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What the failing test actually compared
Wang describes an integration test that passed locally and later failed in CI. Its assertion compared response.text with a fixed expected string. The response contained the same JSON data, but its object members appeared in a different order.
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The example implementation built a dictionary by iterating over set(fields). A set does not preserve the caller’s declared sequence, so converting the fields to a set discarded that intended order. The article shows several observed key permutations as an illustration of the failure; they are not a benchmark or a claim about how often any particular permutation occurs.
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Why JSON object order usually is not the contract
RFC 8259, the JSON standard published in December 2017, defines an object as an unordered collection of name/value pairs. It distinguishes objects from arrays, which are ordered sequences. A consumer should therefore not depend on the order of object members unless an additional API contract explicitly requires it.
That distinction matters in tests: two JSON strings can differ while representing equivalent objects. But order is not universally irrelevant. Array order can carry meaning, and an API, signature scheme, or downstream consumer may impose a separate serialization requirement. Test those requirements when they exist; do not infer them merely from one observed rendering.
Fix construction when input order is intentional
If the code is meant to retain the sequence in which fields were supplied, iterate over that sequence directly rather than converting it to a set:
for field in fields:
result[field] = make_value(field)This keeps construction aligned with the caller’s declared order. It is appropriate when preserving input order is itself useful or required. It does not make JSON object order a standard guarantee, so tests should still reflect the API’s actual promise rather than incidental serializer behavior.
Test parsed values instead of raw text
When the contract concerns JSON data rather than its exact textual encoding, parse the response and assert on the relevant structure and values. For example, in Python:
import json
actual = json.loads(response.text)
assert actual["status"] == "ok"
assert actual["count"] == 3Choose assertions that match the behavior under test. Focused checks are easier to diagnose than a whole-string comparison: a failure points to a changed value or missing key instead of a formatting difference. If object order is immaterial, do not make it a pass condition.
Normalize only irrelevant variation
Wang’s proposed approach parses the JSON, removes a deliberately varying timestamp, recursively sorts object keys, and compares the resulting values. This can make a structural comparison stable, but every normalization step weakens what the test can detect. If a timestamp is part of the behavior being tested, removing it could hide a regression. Likewise, sorting arrays would be wrong if their sequence is meaningful.
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Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Choose assertions by the contract you need to protect
| Test approach | Best for | What it can miss or overstate |
|---|---|---|
| Raw JSON string comparison | Exact serialized output when byte-for-byte representation is explicitly part of the contract. | Can fail on irrelevant whitespace or object-member ordering and obscure which value changed. |
| Focused assertions on parsed JSON | Checking selected keys, values, and behaviors. | Does not by itself detect every unexpected or missing field. |
| Schema or contract validation | Checking the complete response shape, including required fields and types. | Does not replace focused checks for business behavior or values the schema does not constrain. |
These approaches can complement one another. Use focused assertions for the behavior that matters, and add schema or contract validation when the full request or response shape must remain stable. Reserve exact-string assertions for cases where exact serialization is genuinely promised.
Trace nondeterminism before blaming the model
- Repeat the failing test and confirm whether the output varies across runs or environments.
- Inspect code that changes ordering, especially conversions to unordered collections such as sets.
- Separate object-member order from meaningful array order and from changes to actual values.
- Check whether the assertion compares serialized text or parsed data.
- Review variable fields such as timestamps, and exclude them only if the test is not intended to validate them.
A string diff establishes that two representations differ. By itself, it does not establish that the model, response data, or API behavior changed.
Quick Recap
Sources
- Taylor Wang, “The Model Did Not Change: The Test Was Comparing Strings, Not Behavior”. The page displays “Posted on Aug 19” without a year.
- T. Bray, editor, RFC 8259: The JavaScript Object Notation (JSON) Data Interchange Format, December 2017.
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