diff --git a/app/backend/src/common/soroban-errors/soroban-error.codes.js b/app/backend/src/common/soroban-errors/soroban-error.codes.js
index be9303042..f42d00f27 100644
--- a/app/backend/src/common/soroban-errors/soroban-error.codes.js
+++ b/app/backend/src/common/soroban-errors/soroban-error.codes.js
@@ -43,6 +43,12 @@ var SorobanErrorCode;
     SorobanErrorCode["VERSION_MISMATCH"] = "CONTRACT_VERSION_MISMATCH";
     /** WASM hash provided for upgrade is invalid. */
     SorobanErrorCode["INVALID_WASM_HASH"] = "CONTRACT_INVALID_WASM_HASH";
+    /** Upgrade window is not currently active; start_upgrade is blocked. */
+    SorobanErrorCode["UPGRADE_WINDOW_NOT_ACTIVE"] = "CONTRACT_UPGRADE_WINDOW_NOT_ACTIVE";
+    /** An upgrade is already in progress; start_upgrade cannot be called again. */
+    SorobanErrorCode["UPGRADE_ALREADY_IN_PROGRESS"] = "CONTRACT_UPGRADE_ALREADY_IN_PROGRESS";
+    /** No upgrade is currently in progress; upgrade or complete_upgrade cannot proceed. */
+    SorobanErrorCode["UPGRADE_NOT_IN_PROGRESS"] = "CONTRACT_UPGRADE_NOT_IN_PROGRESS";
     // ── Admin ─────────────────────────────────────────────────────────────────
     /** Caller is not the contract admin. */
     SorobanErrorCode["NOT_ADMIN"] = "CONTRACT_NOT_ADMIN";
diff --git a/app/backend/src/common/soroban-errors/soroban-error.codes.ts b/app/backend/src/common/soroban-errors/soroban-error.codes.ts
index 2cdd89856..d6f9bfae5 100644
--- a/app/backend/src/common/soroban-errors/soroban-error.codes.ts
+++ b/app/backend/src/common/soroban-errors/soroban-error.codes.ts
@@ -37,11 +37,17 @@
    /** Amount provided is zero or negative. */
    INVALID_AMOUNT = 'CONTRACT_INVALID_AMOUNT',
 
-   // ── Version / upgrade ─────────────────────────────────────────────────────
-   /** Contract schema version is not supported by this client. */
-   VERSION_MISMATCH = 'CONTRACT_VERSION_MISMATCH',
-   /** WASM hash provided for upgrade is invalid. */
-   INVALID_WASM_HASH = 'CONTRACT_INVALID_WASM_HASH',
+    // ── Version / upgrade ─────────────────────────────────────────────────────
+    /** Contract schema version is not supported by this client. */
+    VERSION_MISMATCH = 'CONTRACT_VERSION_MISMATCH',
+    /** WASM hash provided for upgrade is invalid. */
+    INVALID_WASM_HASH = 'CONTRACT_INVALID_WASM_HASH',
+    /** Upgrade window is not currently active; start_upgrade is blocked. */
+    UPGRADE_WINDOW_NOT_ACTIVE = 'CONTRACT_UPGRADE_WINDOW_NOT_ACTIVE',
+    /** An upgrade is already in progress; start_upgrade cannot be called again. */
+    UPGRADE_ALREADY_IN_PROGRESS = 'CONTRACT_UPGRADE_ALREADY_IN_PROGRESS',
+    /** No upgrade is currently in progress; upgrade or complete_upgrade cannot proceed. */
+    UPGRADE_NOT_IN_PROGRESS = 'CONTRACT_UPGRADE_NOT_IN_PROGRESS',
 
    // ── Admin ─────────────────────────────────────────────────────────────────
    /** Caller is not the contract admin. */
diff --git a/app/backend/src/common/soroban-errors/soroban-error.mapper.js b/app/backend/src/common/soroban-errors/soroban-error.mapper.js
index 1314347b0..ec1d5a4e1 100644
--- a/app/backend/src/common/soroban-errors/soroban-error.mapper.js
+++ b/app/backend/src/common/soroban-errors/soroban-error.mapper.js
@@ -119,6 +119,21 @@ var ERROR_MAPPINGS = [
         code: soroban_error_codes_1.SorobanErrorCode.INVALID_WASM_HASH,
         message: 'The WASM hash provided for the contract upgrade is invalid.',
     },
+    {
+        pattern: /upgrade.*window.*not.*active|upgrade.*window.*inactive/i,
+        code: soroban_error_codes_1.SorobanErrorCode.UPGRADE_WINDOW_NOT_ACTIVE,
+        message: 'The upgrade window is not currently active. Please try again during the scheduled window.',
+    },
+    {
+        pattern: /upgrade.*already.*in.*progress/i,
+        code: soroban_error_codes_1.SorobanErrorCode.UPGRADE_ALREADY_IN_PROGRESS,
+        message: 'An upgrade is already in progress. Please wait for it to complete before starting a new one.',
+    },
+    {
+        pattern: /upgrade.*not.*in.*progress|no.*upgrade.*in.*progress/i,
+        code: soroban_error_codes_1.SorobanErrorCode.UPGRADE_NOT_IN_PROGRESS,
+        message: 'No upgrade is currently in progress. Start an upgrade first before performing this operation.',
+    },
     // ── Input / params ────────────────────────────────────────────────────────
     {
         pattern: /account.*does not exist|account.*not.*found/i,
diff --git a/app/backend/src/common/soroban-errors/soroban-error.mapper.ts b/app/backend/src/common/soroban-errors/soroban-error.mapper.ts
index 6adf23abd..8cd13296f 100644
--- a/app/backend/src/common/soroban-errors/soroban-error.mapper.ts
+++ b/app/backend/src/common/soroban-errors/soroban-error.mapper.ts
@@ -151,6 +151,21 @@ const ERROR_MAPPINGS: Array<{
     code: SorobanErrorCode.INVALID_WASM_HASH,
     message: 'The WASM hash provided for the contract upgrade is invalid.',
   },
+  {
+    pattern: /upgrade.*window.*not.*active|upgrade.*window.*inactive/i,
+    code: SorobanErrorCode.UPGRADE_WINDOW_NOT_ACTIVE,
+    message: 'The upgrade window is not currently active. Please try again during the scheduled window.',
+  },
+  {
+    pattern: /upgrade.*already.*in.*progress/i,
+    code: SorobanErrorCode.UPGRADE_ALREADY_IN_PROGRESS,
+    message: 'An upgrade is already in progress. Please wait for it to complete before starting a new one.',
+  },
+  {
+    pattern: /upgrade.*not.*in.*progress|no.*upgrade.*in.*progress/i,
+    code: SorobanErrorCode.UPGRADE_NOT_IN_PROGRESS,
+    message: 'No upgrade is currently in progress. Start an upgrade first before performing this operation.',
+  },
 
   // ── Input / params ────────────────────────────────────────────────────────
   {
diff --git a/app/backend/src/common/soroban-errors/soroban-error.mapper.unit.spec.ts b/app/backend/src/common/soroban-errors/soroban-error.mapper.unit.spec.ts
index 4e03b1bee..f525de09a 100644
--- a/app/backend/src/common/soroban-errors/soroban-error.mapper.unit.spec.ts
+++ b/app/backend/src/common/soroban-errors/soroban-error.mapper.unit.spec.ts
@@ -112,6 +112,21 @@ describe('mapSorobanError', () => {
     expect(result.code).toBe(SorobanErrorCode.INVALID_WASM_HASH);
   });
 
+  it('maps "upgrade window not active" to UPGRADE_WINDOW_NOT_ACTIVE', () => {
+    const result = mapSorobanError('upgrade window not active');
+    expect(result.code).toBe(SorobanErrorCode.UPGRADE_WINDOW_NOT_ACTIVE);
+  });
+
+  it('maps "upgrade already in progress" to UPGRADE_ALREADY_IN_PROGRESS', () => {
+    const result = mapSorobanError('upgrade already in progress');
+    expect(result.code).toBe(SorobanErrorCode.UPGRADE_ALREADY_IN_PROGRESS);
+  });
+
+  it('maps "no upgrade in progress" to UPGRADE_NOT_IN_PROGRESS', () => {
+    const result = mapSorobanError('no upgrade in progress');
+    expect(result.code).toBe(SorobanErrorCode.UPGRADE_NOT_IN_PROGRESS);
+  });
+
   // ── Admin ───────────────────────────────────────────────────────────────
 
   it('maps "not admin" to NOT_ADMIN', () => {
diff --git a/app/contract/.gitignore b/app/contract/.gitignore
index d74cc01f6..1c81c0d8d 100644
--- a/app/contract/.gitignore
+++ b/app/contract/.gitignore
@@ -4,3 +4,6 @@ target
 # Local settings
 .soroban
 .stellar
+
+# Test snapshots (generated by property-based tests)
+contracts/Folder/test_snapshots/
diff --git a/app/contract/UPGRADE_SAFETY_GATE_IMPLEMENTATION.md b/app/contract/UPGRADE_SAFETY_GATE_IMPLEMENTATION.md
index 141b14bc3..e24a47553 100644
--- a/app/contract/UPGRADE_SAFETY_GATE_IMPLEMENTATION.md
+++ b/app/contract/UPGRADE_SAFETY_GATE_IMPLEMENTATION.md
@@ -25,7 +25,7 @@ Added contract-level safeguards and comprehensive invariant enforcement for safe
 - Function: `storage::is_upgrade_window_active(env)` checks ledger timestamp against `[start, end)`
 - Gating: `upgrade()` now requires `UpgradeInProgress` and active window.
 - Verification: `upgrade()` verifies the WASM hash matches the one stored in `start_upgrade()`.
-- Error: Returns `InvalidAmount` (repurposed as "upgrade window not active") or `InternalError` (not in progress).
+- Error: Returns `UpgradeWindowNotActive` when window is not active or `UpgradeAlreadyInProgress` when already started.
 
 **Test**: `upgrade_safety_gate_blocks_upgrade_outside_window`, `upgrade_safety_gate_blocks_direct_upgrade_without_start`
 
@@ -71,7 +71,7 @@ admin::upgrade(hash)
 
 ```rust
 pub fn complete_upgrade(env, caller, new_version) {
-    if !storage::is_upgrade_in_progress(env) { return Err(InternalError); }
+    if !storage::is_upgrade_in_progress(env) { return Err(UpgradeNotInProgress); }
     if new_version != storage::get_pending_upgrade_version(env) { return Err(InvalidContractVersion); }
     if storage::get_wasm_hash(env) != storage::get_pending_upgrade_wasm_hash(env) { return Err(InternalError); }
     // ... run migrate() ...
@@ -177,7 +177,7 @@ Step 1: Admin calls set_upgrade_window(start, end)
 Step 2: Admin calls start_upgrade(new_version)
         → Check: is_upgrade_window_active()
         → If yes: set UpgradeInProgress = true, emit UpgradeStarted
-        → If no: return Err(InvalidAmount)
+        → If no: return Err(UpgradeWindowNotActive)
 
 Step 3a: (Deploy) update_current_contract_wasm(new_wasm_hash)
          → Caller publishes new WASM; contract code swaps
@@ -191,9 +191,9 @@ Step 3b: Admin calls complete_upgrade(new_version)
 
 **Error Handling**:
 
-- `InvalidAmount`: Used to signal "upgrade window not active"
-- `ContractPaused`: Used to signal "upgrade already in progress" (repurposed)
-- `InternalError`: Used when post-upgrade invariants fail (repurposed)
+- `UpgradeWindowNotActive`: Used to signal "upgrade window not active"
+- `UpgradeAlreadyInProgress`: Used to signal "upgrade already in progress"
+- `InternalError`: Used when post-upgrade invariants fail
 
 ---
 
diff --git a/app/contract/UPGRADE_SAFETY_GATE_QUICK_REFERENCE.md b/app/contract/UPGRADE_SAFETY_GATE_QUICK_REFERENCE.md
index 79a8939f9..df441ebb7 100644
--- a/app/contract/UPGRADE_SAFETY_GATE_QUICK_REFERENCE.md
+++ b/app/contract/UPGRADE_SAFETY_GATE_QUICK_REFERENCE.md
@@ -31,10 +31,10 @@ start_upgrade(env, caller, new_version) -> Result<(),  RustAcademyError>
 - **Must be called during active window** (AC1)
 - Sets `UpgradeInProgress = true`
 - Emits: `UpgradeStarted { admin, old_version, new_version, ... }`
-- **Errors**:
-  - `InvalidAmount`: Window not active
-  - `ContractPaused`: Already in-progress
-  - `InsufficientRole`: Not admin
+ - **Errors**:
+   - `UpgradeWindowNotActive`: Window not active
+   - `UpgradeAlreadyInProgress`: Already in-progress
+   - `InsufficientRole`: Not admin
 
 ### 4. Update WASM
 
@@ -42,9 +42,12 @@ start_upgrade(env, caller, new_version) -> Result<(),  RustAcademyError>
 upgrade(env, caller, new_wasm_hash) -> Result<(),  RustAcademyError>
 ```
 
-- Swaps contract code (no storage changes)
-- Emits: `ContractUpgraded { admin, new_wasm_hash, ... }`
-- **Error**: `InsufficientRole` if not admin
+ - Swaps contract code (no storage changes)
+ - Emits: `ContractUpgraded { admin, new_wasm_hash, ... }`
+ - **Errors**:
+   - `UpgradeNotInProgress`: No upgrade in progress
+   - `UpgradeWindowNotActive`: Window not active
+   - `InsufficientRole`: Not admin
 
 ### 5. Complete Upgrade (Admin)
 
@@ -57,9 +60,10 @@ complete_upgrade(env, caller, new_version) -> Result<u32,  RustAcademyError>
 - Sets `UpgradeInProgress = false`
 - Emits: `UpgradeCompleted { admin, old_version, new_version, ... }`
 - **Returns**: New contract version
-- **Errors**:
-  - `InternalError`: Not in-progress, or invariants failed
-  - `InvalidContractVersion`: Version mismatch
+ - **Errors**:
+   - `UpgradeNotInProgress`: No upgrade in progress
+   - `InternalError`: Invariants failed post-upgrade
+   - `InvalidContractVersion`: Version mismatch
 
 ---
 
@@ -79,13 +83,14 @@ Day 2 @ 14:15: admin.complete_upgrade(2u32)
 
 ---
 
-## Error Codes (Repurposed)
+## Error Codes (Semantic)
 
-| Error            | Code | Meaning                        |
-| ---------------- | ---- | ------------------------------ |
-| `InvalidAmount`  | 100  | Upgrade window not active      |
-| `ContractPaused` | 300  | Upgrade already in-progress    |
-| `InternalError`  | 900  | Invariants failed post-upgrade |
+| Error                     | Code | Meaning                        |
+| ------------------------- | ---- | ------------------------------ |
+| `UpgradeWindowNotActive`  | 502  | Upgrade window not active      |
+| `UpgradeAlreadyInProgress`| 503  | Upgrade already in-progress    |
+| `UpgradeNotInProgress`    | 504  | No upgrade in progress         |
+| `InternalError`           | 900  | Invariants failed post-upgrade |
 
 ---
 
@@ -168,7 +173,7 @@ Post-upgrade checks that **must** hold:
 A: No. The original `migrate()` still works standalone. These are optional extra guards.
 
 **Q: Can I call `complete_upgrade()` without `start_upgrade()`?**  
-A: No. `complete_upgrade()` checks `UpgradeInProgress` flag first → returns `InternalError`.
+A: No. `complete_upgrade()` checks `UpgradeInProgress` flag first → returns `UpgradeNotInProgress`.
 
 **Q: What if invariants fail during `complete_upgrade()`?**  
 A: Contract panics → all storage changes rolled back → upgrade aborted. Retry after fixing.
diff --git a/app/contract/UPGRADE_SAFETY_GATE_TEST_GUIDE.md b/app/contract/UPGRADE_SAFETY_GATE_TEST_GUIDE.md
index c3d492ca0..767845c32 100644
--- a/app/contract/UPGRADE_SAFETY_GATE_TEST_GUIDE.md
+++ b/app/contract/UPGRADE_SAFETY_GATE_TEST_GUIDE.md
@@ -33,7 +33,7 @@
    assert!(result.is_err(), "start_upgrade must fail when no window is set");
    ```
 
-   - Expected: `InvalidAmount` ("upgrade window not active")
+   - Expected: `UpgradeWindowNotActive` ("upgrade window not active")
 
 2. **Before Window**
 
@@ -45,9 +45,9 @@
    assert!(result.is_err(), "start_upgrade must fail before window start");
    ```
 
-   - Expected: `InvalidAmount`
+    - Expected: `UpgradeWindowNotActive`
 
-3. **Within Window**
+ 3. **Within Window**
 
    ```rust
    gs.env.ledger().with_mut(|li| {
@@ -71,7 +71,7 @@
    assert!(result.is_err(), "start_upgrade must fail after window end");
    ```
 
-   - Expected: `InvalidAmount`
+    - Expected: `UpgradeWindowNotActive`
 
 **Acceptance Criterion**: ✅ AC1 – Upgrades are blocked outside the allowed window.
 
@@ -301,7 +301,7 @@ LIMIT 1;
    ```rust
    let result = client.try_start_upgrade(&gs.admin, &3u32);
    assert!(result.is_err(), "start_upgrade must fail when already in progress");
-   // → Expected: ContractPaused ("upgrade already in progress")
+    // → Expected: `UpgradeAlreadyInProgress` ("upgrade already in progress")
    ```
 
 3. **Complete & Retry**
diff --git a/app/contract/contracts/Folder/src/admin.rs b/app/contract/contracts/Folder/src/admin.rs
index f5f121145..3ada4d8af 100644
--- a/app/contract/contracts/Folder/src/admin.rs
+++ b/app/contract/contracts/Folder/src/admin.rs
@@ -354,11 +354,11 @@ pub fn start_upgrade(
 
     // Check upgrade window is active (Issue #432 AC1)
     if !storage::is_upgrade_window_active(env) {
-        return Err(RustAcademyError::InvalidAmount); // Repurpose for "upgrade window not active"
+        return Err(RustAcademyError::UpgradeWindowNotActive);
     }
 
     if storage::is_upgrade_in_progress(env) {
-        return Err(RustAcademyError::ContractPaused); // Reuse for "upgrade in progress"
+        return Err(RustAcademyError::UpgradeAlreadyInProgress);
     }
 
     let old_version = get_version(env);
@@ -398,11 +398,11 @@ pub fn upgrade(
     require_admin(env, caller)?;
 
     if !storage::is_upgrade_in_progress(env) {
-        return Err(RustAcademyError::InternalError);
+        return Err(RustAcademyError::UpgradeNotInProgress);
     }
 
     if !storage::is_upgrade_window_active(env) {
-        return Err(RustAcademyError::InvalidAmount);
+        return Err(RustAcademyError::UpgradeWindowNotActive);
     }
 
     let pending_hash =
@@ -448,7 +448,7 @@ pub fn complete_upgrade(
     new_version: u32,
 ) -> Result<u32, RustAcademyError> {
     if !storage::is_upgrade_in_progress(env) {
-        return Err(RustAcademyError::InternalError); // Not in upgrade state
+        return Err(RustAcademyError::UpgradeNotInProgress);
     }
 
     // Verify version and hash (Issue #432 AC2)
diff --git a/app/contract/contracts/Folder/src/errors.rs b/app/contract/contracts/Folder/src/errors.rs
index 401a0b9c0..9203bf031 100644
--- a/app/contract/contracts/Folder/src/errors.rs
+++ b/app/contract/contracts/Folder/src/errors.rs
@@ -91,4 +91,11 @@ pub enum RustAcademyError {
     NonceAlreadyUsed = 500,
     /// The signature's valid_until timestamp has passed; signature expired.
     SignatureExpired = 501,
+    // Upgrade gating (502-504)
+    /// The upgrade window is not currently active; start_upgrade is blocked.
+    UpgradeWindowNotActive = 502,
+    /// An upgrade is already in progress; start_upgrade cannot be called again.
+    UpgradeAlreadyInProgress = 503,
+    /// No upgrade is currently in progress; upgrade or complete_upgrade cannot proceed.
+    UpgradeNotInProgress = 504,
 }
diff --git a/app/contract/contracts/Folder/src/lib.rs b/app/contract/contracts/Folder/src/lib.rs
index fef4afd25..02293f952 100644
--- a/app/contract/contracts/Folder/src/lib.rs
+++ b/app/contract/contracts/Folder/src/lib.rs
@@ -1211,6 +1211,8 @@ impl RustAcademyContract {
     ///
     /// # Errors
     /// * `Unauthorized` - Caller is not the admin, or admin not set
+    /// * `UpgradeNotInProgress` - no upgrade is currently in progress
+    /// * `UpgradeWindowNotActive` - upgrade window is not currently active
     ///
     /// # Security
     /// Updates the contract's executable code. Call [`migrate`]( RustAcademyContract::migrate)
@@ -1267,8 +1269,8 @@ impl RustAcademyContract {
     /// * `new_wasm_hash` - The target WASM hash
     ///
     /// # Errors
-    /// * `InvalidAmount` - (repurposed) upgrade window not active
-    /// * `ContractPaused` - (repurposed) upgrade already in progress
+    /// * `UpgradeWindowNotActive` - upgrade window is not currently active
+    /// * `UpgradeAlreadyInProgress` - an upgrade is already in progress
     pub fn start_upgrade(
         env: Env,
         caller: Address,
@@ -1297,7 +1299,8 @@ impl RustAcademyContract {
     /// The actual new contract version
     ///
     /// # Errors
-    /// * `InternalError` - no upgrade in progress, or post-upgrade invariants violated
+    /// * `UpgradeNotInProgress` - no upgrade is currently in progress
+    /// * `InternalError` - post-upgrade invariants violated
     pub fn complete_upgrade(
         env: Env,
         caller: Address,
diff --git a/app/contract/docs/UPGRADE_SAFETY_GATE.md b/app/contract/docs/UPGRADE_SAFETY_GATE.md
index 7ea1d8e7b..67b4ffe0a 100644
--- a/app/contract/docs/UPGRADE_SAFETY_GATE.md
+++ b/app/contract/docs/UPGRADE_SAFETY_GATE.md
@@ -133,7 +133,7 @@ admin_account.complete_upgrade(contract, 2u32)?;
 - ✅ `start_upgrade()` fails after window end time
 - ✅ `start_upgrade()` succeeds within window `[start, end)`
 
-**Error Code**: `InvalidAmount` (repurposed as "upgrade window not active")
+**Error Code**: `UpgradeWindowNotActive`
 
 ### AC2: Post-Upgrade Invariants Enforced Deterministically
 
@@ -235,13 +235,15 @@ pub struct UpgradeCompletedEvent {
 
 ## Error Codes
 
-New or repurposed errors:
+New upgrade-specific errors:
 
-| Error            | Code | Trigger                        | Context                                     |
-| ---------------- | ---- | ------------------------------ | ------------------------------------------- |
-| `InvalidAmount`  | 100  | Upgrade window not active      | `start_upgrade()` outside `[start, end)`    |
-| `ContractPaused` | 300  | Upgrade already in progress    | `start_upgrade()` called twice              |
-| `InternalError`  | 900  | Post-upgrade invariants failed | `complete_upgrade()` after failed migration |
+| Error                     | Code | Trigger                        | Context                                     |
+| ------------------------- | ---- | ------------------------------ | ------------------------------------------- |
+| `UpgradeWindowNotActive`  | 502  | Upgrade window not active      | `start_upgrade()` outside `[start, end)`    |
+| `UpgradeAlreadyInProgress`| 503  | Upgrade already in progress    | `start_upgrade()` called twice              |
+| `UpgradeNotInProgress`    | 504  | No upgrade in progress         | `upgrade()` / `complete_upgrade()` without `start_upgrade()` |
+
+Legacy repurposed codes removed; backend now maps these to stable API codes.
 
 ---
 
@@ -291,7 +293,7 @@ let new_version = admin.complete_upgrade(contract, 2u32)?;
 ```rust
 // Same setup as above; but now it's Day 1, 13:00 UTC (before window)
 admin.start_upgrade(contract, 2u32)?;
-// → Err: InvalidAmount ("upgrade window not active")
+// → Err: UpgradeWindowNotActive ("upgrade window not active")
 ```
 
 ---