@@ -20,7 +20,10 @@ development at the same time, such as 4.5.x and 5.0.
 Unreleased
 ----------
 
-Nothing yet.
+- Fix: the JSON report now includes an explicit format version number, closing
+  `issue 1732`_.
+
+.. _issue 1732: https://github.com/nedbat/coveragepy/issues/1732
 
 
 .. scriv-start-here
 
@@ -21,6 +21,10 @@
     from coverage.data import CoverageData
 
 
+# "Version 1" had no format number at all.
+# 2: add the meta.format field.
+FORMAT_VERSION = 2
+
 class JsonReporter:
     """A reporter for writing JSON coverage results."""
 
@@ -44,6 +48,7 @@ def report(self, morfs: Optional[Iterable[TMorf]], outfile: IO[str]) -> float:
         coverage_data = self.coverage.get_data()
         coverage_data.set_query_contexts(self.config.report_contexts)
         self.report_data["meta"] = {
+            "format": FORMAT_VERSION,
             "version": __version__,
             "timestamp": datetime.datetime.now().isoformat(),
             "branch_coverage": coverage_data.has_arcs(),
 
@@ -13,6 +13,7 @@
 
 import coverage
 from coverage import Coverage
+from coverage.jsonreport import FORMAT_VERSION
 
 from tests.coveragetest import UsingModulesMixin, CoverageTest
 
@@ -26,7 +27,7 @@ def _assert_expected_json_report(
         expected_result: Dict[str, Any],
     ) -> None:
         """
-        Helper for tests that handles the common ceremony so the tests can be clearly show the
+        Helper that handles common ceremonies so tests can clearly show the
         consequences of setting various arguments.
         """
         self.make_file("a.py", """\
@@ -49,13 +50,16 @@ def _assert_expected_json_report(
             datetime.strptime(parsed_result['meta']['timestamp'], "%Y-%m-%dT%H:%M:%S.%f")
         )
         del (parsed_result['meta']['timestamp'])
+        expected_result["meta"].update({
+            "format": FORMAT_VERSION,
+            "version": coverage.__version__,
+        })
         assert parsed_result == expected_result
 
     def test_branch_coverage(self) -> None:
         cov = coverage.Coverage(branch=True)
         expected_result = {
             'meta': {
-                "version": coverage.__version__,
                 "branch_coverage": True,
                 "show_contexts": False,
             },
@@ -107,7 +111,6 @@ def test_simple_line_coverage(self) -> None:
         cov = coverage.Coverage()
         expected_result = {
             'meta': {
-                "version": coverage.__version__,
                 "branch_coverage": False,
                 "show_contexts": False,
             },
@@ -152,7 +155,6 @@ def run_context_test(self, relative_files: bool) -> None:
         cov = coverage.Coverage(context="cool_test", config_file="config")
         expected_result = {
             'meta': {
-                "version": coverage.__version__,
                 "branch_coverage": False,
                 "show_contexts": True,
             },