diff --git a/lib/mod/conformance.conf b/lib/mod/conformance.conf
index bac5c834..92e4abac 100644
--- a/lib/mod/conformance.conf
+++ b/lib/mod/conformance.conf
@@ -19,6 +19,7 @@ PRELOADS=(
   lib/mod/api.sx
   lib/mod/fed.sx
   lib/mod/link.sx
+  lib/mod/lint.sx
 )
 
 SUITES=(
@@ -28,4 +29,5 @@ SUITES=(
   "fed:lib/mod/tests/fed.sx:(mod-fed-tests-run!)"
   "extensions:lib/mod/tests/extensions.sx:(mod-extensions-tests-run!)"
   "link:lib/mod/tests/link.sx:(mod-link-tests-run!)"
+  "lint:lib/mod/tests/lint.sx:(mod-lint-tests-run!)"
 )
diff --git a/lib/mod/lint.sx b/lib/mod/lint.sx
new file mode 100644
index 00000000..b3e83597
--- /dev/null
+++ b/lib/mod/lint.sx
@@ -0,0 +1,69 @@
+;; lib/mod/lint.sx — static analysis of a policy rule set.
+;;
+;; Because precedence is "first matching clause wins" (pl-query-one), the rule
+;; order has correctness consequences a moderator can get wrong: a rule placed
+;; after an unconditional (empty :when) rule can never fire, and a rule set with
+;; no unconditional rule may leave some reports undecided. lint-rules surfaces
+;; these without running the engine.
+
+(define mod/rule-unconditional? (fn (r) (empty? (mod/rule-when r))))
+
+;; names of rules that follow the first unconditional rule — structurally dead,
+;; since the unconditional rule always matches first
+(define
+  mod/unreachable-rules
+  (fn
+    (rules)
+    (get
+      (reduce
+        (fn
+          (acc r)
+          (if
+            (get acc :hit)
+            {:dead (append (get acc :dead) (list (mod/rule-name r))) :hit true}
+            (if (mod/rule-unconditional? r) {:dead (get acc :dead) :hit true} acc)))
+        {:dead (list) :hit false}
+        rules)
+      :dead)))
+
+(define
+  mod/has-catchall?
+  (fn (rules) (mod/any? mod/rule-unconditional? rules)))
+
+(define
+  mod/count-eq
+  (fn
+    (x lst)
+    (reduce (fn (a y) (if (= y x) (+ a 1) a)) 0 lst)))
+
+(define
+  mod/duplicate-rule-names
+  (fn
+    (rules)
+    (let
+      ((names (map mod/rule-name rules)))
+      (mod/distinct
+        (reduce
+          (fn
+            (acc n)
+            (if
+              (< 1 (mod/count-eq n names))
+              (append acc (list n))
+              acc))
+          (list)
+          names)))))
+
+(define mod/lint-rules (fn (rules) {:duplicate-names (mod/duplicate-rule-names rules) :has-catchall (mod/has-catchall? rules) :unreachable (mod/unreachable-rules rules)}))
+
+;; a rule set is well-formed when nothing is dead, it has a catch-all, and rule
+;; names are unique
+(define
+  mod/rules-ok?
+  (fn
+    (rules)
+    (let
+      ((l (mod/lint-rules rules)))
+      (if
+        (empty? (get l :unreachable))
+        (if (get l :has-catchall) (empty? (get l :duplicate-names)) false)
+        false))))
diff --git a/lib/mod/scoreboard.json b/lib/mod/scoreboard.json
index 031c5e68..d75cab7b 100644
--- a/lib/mod/scoreboard.json
+++ b/lib/mod/scoreboard.json
@@ -1,15 +1,16 @@
 {
   "lang": "mod",
-  "total_passed": 176,
+  "total_passed": 190,
   "total_failed": 0,
-  "total": 176,
+  "total": 190,
   "suites": [
     {"name":"decide","passed":31,"failed":0,"total":31},
     {"name":"audit","passed":29,"failed":0,"total":29},
     {"name":"escalation","passed":46,"failed":0,"total":46},
     {"name":"fed","passed":26,"failed":0,"total":26},
     {"name":"extensions","passed":32,"failed":0,"total":32},
-    {"name":"link","passed":12,"failed":0,"total":12}
+    {"name":"link","passed":12,"failed":0,"total":12},
+    {"name":"lint","passed":14,"failed":0,"total":14}
   ],
-  "generated": "2026-06-06T18:09:14+00:00"
+  "generated": "2026-06-06T18:15:06+00:00"
 }
diff --git a/lib/mod/scoreboard.md b/lib/mod/scoreboard.md
index e10a739d..ecec4269 100644
--- a/lib/mod/scoreboard.md
+++ b/lib/mod/scoreboard.md
@@ -1,6 +1,6 @@
 # mod scoreboard
 
-**176 / 176 passing** (0 failure(s)).
+**190 / 190 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
@@ -10,3 +10,4 @@
 | fed | 26 | 26 | ok |
 | extensions | 32 | 32 | ok |
 | link | 12 | 12 | ok |
+| lint | 14 | 14 | ok |
diff --git a/lib/mod/tests/lint.sx b/lib/mod/tests/lint.sx
new file mode 100644
index 00000000..2aa14177
--- /dev/null
+++ b/lib/mod/tests/lint.sx
@@ -0,0 +1,122 @@
+;; lib/mod/tests/lint.sx — Ext 5: policy rule-set static analysis.
+
+(define mod-lint-count 0)
+(define mod-lint-pass 0)
+(define mod-lint-fail 0)
+(define mod-lint-failures (list))
+
+(define
+  mod-lint-test!
+  (fn
+    (name got expected)
+    (begin
+      (set! mod-lint-count (+ mod-lint-count 1))
+      (if
+        (= got expected)
+        (set! mod-lint-pass (+ mod-lint-pass 1))
+        (begin
+          (set! mod-lint-fail (+ mod-lint-fail 1))
+          (append!
+            mod-lint-failures
+            (str name "\n    expected: " expected "\n    got:      " got)))))))
+
+;; ── the default rule set is well-formed ──
+
+(mod-lint-test!
+  "default rules: no unreachable"
+  (mod/unreachable-rules mod/default-rules)
+  (list))
+(mod-lint-test!
+  "default rules: has catch-all"
+  (mod/has-catchall? mod/default-rules)
+  true)
+(mod-lint-test!
+  "default rules: no duplicate names"
+  (mod/duplicate-rule-names mod/default-rules)
+  (list))
+(mod-lint-test!
+  "default rules: well-formed"
+  (mod/rules-ok? mod/default-rules)
+  true)
+
+;; ── unreachable detection ──
+
+(define
+  mod-lint-shadowed
+  (list
+    (mod/mk-rule "spam-hide" :hide (list (list :classification "spam")))
+    (mod/mk-rule "catch-all" :keep (list))
+    (mod/mk-rule
+      "abuse-remove"
+      :remove (list (list :classification "abuse")))
+    (mod/mk-rule
+      "repeated"
+      :escalate (list (list :count-at-least 3)))))
+
+(mod-lint-test!
+  "rules after catch-all are unreachable"
+  (mod/unreachable-rules mod-lint-shadowed)
+  (list "abuse-remove" "repeated"))
+(mod-lint-test!
+  "shadowed rule set is not ok"
+  (mod/rules-ok? mod-lint-shadowed)
+  false)
+
+;; ── missing catch-all ──
+
+(define
+  mod-lint-nocatch
+  (list
+    (mod/mk-rule "spam-hide" :hide (list (list :classification "spam")))
+    (mod/mk-rule
+      "abuse-remove"
+      :remove (list (list :classification "abuse")))))
+
+(mod-lint-test!
+  "no catch-all detected"
+  (mod/has-catchall? mod-lint-nocatch)
+  false)
+(mod-lint-test!
+  "no unreachable when no catch-all"
+  (mod/unreachable-rules mod-lint-nocatch)
+  (list))
+(mod-lint-test!
+  "no-catch-all rule set is not ok"
+  (mod/rules-ok? mod-lint-nocatch)
+  false)
+
+;; ── duplicate names ──
+
+(define
+  mod-lint-dups
+  (list
+    (mod/mk-rule "x" :hide (list (list :classification "spam")))
+    (mod/mk-rule "x" :remove (list (list :classification "abuse")))
+    (mod/mk-rule "default" :keep (list))))
+
+(mod-lint-test!
+  "duplicate names detected"
+  (mod/duplicate-rule-names mod-lint-dups)
+  (list "x"))
+(mod-lint-test!
+  "duplicate-name rule set is not ok"
+  (mod/rules-ok? mod-lint-dups)
+  false)
+
+;; ── helpers ──
+
+(mod-lint-test!
+  "rule-unconditional? true for empty when"
+  (mod/rule-unconditional? (mod/mk-rule "d" :keep (list)))
+  true)
+(mod-lint-test!
+  "rule-unconditional? false with conditions"
+  (mod/rule-unconditional?
+    (mod/mk-rule "s" :hide (list (list :classification "spam"))))
+  false)
+(mod-lint-test!
+  "count-eq counts occurrences"
+  (mod/count-eq "a" (list "a" "b" "a"))
+  2)
+
+(define mod-lint-tests-run! (fn () {:failures mod-lint-failures :total mod-lint-count :passed mod-lint-pass :failed mod-lint-fail}))
diff --git a/plans/mod-on-sx.md b/plans/mod-on-sx.md
index cd5b4755..8670105d 100644
--- a/plans/mod-on-sx.md
+++ b/plans/mod-on-sx.md
@@ -16,7 +16,7 @@ federation extension.
 
 ## Status (rolling)
 
-`bash lib/mod/conformance.sh` → **176/176** (roadmap + 4 extensions complete)
+`bash lib/mod/conformance.sh` → **190/190** (roadmap + 5 extensions complete)
 
 ## Ground rules
 
@@ -147,6 +147,11 @@ lib/mod/fed.sx
   derivation goal-by-goal with `[proved]`/`[unproved]` marks and unification
   bindings. E.g. `Report rc: escalate (rule: repeated-escalate)` … `[proved]
   report(rc, B, S), report_count(S, N), N >= 3  {B=ann, N=3, S=dave}`.
+- [x] **Ext 5 — policy lint** (`lib/mod/lint.sx`, +14). Static analysis of a rule
+  set: `mod/unreachable-rules` flags rules placed after an unconditional (always-
+  matching) rule — structurally dead under first-match precedence;
+  `mod/has-catchall?` checks every report gets a decision; `mod/duplicate-rule-names`
+  + `mod/rules-ok?` give a one-call well-formedness verdict. No engine run needed.
 - [x] **Ext 4 — report linking / dedup** (`lib/mod/link.sx`, +12). `mod/related-ids`
   and `mod/reporters-of` find reports about a subject via a Prolog relational query
   (`report(Id, _, 'subject')`) — the policy substrate reused for retrieval.
@@ -155,6 +160,10 @@ lib/mod/fed.sx
 
 ## Progress log
 
+- **Ext 5 — policy lint, 190/190** (+14). Static analysis of the rule set itself,
+  catching the failure modes first-match precedence makes easy: dead rules after a
+  catch-all, missing catch-all (undecided reports), duplicate names. `mod/rules-ok?`
+  is a single well-formedness gate a policy author can assert in their own tests.
 - **Ext 4 — report linking / dedup, 176/176** (+12). Relational retrieval
   (`related-ids`, `reporters-of`) reuses the Prolog substrate for *querying* report
   clusters, not just deciding them — `report(Id, _, 'subject')` by unification.