index 4802975a1b6..f07fcaee013 100644
--- a/docs/src/content/docs/reference/safe-outputs-pull-requests.md
+++ b/docs/src/content/docs/reference/safe-outputs-pull-requests.md
@@ -137,6 +137,8 @@ safe-outputs:
 
 When `update-branch: true` is set, the handler calls the GitHub REST `pulls.updateBranch` API to merge the latest base branch changes into the PR branch before applying title or body updates. This requires `contents: write` permission; without it only `contents: read` is needed. The field can also be used alone (with `title: false` and `body: false`) to update the branch without changing the PR description.
 
+If GitHub reports `There are no new commits on the base branch.` or `merge conflict between base and head`, the branch update is treated as best-effort: the workflow logs a warning and continues processing the safe output.
+
 When using `target: "*"`, the agent must provide `pull_request_number` in the output to identify which pull request to update.
 
 **Operation Types**: Same as `update-issue` (`append`, `prepend`, `replace`). Title updates always replace the existing title. Disable fields by setting to `false`.