diff --git a/src/executor.ts b/src/executor.ts
index f148560..5805440 100644
--- a/src/executor.ts
+++ b/src/executor.ts
@@ -19,10 +19,7 @@ import {
 	buildConflictResolutionPrompt,
 	MAX_DIFF_BYTES,
 } from "./prompts";
-import {
-	compileIgnorePatterns,
-	type DiffOptions,
-} from "./diff";
+import { compileIgnorePatterns } from "./diff";
 import { extractReflection } from "./reflection";
 import {
 	extractReview,
@@ -49,6 +46,7 @@ import {
 	ensureDir,
 	captureGitCommits,
 	captureGitHead,
+	canComputeRange,
 	getCommitRangeDiff,
 	hasUncommittedChanges,
 	getGitStatusPorcelain,
@@ -918,6 +916,15 @@ async function executeTask(
 								);
 								break;
 							}
+							// Cheap guard mirroring canCompareToBase: if the captured base ref no
+							// longer resolves (stale/broken worktree ref), warn explicitly and
+							// never treat the task as review-verified.
+							if (!canComputeRange(worktreeDir, baseRef)) {
+								sendChatMessage?.(
+									`~ review for ${task.id} · ${task.title} — diff could not be computed (base ref ${baseRef} no longer resolves)`,
+								);
+								break;
+							}
 							const rangeDiff = getCommitRangeDiff(worktreeDir, baseRef);
 							if (rangeDiff.kind === "error") {
 								sendChatMessage?.(
diff --git a/src/utils.ts b/src/utils.ts
index 06ca176..275670f 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -930,7 +930,10 @@ export function canComputeRange(projectDir: string, baseRef: string): boolean {
   const { execSync } = require("node:child_process");
   if (!/^[0-9a-f]{7,40}$/i.test(baseRef)) return false;
   try {
-    execSync(`git rev-parse --verify ${baseRef}`, {
+    // git cat-file -e truly verifies the object EXISTS (rev-parse --verify
+    // accepts any 40-hex SHA even if it was never created), so a stale/broken
+    // base ref is caught here rather than silently treated as no-changes.
+    execSync(`git cat-file -e ${baseRef}`, {
       cwd: projectDir,
       stdio: "pipe",
     });
@@ -961,9 +964,11 @@ export function getCommitRangeDiff(
   }
 
   // Verify the base ref resolves before diffing — a stale/unfetched ref is a
-  // computation failure, not a clean "no changes" signal.
+  // computation failure, not a clean "no changes" signal. git cat-file -e
+  // checks the object genuinely exists (rev-parse --verify would accept any
+  // 40-hex SHA even if it was never created).
   try {
-    execSync(`git rev-parse --verify ${baseRef}`, {
+    execSync(`git cat-file -e ${baseRef}`, {
       cwd: projectDir,
       stdio: "pipe",
     });