json/tools/api_checker

API Checker Tools

Tooling to extract, validate, and track changes to the public API surface of nlohmann/json.

Overview

These tools use libclang AST parsing to programmatically derive "the public API" from C++ semantics (class templates, access specifiers, namespace scoping) — independently of documentation status. The extracted surface is the source of truth for what is considered "public API." On top of this, the tools verify that every public entity carries a documentation link and detect API changes between releases. A per-release historical record lives in history/, one file per tagged v3.* release, so past API changes can be derived without re-running libclang against old git refs.

See POLICY.md for the full definition of what counts as public API, what's excluded, how breaking vs. feature changes are classified, and known limitations.

Installation

Install dependencies:

pip install -r tools/api_checker/requirements.txt

Requires:

• Python 3.7+
• libclang 18.1.1 (installed via pip)
• clang++ or clang (system package, for include-path discovery only — does not need to version-match the pinned libclang wheel)

Tools

extract_api.py

Extract the public API surface by parsing C++ AST using libclang. Produces two different outputs for two different consumers — see "Two outputs, one extraction pass" below for why.

Usage:

python3 tools/api_checker/extract_api.py \
  --header include/nlohmann/json.hpp \
  --include include \
  --output api_snapshot.json \
  --surface-output api_surface.json

Options:

• --header PATH — Header file to analyze (default: include/nlohmann/json.hpp)
• --include PATH — Include directory for parsing (default: include)
• --output PATH — Full snapshot output (location + doc status; default: api_snapshot.json)
• --surface-output PATH — Minimal surface output (identity only; omit to skip)
• --extra-isystem PATH — Extra -isystem include path (repeatable), in case system-include discovery via clang++ -E -v ever fails on a given runner image
• --self-test — Run self-tests and exit (validates ABI-tag stripping)

How it works: Parses the header file using libclang with PARSE_DETAILED_PROCESSING_RECORD, discovers system includes via clang++ -E -x c++ -v /dev/null, and walks the AST:

1. For each of the 6 public class templates (basic_json, adl_serializer, byte_container_with_subtype, json_pointer, json_sax, ordered_map): locate the CLASS_TEMPLATE cursor with is_definition() == True — never a CLASS_DECL implicit instantiation, which silently drops SFINAE-guarded overloads
2. Extract public members: CXX_METHOD, CONSTRUCTOR, DESTRUCTOR, CONVERSION_FUNCTION, FUNCTION_TEMPLATE (callable tier — strict @sa requirement), and TYPE_ALIAS_DECL (type tier — with STL-container exemptions)
3. Extract free functions in nlohmann:: namespace (excluding detail:: and std::)
4. Normalize away ABI inline-namespace (regex ::json_abi[a-z_]*_v\d+_\d+_\d+ → ::)
5. Use overload-disambiguating identity keys built from raw source-text signature capture, ABI-tag-stripped — see "Identity keys" below

Two outputs, one extraction pass:

• --output (full snapshot): each entry carries location (file:line) and documentation status (doc_url, has_sa). Consumed by check_docs.py. Not meant to be committed — location shifts on any unrelated code edit and doc_url changes when doc pages move, so a diff of this file mixes real API changes with pure noise.
• --surface-output (minimal surface): each entry has only scope, kind, name, identity_name, tier, signature, and pretty_signature — nothing that can change without the API itself changing. This is the file that gets committed (tools/api_checker/api_surface.json for the current working tree, tools/api_checker/history/<tag>.json per released tag) and diffed by diff_api.py.

Identity keys — three approaches were tried and rejected before arriving at the current one:

1. {scope, name, kind, params} from cursor.get_arguments(): silently collided for any overload set differentiated only by constness, ref-qualifiers, or SFINAE constraints rather than parameter types — confirmed empirically: basic_json's two zero-argument get() overloads (one const, one not) both produced params=[] and silently overwrote each other. A full scan found 59 such silent overwrites across 27 colliding names.
2. libclang's USR: correctly disambiguates every overload (it encodes the full mangled signature) — but also encodes the enclosing class template's own arity into every member's USR. Confirmed empirically against real release tags: basic_json gaining one new defaulted template parameter between v3.11.2 and v3.11.3 (a backward-compatible change) changed literally every member's USR, which made diff_api.py report 228 of 330 entries as "changed" for a release with zero real breaking changes among them.
3. Current approach: raw source-text signature capture — identity = (scope, identity_name, kind, signature), where signature is the declaration's own text (return type, name, parameter list, trailing cv/ref/noexcept qualifiers; comments and constructors' member-initializer-lists stripped; stops before the function body) read directly via the cursor's byte-offset extent. This is what's actually written in source — declared names like ValueType, never a resolved basic_json<T0,...,T10> — so it's immune to the class-arity problem above. Verified by manually cross-referencing every "added"/"removed"/"changed" entry in three real release-to-release diffs (v3.11.2→v3.11.3, v3.11.3→v3.12.0, v3.12.0→HEAD) against the actual git diff of the source — every one confirmed genuine. Full details, including several further edge-case fixes found the same way (a libclang tokenizer gap, comment-stripping, constructor-name arity-poisoning), are in extract_api.py's identity_key()/get_signature_text() docstrings.

Output (full snapshot, from --output):

{
  "meta": {
    "extracted_from": "include/nlohmann/json.hpp"
  },
  "public_api": {
    "<opaque internal key, not meant to be read>": {
      "scope": "nlohmann::basic_json",
      "name": "parse",
      "identity_name": "parse",
      "kind": "CXX_METHOD",
      "tier": "callable",
      "signature": "static basic_json parse ( InputType && i , ... )",
      "location": "include/nlohmann/json.hpp:4104",
      "doc_url": "https://json.nlohmann.me/api/basic_json/parse/",
      "has_sa": true,
      "pretty_signature": "nlohmann::basic_json::parse"
    }
  },
  "documented_non_public": []
}

documented_non_public lists entities not part of the public surface (private/protected members of the six tracked classes) that surprisingly carry a real @sa URL — a genuine documentation leak. It does not list public entries that merely lack @sa; that's check_docs.py's job.

Output (surface, from --surface-output):

{
  "format_version": 1,
  "meta": {
    "extracted_from": "include/nlohmann/json.hpp"
  },
  "public_api": [
    {
      "scope": "nlohmann::basic_json",
      "kind": "CXX_METHOD",
      "name": "parse",
      "identity_name": "parse",
      "tier": "callable",
      "signature": "static basic_json parse ( InputType && i , parser_callback_t cb = nullptr , const bool allow_exceptions = true , const bool ignore_comments = false )",
      "pretty_signature": "nlohmann::basic_json::parse"
    }
  ]
}

A flat, sorted list of self-describing records — signature/identity_name are the same values identity_key() joins into one opaque internal string, exposed here as explicit fields so the file is readable and diffable by inspection, not just by tooling. identity_name differs from name only for constructors/destructors (a canonical "(constructor)"/"(destructor)" placeholder — see get_identity_name()'s docstring for why cursor.spelling isn't used directly there).

format_version guards against a future change to this schema or to the identity-computing algorithm silently corrupting a comparison against an older stored surface — bump it whenever such a change could alter signature/identity_name text for otherwise-unchanged source (see diff_api.py).

check_docs.py

Verify that all public API entries have valid documentation links and that no non-public entities carry @sa comments.

Usage:

python3 tools/api_checker/check_docs.py --snapshot api_snapshot.json

Options:

• --snapshot PATH — Full API snapshot JSON file from extract_api.py --output (default: api_snapshot.json)

How it works: Two-pass validation over the extracted snapshot:

1. For every public_api entry (callable tier, and type tier minus STL exemptions): flag if missing @sa, flag if @sa URL doesn't resolve to an existing documentation file (following docs/mkdocs/mkdocs.yml's redirect_maps when a page has moved, and trying the class-overview conventions used by different classes — flat <class>.md, nested <class>/<class>.md, nested <class>/index.md)
2. For every documented_non_public entry: flag as unexpected @sa on a non-public entity

Output: Prints warnings for each issue, categorized by rule:

• docs/missing_sa_comment — public API without @sa documentation link
• docs/missing_doc_file — @sa URL points to non-existent .md file
• docs/invalid_sa_url — malformed @sa URL
• docs/sa_on_non_public — unexpected @sa on a non-public entity

Exits with status 0 if all checks pass, 1 if issues found.

diff_api.py

Compare the public API surface between two refs and classify changes as feature (added) or breaking (removed / changed overload).

Usage:

python3 tools/api_checker/diff_api.py --old v3.12.0 --new HEAD
python3 tools/api_checker/diff_api.py --old v3.11.2 --new v3.11.3          # both resolved from history/
python3 tools/api_checker/diff_api.py --old v3.12.0 --new HEAD --fail-on-breaking
python3 tools/api_checker/diff_api.py --old-file a.json --new-file b.json  # compare two files directly

Options:

• --old REF / --new REF — Refs to compare (tags, branches, commits). --new defaults to HEAD. Each is resolved by checking tools/api_checker/history/<ref>.json first (fast — no libclang or git-archive needed), falling back to live extraction if no matching file exists there.
• --old-file PATH / --new-file PATH — Load an arbitrary surface JSON file directly, bypassing both git and tools/api_checker/history/. Mutually exclusive with --old/--new respectively.
• --no-history — Force live extraction even when a matching tools/api_checker/history/<ref>.json exists. Useful to check that a stored record is still faithful to a fresh run of the current tool.
• --allow-format-mismatch — Proceed even if the two surfaces have different format_version (otherwise diff_api.py refuses — see below).
• --header PATH / --include PATH — For live extraction only (default: include/nlohmann/json.hpp / include)
• --fail-on-breaking — Exit with status 1 if breaking changes are detected

How it works: For a ref not found in tools/api_checker/history/, checks out the full include/ tree at that ref into a temp directory via git archive (a single-file checkout of json.hpp is not enough — it #includes dozens of other headers that must exist at the same ref), then runs the current extract_api.py --surface-output against it. Diffs the two surfaces by identity (scope, identity_name, kind, signature):

• Identity only in the new surface → feature.
• Identity only in the old surface → breaking (removed).
• Same (scope, name) with a removed identity and an added identity → grouped as a changed overload, breaking by default. No automatic overload-compatibility reasoning is attempted — a human judges whether the change is actually source-compatible.

Before diffing, the two surfaces' format_version fields are compared; a mismatch aborts with an error (override with --allow-format-mismatch) rather than silently producing an unsound diff — see extract_api.py's SURFACE_FORMAT_VERSION docstring for the incident that motivated this guard.

ABI-tag stripping is inherited automatically since both extractions go through the same extract_api.py.

Use case: Run before cutting a release to verify the changelog correctly categorizes changes as breaking vs. features.

snapshot_release.py

Capture an immutable, per-release API surface record into tools/api_checker/history/<tag>.json. This is what makes diff_api.py fast for released tags — see "How it works" above.

Usage:

python3 tools/api_checker/snapshot_release.py --ref v3.12.0
python3 tools/api_checker/snapshot_release.py --ref v3.11.0 --ref v3.12.0   # repeatable
python3 tools/api_checker/snapshot_release.py --all-tags                    # every v3.* tag
python3 tools/api_checker/snapshot_release.py --ref v3.12.0 --force         # overwrite an existing record

Options:

• --ref REF — Git tag to snapshot; repeatable
• --all-tags — Snapshot every v3.* tag (existing files are skipped unless --force)
• --output-dir PATH — Where to write (default: tools/api_checker/history/)
• --force — Overwrite an existing history file. History files are immutable by convention — only pass this for a deliberate, reviewed regeneration; review the resulting diff before committing
• --header PATH / --include PATH — Same as diff_api.py's live-extraction options

How it works: Reuses diff_api.py's extract_surface_for_ref() for the git-archive-and-extract work, then adds generated_at/generator provenance and writes the result to tools/api_checker/history/<ref>.json. Given multiple refs (or --all-tags), does not abort the batch on one failing ref — collects failures and prints a summary at the end, so one unparseable old tag doesn't block backfilling the releases that do work. See tools/api_checker/history/README.md for the currently-known gaps (pre-restructuring tags that predate the include/nlohmann/ layout entirely).

Not CI-automated — this is a manual step in the release checklist (see below), by design.

check_macros.py

Advisory-only cross-check between documented macros and their #define/reference sites. Never blocks CI, regardless of findings.

Usage:

python3 tools/api_checker/check_macros.py

Options:

• --macros-dir PATH — Directory of macro doc pages (default: docs/mkdocs/docs/api/macros)
• --include-dir PATH — Directory to search for #define/reference sites (default: include/nlohmann)

How it works: For each .md page under docs/mkdocs/docs/api/macros/ (excluding index.md), extracts the macro name(s) from the H1 heading — handling both plain single-macro headings and multi-line HTML headings that list a family of related macros (e.g. the NLOHMANN_DEFINE_DERIVED_TYPE_* family) — then checks whether each name is defined or referenced (#define, #ifdef, #ifndef, defined(...)) anywhere under include/nlohmann/. Referenced-but-not-defined is deliberately accepted: macros like JSON_NOEXCEPTION or JSON_THROW_USER are user-supplied overrides that the library only checks for, never defines itself.

Only checks the documented-macro-still-exists direction (catches stale/renamed doc pages). Does not check the converse (undocumented macros) — see POLICY.md's "Known limitations" for why.

Continuous Integration

A GitHub Actions workflow (.github/workflows/check_api_docs.yml) runs on every pull request:

1. Installs clang (system package, for include discovery) and Python dependencies
2. Runs extract_api.py, regenerating both the ephemeral full snapshot and the tracked tools/api_checker/api_surface.json
3. Runs check_docs.py — Phase 1: advisory (continue-on-error: true), surfacing the doc backlog without failing the job while it's burned down. Will flip to blocking once the backlog is cleared.
4. Runs check_macros.py — always advisory
5. Diffs tools/api_checker/api_surface.json against the regenerated copy — blocking from the start (unlike the doc-backlog check, this is purely mechanical regeneration with no backlog to phase in, matching the precedent set by check_amalgamation.yml). Uploads a patch artifact if it differs, so contributors can git apply it instead of installing libclang locally.

Workflow: Adding New Public API

1. Add the new public method/function to the header
2. Add a /// @sa https://json.nlohmann.me/api/<class>/<member>/ comment above it
3. Create the corresponding documentation page at docs/mkdocs/docs/api/<class>/<member>.md and a mkdocs.yml nav entry
4. Regenerate and commit the tracked surface file:

   python3 tools/api_checker/extract_api.py --surface-output tools/api_checker/api_surface.json
   git add tools/api_checker/api_surface.json
   

5. Push your PR — CI verifies both the doc link and that the surface file is up to date

Workflow: Release Checklist

Before cutting a release:

# Diff current API against the previous release
python3 tools/api_checker/diff_api.py --old v3.12.0 --new HEAD

# Review the output to verify the changelog correctly categorizes breaking vs. feature changes

After tagging the release:

# Capture and commit the new tag's API surface, so future diffs against it hit the fast,
# stored-file path instead of live-extracting every time. A manual step, not CI-automated.
python3 tools/api_checker/snapshot_release.py --ref v3.13.0
git add tools/api_checker/history/v3.13.0.json

Troubleshooting

libclang not found:

Error: Could not locate libclang library

→ Ensure libclang is installed: python3 -m pip install libclang==18.1.1

Parse errors in header:

Parse errors encountered:
  ... list of diagnostics ...

→ Check that system includes can be discovered. Run clang++ -E -x c++ -v /dev/null and verify the output includes a section titled #include <...> search starts here: with system paths. If include discovery fails, use --extra-isystem PATH to provide additional paths.

No API entries extracted (or very few):

• Check that the header file exists and is valid C++: ls -la include/nlohmann/json.hpp
• Verify that no parse errors occur above
• Confirm that you're targeting a CLASS_TEMPLATE definition, not an implicit instantiation (the tool logs Found N public API entries — a zero or very small count suggests the wrong cursor kind)

Doc link returns 404:

• Verify the file exists at the expected path: docs/mkdocs/docs/api/<class>/<member>.md
• Check for URL encoding issues (e.g., operator[] → operator%5B%5D)
• Verify the URL structure in the @sa comment: should be https://json.nlohmann.me/api/<path>/
• Check docs/mkdocs/mkdocs.yml's redirect_maps if the page has moved

tools/api_checker/api_surface.json is out of date in CI: → Regenerate and commit it: python3 tools/api_checker/extract_api.py --surface-output tools/api_checker/api_surface.json

diff_api.py refuses with "format_version mismatch": → One side is a stored surface (live api_surface.json or a tools/api_checker/history/*.json file) captured with an older/newer version of extract_api.py's identity-computing algorithm than the other side. Comparing them directly could produce an unsound diff (this is exactly the failure mode that motivated adding the check — see extract_api.py's SURFACE_FORMAT_VERSION docstring). Either regenerate the older side with the current tool, or pass --allow-format-mismatch if you understand the risk and want to proceed anyway.

Contributing

Report bugs or suggest improvements to Discussion #3691. Read POLICY.md first for the definition of public API this tooling enforces.