diff --git a/sealed/README.md b/sealed/README.md
index c582ec7..1826449 100644
--- a/sealed/README.md
+++ b/sealed/README.md
@@ -10,6 +10,7 @@ JDK25=/path/to/jdk-25 JDK27=/path/to/jdk-27 [JDK21=/path/to/jdk-21] ./run.sh
 - `src/` compiles and runs unchanged on 21, 25 and 27: the `Payment` model, the three subclass modifiers, generic and recursive hierarchies, the `default` trap and a visitor for comparison.
 - `broken/` fails to compile on purpose (one rule per file).
 - `sep/` shows what the JVM itself enforces when a class file disagrees with the sealed type it claims to extend.
+- `jackson/` runs the same `Payment` through Jackson 3.2.3 (`./jackson/fetch.sh` downloads and sha1-checks the three jars from Maven Central; they are not committed).
 - `multi/` is a named module, the only place a sealed hierarchy may span packages.
 
 Tested on Temurin 25.0.4.1+1 and 27+35, and OpenJDK 21.0.10 as the baseline.
diff --git a/sealed/jackson/.gitignore b/sealed/jackson/.gitignore
new file mode 100644
index 0000000..c3af857
--- /dev/null
+++ b/sealed/jackson/.gitignore
@@ -0,0 +1 @@
+lib/
diff --git a/sealed/jackson/JsonForgotten.java b/sealed/jackson/JsonForgotten.java
new file mode 100644
index 0000000..c5d4139
--- /dev/null
+++ b/sealed/jackson/JsonForgotten.java
@@ -0,0 +1,25 @@
+import com.fasterxml.jackson.annotation.JsonSubTypes;
+import com.fasterxml.jackson.annotation.JsonTypeInfo;
+import tools.jackson.databind.json.JsonMapper;
+
+public class JsonForgotten {
+    // Wallet is a permitted subtype, and the compiler is happy, but nobody told Jackson about it.
+    @JsonTypeInfo(use = JsonTypeInfo.Id.NAME, property = "type")
+    @JsonSubTypes({
+        @JsonSubTypes.Type(value = Card.class, name = "card"),
+        @JsonSubTypes.Type(value = Upi.class, name = "upi")})
+    sealed interface Payment permits Card, Upi, Wallet {}
+    record Card(long amount) implements Payment {}
+    record Upi(long amount) implements Payment {}
+    record Wallet(long amount) implements Payment {}
+
+    public static void main(String[] args) {
+        JsonMapper mapper = JsonMapper.builder().build();
+        System.out.println("write : " + mapper.writeValueAsString(new Wallet(500)));
+        try {
+            mapper.readValue("{\"type\":\"wallet\",\"amount\":500}", Payment.class);
+        } catch (Exception e) {
+            System.out.println("read  : " + e.getClass().getSimpleName() + ": " + e.getMessage().lines().findFirst().orElse(""));
+        }
+    }
+}
diff --git a/sealed/jackson/JsonNoSubTypes.java b/sealed/jackson/JsonNoSubTypes.java
new file mode 100644
index 0000000..519a9f9
--- /dev/null
+++ b/sealed/jackson/JsonNoSubTypes.java
@@ -0,0 +1,32 @@
+import com.fasterxml.jackson.annotation.JsonTypeInfo;
+import tools.jackson.databind.json.JsonMapper;
+
+public class JsonNoSubTypes {
+    // Only @JsonTypeInfo, no @JsonSubTypes. Does Jackson find the permitted subclasses by itself?
+    @JsonTypeInfo(use = JsonTypeInfo.Id.SIMPLE_NAME, property = "type")
+    sealed interface Payment permits Card, Upi {}
+    record Card(long amount) implements Payment {}
+    record Upi(long amount) implements Payment {}
+
+    // Control: the same shape without the word sealed.
+    @JsonTypeInfo(use = JsonTypeInfo.Id.SIMPLE_NAME, property = "type")
+    interface OpenPayment {}
+    record OpenCard(long amount) implements OpenPayment {}
+
+    static void tryRead(JsonMapper m, String label, String json, Class<?> type) {
+        try {
+            System.out.println(label + ": " + m.readValue(json, type));
+        } catch (Exception e) {
+            System.out.println(label + ": " + e.getClass().getSimpleName() + ": " + e.getMessage().lines().findFirst().orElse(""));
+        }
+    }
+
+    public static void main(String[] args) {
+        JsonMapper m = JsonMapper.builder().build();
+        System.out.println("write         : " + m.writeValueAsString(new Card(500)));
+        tryRead(m, "sealed, Card  ", "{\"type\":\"Card\",\"amount\":500}", Payment.class);
+        tryRead(m, "sealed, Upi   ", "{\"type\":\"Upi\",\"amount\":500}", Payment.class);
+        tryRead(m, "sealed, Cheque", "{\"type\":\"Cheque\",\"amount\":500}", Payment.class);
+        tryRead(m, "plain, OpenCard", "{\"type\":\"OpenCard\",\"amount\":500}", OpenPayment.class);
+    }
+}
diff --git a/sealed/jackson/JsonPayment.java b/sealed/jackson/JsonPayment.java
new file mode 100644
index 0000000..d7ac851
--- /dev/null
+++ b/sealed/jackson/JsonPayment.java
@@ -0,0 +1,39 @@
+import com.fasterxml.jackson.annotation.JsonSubTypes;
+import com.fasterxml.jackson.annotation.JsonTypeInfo;
+import tools.jackson.databind.json.JsonMapper;
+
+public class JsonPayment {
+    // Two annotations tell Jackson which JSON field names the type, and which class each name means.
+    @JsonTypeInfo(use = JsonTypeInfo.Id.NAME, property = "type")
+    @JsonSubTypes({
+        @JsonSubTypes.Type(value = Card.class, name = "card"),
+        @JsonSubTypes.Type(value = Upi.class, name = "upi"),
+        @JsonSubTypes.Type(value = NetBanking.class, name = "netbanking")})
+    sealed interface Payment permits Card, Upi, NetBanking {}
+    record Card(String last4, long amount) implements Payment {}
+    record Upi(String vpa, long amount) implements Payment {}
+    record NetBanking(String bank, long amount) implements Payment {}
+
+    static long fee(Payment p) {
+        return switch (p) {
+            case Card(String last4, long amount) -> amount * 2 / 100;
+            case Upi(String vpa, long amount) -> 0;
+            case NetBanking(String bank, long amount) -> 15;
+        };
+    }
+
+    public static void main(String[] args) {
+        JsonMapper mapper = JsonMapper.builder().build();
+        Payment[] all = { new Card("4242", 500), new Upi("ankur@bank", 500), new NetBanking("HDFC", 500) };
+        for (Payment p : all) {
+            String json = mapper.writeValueAsString(p);
+            Payment back = mapper.readValue(json, Payment.class);
+            System.out.println(json + "  ->  " + back + "  equal=" + back.equals(p) + "  fee=" + fee(back));
+        }
+        try {
+            mapper.readValue("{\"type\":\"cheque\",\"amount\":500}", Payment.class);
+        } catch (Exception e) {
+            System.out.println("unknown type: " + e.getClass().getSimpleName() + ": " + e.getMessage().lines().findFirst().orElse(""));
+        }
+    }
+}
diff --git a/sealed/jackson/fetch.sh b/sealed/jackson/fetch.sh
new file mode 100755
index 0000000..0839e6b
--- /dev/null
+++ b/sealed/jackson/fetch.sh
@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+# Downloads the three Jackson jars from Maven Central into ./lib and checks each against the published .sha1
+set -euo pipefail
+cd "$(dirname "$0")"; mkdir -p lib
+M=https://repo1.maven.org/maven2
+for a in tools/jackson/core/jackson-databind/3.2.3/jackson-databind-3.2.3 \
+         tools/jackson/core/jackson-core/3.2.3/jackson-core-3.2.3 \
+         com/fasterxml/jackson/core/jackson-annotations/2.22/jackson-annotations-2.22; do
+  f="lib/$(basename "$a").jar"
+  [ -f "$f" ] || curl -sfL -o "$f" "$M/$a.jar"
+  want="$(curl -sfL "$M/$a.jar.sha1" | cut -d' ' -f1)"; got="$(sha1sum "$f" | cut -d' ' -f1)"
+  [ "$want" = "$got" ] && echo "ok  $f  $got" || { echo "SHA1 MISMATCH $f"; exit 1; }
+done
diff --git a/sealed/output/09-release-levels.txt b/sealed/output/09-release-levels.txt
new file mode 100644
index 0000000..68c6a86
--- /dev/null
+++ b/sealed/output/09-release-levels.txt
@@ -0,0 +1,17 @@
+$ javac --release 16 src/ImplicitPermits.java     (25.0.4.1+1-LTS)
+src/ImplicitPermits.java:4: error: sealed classes are not supported in -source 16
+public sealed class ImplicitPermits {
+       ^
+  (use -source 17 or higher to enable sealed classes)
+1 error
+exit=1
+
+$ javac --release 17 src/ImplicitPermits.java     (25.0.4.1+1-LTS)
+exit=0
+
+$ javac --release 17 src/PaymentDemo.java     (25.0.4.1+1-LTS)
+src/PaymentDemo.java:11: error: patterns in switch statements are not supported in -source 17
+            case Card(String last4, long amount) -> amount * 2 / 100;     // 2 percent
+                 ^
+  (use -source 21 or higher to enable patterns in switch statements)
+src/PaymentDemo.java:11: error: deconstruction patterns are not supported in -source 17
diff --git a/sealed/output/10-jackson.txt b/sealed/output/10-jackson.txt
new file mode 100644
index 0000000..0e1c0fa
--- /dev/null
+++ b/sealed/output/10-jackson.txt
@@ -0,0 +1,50 @@
+$ javac -cp jackson/lib/* jackson/JsonPayment.java && java JsonPayment     (21.0.10+7-Ubuntu-124.04)
+{"type":"card","last4":"4242","amount":500}  ->  Card[last4=4242, amount=500]  equal=true  fee=10
+{"type":"upi","vpa":"ankur@bank","amount":500}  ->  Upi[vpa=ankur@bank, amount=500]  equal=true  fee=0
+{"type":"netbanking","bank":"HDFC","amount":500}  ->  NetBanking[bank=HDFC, amount=500]  equal=true  fee=15
+unknown type: InvalidTypeIdException: Could not resolve type id 'cheque' as a subtype of `JsonPayment$Payment`: known type ids = [card, netbanking, upi]
+
+$ javac -cp jackson/lib/* jackson/JsonPayment.java && java JsonPayment     (25.0.4.1+1-LTS)
+{"type":"card","last4":"4242","amount":500}  ->  Card[last4=4242, amount=500]  equal=true  fee=10
+{"type":"upi","vpa":"ankur@bank","amount":500}  ->  Upi[vpa=ankur@bank, amount=500]  equal=true  fee=0
+{"type":"netbanking","bank":"HDFC","amount":500}  ->  NetBanking[bank=HDFC, amount=500]  equal=true  fee=15
+unknown type: InvalidTypeIdException: Could not resolve type id 'cheque' as a subtype of `JsonPayment$Payment`: known type ids = [card, netbanking, upi]
+
+$ javac -cp jackson/lib/* jackson/JsonPayment.java && java JsonPayment     (27+35)
+{"type":"card","last4":"4242","amount":500}  ->  Card[last4=4242, amount=500]  equal=true  fee=10
+{"type":"upi","vpa":"ankur@bank","amount":500}  ->  Upi[vpa=ankur@bank, amount=500]  equal=true  fee=0
+{"type":"netbanking","bank":"HDFC","amount":500}  ->  NetBanking[bank=HDFC, amount=500]  equal=true  fee=15
+unknown type: InvalidTypeIdException: Could not resolve type id 'cheque' as a subtype of `JsonPayment$Payment`: known type ids = [card, netbanking, upi]
+
+$ javac -cp jackson/lib/* jackson/JsonForgotten.java && java JsonForgotten     (21.0.10+7-Ubuntu-124.04)
+write : {"type":"JsonForgotten$Wallet","amount":500}
+read  : InvalidTypeIdException: Could not resolve type id 'wallet' as a subtype of `JsonForgotten$Payment`: known type ids = [card, upi]
+
+$ javac -cp jackson/lib/* jackson/JsonForgotten.java && java JsonForgotten     (25.0.4.1+1-LTS)
+write : {"type":"JsonForgotten$Wallet","amount":500}
+read  : InvalidTypeIdException: Could not resolve type id 'wallet' as a subtype of `JsonForgotten$Payment`: known type ids = [card, upi]
+
+$ javac -cp jackson/lib/* jackson/JsonForgotten.java && java JsonForgotten     (27+35)
+write : {"type":"JsonForgotten$Wallet","amount":500}
+read  : InvalidTypeIdException: Could not resolve type id 'wallet' as a subtype of `JsonForgotten$Payment`: known type ids = [card, upi]
+
+$ javac -cp jackson/lib/* jackson/JsonNoSubTypes.java && java JsonNoSubTypes     (21.0.10+7-Ubuntu-124.04)
+write         : {"type":"Card","amount":500}
+sealed, Card  : Card[amount=500]
+sealed, Upi   : Upi[amount=500]
+sealed, Cheque: InvalidTypeIdException: Could not resolve type id 'Cheque' as a subtype of `JsonNoSubTypes$Payment`: known type ids = [Card, Upi]
+plain, OpenCard: InvalidTypeIdException: Could not resolve type id 'OpenCard' as a subtype of `JsonNoSubTypes$OpenPayment`: known type ids = []
+
+$ javac -cp jackson/lib/* jackson/JsonNoSubTypes.java && java JsonNoSubTypes     (25.0.4.1+1-LTS)
+write         : {"type":"Card","amount":500}
+sealed, Card  : Card[amount=500]
+sealed, Upi   : Upi[amount=500]
+sealed, Cheque: InvalidTypeIdException: Could not resolve type id 'Cheque' as a subtype of `JsonNoSubTypes$Payment`: known type ids = [Card, Upi]
+plain, OpenCard: InvalidTypeIdException: Could not resolve type id 'OpenCard' as a subtype of `JsonNoSubTypes$OpenPayment`: known type ids = []
+
+$ javac -cp jackson/lib/* jackson/JsonNoSubTypes.java && java JsonNoSubTypes     (27+35)
+write         : {"type":"Card","amount":500}
+sealed, Card  : Card[amount=500]
+sealed, Upi   : Upi[amount=500]
+sealed, Cheque: InvalidTypeIdException: Could not resolve type id 'Cheque' as a subtype of `JsonNoSubTypes$Payment`: known type ids = [Card, Upi]
+plain, OpenCard: InvalidTypeIdException: Could not resolve type id 'OpenCard' as a subtype of `JsonNoSubTypes$OpenPayment`: known type ids = []
diff --git a/sealed/run.sh b/sealed/run.sh
index 436c54f..2c9ff48 100755
--- a/sealed/run.sh
+++ b/sealed/run.sh
@@ -56,4 +56,19 @@ mm() { local jdk="$1" o; o="$(mk)"
   echo "\$ javac -d out \$(find multi/demo.pay -name '*.java') && java -p out -m demo.pay/pay.app.Main     ($(t "$jdk"))"
   "$jdk/bin/javac" -d "$o" $(find multi/demo.pay -name '*.java') 2>&1 && "$jdk/bin/java" -p "$o" -m demo.pay/pay.app.Main 2>&1; }
 { all3 mm; } > "$OUT/08-modules.txt"
+
+# 09 - which language level each feature needs (sealed is final in 17; a switch over the sealed type needs 21)
+rl() { local jdk="$1" rel="$2" src="$3" o; o="$(mk)"
+  echo "\$ javac --release $rel $src     ($(t "$jdk"))"
+  "$jdk/bin/javac" --release "$rel" -d "$o" "$src" 2>&1 | grep -v '^Note:'; echo "exit=${PIPESTATUS[0]}"; }
+{ rl "$JDK25" 16 src/ImplicitPermits.java; echo; rl "$JDK25" 17 src/ImplicitPermits.java; echo; rl "$JDK25" 17 src/PaymentDemo.java | head -6; } > "$OUT/09-release-levels.txt"
+
+# 10 - Jackson 3.2.3: does a sealed type help JSON polymorphism? (needs jackson/fetch.sh first; jars are not committed)
+jj() { local jdk="$1" src="$2" cls="$3" o cp; o="$(mk)"
+  cp="jackson/lib/jackson-databind-3.2.3.jar:jackson/lib/jackson-core-3.2.3.jar:jackson/lib/jackson-annotations-2.22.jar"
+  echo "\$ javac -cp jackson/lib/* $src && java $cls     ($(t "$jdk"))"
+  "$jdk/bin/javac" -cp "$cp" -d "$o" "$src" 2>&1 && "$jdk/bin/java" -cp "$cp:$o" "$cls" 2>&1; }
+if [ -f jackson/lib/jackson-databind-3.2.3.jar ]; then
+  { all3 jj jackson/JsonPayment.java JsonPayment; echo; all3 jj jackson/JsonForgotten.java JsonForgotten; echo; all3 jj jackson/JsonNoSubTypes.java JsonNoSubTypes; } > "$OUT/10-jackson.txt"
+else echo "skipped 10-jackson.txt: run jackson/fetch.sh first"; fi
 echo done