[network-diag] introduce AnswerBuilder to manage answer messages (#12887)

This commit introduces `AnswerBuilder` class to track and manage
Network Diagnostic answer messages. This class is used when the
response to a query requires multiple CoAP answer messages. It
automatically manages the inclusion of the Query ID and the Answer
TLVs(providing message indexing and "more-to-follow" flags) in each
allocated answer message, while maintaining all answer messages in a
queue. The `NetworkDiagnostic::Server` is updated to use the
`AnswerBuilder`, simplifying the logic for preparing and sending
answers.

The `AnswerBuilder` class is added in a new header file
`network_diagnostic_types.hpp` to allow for its reuse by other
modules in the future.

src/core/BUILD.gn

@@ -755,6 +755,8 @@ openthread_core_files = [
   "thread/network_diagnostic.hpp",
   "thread/network_diagnostic_tlvs.cpp",
   "thread/network_diagnostic_tlvs.hpp",
+  "thread/network_diagnostic_types.cpp",
+  "thread/network_diagnostic_types.hpp",
   "thread/panid_query_server.cpp",
   "thread/panid_query_server.hpp",
   "thread/peer.cpp",

src/core/CMakeLists.txt

@@ -268,6 +268,7 @@ set(COMMON_SOURCES
     thread/network_data_types.cpp
     thread/network_diagnostic.cpp
     thread/network_diagnostic_tlvs.cpp
+    thread/network_diagnostic_types.cpp
     thread/panid_query_server.cpp
     thread/peer.cpp
     thread/peer_table.cpp

src/core/common/message.cpp

@@ -980,6 +980,30 @@ void MessageQueue::DequeueAndFreeAll(void)
     }
 }
 
+void MessageQueue::EnqueueAllFrom(MessageQueue &aOtherQueue)
+{
+    VerifyOrExit(&aOtherQueue != this);
+
+    VerifyOrExit(aOtherQueue.GetHead() != nullptr);
+
+    if (GetHead() == nullptr)
+    {
+        SetHead(aOtherQueue.GetHead());
+    }
+    else
+    {
+        GetTail()->Next()             = aOtherQueue.GetHead();
+        aOtherQueue.GetHead()->Prev() = GetTail();
+    }
+
+    SetTail(aOtherQueue.GetTail());
+
+    aOtherQueue.Clear();
+
+exit:
+    return;
+}
+
 Message::Iterator MessageQueue::begin(void) { return Message::Iterator(GetHead()); }
 
 Message::ConstIterator MessageQueue::begin(void) const { return Message::ConstIterator(GetHead()); }

src/core/common/message.hpp

@@ -1731,6 +1731,15 @@ public:
      */
     void DequeueAndFreeAll(void);
 
+    /**
+     * Enqueues all messages from another message queue at the end of this queue.
+     *
+     * Upon return, @p aOtherQueue will be empty.
+     *
+     * @param[in,out] aOtherQueue  The other message queue to enqueue from.
+     */
+    void EnqueueAllFrom(MessageQueue &aOtherQueue);
+
     /**
      * Gets the information about number of messages and buffers in the queue.
      *

src/core/thread/network_diagnostic.cpp

@@ -539,34 +539,6 @@ exit:
 
 #if OPENTHREAD_FTD
 
-Error Server::AllocateAnswer(Coap::Message *&aAnswer, AnswerInfo &aInfo)
-{
-    // Allocate an `Answer` message, adds it in `mAnswerQueue`,
-    // update the `aInfo.mFirstAnswer` if it is the first allocated
-    // messages, and appends `QueryIdTlv` to the message (if needed).
-
-    Error error = kErrorNone;
-
-    aAnswer = Get<Tmf::Agent>().AllocateAndInitConfirmablePostMessage(kUriDiagnosticGetAnswer);
-    VerifyOrExit(aAnswer != nullptr, error = kErrorNoBufs);
-    IgnoreError(aAnswer->SetPriority(aInfo.mPriority));
-
-    mAnswerQueue.Enqueue(*aAnswer);
-
-    if (aInfo.mFirstAnswer == nullptr)
-    {
-        aInfo.mFirstAnswer = aAnswer;
-    }
-
-    if (aInfo.mHasQueryId)
-    {
-        SuccessOrExit(error = Tlv::Append<QueryIdTlv>(*aAnswer, aInfo.mQueryId));
-    }
-
-exit:
-    return error;
-}
-
 bool Server::IsLastAnswer(const Coap::Message &aAnswer) const
 {
     // Indicates whether `aAnswer` is the last one associated with
@@ -601,23 +573,29 @@ void Server::FreeAllRelatedAnswers(Coap::Message &aFirstAnswer)
     }
 }
 
-void Server::PrepareAndSendAnswers(const Ip6::Address &aDestination, const Message &aRequest)
+void Server::PrepareAndSendAnswers(const Ip6::Address &aDestination, const Coap::Message &aRequest)
+{
+    AnswerBuilder  answerBuilder(GetInstance());
+    Coap::Message *firstAnswer;
+
+    SuccessOrExit(PrepareAnswers(aRequest, answerBuilder));
+
+    firstAnswer = answerBuilder.GetAnswers().GetHead();
+    mAnswerQueue.EnqueueAllFrom(answerBuilder.GetAnswers());
+
+    SendNextAnswer(*firstAnswer, aDestination);
+
+exit:
+    return;
+}
+
+Error Server::PrepareAnswers(const Coap::Message &aRequest, AnswerBuilder &aAnswerBuilder)
 {
-    Coap::Message      *answer;
     Error               error;
-    AnswerInfo          info;
     uint8_t             tlvType;
     TlvTypeListIterator iterator;
-    AnswerTlvValue      answerTlvValue;
 
-    if (Tlv::Find<QueryIdTlv>(aRequest, info.mQueryId) == kErrorNone)
-    {
-        info.mHasQueryId = true;
-    }
-
-    info.mPriority = aRequest.GetPriority();
-
-    SuccessOrExit(error = AllocateAnswer(answer, info));
+    SuccessOrExit(error = aAnswerBuilder.Start(aRequest));
 
     SuccessOrExit(error = iterator.InitForTypeListTlv(aRequest));
 
@@ -626,51 +604,23 @@ void Server::PrepareAndSendAnswers(const Ip6::Address &aDestination, const Messa
         switch (tlvType)
         {
         case ChildTlv::kType:
-            SuccessOrExit(error = AppendChildTableAsChildTlvs(answer, info));
+            SuccessOrExit(error = AppendChildTableAsChildTlvs(aAnswerBuilder));
             break;
         case ChildIp6AddressListTlv::kType:
-            SuccessOrExit(error = AppendChildTableIp6AddressList(answer, info));
+            SuccessOrExit(error = AppendChildTableIp6AddressList(aAnswerBuilder));
             break;
         case RouterNeighborTlv::kType:
-            SuccessOrExit(error = AppendRouterNeighborTlvs(answer, info));
+            SuccessOrExit(error = AppendRouterNeighborTlvs(aAnswerBuilder));
             break;
         default:
-            SuccessOrExit(error = AppendDiagTlv(tlvType, *answer));
+            SuccessOrExit(error = AppendDiagTlv(tlvType, aAnswerBuilder.GetAnswer()));
             break;
         }
 
-        SuccessOrExit(error = CheckAnswerLength(answer, info));
+        SuccessOrExit(error = aAnswerBuilder.CheckAnswerLength());
     }
 
-    answerTlvValue.Init(info.mAnswerIndex, AnswerTlvValue::kIsLast);
-    SuccessOrExit(error = Tlv::Append<AnswerTlv>(*answer, answerTlvValue));
-
-    SendNextAnswer(*info.mFirstAnswer, aDestination);
-
-exit:
-    if ((error != kErrorNone) && (info.mFirstAnswer != nullptr))
-    {
-        FreeAllRelatedAnswers(*info.mFirstAnswer);
-    }
-}
-
-Error Server::CheckAnswerLength(Coap::Message *&aAnswer, AnswerInfo &aInfo)
-{
-    // This method checks the length of the `aAnswer` message and if it
-    // is above the threshold, it enqueues the message for transmission
-    // after appending an Answer TLV with the current index to the
-    // message. In this case, it will also allocate a new answer
-    // message.
-
-    Error          error = kErrorNone;
-    AnswerTlvValue answerTlvValue;
-
-    VerifyOrExit(aAnswer->GetLength() >= kAnswerMessageLengthThreshold);
-
-    answerTlvValue.Init(aInfo.mAnswerIndex++, AnswerTlvValue::kMoreToFollow);
-    SuccessOrExit(error = Tlv::Append<AnswerTlv>(*aAnswer, answerTlvValue));
-
-    error = AllocateAnswer(aAnswer, aInfo);
+    SuccessOrExit(error = aAnswerBuilder.Finish());
 
 exit:
     return error;
@@ -734,7 +684,7 @@ exit:
     }
 }
 
-Error Server::AppendChildTableAsChildTlvs(Coap::Message *&aAnswer, AnswerInfo &aInfo)
+Error Server::AppendChildTableAsChildTlvs(AnswerBuilder &aAnswerBuilder)
 {
     Error         error = kErrorNone;
     ChildTlvValue childTlvValue;
@@ -743,17 +693,17 @@ Error Server::AppendChildTableAsChildTlvs(Coap::Message *&aAnswer, AnswerInfo &a
     {
         childTlvValue.InitFrom(child);
 
-        SuccessOrExit(error = Tlv::Append<ChildTlv>(*aAnswer, childTlvValue));
-        SuccessOrExit(error = CheckAnswerLength(aAnswer, aInfo));
+        SuccessOrExit(error = Tlv::Append<ChildTlv>(aAnswerBuilder.GetAnswer(), childTlvValue));
+        SuccessOrExit(error = aAnswerBuilder.CheckAnswerLength());
     }
 
-    error = Tlv::AppendEmpty<ChildTlv>(*aAnswer);
+    error = Tlv::AppendEmpty<ChildTlv>(aAnswerBuilder.GetAnswer());
 
 exit:
     return error;
 }
 
-Error Server::AppendRouterNeighborTlvs(Coap::Message *&aAnswer, AnswerInfo &aInfo)
+Error Server::AppendRouterNeighborTlvs(AnswerBuilder &aAnswerBuilder)
 {
     Error                  error = kErrorNone;
     RouterNeighborTlvValue neighborTlvValue;
@@ -767,27 +717,27 @@ Error Server::AppendRouterNeighborTlvs(Coap::Message *&aAnswer, AnswerInfo &aInf
 
         neighborTlvValue.InitFrom(router);
 
-        SuccessOrExit(error = Tlv::Append<RouterNeighborTlv>(*aAnswer, neighborTlvValue));
-        SuccessOrExit(error = CheckAnswerLength(aAnswer, aInfo));
+        SuccessOrExit(error = Tlv::Append<RouterNeighborTlv>(aAnswerBuilder.GetAnswer(), neighborTlvValue));
+        SuccessOrExit(error = aAnswerBuilder.CheckAnswerLength());
     }
 
-    error = Tlv::AppendEmpty<RouterNeighborTlv>(*aAnswer);
+    error = Tlv::AppendEmpty<RouterNeighborTlv>(aAnswerBuilder.GetAnswer());
 
 exit:
     return error;
 }
 
-Error Server::AppendChildTableIp6AddressList(Coap::Message *&aAnswer, AnswerInfo &aInfo)
+Error Server::AppendChildTableIp6AddressList(AnswerBuilder &aAnswerBuilder)
 {
     Error error = kErrorNone;
 
     for (const Child &child : Get<ChildTable>().Iterate(Child::kInStateValid))
     {
-        SuccessOrExit(error = AppendChildIp6AddressListTlv(*aAnswer, child));
-        SuccessOrExit(error = CheckAnswerLength(aAnswer, aInfo));
+        SuccessOrExit(error = AppendChildIp6AddressListTlv(aAnswerBuilder.GetAnswer(), child));
+        SuccessOrExit(error = aAnswerBuilder.CheckAnswerLength());
     }
 
-    error = Tlv::AppendEmpty<ChildIp6AddressListTlv>(*aAnswer);
+    error = Tlv::AppendEmpty<ChildIp6AddressListTlv>(aAnswerBuilder.GetAnswer());
 
 exit:
     return error;

src/core/thread/network_diagnostic.hpp

@@ -45,6 +45,7 @@
 #include "common/numeric_limits.hpp"
 #include "net/udp6.hpp"
 #include "thread/network_diagnostic_tlvs.hpp"
+#include "thread/network_diagnostic_types.hpp"
 #include "thread/tmf.hpp"
 #include "thread/uri_paths.hpp"
 
@@ -119,8 +120,7 @@ public:
     }
 
 private:
-    static constexpr uint16_t kMaxChildEntries              = 398;
-    static constexpr uint16_t kAnswerMessageLengthThreshold = 800;
+    static constexpr uint16_t kMaxChildEntries = 398;
 
     class TlvTypeListIterator
     {
@@ -139,25 +139,6 @@ private:
         BitSet<NumericLimits<uint8_t>::kMax + 1> mProcessedTlvs;
     };
 
-#if OPENTHREAD_FTD
-    struct AnswerInfo
-    {
-        AnswerInfo(void)
-            : mAnswerIndex(0)
-            , mQueryId(0)
-            , mHasQueryId(false)
-            , mFirstAnswer(nullptr)
-        {
-        }
-
-        uint16_t          mAnswerIndex;
-        uint16_t          mQueryId;
-        bool              mHasQueryId;
-        Message::Priority mPriority;
-        Coap::Message    *mFirstAnswer;
-    };
-#endif
-
     Error AppendDiagTlv(uint8_t aTlvType, Message &aMessage);
     Error AppendIp6AddressList(Message &aMessage);
     Error AppendRequestedTlvs(const Message &aRequest, Message &aResponse);
@@ -169,16 +150,15 @@ private:
 #if OPENTHREAD_MTD
     void SendAnswer(const Ip6::Address &aDestination, const Message &aRequest);
 #elif OPENTHREAD_FTD
-    Error AllocateAnswer(Coap::Message *&aAnswer, AnswerInfo &aInfo);
-    Error CheckAnswerLength(Coap::Message *&aAnswer, AnswerInfo &aInfo);
     bool  IsLastAnswer(const Coap::Message &aAnswer) const;
     void  FreeAllRelatedAnswers(Coap::Message &aFirstAnswer);
-    void  PrepareAndSendAnswers(const Ip6::Address &aDestination, const Message &aRequest);
+    void  PrepareAndSendAnswers(const Ip6::Address &aDestination, const Coap::Message &aRequest);
+    Error PrepareAnswers(const Coap::Message &aRequest, AnswerBuilder &aAnswerBuilder);
     void  SendNextAnswer(Coap::Message &aAnswer, const Ip6::Address &aDestination);
     Error AppendChildTable(Message &aMessage);
-    Error AppendChildTableAsChildTlvs(Coap::Message *&aAnswer, AnswerInfo &aInfo);
-    Error AppendRouterNeighborTlvs(Coap::Message *&aAnswer, AnswerInfo &aInfo);
-    Error AppendChildTableIp6AddressList(Coap::Message *&aAnswer, AnswerInfo &aInfo);
+    Error AppendChildTableAsChildTlvs(AnswerBuilder &aAnswerBuilder);
+    Error AppendRouterNeighborTlvs(AnswerBuilder &aAnswerBuilder);
+    Error AppendChildTableIp6AddressList(AnswerBuilder &aAnswerBuilder);
     Error AppendChildIp6AddressListTlv(Message &aAnswer, const Child &aChild);
     Error AppendEnhancedRoute(Message &aMessage);
 

src/core/thread/network_diagnostic_types.cpp

@@ -0,0 +1,108 @@
+/*
+ *  Copyright (c) 2026, The OpenThread Authors.
+ *  All rights reserved.
+ *
+ *  Redistribution and use in source and binary forms, with or without
+ *  modification, are permitted provided that the following conditions are met:
+ *  1. Redistributions of source code must retain the above copyright
+ *     notice, this list of conditions and the following disclaimer.
+ *  2. Redistributions in binary form must reproduce the above copyright
+ *     notice, this list of conditions and the following disclaimer in the
+ *     documentation and/or other materials provided with the distribution.
+ *  3. Neither the name of the copyright holder nor the
+ *     names of its contributors may be used to endorse or promote products
+ *     derived from this software without specific prior written permission.
+ *
+ *  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ *  AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ *  IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ *  ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+ *  LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ *  CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ *  SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ *  INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ *  CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ *  ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ *  POSSIBILITY OF SUCH DAMAGE.
+ */
+
+/**
+ * @file
+ *   This file implements methods for Network Diagnostics helper types and classes.
+ */
+
+#include "network_diagnostic_types.hpp"
+
+#include "instance/instance.hpp"
+
+namespace ot {
+namespace NetworkDiagnostic {
+
+AnswerBuilder::AnswerBuilder(Instance &aInstance)
+    : InstanceLocator(aInstance)
+    , mAnswer(nullptr)
+    , mAnswerIndex(0)
+    , mQueryId(0)
+    , mHasQueryId(false)
+    , mPriority(Message::kPriorityNormal)
+{
+}
+
+AnswerBuilder::~AnswerBuilder(void) { mAnswers.DequeueAndFreeAll(); }
+
+Error AnswerBuilder::Start(const Coap::Message &aRequest)
+{
+    mAnswerIndex = 0;
+    mHasQueryId  = (Tlv::Find<QueryIdTlv>(aRequest, mQueryId) == kErrorNone);
+    mPriority    = aRequest.GetPriority();
+
+    return Allocate();
+}
+
+Error AnswerBuilder::Finish(void) { return AppendAnswerTlv(AnswerTlvValue::kIsLast); }
+
+Error AnswerBuilder::Allocate(void)
+{
+    Error error = kErrorNone;
+
+    mAnswer = Get<Tmf::Agent>().AllocateAndInitConfirmablePostMessage(kUriDiagnosticGetAnswer);
+    VerifyOrExit(mAnswer != nullptr, error = kErrorNoBufs);
+
+    IgnoreError(mAnswer->SetPriority(mPriority));
+
+    mAnswers.Enqueue(*mAnswer);
+
+    if (mHasQueryId)
+    {
+        SuccessOrExit(error = Tlv::Append<QueryIdTlv>(*mAnswer, mQueryId));
+    }
+
+exit:
+    return error;
+}
+
+Error AnswerBuilder::AppendAnswerTlv(AnswerTlvValue::IsLastFlag aFlag)
+{
+    AnswerTlvValue value;
+
+    value.Init(mAnswerIndex++, aFlag);
+
+    return Tlv::Append<AnswerTlv>(*mAnswer, value);
+}
+
+Error AnswerBuilder::CheckAnswerLength(void)
+{
+    Error error = kErrorNone;
+
+    VerifyOrExit(mAnswer->GetLength() >= kAnswerMessageLengthThreshold);
+
+    SuccessOrExit(error = AppendAnswerTlv(AnswerTlvValue::kMoreToFollow));
+
+    error = Allocate();
+
+exit:
+    return error;
+}
+
+} // namespace NetworkDiagnostic
+} // namespace ot

src/core/thread/network_diagnostic_types.hpp

@@ -0,0 +1,139 @@
+/*
+ *  Copyright (c) 2026, The OpenThread Authors.
+ *  All rights reserved.
+ *
+ *  Redistribution and use in source and binary forms, with or without
+ *  modification, are permitted provided that the following conditions are met:
+ *  1. Redistributions of source code must retain the above copyright
+ *     notice, this list of conditions and the following disclaimer.
+ *  2. Redistributions in binary form must reproduce the above copyright
+ *     notice, this list of conditions and the following disclaimer in the
+ *     documentation and/or other materials provided with the distribution.
+ *  3. Neither the name of the copyright holder nor the
+ *     names of its contributors may be used to endorse or promote products
+ *     derived from this software without specific prior written permission.
+ *
+ *  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ *  AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ *  IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ *  ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+ *  LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ *  CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ *  SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ *  INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ *  CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ *  ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ *  POSSIBILITY OF SUCH DAMAGE.
+ */
+
+/**
+ * @file
+ *   This file includes definitions for Network Diagnostics helper types and classes.
+ */
+
+#ifndef OT_CORE_THREAD_NETWORK_DIAGNOSTIC_TYPES_HPP_
+#define OT_CORE_THREAD_NETWORK_DIAGNOSTIC_TYPES_HPP_
+
+#include "openthread-core-config.h"
+
+#include "network_diagnostic_tlvs.hpp"
+#include "coap/coap_message.hpp"
+#include "common/error.hpp"
+#include "common/locator.hpp"
+#include "common/time.hpp"
+#include "thread/uri_paths.hpp"
+
+namespace ot {
+namespace NetworkDiagnostic {
+
+/**
+ * Represents an object for tracking and managing Network Diagnostic answer messages.
+ *
+ * This class is used when the response to a Network Diagnostic query requires multiple CoAP answer messages . It
+ * manages the inclusion of the Query ID and the Answer TLV (providing message indexing and "more-to-follow" flags)
+ * in each allocated answer message, while maintaining all answer messages in a queue.
+ */
+class AnswerBuilder : public InstanceLocator
+{
+public:
+    /**
+     * Initializes the `AnswerBuilder`.
+     *
+     * @param[in] aInstance  The OpenThread instance.
+     */
+    explicit AnswerBuilder(Instance &aInstance);
+
+    /**
+     * Destructor for `AnswerBuilder`.
+     *
+     * Any remaining messages in the answers queue are freed.
+     */
+    ~AnswerBuilder(void);
+
+    /**
+     * Starts the building of answer messages based on a given Network Diagnostic request.
+     *
+     * This method searches for a Query ID TLV within @p aRequest. If present, the Query ID is captured and
+     * automatically included in all allocated answer messages. It also captures the priority from @p aRequest and
+     * uses it for all answer messages.
+     *
+     * @param[in] aRequest  The Network Diagnostic request message.
+     *
+     * @retval kErrorNone   Successfully started.
+     * @retval kErrorNoBufs Insufficient message buffers to allocate the first answer.
+     */
+    Error Start(const Coap::Message &aRequest);
+
+    /**
+     * Gets the current answer message.
+     *
+     * @returns The current answer message.
+     */
+    Coap::Message &GetAnswer(void) { return *mAnswer; }
+
+    /**
+     * Checks the current answer message length and allocates a new one if it exceeds the threshold.
+     *
+     * If the current answer message length is above the threshold, this method appends an Answer TLV with the
+     * "more to follow" flag set, and then allocates a new answer message, adding it to the answer queue.
+     *
+     * @retval kErrorNone   Successfully checked and handled (possibly allocating a new message).
+     * @retval kErrorNoBufs Insufficient message buffers to append Answer TLV or allocate a new message.
+     */
+    Error CheckAnswerLength(void);
+
+    /**
+     * Finishes the answer generation.
+     *
+     * This method appends an Answer TLV to the current answer message with the "is last" flag set.
+     *
+     * @retval kErrorNone   Successfully finished.
+     * @retval kErrorNoBufs Insufficient message buffers to append the Answer TLV.
+     */
+    Error Finish(void);
+
+    /**
+     * Gets the queue containing all answer messages.
+     *
+     * @returns The message queue containing all answer messages.
+     */
+    Coap::MessageQueue &GetAnswers(void) { return mAnswers; }
+
+private:
+    static constexpr uint16_t kAnswerMessageLengthThreshold = 800;
+
+    Error Allocate(void);
+    Error AppendAnswerTlv(AnswerTlvValue::IsLastFlag aFlag);
+
+    Coap::Message     *mAnswer;
+    Coap::MessageQueue mAnswers;
+    uint16_t           mAnswerIndex;
+    uint16_t           mQueryId;
+    bool               mHasQueryId;
+    Message::Priority  mPriority;
+};
+
+} // namespace NetworkDiagnostic
+} // namespace ot
+
+#endif // OT_CORE_THREAD_NETWORK_DIAGNOSTIC_TYPES_HPP_

tests/unit/test_message_queue.cpp

@@ -286,6 +286,59 @@ void TestMessageQueue(void)
     testFreeInstance(sInstance);
 }
 
+void TestMessageQueueEnqueueAllFrom(void)
+{
+    // Validates the behavior of `MessageQueue::EnqueueAllFrom()` method.
+
+    MessageQueue messageQueue1;
+    MessageQueue messageQueue2;
+    Message     *messages[kNumTestMessages];
+
+    sInstance = testInitInstance();
+    VerifyOrQuit(sInstance != nullptr);
+
+    sMessagePool = &sInstance->Get<MessagePool>();
+
+    for (Message *&msg : messages)
+    {
+        msg = sMessagePool->Allocate(Message::kTypeIp6);
+        VerifyOrQuit(msg != nullptr, "Message::Allocate() failed");
+    }
+
+    // Case 1: Both queues are empty
+    messageQueue1.EnqueueAllFrom(messageQueue2);
+    VerifyMessageQueueContent(messageQueue1, 0);
+    VerifyMessageQueueContent(messageQueue2, 0);
+
+    // Case 2: Target queue (Q1) is empty, input queue (Q2) is non-empty
+    messageQueue2.Enqueue(*messages[0]);
+    messageQueue2.Enqueue(*messages[1]);
+    VerifyMessageQueueContent(messageQueue2, 2, messages[0], messages[1]);
+    messageQueue1.EnqueueAllFrom(messageQueue2);
+    VerifyMessageQueueContent(messageQueue1, 2, messages[0], messages[1]);
+    VerifyMessageQueueContent(messageQueue2, 0);
+
+    // Case 3: Target queue (Q1) is non-empty, input queue (Q2) is empty
+    messageQueue1.EnqueueAllFrom(messageQueue2);
+    VerifyMessageQueueContent(messageQueue1, 2, messages[0], messages[1]);
+    VerifyMessageQueueContent(messageQueue2, 0);
+
+    // Case 4: Both queues are non-empty
+    messageQueue2.Enqueue(*messages[2]);
+    VerifyMessageQueueContent(messageQueue2, 1, messages[2]);
+    messageQueue1.EnqueueAllFrom(messageQueue2);
+    VerifyMessageQueueContent(messageQueue1, 3, messages[0], messages[1], messages[2]);
+    VerifyMessageQueueContent(messageQueue2, 0);
+
+    // Case 5: Using the same queue as both the target and input queues
+    messageQueue1.EnqueueAllFrom(messageQueue1);
+    VerifyMessageQueueContent(messageQueue1, 3, messages[0], messages[1], messages[2]);
+    messageQueue2.EnqueueAllFrom(messageQueue2);
+    VerifyMessageQueueContent(messageQueue2, 0);
+
+    testFreeInstance(sInstance);
+}
+
 // This function verifies the content of the message queue to match the passed in messages
 void VerifyMessageQueueContentUsingOtApi(otMessageQueue *aQueue, int aExpectedLength, ...)
 {
@@ -386,6 +439,7 @@ void TestMessageQueueOtApis(void)
 int main(void)
 {
     ot::TestMessageQueue();
+    ot::TestMessageQueueEnqueueAllFrom();
     ot::TestMessageQueueOtApis();
     printf("All tests passed\n");
     return 0;