diff --git a/lib/hypatia/cli.ex b/lib/hypatia/cli.ex
index e3e4a7fc..83b10472 100644
--- a/lib/hypatia/cli.ex
+++ b/lib/hypatia/cli.ex
@@ -1202,7 +1202,7 @@ defmodule Hypatia.CLI do
   # ─── Output formatting ───────────────────────────────────────────────
 
   defp output(findings, "json") do
-    IO.puts(Jason.encode!(findings, pretty: true))
+    IO.puts(Jason.encode!(Enum.map(findings, &json_compat_severity/1), pretty: true))
   end
 
   defp output(findings, "sarif") do
@@ -1251,6 +1251,15 @@ defmodule Hypatia.CLI do
     end
   end
 
+  # Compatibility shim (standards#787 D260). Callers pinned to standards'
+  # hypatia-scan-reusable.yml before bd9313a6 validate the JSON against
+  # critical/high/medium/low/info and reject the whole array on one "warn",
+  # so ~350 repos failed with "Hypatia did not produce one valid findings
+  # array". warn already ranks with medium (@severity_order), so this loses
+  # no information. Remove once no caller is pinned to a rejecting copy.
+  defp json_compat_severity(%{severity: "warn"} = f), do: %{f | severity: "medium"}
+  defp json_compat_severity(f), do: f
+
   defp output_report(findings, repo_path) do
     IO.puts("=" |> String.duplicate(72))
     IO.puts("  Hypatia Security & Policy Report")
diff --git a/test/research_extensions_wiring_test.exs b/test/research_extensions_wiring_test.exs
index b232befd..69cbe228 100644
--- a/test/research_extensions_wiring_test.exs
+++ b/test/research_extensions_wiring_test.exs
@@ -161,6 +161,40 @@ defmodule Hypatia.Rules.ResearchExtensionsWiringTest do
       assert summary =~ ~r/warn=[1-9][0-9]*/
     end
 
+    # Callers on standards' hypatia-scan-reusable.yml before bd9313a6 reject
+    # the whole JSON array if any finding carries severity "warn" (D260).
+    test "CLI JSON output reports warn findings as medium, never as warn" do
+      repo = tripwire_repo()
+
+      json =
+        ExUnit.CaptureIO.capture_io(fn ->
+          ExUnit.CaptureIO.capture_io(:stderr, fn ->
+            CLI.main([
+              "scan",
+              repo,
+              "--rules",
+              "research_extensions",
+              "--format",
+              "json",
+              "--exit-zero"
+            ])
+          end)
+        end)
+
+      findings = Jason.decode!(json)
+      severities = Enum.map(findings, & &1["severity"])
+
+      assert "medium" in severities,
+             "the tripwire repo emits warn-tier RE findings; none reached JSON"
+
+      refute "warn" in severities,
+             "a single \"warn\" makes every pre-bd9313a6 standards validator " <>
+               "reject the whole findings array"
+
+      allowed = ["critical", "high", "medium", "low", "info"]
+      assert Enum.all?(severities, &(&1 in allowed))
+    end
+
     # Six of the ten RE rules emit `severity: :warn`. "warn" was absent from
     # CLI's @severity_order, so `Map.get(@severity_order, "warn", 5)` gave it
     # rank 5; the filter `rank <= threshold` at the default threshold of