diff --git a/tools/test_budget_report.py b/tools/test_budget_report.py
new file mode 100644
index 0000000..c4788e7
--- /dev/null
+++ b/tools/test_budget_report.py
@@ -0,0 +1,133 @@
+"""Offline failure-path tests for AirDef's opt-in WASM budget report."""
+import hashlib
+import json
+import os
+import tempfile
+import unittest
+from pathlib import Path
+from subprocess import CompletedProcess
+from unittest import mock
+
+import airdef
+
+
+class BudgetReportFailureTests(unittest.TestCase):
+    def setUp(self):
+        self.temp = tempfile.TemporaryDirectory(prefix="airdef-budget-report-test-")
+        self.addCleanup(self.temp.cleanup)
+        self.root = Path(self.temp.name)
+        self.fork = self.root / "horizon"
+        self.mod_root = self.root / "airdef-mod"
+        self.artifact = self.mod_root / "target" / "horizon-mod" / "airdef.hmod"
+        self.report = self.root / "report.json"
+
+        (self.fork / "crates/horizon-mod-sdk/wit").mkdir(parents=True)
+        (self.fork / "crates/horizon-mods/src").mkdir(parents=True)
+        (self.mod_root / "tests/budgets").mkdir(parents=True)
+        (self.mod_root / "src").mkdir(parents=True)
+        self.artifact.parent.mkdir(parents=True)
+        (self.fork / "crates/horizon-mod-sdk/wit/horizon.wit").write_text("package test:mods;\n")
+        (self.fork / "crates/horizon-mods/src/runtime.rs").write_text(
+            "const MEMORY: usize = 64 << 20;\n"
+            "const FUEL_INIT: u64 = 2000000000;\n"
+            "const FUEL_TICK: u64 = 300000000;\n"
+            "const FUEL_FRAME: u64 = 10000000;\n"
+            "const FUEL_EVENT: u64 = 1000000000;\n"
+            "wasm_component_model(true); consume_fuel(true);\n"
+        )
+        (self.fork / "Cargo.lock").write_text(
+            '[[package]]\nname = "wasmtime"\nversion = "48.0.7"\n'
+        )
+        (self.mod_root / "src/layout.rs").write_text("pub const MAX_UNITS: usize = 2000;\n")
+        (self.mod_root / "tests/budgets/wasm_budget.toml").write_text(
+            '[[step]]\ncommand = "airdef test"\n'
+        )
+        self.artifact.write_bytes(b"minimal synthetic hmod")
+        self.diff_sha = hashlib.sha256(b"synthetic diff").hexdigest()
+        self.wit_sha = hashlib.sha256((self.fork / "crates/horizon-mod-sdk/wit/horizon.wit").read_bytes()).hexdigest()
+        self.identity = {
+            "airdef": "airdef-sha",
+            "airdef_dirty": False,
+            "airdef_diff_sha256": self.diff_sha,
+            "engine": "engine-id",
+            "engine_pin": "engine-pin",
+            "wit_sha256": self.wit_sha,
+            "artifact_sha256": hashlib.sha256(self.artifact.read_bytes()).hexdigest(),
+        }
+        (self.artifact.parent / "build-identity.json").write_text(json.dumps(self.identity))
+
+        self.patches = [
+            mock.patch.object(airdef, "ROOT", self.root),
+            mock.patch.object(airdef, "FORK", self.fork),
+            mock.patch.object(airdef, "engine_commit", return_value="engine-pin"),
+            mock.patch.object(airdef, "engine_identity", return_value="engine-id"),
+            mock.patch.object(airdef, "fork_api", return_value=(27, [])),
+            mock.patch.object(airdef, "hmod", return_value=self.artifact),
+            mock.patch.object(airdef, "cargo", return_value="cargo"),
+            mock.patch.dict(os.environ, {"AIRDEF_BUDGET_REPORT": str(self.report)}),
+            mock.patch.object(airdef.subprocess, "check_output", side_effect=self.fake_git),
+        ]
+        for patcher in self.patches:
+            patcher.start()
+            self.addCleanup(patcher.stop)
+
+    @staticmethod
+    def fake_git(command, **kwargs):
+        if "rev-parse" in command:
+            result = "airdef-sha\n"
+        elif "status" in command:
+            result = ""
+        elif "diff" in command:
+            result = b"synthetic diff" if kwargs.get("text") is False else "synthetic diff"
+        else:
+            raise AssertionError(f"unexpected git command: {command}")
+        return result
+
+    def read_report(self):
+        return json.loads(self.report.read_text())
+
+    def invoke_and_require_failure(self):
+        with self.assertRaises(SystemExit):
+            airdef.test_budgets()
+        self.assertTrue(self.report.is_file(), "failure path must persist its report")
+        report = self.read_report()
+        self.assertEqual(report["status"], "failed")
+        return report
+
+    def test_early_system_exit_writes_report_and_skips_backend(self):
+        backend = mock.Mock()
+        with mock.patch.object(airdef, "mod", side_effect=SystemExit("synthetic build failure")), \
+             mock.patch.object(airdef.subprocess, "run", backend):
+            report = self.invoke_and_require_failure()
+
+        self.assertIn("SystemExit", report["error"])
+        backend.assert_not_called()
+
+    def test_stale_artifact_identity_blocks_backend(self):
+        stale = dict(self.identity, engine_pin="old-engine-pin")
+        (self.artifact.parent / "build-identity.json").write_text(json.dumps(stale))
+        backend = mock.Mock()
+        with mock.patch.object(airdef, "mod"), \
+             mock.patch.object(airdef.subprocess, "run", backend):
+            report = self.invoke_and_require_failure()
+
+        self.assertFalse(report["build_identity"]["matches"])
+        self.assertFalse(report["build_identity"]["checks"]["engine_pin"])
+        self.assertIn("stale", report["error"])
+        backend.assert_not_called()
+
+    def test_runtime_fuel_trap_persists_nonzero_exit_and_output(self):
+        backend = mock.Mock(return_value=CompletedProcess(
+            args=["cargo", "run"], returncode=137, stdout="error: all fuel consumed (fuel trap)\n"
+        ))
+        with mock.patch.object(airdef, "mod"), \
+             mock.patch.object(airdef.subprocess, "run", backend):
+            report = self.invoke_and_require_failure()
+
+        backend.assert_called_once()
+        self.assertEqual(report["exit_code"], 137)
+        self.assertIn("fuel trap", report["output"])
+
+
+if __name__ == "__main__":
+    unittest.main()