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contracts/escrow/docs/fuzzing.md

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+# Fuzz Testing the Escrow Contract
+
+## Overview
+
+Fuzz testing (property‑based testing) is used to automatically generate a wide range of inputs for the **Escrow** contract and verify that the implementation maintains its invariants under all possible edge‑cases.  
+
+The escrow module includes a dedicated fuzz suite in `src/fuzz_test.rs`. The suite exercises:
+
+| Property | Description |
+|----------|-------------|
+| **Milestone total** | The sum of all milestone amounts must never exceed `MAX_TOTAL_ESCROW_STROOPS`. |
+| **Milestone count** | The number of milestones must stay ≤ `MAX_MILESTONES`. |
+| **Deposit limits** | Deposits cannot over‑fund the contract and must respect the total cap. |
+| **Unauthorized actions** | Only authorized roles (client, freelancer, arbiter) can call privileged functions. |
+| **State transitions** | Contract moves through the correct states (`Created → Funded → Completed/Refunded`). |
+| **Overflow safety** | All arithmetic uses `checked_*` helpers to prevent i128 overflow. |
+| **Boundary handling** | Values at, just below and just above limits are explicitly tested. |
+
+## How the Fuzz Suite Works
+
+1. **Input Generation** – The `proptest` crate generates random vectors of `i128` milestone amounts, random deposit amounts, and random role combinations.
+2. **Invariant Checks** – After each operation the suite calls the contract’s public methods and asserts that:
+   * No panic occurs unless an expected error is triggered.
+   * The contract’s internal balances (`funded_amount`, `released_amount`, `refunded_amount`) stay within the legal range.
+   * The contract’s status matches the expected lifecycle phase.
+3. **Boundary Cases** – Dedicated tests (`fuzz_milestone_total_exact`, `fuzz_milestone_total_over`) explicitly cover:
+   * **Exact cap** (`MAX_TOTAL_ESCROW_STROOPS`) – should succeed.
+   * **One‑above cap** (`MAX_TOTAL_ESCROW_STROOPS + 1`) – must be rejected with `EscrowError::InvalidMilestoneAmount`.
+4. **State‑ful Fuzzing** – The suite runs a series of actions in random order (deposit, approve, release, refund) to ensure that the contract never reaches an illegal state.
+
+## Running the Fuzz Tests
+
+```bash
+# From the escrow contract directory
+cargo test --manifest-path Cargo.toml fuzz_test -- --nocapture
+```
+
+*The `--nocapture` flag prints the generated inputs when a test fails, making debugging easier.*
+
+## Adding New Fuzz Scenarios
+
+When adding new functionality (e.g., dispute resolution or fee logic) follow these steps:
+
+1. **Define the property** you want to guarantee (e.g., “dispute can only be opened by the arbiter”).
+2. **Add a new `proptest!` block** in `src/fuzz_test.rs` that:
+   * Generates random role‑addresses and contract IDs.
+   * Calls the new method.
+   * Asserts the expected error or state transition.
+3. **Update `docs/fuzzing.md`** with a short description of the new scenario and the property it checks.
+
+### Example: New Fee Validation
+
+```rust
+proptest! {
+    #[test]
+    fn fuzz_fee_validation(
+        fee_bps in 0u32..=10_000, // 0% – 100%
+        total in 1i128..=MAX_TOTAL_ESCROW_STROOPS
+    ) {
+        // Setup contract with max total and fee
+        let mut contract = create_contract_with_fee(fee_bps, total);
+        // Deposit the full amount
+        contract.deposit_funds(total).unwrap();
+        // Verify that the fee stored equals total * fee_bps / 10_000
+        assert_eq!(contract.protocol_fee(), total * fee_bps as i128 / 10_000);
+    }
+}
+```
+
+After adding the test, run the fuzz suite as described above to confirm that the fee logic never over‑charges or under‑charges.
+
+## Documentation Generation
+
+The fuzz suite doubles as an executable specification. Each test’s name and comments are extracted automatically for the docs page using the `cargo doc` system. Ensure that every test has a concise comment explaining the invariant it validates.
+
+---
+
+**Next Steps**
+
+* Review `src/fuzz_test.rs` for any missing edge‑cases.
+* Add the new fee‑validation test (or any other feature you plan).
+* Regenerate the documentation with `cargo doc` and verify that the updated `fuzzing.md` reflects the new tests.
+
+Feel free to ask if you need help writing a specific fuzz scenario or updating the documentation.