diff --git a/lib/mod/conformance.conf b/lib/mod/conformance.conf
index 75243257..517be3c0 100644
--- a/lib/mod/conformance.conf
+++ b/lib/mod/conformance.conf
@@ -18,6 +18,7 @@ PRELOADS=(
   lib/mod/offenders.sx
   lib/mod/quorum.sx
   lib/mod/trace.sx
+  lib/mod/whatif.sx
   lib/mod/lifecycle.sx
   lib/mod/audit.sx
   lib/mod/api.sx
@@ -38,4 +39,5 @@ SUITES=(
   "offenders:lib/mod/tests/offenders.sx:(mod-offenders-tests-run!)"
   "quorum:lib/mod/tests/quorum.sx:(mod-quorum-tests-run!)"
   "trace:lib/mod/tests/trace.sx:(mod-trace-tests-run!)"
+  "whatif:lib/mod/tests/whatif.sx:(mod-whatif-tests-run!)"
 )
diff --git a/lib/mod/scoreboard.json b/lib/mod/scoreboard.json
index 6282142f..3c9acdc6 100644
--- a/lib/mod/scoreboard.json
+++ b/lib/mod/scoreboard.json
@@ -1,8 +1,8 @@
 {
   "lang": "mod",
-  "total_passed": 247,
+  "total_passed": 260,
   "total_failed": 0,
-  "total": 247,
+  "total": 260,
   "suites": [
     {"name":"decide","passed":31,"failed":0,"total":31},
     {"name":"audit","passed":29,"failed":0,"total":29},
@@ -14,7 +14,8 @@
     {"name":"severity","passed":14,"failed":0,"total":14},
     {"name":"offenders","passed":19,"failed":0,"total":19},
     {"name":"quorum","passed":9,"failed":0,"total":9},
-    {"name":"trace","passed":15,"failed":0,"total":15}
+    {"name":"trace","passed":15,"failed":0,"total":15},
+    {"name":"whatif","passed":13,"failed":0,"total":13}
   ],
-  "generated": "2026-06-06T18:48:10+00:00"
+  "generated": "2026-06-06T18:51:15+00:00"
 }
diff --git a/lib/mod/scoreboard.md b/lib/mod/scoreboard.md
index 04d8ae2d..365c2351 100644
--- a/lib/mod/scoreboard.md
+++ b/lib/mod/scoreboard.md
@@ -1,6 +1,6 @@
 # mod scoreboard
 
-**247 / 247 passing** (0 failure(s)).
+**260 / 260 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
@@ -15,3 +15,4 @@
 | offenders | 19 | 19 | ok |
 | quorum | 9 | 9 | ok |
 | trace | 15 | 15 | ok |
+| whatif | 13 | 13 | ok |
diff --git a/lib/mod/tests/whatif.sx b/lib/mod/tests/whatif.sx
new file mode 100644
index 00000000..d4eb8099
--- /dev/null
+++ b/lib/mod/tests/whatif.sx
@@ -0,0 +1,117 @@
+;; lib/mod/tests/whatif.sx — Ext 10: policy what-if / impact analysis.
+
+(define mod-wi-count 0)
+(define mod-wi-pass 0)
+(define mod-wi-fail 0)
+(define mod-wi-failures (list))
+
+(define
+  mod-wi-test!
+  (fn
+    (name got expected)
+    (begin
+      (set! mod-wi-count (+ mod-wi-count 1))
+      (if
+        (= got expected)
+        (set! mod-wi-pass (+ mod-wi-pass 1))
+        (begin
+          (set! mod-wi-fail (+ mod-wi-fail 1))
+          (append!
+            mod-wi-failures
+            (str name "\n    expected: " expected "\n    got:      " got)))))))
+
+;; rules-b is the default policy with spam-hide removed: spam now falls through
+;; to default-keep. A spam report flips hide → keep; everything else is unchanged.
+(define mod-wi-rules-a mod/default-rules)
+(define
+  mod-wi-rules-b
+  (list
+    (mod/mk-rule
+      "reviewer-remove"
+      :remove (list (list :evidence "confirmed-abuse")))
+    (mod/mk-rule
+      "abuse-remove"
+      :remove (list (list :classification "abuse")))
+    (mod/mk-rule
+      "repeated-escalate"
+      :escalate (list (list :count-at-least 3)))
+    (mod/mk-rule "default-keep" :keep (list))))
+
+(define mod-wi-spam (mod/mk-report "r1" "a" "bob" "this is spam"))
+(define mod-wi-abuse (mod/mk-report "r2" "a" "carol" "harassment here"))
+(define mod-wi-clean (mod/mk-report "r3" "a" "dave" "a fine post"))
+
+;; ── single-report diff ──
+
+(define
+  mod-wi-d
+  (mod/decision-diff
+    mod-wi-spam
+    (list mod-wi-spam)
+    mod-wi-rules-a
+    mod-wi-rules-b))
+(mod-wi-test! "spam before = hide" (get mod-wi-d :before) "hide")
+(mod-wi-test! "spam after = keep" (get mod-wi-d :after) "keep")
+(mod-wi-test! "spam decision flips" (get mod-wi-d :changed) true)
+(mod-wi-test! "diff carries report id" (get mod-wi-d :report-id) "r1")
+
+(define
+  mod-wi-da
+  (mod/decision-diff
+    mod-wi-abuse
+    (list mod-wi-abuse)
+    mod-wi-rules-a
+    mod-wi-rules-b))
+(mod-wi-test! "abuse unchanged (remove both)" (get mod-wi-da :changed) false)
+(mod-wi-test! "abuse stays remove" (get mod-wi-da :after) "remove")
+
+(define
+  mod-wi-dc
+  (mod/decision-diff
+    mod-wi-clean
+    (list mod-wi-clean)
+    mod-wi-rules-a
+    mod-wi-rules-b))
+(mod-wi-test! "clean unchanged (keep both)" (get mod-wi-dc :changed) false)
+
+;; ── batch impact ──
+
+(define mod-wi-batch (list mod-wi-spam mod-wi-abuse mod-wi-clean))
+(define
+  mod-wi-impact
+  (mod/policy-impact mod-wi-batch mod-wi-rules-a mod-wi-rules-b))
+
+(mod-wi-test!
+  "impact lists only changed reports"
+  (len mod-wi-impact)
+  1)
+(mod-wi-test!
+  "impacted report is the spam one"
+  (get (first mod-wi-impact) :report-id)
+  "r1")
+(mod-wi-test!
+  "impact-count agrees"
+  (mod/impact-count mod-wi-batch mod-wi-rules-a mod-wi-rules-b)
+  1)
+
+;; ── identical rule sets → no impact ──
+
+(mod-wi-test!
+  "same rules → zero impact"
+  (mod/impact-count mod-wi-batch mod-wi-rules-a mod-wi-rules-a)
+  0)
+(mod-wi-test!
+  "same rules → empty report"
+  (mod/impact-report mod-wi-batch mod-wi-rules-a mod-wi-rules-a)
+  "No decisions change.")
+
+;; ── rendering ──
+
+(mod-wi-test!
+  "impact-report renders the flip"
+  (mod/str-contains?
+    (mod/impact-report mod-wi-batch mod-wi-rules-a mod-wi-rules-b)
+    "r1: hide → keep")
+  true)
+
+(define mod-whatif-tests-run! (fn () {:failures mod-wi-failures :total mod-wi-count :passed mod-wi-pass :failed mod-wi-fail}))
diff --git a/lib/mod/whatif.sx b/lib/mod/whatif.sx
new file mode 100644
index 00000000..23b24eac
--- /dev/null
+++ b/lib/mod/whatif.sx
@@ -0,0 +1,56 @@
+;; lib/mod/whatif.sx — policy what-if / impact analysis.
+;;
+;; Before shipping a policy change, a moderation team needs to know which past or
+;; pending reports would decide differently. mod/decision-diff compares one
+;; report's action under two rule sets; mod/policy-impact runs a whole batch and
+;; returns only the reports whose decision flips. Pure SX over decide-report.
+
+(define
+  mod/decision-diff
+  (fn
+    (r reports rules-a rules-b)
+    (let
+      ((a (get (mod/decide-report r reports rules-a) :action))
+        (b (get (mod/decide-report r reports rules-b) :action)))
+      {:after b :changed (if (= a b) false true) :report-id (mod/report-id r) :before a})))
+
+(define
+  mod/policy-impact
+  (fn
+    (reports rules-a rules-b)
+    (reduce
+      (fn
+        (acc r)
+        (let
+          ((d (mod/decision-diff r reports rules-a rules-b)))
+          (if (get d :changed) (append acc (list d)) acc)))
+      (list)
+      reports)))
+
+(define
+  mod/impact-count
+  (fn
+    (reports rules-a rules-b)
+    (len (mod/policy-impact reports rules-a rules-b))))
+
+(define
+  mod/impact-report
+  (fn
+    (reports rules-a rules-b)
+    (let
+      ((changed (mod/policy-impact reports rules-a rules-b)))
+      (if
+        (empty? changed)
+        "No decisions change."
+        (mod/join-with
+          "\n"
+          (map
+            (fn
+              (d)
+              (str
+                (get d :report-id)
+                ": "
+                (get d :before)
+                " → "
+                (get d :after)))
+            changed))))))
diff --git a/plans/mod-on-sx.md b/plans/mod-on-sx.md
index 7ff76ea3..5e074024 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` → **247/247** (roadmap + 9 extensions complete)
+`bash lib/mod/conformance.sh` → **260/260** (roadmap + 10 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 10 — policy what-if / impact** (`lib/mod/whatif.sx`, +13).
+  `mod/decision-diff` compares one report's action under two rule sets;
+  `mod/policy-impact` runs a batch and returns only the reports whose decision
+  flips; `mod/impact-count` / `mod/impact-report` summarize. Lets a team measure a
+  policy change before shipping it (e.g. "removing spam-hide flips r1 hide→keep").
 - [x] **Ext 9 — policy dry-run trace** (`lib/mod/trace.sx`, +15). `mod/trace-rules`
   evaluates a report against every rule and returns each rule's proved/unproved
   status + its goal-by-goal derivation, so an unproved rule shows which goal
@@ -186,6 +191,12 @@ lib/mod/fed.sx
 
 ## Progress log
 
+- **Ext 10 — policy what-if / impact, 260/260** (+13). Decisions are now
+  comparable across rule sets — diff one report, or batch a whole set and surface
+  only the flips. Pure SX over `decide-report`, no engine change. Closes the
+  policy-authoring loop alongside lint (Ext 5) and trace (Ext 9): lint checks
+  well-formedness, trace explains one report, what-if measures a change's blast
+  radius before it ships.
 - **Ext 9 — policy dry-run trace, 247/247** (+15). Whole-rule-set diagnostics over
   the proof machinery: every rule's fire/no-fire and the goal that decided it. The
   winner agrees with `decide-report` by construction (first proved = pl-query-one),