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legacy-code-summarizer

Summarizes undocumented legacy code by inferring intent from structure, naming, data flow, and calling context — explicitly flagging what's inferred vs. what's certain. Use when onboarding to inherited code, when documentation is missing or wrong, or when deciding whether legacy code is safe to change.

Install with Tessl CLI

npx tessl i github:santosomar/general-secure-coding-agent-skills --skill legacy-code-summarizer
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SKILL.md
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Legacy Code Summarizer

Legacy code has no docs, misleading names, and commit history that says fix. The summary you produce is archaeology — reconstructing intent from artifacts. Be honest about what's deduction vs. what's guesswork.

Differs from → code-summarizer: that one assumes the code is readable. This one assumes it isn't.

Evidence sources — in order of reliability

SourceReliabilityWhat it tells you
What the code does (dataflow)HighActual behavior. Can't lie.
How it's called (grep callers)HighWhat callers pass in, what they do with the result
Tests (if any)HighSomeone wrote down an expectation
Variable/function namesMediumMight be right. Might be copy-pasted from elsewhere.
CommentsLowOften stale. Trust the code over the comment.
Commit messagesLow"fix bug", "wip", "asdf"

Step 1 — Trace the data, not the names

Start from inputs, follow to outputs. Ignore what things are called; watch what happens to them.

int proc_rec(char *buf, int n) {       // name tells you nothing
    int k = 0;
    for (int i = 0; i < n; i++) {
        if (buf[i] == 0x1F) k++;        // 0x1F = ASCII unit separator
    }
    return k + 1;
}

Data trace: counts 0x1F bytes in buf[0..n], returns count + 1. That's "count fields in a unit-separator-delimited record." The +1 is fence-post: N separators → N+1 fields. Now you know what proc_rec means: "process record" → actually "count fields in record."

Step 2 — Look at callers

int nf = proc_rec(line, len);
char **fields = malloc(nf * sizeof(char*));
split_on(line, len, 0x1F, fields);

Caller uses the result to size an allocation for split_on with the same delimiter. Confirms: proc_rec is pre-counting fields so the split can allocate exactly. The abstraction is "two-pass field split: count pass, then split pass."

Step 3 — Mark confidence explicitly

Every claim in the summary gets a confidence tag:

  • [observed] — this is what the code does, period.
  • [inferred] — this is probably why, based on callers/structure.
  • [guess] — name/comment suggests this, unverified.
  • [unknown] — can't tell. Flag for a human who knows the history.

Worked example output

Code: 400 lines of Perl, sub munge_data, no comments, written 2009.

Summary:

## munge_data($ref, $mode)

### Behavior [observed]
- Walks a nested hashref `$ref` in-place.
- In mode 'C': lowercases all string leaf values, trims whitespace.
- In mode 'V': validates leaf values against regexes in the package-global
  %PATTERNS (keyed by parent hash key). Dies on mismatch.
- In any other mode: no-op (falls through the if/elsif chain).
- Mutates `$ref` directly. Returns nothing useful ($ref, but caller ignores it).

### Intent [inferred]
- 'C' = "canonicalize", 'V' = "validate" — based on the operations, not the letters.
- Called in sequence: `munge_data($d, 'C'); munge_data($d, 'V');` in 3 of 4 call
  sites. The 4th calls only 'V' — that input is apparently pre-canonicalized.
- %PATTERNS is populated from `config/field_rules.txt` at module load.

### Hazards [observed]
- Any mode other than 'C'/'V' silently does nothing — no error. Typo in mode
  string = silent skip. [→ seen in git log: commit a3f891 "fix: was passing 'c'
  not 'C'" — this bit someone]
- Deep recursion, no depth limit. Cyclic `$ref` → infinite loop.
- %PATTERNS global — if two threads load different configs, races.

### Unknowns
- [unknown] Why mode 'V' dies instead of returning an error list. Every caller
  wraps it in eval{} — suggests dying was a mistake, worked around.
- [guess] The 4th call site (V-only, in `import_legacy.pl`) might be handling
  data that's already canonical from an upstream system. Or might be a bug.

Archaeology tools

QuestionCommand
Who calls this?rg -w funcname across the repo
When was this line last touched?git blame -w <file>
What did it look like before?git log -p --follow -- <file>
Who wrote it originally?git log --diff-filter=A -- <file> (first commit adding)
Was there ever a comment here?git log -p -L <line>,<line>:<file>
What else changed in the same commit?git show <sha> — context for why

Do not

  • Do not trust names over behavior. validate() might mutate. get_x() might have side effects. Read the body.
  • Do not write a summary that sounds confident about guesses. "This function validates input" [guess] is very different from "[observed]." Tag it.
  • Do not summarize in isolation. The callers are half the story — they tell you which inputs actually occur and what's done with outputs.
  • Do not clean up the code while summarizing. Understand first, change later. Changing what you don't understand is how legacy code gets worse.

Output format

## <function/module name>

### Behavior [observed]
<what it does — traced from code, no interpretation>

### Intent [inferred]
<why — based on callers, structure, git history>

### Hazards [observed]
<footguns, silent failures, global state, thread-unsafety>

### Unknowns
<[unknown] and [guess] items — what a human with context should confirm>

### Evidence
<callers found, git commits consulted, tests referenced>
Repository
santosomar/general-secure-coding-agent-skills
Commit

47d56bb

Last updated
Created

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