diff --git a/ARCHITECTURE.md b/ARCHITECTURE.md
index fc806e0..0dc433c 100644
--- a/ARCHITECTURE.md
+++ b/ARCHITECTURE.md
@@ -153,3 +153,24 @@ All payout math uses `checked_mul`/`checked_div`/`checked_add`/`checked_sub` to
 - `contracts/sharpy/src/events.rs` — typed event helpers
 - `contracts/sharpy/src/types.rs` — `Invoice`, `SplitRule`, `DisputeState`, etc.
 - `contracts/sharpy/src/test.rs` — 184 unit/integration tests
+
+## Auth Matrix (audit harness, closes #199)
+
+| Entry | Auth | Rationale |
+|-------|------|-----------|
+| `initialize` | once, no auth | sets admin/treasury singletons |
+| `pause`/`unpause` | admin | circuit breaker |
+| `create_*` | creator | creator-indexed |
+| `pay`/`pool_pay`/`pay_with_tip` | payer + whitelist | sequential guard prevents double-spend |
+| `release`/`refund`/`refund_batch` | permissionless | deadline/status gates; payouts fixed to stored parties |
+| `release_escrow`/`dispute`/`resolve` | creator/arbitrator | escrow state gates |
+| `cancel`/`freeze`/`set_*`/`extend`/`archive` | creator (freeze admin) | mutators append audit + `inv_upd` |
+| `claim` | permissionless | CEI: remove before transfer |
+| `create_stream`/`withdraw`/`top_up`/`cancel_stream` | permissionless (documented) | scratch state keyed by id; future major may gate |
+| `set_route` | any authed caller | repoint allowed; self/2-cycle panic; bounded resolve |
+
+## CEI Checklist
+
+- `claim`: checks balance>0, effects remove, interactions transfer last.
+- `_release`: status assert first, distribution via try_transfer+fallback, status write after.
+- `_refund_payers`: aggregation then transfers; status transition makes retry panic.
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 59ead39..c32796e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,7 @@
 All notable changes to the Sharpy smart contract are documented here.
 
 ## [Unreleased]
+- feat(contract): `get_funding_remaining(id)` invariant view + auth-matrix/CEI docs; double-release/refund/claim + remaining guards — feat/audit-harness (closes #199)
 - feat(contract): `get_audit_count(id)` + whitelist audit entries (`wlist`/`wrem`) + `inv_upd` ordering docs; per-path emission proof — feat/invupd-coverage (closes #198)
 - feat(contract): `is_invoice_terminal(id)` view + ttl-hint terminal/expired-mirror coverage (countdown, Released/Refunded/Cancelled zeros, extend revives) — feat/ttl-terminal (closes #197)
 - feat(contract): `get_creator_invoice_total` + `get_payer_invoice_total` O(1) totals; empty-page/total consistency proof — feat/paged-totals (closes #196)
diff --git a/contracts/sharpy/src/lib.rs b/contracts/sharpy/src/lib.rs
index f6d46bb..d411d6e 100644
--- a/contracts/sharpy/src/lib.rs
+++ b/contracts/sharpy/src/lib.rs
@@ -695,6 +695,15 @@ impl SharpyContract {
         load_invoice(&env, invoice_id)
     }
 
+    /// Remaining fundable amount (`total - funded`), floored at 0. Pure view.
+    /// Invariant harness: `funded + remaining == total` and `funded <= total`
+    /// always; sequential pays never exceed the remainder (see `pay` guard).
+    pub fn get_funding_remaining(env: Env, invoice_id: u64) -> i128 {
+        let inv = load_invoice(&env, invoice_id);
+        let total: i128 = inv.amounts.iter().sum();
+        total.checked_sub(inv.funded).expect("remaining: underflow").max(0i128)
+    }
+
     /// Number of audit entries for `invoice_id` (0 when none).
     /// Pairs with `get_audit_log`: cheap length check for emission-coverage
     /// assertions — every mutator that appends an audit entry bumps this count.
diff --git a/contracts/sharpy/src/test.rs b/contracts/sharpy/src/test.rs
index fdd4c0e..5562c9a 100644
--- a/contracts/sharpy/src/test.rs
+++ b/contracts/sharpy/src/test.rs
@@ -6295,3 +6295,139 @@ mod test_invupd_c {
         assert_eq!(client.get_whitelist(&id).unwrap().payers.len(), 0u32);
     }
 }
+
+#[cfg(test)]
+mod test_invariant_a {
+    use soroban_sdk::{testutils::Address as _, token, Address, Env, Vec};
+    use crate::SharpyContractClient;
+    fn setup() -> (Env, SharpyContractClient<'static>) {
+        let env = Env::default();
+        env.mock_all_auths();
+        let cid = env.register(crate::SharpyContract, ());
+        let c = SharpyContractClient::new(&env, &cid);
+        let a = Address::generate(&env);
+        let t = Address::generate(&env);
+        c.initialize(&a, &t);
+        (env, c)
+    }
+    #[test]
+    fn test_funded_plus_remaining_equals_total() {
+        let (env, client) = setup();
+        let creator = Address::generate(&env);
+        let payer = Address::generate(&env);
+        let admin = Address::generate(&env);
+        let tok = env.register_stellar_asset_contract(admin);
+        token::StellarAssetClient::new(&env, &tok).mint(&payer, &5000i128);
+        let r = Address::generate(&env);
+        let dl = env.ledger().timestamp() + 86400;
+        let opts = crate::types::InvoiceOptions { escrow_enabled: false, escrow_release_delay: None, split_rules: Vec::new(&env), auto_resolve_rules: Vec::new(&env), arbitrator: None };
+        let id = client.create_invoice(&creator, &Vec::from_array(&env, [r]), &Vec::from_array(&env, [1000i128]), &Vec::from_array(&env, [tok]), &dl, &opts);
+        assert_eq!(client.get_funding_remaining(&id), 1000i128);
+        client.pay(&payer, &id, &400i128);
+        assert_eq!(client.get_funding_remaining(&id), 600i128);
+        assert_eq!(client.get_invoice(&id).funded + client.get_funding_remaining(&id), 1000i128);
+    }
+    #[test]
+    #[should_panic(expected = "payment exceeds remaining balance")]
+    fn test_overpay_panics() {
+        let (env, client) = setup();
+        let creator = Address::generate(&env);
+        let payer = Address::generate(&env);
+        let admin = Address::generate(&env);
+        let tok = env.register_stellar_asset_contract(admin);
+        token::StellarAssetClient::new(&env, &tok).mint(&payer, &5000i128);
+        let r = Address::generate(&env);
+        let dl = env.ledger().timestamp() + 86400;
+        let opts = crate::types::InvoiceOptions { escrow_enabled: false, escrow_release_delay: None, split_rules: Vec::new(&env), auto_resolve_rules: Vec::new(&env), arbitrator: None };
+        let id = client.create_invoice(&creator, &Vec::from_array(&env, [r]), &Vec::from_array(&env, [500i128]), &Vec::from_array(&env, [tok]), &dl, &opts);
+        client.pay(&payer, &id, &400i128);
+        client.pay(&payer, &id, &200i128);
+    }
+}
+
+#[cfg(test)]
+mod test_invariant_b {
+    use soroban_sdk::{testutils::Address as _, token, Address, Env, Vec};
+    use crate::SharpyContractClient;
+    fn setup() -> (Env, SharpyContractClient<'static>) {
+        let env = Env::default();
+        env.mock_all_auths();
+        let cid = env.register(crate::SharpyContract, ());
+        let c = SharpyContractClient::new(&env, &cid);
+        let a = Address::generate(&env);
+        let t = Address::generate(&env);
+        c.initialize(&a, &t);
+        (env, c)
+    }
+    #[test]
+    #[should_panic(expected = "invoice is not pending")]
+    fn test_double_release_panics() {
+        let (env, client) = setup();
+        let creator = Address::generate(&env);
+        let payer = Address::generate(&env);
+        let admin = Address::generate(&env);
+        let tok = env.register_stellar_asset_contract(admin);
+        token::StellarAssetClient::new(&env, &tok).mint(&payer, &5000i128);
+        let r = Address::generate(&env);
+        let dl = env.ledger().timestamp() + 86400;
+        let opts = crate::types::InvoiceOptions { escrow_enabled: false, escrow_release_delay: None, split_rules: Vec::new(&env), auto_resolve_rules: Vec::new(&env), arbitrator: None };
+        let id = client.create_invoice(&creator, &Vec::from_array(&env, [r]), &Vec::from_array(&env, [500i128]), &Vec::from_array(&env, [tok]), &dl, &opts);
+        client.pay(&payer, &id, &500i128);
+        client.release(&id);
+    }
+    #[test]
+    #[should_panic(expected = "no claimable balance")]
+    fn test_double_claim_panics() {
+        let (env, client) = setup();
+        let who = Address::generate(&env);
+        let tok = Address::generate(&env);
+        client.claim(&who, &tok);
+    }
+}
+
+#[cfg(test)]
+mod test_invariant_c {
+    use soroban_sdk::{testutils::Address as _, testutils::Ledger as _, token, Address, Env, Vec};
+    use crate::SharpyContractClient;
+    fn setup() -> (Env, SharpyContractClient<'static>) {
+        let env = Env::default();
+        env.mock_all_auths();
+        let cid = env.register(crate::SharpyContract, ());
+        let c = SharpyContractClient::new(&env, &cid);
+        let a = Address::generate(&env);
+        let t = Address::generate(&env);
+        c.initialize(&a, &t);
+        (env, c)
+    }
+    #[test]
+    #[should_panic(expected = "invoice is not pending")]
+    fn test_double_refund_panics() {
+        let (env, client) = setup();
+        let creator = Address::generate(&env);
+        let r = Address::generate(&env);
+        let tok = Address::generate(&env);
+        let now = env.ledger().timestamp();
+        let opts = crate::types::InvoiceOptions { escrow_enabled: false, escrow_release_delay: None, split_rules: Vec::new(&env), auto_resolve_rules: Vec::new(&env), arbitrator: None };
+        let id = client.create_invoice(&creator, &Vec::from_array(&env, [r]), &Vec::from_array(&env, [100i128]), &Vec::from_array(&env, [tok]), &(now + 10), &opts);
+        env.ledger().set_timestamp(now + 11);
+        client.refund(&id);
+        client.refund(&id);
+    }
+    #[test]
+    fn test_sequential_pays_respect_remaining() {
+        let (env, client) = setup();
+        let creator = Address::generate(&env);
+        let payer = Address::generate(&env);
+        let admin = Address::generate(&env);
+        let tok = env.register_stellar_asset_contract(admin);
+        token::StellarAssetClient::new(&env, &tok).mint(&payer, &5000i128);
+        let r = Address::generate(&env);
+        let dl = env.ledger().timestamp() + 86400;
+        let opts = crate::types::InvoiceOptions { escrow_enabled: false, escrow_release_delay: None, split_rules: Vec::new(&env), auto_resolve_rules: Vec::new(&env), arbitrator: None };
+        let id = client.create_invoice(&creator, &Vec::from_array(&env, [r]), &Vec::from_array(&env, [1000i128]), &Vec::from_array(&env, [tok]), &dl, &opts);
+        client.pay(&payer, &id, &600i128);
+        client.pay(&payer, &id, &400i128);
+        assert_eq!(client.get_funding_remaining(&id), 0i128);
+        assert!(client.is_invoice_terminal(&id));
+    }
+}