@pipeworx/edge-case-museum

Connect: https://pipeworx.io/mcp — every tool in the catalog, including @pipeworx/edge-case-museum’s. Install: one-click buttons

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Tools: 4

Your code works. Except for these wonderfully terrible inputs.

By Mojibake, creators of Pipeworx.

Tell it what you are building (“username uniqueness”, “parse a CSV upload”, “store and add up money amounts”, “schedule a reminder in the user’s timezone”) and it hands back the inputs that quietly break that kind of code: the assumption each one breaks, the exact bytes, what goes wrong, why, the safer pattern, and a repro in Python and JavaScript whose output came from actually running it. Then it writes you a test file of those inputs, pointed at your own function.

Three exhibits

Every output below was printed by running the code, by the same harness that checks the whole collection (Python 3.14.2, Node.js v22.22.2).

1. One NaN can leave the rest of your list unsorted

edge_case_get({ id: "nan-in-sort" })

nan = float("nan")
print(sorted([3, nan, 1]))

[3, nan, 1]

console.log([3, NaN, 1].sort((a, b) => a - b));

[ 3, NaN, 1 ]

Not “NaN ends up somewhere odd”. The 3 and the 1 are left out of order. Every comparison with NaN is false, so the sort sees NaN as equal to everything. That breaks transitivity (3 “equals” NaN, NaN “equals” 1, so 3 “equals” 1), and the algorithm concludes the list is already sorted. Python promises nothing for inconsistent comparisons. ECMAScript makes the order implementation-defined.

2. “01/01/68” is 2068 in Python and 1968 in Node.js

edge_case_get({ id: "two-digit-year-century-pivot-mismatch" })

for s in ("01/01/49", "01/01/50", "01/01/68", "01/01/69"):
    print(s, "->", datetime.strptime(s, "%m/%d/%y").year)

01/01/49 -> 2049
01/01/50 -> 2050
01/01/68 -> 2068
01/01/69 -> 1969

for (const s of ['01/01/49', '01/01/50', '01/01/68', '01/01/69'])
  console.log(s, '->', new Date(s).getFullYear());

01/01/49 -> 2049
01/01/50 -> 1950
01/01/68 -> 1968
01/01/69 -> 1969

Python follows the POSIX pivot at 69. V8 pivots at 50. The two agree on every two-digit year except 50 through 68, where they are a century apart. Test a handful of dates and you will probably never hit one.

3. Intl says the Iraqi dinar has no decimals. ISO 4217 says it has three.

edge_cases_find({ context: "store and add up money amounts", language: "javascript" })

for (const c of ["USD", "JPY", "BHD", "KWD", "CLF", "IQD", "HUF"]) {
  const digits = new Intl.NumberFormat("en-US", { style: "currency", currency: c })
    .resolvedOptions().maximumFractionDigits;
  console.log(c, digits);
}

USD 2
JPY 0
BHD 3
KWD 3
CLF 4
IQD 0
HUF 0

The first lesson is that amount * 100 is wrong for JPY (a 100x overcharge) and for BHD (a 10x undercharge). The second is quieter: the obvious fix, asking Intl how many decimals a currency has, returns CLDR display data, not ISO 4217. IQD is 3 in ISO 4217 and HUF is 2, and Intl reports 0 for both. Store the exponent your payment processor uses. Don’t derive it from a formatter.

And then it writes the test

edge_cases_test_file({ context: "parse a CSV upload", language: "python" }) returns a pytest file of eight cases from seven CSV exhibits. Pointed at the parser most of us write first, list(csv.reader(io.StringIO(text))):

PASSED test_edge_case[csv-formula-injection-excel]
PASSED test_edge_case[csv-crlf-trailing-blank-line0]
PASSED test_edge_case[csv-crlf-trailing-blank-line1]
PASSED test_edge_case[csv-quoted-field-with-newline]
PASSED test_edge_case[csv-doubled-quote-escaping]
PASSED test_edge_case[csv-quoted-field-with-comma]
FAILED test_edge_case[csv-utf8-bom-first-header]
FAILED test_edge_case[csv-semicolon-delimited-european-excel]
2 failed, 6 passed

Two real bugs: the first header comes back as "name" from a file saved as Excel’s “CSV UTF-8”, and a semicolon-delimited export from a decimal-comma locale parses as one column. Each failure names the exhibit that explains it.

What is in it

122 exhibits, every one with at least one repro that was executed:

Tools

• edge_cases_find({ context, language?, limit? }) — what you are implementing, in plain words, to the exhibits that matter for it. Each comes with the repro for your language and a refine_tags list to narrow down.
• edge_case_get({ id }) — one exhibit in full: every repro with its captured output and runtime version, the why, the fix, related exhibits, sources.
• edge_cases_browse({ category?, tag? }) — the collection by category (unicode, datetime, float-money, sorting, filenames, parsing, integers, names-addresses) or by tag (username, csv, money, timezone, …).
• edge_cases_test_file({ context, language }) — one runnable pytest or node:test file bundling the relevant inputs as cases. It runs as generated against a stub; replace the stub (marked TODO) with your function and the failures tell you which exhibit to read.

Install

Remote (no setup)

{
  "mcpServers": {
    "edge-case-museum": {
      "url": "https://gateway.pipeworx.io/edge-case-museum/mcp"
    }
  }
}

This endpoint also carries the shared Pipeworx tools (ask_pipeworx, discover_tools, and friends).

Local

{
  "mcpServers": {
    "edge-case-museum": {
      "command": "npx",
      "args": ["-y", "@pipeworx/mcp-edge-case-museum"]
    }
  }
}

The whole collection ships inside the package. No network calls, no account.

Auth

Keyless.

Why trust it

• No output is typed from memory. Every Python and JavaScript repro is executed by a committed harness, and its expected_output is what the run printed. verified_with names the runtime (for example Python 3.14.2, Node.js v22.22.2), because some of these behaviours change between versions and you should know which one we saw.
• The harness can fail, and has. Change one character of an expected output and the run goes red with the diff. A repro whose code or output was edited without re-running it cannot be baked into the pack at all: the bake checks a hash the harness recorded.
• Stacks we did not run are labelled. Postgres, Windows and macOS filesystem behaviour appear as documented, with the source that says so, never as verified.
• Every exhibit cites its spec or docs. Unicode annexes, RFC sections, the ECMAScript spec, IEEE 754 references, ISO 4217, IANA tz, the language docs. Written in our own words.

Data sources

• Exhibits written for this collection by Mojibake, each citing the standard or documentation it rests on.
• Some inputs come from the Big List of Naughty Strings by Max Woolf (MIT licence); exhibits that use one say so in their sources.
• No real person’s name, address or contact detail appears anywhere. The names-and-addresses exhibits use invented examples and state facts about naming and addressing systems.

Contribute an exhibit

Add one file, src/exhibits/CATEGORY/ID.json (copy any existing exhibit for the shape), with your repros’ code but no expected_output. Run node src/harness.mjs --only ID --capture: it executes each repro, writes the real output into your file, and records it. Read what it captured; if the output does not say what your explanation says, the explanation is wrong. Then node src/bake.mjs and open a pull request. An exhibit whose repro has not been run by the harness will not bake.

Tools

• edge_cases_find — Edge Case Museum: describe what you are implementing in plain words (“username uniqueness”, “parse a CSV upload”, “store and add up money amounts”, “schedule a reminder in the user’s timezone”, “sort
• edge_case_get — Edge Case Museum: one exhibit in full by id (from edge_cases_find or edge_cases_browse): the assumption it breaks, the exact input, what goes wrong and why, every repro (executed Python/Node.js repros
• edge_cases_browse — Edge Case Museum: browse the collection. With no arguments, lists the categories (unicode, datetime, float-money, sorting, filenames, parsing, integers, names-addresses) with counts and their most com
• edge_cases_test_file — Edge Case Museum: one runnable test file (pytest for python, node:test for javascript) bundling the inputs from the exhibits relevant to what you are implementing, as parametrized cases you point at y