vibe-fuzz-parser-inputs

Every parser will eventually see input you didn't expect. Fuzz testing finds the crashes before production does.

When to Use This Skill

• Implementing any parser (YAML, JSON, XML, config, DSL)
• Processing user-supplied input
• Handling webhook payloads or API responses
• Parsing file formats

When NOT to Use This Skill

• The parser is a well-tested standard library (e.g., encoding/json)
• You're only reading known, controlled input
• The parser is trivial (e.g., splitting a string by comma)

Steps

Go Fuzz Tests

1. Create fuzz test file (parser_fuzz_test.go):

   func FuzzParseConfig(f *testing.F) {
       // Seed corpus from existing test fixtures
       files, _ := filepath.Glob("testdata/*.yaml")
       for _, file := range files {
           data, _ := os.ReadFile(file)
           f.Add(data)
       }
   
       // Add targeted seeds
       f.Add([]byte(""))           // empty
       f.Add([]byte("{}"))         // minimal valid
       f.Add([]byte("\x00\x00"))   // binary
   
       f.Fuzz(func(t *testing.T, data []byte) {
           // Should never panic
           result, err := ParseConfig(data)
           if err != nil {
               return // errors are fine
           }
           // If no error, result should be valid
           if result.Name == "" {
               t.Error("parsed successfully but Name is empty")
           }
       })
   }
   

2. Seed the corpus from:

   • Existing test fixtures
   • Real production examples
   • Known edge cases
   • Minimally valid inputs
   • Binary/garbage data

3. Run initial fuzz:

   go test -fuzz=FuzzParseConfig -fuzztime=30s
   

4. Record results:

   • Crashes found
   • New corpus entries generated
   • Edge cases discovered

5. Fix crashes -- Every panic or unexpected behavior becomes a permanent test case

Other Languages

• JavaScript/TypeScript: Use jest-fuzz or fast-check property-based testing
• Python: Use hypothesis for property-based testing
• Rust: Use cargo-fuzz with libfuzzer

Output Format

Fuzz Test: [Parser Name]

Seeds: X (Y from fixtures, Z manual) Duration: [fuzz time] Corpus growth: X -> Y entries (Z% expansion) Crashes found: N

#	Input (hex)	Crash Type	Fix
1	\x00\xff...	nil panic in tokenizer	Added nil check at line 42

New Edge Cases Discovered

1. [Description] -- added as permanent test case