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json-semantic-diff

Semantic similarity scoring for JSON structures — not just whether they differ, but how similar they are.

Python 3.11+ License: MIT Tests Type Checked

Overview

json-semantic-diff compares two JSON documents and returns a normalised similarity score in [0.0, 1.0], along with an audit trail of which keys matched, which were renamed, and which are missing. It handles naming convention differences (camelCase, snake_case, PascalCase, kebab-case) transparently.

Traditional JSON diff tools are binary: match or no match. json-semantic-diff tells you how similar two documents are, which is what you actually need when testing LLM outputs, validating API migrations, or measuring generator stability.

Core algorithm: STED (Semantic Tree Edit Distance) — tree edit distance extended with semantic key matching via the Hungarian algorithm and per-level normalisation.

Key Features

  • Similarity scoring — Normalised float in [0.0, 1.0] instead of binary match/no-match
  • Naming convention tolerance — user_name, userName, UserName, user-name all score 1.0 against each other
  • Rich audit trail — Key mappings, matched pairs, unmatched paths, computation time
  • Generator consistency — Single-number stability metric for LLM outputs
  • Three backends — Zero-dependency Levenshtein, local ONNX embeddings, or OpenAI cloud
  • Evaluation platform adapters — pytest, LangSmith, Braintrust, W&B Weave
  • Configurable — Structural/content weights, array comparison modes, type coercion, null handling
  • Zero global state — Each comparator instance owns its own cache; thread-safe by design

Installation

pip install json-semantic-diff

With optional backends:

pip install json-semantic-diff[fastembed]   # Local ONNX embeddings (384-dim)
pip install json-semantic-diff[openai]      # OpenAI cloud embeddings (1536-dim)

With evaluation platform adapters:

pip install json-semantic-diff[langsmith]
pip install json-semantic-diff[braintrust]
pip install json-semantic-diff[weave]

Quick Start

Compare two JSON documents

from json_semantic_diff import compare

result = compare(
    {"user_name": "Alice", "email_address": "alice@corp.com"},
    {"userName": "Alice", "emailAddress": "alice@corp.com"},
)

print(result.similarity_score)   # 0.97
print(result.key_mappings)       # {"user_name": "userName", "email_address": "emailAddress"}
print(result.unmatched_left)     # []
print(result.unmatched_right)    # []
print(result.computation_time_ms)  # 0.5

Quick similarity score

from json_semantic_diff import similarity_score

score = similarity_score(
    {"first_name": "Bob", "last_name": "Smith"},
    {"firstName": "Bob", "lastName": "Smith"},
)
print(score)  # 0.97

Boolean equivalence check

from json_semantic_diff import is_equivalent

# Passes — naming convention difference only
is_equivalent({"user_name": "Alice"}, {"userName": "Alice"})  # True

# Fails — different structure entirely
is_equivalent({"name": "Alice"}, {"product": "Widget"})  # False

# Custom threshold
is_equivalent({"user_name": "Alice"}, {"userName": "Alice"}, threshold=0.99)

Measure generator consistency

from json_semantic_diff import consistency_score

# Stable generator → 1.0
docs = [
    {"name": "Alice", "age": 30},
    {"name": "Alice", "age": 30},
    {"name": "Alice", "age": 30},
]
print(consistency_score(docs))  # 1.0

# Erratic generator → low score
erratic = [
    {"name": "Alice", "age": 30},
    {"fullName": "Alice", "years": 30},
    {"person": "Alice"},
]
print(consistency_score(erratic))  # < 0.4

API Reference

Public Functions

Function Signature Returns
compare(left, right, config=None) Two JSON values + optional config ComparisonResult
similarity_score(left, right, config=None) Two JSON values + optional config float in [0.0, 1.0]
is_equivalent(left, right, threshold=0.85, config=None) Two JSON values + threshold + config bool
consistency_score(docs, config=None) List of JSON values + optional config float in [0.0, 1.0]

ComparisonResult

Returned by compare(). Frozen dataclass with six fields:

Field Type Description
similarity_score float Normalised similarity in [0.0, 1.0]. 1.0 = identical.
matched_pairs list[tuple[str, str]] JSON Pointer pairs for matched KEY nodes
key_mappings dict[str, str] Raw left key name → raw right key name
unmatched_left list[str] JSON Pointer paths with no match in right document
unmatched_right list[str] JSON Pointer paths with no match in left document
computation_time_ms float Wall-clock duration in milliseconds

STEDConfig

Immutable configuration for the algorithm. All parameters have sensible defaults.

from json_semantic_diff import STEDConfig, ArrayComparisonMode

config = STEDConfig(
    w_s=0.5,                                          # Structural weight [0, 1]
    w_c=0.5,                                          # Content weight [0, 1] (must sum to 1.0)
    lambda_unmatched=0.1,                              # Penalty for unmatched children (>= 0)
    array_comparison_mode=ArrayComparisonMode.ORDERED,  # ORDERED | UNORDERED | AUTO
    type_coercion=False,                               # "42" == 42?
    null_equals_missing=False,                         # {x: null} == {}?
)

result = compare(doc1, doc2, config=config)
Parameter Type Default Description
w_s float 0.5 Structural weight. Higher → structure matters more.
w_c float 0.5 Content weight. Higher → values matter more. Must sum to 1.0 with w_s.
lambda_unmatched float 0.1 Penalty per unmatched child. 0.0 = ignore extras. 1.0 = full penalty.
array_comparison_mode ArrayComparisonMode ORDERED How arrays are compared.
type_coercion bool False When True, "42" and 42 compare as equal.
null_equals_missing bool False When True, {"x": null} and {} compare as equal.

Array Comparison Modes

Mode Strategy Use case
ORDERED Positional DP alignment Sequences, logs, ordered lists
UNORDERED Hungarian matching Tags, feature flags, sets
AUTO Infer from content (scalars → unordered, objects → ordered) When array semantics vary

Backends

Three embedding backends for key matching, each optimising for different constraints:

Backend Dependencies Latency Key matching quality Cost
StaticBackend (default) None ~0.1ms Naming conventions Free
FastEmbedBackend fastembed ~1-2s cold start Semantic understanding Free
OpenAIBackend openai, tenacity ~100-500ms Best quality API costs

StaticBackend (default)

Uses Levenshtein edit distance on normalised keys. No ML model, no API calls.

from json_semantic_diff.backends import StaticBackend

backend = StaticBackend()
backend.similarity("user_name", "userName")   # 1.0
backend.similarity("user_name", "address")    # < 0.5

FastEmbedBackend

Local ONNX embeddings via sentence-transformers/all-MiniLM-L6-v2 (384-dim).

from json_semantic_diff.backends.fastembed import FastEmbedBackend
from json_semantic_diff.comparator import STEDComparator

backend = FastEmbedBackend()  # ~1-2s cold start for model loading
cmp = STEDComparator(backend=backend)
result = cmp.compare(doc1, doc2)

OpenAIBackend

Cloud embeddings via text-embedding-3-small (1536-dim). API key from OPENAI_API_KEY environment variable.

from json_semantic_diff.backends.openai import OpenAIBackend
from json_semantic_diff.comparator import STEDComparator

backend = OpenAIBackend()  # Reads OPENAI_API_KEY from env
cmp = STEDComparator(backend=backend)
result = cmp.compare(doc1, doc2)

Auto-retries rate-limited requests with jittered exponential backoff (up to 6 attempts via tenacity).

Using STEDComparator directly

For batch comparisons where you want cache reuse across calls:

from json_semantic_diff.comparator import STEDComparator

cmp = STEDComparator()  # Or with backend= and config=
for doc in documents:
    result = cmp.compare(doc, reference)
    # Embedding cache reused across all calls

Integrations

pytest

Auto-discovered fixture — install the package and it's available in every test:

def test_api_response(assert_json_equivalent):
    actual = get_user_from_api()
    expected = {"user_name": "Alice", "age": 30}

    # Passes even if API returns {"userName": "Alice", "age": 30}
    assert_json_equivalent(actual, expected)

Custom threshold and config:

from json_semantic_diff import STEDConfig

def test_strict(assert_json_equivalent):
    config = STEDConfig(type_coercion=True)
    assert_json_equivalent(actual, expected, threshold=0.95, config=config)

Failure messages include full context:

AssertionError: JSON documents not equivalent:
  similarity=0.42 < threshold=0.85
  actual:   {"product": "Widget", "price": 19.99}
  expected: {"user_name": "Alice", "age": 30}
  key_mappings: {"product": "user_name"}
  unmatched_left: ["/price"]
  unmatched_right: ["/age"]

LangSmith

from json_semantic_diff.comparator import STEDComparator
from json_semantic_diff.integrations import LangSmithEvaluator

comparator = STEDComparator()
evaluator = LangSmithEvaluator(comparator, output_key="response")
# Pass directly to langsmith.evaluate()

Braintrust

from json_semantic_diff.comparator import STEDComparator
from json_semantic_diff.integrations import BraintrustScorer

comparator = STEDComparator()
scorer = BraintrustScorer(comparator)
# Returns float in [0.0, 1.0] or None when no expected value

W&B Weave

from json_semantic_diff.comparator import STEDComparator
from json_semantic_diff.integrations import WeaveScorer

comparator = STEDComparator()
scorer = WeaveScorer(comparator)
# Returns {"semantic_similarity": float}

How It Works

The STED Algorithm

  1. JSON → Typed Tree. Each JSON value becomes a tree node with a type (OBJECT, KEY, ARRAY, ELEMENT, SCALAR). KEY nodes store both raw and normalised labels.

  2. Key Normalisation. A five-pass regex pipeline converts all naming conventions to a canonical form: userName → "user name", user_name → "user name", UserName → "user name", user-name → "user name".

  3. Hungarian Key Matching. For each pair of OBJECT nodes, a cost matrix is built using key similarity scores. The Hungarian algorithm finds the optimal key-to-key assignment.

  4. Blended Cost. Each node comparison blends structural and content distance: cost = w_s * structural_distance + w_c * content_distance.

  5. Per-Level Normalisation. At each tree level: similarity = 1.0 - min(1.0, [d_matched + λ·|n_left - n_right|] / max(n_left, n_right, 1)). This keeps scores in [0.0, 1.0] regardless of document depth.

Consistency Score Formula

pairwise_scores = [compare(docs[i], docs[j]).similarity_score for all unique (i, j) pairs]
consistency = max(0, mean(pairwise_scores) - std(pairwise_scores))

Penalises both low average similarity (different outputs) and high variance (erratic outputs).

Embedding Cache

All unique KEY labels are collected before the algorithm runs and embedded in a single batch call. The algorithm then runs entirely from cache — zero additional backend calls per comparison.

Each STEDComparator instance owns its own cache (no global state). The public API functions create a fresh comparator per call for isolation. Use STEDComparator directly for batch comparisons where cache reuse matters.

Development

git clone https://github.com/mokhld/json-semantic-diff.git
cd json-semantic-diff
poetry install --with dev

Running tests

poetry run pytest                    # All 661 tests
poetry run pytest -x                 # Stop on first failure
poetry run pytest tests/unit/        # Unit tests only
poetry run pytest tests/algorithm/   # Algorithm tests only

Type checking

poetry run mypy src/          # Strict mode

Linting

poetry run ruff check src/ tests/
poetry run ruff format src/ tests/

Project structure

src/json_semantic_diff/
├── __init__.py              # Public API exports
├── api.py                   # 4 public functions
├── comparator.py            # STEDComparator orchestrator
├── scorer.py                # ConsistencyScorer
├── result.py                # ComparisonResult dataclass
├── protocols.py             # EmbeddingBackend Protocol
├── cache.py                 # EmbeddingCache (LRU)
├── algorithm/
│   ├── config.py            # STEDConfig + ArrayComparisonMode
│   ├── sted.py              # STEDAlgorithm (recursive core)
│   ├── costs.py             # Insert/delete/update cost functions
│   ├── matcher.py           # Hungarian algorithm wrapper
│   └── normalizer.py        # Per-level similarity normalisation
├── backends/
│   ├── static.py            # Levenshtein (zero dependencies)
│   ├── fastembed.py         # ONNX (local embeddings)
│   └── openai.py            # OpenAI (cloud embeddings)
├── tree/
│   ├── nodes.py             # TreeNode + NodeType
│   ├── builder.py           # JSON → Tree conversion
│   └── normalizer.py        # Key normalisation pipeline
└── integrations/
    ├── _pytest_plugin.py    # pytest fixture (auto-discovered)
    ├── _langsmith.py        # LangSmith evaluator adapter
    ├── _braintrust.py       # Braintrust scorer adapter
    └── _weave.py            # W&B Weave scorer adapter

License

MIT

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v0.1.0 Latest
Feb 24, 2026

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