diff --git a/.release-please-manifest.json b/.release-please-manifest.json
index 59bb50ff..b9e06599 100644
--- a/.release-please-manifest.json
+++ b/.release-please-manifest.json
@@ -1,3 +1,3 @@
 {
-  ".": "4.20.2"
+  ".": "4.21.0"
 }
\ No newline at end of file
diff --git a/.stats.yml b/.stats.yml
index 9e3b99b6..809d9f7f 100644
--- a/.stats.yml
+++ b/.stats.yml
@@ -1,4 +1,4 @@
-configured_endpoints: 138
-openapi_spec_url: https://storage.googleapis.com/stainless-sdk-openapi-specs/courier/courier-11e1931d767cef402bbed5a965746d1933ae72db0f41881860593a0b1424fd80.yml
-openapi_spec_hash: 9def941805f233f8761c4bdae88f6137
-config_hash: 6c3a754258f0f77e9032a90a478ab76d
+configured_endpoints: 139
+openapi_spec_url: https://storage.googleapis.com/stainless-sdk-openapi-specs/courier/courier-def1e982add0411bf64b48cdf5e4f2c1b85b9bd0ada0d5e6fc4869afa85e0bff.yml
+openapi_spec_hash: 5d129cdf800798bcd4a93585ec89c696
+config_hash: dcd58abf1e56b114bdce22336ca3d570
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 58efece7..d12950c9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,14 @@
 # Changelog
 
+## 4.21.0 (2026-07-14)
+
+Full Changelog: [v4.20.2...v4.21.0](https://github.com/trycourier/courier-java/compare/v4.20.2...v4.21.0)
+
+### Features
+
+* Document POST /notifications/{id}/duplicate [C-18607] ([#165](https://github.com/trycourier/courier-java/issues/165)) ([2d6de78](https://github.com/trycourier/courier-java/commit/2d6de78b8dca5f167d42c3e2d136998278ddc80d))
+* Merge pull request [#149](https://github.com/trycourier/courier-java/issues/149) from trycourier/geraldosilva/c-19201-notification-template-subscription-topic-id ([74a301f](https://github.com/trycourier/courier-java/commit/74a301ff09a28e668a231af1ba87417bd637b8dd))
+
 ## 4.20.2 (2026-07-10)
 
 Full Changelog: [v4.20.1...v4.20.2](https://github.com/trycourier/courier-java/compare/v4.20.1...v4.20.2)
diff --git a/build.gradle.kts b/build.gradle.kts
index 13572d6b..58362a3f 100644
--- a/build.gradle.kts
+++ b/build.gradle.kts
@@ -8,7 +8,7 @@ repositories {
 
 allprojects {
     group = "com.courier"
-    version = "4.20.2" // x-release-please-version
+    version = "4.21.0" // x-release-please-version
 }
 
 subprojects {
diff --git a/courier-java-core/src/main/kotlin/com/courier/models/notifications/NotificationDuplicateParams.kt b/courier-java-core/src/main/kotlin/com/courier/models/notifications/NotificationDuplicateParams.kt
new file mode 100644
index 00000000..5352b9d3
--- /dev/null
+++ b/courier-java-core/src/main/kotlin/com/courier/models/notifications/NotificationDuplicateParams.kt
@@ -0,0 +1,238 @@
+// File generated from our OpenAPI spec by Stainless.
+
+package com.courier.models.notifications
+
+import com.courier.core.JsonValue
+import com.courier.core.Params
+import com.courier.core.http.Headers
+import com.courier.core.http.QueryParams
+import com.courier.core.toImmutable
+import java.util.Objects
+import java.util.Optional
+import kotlin.jvm.optionals.getOrNull
+
+/**
+ * Duplicate a notification template. Creates a standalone copy within the same workspace and
+ * environment, with " COPY" appended to the title. The copy clones the source draft's tags, brand,
+ * subscription topic, routing strategy, channels, and content, and is always created as a
+ * standalone template (it is not linked to any journey or broadcast, even if the source was).
+ * Templates that are scoped to a journey or a broadcast cannot be duplicated through this endpoint.
+ */
+class NotificationDuplicateParams
+private constructor(
+    private val id: String?,
+    private val additionalHeaders: Headers,
+    private val additionalQueryParams: QueryParams,
+    private val additionalBodyProperties: Map<String, JsonValue>,
+) : Params {
+
+    fun id(): Optional<String> = Optional.ofNullable(id)
+
+    /** Additional body properties to send with the request. */
+    fun _additionalBodyProperties(): Map<String, JsonValue> = additionalBodyProperties
+
+    /** Additional headers to send with the request. */
+    fun _additionalHeaders(): Headers = additionalHeaders
+
+    /** Additional query param to send with the request. */
+    fun _additionalQueryParams(): QueryParams = additionalQueryParams
+
+    fun toBuilder() = Builder().from(this)
+
+    companion object {
+
+        @JvmStatic fun none(): NotificationDuplicateParams = builder().build()
+
+        /**
+         * Returns a mutable builder for constructing an instance of [NotificationDuplicateParams].
+         */
+        @JvmStatic fun builder() = Builder()
+    }
+
+    /** A builder for [NotificationDuplicateParams]. */
+    class Builder internal constructor() {
+
+        private var id: String? = null
+        private var additionalHeaders: Headers.Builder = Headers.builder()
+        private var additionalQueryParams: QueryParams.Builder = QueryParams.builder()
+        private var additionalBodyProperties: MutableMap<String, JsonValue> = mutableMapOf()
+
+        @JvmSynthetic
+        internal fun from(notificationDuplicateParams: NotificationDuplicateParams) = apply {
+            id = notificationDuplicateParams.id
+            additionalHeaders = notificationDuplicateParams.additionalHeaders.toBuilder()
+            additionalQueryParams = notificationDuplicateParams.additionalQueryParams.toBuilder()
+            additionalBodyProperties =
+                notificationDuplicateParams.additionalBodyProperties.toMutableMap()
+        }
+
+        fun id(id: String?) = apply { this.id = id }
+
+        /** Alias for calling [Builder.id] with `id.orElse(null)`. */
+        fun id(id: Optional<String>) = id(id.getOrNull())
+
+        fun additionalHeaders(additionalHeaders: Headers) = apply {
+            this.additionalHeaders.clear()
+            putAllAdditionalHeaders(additionalHeaders)
+        }
+
+        fun additionalHeaders(additionalHeaders: Map<String, Iterable<String>>) = apply {
+            this.additionalHeaders.clear()
+            putAllAdditionalHeaders(additionalHeaders)
+        }
+
+        fun putAdditionalHeader(name: String, value: String) = apply {
+            additionalHeaders.put(name, value)
+        }
+
+        fun putAdditionalHeaders(name: String, values: Iterable<String>) = apply {
+            additionalHeaders.put(name, values)
+        }
+
+        fun putAllAdditionalHeaders(additionalHeaders: Headers) = apply {
+            this.additionalHeaders.putAll(additionalHeaders)
+        }
+
+        fun putAllAdditionalHeaders(additionalHeaders: Map<String, Iterable<String>>) = apply {
+            this.additionalHeaders.putAll(additionalHeaders)
+        }
+
+        fun replaceAdditionalHeaders(name: String, value: String) = apply {
+            additionalHeaders.replace(name, value)
+        }
+
+        fun replaceAdditionalHeaders(name: String, values: Iterable<String>) = apply {
+            additionalHeaders.replace(name, values)
+        }
+
+        fun replaceAllAdditionalHeaders(additionalHeaders: Headers) = apply {
+            this.additionalHeaders.replaceAll(additionalHeaders)
+        }
+
+        fun replaceAllAdditionalHeaders(additionalHeaders: Map<String, Iterable<String>>) = apply {
+            this.additionalHeaders.replaceAll(additionalHeaders)
+        }
+
+        fun removeAdditionalHeaders(name: String) = apply { additionalHeaders.remove(name) }
+
+        fun removeAllAdditionalHeaders(names: Set<String>) = apply {
+            additionalHeaders.removeAll(names)
+        }
+
+        fun additionalQueryParams(additionalQueryParams: QueryParams) = apply {
+            this.additionalQueryParams.clear()
+            putAllAdditionalQueryParams(additionalQueryParams)
+        }
+
+        fun additionalQueryParams(additionalQueryParams: Map<String, Iterable<String>>) = apply {
+            this.additionalQueryParams.clear()
+            putAllAdditionalQueryParams(additionalQueryParams)
+        }
+
+        fun putAdditionalQueryParam(key: String, value: String) = apply {
+            additionalQueryParams.put(key, value)
+        }
+
+        fun putAdditionalQueryParams(key: String, values: Iterable<String>) = apply {
+            additionalQueryParams.put(key, values)
+        }
+
+        fun putAllAdditionalQueryParams(additionalQueryParams: QueryParams) = apply {
+            this.additionalQueryParams.putAll(additionalQueryParams)
+        }
+
+        fun putAllAdditionalQueryParams(additionalQueryParams: Map<String, Iterable<String>>) =
+            apply {
+                this.additionalQueryParams.putAll(additionalQueryParams)
+            }
+
+        fun replaceAdditionalQueryParams(key: String, value: String) = apply {
+            additionalQueryParams.replace(key, value)
+        }
+
+        fun replaceAdditionalQueryParams(key: String, values: Iterable<String>) = apply {
+            additionalQueryParams.replace(key, values)
+        }
+
+        fun replaceAllAdditionalQueryParams(additionalQueryParams: QueryParams) = apply {
+            this.additionalQueryParams.replaceAll(additionalQueryParams)
+        }
+
+        fun replaceAllAdditionalQueryParams(additionalQueryParams: Map<String, Iterable<String>>) =
+            apply {
+                this.additionalQueryParams.replaceAll(additionalQueryParams)
+            }
+
+        fun removeAdditionalQueryParams(key: String) = apply { additionalQueryParams.remove(key) }
+
+        fun removeAllAdditionalQueryParams(keys: Set<String>) = apply {
+            additionalQueryParams.removeAll(keys)
+        }
+
+        fun additionalBodyProperties(additionalBodyProperties: Map<String, JsonValue>) = apply {
+            this.additionalBodyProperties.clear()
+            putAllAdditionalBodyProperties(additionalBodyProperties)
+        }
+
+        fun putAdditionalBodyProperty(key: String, value: JsonValue) = apply {
+            additionalBodyProperties.put(key, value)
+        }
+
+        fun putAllAdditionalBodyProperties(additionalBodyProperties: Map<String, JsonValue>) =
+            apply {
+                this.additionalBodyProperties.putAll(additionalBodyProperties)
+            }
+
+        fun removeAdditionalBodyProperty(key: String) = apply {
+            additionalBodyProperties.remove(key)
+        }
+
+        fun removeAllAdditionalBodyProperties(keys: Set<String>) = apply {
+            keys.forEach(::removeAdditionalBodyProperty)
+        }
+
+        /**
+         * Returns an immutable instance of [NotificationDuplicateParams].
+         *
+         * Further updates to this [Builder] will not mutate the returned instance.
+         */
+        fun build(): NotificationDuplicateParams =
+            NotificationDuplicateParams(
+                id,
+                additionalHeaders.build(),
+                additionalQueryParams.build(),
+                additionalBodyProperties.toImmutable(),
+            )
+    }
+
+    fun _body(): Optional<Map<String, JsonValue>> =
+        Optional.ofNullable(additionalBodyProperties.ifEmpty { null })
+
+    fun _pathParam(index: Int): String =
+        when (index) {
+            0 -> id ?: ""
+            else -> ""
+        }
+
+    override fun _headers(): Headers = additionalHeaders
+
+    override fun _queryParams(): QueryParams = additionalQueryParams
+
+    override fun equals(other: Any?): Boolean {
+        if (this === other) {
+            return true
+        }
+
+        return other is NotificationDuplicateParams &&
+            id == other.id &&
+            additionalHeaders == other.additionalHeaders &&
+            additionalQueryParams == other.additionalQueryParams &&
+            additionalBodyProperties == other.additionalBodyProperties
+    }
+
+    override fun hashCode(): Int =
+        Objects.hash(id, additionalHeaders, additionalQueryParams, additionalBodyProperties)
+
+    override fun toString() =
+        "NotificationDuplicateParams{id=$id, additionalHeaders=$additionalHeaders, additionalQueryParams=$additionalQueryParams, additionalBodyProperties=$additionalBodyProperties}"
+}
diff --git a/courier-java-core/src/main/kotlin/com/courier/models/notifications/NotificationTemplateSummary.kt b/courier-java-core/src/main/kotlin/com/courier/models/notifications/NotificationTemplateSummary.kt
index 9df42446..fecacd3d 100644
--- a/courier-java-core/src/main/kotlin/com/courier/models/notifications/NotificationTemplateSummary.kt
+++ b/courier-java-core/src/main/kotlin/com/courier/models/notifications/NotificationTemplateSummary.kt
@@ -30,6 +30,7 @@ private constructor(
     private val name: JsonField<String>,
     private val state: JsonField<State>,
     private val tags: JsonField<List<String>>,
+    private val subscriptionTopicId: JsonField<String>,
     private val updated: JsonField<Long>,
     private val updater: JsonField<String>,
     private val additionalProperties: MutableMap<String, JsonValue>,
@@ -43,9 +44,23 @@ private constructor(
         @JsonProperty("name") @ExcludeMissing name: JsonField<String> = JsonMissing.of(),
         @JsonProperty("state") @ExcludeMissing state: JsonField<State> = JsonMissing.of(),
         @JsonProperty("tags") @ExcludeMissing tags: JsonField<List<String>> = JsonMissing.of(),
+        @JsonProperty("subscription_topic_id")
+        @ExcludeMissing
+        subscriptionTopicId: JsonField<String> = JsonMissing.of(),
         @JsonProperty("updated") @ExcludeMissing updated: JsonField<Long> = JsonMissing.of(),
         @JsonProperty("updater") @ExcludeMissing updater: JsonField<String> = JsonMissing.of(),
-    ) : this(id, created, creator, name, state, tags, updated, updater, mutableMapOf())
+    ) : this(
+        id,
+        created,
+        creator,
+        name,
+        state,
+        tags,
+        subscriptionTopicId,
+        updated,
+        updater,
+        mutableMapOf(),
+    )
 
     /**
      * @throws CourierInvalidDataException if the JSON field has an unexpected type or is
@@ -87,6 +102,16 @@ private constructor(
      */
     fun tags(): List<String> = tags.getRequired("tags")
 
+    /**
+     * The linked subscription (preference) topic of the published version. Omitted when no topic is
+     * linked or the template has never been published.
+     *
+     * @throws CourierInvalidDataException if the JSON field has an unexpected type (e.g. if the
+     *   server responded with an unexpected value).
+     */
+    fun subscriptionTopicId(): Optional<String> =
+        subscriptionTopicId.getOptional("subscription_topic_id")
+
     /**
      * Epoch milliseconds of last update.
      *
@@ -145,6 +170,16 @@ private constructor(
      */
     @JsonProperty("tags") @ExcludeMissing fun _tags(): JsonField<List<String>> = tags
 
+    /**
+     * Returns the raw JSON value of [subscriptionTopicId].
+     *
+     * Unlike [subscriptionTopicId], this method doesn't throw if the JSON field has an unexpected
+     * type.
+     */
+    @JsonProperty("subscription_topic_id")
+    @ExcludeMissing
+    fun _subscriptionTopicId(): JsonField<String> = subscriptionTopicId
+
     /**
      * Returns the raw JSON value of [updated].
      *
@@ -198,6 +233,7 @@ private constructor(
         private var name: JsonField<String>? = null
         private var state: JsonField<State>? = null
         private var tags: JsonField<MutableList<String>>? = null
+        private var subscriptionTopicId: JsonField<String> = JsonMissing.of()
         private var updated: JsonField<Long> = JsonMissing.of()
         private var updater: JsonField<String> = JsonMissing.of()
         private var additionalProperties: MutableMap<String, JsonValue> = mutableMapOf()
@@ -210,6 +246,7 @@ private constructor(
             name = notificationTemplateSummary.name
             state = notificationTemplateSummary.state
             tags = notificationTemplateSummary.tags.map { it.toMutableList() }
+            subscriptionTopicId = notificationTemplateSummary.subscriptionTopicId
             updated = notificationTemplateSummary.updated
             updater = notificationTemplateSummary.updater
             additionalProperties = notificationTemplateSummary.additionalProperties.toMutableMap()
@@ -289,6 +326,24 @@ private constructor(
             tags = (tags ?: JsonField.of(mutableListOf())).also { checkKnown("tags", it).add(tag) }
         }
 
+        /**
+         * The linked subscription (preference) topic of the published version. Omitted when no
+         * topic is linked or the template has never been published.
+         */
+        fun subscriptionTopicId(subscriptionTopicId: String) =
+            subscriptionTopicId(JsonField.of(subscriptionTopicId))
+
+        /**
+         * Sets [Builder.subscriptionTopicId] to an arbitrary JSON value.
+         *
+         * You should usually call [Builder.subscriptionTopicId] with a well-typed [String] value
+         * instead. This method is primarily for setting the field to an undocumented or not yet
+         * supported value.
+         */
+        fun subscriptionTopicId(subscriptionTopicId: JsonField<String>) = apply {
+            this.subscriptionTopicId = subscriptionTopicId
+        }
+
         /** Epoch milliseconds of last update. */
         fun updated(updated: Long) = updated(JsonField.of(updated))
 
@@ -355,6 +410,7 @@ private constructor(
                 checkRequired("name", name),
                 checkRequired("state", state),
                 checkRequired("tags", tags).map { it.toImmutable() },
+                subscriptionTopicId,
                 updated,
                 updater,
                 additionalProperties.toMutableMap(),
@@ -382,6 +438,7 @@ private constructor(
         name()
         state().validate()
         tags()
+        subscriptionTopicId()
         updated()
         updater()
         validated = true
@@ -408,6 +465,7 @@ private constructor(
             (if (name.asKnown().isPresent) 1 else 0) +
             (state.asKnown().getOrNull()?.validity() ?: 0) +
             (tags.asKnown().getOrNull()?.size ?: 0) +
+            (if (subscriptionTopicId.asKnown().isPresent) 1 else 0) +
             (if (updated.asKnown().isPresent) 1 else 0) +
             (if (updater.asKnown().isPresent) 1 else 0)
 
@@ -557,6 +615,7 @@ private constructor(
             name == other.name &&
             state == other.state &&
             tags == other.tags &&
+            subscriptionTopicId == other.subscriptionTopicId &&
             updated == other.updated &&
             updater == other.updater &&
             additionalProperties == other.additionalProperties
@@ -570,6 +629,7 @@ private constructor(
             name,
             state,
             tags,
+            subscriptionTopicId,
             updated,
             updater,
             additionalProperties,
@@ -579,5 +639,5 @@ private constructor(
     override fun hashCode(): Int = hashCode
 
     override fun toString() =
-        "NotificationTemplateSummary{id=$id, created=$created, creator=$creator, name=$name, state=$state, tags=$tags, updated=$updated, updater=$updater, additionalProperties=$additionalProperties}"
+        "NotificationTemplateSummary{id=$id, created=$created, creator=$creator, name=$name, state=$state, tags=$tags, subscriptionTopicId=$subscriptionTopicId, updated=$updated, updater=$updater, additionalProperties=$additionalProperties}"
 }
diff --git a/courier-java-core/src/main/kotlin/com/courier/services/async/NotificationServiceAsync.kt b/courier-java-core/src/main/kotlin/com/courier/services/async/NotificationServiceAsync.kt
index 37cfa66a..795006ee 100644
--- a/courier-java-core/src/main/kotlin/com/courier/services/async/NotificationServiceAsync.kt
+++ b/courier-java-core/src/main/kotlin/com/courier/services/async/NotificationServiceAsync.kt
@@ -9,6 +9,7 @@ import com.courier.core.http.HttpResponseFor
 import com.courier.models.notifications.NotificationArchiveParams
 import com.courier.models.notifications.NotificationContentMutationResponse
 import com.courier.models.notifications.NotificationCreateParams
+import com.courier.models.notifications.NotificationDuplicateParams
 import com.courier.models.notifications.NotificationListParams
 import com.courier.models.notifications.NotificationListResponse
 import com.courier.models.notifications.NotificationListVersionsParams
@@ -162,6 +163,50 @@ interface NotificationServiceAsync {
     fun archive(id: String, requestOptions: RequestOptions): CompletableFuture<Void?> =
         archive(id, NotificationArchiveParams.none(), requestOptions)
 
+    /**
+     * Duplicate a notification template. Creates a standalone copy within the same workspace and
+     * environment, with " COPY" appended to the title. The copy clones the source draft's tags,
+     * brand, subscription topic, routing strategy, channels, and content, and is always created as
+     * a standalone template (it is not linked to any journey or broadcast, even if the source was).
+     * Templates that are scoped to a journey or a broadcast cannot be duplicated through this
+     * endpoint.
+     */
+    fun duplicate(id: String): CompletableFuture<NotificationTemplateResponse> =
+        duplicate(id, NotificationDuplicateParams.none())
+
+    /** @see duplicate */
+    fun duplicate(
+        id: String,
+        params: NotificationDuplicateParams = NotificationDuplicateParams.none(),
+        requestOptions: RequestOptions = RequestOptions.none(),
+    ): CompletableFuture<NotificationTemplateResponse> =
+        duplicate(params.toBuilder().id(id).build(), requestOptions)
+
+    /** @see duplicate */
+    fun duplicate(
+        id: String,
+        params: NotificationDuplicateParams = NotificationDuplicateParams.none(),
+    ): CompletableFuture<NotificationTemplateResponse> =
+        duplicate(id, params, RequestOptions.none())
+
+    /** @see duplicate */
+    fun duplicate(
+        params: NotificationDuplicateParams,
+        requestOptions: RequestOptions = RequestOptions.none(),
+    ): CompletableFuture<NotificationTemplateResponse>
+
+    /** @see duplicate */
+    fun duplicate(
+        params: NotificationDuplicateParams
+    ): CompletableFuture<NotificationTemplateResponse> = duplicate(params, RequestOptions.none())
+
+    /** @see duplicate */
+    fun duplicate(
+        id: String,
+        requestOptions: RequestOptions,
+    ): CompletableFuture<NotificationTemplateResponse> =
+        duplicate(id, NotificationDuplicateParams.none(), requestOptions)
+
     /** List versions of a notification template. */
     fun listVersions(id: String): CompletableFuture<NotificationTemplateVersionListResponse> =
         listVersions(id, NotificationListVersionsParams.none())
@@ -542,6 +587,49 @@ interface NotificationServiceAsync {
         fun archive(id: String, requestOptions: RequestOptions): CompletableFuture<HttpResponse> =
             archive(id, NotificationArchiveParams.none(), requestOptions)
 
+        /**
+         * Returns a raw HTTP response for `post /notifications/{id}/duplicate`, but is otherwise
+         * the same as [NotificationServiceAsync.duplicate].
+         */
+        fun duplicate(
+            id: String
+        ): CompletableFuture<HttpResponseFor<NotificationTemplateResponse>> =
+            duplicate(id, NotificationDuplicateParams.none())
+
+        /** @see duplicate */
+        fun duplicate(
+            id: String,
+            params: NotificationDuplicateParams = NotificationDuplicateParams.none(),
+            requestOptions: RequestOptions = RequestOptions.none(),
+        ): CompletableFuture<HttpResponseFor<NotificationTemplateResponse>> =
+            duplicate(params.toBuilder().id(id).build(), requestOptions)
+
+        /** @see duplicate */
+        fun duplicate(
+            id: String,
+            params: NotificationDuplicateParams = NotificationDuplicateParams.none(),
+        ): CompletableFuture<HttpResponseFor<NotificationTemplateResponse>> =
+            duplicate(id, params, RequestOptions.none())
+
+        /** @see duplicate */
+        fun duplicate(
+            params: NotificationDuplicateParams,
+            requestOptions: RequestOptions = RequestOptions.none(),
+        ): CompletableFuture<HttpResponseFor<NotificationTemplateResponse>>
+
+        /** @see duplicate */
+        fun duplicate(
+            params: NotificationDuplicateParams
+        ): CompletableFuture<HttpResponseFor<NotificationTemplateResponse>> =
+            duplicate(params, RequestOptions.none())
+
+        /** @see duplicate */
+        fun duplicate(
+            id: String,
+            requestOptions: RequestOptions,
+        ): CompletableFuture<HttpResponseFor<NotificationTemplateResponse>> =
+            duplicate(id, NotificationDuplicateParams.none(), requestOptions)
+
         /**
          * Returns a raw HTTP response for `get /notifications/{id}/versions`, but is otherwise the
          * same as [NotificationServiceAsync.listVersions].
diff --git a/courier-java-core/src/main/kotlin/com/courier/services/async/NotificationServiceAsyncImpl.kt b/courier-java-core/src/main/kotlin/com/courier/services/async/NotificationServiceAsyncImpl.kt
index dd19c526..b0f46fb9 100644
--- a/courier-java-core/src/main/kotlin/com/courier/services/async/NotificationServiceAsyncImpl.kt
+++ b/courier-java-core/src/main/kotlin/com/courier/services/async/NotificationServiceAsyncImpl.kt
@@ -20,6 +20,7 @@ import com.courier.core.prepareAsync
 import com.courier.models.notifications.NotificationArchiveParams
 import com.courier.models.notifications.NotificationContentMutationResponse
 import com.courier.models.notifications.NotificationCreateParams
+import com.courier.models.notifications.NotificationDuplicateParams
 import com.courier.models.notifications.NotificationListParams
 import com.courier.models.notifications.NotificationListResponse
 import com.courier.models.notifications.NotificationListVersionsParams
@@ -83,6 +84,13 @@ class NotificationServiceAsyncImpl internal constructor(private val clientOption
         // delete /notifications/{id}
         withRawResponse().archive(params, requestOptions).thenAccept {}
 
+    override fun duplicate(
+        params: NotificationDuplicateParams,
+        requestOptions: RequestOptions,
+    ): CompletableFuture<NotificationTemplateResponse> =
+        // post /notifications/{id}/duplicate
+        withRawResponse().duplicate(params, requestOptions).thenApply { it.parse() }
+
     override fun listVersions(
         params: NotificationListVersionsParams,
         requestOptions: RequestOptions,
@@ -272,6 +280,40 @@ class NotificationServiceAsyncImpl internal constructor(private val clientOption
                 }
         }
 
+        private val duplicateHandler: Handler<NotificationTemplateResponse> =
+            jsonHandler<NotificationTemplateResponse>(clientOptions.jsonMapper)
+
+        override fun duplicate(
+            params: NotificationDuplicateParams,
+            requestOptions: RequestOptions,
+        ): CompletableFuture<HttpResponseFor<NotificationTemplateResponse>> {
+            // We check here instead of in the params builder because this can be specified
+            // positionally or in the params class.
+            checkRequired("id", params.id().getOrNull())
+            val request =
+                HttpRequest.builder()
+                    .method(HttpMethod.POST)
+                    .baseUrl(clientOptions.baseUrl())
+                    .addPathSegments("notifications", params._pathParam(0), "duplicate")
+                    .apply { params._body().ifPresent { body(json(clientOptions.jsonMapper, it)) } }
+                    .build()
+                    .prepareAsync(clientOptions, params)
+            val requestOptions = requestOptions.applyDefaults(RequestOptions.from(clientOptions))
+            return request
+                .thenComposeAsync { clientOptions.httpClient.executeAsync(it, requestOptions) }
+                .thenApply { response ->
+                    errorHandler.handle(response).parseable {
+                        response
+                            .use { duplicateHandler.handle(it) }
+                            .also {
+                                if (requestOptions.responseValidation!!) {
+                                    it.validate()
+                                }
+                            }
+                    }
+                }
+        }
+
         private val listVersionsHandler: Handler<NotificationTemplateVersionListResponse> =
             jsonHandler<NotificationTemplateVersionListResponse>(clientOptions.jsonMapper)
 
diff --git a/courier-java-core/src/main/kotlin/com/courier/services/blocking/NotificationService.kt b/courier-java-core/src/main/kotlin/com/courier/services/blocking/NotificationService.kt
index e2b77916..85f2a049 100644
--- a/courier-java-core/src/main/kotlin/com/courier/services/blocking/NotificationService.kt
+++ b/courier-java-core/src/main/kotlin/com/courier/services/blocking/NotificationService.kt
@@ -9,6 +9,7 @@ import com.courier.core.http.HttpResponseFor
 import com.courier.models.notifications.NotificationArchiveParams
 import com.courier.models.notifications.NotificationContentMutationResponse
 import com.courier.models.notifications.NotificationCreateParams
+import com.courier.models.notifications.NotificationDuplicateParams
 import com.courier.models.notifications.NotificationListParams
 import com.courier.models.notifications.NotificationListResponse
 import com.courier.models.notifications.NotificationListVersionsParams
@@ -153,6 +154,44 @@ interface NotificationService {
     fun archive(id: String, requestOptions: RequestOptions) =
         archive(id, NotificationArchiveParams.none(), requestOptions)
 
+    /**
+     * Duplicate a notification template. Creates a standalone copy within the same workspace and
+     * environment, with " COPY" appended to the title. The copy clones the source draft's tags,
+     * brand, subscription topic, routing strategy, channels, and content, and is always created as
+     * a standalone template (it is not linked to any journey or broadcast, even if the source was).
+     * Templates that are scoped to a journey or a broadcast cannot be duplicated through this
+     * endpoint.
+     */
+    fun duplicate(id: String): NotificationTemplateResponse =
+        duplicate(id, NotificationDuplicateParams.none())
+
+    /** @see duplicate */
+    fun duplicate(
+        id: String,
+        params: NotificationDuplicateParams = NotificationDuplicateParams.none(),
+        requestOptions: RequestOptions = RequestOptions.none(),
+    ): NotificationTemplateResponse = duplicate(params.toBuilder().id(id).build(), requestOptions)
+
+    /** @see duplicate */
+    fun duplicate(
+        id: String,
+        params: NotificationDuplicateParams = NotificationDuplicateParams.none(),
+    ): NotificationTemplateResponse = duplicate(id, params, RequestOptions.none())
+
+    /** @see duplicate */
+    fun duplicate(
+        params: NotificationDuplicateParams,
+        requestOptions: RequestOptions = RequestOptions.none(),
+    ): NotificationTemplateResponse
+
+    /** @see duplicate */
+    fun duplicate(params: NotificationDuplicateParams): NotificationTemplateResponse =
+        duplicate(params, RequestOptions.none())
+
+    /** @see duplicate */
+    fun duplicate(id: String, requestOptions: RequestOptions): NotificationTemplateResponse =
+        duplicate(id, NotificationDuplicateParams.none(), requestOptions)
+
     /** List versions of a notification template. */
     fun listVersions(id: String): NotificationTemplateVersionListResponse =
         listVersions(id, NotificationListVersionsParams.none())
@@ -523,6 +562,52 @@ interface NotificationService {
         fun archive(id: String, requestOptions: RequestOptions): HttpResponse =
             archive(id, NotificationArchiveParams.none(), requestOptions)
 
+        /**
+         * Returns a raw HTTP response for `post /notifications/{id}/duplicate`, but is otherwise
+         * the same as [NotificationService.duplicate].
+         */
+        @MustBeClosed
+        fun duplicate(id: String): HttpResponseFor<NotificationTemplateResponse> =
+            duplicate(id, NotificationDuplicateParams.none())
+
+        /** @see duplicate */
+        @MustBeClosed
+        fun duplicate(
+            id: String,
+            params: NotificationDuplicateParams = NotificationDuplicateParams.none(),
+            requestOptions: RequestOptions = RequestOptions.none(),
+        ): HttpResponseFor<NotificationTemplateResponse> =
+            duplicate(params.toBuilder().id(id).build(), requestOptions)
+
+        /** @see duplicate */
+        @MustBeClosed
+        fun duplicate(
+            id: String,
+            params: NotificationDuplicateParams = NotificationDuplicateParams.none(),
+        ): HttpResponseFor<NotificationTemplateResponse> =
+            duplicate(id, params, RequestOptions.none())
+
+        /** @see duplicate */
+        @MustBeClosed
+        fun duplicate(
+            params: NotificationDuplicateParams,
+            requestOptions: RequestOptions = RequestOptions.none(),
+        ): HttpResponseFor<NotificationTemplateResponse>
+
+        /** @see duplicate */
+        @MustBeClosed
+        fun duplicate(
+            params: NotificationDuplicateParams
+        ): HttpResponseFor<NotificationTemplateResponse> = duplicate(params, RequestOptions.none())
+
+        /** @see duplicate */
+        @MustBeClosed
+        fun duplicate(
+            id: String,
+            requestOptions: RequestOptions,
+        ): HttpResponseFor<NotificationTemplateResponse> =
+            duplicate(id, NotificationDuplicateParams.none(), requestOptions)
+
         /**
          * Returns a raw HTTP response for `get /notifications/{id}/versions`, but is otherwise the
          * same as [NotificationService.listVersions].
diff --git a/courier-java-core/src/main/kotlin/com/courier/services/blocking/NotificationServiceImpl.kt b/courier-java-core/src/main/kotlin/com/courier/services/blocking/NotificationServiceImpl.kt
index 52ae5eb6..9b6d39da 100644
--- a/courier-java-core/src/main/kotlin/com/courier/services/blocking/NotificationServiceImpl.kt
+++ b/courier-java-core/src/main/kotlin/com/courier/services/blocking/NotificationServiceImpl.kt
@@ -20,6 +20,7 @@ import com.courier.core.prepare
 import com.courier.models.notifications.NotificationArchiveParams
 import com.courier.models.notifications.NotificationContentMutationResponse
 import com.courier.models.notifications.NotificationCreateParams
+import com.courier.models.notifications.NotificationDuplicateParams
 import com.courier.models.notifications.NotificationListParams
 import com.courier.models.notifications.NotificationListResponse
 import com.courier.models.notifications.NotificationListVersionsParams
@@ -80,6 +81,13 @@ class NotificationServiceImpl internal constructor(private val clientOptions: Cl
         withRawResponse().archive(params, requestOptions)
     }
 
+    override fun duplicate(
+        params: NotificationDuplicateParams,
+        requestOptions: RequestOptions,
+    ): NotificationTemplateResponse =
+        // post /notifications/{id}/duplicate
+        withRawResponse().duplicate(params, requestOptions).parse()
+
     override fun listVersions(
         params: NotificationListVersionsParams,
         requestOptions: RequestOptions,
@@ -255,6 +263,37 @@ class NotificationServiceImpl internal constructor(private val clientOptions: Cl
             }
         }
 
+        private val duplicateHandler: Handler<NotificationTemplateResponse> =
+            jsonHandler<NotificationTemplateResponse>(clientOptions.jsonMapper)
+
+        override fun duplicate(
+            params: NotificationDuplicateParams,
+            requestOptions: RequestOptions,
+        ): HttpResponseFor<NotificationTemplateResponse> {
+            // We check here instead of in the params builder because this can be specified
+            // positionally or in the params class.
+            checkRequired("id", params.id().getOrNull())
+            val request =
+                HttpRequest.builder()
+                    .method(HttpMethod.POST)
+                    .baseUrl(clientOptions.baseUrl())
+                    .addPathSegments("notifications", params._pathParam(0), "duplicate")
+                    .apply { params._body().ifPresent { body(json(clientOptions.jsonMapper, it)) } }
+                    .build()
+                    .prepare(clientOptions, params)
+            val requestOptions = requestOptions.applyDefaults(RequestOptions.from(clientOptions))
+            val response = clientOptions.httpClient.execute(request, requestOptions)
+            return errorHandler.handle(response).parseable {
+                response
+                    .use { duplicateHandler.handle(it) }
+                    .also {
+                        if (requestOptions.responseValidation!!) {
+                            it.validate()
+                        }
+                    }
+            }
+        }
+
         private val listVersionsHandler: Handler<NotificationTemplateVersionListResponse> =
             jsonHandler<NotificationTemplateVersionListResponse>(clientOptions.jsonMapper)
 
diff --git a/courier-java-core/src/test/kotlin/com/courier/models/notifications/NotificationDuplicateParamsTest.kt b/courier-java-core/src/test/kotlin/com/courier/models/notifications/NotificationDuplicateParamsTest.kt
new file mode 100644
index 00000000..48c3c0e3
--- /dev/null
+++ b/courier-java-core/src/test/kotlin/com/courier/models/notifications/NotificationDuplicateParamsTest.kt
@@ -0,0 +1,23 @@
+// File generated from our OpenAPI spec by Stainless.
+
+package com.courier.models.notifications
+
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.jupiter.api.Test
+
+internal class NotificationDuplicateParamsTest {
+
+    @Test
+    fun create() {
+        NotificationDuplicateParams.builder().id("id").build()
+    }
+
+    @Test
+    fun pathParams() {
+        val params = NotificationDuplicateParams.builder().id("id").build()
+
+        assertThat(params._pathParam(0)).isEqualTo("id")
+        // out-of-bound path param
+        assertThat(params._pathParam(1)).isEqualTo("")
+    }
+}
diff --git a/courier-java-core/src/test/kotlin/com/courier/models/notifications/NotificationTemplateSummaryTest.kt b/courier-java-core/src/test/kotlin/com/courier/models/notifications/NotificationTemplateSummaryTest.kt
index b03e82bf..df730f63 100644
--- a/courier-java-core/src/test/kotlin/com/courier/models/notifications/NotificationTemplateSummaryTest.kt
+++ b/courier-java-core/src/test/kotlin/com/courier/models/notifications/NotificationTemplateSummaryTest.kt
@@ -19,6 +19,7 @@ internal class NotificationTemplateSummaryTest {
                 .name("name")
                 .state(NotificationTemplateSummary.State.DRAFT)
                 .addTag("string")
+                .subscriptionTopicId("subscription_topic_id")
                 .updated(0L)
                 .updater("updater")
                 .build()
@@ -30,6 +31,8 @@ internal class NotificationTemplateSummaryTest {
         assertThat(notificationTemplateSummary.state())
             .isEqualTo(NotificationTemplateSummary.State.DRAFT)
         assertThat(notificationTemplateSummary.tags()).containsExactly("string")
+        assertThat(notificationTemplateSummary.subscriptionTopicId())
+            .contains("subscription_topic_id")
         assertThat(notificationTemplateSummary.updated()).contains(0L)
         assertThat(notificationTemplateSummary.updater()).contains("updater")
     }
@@ -45,6 +48,7 @@ internal class NotificationTemplateSummaryTest {
                 .name("name")
                 .state(NotificationTemplateSummary.State.DRAFT)
                 .addTag("string")
+                .subscriptionTopicId("subscription_topic_id")
                 .updated(0L)
                 .updater("updater")
                 .build()
diff --git a/courier-java-core/src/test/kotlin/com/courier/models/routingstrategies/AssociatedNotificationListResponseTest.kt b/courier-java-core/src/test/kotlin/com/courier/models/routingstrategies/AssociatedNotificationListResponseTest.kt
index 92ed3f51..fc48cf1a 100644
--- a/courier-java-core/src/test/kotlin/com/courier/models/routingstrategies/AssociatedNotificationListResponseTest.kt
+++ b/courier-java-core/src/test/kotlin/com/courier/models/routingstrategies/AssociatedNotificationListResponseTest.kt
@@ -24,6 +24,7 @@ internal class AssociatedNotificationListResponseTest {
                         .name("name")
                         .state(NotificationTemplateSummary.State.DRAFT)
                         .addTag("string")
+                        .subscriptionTopicId("subscription_topic_id")
                         .updated(0L)
                         .updater("updater")
                         .build()
@@ -41,6 +42,7 @@ internal class AssociatedNotificationListResponseTest {
                     .name("name")
                     .state(NotificationTemplateSummary.State.DRAFT)
                     .addTag("string")
+                    .subscriptionTopicId("subscription_topic_id")
                     .updated(0L)
                     .updater("updater")
                     .build()
@@ -61,6 +63,7 @@ internal class AssociatedNotificationListResponseTest {
                         .name("name")
                         .state(NotificationTemplateSummary.State.DRAFT)
                         .addTag("string")
+                        .subscriptionTopicId("subscription_topic_id")
                         .updated(0L)
                         .updater("updater")
                         .build()
diff --git a/courier-java-core/src/test/kotlin/com/courier/services/async/NotificationServiceAsyncTest.kt b/courier-java-core/src/test/kotlin/com/courier/services/async/NotificationServiceAsyncTest.kt
index a103ee60..b06be93b 100644
--- a/courier-java-core/src/test/kotlin/com/courier/services/async/NotificationServiceAsyncTest.kt
+++ b/courier-java-core/src/test/kotlin/com/courier/services/async/NotificationServiceAsyncTest.kt
@@ -122,6 +122,18 @@ internal class NotificationServiceAsyncTest {
         val response = future.get()
     }
 
+    @Disabled("Mock server tests are disabled")
+    @Test
+    fun duplicate() {
+        val client = CourierOkHttpClientAsync.builder().apiKey("My API Key").build()
+        val notificationServiceAsync = client.notifications()
+
+        val notificationTemplateResponseFuture = notificationServiceAsync.duplicate("id")
+
+        val notificationTemplateResponse = notificationTemplateResponseFuture.get()
+        notificationTemplateResponse.validate()
+    }
+
     @Disabled("Mock server tests are disabled")
     @Test
     fun listVersions() {
diff --git a/courier-java-core/src/test/kotlin/com/courier/services/blocking/NotificationServiceTest.kt b/courier-java-core/src/test/kotlin/com/courier/services/blocking/NotificationServiceTest.kt
index 1f525e00..4d405dcb 100644
--- a/courier-java-core/src/test/kotlin/com/courier/services/blocking/NotificationServiceTest.kt
+++ b/courier-java-core/src/test/kotlin/com/courier/services/blocking/NotificationServiceTest.kt
@@ -117,6 +117,17 @@ internal class NotificationServiceTest {
         notificationService.archive("id")
     }
 
+    @Disabled("Mock server tests are disabled")
+    @Test
+    fun duplicate() {
+        val client = CourierOkHttpClient.builder().apiKey("My API Key").build()
+        val notificationService = client.notifications()
+
+        val notificationTemplateResponse = notificationService.duplicate("id")
+
+        notificationTemplateResponse.validate()
+    }
+
     @Disabled("Mock server tests are disabled")
     @Test
     fun listVersions() {