Issues

• Release build fails when JSON diagnostics enabled

  confirmedkind: bugsolution: proposed fix
  nlohmann/json#5742 · Filter by author IvanPizhenko opened on Sep 30, 2026 · Release 3.13.0
  4

• basic_json and ordered_map: insert(pos, json&&) copies, duplicated from_*/lookup/value()/null-to-container bodies, avoidable lint suppressions, stale comments

  kind: enhancement/improvement
  nlohmann/json#5724 · Filter by author nlohmann opened on Sep 30, 2026
  2

  

• Packaging metadata: incomplete json.tar.xz and Bazel singleheader target, SwiftPM docs, REUSE dep5, stale README compiler list and CITATION.cff

  CICMakedocumentationkind: enhancement/improvement
  nlohmann/json#5716 · Filter by author nlohmann opened on Sep 30, 2026
  

• Type traits, macros, conversions, json_pointer and iterators: dead metaprogramming, duplicated bodies, stale comments

  kind: enhancement/improvement
  nlohmann/json#5708 · Filter by author nlohmann opened on Sep 30, 2026
  

• sax_parse cannot read CBOR that contains tags, including binary values with a subtype written by to_cbor

  aspect: binary formatskind: enhancement/improvement
  nlohmann/json#5676 · Filter by author nlohmann opened on Sep 29, 2026
  1

• Copy-constructing basic_json now requires an assignable CustomBaseClass (compile regression)

  kind: bug
  nlohmann/json#5674 · Filter by author nlohmann opened on Sep 29, 2026
  

• ordered_json::emplace(key, value) does not compile when value is an lvalue

  kind: bug
  nlohmann/json#5673 · Filter by author nlohmann opened on Sep 29, 2026
  

• With JSON_NOEXCEPTION, value(json_pointer, default) aborts for some array tokens

  kind: bug
  nlohmann/json#5672 · Filter by author nlohmann opened on Sep 29, 2026
  

• Legacy discarded comparison in C++20: scalar <= discarded and scalar >= discarded yield false

  kind: bug
  nlohmann/json#5665 · Filter by author nlohmann opened on Sep 29, 2026
  

• Keys convertible to std::string_view break at/find/contains, including keys that worked in 3.12.0

  kind: bug
  nlohmann/json#5663 · Filter by author nlohmann opened on Sep 29, 2026
  

• JSON_BRACE_INIT_COPY_SEMANTICS: json j{arr} turns an array like ["key", 42] into an object

  kind: bug
  nlohmann/json#5662 · Filter by author nlohmann opened on Sep 29, 2026
  

• BJData ND-array annotations are not parsed back into the same object ("single" precision, ordered_json key order)

  aspect: binary formatskind: bug
  nlohmann/json#5661 · Filter by author nlohmann opened on Sep 29, 2026
  

• Floats are truncated at the decimal point under locales with a multi-byte decimal point (fa_IR.UTF-8)

  kind: bug
  nlohmann/json#5660 · Filter by author nlohmann opened on Sep 29, 2026
  3

  

• A NUL byte that ends a // comment does not end the input with the default NUL handling

  kind: bug
  nlohmann/json#5659 · Filter by author nlohmann opened on Sep 29, 2026
  

• C++20 operator<=> result depends on nesting depth when binary values differ only in subtype

  kind: bug
  nlohmann/json#5654 · Filter by author nlohmann opened on Sep 29, 2026
  

• Binary writers emit strings with ill-formed UTF-8 that the corresponding readers reject

  aspect: binary formatskind: bugsolution: proposed fix
  nlohmann/json#5651 · Filter by author nlohmann opened on Sep 29, 2026
  2

  

• Deep copy (nested 128+ levels) drops the object comparator's state: keys reordered or lost

  kind: bug
  nlohmann/json#5649 · Filter by author nlohmann opened on Sep 29, 2026
  

• from_bon8(ptr, len) and from_bjdata(ptr, len) compile without a warning and read ptr as a NUL-terminated string

  aspect: binary formatskind: bug
  nlohmann/json#5648 · Filter by author nlohmann opened on Sep 29, 2026
  

• Wide-string input: lone UTF-16 surrogates and negative wchar_t units are accepted or end the input

  kind: bug
  nlohmann/json#5645 · Filter by author nlohmann opened on Sep 29, 2026
  

• from_cbor()/from_msgpack() do not validate UTF-8 in text strings at decode time (only dump() does)

  aspect: binary formatskind: bugsolution: proposed fixstate: please discuss
  nlohmann/json#5529 · Filter by author nlohmann opened on Sep 15, 2026 · Release 3.13.0
  6

• Test suite: reduce runtime — 52 % is Unicode byte sweeps, plus redundant re-parsing in binary roundtrip tests

  solution: proposed fix
  nlohmann/json#5418 · Filter by author nlohmann opened on Aug 26, 2026
  1

  

• std::hash<basic_json> is inconsistent with operator== for equal cross-type numbers (breaks unordered containers)

  kind: bugsolution: proposed fix
  nlohmann/json#5400 · Filter by author nlohmann opened on Aug 25, 2026

• Stack overflow in binary format writers (CBOR/MessagePack/UBJSON/BSON) due to unbounded recursion

  nlohmann/json#5392 · Filter by author nlohmann opened on Aug 21, 2026
  2

  

• Behavior differences in operator[] object creation with a json_pointer

  state: please discuss
  nlohmann/json#5357 · Filter by author nlohmann opened on Aug 3, 2026
  1

• to_cbor()/to_msgpack() allocate a temporary basic_json for every object key

  aspect: binary formatsgood first issuekind: enhancement/improvement
  nlohmann/json#5318 · Filter by author nlohmann opened on Jul 27, 2026
  2