Crap Score

---
name: crap-score
description: >
  Calculates CRAP (Change Risk Anti-Patterns) for a named .NET method, class, or
  file. USE FOR: explicit CRAP calculation or coverage-and-complexity risk
  within that named target, including which tests to prioritize. DO NOT USE
  FOR: project-wide coverage/CRAP, plateaus, or project-wide blockers/priorities
  (coverage-analysis); behavioral/pseudo-mutation gaps (test-gap-analysis);
  writing tests; test runs without CRAP context.
license: MIT
---

# CRAP Score Analysis

Calculate CRAP (Change Risk Anti-Patterns) scores for .NET methods to identify code that is both complex and undertested.

## Background

The CRAP score combines **cyclomatic complexity** and **code coverage** into a single metric:

$$\text{CRAP}(m) = \text{comp}(m)^2 \times (1 - \text{cov}(m))^3 + \text{comp}(m)$$

Where:

- $\text{comp}(m)$ = cyclomatic complexity of method $m$
- $\text{cov}(m)$ = code coverage ratio (0.0 to 1.0) of method $m$

| CRAP Score | Risk Level | Interpretation |
|------------|------------|----------------|
| < 5        | Low        | Simple and well-tested |
| 5 to < 15  | Moderate   | Acceptable for most code |
| 15 to 30   | High       | Needs more tests or simplification |
| > 30       | Critical   | Refactor and add coverage urgently |

A method with 100% coverage has CRAP = complexity (the minimum). A method with 0% coverage has CRAP = complexity^2 + complexity.

## When to Use

- User wants to assess which methods are risky due to low coverage and high complexity
- User asks for CRAP score of specific methods, classes, or files
- User wants to prioritize what to test next within a named method, class, or file based on coverage-and-complexity risk
- User wants to evaluate test quality beyond simple coverage percentages

## When Not to Use

- User just wants to run tests (use `run-tests` skill)
- User wants to write new tests (use `code-testing-agent`)
- User only wants a coverage percentage without complexity analysis
- User wants project-wide coverage/CRAP analysis or priorities (use `coverage-analysis`)

## Inputs

| Input | Required | Description |
|-------|----------|-------------|
| Target scope | Yes | Method name, class name, or file path to analyze |
| Test project path | No | Path to the test project. Defaults to discovering test projects in the solution. |
| Source project path | No | Path to the source project under analysis |

## Workflow

### Step 1: Collect code coverage data

If no coverage data exists yet, classify the test project first. For SDK-style
projects, run `dotnet test` with coverage collection. For classic non-SDK
projects (`ToolsVersion`, explicit compile items, or `packages.config`), use only
a repository-provided coverage command that emits Cobertura. If none exists,
ask for Cobertura XML and stop; do not migrate the project or inject an SDK-style
coverage package. CRAP scores always require real coverage data.

Check the test project's `.csproj` for the coverage package, then run the appropriate command:

| Coverage Package | Command | Output Location |
|---|---|---|
| `coverlet.collector` | `dotnet test --collect:"XPlat Code Coverage" --results-directory ./TestResults` | Typically under `TestResults/<guid>/coverage.cobertura.xml`. Search recursively under the results directory (for example, `TestResults/**/coverage.cobertura.xml`) or use any explicit coverage path the user provides. |
| `Microsoft.Testing.Extensions.CodeCoverage` (.NET 9) | `dotnet test -- --coverage --coverage-output-format cobertura --coverage-output ./TestResults` | `--coverage-output` path |
| `Microsoft.Testing.Extensions.CodeCoverage` (.NET 10+) | `dotnet test --coverage --coverage-output-format cobertura --coverage-output ./TestResults` | `--coverage-output` path |

#### Never estimate coverage

**Guessed coverage produces wrong CRAP scores, which is worse than no answer.**
For a classic project with no repository coverage command or existing report,
stop here and request Cobertura; do not use any collection fallback below.

For SDK-style projects, if the first command yields no Cobertura XML, work down
this collection list before giving up:

1. For SDK-style projects only, add a provider if none is referenced:
   `dotnet add <test.csproj> package coverlet.collector`, then re-run. Never use
   this fallback for `packages.config` or classic non-SDK projects.
2. Use the standalone collector, which works even when the test host or a shared assembly blocks the in-proc collector:
   `dotnet tool install --global dotnet-coverage` then
   `dotnet-coverage collect -f cobertura -o coverage.cobertura.xml "dotnet test <test.csproj>"`.

For any project type, if a real binary `.coverage` report already exists, convert
or summarize that existing data with ReportGenerator:

3. Convert the existing report:
   `dotnet tool install --global dotnet-reportgenerator-globaltool` then
   `reportgenerator -reports:<file> -targetdir:cov -reporttypes:Cobertura`.
4. Tests fail but still run? Coverage is collected from the tests that executed — continue with that data and note the failures.

If every path fails, **report that coverage could not be collected, show the commands you tried and their errors, and stop.** Report complexity on its own if useful, but never publish a CRAP number derived from an assumed coverage percentage.

Before using a report, verify that it parses, contains at least one class and
method, and contains the requested target. An empty report or a report that
omits the target is failed collection or filtering, not 0% coverage. Regenerate
coverage when possible; otherwise stop without publishing a CRAP score.

If the user supplies an existing report, state that it was not regenerated.
Do not describe its data as current unless its provenance is established by
running the repository's coverage command in this analysis.

### Step 2: Compute cyclomatic complexity

Prefer a machine-produced per-method complexity from a repository-provided code
metrics report or from the Cobertura method's `complexity` attribute when that
report maps to the current source. Microsoft.CodeAnalysis.Metrics can generate
method-level `CyclomaticComplexity` data through `msbuild /t:Metrics`, but do
not add the package or modify the project without user approval.

If no machine-produced metric exists, analyze the current target source and
label the result as a manual complexity count. Count the following decision
points (each adds 1 to the base complexity of 1):

| Construct | Example |
|-----------|---------|
| `if` | `if (x > 0)` |
| `else if` | `else if (y < 0)` |
| `case` (each) | `case 1:` |
| `for` | `for (int i = 0; ...)` |
| `foreach` | `foreach (var item in list)` |
| `while` | `while (running)` |
| `do...while` | `do { } while (cond)` |
| `catch` (each) | `catch (Exception ex)` |
| `&&` | `if (a && b)` |
| `\|\|` (OR) | `if (a \|\| b)` |
| `??` | `value ?? fallback` |
| `?.` | `obj?.Method()` |
| `? :` (ternary) | `x > 0 ? a : b` |
| Pattern match arm | `x is > 0 and < 10` |

Base complexity is 1 for every method. Each decision point adds 1.

When counting manually, read the source file, report the construct-by-construct
breakdown, and do not use a source comment as evidence. If the report's
complexity attribute disagrees with the current-source count, report the
conflict and do not present either resulting CRAP score as authoritative.

### Step 3: Extract per-method coverage from Cobertura XML

Parse the Cobertura XML to find each method's `line-rate` attribute under the target `<class>` element. If `line-rate` is not available at method level, compute it from the `<lines>` elements:

$$\text{cov}(m) = \frac{\text{lines with hits} > 0}{\text{total lines}}$$

Method names in Cobertura may differ from source (async methods, lambdas). Match by line ranges when names don't align.

When both `line-rate` and `<lines>` exist, recompute the hit ratio and compare
them. Allow only normal report rounding (one percentage point); if they differ
more, the report contradicts itself. Regenerate it or report the conflict and
stop without calculating CRAP. Never silently choose whichever value produces
the expected score.

### Step 4: Calculate CRAP scores

For each method in scope, apply the formula:

$$\text{CRAP}(m) = \text{comp}(m)^2 \times (1 - \text{cov}(m))^3 + \text{comp}(m)$$

Use a calculator or script for the arithmetic and show the substituted
complexity and coverage. Do not calculate the formula mentally.

### Step 5: Present results

Present a sorted table (highest CRAP first):

```text
| Method                          | Complexity | Coverage | CRAP Score | Risk     |
|---------------------------------|------------|----------|------------|----------|
| OrderService.ProcessOrder       | 10         | 45%      | 26.6       | High     |
| OrderService.ValidateItems      | 8          | 90%      | 8.1        | Moderate |
| OrderService.CalculateTotal     | 3          | 100%     | 3.0        | Low      |
```

Include:

- **Summary**: total methods analyzed, how many in each risk category
- **Top offenders**: methods with CRAP > 30, with specific recommendations
- **Quick wins**: methods with high complexity but where small coverage improvements would drop the score significantly

### Step 6: Provide actionable recommendations

For high-CRAP methods, suggest one or both:

1. **Add tests** -- identify uncovered branches and suggest specific test cases
2. **Reduce complexity** -- suggest extract-method refactoring for deeply nested logic

Calculate the **coverage needed** to bring a method below a CRAP threshold of 15:

$$\text{cov}_{\text{needed}} = 1 - \left(\frac{15 - \text{comp}}{\text{comp}^2}\right)^{1/3}$$

This formula only applies when comp < 15. When comp >= 15, the minimum possible CRAP score (at 100% coverage) is comp itself, which already meets or exceeds the threshold. In that case, **coverage alone cannot bring the CRAP score below the threshold** -- the method must be refactored to reduce its cyclomatic complexity first.

Report this as: "To bring `ProcessOrder` (complexity 10) below CRAP 15, increase coverage from 45% to more than 63.2% (at least 64% when reporting whole percentages)." For methods where complexity alone exceeds the threshold, report: "`ComplexMethod` (complexity 18) cannot reach CRAP < 15 through testing alone -- reduce complexity by extracting sub-methods."

## Validation

- Verify that coverage data was collected successfully (Cobertura XML exists and contains data)
- Confirm the target method is present; absence is not evidence of 0% coverage
- Confirm every coverage figure came from that XML — no estimated, assumed, or source-comment-derived values
- Cross-check method `line-rate` against its line-hit ratio when both exist
- Cross-check that method names in coverage data match the source code
- Confirm CRAP scores with calculator or script output
- Ensure a 100%-covered method's CRAP equals its complexity exactly

## Common Pitfalls

- **Estimating coverage when collection fails**: never do it — the resulting CRAP scores are wrong in the direction that matters. Work through the fallbacks in Step 1, then report the blocker instead.
- **Treating an empty report or missing method as 0% coverage**: this is failed collection, filtering, or method mapping; do not manufacture a score.
- **Trusting contradictory Cobertura fields**: compare `line-rate` with the line-hit ratio and stop if they disagree beyond rounding.
- **Trusting a stale complexity comment in the source**: compute cyclomatic complexity from the current code; a `// complexity: 7` comment left by a previous author is not evidence.
- **Mental CRAP arithmetic**: use a calculator or script and show the substituted inputs.
- **Giving up on a shared-assembly or test-host collector error**: `dotnet-coverage collect` runs out of process and usually succeeds where the in-proc collector fails.
- **Stale coverage data**: regenerate when the user asks for current results or the source/binaries changed; otherwise disclose that a supplied report was not regenerated.
- **Method name mismatches**: Cobertura XML may use mangled/compiler-generated names for async methods, lambdas, or local functions. Match by line ranges when names don't align.
- **Generated code**: Exclude auto-generated files (e.g., `*.Designer.cs`, `*.g.cs`) from analysis unless explicitly requested.