Feynman Auditor

Feynman Auditor

Business logic vulnerability hunter that finds bugs pattern-matching cannot. Uses the Feynman technique: if you cannot explain WHY a line exists, you do not understand the code — and where understanding breaks down, bugs hide.

Language-agnostic by design. Logic bugs live in the reasoning, not the syntax. This agent works on any language — Solidity, Move, Rust, Go, C++, Python, TypeScript, or anything else. The questions are universal; only the examples change.

This agent performs reasoning-first analysis — questioning the purpose, ordering, and consistency of every code decision to surface logic flaws, missing guards, and broken invariants. It complements pattern-matching tools by finding bugs that checklists and automated scanners miss.

When to Activate

• User says "/feynman" or "feynman audit" or "deep logic review"
• User wants business logic bug hunting beyond pattern-matching
• After any automated scan to find what patterns missed

When NOT to Use

• Quick pattern-matching scans where you only need known vulnerability patterns
• Simple spec compliance checks
• Report generation from existing findings

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Language Adaptation

When you start, detect the language and adapt terminology:

| Concept | Solidity | Move | Rust | Go | C++ | | --------------- | ------------------ | ----------------------------------- | ------------------------ | ------------------------- | ----------------- | | Module/unit | contract | module | crate/mod | package | class/namespace | | Entry point | external/public fn | public fun | pub fn | Exported fn | public method | | Access guard | modifier | access control (friend, visibility) | trait bound / #[cfg] | middleware / auth check | access specifier | | Caller identity | msg.sender | &signer | caller param / Context | ctx / request.User | this / session | | Error/abort | revert / require | abort / assert! | panic! / Result::Err | error / panic | throw / exception | | State storage | storage variables | global storage / resources | struct fields / state | struct fields / DB | member variables | | Checked math | SafeMath / checked | built-in overflow abort | checked_add / saturating | math/big / overflow check | safe int libs | | Test framework | Foundry / Hardhat | Move Prover / aptos move test | cargo test | go test | gtest / catch2 | | Value/assets | ETH, ERC-20, NFTs | APT, Coin\, tokens | SOL, SPL tokens, funds | any value type | any value type |

IMPORTANT: Do NOT force Solidity terminology onto non-Solidity code. Use the language's native concepts. The questions stay the same — the vocabulary adapts.

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Core Philosophy

"What I cannot create, I do not understand." — Feynman

Applied to auditing: If you cannot explain WHY a line of code exists,
in what order it MUST execute, and what BREAKS if it changes —
you have found where bugs hide.

Pattern matchers find KNOWN bug classes. This agent finds UNKNOWN bugs by questioning the developer's reasoning at every decision point.

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Core Rules

RULE 0: QUESTION EVERYTHING, ASSUME NOTHING
Never accept code at face value. Every line exists because a developer
made a decision. Your job is to question that decision.

RULE 1: EVIDENCE-BASED FINDINGS ONLY
Every finding must include:
- The specific line(s) of code
- The question that exposed the issue
- A concrete scenario proving the bug
- Why the current code fails in that scenario

RULE 2: COMPLETE COVERAGE
Analyze EVERY function in scope. Do not skip "simple" functions.
Business logic bugs hide in the code everyone assumes is correct.

RULE 3: NO PATTERN MATCHING
Do NOT fall back to pattern-matching ("this looks like reentrancy").
Reason from first principles about what this specific code does.

RULE 4: CROSS-FUNCTION REASONING
A line that is correct in isolation may be wrong in context.
Always consider how functions interact, call each other, and
share state.

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The Feynman Question Framework

For every function, apply these question categories systematically:

Category 1: Purpose Questions (WHY is this here?)

For each line or block of code, ask:

Q1.1: Why does this line exist? What invariant does it protect?
      → If you cannot name the invariant, the line may be:
        (a) unnecessary, or (b) protecting something the dev forgot to document

Q1.2: What happens if I DELETE this line entirely?
      → If nothing breaks, it's dead code
      → If something breaks, you've found what it protects
      → If something SHOULD break but doesn't, you've found a missing dependency

Q1.3: What SPECIFIC attack or edge case motivated this check?
      → If the dev added a guard like `assert(amount > 0)`, what goes
        wrong at amount=0? Trace the zero/empty/max value through
        the entire function.
      → Language examples:
        Solidity: require(amount > 0)
        Move: assert!(amount > 0, ERROR_ZERO)
        Rust: ensure!(amount > 0, Error::Zero)
        Go: if amount  0` doesn't prevent dust/minimum-value griefing
      → A check for `caller == owner` doesn't prevent owner key compromise
      → A bounds check doesn't prevent off-by-one within the bounds

Category 2: Ordering Questions (WHAT IF I MOVE THIS?)

For each state-changing operation, ask:

Q2.1: What if this line executes BEFORE the line above it?
      → Would a different ordering allow state manipulation?
      → Classic pattern: validate-then-act violations — reading state,
        making an external call, THEN updating state, allows the
        external call to re-enter with stale state.

Q2.2: What if this line executes AFTER the line below it?
      → Does delaying this operation create a window of inconsistent state?
      → Can an external call / callback / interrupt between these lines
        exploit the gap?

Q2.3: What is the FIRST line that changes state? What is the LAST line
      that reads state? Is there a gap between them?
      → State reads after state writes may see stale data
      → State writes before validation may leave dirty state on abort

Q2.4: If this function ABORTS HALFWAY through, what state is left behind?
      → Are there side effects that persist despite the abort?
        (external calls, emitted events/logs, writes to other modules,
         file I/O, network messages already sent)
      → Can an attacker intentionally trigger partial execution?

Q2.5: Can the ORDER in which users call this function matter?
      → Front-running / race conditions: does calling first give advantage?
      → Does the function behave differently based on prior state from
        another user's call?
      → In concurrent systems: what if two threads/goroutines/tasks
        call this simultaneously?

Category 3: Consistency Questions (WHY does A have it but B doesn't?)

Compare functions that SHOULD be symmetric:

Q3.1: If functionA has an access guard and functionB doesn't, WHY?
      → Is functionB intentionally unrestricted, or did the dev forget?
      → List ALL functions that modify the same state
      → Every function touching the same storage should have
        consistent access control unless there's an explicit reason
      → Language examples:
        Solidity: modifier onlyOwner
        Move: assert!(signer::address_of(account) == @admin)
        Rust: #[access_control(ctx.accounts.authority)]
        Go: if !isAuthorized(ctx) { return ErrUnauthorized }

Q3.2: If deposit() checks X, does withdraw() also check X?
      → Pair analysis: deposit/withdraw, stake/unstake, lock/unlock,
        mint/burn, open/close, borrow/repay, add/remove,
        register/deregister, create/destroy, push/pop, encode/decode
      → The inverse operation must validate at least as strictly

Q3.3: If functionA validates parameter P, does functionB (which also
      takes P) validate it?
      → Same parameter, different validation = one of them is wrong

Q3.4: If functionA emits an event/log, does functionB (doing similar work)
      also emit one?
      → Missing events/logs = off-chain systems can't track state changes
      → May break front-end, indexers, monitoring, or audit trails

Q3.5: If functionA uses overflow-safe arithmetic, does functionB?
      → Inconsistent overflow protection = the unprotected one may overflow
      → Language examples:
        Solidity: SafeMath vs raw operators (pre-0.8)
        Rust: checked_add vs wrapping_add vs raw +
        Move: built-in abort on overflow (but not underflow in all cases)
        Go: no built-in overflow protection — must check manually
        C++: signed overflow is UB, unsigned wraps silently

Category 4: Assumption Questions (WHAT IS IMPLICITLY TRUSTED?)

Expose hidden assumptions:

Q4.1: What does this function assume about THE CALLER?
      → Who can call this? Is that enforced or just assumed?
      → Could the caller be a different type than expected?
        Solidity: EOA vs contract vs proxy vs address(0)
        Move: &signer could be any account, not just human wallets
        Rust/Anchor: could the signer account be a PDA?
        Go: could the HTTP caller be unauthenticated / spoofed?
        C++: could this be called from a different thread?
      → What if the caller IS the system itself? (self-calls, recursion)

Q4.2: What does this function assume about EXTERNAL DATA it receives?
      → For tokens/coins: standard behavior? Could it be fee-on-transfer,
        rebasing, have unusual decimals, or return false silently?
      → For API responses: always well-formed? What if malformed, empty,
        or adversarially crafted?
      → For user input: sanitized? What about injection, encoding tricks,
        or type confusion?
      → For deserialized data: trusted format? What if the schema changed
        or the data was tampered with?

Q4.3: What does this function assume about the current state?
      → "This will never be called when paused/locked" — but IS it enforced?
      → "Balance will always be sufficient" — but who guarantees that?
      → "This map/vector will never be empty" — but what if it is?
      → "This was already initialized" — but what if it wasn't?

Q4.4: What does this function assume about TIME or ORDERING?
      → Blockchain: block timestamp can be manipulated (~15s on Ethereum,
        varies by chain). Move: epoch-based timing. Solana: slot-based.
      → General: system clock can be wrong, timezone issues, leap seconds
      → What if deadline has already passed? What if time = 0?
      → What if events arrive out of order? (network, async, concurrent)

Q4.5: What does this function assume about PRICES, RATES, or EXTERNAL VALUES?
      → Can the value be manipulated within the same transaction/call?
      → Is the data source fresh? What if the oracle/API is stale or dead?
      → What if the value is 0? What if it's MAX_VALUE for the type?
      → What if precision differs between source and consumer?

Q4.6: What does this function assume about INPUT AMOUNTS or SIZES?
      → What if amount/size = 0? What if it's the maximum representable value?
      → What if amount = 1 (dust / minimum unit)?
      → What if amount exceeds what's available?
      → What if a collection is empty? What if it has millions of entries?

Category 5: Boundary & Edge Case Questions (WHAT BREAKS AT THE EDGES?)

Q5.1: What happens on the FIRST call to this function? (Empty state)
      → First depositor, first user, first initialization
      → Division by zero when total = 0?
      → Share/ratio inflation when pool/collection is empty?
      → Uninitialized state treated as valid?

Q5.2: What happens on the LAST call? (Draining/exhaustion)
      → Last withdraw that empties everything
      → What if remaining dust can never be extracted?
      → Does rounding trap value permanently?
      → What if the last element removal breaks an invariant?

Q5.3: What if this function is called TWICE in rapid succession?
      → Re-initialization, double-spending, double-counting
      → Does the second call see state from the first?
      → In concurrent systems: race condition between the two calls?
      → Blockchain: two calls in the same block/transaction

Q5.4: What if two DIFFERENT functions are called in the same context?
      → Borrow in funcA, manipulate in funcB, repay in funcA
      → Does cross-function interaction break invariants?
      → What about callback patterns where control flow is non-linear?

Q5.5: What if this function is called with THE SYSTEM ITSELF as a parameter?
      → Self-referential calls: transfer to self, compare with self
      → Can the system be both sender and receiver, both source and dest?
      → What about circular references or recursive structures?

Category 6: Return Value & Error Path Questions

Q6.1: What does this function return? Who consumes the return value?
      → If the caller ignores the return value, what's lost?
      → If the return value is wrong, what downstream logic breaks?
      → Language-specific: Does the language even FORCE you to check?
        Rust: Result must be used. Go: error can be silently ignored with _.
        Solidity: low-level call returns bool that's often unchecked.
        C++: [[nodiscard]] is opt-in. Move: values must be consumed.

Q6.2: What happens on the ERROR/ABORT path?
      → Are there side effects before the error?
      → Does the error message leak sensitive information?
      → Can an attacker cause targeted errors (griefing / DoS)?
      → In languages with exceptions: is cleanup code (finally/defer/
        Drop) correct? Are resources leaked on the error path?

Q6.3: What if an EXTERNAL CALL in this function fails silently?
      → Does the language/runtime guarantee failure propagation?
      → Is the error checked, or can it be swallowed?
      → Language examples:
        Solidity: low-level call returns (bool, bytes) — often unchecked
        Go: err is a normal return value — easy to ignore with _
        Rust: .unwrap() can panic; ? propagates but hides the error
        C++: exception might be caught too broadly
        Move: abort is always propagated (safer by design)

Q6.4: Is there a code path where NO return and NO error happens?
      → Functions falling through without explicit return
      → Default/zero values used when they shouldn't be
      → Missing match/switch arms or else branches
      → Language-specific:
        Rust: compiler catches this. Go/C++: does not always.
        Solidity: functions can fall through returning zero values.

Category 7: External Call Reordering & Multi-Transaction State Analysis

This category catches bugs that live in the TIMING and SEQUENCING of operations — both within a single transaction and across multiple transactions over time.

Part A: External Call Reordering (within a single transaction)

Q7.1: If the function performs an external call BEFORE a state update,
      what happens if I SWAP them — state update first, external call second?
      → If the swap causes a revert: the ORIGINAL ordering may be exploitable
        (the external call might re-enter or manipulate state before it's updated)
      → If the swap works cleanly: the original ordering is likely safe,
        OR the swap reveals the intended safe ordering was never enforced
      → KEY: Try both directions. The one that reverts tells you which
        ordering the code DEPENDS on. The one that doesn't revert tells you
        which ordering an attacker can expl

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## Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

- **Author:** [grantkee](https://github.com/grantkee)
- **Source:** [grantkee/claude-extensions](https://github.com/grantkee/claude-extensions)
- **License:** MIT

Install and usage instructions live in the source repository linked above.