Free

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Released
October 1, 2026
Updated
November 8, 2026

Features

Stderr screenshot #1 for iPhone
Stderr screenshot #2 for iPhone
Stderr screenshot #3 for iPhone
Stderr screenshot #4 for iPhone
iphone
iphoneDuo
ipad
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About

Some bugs announce themselves. A compiler refuses, a traceback prints, and at least you know where to look. The expensive ones do not: the program runs, exits zero, and quietly prints the wrong number. Stderr is a catalogue of both kinds. Each entry is a short Java or Python program, the output it actually produced when it was run, and the belief that makes people write it that way in the first place. You do not work through it. You arrive with something you have just seen — a compiler message, an exception, a value that is not what you expected — and you find it. Paste the error exactly as your terminal printed it; search runs over the captured output itself, not over lesson titles. The entry opens on the output, because the output is the reason you came. Underneath it is the program that produced it, then the beliefs that make the wrong answer look right — "you divided the way a calculator would" — then what actually happens and why. Nothing here is asserted. Every program was compiled and run on real Java and Python installations before the app shipped, and the printed lines are what those runs produced, character for character. The authoring format has no field for a hand-written output, so a plausible-looking wrong one cannot be introduced by mistake. It covers Java and Python today, on iPhone, iPad and Mac. Entries are grouped by what happened to you — it refused to run, or it ran and lied — rather than by chapter. Free. No accounts, no advertising, no analytics, no tracking, and no networking code at all, so it works the same with the network off.
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What's New in Stderr

1.0.1

November 8, 2026

=== iPhone and iPad (live: 1.0, build 3) === Stderr is ready for iPhone Duo, and reads better at the largest text sizes: • In decoded errors and on entries, class and file names no longer pick up hyphens that were never in them, such as "NullPointer-Exception" or "Settings.ja-va:42". • Labels that sat side by side now stack when the text is large, so none is squeezed into a few letters: the samples on Decode, javac's details, the tags on each call, Copy on a program and Clear history in Saved. • Rows that point to the Outputs wall show their whole line and reason at those sizes. • In Saved, a long error name now fits its line at the standard text sizes instead of breaking in the middle. As before: no account, no tracking, no networking code. What you paste never leaves your device. === Mac (live: 1.0, build 2, which has only the Outputs wall) === Stderr 1.0.1 brings the Mac everything the iPhone and iPad version gained: • Decode: paste a Python traceback, a Python syntax error, a Java stack trace or javac output, and see which error it is, what it means, where it happened, how the program got there and what usually fixes it. • Errors: a reference of 72 common Java and Python errors, each with its causes, its fixes and a real program with the output it printed. • Saved and Recently viewed, to find entries again. • Share on every entry, and Share and Copy on a decoded error. • Entries can be found with Spotlight. • On Outputs, nearly every entry that ended in an error links to that error in the reference. As before: no account, no tracking, no networking code. What you paste never leaves your Mac.

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