diff --git a/Sources/DHKit/AdversaryState.swift b/Sources/DHKit/AdversaryState.swift
index a1362d7..726f200 100644
--- a/Sources/DHKit/AdversaryState.swift
+++ b/Sources/DHKit/AdversaryState.swift
@@ -24,7 +24,8 @@ import Foundation
 ///
 /// All properties are immutable. Mutations are performed by ``EncounterSession``,
 /// which replaces values wholesale (copy-with-update pattern).
-nonisolated public struct AdversaryState: CombatParticipant, Sendable, Equatable, Hashable {
+nonisolated public struct AdversaryState: CombatParticipant, Codable, Sendable, Equatable, Hashable
+{
   public let id: UUID
   /// The slug that identifies this adversary in the ``Compendium``.
   public let adversaryID: String
diff --git a/Sources/DHKit/EncounterSession.swift b/Sources/DHKit/EncounterSession.swift
index 3e81cb3..344a206 100644
--- a/Sources/DHKit/EncounterSession.swift
+++ b/Sources/DHKit/EncounterSession.swift
@@ -60,6 +60,14 @@ public final class EncounterSession: Identifiable, Hashable {
   public let id: UUID
   public var name: String
 
+  /// The ID of the ``EncounterDefinition`` this session was created from.
+  /// `nil` for sessions created directly (tests, blank sessions).
+  public let definitionID: UUID?
+
+  /// The modification date of the source definition at the time this session was created.
+  /// Used by the registry to detect stale sessions. `nil` if not definition-backed.
+  public let definitionSnapshotDate: Date?
+
   // MARK: Participants (private backing stores)
   private var _adversarySlots: [AdversaryState]
   private var _playerSlots: [PlayerState]
@@ -102,8 +110,11 @@ public final class EncounterSession: Identifiable, Hashable {
     environmentSlots: [EnvironmentState] = [],
     fearPool: Int = 0,
     hopePool: Int = 0,
+    spotlightedSlotID: UUID? = nil,
     spotlightCount: Int = 0,
-    gmNotes: String = ""
+    gmNotes: String = "",
+    definitionID: UUID? = nil,
+    definitionSnapshotDate: Date? = nil
   ) {
     self.id = id
     self.name = name
@@ -112,9 +123,11 @@ public final class EncounterSession: Identifiable, Hashable {
     self._environmentSlots = environmentSlots
     self.fearPool = fearPool
     self.hopePool = hopePool
-    self.spotlightedSlotID = nil
+    self.spotlightedSlotID = spotlightedSlotID
     self.spotlightCount = spotlightCount
     self.gmNotes = gmNotes
+    self.definitionID = definitionID
+    self.definitionSnapshotDate = definitionSnapshotDate
   }
 
   // MARK: - Roster Management
@@ -347,9 +360,20 @@ public final class EncounterSession: Identifiable, Hashable {
     from definition: EncounterDefinition,
     using compendium: Compendium
   ) -> EncounterSession {
+    // Count occurrences of each adversary ID so duplicates can be named.
+    var counts: [String: Int] = [:]
+    for id in definition.adversaryIDs { counts[id, default: 0] += 1 }
+
+    // Assign sequential custom names only when the same adversary appears more than once.
+    var counters: [String: Int] = [:]
     let adversarySlots: [AdversaryState] = definition.adversaryIDs.compactMap { id in
       guard let adversary = compendium.adversary(id: id) else { return nil }
-      return AdversaryState(from: adversary)
+      guard (counts[id] ?? 0) > 1 else {
+        return AdversaryState(from: adversary)
+      }
+      let n = (counters[id] ?? 0) + 1
+      counters[id] = n
+      return AdversaryState(from: adversary, customName: "\(adversary.name) \(n)")
     }
 
     let environmentSlots: [EnvironmentState] = definition.environmentIDs.compactMap { id in
@@ -374,7 +398,61 @@ public final class EncounterSession: Identifiable, Hashable {
       adversarySlots: adversarySlots,
       playerSlots: playerSlots,
       environmentSlots: environmentSlots,
-      gmNotes: definition.gmNotes
+      gmNotes: definition.gmNotes,
+      definitionID: definition.id,
+      definitionSnapshotDate: definition.modifiedAt
+    )
+  }
+}
+
+// MARK: - Codable
+
+extension EncounterSession: @MainActor Codable {
+
+  enum CodingKeys: String, CodingKey {
+    case id, name
+    case adversarySlots, playerSlots, environmentSlots
+    case fearPool, hopePool, spotlightedSlotID, spotlightCount, gmNotes
+    case definitionID, definitionSnapshotDate
+  }
+
+  public func encode(to encoder: any Encoder) throws {
+    var c = encoder.container(keyedBy: CodingKeys.self)
+    try c.encode(id, forKey: .id)
+    try c.encode(name, forKey: .name)
+    try c.encode(_adversarySlots, forKey: .adversarySlots)
+    try c.encode(_playerSlots, forKey: .playerSlots)
+    try c.encode(_environmentSlots, forKey: .environmentSlots)
+    try c.encode(fearPool, forKey: .fearPool)
+    try c.encode(hopePool, forKey: .hopePool)
+    try c.encodeIfPresent(spotlightedSlotID, forKey: .spotlightedSlotID)
+    try c.encode(spotlightCount, forKey: .spotlightCount)
+    try c.encode(gmNotes, forKey: .gmNotes)
+    try c.encodeIfPresent(definitionID, forKey: .definitionID)
+    try c.encodeIfPresent(definitionSnapshotDate, forKey: .definitionSnapshotDate)
+  }
+
+  public convenience init(from decoder: any Decoder) throws {
+    let c = try decoder.container(keyedBy: CodingKeys.self)
+    let id = try c.decode(UUID.self, forKey: .id)
+    let name = try c.decode(String.self, forKey: .name)
+    let adversarySlots = try c.decode([AdversaryState].self, forKey: .adversarySlots)
+    let playerSlots = try c.decode([PlayerState].self, forKey: .playerSlots)
+    let environmentSlots = try c.decode([EnvironmentState].self, forKey: .environmentSlots)
+    let fearPool = try c.decode(Int.self, forKey: .fearPool)
+    let hopePool = try c.decode(Int.self, forKey: .hopePool)
+    let spotlightedSlotID = try c.decodeIfPresent(UUID.self, forKey: .spotlightedSlotID)
+    let spotlightCount = try c.decode(Int.self, forKey: .spotlightCount)
+    let gmNotes = try c.decode(String.self, forKey: .gmNotes)
+    let definitionID = try c.decodeIfPresent(UUID.self, forKey: .definitionID)
+    let definitionSnapshotDate = try c.decodeIfPresent(Date.self, forKey: .definitionSnapshotDate)
+    self.init(
+      id: id, name: name,
+      adversarySlots: adversarySlots, playerSlots: playerSlots,
+      environmentSlots: environmentSlots,
+      fearPool: fearPool, hopePool: hopePool,
+      spotlightedSlotID: spotlightedSlotID, spotlightCount: spotlightCount, gmNotes: gmNotes,
+      definitionID: definitionID, definitionSnapshotDate: definitionSnapshotDate
     )
   }
 }
diff --git a/Sources/DHKit/EnvironmentState.swift b/Sources/DHKit/EnvironmentState.swift
index d0ce086..04b4206 100644
--- a/Sources/DHKit/EnvironmentState.swift
+++ b/Sources/DHKit/EnvironmentState.swift
@@ -18,7 +18,9 @@ import Foundation
 ///
 /// All properties are immutable. Mutations are performed by ``EncounterSession``,
 /// which replaces values wholesale (copy-with-update pattern).
-nonisolated public struct EnvironmentState: EncounterParticipant, Sendable, Equatable, Hashable {
+nonisolated public struct EnvironmentState: EncounterParticipant, Codable, Sendable, Equatable,
+  Hashable
+{
   public let id: UUID
   /// The slug identifying this environment in the ``Compendium``.
   public let environmentID: String
diff --git a/Sources/DHKit/PlayerState.swift b/Sources/DHKit/PlayerState.swift
index 1b39805..7dcf55c 100644
--- a/Sources/DHKit/PlayerState.swift
+++ b/Sources/DHKit/PlayerState.swift
@@ -24,7 +24,7 @@ import Foundation
 ///
 /// Tracks combat-relevant PC stats the GM needs to resolve hits and
 /// track health during play. The full character sheet remains with the player.
-nonisolated public struct PlayerState: CombatParticipant, Sendable, Equatable, Hashable {
+nonisolated public struct PlayerState: CombatParticipant, Codable, Sendable, Equatable, Hashable {
   public let id: UUID
   public let name: String
 
diff --git a/Tests/DHKitTests/EncounterSessionTests.swift b/Tests/DHKitTests/EncounterSessionTests.swift
index d482f3e..a41a75b 100644
--- a/Tests/DHKitTests/EncounterSessionTests.swift
+++ b/Tests/DHKitTests/EncounterSessionTests.swift
@@ -470,6 +470,140 @@ import Testing
     #expect(session.spotlightedSlotID == nil)
     #expect(session.spotlightCount == 0)
   }
+
+  // MARK: Definition provenance
+
+  @Test func makeFromDefinitionSetsDefinitionProvenance() {
+    let compendium = makeCompendium()
+    let def = EncounterDefinition(name: "Provenance Test")
+
+    let session = EncounterSession.make(from: def, using: compendium)
+
+    #expect(session.definitionID == def.id)
+    #expect(session.definitionSnapshotDate == def.modifiedAt)
+  }
+
+  // MARK: Adversary auto-numbering (#17)
+
+  @Test func singleAdversaryHasNoCustomName() {
+    let compendium = makeCompendium()
+    var def = EncounterDefinition(name: "Test")
+    def.adversaryIDs = ["ironguard-soldier"]
+
+    let session = EncounterSession.make(from: def, using: compendium)
+
+    #expect(session.adversarySlots[0].customName == nil)
+  }
+
+  @Test func duplicateAdversariesGetNumberedNames() {
+    let compendium = makeCompendium()
+    var def = EncounterDefinition(name: "Test")
+    def.adversaryIDs = ["ironguard-soldier", "ironguard-soldier"]
+
+    let session = EncounterSession.make(from: def, using: compendium)
+
+    #expect(session.adversarySlots[0].customName == "Ironguard Soldier 1")
+    #expect(session.adversarySlots[1].customName == "Ironguard Soldier 2")
+  }
+
+  @Test func triplicateAdversariesGetNumberedNames() {
+    let comp = Compendium()
+    comp.addAdversary(
+      Adversary(
+        id: "goblin", name: "Goblin",
+        tier: 1, role: .minion, flavorText: "A small menace.",
+        difficulty: 8, thresholdMajor: 3, thresholdSevere: 6,
+        hp: 3, stress: 2, attackModifier: "+1", attackName: "Shiv",
+        attackRange: .veryClose, damage: "1d6 phy"
+      ))
+    var def = EncounterDefinition(name: "Test")
+    def.adversaryIDs = ["goblin", "goblin", "goblin"]
+
+    let session = EncounterSession.make(from: def, using: comp)
+
+    #expect(session.adversarySlots[0].customName == "Goblin 1")
+    #expect(session.adversarySlots[1].customName == "Goblin 2")
+    #expect(session.adversarySlots[2].customName == "Goblin 3")
+  }
+
+  @Test func mixedAdversariesOnlyDuplicatesAreNumbered() {
+    let comp = Compendium()
+    comp.addAdversary(
+      Adversary(
+        id: "goblin", name: "Goblin",
+        tier: 1, role: .minion, flavorText: "A small menace.",
+        difficulty: 8, thresholdMajor: 3, thresholdSevere: 6,
+        hp: 3, stress: 2, attackModifier: "+1", attackName: "Shiv",
+        attackRange: .veryClose, damage: "1d6 phy"
+      ))
+    comp.addAdversary(
+      Adversary(
+        id: "ironguard-soldier", name: "Ironguard Soldier",
+        tier: 1, role: .bruiser, flavorText: "A disciplined mercenary.",
+        difficulty: 11, thresholdMajor: 5, thresholdSevere: 10,
+        hp: 6, stress: 3, attackModifier: "+3", attackName: "Longsword",
+        attackRange: .veryClose, damage: "1d10+3 phy"
+      ))
+    var def = EncounterDefinition(name: "Test")
+    def.adversaryIDs = ["goblin", "ironguard-soldier", "ironguard-soldier"]
+
+    let session = EncounterSession.make(from: def, using: comp)
+
+    // goblin appears once — no custom name
+    #expect(session.adversarySlots[0].customName == nil)
+    // soldiers appear twice — numbered
+    #expect(session.adversarySlots[1].customName == "Ironguard Soldier 1")
+    #expect(session.adversarySlots[2].customName == "Ironguard Soldier 2")
+  }
+}
+
+// MARK: - EncounterSession Codable
+
+@MainActor struct EncounterSessionCodableTests {
+
+  @Test func sessionCodableRoundTrip() throws {
+    let adversarySlot = AdversaryState(adversaryID: "ironguard-soldier", maxHP: 6, maxStress: 3)
+    let session = EncounterSession(
+      name: "Bandit Ambush",
+      adversarySlots: [adversarySlot],
+      playerSlots: [
+        PlayerState(
+          name: "Aldric", maxHP: 6, maxStress: 6,
+          evasion: 12, thresholdMajor: 8, thresholdSevere: 15, armorSlots: 3
+        )
+      ],
+      fearPool: 2,
+      hopePool: 3,
+      spotlightedSlotID: adversarySlot.id,
+      spotlightCount: 1,
+      gmNotes: "Remember the secret door.",
+      definitionID: UUID(),
+      definitionSnapshotDate: Date(timeIntervalSince1970: 1_000_000)
+    )
+
+    let data = try JSONEncoder().encode(session)
+    let decoded = try JSONDecoder().decode(EncounterSession.self, from: data)
+
+    #expect(decoded.id == session.id)
+    #expect(decoded.name == "Bandit Ambush")
+    #expect(decoded.adversarySlots.count == 1)
+    #expect(decoded.adversarySlots[0].adversaryID == "ironguard-soldier")
+    #expect(decoded.playerSlots.count == 1)
+    #expect(decoded.playerSlots[0].name == "Aldric")
+    #expect(decoded.fearPool == 2)
+    #expect(decoded.hopePool == 3)
+    #expect(decoded.spotlightedSlotID == adversarySlot.id)
+    #expect(decoded.spotlightCount == 1)
+    #expect(decoded.gmNotes == "Remember the secret door.")
+    #expect(decoded.definitionID == session.definitionID)
+    #expect(decoded.definitionSnapshotDate == session.definitionSnapshotDate)
+  }
+
+  @Test func directInitHasNilDefinitionFields() {
+    let session = EncounterSession(name: "Ad-hoc Encounter")
+    #expect(session.definitionID == nil)
+    #expect(session.definitionSnapshotDate == nil)
+  }
 }
 
 // MARK: - AdversaryState stat snapshot
diff --git a/Tests/DHKitTests/StateTests.swift b/Tests/DHKitTests/StateTests.swift
index b624357..d2e2414 100644
--- a/Tests/DHKitTests/StateTests.swift
+++ b/Tests/DHKitTests/StateTests.swift
@@ -48,6 +48,25 @@ struct PlayerStateTests {
     )
     #expect(slot1 == slot2)
   }
+
+  @Test func playerStateCodableRoundTrip() throws {
+    let slot = PlayerState(
+      name: "Aldric", maxHP: 6, currentHP: 4,
+      maxStress: 6, currentStress: 2,
+      evasion: 12, thresholdMajor: 8, thresholdSevere: 15,
+      armorSlots: 3, currentArmorSlots: 1,
+      conditions: [.restrained]
+    )
+    let data = try JSONEncoder().encode(slot)
+    let decoded = try JSONDecoder().decode(PlayerState.self, from: data)
+
+    #expect(decoded.id == slot.id)
+    #expect(decoded.name == "Aldric")
+    #expect(decoded.currentHP == 4)
+    #expect(decoded.currentStress == 2)
+    #expect(decoded.currentArmorSlots == 1)
+    #expect(decoded.conditions == [.restrained])
+  }
 }
 
 // MARK: - AdversaryState
@@ -86,6 +105,27 @@ struct AdversaryStateTests {
     #expect(updated.adversaryID == "orc")
     #expect(updated.id == slot.id)
   }
+
+  @Test func adversaryCodableRoundTrip() throws {
+    let slot = AdversaryState(
+      adversaryID: "goblin",
+      customName: "Grimfang",
+      maxHP: 4, maxStress: 2,
+      currentHP: 2, currentStress: 1,
+      isDefeated: false,
+      conditions: [.vulnerable]
+    )
+    let data = try JSONEncoder().encode(slot)
+    let decoded = try JSONDecoder().decode(AdversaryState.self, from: data)
+
+    #expect(decoded.id == slot.id)
+    #expect(decoded.adversaryID == "goblin")
+    #expect(decoded.customName == "Grimfang")
+    #expect(decoded.currentHP == 2)
+    #expect(decoded.currentStress == 1)
+    #expect(decoded.isDefeated == false)
+    #expect(decoded.conditions == [.vulnerable])
+  }
 }
 
 // MARK: - EnvironmentState
@@ -105,4 +145,14 @@ struct EnvironmentStateTests {
     #expect(deactivated.id == slot.id)
     #expect(deactivated.environmentID == slot.environmentID)
   }
+
+  @Test func environmentStateCodableRoundTrip() throws {
+    let slot = EnvironmentState(environmentID: "arcane-storm", isActive: false)
+    let data = try JSONEncoder().encode(slot)
+    let decoded = try JSONDecoder().decode(EnvironmentState.self, from: data)
+
+    #expect(decoded.id == slot.id)
+    #expect(decoded.environmentID == "arcane-storm")
+    #expect(decoded.isActive == false)
+  }
 }