diff --git a/src/atom.h b/src/atom.h
index 86d81c0148..667da04823 100644
--- a/src/atom.h
+++ b/src/atom.h
@@ -16,7 +16,7 @@
 
 #include "pointers.h"
 
-#include "json.h"
+#include "json_fwd.h"
 
 #include <map>
 #include <set>
diff --git a/src/info.h b/src/info.h
index 8ef38b3c9d..fc7267dbda 100644
--- a/src/info.h
+++ b/src/info.h
@@ -21,6 +21,7 @@ CommandStyle(info,Info);
 #define LMP_INFO_H
 
 #include "command.h"
+#include "json_fwd.h"
 
 #include <vector>
 
diff --git a/src/json.h b/src/json.h
index 2f364b445e..46aae1c966 100644
--- a/src/json.h
+++ b/src/json.h
@@ -20,6 +20,6 @@
 #include "nlohmann/json.hpp"
 
 namespace LAMMPS_NS {
-using json = ::nlohmann_lmp::json;
+using json = ::nlohmann_lmp::basic_json<>;
 }
 #endif
diff --git a/src/json_fwd.h b/src/json_fwd.h
new file mode 100644
index 0000000000..b63d41bd4d
--- /dev/null
+++ b/src/json_fwd.h
@@ -0,0 +1,25 @@
+/* -*- c++ -*- ----------------------------------------------------------
+   LAMMPS - Large-scale Atomic/Molecular Massively Parallel Simulator
+   https://www.lammps.org/, Sandia National Laboratories
+   LAMMPS development team: developers@lammps.org
+
+   Copyright (2003) Sandia Corporation.  Under the terms of Contract
+   DE-AC04-94AL85000 with Sandia Corporation, the U.S. Government retains
+   certain rights in this software.  This software is distributed under
+   the GNU General Public License.
+
+   See the README file in the top-level LAMMPS directory.
+------------------------------------------------------------------------- */
+
+#ifndef LMP_JSON_FWD_H
+#define LMP_JSON_FWD_H
+
+// Forward declarations for header-only JSON class
+// For use in headers
+
+#include "nlohmann/json_fwd.hpp"
+
+namespace LAMMPS_NS {
+using json = ::nlohmann_lmp::basic_json<>;
+}
+#endif
diff --git a/src/molecule.h b/src/molecule.h
index 7444342357..f384c6e003 100644
--- a/src/molecule.h
+++ b/src/molecule.h
@@ -16,7 +16,7 @@
 
 #include "pointers.h"
 
-#include "json.h"
+#include "json_fwd.h"
 
 namespace LAMMPS_NS {
 
diff --git a/src/nlohmann/json.hpp b/src/nlohmann/json.hpp
index 75014cae41..58bcefba46 100644
--- a/src/nlohmann/json.hpp
+++ b/src/nlohmann/json.hpp
@@ -134,6 +134,7 @@
 #define NLOHMANN_JSON_NAMESPACE_CONCAT(a, b) \
     NLOHMANN_JSON_NAMESPACE_CONCAT_EX(a, b)
 
+// LAMMPS CUSTOMIZATION: to avoid namespace clashes we use "nlohmann_lmp"
 #ifndef NLOHMANN_JSON_NAMESPACE
 #define NLOHMANN_JSON_NAMESPACE               \
     nlohmann_lmp::NLOHMANN_JSON_NAMESPACE_CONCAT( \
@@ -160,10 +161,10 @@
 // #include <nlohmann/detail/conversions/from_json.hpp>
 //     __ _____ _____ _____
 //  __|  |   __|     |   | |  JSON for Modern C++
-// |  |  |__   |  |  | | | |  version 3.11.3
+// |  |  |__   |  |  | | | |  version 3.12.0
 // |_____|_____|_____|_|___|  https://github.com/nlohmann/json
 //
-// SPDX-FileCopyrightText: 2013-2023 Niels Lohmann <https://nlohmann.me>
+// SPDX-FileCopyrightText: 2013 - 2025 Niels Lohmann <https://nlohmann.me>
 // SPDX-License-Identifier: MIT
 
 
@@ -173,6 +174,9 @@
 #include <forward_list> // forward_list
 #include <iterator> // inserter, front_inserter, end
 #include <map> // map
+#ifdef JSON_HAS_CPP_17
+    #include <optional> // optional
+#endif
 #include <string> // string
 #include <tuple> // tuple, make_tuple
 #include <type_traits> // is_arithmetic, is_same, is_enum, underlying_type, is_convertible
@@ -183,10 +187,10 @@
 // #include <nlohmann/detail/exceptions.hpp>
 //     __ _____ _____ _____
 //  __|  |   __|     |   | |  JSON for Modern C++
-// |  |  |__   |  |  | | | |  version 3.11.3
+// |  |  |__   |  |  | | | |  version 3.12.0
 // |_____|_____|_____|_|___|  https://github.com/nlohmann/json
 //
-// SPDX-FileCopyrightText: 2013-2023 Niels Lohmann <https://nlohmann.me>
+// SPDX-FileCopyrightText: 2013 - 2025 Niels Lohmann <https://nlohmann.me>
 // SPDX-License-Identifier: MIT
 
 
@@ -3489,8 +3493,9 @@ NLOHMANN_JSON_NAMESPACE_END
 // SPDX-FileCopyrightText: 2013 - 2025 Niels Lohmann <https://nlohmann.me>
 // SPDX-License-Identifier: MIT
 
-#ifndef INCLUDE_NLOHMANN_JSON_FWD_HPP_
-    #define INCLUDE_NLOHMANN_JSON_FWD_HPP_
+// LAMMPS customization. We changed the include guard with the namespace name
+#ifndef LMP_NLOHMANN_JSON_FWD_HPP_
+    #define LMP_NLOHMANN_JSON_FWD_HPP_
 
     #include <cstdint> // int64_t, uint64_t
     #include <map> // map
@@ -3556,7 +3561,7 @@ NLOHMANN_JSON_NAMESPACE_END
 
     NLOHMANN_JSON_NAMESPACE_END
 
-#endif  // INCLUDE_NLOHMANN_JSON_FWD_HPP_
+#endif  // LMP_NLOHMANN_JSON_FWD_HPP_
 
 
 NLOHMANN_JSON_NAMESPACE_BEGIN
diff --git a/src/nlohmann/json_fwd.hpp b/src/nlohmann/json_fwd.hpp
new file mode 100644
index 0000000000..8ebbf1d4e5
--- /dev/null
+++ b/src/nlohmann/json_fwd.hpp
@@ -0,0 +1,188 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013 - 2025 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+#ifndef LMP_NLOHMANN_JSON_FWD_HPP_
+#define LMP_NLOHMANN_JSON_FWD_HPP_
+
+#include <cstdint> // int64_t, uint64_t
+#include <map> // map
+#include <memory> // allocator
+#include <string> // string
+#include <vector> // vector
+
+// #include <nlohmann/detail/abi_macros.hpp>
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013 - 2025 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+
+
+// This file contains all macro definitions affecting or depending on the ABI
+
+#ifndef JSON_SKIP_LIBRARY_VERSION_CHECK
+    #if defined(NLOHMANN_JSON_VERSION_MAJOR) && defined(NLOHMANN_JSON_VERSION_MINOR) && defined(NLOHMANN_JSON_VERSION_PATCH)
+        #if NLOHMANN_JSON_VERSION_MAJOR != 3 || NLOHMANN_JSON_VERSION_MINOR != 12 || NLOHMANN_JSON_VERSION_PATCH != 0
+            #warning "Already included a different version of the library!"
+        #endif
+    #endif
+#endif
+
+#define NLOHMANN_JSON_VERSION_MAJOR 3   // NOLINT(modernize-macro-to-enum)
+#define NLOHMANN_JSON_VERSION_MINOR 12  // NOLINT(modernize-macro-to-enum)
+#define NLOHMANN_JSON_VERSION_PATCH 0   // NOLINT(modernize-macro-to-enum)
+
+#ifndef JSON_DIAGNOSTICS
+    #define JSON_DIAGNOSTICS 0
+#endif
+
+#ifndef JSON_DIAGNOSTIC_POSITIONS
+    #define JSON_DIAGNOSTIC_POSITIONS 0
+#endif
+
+#ifndef JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON
+    #define JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON 0
+#endif
+
+#if JSON_DIAGNOSTICS
+    #define NLOHMANN_JSON_ABI_TAG_DIAGNOSTICS _diag
+#else
+    #define NLOHMANN_JSON_ABI_TAG_DIAGNOSTICS
+#endif
+
+#if JSON_DIAGNOSTIC_POSITIONS
+    #define NLOHMANN_JSON_ABI_TAG_DIAGNOSTIC_POSITIONS _dp
+#else
+    #define NLOHMANN_JSON_ABI_TAG_DIAGNOSTIC_POSITIONS
+#endif
+
+#if JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON
+    #define NLOHMANN_JSON_ABI_TAG_LEGACY_DISCARDED_VALUE_COMPARISON _ldvcmp
+#else
+    #define NLOHMANN_JSON_ABI_TAG_LEGACY_DISCARDED_VALUE_COMPARISON
+#endif
+
+#ifndef NLOHMANN_JSON_NAMESPACE_NO_VERSION
+    #define NLOHMANN_JSON_NAMESPACE_NO_VERSION 0
+#endif
+
+// Construct the namespace ABI tags component
+#define NLOHMANN_JSON_ABI_TAGS_CONCAT_EX(a, b, c) json_abi ## a ## b ## c
+#define NLOHMANN_JSON_ABI_TAGS_CONCAT(a, b, c) \
+    NLOHMANN_JSON_ABI_TAGS_CONCAT_EX(a, b, c)
+
+#define NLOHMANN_JSON_ABI_TAGS                                       \
+    NLOHMANN_JSON_ABI_TAGS_CONCAT(                                   \
+            NLOHMANN_JSON_ABI_TAG_DIAGNOSTICS,                       \
+            NLOHMANN_JSON_ABI_TAG_LEGACY_DISCARDED_VALUE_COMPARISON, \
+            NLOHMANN_JSON_ABI_TAG_DIAGNOSTIC_POSITIONS)
+
+// Construct the namespace version component
+#define NLOHMANN_JSON_NAMESPACE_VERSION_CONCAT_EX(major, minor, patch) \
+    _v ## major ## _ ## minor ## _ ## patch
+#define NLOHMANN_JSON_NAMESPACE_VERSION_CONCAT(major, minor, patch) \
+    NLOHMANN_JSON_NAMESPACE_VERSION_CONCAT_EX(major, minor, patch)
+
+#if NLOHMANN_JSON_NAMESPACE_NO_VERSION
+#define NLOHMANN_JSON_NAMESPACE_VERSION
+#else
+#define NLOHMANN_JSON_NAMESPACE_VERSION                                 \
+    NLOHMANN_JSON_NAMESPACE_VERSION_CONCAT(NLOHMANN_JSON_VERSION_MAJOR, \
+                                           NLOHMANN_JSON_VERSION_MINOR, \
+                                           NLOHMANN_JSON_VERSION_PATCH)
+#endif
+
+// Combine namespace components
+#define NLOHMANN_JSON_NAMESPACE_CONCAT_EX(a, b) a ## b
+#define NLOHMANN_JSON_NAMESPACE_CONCAT(a, b) \
+    NLOHMANN_JSON_NAMESPACE_CONCAT_EX(a, b)
+
+// LAMMPS CUSTOMIZATION: to avoid namespace clashes we use "nlohmann_lmp".
+#ifndef NLOHMANN_JSON_NAMESPACE
+#define NLOHMANN_JSON_NAMESPACE                   \
+    nlohmann_lmp::NLOHMANN_JSON_NAMESPACE_CONCAT( \
+            NLOHMANN_JSON_ABI_TAGS,               \
+            NLOHMANN_JSON_NAMESPACE_VERSION)
+#endif
+
+#ifndef NLOHMANN_JSON_NAMESPACE_BEGIN
+#define NLOHMANN_JSON_NAMESPACE_BEGIN                \
+    namespace nlohmann_lmp                           \
+    {                                                \
+    inline namespace NLOHMANN_JSON_NAMESPACE_CONCAT( \
+                NLOHMANN_JSON_ABI_TAGS,              \
+                NLOHMANN_JSON_NAMESPACE_VERSION)     \
+    {
+#endif
+
+#ifndef NLOHMANN_JSON_NAMESPACE_END
+#define NLOHMANN_JSON_NAMESPACE_END                                     \
+    }  /* namespace (inline namespace) NOLINT(readability/namespace) */ \
+    }  // namespace nlohmann_lmp
+#endif
+
+
+/*!
+@brief namespace for Niels Lohmann
+@see https://github.com/nlohmann
+@since version 1.0.0
+*/
+NLOHMANN_JSON_NAMESPACE_BEGIN
+
+/*!
+@brief default JSONSerializer template argument
+
+This serializer ignores the template arguments and uses ADL
+([argument-dependent lookup](https://en.cppreference.com/w/cpp/language/adl))
+for serialization.
+*/
+template<typename T = void, typename SFINAE = void>
+struct adl_serializer;
+
+/// a class to store JSON values
+/// @sa https://json.nlohmann.me/api/basic_json/
+template<template<typename U, typename V, typename... Args> class ObjectType =
+         std::map,
+         template<typename U, typename... Args> class ArrayType = std::vector,
+         class StringType = std::string, class BooleanType = bool,
+         class NumberIntegerType = std::int64_t,
+         class NumberUnsignedType = std::uint64_t,
+         class NumberFloatType = double,
+         template<typename U> class AllocatorType = std::allocator,
+         template<typename T, typename SFINAE = void> class JSONSerializer =
+         adl_serializer,
+         class BinaryType = std::vector<std::uint8_t>, // cppcheck-suppress syntaxError
+         class CustomBaseClass = void>
+class basic_json;
+
+/// @brief JSON Pointer defines a string syntax for identifying a specific value within a JSON document
+/// @sa https://json.nlohmann.me/api/json_pointer/
+template<typename RefStringType>
+class json_pointer;
+
+/*!
+@brief default specialization
+@sa https://json.nlohmann.me/api/json/
+*/
+using json = basic_json<>;
+
+/// @brief a minimal map-like container that preserves insertion order
+/// @sa https://json.nlohmann.me/api/ordered_map/
+template<class Key, class T, class IgnoredLess, class Allocator>
+struct ordered_map;
+
+/// @brief specialization that maintains the insertion order of object keys
+/// @sa https://json.nlohmann.me/api/ordered_json/
+using ordered_json = basic_json<nlohmann_lmp::ordered_map>;
+
+NLOHMANN_JSON_NAMESPACE_END
+
+#endif  // LMP_NLOHMANN_JSON_FWD_HPP_