diff --git a/action.yml b/action.yml
index 6e25c6ba5..3c18932b8 100644
--- a/action.yml
+++ b/action.yml
@@ -131,6 +131,7 @@ runs:
       env:
         INPUT_DIRECTORY: ${{ inputs.directory }}
         BASE_SHA: ${{ github.event.pull_request.base.sha }}
+        HEAD_SHA: ${{ github.event.pull_request.head.sha }}
         CHANGED_FILES_FILE: ${{ runner.temp }}/react-doctor-changed-files.txt
       run: |
         # Best-effort fetch of the PR base commit so react-doctor can read base
@@ -169,13 +170,15 @@ runs:
 
         # Derive the changed set locally. Three-dot (merge-base) + AMR
         # (added/modified/renamed) match the `pulls.listFiles` API contract and
-        # the CLI's baseline semantics. Attempt it whenever a base SHA is known —
-        # the base may be reachable in history even when the fetch above was a
-        # no-op or failed. On any failure (shallow history with no reachable
-        # merge-base), leave the file unwritten so the API fallback runs — no
-        # regression vs the previous API-only behavior.
+        # the CLI's baseline semantics. Compare the event's base SHA to the
+        # event's head SHA (not the synthetic merge-ref HEAD, which includes
+        # base-branch drift). Attempt it whenever both SHAs are known — the base
+        # may be reachable in history even when the fetch above was a no-op or
+        # failed. On any failure (shallow history with no reachable merge-base),
+        # leave the file unwritten so the API fallback runs — no regression vs
+        # the previous API-only behavior.
         RAW_CHANGED="${RUNNER_TEMP:-/tmp}/react-doctor-raw-changed.txt"
-        if [ -n "$BASE_SHA" ] && git -C "$INPUT_DIRECTORY" diff --name-only --diff-filter=AMR "$BASE_SHA...HEAD" > "$RAW_CHANGED" 2>/dev/null; then
+        if [ -n "$BASE_SHA" ] && [ -n "$HEAD_SHA" ] && git -C "$INPUT_DIRECTORY" diff --name-only --diff-filter=AMR "$BASE_SHA...$HEAD_SHA" > "$RAW_CHANGED" 2>/dev/null; then
           node "$GITHUB_ACTION_PATH/scripts/normalize-changed-files.mjs" "$SCAN_PREFIX" "$CHANGED_FILES_FILE" < "$RAW_CHANGED"
           echo "path=$CHANGED_FILES_FILE" >> "$GITHUB_OUTPUT"
         else
diff --git a/packages/react-doctor/tests/github-action.test.ts b/packages/react-doctor/tests/github-action.test.ts
index 0f41712aa..380d3f783 100644
--- a/packages/react-doctor/tests/github-action.test.ts
+++ b/packages/react-doctor/tests/github-action.test.ts
@@ -287,6 +287,126 @@ describe("GitHub Action contract", () => {
     30_000,
   );
 
+  itOnPosix(
+    "compares base to PR head (not synthetic merge-ref HEAD) to exclude base-branch drift",
+    () => {
+      const fixtureRoot = fs.mkdtempSync(path.join(os.tmpdir(), "react-doctor-action-"));
+      try {
+        const originDirectory = path.join(fixtureRoot, "origin");
+        const checkoutDirectory = path.join(fixtureRoot, "checkout");
+        const runnerTemp = path.join(fixtureRoot, "runner-temp");
+        fs.mkdirSync(originDirectory);
+        fs.mkdirSync(checkoutDirectory);
+        fs.mkdirSync(runnerTemp);
+
+        runGit(originDirectory, "init");
+        fs.mkdirSync(path.join(originDirectory, "src"), { recursive: true });
+        fs.writeFileSync(
+          path.join(originDirectory, "src", "app.tsx"),
+          "export const App = () => null;\n",
+        );
+        runGit(originDirectory, "add", ".");
+        runGit(originDirectory, "-c", "commit.gpgsign=false", "commit", "-m", "initial");
+        const baseSha = runGit(originDirectory, "rev-parse", "HEAD");
+
+        runGit(originDirectory, "checkout", "-b", "pr");
+        fs.writeFileSync(
+          path.join(originDirectory, "src", "feature.tsx"),
+          "export const Feature = () => null;\n",
+        );
+        runGit(originDirectory, "add", ".");
+        runGit(originDirectory, "-c", "commit.gpgsign=false", "commit", "-m", "feature");
+        const prHeadSha = runGit(originDirectory, "rev-parse", "HEAD");
+
+        runGit(originDirectory, "checkout", "-");
+        for (let index = 0; index < 15; index += 1) {
+          fs.writeFileSync(
+            path.join(originDirectory, "src", `drift-${index}.tsx`),
+            `export const Drift${index} = () => null;\n`,
+          );
+          runGit(originDirectory, "add", ".");
+          runGit(originDirectory, "-c", "commit.gpgsign=false", "commit", "-m", `drift ${index}`);
+        }
+
+        runGit(originDirectory, "-c", "advice.detachedHead=false", "checkout", "--detach");
+        runGit(
+          originDirectory,
+          "-c",
+          "commit.gpgsign=false",
+          "merge",
+          "--no-ff",
+          "pr",
+          "-m",
+          "merge",
+        );
+        runGit(originDirectory, "update-ref", "refs/pull/1/merge", "HEAD");
+
+        runGit(checkoutDirectory, "init");
+        runGit(checkoutDirectory, "remote", "add", "origin", pathToFileURL(originDirectory).href);
+        runGit(
+          checkoutDirectory,
+          "fetch",
+          "--no-tags",
+          "origin",
+          "+refs/pull/1/merge:refs/remotes/pull/1/merge",
+        );
+        runGit(
+          checkoutDirectory,
+          "-c",
+          "advice.detachedHead=false",
+          "checkout",
+          "--force",
+          "refs/remotes/pull/1/merge",
+        );
+
+        const changedFilesFile = path.join(runnerTemp, "react-doctor-changed-files.txt");
+        const githubOutputFile = path.join(runnerTemp, "github-output.txt");
+        fs.writeFileSync(githubOutputFile, "");
+        const scriptOutput = execFileSync(
+          "bash",
+          [
+            "--noprofile",
+            "--norc",
+            "-e",
+            "-o",
+            "pipefail",
+            "-c",
+            extractBaseStepScript(readActionYaml()),
+          ],
+          {
+            cwd: checkoutDirectory,
+            encoding: "utf8",
+            env: {
+              ...GIT_TEST_ENV,
+              INPUT_DIRECTORY: ".",
+              BASE_SHA: baseSha,
+              HEAD_SHA: prHeadSha,
+              CHANGED_FILES_FILE: changedFilesFile,
+              RUNNER_TEMP: runnerTemp,
+              GITHUB_OUTPUT: githubOutputFile,
+              GITHUB_ACTION_PATH: REPOSITORY_ROOT,
+            },
+          },
+        );
+
+        expect(scriptOutput).not.toContain("could not derive");
+        expect(fs.existsSync(changedFilesFile)).toBe(true);
+        const changedFiles = fs
+          .readFileSync(changedFilesFile, "utf8")
+          .split("\n")
+          .filter((line) => line.trim() !== "")
+          .sort();
+
+        expect(changedFiles).toEqual(["src/feature.tsx"]);
+        expect(changedFiles).not.toContain("src/drift-0.tsx");
+        expect(changedFiles).not.toContain("src/drift-14.tsx");
+      } finally {
+        fs.rmSync(fixtureRoot, { recursive: true, force: true });
+      }
+    },
+    30_000,
+  );
+
   it("falls back to a full-project scan when listing PR files is not permitted", () => {
     const prFilesStep = normalizeWhitespace(extractStep(readActionYaml(), "- id: pr-files"));