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Kill the mutant

You will learn

How mutation testing plants a bug on purpose and asks the tests to notice.

Mutation testing inverts the whole game: instead of asking whether the tests pass, break the design on purpose and ask whether the tests notice. The widget is the tool doing it to a bitstream path -- plant a bug, watch the tests catch it -- and the lesson's spec is the command that owns the planting. A test suite that passes everything and catches no mutant is a suite that cannot fail, and a suite that cannot fail is not protecting anything.

Try it

Plant a bug with the mutate tool and watch which test catches it; then read the spec and find where the planted fault is named.

Open the interactive lesson →

tri x7-mutate · plant a bug, the tests must catch it
tri x7-mutate · plant a bug, the tests must catch it ↗

Five planted bugs in packets.t27 (CRC polynomial bit, CRC address width, LFRM no-ops, COR0 bit 0, type-2 word count). Each mutant is rebuilt and re-swept; every one is CAUGHT by its tests and by the sweeps. mutate: all 5 caught.

specs/tools/trios/tri/x7-mutate.t27

// SPDX-License-Identifier: Apache-2.0
; specs/tools/trios/tri/x7-mutate.t27 -- tool gHashTag/BrowserOS:tri/x7-mutate, the `tri x7-mutate` command of the trios loop CLI
; Generated by apps/website/scripts/tools-from-trios-tri.mjs from gHashTag/BrowserOS:trios/bin/tri at 7366096248df; do not edit.
; The CLI the loop timers run (`tri drift` holds ~/.local/bin/tri to the tracked copy). It is another program than
; the Rust tri of gHashTag/t27 and the Zig tri of gHashTag/trinity, so the ID is repository-qualified.
; A card is data and carries no test block. ASCII only (L3). phi^2 + 1/phi^2 = 3 | TRINITY

module tool_trios_tri_x7_mutate;

pub const KIND : str = "tool";
pub const FAMILY : str = "tri-cli";
pub const ID : str = "gHashTag/BrowserOS:tri/x7-mutate";
pub const REPO : str = "gHashTag/BrowserOS";
pub const QUALIFIED_ID : str = "gHashTag/BrowserOS:tri/x7-mutate";
pub const SCHEMA : u32 = 2;
pub const COMMAND : str = "tri x7-mutate";
; The case arm of the dispatcher and the first line of it that does the work.
pub const VARIANT : str = "case arm `x7-mutate)`, line 1900";
pub const SOURCE : str = "trios/bin/tri";
pub const ENTRY : str = "trios/bin/tri";
pub const SOURCE_COMMIT : str = "7366096248dfb09893e9a7d1a0f1c56ced13d16e";
pub const ROUTED : bool = true;
pub const DISPATCH : str = "exec python3 \"$HOME/skills/xilinx7-bitstream-loop/scripts/x7.py\" mutate \"$@\"";
pub const DOCUMENTED : bool = true;
pub const HELP_LINE : str = "tri x7-mutate [SPEC] -- mutation catalogue: every defect must fail the spec tests AND the Vivado sweep";
pub const CATEGORY : str = "xilinx7 MVP loop (bitstream back half as .t27, skill xilinx7-bitstream-loop)";
pub const ABOUT : str = "mutation catalogue: every defect must fail the spec tests AND the Vivado sweep";
pub const ABOUT_SOURCE : str = "`tri help` (the heredoc under the help arm of trios/bin/tri)";
pub const ACTIONS : [0]str = [];
pub const ACTIONS_ABOUT : [0]str = [];
pub const ARGS : [1]str = ["[SPEC]"];
pub const AGENTS : [0]str = [];
pub const AGENTS_NOTE : str = "No source binds an agent letter to this command: the trios CLI is not named by docs/agents/AGENTS_ALPHABET.md or .claude/agents/*.md of gHashTag/t27.";
pub const WHEN_TO_USE : str = "mutation catalogue: every defect must fail the spec tests AND the Vivado sweep";
; public/term/tri-x7-mutate/meta.json lists 1 run(s) of `tri x7-mutate`; recorded with the live ~/.local/bin/tri,
; not a build of SOURCE_COMMIT. The site plays it at the end of this card.
pub const CAST : str = "term/tri-x7-mutate/session.cast";
pub const WITNESS : str = "source-parse";
pub const WITNESS_SOURCE : str = "gHashTag/BrowserOS:trios/bin/tri at 7366096248dfb09893e9a7d1a0f1c56ced13d16e, read as text (the help heredoc and the top-level case arms); the CLI was not run";
pub const ENABLED : bool = true;

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