nlohmann/json

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JSON for Modern C++

Stars: 50.6k
Forks: 7.5k
Open beginner issues: 0
Indexed issues: 39
Avg merge: 4d 17h
Merged PRs (30d): 58

Dominant language: C++
License: MIT
Last GitHub push: Sep 16, 2026
Latest indexed: Sep 19, 2026
Contributing guide: Contributing guide
Code of conduct: Code of conduct
Beginner labels: good first issue

Beginner friendly Everything

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39 open issues indexed

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

aspect: binary formats kind: bug solution: proposed fix state: please discuss

Difficulty 4/5 3-5 days Newbie friendliness 55/100

nlohmann/json#5529 · 6 comments · Opened Sep 15, 2026
[GLD Serializer Benchmark] C++ nlohmann_ubjson 3.12.0 — spec compliance misses Open

Difficulty 4/5 3-5 days Newbie friendliness 38/100

nlohmann/json#5528 · 1 comment · Opened Sep 14, 2026
[GLD Serializer Benchmark] C++ nlohmann_msgpack 3.12.0 — spec compliance misses Open

Difficulty 4/5 3-5 days Newbie friendliness 35/100

nlohmann/json#5527 · 1 comment · Opened Sep 14, 2026
[GLD Serializer Benchmark] C++ nlohmann_json 3.12.0 — spec compliance misses Open

Difficulty 4/5 3-5 days Newbie friendliness 38/100

nlohmann/json#5526 · 2 comments · Opened Sep 14, 2026
[GLD Serializer Benchmark] C++ nlohmann_cbor 3.12.0 — spec compliance misses Open

Difficulty 4/5 3-5 days Newbie friendliness 35/100

nlohmann/json#5525 · 2 comments · Opened Sep 14, 2026
[GLD Serializer Benchmark] C++ nlohmann_bson 3.12.0 — spec compliance misses Open

Difficulty 4/5 3-5 days Newbie friendliness 35/100

nlohmann/json#5524 · 2 comments · Opened Sep 14, 2026
Test suite: reduce runtime — 52 % is Unicode byte sweeps, plus redundant re-parsing in binary roundtrip tests Open

Difficulty 4/5 3-5 days Newbie friendliness 48/100

nlohmann/json#5418 · 1 comment · Opened Aug 26, 2026
std::hash<basic_json> is inconsistent with operator== for equal cross-type numbers (breaks unordered containers) Open

kind: bug solution: proposed fix

Difficulty 4/5 3-5 days Newbie friendliness 48/100

nlohmann/json#5400 · Opened Aug 25, 2026
Stack overflow in diff(), merge_patch(), and flatten() due to unbounded recursion Open

Difficulty 4/5 3-5 days Newbie friendliness 55/100

nlohmann/json#5393 · 1 comment · Opened Aug 21, 2026
Stack overflow in binary format writers (CBOR/MessagePack/UBJSON/BSON) due to unbounded recursion Open

Difficulty 5/5 Over a week Newbie friendliness 45/100

nlohmann/json#5392 · 1 comment · Opened Aug 21, 2026
Behavior differences in operator[] object creation with a json_pointer Open

state: please discuss

Difficulty 4/5 3-5 days Newbie friendliness 45/100

nlohmann/json#5357 · 1 comment · Opened Aug 3, 2026
operator>> does not restore the character that terminates a number Open

state: please discuss

Difficulty 4/5 3-5 days Newbie friendliness 45/100

nlohmann/json#5340 · 2 comments · Opened Jul 31, 2026
to_msgpack() silently emits corrupt output (or reads out of bounds) above the documented 4294967295 size limit Open

aspect: binary formats solution: proposed fix

nlohmann/json#5320 · 3 comments · 1 assignee · Opened Jul 27, 2026
to_cbor()/to_msgpack() allocate a temporary basic_json for every object key Open

aspect: binary formats good first issue kind: enhancement/improvement

nlohmann/json#5318 · 2 comments · 1 assignee · Opened Jul 27, 2026
CBOR: nested indefinite-length strings/byte strings are accepted (RFC 8949 3.2.3 requires definite-length chunks) Open

aspect: binary formats good first issue

nlohmann/json#5317 · 1 comment · 1 assignee · Opened Jul 27, 2026
CBOR: cbor_tag_handler_t::store rejects documents that ignore accepts, and drops tag numbers 6-20 Open

aspect: binary formats state: please discuss

Difficulty 3/5 1-2 days Newbie friendliness 68/100

nlohmann/json#5316 · Opened Jul 27, 2026
to_bson() is O(size × nesting depth), not linear as documented Open

aspect: binary formats documentation

Difficulty 4/5 3-5 days Newbie friendliness 52/100

nlohmann/json#5308 · Opened Jul 26, 2026
Idea: arena / PMR document mode (O(1) teardown, allocation-free destruction) Open

Difficulty 5/5 Over a week Newbie friendliness 25/100

nlohmann/json#5298 · 2 comments · Opened Jul 22, 2026
Idea: canonical / deterministic serialization (RFC 8785 JCS) Open

Difficulty 5/5 Over a week Newbie friendliness 25/100

nlohmann/json#5297 · 1 comment · Opened Jul 22, 2026
Idea: lossless number round-tripping (raw / arbitrary-precision number mode) Open

Difficulty 5/5 Over a week Newbie friendliness 25/100

nlohmann/json#5296 · Opened Jul 22, 2026
Idea: borrowed / zero-copy read-only view over a source buffer Open

Difficulty 5/5 Over a week Newbie friendliness 25/100

nlohmann/json#5295 · Opened Jul 22, 2026
Ideas: exploring ways to reduce compilation time Open

Difficulty 5/5 Over a week Newbie friendliness 25/100

nlohmann/json#5294 · 2 comments · Opened Jul 22, 2026
Int and uint compare equal but hashes do not Open

confirmed kind: bug solution: proposed fix state: please discuss

Difficulty 3/5 1-2 days Newbie friendliness 35/100

nlohmann/json#5256 · 4 comments · Opened Jul 9, 2026
std::optional<T> direct construction from JSON null throws instead of yielding std::nullopt Open

Difficulty 5/5 Over a week Newbie friendliness 35/100

nlohmann/json#5246 · 2 comments · Opened Jul 8, 2026
TOCTOU race between lexer construction and locale changes causes float truncation Open

kind: bug

Difficulty 5/5 Over a week Newbie friendliness 35/100

nlohmann/json#5198 · 10 comments · Opened May 30, 2026
basic_json destructor allocates memory, violating noexcept semantics Open

kind: bug state: please discuss

Difficulty 4/5 3-5 days Newbie friendliness 35/100

nlohmann/json#5135 · 8 comments · Opened Apr 13, 2026
Stack overflow in binary format parsers (CBOR/MessagePack/UBJSON/BSON) due to unbounded recursion Open

Difficulty 4/5 3-5 days Newbie friendliness 45/100

nlohmann/json#5104 · 4 comments · Opened Mar 11, 2026
MSVC: unexpected behaviour converting from json to a rvalue reference of a variant containing a type that can be constructed from a string Open

kind: bug

Difficulty 4/5 3-5 days Newbie friendliness 45/100

nlohmann/json#5066 · 16 comments · Opened Jan 30, 2026
Binary formats invalid encoding for <discarded> values in arrays and objects Open

aspect: binary formats kind: bug

Difficulty 4/5 3-5 days Newbie friendliness 45/100

nlohmann/json#4714 · 14 comments · Opened Apr 1, 2025
NLOHMANN_DEFINE_TYPE_* fails with zero members Open

confirmed kind: bug solution: proposed fix

Difficulty 3/5 1-2 days Newbie friendliness 45/100

nlohmann/json#4041 · 7 comments · Opened May 23, 2023
meson build does not install nlohmann_json*.cmake files Open

good first issue kind: bug state: help needed state: waiting for PR

Difficulty 3/5 1-2 days Newbie friendliness 38/100

nlohmann/json#3885 · 10 comments · Opened Dec 17, 2022
invalid use of incomplete type (boost::optional) / compile error Open

kind: bug

Difficulty 4/5 3-5 days Newbie friendliness 32/100

nlohmann/json#3669 · 3 comments · Opened Aug 4, 2022
json destructor quite slow Open

kind: bug

Difficulty 4/5 3-5 days Newbie friendliness 35/100

nlohmann/json#3583 · 7 comments · Opened Jul 16, 2022
Unable to use gnu mpz types for NumberIntegerType Open

kind: bug

Difficulty 5/5 Over a week Newbie friendliness 25/100

nlohmann/json#3578 · 17 comments · Opened Jul 11, 2022
Define roadmap for 4.0.0 Open

documentation

Difficulty 5/5 Over a week Newbie friendliness 35/100

nlohmann/json#3453 · 7 comments · Opened Apr 24, 2022
msgpack parser failed to parse null as Map key Open

aspect: binary formats kind: bug state: please discuss

Difficulty 4/5 3-5 days Newbie friendliness 35/100

nlohmann/json#3381 · 9 comments · Opened Mar 8, 2022
Support for integer keys in the SAX interface when parsing CBOR Open

aspect: binary formats

Difficulty 5/5 Over a week Newbie friendliness 35/100

nlohmann/json#2766 · 3 comments · Opened May 9, 2021
String type change breaks C++ type matching Open

kind: bug state: help needed

Difficulty 4/5 3-5 days Newbie friendliness 42/100

nlohmann/json#2649 · 18 comments · Opened Feb 18, 2021
std::tuple dangling reference - implicit conversion Open

kind: bug state: help needed

Difficulty 4/5 3-5 days Newbie friendliness 35/100

nlohmann/json#2226 · 42 comments · Opened Jun 27, 2020

nlohmann/json

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