diff --git a/eng/tools/azure-sdk-tools/azpysdk/breaking.py b/eng/tools/azure-sdk-tools/azpysdk/breaking.py
index f204f1794e39..83f289963cdd 100644
--- a/eng/tools/azure-sdk-tools/azpysdk/breaking.py
+++ b/eng/tools/azure-sdk-tools/azpysdk/breaking.py
@@ -128,8 +128,15 @@ def run(self, args: argparse.Namespace) -> int:
             )
             logger.info(f"Processing {package_name} for breaking check...")
 
-            # install dependencies
-            self.install_dev_reqs(executable, args, package_dir)
+            use_apistub = getattr(args, "use_apistub", False)
+
+            # The apistub path builds the code report via static analysis (apistub generates
+            # api.md and it is converted to a report); the target package is never imported.
+            # So installing dev requirements and building/installing the package sdist is only
+            # needed for the default (import-based) path. Skip both in apistub mode.
+            if not use_apistub:
+                # install dependencies
+                self.install_dev_reqs(executable, args, package_dir)
 
             try:
                 install_into_venv(
@@ -144,17 +151,18 @@ def run(self, args: argparse.Namespace) -> int:
                 results.append(1)
                 continue
 
-            create_package_and_install(
-                distribution_directory=staging_directory,
-                target_setup=package_dir,
-                skip_install=False,
-                cache_dir=None,
-                work_dir=staging_directory,
-                force_create=False,
-                package_type="sdist",
-                pre_download_disabled=False,
-                python_executable=executable,
-            )
+            if not use_apistub:
+                create_package_and_install(
+                    distribution_directory=staging_directory,
+                    target_setup=package_dir,
+                    skip_install=False,
+                    cache_dir=None,
+                    work_dir=staging_directory,
+                    force_create=False,
+                    package_type="sdist",
+                    pre_download_disabled=False,
+                    python_executable=executable,
+                )
 
             try:
                 cmd = [
diff --git a/eng/tools/azure-sdk-tools/tests/test_breaking.py b/eng/tools/azure-sdk-tools/tests/test_breaking.py
new file mode 100644
index 000000000000..15049696ee40
--- /dev/null
+++ b/eng/tools/azure-sdk-tools/tests/test_breaking.py
@@ -0,0 +1,100 @@
+import argparse
+import os
+import sys
+
+from unittest.mock import MagicMock, patch
+
+from azpysdk.breaking import breaking
+
+
+def _make_args(use_apistub=False, changelog=True, isolate=False):
+    """Build an argparse.Namespace with every attribute breaking.run() reads."""
+    return argparse.Namespace(
+        target=".",
+        isolate=isolate,
+        command="breaking",
+        service=None,
+        target_module=None,
+        in_venv=False,
+        stable_version=None,
+        changelog=changelog,
+        code_report=False,
+        source_report=None,
+        target_report=None,
+        latest_pypi_version=False,
+        use_apistub=use_apistub,
+        debug=False,
+    )
+
+
+def _run_breaking(args, tmp_path):
+    """Invoke breaking.run() with all external side effects mocked out.
+
+    Returns a dict of the mocks that assertions can inspect.
+    """
+    chk = breaking()
+    staging = str(tmp_path / "staging")
+    os.makedirs(staging, exist_ok=True)
+    fake_parsed = MagicMock()
+    fake_parsed.folder = str(tmp_path)
+    fake_parsed.name = "azure-core"
+
+    original_cwd = os.getcwd()
+    with patch("azpysdk.breaking.set_envvar_defaults"), patch(
+        "azpysdk.breaking.install_into_venv"
+    ) as install_into_venv, patch("azpysdk.breaking.create_package_and_install") as create_package_and_install, patch(
+        "azpysdk.breaking.check_call"
+    ) as check_call, patch.object(
+        chk, "get_targeted_directories", return_value=[fake_parsed]
+    ), patch.object(
+        chk, "get_executable", return_value=(sys.executable, staging)
+    ), patch.object(
+        chk, "install_dev_reqs"
+    ) as install_dev_reqs:
+        try:
+            result = chk.run(args)
+        finally:
+            os.chdir(original_cwd)
+
+    return {
+        "result": result,
+        "install_dev_reqs": install_dev_reqs,
+        "install_into_venv": install_into_venv,
+        "create_package_and_install": create_package_and_install,
+        "check_call": check_call,
+    }
+
+
+class TestBreakingUseApistubGuard:
+    """The apistub path builds the report via static analysis, so it must not install the
+    package's dev requirements nor build/install the target package sdist."""
+
+    def test_use_apistub_skips_dev_reqs_and_sdist_install(self, tmp_path):
+        mocks = _run_breaking(_make_args(use_apistub=True), tmp_path)
+
+        # The failing/expensive steps are skipped in apistub mode.
+        mocks["install_dev_reqs"].assert_not_called()
+        mocks["create_package_and_install"].assert_not_called()
+
+        # jsondiff + breaking-change checker are still required by the detector.
+        mocks["install_into_venv"].assert_called_once()
+
+        # The detector still runs, and it receives --use-apistub.
+        mocks["check_call"].assert_called_once()
+        detector_cmd = mocks["check_call"].call_args.args[0]
+        assert "--use-apistub" in detector_cmd
+        assert mocks["result"] == 0
+
+    def test_default_path_installs_dev_reqs_and_sdist(self, tmp_path):
+        mocks = _run_breaking(_make_args(use_apistub=False), tmp_path)
+
+        # The import-based path needs the package (and its deps) installed.
+        mocks["install_dev_reqs"].assert_called_once()
+        mocks["create_package_and_install"].assert_called_once()
+
+        mocks["install_into_venv"].assert_called_once()
+
+        mocks["check_call"].assert_called_once()
+        detector_cmd = mocks["check_call"].call_args.args[0]
+        assert "--use-apistub" not in detector_cmd
+        assert mocks["result"] == 0