diff --git a/src/github/operations/branch.ts b/src/github/operations/branch.ts
index 880c91ce..bfd68ea7 100644
--- a/src/github/operations/branch.ts
+++ b/src/github/operations/branch.ts
@@ -9,6 +9,8 @@
 import { $ } from "bun";
 import { execFileSync } from "child_process";
 import type { ParsedGitHubContext } from "../context";
+import { isPullRequestEvent } from "../context";
+import { retryWithBackoff } from "../../utils/retry";
 import type { GitHubPullRequest } from "../types";
 import type { Octokits } from "../api/client";
 import type { FetchDataResult } from "../data/fetcher";
@@ -156,6 +158,7 @@ export async function setupBranch(
   octokits: Octokits,
   githubData: FetchDataResult,
   context: ParsedGitHubContext,
+  triggerTime?: string,
 ): Promise<BranchInfo> {
   const { owner, repo } = context.repository;
   const entityNumber = context.entityNumber;
@@ -177,6 +180,7 @@ export async function setupBranch(
       console.log("This is an open PR, checking out PR branch...");
 
       const branchName = prData.headRefName;
+      const headSha = prData.headRefOid;
 
       // Determine optimal fetch depth based on PR commit count, with a minimum
       // of 20. Only applied to a checkout that is already shallow — see
@@ -195,25 +199,56 @@ export async function setupBranch(
 
       // Validate branch names before use to prevent command injection
       validateBranchName(branchName);
+      if (!/^[0-9a-f]{40}$/.test(headSha)) {
+        throw new Error(
+          `Unexpected head SHA for PR #${entityNumber}: ${headSha}`,
+        );
+      }
 
-      // For cross-repository (fork) PRs, fetch via the pull ref since the
-      // branch only exists on the fork's remote, not on origin.
-      if (prData.isCrossRepository) {
-        console.log(
-          `PR #${entityNumber} is from a fork, fetching via refs/pull/${entityNumber}/head...`,
+      if (isPullRequestEvent(context)) {
+        const eventSha = context.payload.pull_request.head.sha;
+        if (eventSha !== headSha) {
+          throw new Error(
+            `PR #${entityNumber} was updated after the trigger (${eventSha} -> ${headSha}). Push again or mention Claude to run on the latest version.`,
+          );
+        }
+      } else if (triggerTime) {
+        if (!prData.headRepository) {
+          throw new Error(
+            `PR #${entityNumber}: head repository is not available, cannot pin the PR version`,
+          );
+        }
+        const pushedAt = await headPushedAt(
+          octokits,
+          {
+            owner: prData.headRepository.owner.login,
+            repo: prData.headRepository.name,
+          },
+          branchName,
+          headSha,
+        );
+        if (new Date(pushedAt).getTime() >= new Date(triggerTime).getTime()) {
+          throw new Error(
+            `PR #${entityNumber} was updated after the trigger (${pushedAt} >= ${triggerTime}). Mention Claude again to run on the latest version.`,
+          );
+        }
+      }
+
+      const localBranch = prData.isCrossRepository
+        ? `pr-${entityNumber}`
+        : branchName;
+      console.log(`PR #${entityNumber}: pinning ${localBranch} to ${headSha}`);
+      execGit(["fetch", "origin", ...depthArgs, headSha]);
+      execGit(["checkout", "-B", localBranch, headSha, "--"]);
+
+      const checkedOut = execFileSync("git", ["rev-parse", "HEAD"], {
+        encoding: "utf8",
+      }).trim();
+      if (checkedOut !== headSha) {
+        throw new Error(
+          `PR #${entityNumber}: checked out ${checkedOut}, expected ${headSha}`,
         );
-        execGit([
-          "fetch",
-          "origin",
-          ...depthArgs,
-          `pull/${entityNumber}/head:${branchName}`,
-        ]);
-      } else {
-        // Execute git commands to checkout PR branch (dynamic depth based on PR size)
-        // Using execFileSync instead of shell template literals for security
-        execGit(["fetch", "origin", ...depthArgs, branchName]);
       }
-      execGit(["checkout", branchName, "--"]);
 
       console.log(`Successfully checked out PR branch for PR #${entityNumber}`);
 
@@ -223,7 +258,7 @@ export async function setupBranch(
 
       return {
         baseBranch,
-        currentBranch: branchName,
+        currentBranch: localBranch,
       };
     }
   }
@@ -350,3 +385,26 @@ export async function setupBranch(
     process.exit(1);
   }
 }
+
+async function headPushedAt(
+  octokits: Octokits,
+  headRepo: { owner: string; repo: string },
+  branchName: string,
+  headSha: string,
+): Promise<string> {
+  const { data } = await retryWithBackoff(() =>
+    octokits.rest.request("GET /repos/{owner}/{repo}/activity", {
+      owner: headRepo.owner,
+      repo: headRepo.repo,
+      ref: `refs/heads/${branchName}`,
+      per_page: 1,
+    }),
+  );
+  const latest = (data as Array<{ after: string; timestamp: string }>)[0];
+  if (!latest || latest.after !== headSha) {
+    throw new Error(
+      `Could not determine when ${headSha} was pushed to ${headRepo.owner}/${headRepo.repo}:${branchName}`,
+    );
+  }
+  return latest.timestamp;
+}
diff --git a/src/modes/tag/index.ts b/src/modes/tag/index.ts
index 54e838a4..e827b416 100644
--- a/src/modes/tag/index.ts
+++ b/src/modes/tag/index.ts
@@ -14,7 +14,12 @@ import {
   extractOriginalBody,
 } from "../../github/data/fetcher";
 import { createPrompt } from "../../create-prompt";
-import { isEntityContext } from "../../github/context";
+import {
+  isEntityContext,
+  isIssueCommentEvent,
+  isPullRequestReviewCommentEvent,
+} from "../../github/context";
+import { updateClaudeComment } from "../../github/operations/comments/update-claude-comment";
 import type { GitHubContext } from "../../github/context";
 import type { Octokits } from "../../github/api/client";
 import { parseAllowedTools } from "../agent/parse-tools";
@@ -64,7 +69,30 @@ export async function prepareTagMode({
   });
 
   // Setup branch
-  const branchInfo = await setupBranch(octokit, githubData, context);
+  const editedAt =
+    context.eventAction === "edited" &&
+    (isIssueCommentEvent(context) || isPullRequestReviewCommentEvent(context))
+      ? context.payload.comment.updated_at
+      : undefined;
+  let branchInfo;
+  try {
+    branchInfo = await setupBranch(
+      octokit,
+      githubData,
+      context,
+      editedAt || triggerTime,
+    );
+  } catch (error) {
+    const message = error instanceof Error ? error.message : String(error);
+    await updateClaudeComment(octokit.rest, {
+      owner: context.repository.owner,
+      repo: context.repository.repo,
+      commentId,
+      body: message,
+      isPullRequestReviewComment: isPullRequestReviewCommentEvent(context),
+    }).catch((e) => console.error(`Failed to update tracking comment: ${e}`));
+    throw error;
+  }
 
   // Configure git authentication
   // SSH signing takes precedence if provided