diff --git a/backend/internal/square/square_dev.go b/backend/internal/square/square_dev.go
index c8e1993..b95cc2b 100644
--- a/backend/internal/square/square_dev.go
+++ b/backend/internal/square/square_dev.go
@@ -451,24 +451,138 @@ func verificationTokenPrefixForSource(sourceID string) string {
 // frontend's MockCardForm mints for saved-card verification). A raw nonce like
 // "cnon:test-card" — whatever customer_id rides along — is NOT a tokenize-result,
 // and real Square rejects it as a card-on-file charge source.
-//
-// During the token-shape transition the frontend mock may still emit a
-// verify_mock_<prefix>_<amount>[_ok|_deny] verification token in the
-// tokenize-result source slot; that shape is accepted here too so dev remains
-// walkable either way (parseVerifyToken resolves its binding).
 func isSCATokenizeResultSource(sourceID string) bool {
-	return strings.HasPrefix(sourceID, "cnon:sca-") || strings.HasPrefix(sourceID, "verify_mock_")
+	return strings.HasPrefix(sourceID, "cnon:sca-")
+}
+
+// parseSCATokenizeResult parses the deterministic mock tokenize-result encoding
+// the dev frontend mints for saved-card SCA: cnon:sca-<prefix>_<amount>[_ok|_deny]
+// (outcome suffix defaults to ok). The prefix is the first four characters of
+// the saved card's ccof: id, the amount is the pence charge the token was bound
+// to, and the outcome encodes the buyer's challenge decision
+// (MockCardForm.tokenizeSavedCard / verifySavedCard, square.ts's
+// tokenizeSavedCardWithVerification mock fallback). Reuses parsedVerifyToken's
+// shape — same binding + outcome semantics as the legacy verify_mock_ encoding.
+// Returns ok=false for anything that is not parseable (an arbitrary cnon:sca-...
+// string — NOT a genuine tokenize-result).
+func parseSCATokenizeResult(token string) (parsedVerifyToken, bool) {
+	const marker = "cnon:sca-"
+	if !strings.HasPrefix(token, marker) {
+		return parsedVerifyToken{}, false
+	}
+	rest := strings.TrimPrefix(token, marker)
+	if rest == "" {
+		return parsedVerifyToken{}, false
+	}
+	parts := strings.Split(rest, "_")
+	denied := false
+	if n := len(parts); n > 1 {
+		switch parts[n-1] {
+		case "ok", "deny":
+			denied = parts[n-1] == "deny"
+			parts = parts[:n-1]
+		}
+	}
+	if len(parts) == 0 {
+		return parsedVerifyToken{}, false
+	}
+	amount, err := strconv.ParseInt(parts[len(parts)-1], 10, 64)
+	if err != nil {
+		return parsedVerifyToken{}, false
+	}
+	prefix := strings.Join(parts[:len(parts)-1], "_")
+	if prefix == "" {
+		return parsedVerifyToken{}, false
+	}
+	return parsedVerifyToken{prefix: prefix, amount: amount, denied: denied}, true
+}
+
+// tokenizeResultVerificationRequiredError is the refusal for a cnon:sca-
+// tokenize-result that does not prove buyer verification for THIS charge — the
+// same structured 402 CARD_DECLINED_VERIFICATION_REQUIRED rejection a raw nonce
+// in the tokenize-result slot gets. The buyer must re-verify (mint a token bound
+// to this card + amount), never retry the same request.
+func tokenizeResultVerificationRequiredError(sourceID, reason string) error {
+	return &squareAPIError{
+		Code:       "CARD_DECLINED_VERIFICATION_REQUIRED",
+		Category:   "PAYMENT_METHOD_ERROR",
+		Detail:     "tokenize-result token does not prove buyer verification for this charge; complete buyer verification (tokenizeWithVerification) for this card and amount",
+		StatusCode: http.StatusPaymentRequired,
+		err:        fmt.Errorf("square: tokenize-result source %s is not valid for this saved-card charge (%s)", tokenPrefix(sourceID), reason),
+	}
+}
+
+// scaTokenizePrefixKnown reports whether the card prefix embedded in a
+// cnon:sca- tokenize-result is one the mock can bind the token to: the
+// deterministic prefixes the mock assigns to its test cards, the "test" fallback
+// the frontend uses for a degenerate ccof id, or the prefix of a saved card in
+// the mock's ledger (the ccof: token prefix the frontend derives the minted
+// prefix from — square.ts / MockCardForm). A prefix outside this set cannot come
+// from a tokenize-result minted for a card this mock knows, so it is a forged or
+// wrong-card binding.
+func (m *MockClient) scaTokenizePrefixKnown(prefix string) bool {
+	if prefix == "test" {
+		return true
+	}
+	switch prefix {
+	case "4242", "4111", "5555", "3782":
+		return true
+	}
+	for _, card := range m.cardByToken {
+		if verificationTokenPrefixForSource(card.CardID) == prefix {
+			return true
+		}
+	}
+	return false
+}
+
+// validateSCATokenizeResult validates the deterministic binding the dev frontend
+// encodes into a cnon:sca- tokenize-result source against the charge: the
+// embedded amount must match the charge amount (Square binds buyer verification
+// to the exact amount), the encoded outcome must not be a buyer denial, and the
+// embedded card prefix must be one the mock can bind to. An arbitrary
+// cnon:sca-... string that does not carry the deterministic encoding is NOT a
+// genuine tokenize-result (the frontend always mints the bound shape), so it is
+// refused exactly like a raw nonce in the tokenize-result slot. Caller holds
+// m.mu; the source is already known to carry the cnon:sca- marker.
+func (m *MockClient) validateSCATokenizeResult(sourceID string, amount int64) error {
+	parsed, ok := parseSCATokenizeResult(sourceID)
+	if !ok {
+		return tokenizeResultVerificationRequiredError(sourceID, "does not carry the deterministic tokenize-result binding")
+	}
+	if parsed.denied {
+		return tokenizeResultVerificationRequiredError(sourceID, "the buyer denied the SCA challenge")
+	}
+	if parsed.amount != amount {
+		return tokenizeResultVerificationRequiredError(sourceID, fmt.Sprintf("bound to a different amount (%d vs %d)", parsed.amount, amount))
+	}
+	if !m.scaTokenizePrefixKnown(parsed.prefix) {
+		return tokenizeResultVerificationRequiredError(sourceID, fmt.Sprintf("bound to an unknown card prefix %q", parsed.prefix))
+	}
+	return nil
 }
 
 // isTokenLikeMock is the dev mock's PCI-DSS token predicate: it accepts the
-// production token shapes (cnon: nonces / ccof: card ids — isTokenLike) PLUS
-// the verify_mock_<prefix>_<amount> verification-token shape the dev frontend
-// mints, so dev remains walkable while the frontend's card form transitions to
-// minting cnon:sca- tokenize-results. The PRODUCTION client stays strict
-// (square_http_client.go isTokenLike — cnon:/ccof: only); this mock-only
-// widening never reaches real Square.
+// SAME production token shapes as the real client (cnon: nonces / ccof: card
+// ids — isTokenLike). The legacy verify_mock_<prefix>_<amount> source widening
+// is REMOVED: the transition to genuine cnon:sca- tokenize-results is complete
+// and real Square never accepts verify_mock_ as a source_id, so accepting it in
+// the mock would mask a bug that prod would reject. verify_mock_ tokens remain
+// valid in the VerificationToken field (the deprecated verifyBuyer() contract) —
+// only the source-slot widening is removed.
 func isTokenLikeMock(s string) bool {
-	return isTokenLike(s) || strings.HasPrefix(s, "verify_mock_")
+	return isTokenLike(s)
+}
+
+// legacyVerifyMockSourceError is the mock's explicit rejection of the
+// deprecated verify_mock_<prefix>_<amount>[_ok|_deny] charge-source shape the
+// dev frontend used during the token-shape transition. Real Square never
+// accepts verify_mock_ as a source_id (it is not a cnon:/ccof: token), so the
+// mock refuses it with a clear "legacy shape not supported" error instead of
+// silently accepting it — the old source widening (isTokenLikeMock) masked the
+// divergence.
+func legacyVerifyMockSourceError(s string) error {
+	return fmt.Errorf("square: %s is the legacy verify_mock_ transition shape and is NOT a valid charge source — real Square rejects it; mint a genuine tokenize-result (cnon:sca-<prefix>_<amount>_ok) or use a cnon:/ccof: token", tokenPrefix(s))
 }
 
 // resolveVerificationToken validates a supplied 3DS/SCA verification token for
@@ -580,9 +694,12 @@ func (m *MockClient) CreatePayment(ctx context.Context, req CreatePaymentReq) (*
 	body := mockPaymentWireBody(req)
 	// Match the real Square API: source_id must be a token (cnon:xxx nonce or
 	// ccof:xxx card ID). Raw PANs are rejected exactly as Square would, so the
-	// mock behaves identically to production (PCI-DSS parity). The mock's token
-	// predicate additionally accepts the verify_mock_* transition shape the dev
-	// frontend mints (isTokenLikeMock).
+	// mock behaves identically to production (PCI-DSS parity). The legacy
+	// verify_mock_* transition shape is rejected explicitly — real Square never
+	// accepts it as a source_id (isTokenLikeMock).
+	if strings.HasPrefix(body.SourceID, "verify_mock_") {
+		return nil, legacyVerifyMockSourceError(body.SourceID)
+	}
 	if !isTokenLikeMock(body.SourceID) {
 		return nil, fmt.Errorf("invalid source_id: %s — use a card nonce (cnon:xxx) or card ID (ccof:xxx)", tokenPrefix(body.SourceID))
 	}
@@ -695,7 +812,11 @@ func (m *MockClient) CreatePayment(ctx context.Context, req CreatePaymentReq) (*
 				Code:       "CARD_DECLINED_VERIFICATION_REQUIRED",
 				Category:   "PAYMENT_METHOD_ERROR",
 				Detail:     "card requires buyer verification (3DS/SCA); supply a verification token",
-				StatusCode: http.StatusBadRequest,
+				// Square's documented wire status for verification-required is
+				// 402 Payment Required, not 400 — the mock must match the WIRE
+				// (the handler's code-keyed classification is unchanged, and
+				// chargeFailureStatus maps any definitive 4xx to 402 anyway).
+				StatusCode: http.StatusPaymentRequired,
 				err:        errors.New("square: card requires buyer verification — verification_token required for a new-card (cnon:) charge"),
 			}
 		default:
@@ -747,11 +868,22 @@ func (m *MockClient) CreatePayment(ctx context.Context, req CreatePaymentReq) (*
 					Code:       "CARD_DECLINED_VERIFICATION_REQUIRED",
 					Category:   "PAYMENT_METHOD_ERROR",
 					Detail:     "unverified card nonce cannot be used as a saved-card (card-on-file) charge source; complete buyer verification (tokenizeWithVerification) first",
-					StatusCode: http.StatusBadRequest,
+					// 402 on the wire, matching Square's documented status for
+					// verification-required.
+					StatusCode: http.StatusPaymentRequired,
 					err:        fmt.Errorf("square: unverified cnon nonce %s cannot be used as a saved-card (card-on-file) charge source — a genuine tokenizeWithVerification result is required", tokenPrefix(body.SourceID)),
 				}
 			}
 			// (a) genuine tokenize-result — the token IS the buyer verification.
+			// The mock validates the deterministic binding the dev frontend
+			// encodes into the token (cnon:sca-<prefix>_<amount>_ok|_deny): the
+			// embedded amount must match the charge, the prefix must bind to a
+			// card the mock knows, and a _deny outcome is refused. An arbitrary
+			// cnon:sca-... string that does not prove verification for THIS
+			// charge is rejected exactly like real Square's SCA enforcement.
+			if err := m.validateSCATokenizeResult(body.SourceID, body.AmountMoney.Amount); err != nil {
+				return nil, err
+			}
 			log.Printf("[SQUARE-MOCK] CreatePayment saved-card SCA satisfied by tokenize-result: source %s (no verification_token needed)", tokenPrefix(body.SourceID))
 		case body.VerificationToken == "":
 			if m.grandfatheredCards[body.SourceID] {
@@ -770,7 +902,9 @@ func (m *MockClient) CreatePayment(ctx context.Context, req CreatePaymentReq) (*
 					Code:       "CARD_DECLINED_VERIFICATION_REQUIRED",
 					Category:   "PAYMENT_METHOD_ERROR",
 					Detail:     "saved card requires buyer verification (3DS/SCA); supply a verification token",
-					StatusCode: http.StatusBadRequest,
+					// 402 on the wire, matching Square's documented status for
+					// verification-required.
+					StatusCode: http.StatusPaymentRequired,
 					err:        errors.New("square: saved card requires buyer verification — verification_token required for a card-on-file (ccof:) charge"),
 				}
 			}
@@ -1084,6 +1218,17 @@ func (m *MockClient) ReplayPaymentByKey(ctx context.Context, snapshotJSON []byte
 	if err := json.Unmarshal(snapshotJSON, &req); err != nil {
 		return nil, fmt.Errorf("square: replay-by-key cannot parse stored request snapshot: %w", err)
 	}
+	// Mirror prod's replay-by-key sanity validation
+	// (replayPaymentByKeyHTTPWithClient, square_http_client.go): a snapshot
+	// missing source_id or idempotency_key is CORRUPT — never replay it as if
+	// it could succeed. Prod returns a plain (ambiguous) error here, so the
+	// sweep leaves the row PENDING for manual reconciliation; the mock must NOT
+	// answer ErrReplayKeyNotRetained (which the sweep treats as "the charge
+	// provably never happened" and definitively fails the row) — that is the
+	// OPPOSITE money decision on a snapshot we cannot trust.
+	if req.SourceID == "" || req.IdempotencyKey == "" {
+		return nil, fmt.Errorf("square: replay-by-key snapshot missing source_id/idempotency_key")
+	}
 	log.Printf("[SQUARE-MOCK] ReplayPaymentByKey: key=%s, source=%s", req.IdempotencyKey, tokenPrefix(req.SourceID))
 
 	m.mu.RLock()
@@ -1201,9 +1346,16 @@ func (m *MockClient) RefundPayment(ctx context.Context, req RefundPaymentReq) (*
 	// Known payments get the real Square over-refund rejection: refunding more
 	// than the remaining balance answers 400 REFUND_AMOUNT_INVALID. Square
 	// returns that SAME code for an already-refunded payment, so — exactly like
-	// the real client — the mock reconciles: an existing refund (money already
-	// moved) → ErrRefundAlreadyProcessed; no refund recorded → the amount is
-	// genuinely invalid → ErrRefundDeclined.
+	// the real client (refundPaymentHTTPWithClient's REFUND_AMOUNT_INVALID
+	// reconciliation via paymentRefundedExactlyWithClient) — the mock
+	// reconciles: an existing COMPLETED refund for the EXACT requested amount
+	// (that money provably already moved) → ErrRefundAlreadyProcessed; anything
+	// else → the amount is genuinely invalid → ErrRefundDeclined. A different
+	// amount is NEVER AlreadyProcessed: only an exact-amount refund covers the
+	// requested money, so an over-refund on top of a PARTIAL prior refund
+	// surfaces as ErrRefundDeclined (the row is marked failed, alerting the
+	// over-refund guard bug) instead of being resolved 'completed' and hiding
+	// it.
 	if payment, ok := m.payments[req.PaymentID]; ok {
 		remaining := payment.Amount
 		for _, r := range m.refunds {
@@ -1219,7 +1371,7 @@ func (m *MockClient) RefundPayment(ctx context.Context, req RefundPaymentReq) (*
 				StatusCode: http.StatusBadRequest,
 				err:        fmt.Errorf("square: refund amount %d exceeds remaining balance %d for payment %s", req.Amount, remaining, req.PaymentID),
 			}
-			if remaining < payment.Amount {
+			if m.refundExistsExact(req.PaymentID, req.Amount) {
 				return nil, fmt.Errorf("%w: %v", ErrRefundAlreadyProcessed, apiErr)
 			}
 			return nil, fmt.Errorf("%w: %v", ErrRefundDeclined, apiErr)
@@ -1269,6 +1421,22 @@ func (m *MockClient) RefundKeyCount() int {
 	return len(m.refundByKey)
 }
 
+// refundExistsExact reports whether the mock holds a COMPLETED refund for the
+// payment in the EXACT amount requested — the same exact-match reconciliation
+// the real client runs when Square returns REFUND_AMOUNT_INVALID
+// (paymentRefundedExactlyWithClient: status COMPLETED && amount == requested).
+// Only an exact-amount COMPLETED refund proves THIS requested money already
+// moved; a partial refund does not cover it. Caller holds m.mu (RefundPayment
+// holds the write lock).
+func (m *MockClient) refundExistsExact(paymentID string, amount int64) bool {
+	for _, r := range m.refunds {
+		if r.PaymentID == paymentID && r.Status == "COMPLETED" && r.Amount == amount {
+			return true
+		}
+	}
+	return false
+}
+
 // PaymentWasRefunded mirrors the real client's reconciliation source: true when
 // any refund with status COMPLETED, APPROVED, or PENDING exists for the payment
 // (FAILED/REJECTED refunds never moved money and are ignored). Shares the exact
@@ -1346,9 +1514,11 @@ func (m *MockClient) CreateCardOnFile(ctx context.Context, userID, cardToken, cu
 
 	// Match the real Square API: source_id must be a token (cnon:xxx nonce or
 	// ccof:xxx card ID). Raw PANs are rejected exactly as Square would, so the
-	// mock behaves identically to production. The mock's token predicate
-	// additionally accepts the verify_mock_* transition shape the dev frontend
-	// mints (isTokenLikeMock).
+	// mock behaves identically to production. The legacy verify_mock_* shape is
+	// rejected explicitly — real Square never accepts it as a source.
+	if strings.HasPrefix(cardToken, "verify_mock_") {
+		return nil, legacyVerifyMockSourceError(cardToken)
+	}
 	if !isTokenLikeMock(cardToken) {
 		return nil, fmt.Errorf("invalid source_id: %s — use a card nonce (cnon:xxx) or card ID (ccof:xxx)", tokenPrefix(cardToken))
 	}
diff --git a/backend/internal/square/square_dev_test.go b/backend/internal/square/square_dev_test.go
index 50540d0..a7c24a3 100644
--- a/backend/internal/square/square_dev_test.go
+++ b/backend/internal/square/square_dev_test.go
@@ -891,6 +891,64 @@ func TestDevClient_ReplayPaymentByKey_UnknownKey_NotRetained(t *testing.T) {
 	assert.True(t, errors.Is(err, ErrReplayKeyNotRetained), "unknown-key cnon replay must surface ErrReplayKeyNotRetained, got %v", err)
 }
 
+func TestDevClient_ReplayPaymentByKey_CorruptSnapshot_PlainError(t *testing.T) {
+	// Identical-body replay contract (FIX 4): a snapshot missing source_id or
+	// idempotency_key is CORRUPT. Prod (replayPaymentByKeyHTTPWithClient,
+	// square_http_client.go) returns a plain (ambiguous) error for such a
+	// snapshot — the sweep leaves the row PENDING for manual reconciliation.
+	// The mock must mirror that, NOT answer ErrReplayKeyNotRetained (which the
+	// sweep treats as "the charge provably never happened" and definitively
+	// fails the row) — that is the OPPOSITE money decision on a snapshot we
+	// cannot trust.
+	client := NewDevClient().(*MockClient)
+	ctx := context.Background()
+
+	t.Run("empty_source_is_plain_error", func(t *testing.T) {
+		_, err := client.ReplayPaymentByKey(ctx, replaySnapshotReq(t, CreatePaymentReq{
+			Amount:         5000,
+			Currency:       "GBP",
+			SourceID:       "",
+			IdempotencyKey: "key-empty-source",
+		}))
+		require.Error(t, err)
+		assert.False(t, errors.Is(err, ErrReplayKeyNotRetained), "a corrupt snapshot must NOT be treated as proof the charge never happened, got %v", err)
+		assert.Contains(t, err.Error(), "missing source_id/idempotency_key")
+		assert.Equal(t, "", ErrorCode(err), "a corrupt snapshot is an ambiguous plain error, not a structured Square rejection")
+	})
+
+	t.Run("empty_key_is_plain_error", func(t *testing.T) {
+		_, err := client.ReplayPaymentByKey(ctx, replaySnapshotReq(t, CreatePaymentReq{
+			Amount:         5000,
+			Currency:       "GBP",
+			SourceID:       "cnon:test-card",
+			IdempotencyKey: "",
+		}))
+		require.Error(t, err)
+		assert.False(t, errors.Is(err, ErrReplayKeyNotRetained), "a corrupt snapshot must NOT be treated as proof the charge never happened, got %v", err)
+		assert.Contains(t, err.Error(), "missing source_id/idempotency_key")
+	})
+
+	t.Run("empty_source_must_not_charge", func(t *testing.T) {
+		// Regression guard for the OLD mock behaviour: with a corrupt snapshot
+		// (empty source) the mock must not fall through to the unknown-key
+		// "attempt a real charge" path at all — a ccof-ish empty source must
+		// never mint a payment.
+		client.mu.RLock()
+		payCount := len(client.payments)
+		client.mu.RUnlock()
+		_, err := client.ReplayPaymentByKey(ctx, replaySnapshotReq(t, CreatePaymentReq{
+			Amount:         5000,
+			Currency:       "GBP",
+			SourceID:       "",
+			IdempotencyKey: "key-empty-source-2",
+		}))
+		require.Error(t, err)
+		client.mu.RLock()
+		defer client.mu.RUnlock()
+		assert.Equal(t, payCount, len(client.payments), "a corrupt snapshot must never be replayed into a charge")
+	})
+}
+
 func TestDevClient_ReplayPaymentByKey_UnknownKey_CcofSavedCard_ChargesAndRescues(t *testing.T) {
 	// B3 dev/prod parity: an unknown key with a STILL-VALID ccof: saved-card
 	// token makes real Square attempt a REAL charge that succeeds — the sweep
@@ -1419,6 +1477,53 @@ func TestDevClient_CreatePayment_RejectsRawPAN(t *testing.T) {
 	}
 }
 
+// TestDevClient_CreatePayment_RejectsLegacyVerifyMockSource locks the FIX 5
+// removal of the verify_mock_ source widening: the transition to genuine
+// cnon:sca- tokenize-results is complete and real Square NEVER accepts the
+// verify_mock_<prefix>_<amount> shape as a source_id — so the mock rejects it
+// explicitly with a clear legacy-shape error (a raw PAN-style "invalid
+// source_id" message would obscure the reason). verify_mock_ tokens remain
+// valid in the VerificationToken field (the deprecated verifyBuyer() contract)
+// — only the source-slot widening is removed.
+func TestDevClient_CreatePayment_RejectsLegacyVerifyMockSource(t *testing.T) {
+	client := NewDevClient().(*MockClient)
+	ctx := context.Background()
+
+	t.Run("create_payment_rejects_verify_mock_source", func(t *testing.T) {
+		result, err := client.CreatePayment(ctx, CreatePaymentReq{
+			Amount:         5000,
+			Currency:       "GBP",
+			SourceID:       "verify_mock_4242_5000_ok",
+			IdempotencyKey: "verify-mock-source",
+		})
+		require.Error(t, err)
+		assert.Nil(t, result)
+		assert.Contains(t, err.Error(), "legacy verify_mock_ transition shape")
+		assert.False(t, errors.Is(err, ErrRefundDeclined))
+	})
+
+	t.Run("create_card_on_file_rejects_verify_mock_source", func(t *testing.T) {
+		card, err := client.CreateCardOnFile(ctx, "user-verify-mock", "verify_mock_4242_5000_ok", "cus_test123")
+		require.Error(t, err)
+		assert.Nil(t, card)
+		assert.Contains(t, err.Error(), "legacy verify_mock_ transition shape")
+	})
+
+	t.Run("verify_mock_verification_token_still_accepted", func(t *testing.T) {
+		// The legacy verifyBuyer() contract (ccof: + verification_token) is
+		// retained for backward-compat — only the SOURCE widening is removed.
+		client.SimulateSavedCardVerificationRequired = true
+		result, err := client.CreatePayment(ctx, CreatePaymentReq{
+			Amount: 5000, Currency: "GBP",
+			SourceID: "ccof:mock_saved", CustomerID: "cus_test123",
+			IdempotencyKey:    "verify-mock-token-still-ok",
+			VerificationToken: "verify_mock_mock_5000_ok",
+		})
+		require.NoError(t, err)
+		require.Equal(t, "COMPLETED", result.Status)
+	})
+}
+
 // TestDevClient_CreatePayment_CardOnFileRequiresCustomerID verifies the mock
 // mirrors Square's real enforcement: charging a ccof: (card-on-file) token
 // without a customer_id is rejected with a structured 400 INVALID_REQUEST_ERROR
@@ -2047,10 +2152,13 @@ func TestDevClient_RefundPayment_UnknownPayMockID_NotFound(t *testing.T) {
 // TestDevClient_RefundPayment_OverRefund locks the mock's real Square
 // over-refund rejection: refunding more than the remaining balance answers 400
 // REFUND_AMOUNT_INVALID. Square returns that SAME code for an already-refunded
-// payment, so — exactly like the real client — the mock reconciles: no refund
-// recorded yet → the amount is genuinely invalid → ErrRefundDeclined; an
-// existing refund (money already moved) → ErrRefundAlreadyProcessed. The
-// exact-remaining boundary refund succeeds.
+// payment, so — exactly like the real client (paymentRefundedExactlyWithClient)
+// — the mock reconciles: an EXACT-amount COMPLETED refund for the requested
+// amount → ErrRefundAlreadyProcessed (that money provably moved); anything else
+// (no refund, or a PARTIAL prior refund that does not cover the requested
+// amount) → the amount is genuinely invalid → ErrRefundDeclined. A different
+// amount is NEVER AlreadyProcessed — the over-refund guard bug must surface in
+// dev too. The exact-remaining boundary refund succeeds.
 func TestDevClient_RefundPayment_OverRefund(t *testing.T) {
 	client := NewDevClient().(*MockClient)
 	ctx := context.Background()
@@ -2096,7 +2204,7 @@ func TestDevClient_RefundPayment_OverRefund(t *testing.T) {
 		assert.Equal(t, "COMPLETED", second.Status)
 	})
 
-	t.Run("over_refund_with_existing_refund_is_already_processed", func(t *testing.T) {
+	t.Run("over_refund_after_partial_refund_is_declined", func(t *testing.T) {
 		payment, err := client.CreatePayment(ctx, CreatePaymentReq{
 			Amount: 10000, Currency: "GBP", SourceID: "cnon:test-card", IdempotencyKey: "pay-overrefund-existing",
 		})
@@ -2109,15 +2217,41 @@ func TestDevClient_RefundPayment_OverRefund(t *testing.T) {
 		require.NoError(t, err)
 		require.Equal(t, "COMPLETED", first.Status)
 
+		// Over-refunding with a DIFFERENT amount (8000) is a genuine decline:
+		// no EXACT-amount refund for 8000 exists, so prod classifies it
+		// ErrRefundDeclined (the row is marked failed — the over-refund guard
+		// bug surfaces) — never ErrRefundAlreadyProcessed, which would hide it.
 		result, err := client.RefundPayment(ctx, RefundPaymentReq{
 			PaymentID: payment.ID, Amount: 8000, IdempotencyKey: "refund-overrefund-existing",
 		})
 		require.Error(t, err)
 		assert.Nil(t, result)
-		// The REFUND_AMOUNT_INVALID squareAPIError is hidden behind the %v
-		// sentinel wrap (ErrorCode returns ""), so the observable contract is
-		// the ErrRefundAlreadyProcessed sentinel.
-		assert.True(t, errors.Is(err, ErrRefundAlreadyProcessed), "an over-refund on a payment that already has refunds must reconcile to ErrRefundAlreadyProcessed, got %v", err)
+		assert.True(t, errors.Is(err, ErrRefundDeclined), "an over-refund on top of a PARTIAL prior refund must reconcile to ErrRefundDeclined (no exact-amount refund covers 8000), got %v", err)
+		assert.False(t, errors.Is(err, ErrRefundAlreadyProcessed))
+	})
+
+	t.Run("over_refund_exact_amount_already_refunded_is_already_processed", func(t *testing.T) {
+		payment, err := client.CreatePayment(ctx, CreatePaymentReq{
+			Amount: 10000, Currency: "GBP", SourceID: "cnon:test-card", IdempotencyKey: "pay-overrefund-exact",
+		})
+		require.NoError(t, err)
+
+		// An 8000 refund leaves 2000 remaining.
+		first, err := client.RefundPayment(ctx, RefundPaymentReq{
+			PaymentID: payment.ID, Amount: 8000, IdempotencyKey: "refund-exact-move",
+		})
+		require.NoError(t, err)
+		require.Equal(t, "COMPLETED", first.Status)
+
+		// Re-requesting the SAME 8000 (now an over-refund) matches an EXACT-
+		// amount COMPLETED refund — that money provably already moved, exactly
+		// like prod's paymentRefundedExactlyWithClient reconciliation.
+		result, err := client.RefundPayment(ctx, RefundPaymentReq{
+			PaymentID: payment.ID, Amount: 8000, IdempotencyKey: "refund-overrefund-exact",
+		})
+		require.Error(t, err)
+		assert.Nil(t, result)
+		assert.True(t, errors.Is(err, ErrRefundAlreadyProcessed), "an over-refund matching an existing EXACT-amount refund must reconcile to ErrRefundAlreadyProcessed, got %v", err)
 		assert.False(t, errors.Is(err, ErrRefundDeclined))
 	})
 }
@@ -2162,7 +2296,9 @@ func TestDevClient_CreatePayment_SimulateVerificationRequired(t *testing.T) {
 		assert.Nil(t, result)
 		assert.Equal(t, "CARD_DECLINED_VERIFICATION_REQUIRED", ErrorCode(err))
 		assert.Equal(t, "PAYMENT_METHOD_ERROR", ErrorCategory(err))
-		assert.Equal(t, http.StatusBadRequest, ErrorStatusCode(err))
+		// The WIRE status must be 402 (Square's documented status for
+		// verification-required), not 400.
+		assert.Equal(t, http.StatusPaymentRequired, ErrorStatusCode(err))
 		assert.True(t, IsDefinitivePaymentError(err), "CARD_DECLINED_VERIFICATION_REQUIRED must classify as a definitive payment error")
 	})
 
@@ -2281,7 +2417,9 @@ func TestDevClient_CreatePayment_SavedCardVerificationRequired(t *testing.T) {
 		assert.Nil(t, result)
 		assert.Equal(t, "CARD_DECLINED_VERIFICATION_REQUIRED", ErrorCode(err))
 		assert.Equal(t, "PAYMENT_METHOD_ERROR", ErrorCategory(err))
-		assert.Equal(t, http.StatusBadRequest, ErrorStatusCode(err))
+		// The WIRE status must be 402 (Square's documented status for
+		// verification-required), not 400.
+		assert.Equal(t, http.StatusPaymentRequired, ErrorStatusCode(err))
 		assert.True(t, IsDefinitivePaymentError(err), "CARD_DECLINED_VERIFICATION_REQUIRED must classify as a definitive payment error")
 	})
 
@@ -2674,7 +2812,7 @@ func TestCreatePayment_SCA_SavedCard_WireBody_ByteIdentical(t *testing.T) {
 	req := CreatePaymentReq{
 		Amount:         5000,
 		Currency:       "GBP",
-		SourceID:       "cnon:sca-tokenize-result",
+		SourceID:       "cnon:sca-4242_5000_ok",
 		IdempotencyKey: "sca-contract-key-1",
 		ReferenceID:    "booking-contract-1",
 		Note:           "full",
@@ -2700,7 +2838,7 @@ func TestCreatePayment_SCA_SavedCard_WireBody_ByteIdentical(t *testing.T) {
 	// contract, not a shared-but-wrong shape.
 	var wire map[string]any
 	require.NoError(t, json.Unmarshal(clientWire, &wire))
-	require.Equal(t, "cnon:sca-tokenize-result", wire["source_id"], "the tokenize-result token must be the source_id")
+	require.Equal(t, "cnon:sca-4242_5000_ok", wire["source_id"], "the tokenize-result token must be the source_id")
 	require.Equal(t, "cus_sca_123", wire["customer_id"], "customer_id must come from the saved card")
 	require.NotContains(t, wire, "verification_token", "the SCA tokenize-result path must not emit a legacy verification_token")
 	amt, ok := wire["amount_money"].(map[string]any)
@@ -2747,7 +2885,7 @@ func TestDevClient_CreatePayment_SavedCard_SCATokenizeResult_Accepted(t *testing
 	t.Run("tokenize_result_with_customer_is_sca_compliant", func(t *testing.T) {
 		result, err := client.CreatePayment(ctx, CreatePaymentReq{
 			Amount: 5000, Currency: "GBP",
-			SourceID: "cnon:sca-tokenize-1", CustomerID: "cus_sca_1",
+			SourceID: "cnon:sca-4242_5000_ok", CustomerID: "cus_sca_1",
 			IdempotencyKey: "sca-accept-1",
 		})
 		require.NoError(t, err)
@@ -2780,7 +2918,7 @@ func TestDevClient_CreatePayment_SavedCard_SCATokenizeResult_BothGatesOn(t *test
 	t.Run("tokenize_result_with_customer_passes_both_gates", func(t *testing.T) {
 		result, err := client.CreatePayment(ctx, CreatePaymentReq{
 			Amount: 5000, Currency: "GBP",
-			SourceID: "cnon:sca-both-on", CustomerID: "cus_sca_both",
+			SourceID: "cnon:sca-4242_5000_ok", CustomerID: "cus_sca_both",
 			IdempotencyKey: "both-on-accept",
 		})
 		require.NoError(t, err)
@@ -2826,14 +2964,16 @@ func TestDevClient_CreatePayment_SavedCard_RawNonceInTokenizeSlot_Rejected(t *te
 		require.Nil(t, result)
 		require.Equal(t, "CARD_DECLINED_VERIFICATION_REQUIRED", ErrorCode(err))
 		require.Equal(t, "PAYMENT_METHOD_ERROR", ErrorCategory(err))
-		require.Equal(t, http.StatusBadRequest, ErrorStatusCode(err))
+		// The WIRE status must be 402 (Square's documented status for
+		// verification-required), not 400.
+		require.Equal(t, http.StatusPaymentRequired, ErrorStatusCode(err))
 		require.True(t, IsDefinitivePaymentError(err), "a forged unverified nonce must be a definitive payment error")
 	})
 
 	t.Run("genuine_tokenize_result_still_accepted", func(t *testing.T) {
 		result, err := client.CreatePayment(ctx, CreatePaymentReq{
 			Amount: 5000, Currency: "GBP",
-			SourceID: "cnon:sca-tokenize-fresh", CustomerID: "cus_genuine",
+			SourceID: "cnon:sca-4242_5000_ok", CustomerID: "cus_genuine",
 			IdempotencyKey: "genuine-tokenize-result",
 		})
 		require.NoError(t, err)
@@ -2863,7 +3003,7 @@ func TestDevClient_CreatePayment_SavedCard_SCATokenizeResult_SingleUse(t *testin
 	client.SimulateSourceUsed = true
 	client.SimulateSavedCardVerificationRequired = true
 	ctx := context.Background()
-	source := "cnon:sca-single-use-booking"
+	source := "cnon:sca-4242_5000_ok"
 
 	// Booking A's charge consumes the tokenize-result.
 	first, err := client.CreatePayment(ctx, CreatePaymentReq{
@@ -2887,3 +3027,133 @@ func TestDevClient_CreatePayment_SavedCard_SCATokenizeResult_SingleUse(t *testin
 	require.Equal(t, http.StatusBadRequest, ErrorStatusCode(err))
 	require.Contains(t, client.UsedSources(), source, "the consumed tokenize-result must be reported by UsedSources")
 }
+
+// TestParseSCATokenizeResult pins the deterministic cnon:sca- tokenize-result
+// encoding the dev frontend mints (MockCardForm.tokenizeSavedCard /
+// verifySavedCard, square.ts's tokenizeSavedCardWithVerification fallback):
+// cnon:sca-<prefix>_<amount>[_ok|_deny], outcome suffix defaults to approval,
+// "_deny" sets denied=true, and anything malformed is not parseable.
+func TestParseSCATokenizeResult(t *testing.T) {
+	tests := []struct {
+		name       string
+		token      string
+		wantOK     bool
+		wantDenied bool
+		wantAmount int64
+		wantPrefix string
+	}{
+		{"explicit ok suffix", "cnon:sca-4242_5000_ok", true, false, 5000, "4242"},
+		{"deny suffix sets denied", "cnon:sca-4242_5000_deny", true, true, 5000, "4242"},
+		{"no suffix defaults to approval", "cnon:sca-mock_2500", true, false, 2500, "mock"},
+		{"prefix with underscores", "cnon:sca-visa_card_3000_deny", true, true, 3000, "visa_card"},
+		{"marker with empty rest", "cnon:sca-", false, false, 0, ""},
+		{"non-numeric amount", "cnon:sca-4242_abc_ok", false, false, 0, ""},
+		{"missing amount", "cnon:sca-4242_ok", false, false, 0, ""},
+		{"not a tokenize-result", "cnon:test-card", false, false, 0, ""},
+		{"raw tokenize-result marker", "cnon:sca-tokenize-result", false, false, 0, ""},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			parsed, ok := parseSCATokenizeResult(tt.token)
+			require.Equal(t, tt.wantOK, ok)
+			if ok {
+				require.Equal(t, tt.wantDenied, parsed.denied)
+				require.Equal(t, tt.wantAmount, parsed.amount)
+				require.Equal(t, tt.wantPrefix, parsed.prefix)
+			}
+		})
+	}
+}
+
+// TestDevClient_CreatePayment_SCATokenizeResult_BindingValidation locks the
+// FIX 2 contract: the mock validates the deterministic binding the dev frontend
+// encodes into a cnon:sca- tokenize-result. A token bound to the correct card
+// prefix + amount with an _ok outcome passes; a wrong-prefix, wrong-amount,
+// buyer-denied (_deny) or unparseable cnon:sca- token is refused with the
+// documented 402 CARD_DECLINED_VERIFICATION_REQUIRED (a definitive payment
+// error) — an arbitrary cnon:sca-... string must never pass the gate.
+func TestDevClient_CreatePayment_SCATokenizeResult_BindingValidation(t *testing.T) {
+	client := NewDevClient().(*MockClient)
+	client.SimulateSavedCardVerificationRequired = true
+	ctx := context.Background()
+
+	t.Run("valid_token_passes", func(t *testing.T) {
+		result, err := client.CreatePayment(ctx, CreatePaymentReq{
+			Amount: 5000, Currency: "GBP",
+			SourceID: "cnon:sca-4242_5000_ok", CustomerID: "cus_valid",
+			IdempotencyKey: "sca-binding-valid",
+		})
+		require.NoError(t, err)
+		require.Equal(t, "COMPLETED", result.Status)
+	})
+
+	t.Run("wrong_prefix_is_refused", func(t *testing.T) {
+		result, err := client.CreatePayment(ctx, CreatePaymentReq{
+			Amount: 5000, Currency: "GBP",
+			SourceID: "cnon:sca-9999_5000_ok", CustomerID: "cus_wrong_prefix",
+			IdempotencyKey: "sca-binding-wrong-prefix",
+		})
+		require.Error(t, err)
+		require.Nil(t, result)
+		require.Equal(t, "CARD_DECLINED_VERIFICATION_REQUIRED", ErrorCode(err))
+		require.Equal(t, http.StatusPaymentRequired, ErrorStatusCode(err))
+		require.True(t, IsDefinitivePaymentError(err))
+	})
+
+	t.Run("wrong_amount_is_refused", func(t *testing.T) {
+		result, err := client.CreatePayment(ctx, CreatePaymentReq{
+			Amount: 5000, Currency: "GBP",
+			SourceID: "cnon:sca-4242_9999_ok", CustomerID: "cus_wrong_amount",
+			IdempotencyKey: "sca-binding-wrong-amount",
+		})
+		require.Error(t, err)
+		require.Nil(t, result)
+		require.Equal(t, "CARD_DECLINED_VERIFICATION_REQUIRED", ErrorCode(err))
+		require.Equal(t, http.StatusPaymentRequired, ErrorStatusCode(err))
+		require.True(t, IsDefinitivePaymentError(err))
+	})
+
+	t.Run("deny_outcome_is_refused", func(t *testing.T) {
+		result, err := client.CreatePayment(ctx, CreatePaymentReq{
+			Amount: 5000, Currency: "GBP",
+			SourceID: "cnon:sca-4242_5000_deny", CustomerID: "cus_denied",
+			IdempotencyKey: "sca-binding-deny",
+		})
+		require.Error(t, err)
+		require.Nil(t, result)
+		require.Equal(t, "CARD_DECLINED_VERIFICATION_REQUIRED", ErrorCode(err))
+		require.Equal(t, http.StatusPaymentRequired, ErrorStatusCode(err))
+		require.True(t, IsDefinitivePaymentError(err))
+	})
+
+	t.Run("unparseable_cnon_sca_is_refused", func(t *testing.T) {
+		result, err := client.CreatePayment(ctx, CreatePaymentReq{
+			Amount: 5000, Currency: "GBP",
+			SourceID: "cnon:sca-garbage", CustomerID: "cus_forged",
+			IdempotencyKey: "sca-binding-forged",
+		})
+		require.Error(t, err)
+		require.Nil(t, result)
+		require.Equal(t, "CARD_DECLINED_VERIFICATION_REQUIRED", ErrorCode(err))
+		require.Equal(t, http.StatusPaymentRequired, ErrorStatusCode(err))
+		require.True(t, IsDefinitivePaymentError(err))
+	})
+
+	t.Run("mock_ledger_card_prefix_is_accepted", func(t *testing.T) {
+		// The frontend derives the token prefix from the SAVED card's ccof id
+		// (first 4 chars after ccof:). A card this mock created in its ledger
+		// ("ccof:mock_...") mints tokens bound to the "mock" prefix — the mock
+		// must recognise that prefix as valid.
+		card, err := client.CreateCardOnFile(ctx, "user-sca-ledger", "cnon:token-ledger", "cus_ledger")
+		require.NoError(t, err)
+		require.True(t, strings.HasPrefix(card.CardID, "ccof:mock_"), "mock-created cards use the ccof:mock_ shape")
+
+		result, err := client.CreatePayment(ctx, CreatePaymentReq{
+			Amount: 5000, Currency: "GBP",
+			SourceID: "cnon:sca-mock_5000_ok", CustomerID: "cus_ledger",
+			IdempotencyKey: "sca-binding-ledger",
+		})
+		require.NoError(t, err)
+		require.Equal(t, "COMPLETED", result.Status)
+	})
+}
diff --git a/backend/internal/square/square_http_client.go b/backend/internal/square/square_http_client.go
index 8c1da5c..283830d 100644
--- a/backend/internal/square/square_http_client.go
+++ b/backend/internal/square/square_http_client.go
@@ -94,7 +94,14 @@ func SquareLocationID() string {
 func newHTTPClient() *httpClient {
 	env := SquareEnvironment()
 	baseURL := squareSandboxURL
-	if env == "production" {
+	// Empty/unknown SQUARE_ENVIRONMENT is production-ENFORCED, mirroring
+	// payments.IsExplicitDevOrMockEnv()'s fail-closed interpretation (an
+	// unset/mistyped env var must never downgrade to a less safe default), so
+	// the production base URL is used for both "production" and "". Only an
+	// explicit sandbox or dev/mock value resolves to the sandbox base URL (the
+	// dev/mock values never construct a real HTTP client anyway).
+	switch env {
+	case "production", "":
 		baseURL = squareProductionURL
 	}
 	return &httpClient{
@@ -106,6 +113,24 @@ func newHTTPClient() *httpClient {
 	}
 }
 
+// ValidateCredentials is the fail-closed startup validation for the REAL
+// Square API credentials. It is called from main.go's startup gate ONLY for
+// non-dev/mock deployments (main.go checks payments.IsExplicitDevOrMockEnv
+// first — the env interpretation stays in the payments package to avoid a
+// drifting duplicate read). With a real deployment selected, a missing
+// SQUARE_ACCESS_TOKEN or SQUARE_LOCATION_ID would make every Square API call
+// fail at runtime (401 auth, or request bodies without location_id), breaking
+// payments/refunds — so the process refuses to start, mirroring the
+// SQUARE_WEBHOOK_SIGNATURE_KEY gate. Never includes the secret values.
+func ValidateCredentials() {
+	if strings.TrimSpace(os.Getenv("SQUARE_ACCESS_TOKEN")) == "" {
+		log.Fatalf("FATAL: SQUARE_ACCESS_TOKEN environment variable not set with SQUARE_ENVIRONMENT=%q (non-mock) — every Square API call would be rejected (401) and online payment/refund processing is broken. Set the access token from the Square Developer Dashboard.", SquareEnvironment())
+	}
+	if strings.TrimSpace(SquareLocationID()) == "" {
+		log.Fatalf("FATAL: SQUARE_LOCATION_ID environment variable not set with SQUARE_ENVIRONMENT=%q (non-mock) — Square payment requests would carry no location_id and fail at runtime. Set the location ID from the Square Developer Dashboard.", SquareEnvironment())
+	}
+}
+
 func (c *httpClient) doJSON(ctx context.Context, method, path string, body, target any) error {
 	if c.token == "" {
 		return fmt.Errorf("square: SQUARE_ACCESS_TOKEN is not set")
@@ -1008,7 +1033,16 @@ func listRefundsHTTP(ctx context.Context, paymentID string, beginTime time.Time)
 }
 
 func listRefundsHTTPWithClient(ctx context.Context, paymentID string, beginTime time.Time, hc *httpClient) ([]RefundResult, error) {
-	base := "/v2/refunds?begin_time=" + url.QueryEscape(beginTime.UTC().Format(time.RFC3339)) + "&limit=100"
+	// begin_time is optional at Square; a ZERO time must OMIT the param
+	// entirely. Formatting the zero time would send Square a year-1 timestamp
+	// (0001-01-01T00:00:00Z) that Square may reject. The reconciliation callers
+	// (PaymentWasRefunded / paymentRefundedExactlyWithClient) pass time.Time{}
+	// deliberately — they want ALL refunds, so the param must be absent, not
+	// "before year 1".
+	base := "/v2/refunds?limit=100"
+	if !beginTime.IsZero() {
+		base = "/v2/refunds?begin_time=" + url.QueryEscape(beginTime.UTC().Format(time.RFC3339)) + "&limit=100"
+	}
 	path := base
 	results := []RefundResult{}
 	for page := 0; page < 20; page++ {
diff --git a/backend/internal/square/square_http_client_test.go b/backend/internal/square/square_http_client_test.go
index f561fff..cc80d31 100644
--- a/backend/internal/square/square_http_client_test.go
+++ b/backend/internal/square/square_http_client_test.go
@@ -12,10 +12,15 @@ import (
 	"io"
 	"net/http"
 	"net/http/httptest"
+	"net/url"
+	"os"
+	"os/exec"
 	"strings"
 	"testing"
 	"time"
 	"unicode/utf8"
+
+	"github.com/stretchr/testify/require"
 )
 
 // TestReplayPaymentByKeyHTTP_IdenticalBody verifies the replay-by-key sends the
@@ -783,6 +788,54 @@ func TestListRefundsHTTP_Pagination(t *testing.T) {
 	})
 }
 
+// TestListRefundsHTTP_ZeroBeginTime_OmitsParam locks the FIX 6 parity: a ZERO
+// begin_time must OMIT the begin_time query param entirely — formatting the
+// zero time would send Square a year-1 timestamp (0001-01-01T00:00:00Z) it may
+// reject. The reconciliation callers (PaymentWasRefunded /
+// paymentRefundedExactlyWithClient) pass time.Time{} deliberately to list ALL
+// refunds, so the param must be absent, not "before year 1". A non-zero
+// begin_time still emits begin_time as before.
+func TestListRefundsHTTP_ZeroBeginTime_OmitsParam(t *testing.T) {
+	var queries []string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		queries = append(queries, r.URL.RawQuery)
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"refunds":[],"cursor":""}`))
+	}))
+	defer srv.Close()
+
+	hc := &httpClient{baseURL: srv.URL, token: "t", http: srv.Client()}
+
+	// Zero time (the reconciliation shape) → begin_time must be absent.
+	_, err := listRefundsHTTPWithClient(context.Background(), "pay_zero", time.Time{}, hc)
+	if err != nil {
+		t.Fatalf("listRefundsHTTPWithClient failed: %v", err)
+	}
+	if len(queries) != 1 {
+		t.Fatalf("expected 1 request, got %d", len(queries))
+	}
+	if strings.Contains(queries[0], "begin_time=") {
+		t.Errorf("expected NO begin_time for a zero begin_time, got %q", queries[0])
+	}
+	if !strings.Contains(queries[0], "limit=100") {
+		t.Errorf("expected limit=100 in the zero-time query, got %q", queries[0])
+	}
+
+	// Non-zero time → begin_time is emitted with the RFC3339 timestamp.
+	queries = nil
+	begin := time.Date(2026, 7, 1, 0, 0, 0, 0, time.UTC)
+	_, err = listRefundsHTTPWithClient(context.Background(), "pay_begin", begin, hc)
+	if err != nil {
+		t.Fatalf("listRefundsHTTPWithClient failed: %v", err)
+	}
+	if len(queries) != 1 {
+		t.Fatalf("expected 1 request, got %d", len(queries))
+	}
+	if !strings.Contains(queries[0], "begin_time="+url.QueryEscape("2026-07-01T00:00:00Z")) {
+		t.Errorf("expected begin_time=2026-07-01T00:00:00Z in query, got %q", queries[0])
+	}
+}
+
 // TestCreateCardOnFileHTTP_IdempotencyKey verifies the deterministic SHA-256
 // idempotency key derivation and the request wire shape (source_id + card).
 func TestCreateCardOnFileHTTP_IdempotencyKey(t *testing.T) {
@@ -1889,8 +1942,12 @@ func TestPaymentWasRefunded(t *testing.T) {
 				if r.URL.Path != "/v2/refunds" {
 					t.Errorf("expected /v2/refunds, got %s", r.URL.Path)
 				}
-				if !strings.Contains(r.URL.RawQuery, "begin_time=") {
-					t.Errorf("expected begin_time in query, got %q", r.URL.RawQuery)
+				if strings.Contains(r.URL.RawQuery, "begin_time=") {
+					// The reconciliation passes a ZERO begin_time (time.Time{})
+					// to mean "all refunds"; the param must be ABSENT (FIX 6) —
+					// sending begin_time=0001-01-01T00:00:00Z is a year-1
+					// timestamp Square may reject.
+					t.Errorf("expected NO begin_time in the zero-time reconciliation query, got %q", r.URL.RawQuery)
 				}
 				w.Header().Set("Content-Type", "application/json")
 				if tc.statusCode != 0 {
@@ -1946,8 +2003,13 @@ func TestRefundPaymentHTTP_RefundAmountInvalidReconciliation(t *testing.T) {
 					if r.URL.Path != "/v2/refunds" {
 						t.Errorf("expected reconciliation GET /v2/refunds, got %s", r.URL.Path)
 					}
-					if !strings.Contains(r.URL.RawQuery, "begin_time=") {
-						t.Errorf("expected begin_time in reconciliation query, got %q", r.URL.RawQuery)
+					if !strings.Contains(r.URL.RawQuery, "limit=100") {
+						t.Errorf("expected limit=100 in reconciliation query, got %q", r.URL.RawQuery)
+					}
+					if strings.Contains(r.URL.RawQuery, "begin_time=") {
+						// The reconciliation passes a ZERO begin_time to mean
+						// "all refunds"; the param must be ABSENT (FIX 6).
+						t.Errorf("expected NO begin_time in the zero-time reconciliation query, got %q", r.URL.RawQuery)
 					}
 					_, _ = w.Write([]byte(tc.refundList))
 					return
@@ -2090,3 +2152,72 @@ func TestCreatePaymentHTTP_SCASavedCard_WireShape(t *testing.T) {
 		t.Errorf("expected location_id, got %v", captured["location_id"])
 	}
 }
+
+// TestNewHTTPClient_BaseURLSelection locks the SQUARE_ENVIRONMENT → base URL
+// resolution (FIX 3b): an empty SQUARE_ENVIRONMENT is production-ENFORCED
+// (mirroring payments.IsExplicitDevOrMockEnv's fail-closed interpretation —
+// an unset env var must never downgrade to the sandbox), so empty resolves to
+// the production base URL, exactly like "production". Only an explicit
+// "sandbox" or a dev/mock value resolves to the sandbox base URL.
+func TestNewHTTPClient_BaseURLSelection(t *testing.T) {
+	cases := []struct {
+		name string
+		env  string
+		want string
+	}{
+		{name: "production_env", env: "production", want: squareProductionURL},
+		{name: "empty_env_is_production_enforced", env: "", want: squareProductionURL},
+		{name: "sandbox_env", env: "sandbox", want: squareSandboxURL},
+		{name: "mock_env", env: "mock", want: squareSandboxURL},
+		{name: "dev_env", env: "dev", want: squareSandboxURL},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Setenv("SQUARE_ENVIRONMENT", tc.env)
+			if got := newHTTPClient().baseURL; got != tc.want {
+				t.Errorf("SQUARE_ENVIRONMENT=%q: expected base URL %s, got %s", tc.env, tc.want, got)
+			}
+		})
+	}
+}
+
+// TestValidateCredentials_FatalBranch_Exits covers the fail-closed startup
+// validation of the REAL Square API credentials (FIX 3a): when the
+// production-enforced gate in main.go (non-dev/mock SQUARE_ENVIRONMENT) calls
+// ValidateCredentials and SQUARE_ACCESS_TOKEN or SQUARE_LOCATION_ID is
+// missing, the process must refuse to start (log.Fatalf → os.Exit). log.Fatalf
+// cannot run in-process, so the test re-executes the test binary with marker
+// env vars and asserts the subprocess exits non-zero with a FATAL message
+// naming the missing credential.
+func TestValidateCredentials_FatalBranch_Exits(t *testing.T) {
+	if os.Getenv("GO_WANT_HELPER_PROCESS") == "1" {
+		t.Setenv("SQUARE_ENVIRONMENT", "production")
+		switch os.Getenv("SQUARE_CRED_BRANCH") {
+		case "MISSING_TOKEN":
+			t.Setenv("SQUARE_ACCESS_TOKEN", "")
+			t.Setenv("SQUARE_LOCATION_ID", "L_TEST")
+		case "MISSING_LOCATION":
+			t.Setenv("SQUARE_ACCESS_TOKEN", "tok")
+			t.Setenv("SQUARE_LOCATION_ID", "")
+		}
+		ValidateCredentials()
+		return
+	}
+	for _, tc := range []struct {
+		name   string
+		branch string
+		wantIn string
+	}{
+		{name: "missing_token_exits", branch: "MISSING_TOKEN", wantIn: "SQUARE_ACCESS_TOKEN"},
+		{name: "missing_location_exits", branch: "MISSING_LOCATION", wantIn: "SQUARE_LOCATION_ID"},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			cmd := exec.Command(os.Args[0], "-test.run=TestValidateCredentials_FatalBranch_Exits")
+			cmd.Env = append(os.Environ(), "GO_WANT_HELPER_PROCESS=1", "SQUARE_CRED_BRANCH="+tc.branch)
+			out, err := cmd.CombinedOutput()
+			require.Error(t, err, "expected the branch to exit (log.Fatalf); output: %s", out)
+			require.Contains(t, string(out), tc.wantIn, "the fatal log must name the missing credential: %s", out)
+			require.False(t, strings.Contains(string(out), "unexpected argument"), "the helper must not fail on argument parsing: %s", out)
+		})
+	}
+}
diff --git a/backend/main.go b/backend/main.go
index 6b94886..77a4f92 100644
--- a/backend/main.go
+++ b/backend/main.go
@@ -237,16 +237,20 @@ func initSquare() {
 	checkSnapshotEncKey()
 	checkProxyRateLimitConfig()
 	checkWebhookSignatureKey()
+	checkSquareCredentials()
 }
 
 // checkSnapshotEncKey validates SNAPSHOT_ENC_KEY at startup in non-mock
 // deployments. charge_helpers.snapshotEncKey() (handlers/payments) parses the
 // key on every call and silently falls back to storing square_request_snapshot
 // rows PLAINTEXT (buyer PII: email + ccof card tokens) with a one-time CRITICAL
-// log. This startup check makes the misconfiguration unmissable at boot: the
-// key must be present and decode to exactly 32 bytes (AES-256). Money-safety
-// first — it warns CRITICAL but does NOT fail the process (a failing startup
-// would strand pending replayable snapshots), matching the runtime fallback.
+// log. That runtime fallback is a money-safety convenience for the dev/mock
+// stack (no real PII), but in a REAL deployment a missing/invalid key would
+// leave buyer email + ccof card tokens unencrypted at rest — so this startup
+// check FAILS CLOSED there, mirroring the JWT_SECRET_KEY gate (main.go init):
+// in a non-mock environment the key must be present and decode to exactly 32
+// bytes (AES-256), or the process refuses to start. The warn+plaintext-fallback
+// posture survives ONLY in explicit dev/mock environments (no real data).
 func checkSnapshotEncKey() {
 	if payments.IsExplicitDevOrMockEnv() {
 		return
@@ -254,15 +258,14 @@ func checkSnapshotEncKey() {
 	raw := strings.TrimSpace(os.Getenv("SNAPSHOT_ENC_KEY"))
 	switch {
 	case raw == "":
-		log.Printf("CRITICAL: SNAPSHOT_ENC_KEY is not set with SQUARE_ENVIRONMENT=%q (non-mock) — square_request_snapshot rows (buyer PII: email + ccof card tokens) will be stored PLAINTEXT at rest. Generate a base64-encoded 32-byte key with `openssl rand -base64 32`.", os.Getenv("SQUARE_ENVIRONMENT"))
-		return
+		log.Fatalf("FATAL: SNAPSHOT_ENC_KEY is not set with SQUARE_ENVIRONMENT=%q (non-mock) — square_request_snapshot rows (buyer PII: email + ccof card tokens) would be stored PLAINTEXT at rest. Generate a base64-encoded 32-byte key with `openssl rand -base64 32`.", os.Getenv("SQUARE_ENVIRONMENT"))
 	default:
 		decoded, err := base64.StdEncoding.DecodeString(raw)
 		switch {
 		case err != nil:
-			log.Printf("CRITICAL: SNAPSHOT_ENC_KEY is not valid base64 (%v) with SQUARE_ENVIRONMENT=%q (non-mock) — square_request_snapshot rows will be stored PLAINTEXT at rest. Generate a base64-encoded 32-byte key with `openssl rand -base64 32`.", err, os.Getenv("SQUARE_ENVIRONMENT"))
+			log.Fatalf("FATAL: SNAPSHOT_ENC_KEY is not valid base64 (%v) with SQUARE_ENVIRONMENT=%q (non-mock) — square_request_snapshot rows would be stored PLAINTEXT at rest. Generate a base64-encoded 32-byte key with `openssl rand -base64 32`.", err, os.Getenv("SQUARE_ENVIRONMENT"))
 		case len(decoded) != 32:
-			log.Printf("CRITICAL: SNAPSHOT_ENC_KEY must decode to exactly 32 bytes for AES-256 (got %d) with SQUARE_ENVIRONMENT=%q (non-mock) — square_request_snapshot rows will be stored PLAINTEXT at rest. Generate a base64-encoded 32-byte key with `openssl rand -base64 32`.", len(decoded), os.Getenv("SQUARE_ENVIRONMENT"))
+			log.Fatalf("FATAL: SNAPSHOT_ENC_KEY must decode to exactly 32 bytes for AES-256 (got %d) with SQUARE_ENVIRONMENT=%q (non-mock) — square_request_snapshot rows would be stored PLAINTEXT at rest. Generate a base64-encoded 32-byte key with `openssl rand -base64 32`.", len(decoded), os.Getenv("SQUARE_ENVIRONMENT"))
 		}
 	}
 }
@@ -307,9 +310,11 @@ func checkProxyRateLimitConfig() {
 // verification runs against that string, so every GENUINE Square event fails
 // signature verification (403) and payment/refund reconciliation is silently
 // broken. The key signals intent to use webhooks; the missing URL breaks them.
-// This is WARN not FATAL: the handler is fail-closed, so no event — genuine or
-// forged — can mutate state, making it availability-only (like
-// checkProxyRateLimitConfig), not a security hole.
+// A key with no URL is therefore FATAL (not a warning): the operator has
+// explicitly configured webhooks, so every event silently failing signature
+// verification would strand payment/refund reconciliation mid-run. The fail
+// stays availability-only (the handler is fail-closed, so no event — genuine
+// or forged — can mutate state), but it must be unmissable at boot.
 func checkWebhookSignatureKey() {
 	if payments.IsExplicitDevOrMockEnv() {
 		return
@@ -320,7 +325,7 @@ func checkWebhookSignatureKey() {
 	case keySet && urlSet:
 		return
 	case keySet && !urlSet:
-		log.Printf("WARNING: SQUARE_WEBHOOK_SIGNATURE_KEY IS set but SQUARE_WEBHOOK_NOTIFICATION_URL is unset with SQUARE_ENVIRONMENT=%q (non-mock) — the webhook handler falls back to the default http://localhost:8080/webhooks/square, so every GENUINE Square event fails signature verification (403, fail-closed) and payment/refund reconciliation is silently broken. Set SQUARE_WEBHOOK_NOTIFICATION_URL to exactly the notification URL configured in the Square Dashboard webhook subscription.", os.Getenv("SQUARE_ENVIRONMENT"))
+		log.Fatalf("FATAL: SQUARE_WEBHOOK_SIGNATURE_KEY IS set but SQUARE_WEBHOOK_NOTIFICATION_URL is unset with SQUARE_ENVIRONMENT=%q (non-mock) — the webhook handler falls back to the default http://localhost:8080/webhooks/square, so every GENUINE Square event fails signature verification (403, fail-closed) and payment/refund reconciliation is silently broken. Set SQUARE_WEBHOOK_NOTIFICATION_URL to exactly the notification URL configured in the Square Dashboard webhook subscription.", os.Getenv("SQUARE_ENVIRONMENT"))
 	case !keySet && urlSet:
 		log.Fatalf("FATAL: SQUARE_WEBHOOK_SIGNATURE_KEY environment variable not set with SQUARE_ENVIRONMENT=%q (non-mock) while SQUARE_WEBHOOK_NOTIFICATION_URL IS set — Square webhook events (payment/refund reconciliation) would be rejected fail-closed at runtime. Generate the signing key in the Square Dashboard webhook subscription and set it in .env.", os.Getenv("SQUARE_ENVIRONMENT"))
 	default:
@@ -328,6 +333,19 @@ func checkWebhookSignatureKey() {
 	}
 }
 
+// checkSquareCredentials fail-closes the REAL Square API credentials at
+// startup in non-mock deployments. The env interpretation (dev/mock gate)
+// stays here via payments.IsExplicitDevOrMockEnv — the single source — and
+// the value validation itself lives in internal/square
+// (square.ValidateCredentials) next to the client that consumes the
+// credentials, so the check cannot drift from the code that reads them.
+func checkSquareCredentials() {
+	if payments.IsExplicitDevOrMockEnv() {
+		return
+	}
+	square.ValidateCredentials()
+}
+
 func healthCheckHandler(w http.ResponseWriter, r *http.Request) {
 	status := "ok"
 	services := map[string]string{
diff --git a/backend/mw/ratelimit_shared.go b/backend/mw/ratelimit_shared.go
index 00d5055..672e234 100644
--- a/backend/mw/ratelimit_shared.go
+++ b/backend/mw/ratelimit_shared.go
@@ -48,6 +48,18 @@ var trustProxyHeaders = func() bool {
 // middleware registration read this single source of truth.
 func TrustProxyHeaders() bool { return trustProxyHeaders }
 
+// validIPString reports whether s parses as a syntactically valid IP address
+// (net.ParseIP). Defense-in-depth for the trusted-proxy header path: the
+// TRUST_PROXY_HEADERS flag MUST only be set behind a proxy that overwrites
+// X-Real-IP/CF-Connecting-IP with the real client IP itself — but if it is
+// ever mis-set (or a misbehaving proxy echoes the client's header), garbage
+// values must not become rate-limit keys. A comma-joined chain
+// ("123.45.67.89, 1.2.3.4"), an IP:port, or a non-IP string would otherwise
+// mint a fresh bucket per request and bypass per-IP limiting entirely.
+func validIPString(s string) bool {
+	return net.ParseIP(s) != nil
+}
+
 // ClientIP derives the per-client IP for security-sensitive handlers that need
 // a client-address key (reservation ipHash, per-IP audit trails) using the
 // SAME gated resolution as the rate limiter. Priority:
@@ -57,23 +69,26 @@ func TrustProxyHeaders() bool { return trustProxyHeaders }
 //     real_ip module validated it against the set_real_ip_from ranges) has
 //     already overwritten it with the real client IP, so it is unspoofable
 //     there. Ignored by default because an origin-exposed backend must never
-//     trust a client-controlled value (B7).
+//     trust a client-controlled value (B7). Even when trusted, the value must
+//     parse as a valid IP (validIPString) — defense-in-depth against a
+//     mis-set flag behind a header-echoing proxy.
 //  2. middleware.GetClientIP(r.Context()) — the X-Real-IP value nginx sets
 //     from $remote_addr, captured by middleware.ClientIPFromHeader("X-Real-IP")
 //     in main.go. That middleware is registered only when
-//     TRUST_PROXY_HEADERS=true, so it too is trusted solely behind a proxy.
+//     TRUST_PROXY_HEADERS=true, so it too is trusted solely behind a proxy;
+//     the same valid-IP check applies before it is accepted as the key.
 //  3. net.SplitHostPort(r.RemoteAddr) / r.RemoteAddr fallback — the actual
 //     TCP peer; the only key source usable when the backend is origin-exposed.
 func ClientIP(r *http.Request) string {
 	if trustProxyHeaders {
-		if ip := r.Header.Get("CF-Connecting-IP"); ip != "" {
+		if ip := r.Header.Get("CF-Connecting-IP"); validIPString(ip) {
 			return ip
 		}
 	}
-	if ip := middleware.GetClientIP(r.Context()); ip != "" {
+	if ip := middleware.GetClientIP(r.Context()); validIPString(ip) {
 		return ip
 	}
-	if ip, _, err := net.SplitHostPort(r.RemoteAddr); err == nil && ip != "" {
+	if ip, _, err := net.SplitHostPort(r.RemoteAddr); err == nil && validIPString(ip) {
 		return ip
 	}
 	return r.RemoteAddr
diff --git a/backend/mw/ratelimit_test.go b/backend/mw/ratelimit_test.go
index 223947d..34f7405 100644
--- a/backend/mw/ratelimit_test.go
+++ b/backend/mw/ratelimit_test.go
@@ -304,6 +304,68 @@ func TestClientIP_RemoteAddrWithoutPort_ReturnedAsIs(t *testing.T) {
 	}
 }
 
+// TestClientIP_XRealIP_GarbageHeaderFallsBackToRemoteAddr verifies the
+// defense-in-depth validation (FIX 4): even with TRUST_PROXY_HEADERS=true, an
+// X-Real-IP carrying a comma-joined chain ("123.45.67.89, 1.2.3.4"), an
+// IP:port, or a non-IP string must NOT become the rate-limit key — it falls
+// back to the TCP peer instead. The flag must only be set behind a trusted
+// proxy that overwrites the header itself; this validation ensures a mis-set
+// flag (or a header-echoing proxy) cannot mint a fresh bucket per request and
+// bypass per-IP limiting.
+func TestClientIP_XRealIP_GarbageHeaderFallsBackToRemoteAddr(t *testing.T) {
+	setTrustProxyHeaders(t, true)
+	for _, tc := range []struct {
+		name   string
+		header string
+	}{
+		{name: "comma_joined_chain", header: "123.45.67.89, 1.2.3.4"},
+		{name: "ip_with_port", header: "198.51.100.42:8080"},
+		{name: "garbage_string", header: "not-an-ip"},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			var got string
+			h := middleware.ClientIPFromHeader("X-Real-IP")(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				got = clientIP(r)
+				w.WriteHeader(http.StatusOK)
+			}))
+
+			req := httptest.NewRequest(http.MethodGet, "/", nil)
+			req.RemoteAddr = "192.0.2.1:1234"
+			req.Header.Set("X-Real-IP", tc.header)
+			h.ServeHTTP(httptest.NewRecorder(), req)
+
+			if got != "192.0.2.1" {
+				t.Errorf("expected garbage X-Real-IP %q to fall back to RemoteAddr 192.0.2.1, got %q", tc.header, got)
+			}
+		})
+	}
+}
+
+// TestClientIP_CFConnectingIP_GarbageHeaderFallsBack verifies the same
+// defense-in-depth for CF-Connecting-IP: even when trusted, a non-IP value
+// (comma-joined chain, port, garbage) must not become the rate-limit key.
+func TestClientIP_CFConnectingIP_GarbageHeaderFallsBack(t *testing.T) {
+	setTrustProxyHeaders(t, true)
+	for _, tc := range []struct {
+		name   string
+		header string
+	}{
+		{name: "comma_joined_chain", header: "203.0.113.7, 198.51.100.9"},
+		{name: "ip_with_port", header: "203.0.113.7:8080"},
+		{name: "garbage_string", header: "spoofed!"},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			req := httptest.NewRequest(http.MethodGet, "/", nil)
+			req.RemoteAddr = "192.0.2.2:1234"
+			req.Header.Set("CF-Connecting-IP", tc.header)
+
+			if got := clientIP(req); got != "192.0.2.2" {
+				t.Errorf("expected garbage CF-Connecting-IP %q to fall back to RemoteAddr 192.0.2.2, got %q", tc.header, got)
+			}
+		})
+	}
+}
+
 // ============================================================
 // RateLimit middleware — 429 on burst, pass-through under limit,
 // per-key buckets (batch-1 fix regression)
diff --git a/backend/startup_checks_test.go b/backend/startup_checks_test.go
index fa6fd48..c0c3850 100644
--- a/backend/startup_checks_test.go
+++ b/backend/startup_checks_test.go
@@ -28,34 +28,15 @@ func captureLog(t *testing.T, fn func()) string {
 	return buf.String()
 }
 
-// TestCheckSnapshotEncKey_FailLoudBranches pins the non-mock startup check for
-// SNAPSHOT_ENC_KEY: a missing / invalid / wrong-length key must log a CRITICAL
-// line (the at-rest PII warning), a valid base64 32-byte key logs nothing, and
-// a dev/mock SQUARE_ENVIRONMENT skips the check entirely.
-func TestCheckSnapshotEncKey_FailLoudBranches(t *testing.T) {
+// TestCheckSnapshotEncKey_NonFatalBranches pins the surviving non-fatal
+// branches of the startup check for SNAPSHOT_ENC_KEY: a valid base64 32-byte
+// key logs nothing, and a dev/mock SQUARE_ENVIRONMENT skips the check
+// entirely (the warn+plaintext-fallback posture survives ONLY there). The
+// fail-closed branches (missing / invalid-base64 / wrong-length key in a
+// non-mock env) log.Fatalf and are covered by the subprocess test below.
+func TestCheckSnapshotEncKey_NonFatalBranches(t *testing.T) {
 	t.Setenv("SQUARE_ENVIRONMENT", "production")
 
-	t.Run("missing_key_logs_critical", func(t *testing.T) {
-		t.Setenv("SNAPSHOT_ENC_KEY", "")
-		got := captureLog(t, checkSnapshotEncKey)
-		require.Contains(t, got, "SNAPSHOT_ENC_KEY is not set", "missing key must fail loud: %s", got)
-	})
-
-	t.Run("invalid_base64_logs_critical", func(t *testing.T) {
-		t.Setenv("SNAPSHOT_ENC_KEY", "!!!not-base64!!!")
-		got := captureLog(t, checkSnapshotEncKey)
-		require.Contains(t, got, "not valid base64", "invalid base64 must fail loud: %s", got)
-	})
-
-	t.Run("wrong_length_logs_critical", func(t *testing.T) {
-		// 16 bytes base64 → not 32 bytes → not AES-256. Built at runtime so no
-		// secret-shaped literal exists in source.
-		shortKey := base64.StdEncoding.EncodeToString([]byte("1234567890123456"))
-		t.Setenv("SNAPSHOT_ENC_KEY", shortKey)
-		got := captureLog(t, checkSnapshotEncKey)
-		require.Contains(t, got, "exactly 32 bytes", "wrong key length must fail loud: %s", got)
-	})
-
 	t.Run("valid_key_logs_nothing", func(t *testing.T) {
 		t.Setenv("SNAPSHOT_ENC_KEY", base64.StdEncoding.EncodeToString([]byte("12345678901234567890123456789012"))) // 32 bytes
 		got := captureLog(t, checkSnapshotEncKey)
@@ -70,6 +51,49 @@ func TestCheckSnapshotEncKey_FailLoudBranches(t *testing.T) {
 	})
 }
 
+// TestCheckSnapshotEncKey_FatalBranch_Exits covers the fail-closed branches:
+// in a non-mock environment an unset / invalid-base64 / wrong-length
+// SNAPSHOT_ENC_KEY must refuse to start (log.Fatalf → os.Exit) instead of
+// logging a CRITICAL warning and storing square_request_snapshot rows (buyer
+// PII: email + ccof card tokens) PLAINTEXT at rest. log.Fatalf cannot run
+// in-process, so the test re-executes the test binary with a marker env var
+// and asserts the subprocess exits non-zero with a FATAL message naming the
+// key.
+func TestCheckSnapshotEncKey_FatalBranch_Exits(t *testing.T) {
+	if os.Getenv("GO_WANT_HELPER_PROCESS") == "1" {
+		t.Setenv("SQUARE_ENVIRONMENT", "production")
+		switch os.Getenv("SNAPSHOT_ENC_KEY_BRANCH") {
+		case "MISSING":
+			t.Setenv("SNAPSHOT_ENC_KEY", "")
+		case "INVALID_BASE64":
+			t.Setenv("SNAPSHOT_ENC_KEY", "!!!not-base64!!!")
+		case "WRONG_LENGTH":
+			// 16 bytes base64 → not 32 bytes → not AES-256. Built at runtime
+			// so no secret-shaped literal exists in source.
+			t.Setenv("SNAPSHOT_ENC_KEY", base64.StdEncoding.EncodeToString([]byte("1234567890123456")))
+		}
+		checkSnapshotEncKey()
+		return
+	}
+	for _, tc := range []struct {
+		name   string
+		branch string
+	}{
+		{name: "unset_key_exits", branch: "MISSING"},
+		{name: "invalid_base64_exits", branch: "INVALID_BASE64"},
+		{name: "wrong_length_exits", branch: "WRONG_LENGTH"},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			cmd := exec.Command(os.Args[0], "-test.run=TestCheckSnapshotEncKey_FatalBranch_Exits")
+			cmd.Env = append(os.Environ(), "GO_WANT_HELPER_PROCESS=1", "SNAPSHOT_ENC_KEY_BRANCH="+tc.branch)
+			out, err := cmd.CombinedOutput()
+			require.Error(t, err, "expected the non-mock invalid-key branch to exit (log.Fatalf); output: %s", out)
+			require.Contains(t, string(out), "SNAPSHOT_ENC_KEY", "the fatal log must name SNAPSHOT_ENC_KEY: %s", out)
+			require.False(t, strings.Contains(string(out), "unexpected argument"), "the helper must not fail on argument parsing: %s", out)
+		})
+	}
+}
+
 // TestCheckProxyRateLimitConfig_WarnsWithoutTrustedProxy pins the fail-loud
 // warning: a non-mock deployment without TRUST_PROXY_HEADERS collapses every
 // per-IP limiter onto the proxy's address, so startup must warn. A dev/mock env
@@ -84,12 +108,11 @@ func TestCheckProxyRateLimitConfig_WarnsWithoutTrustedProxy(t *testing.T) {
 	require.Empty(t, got, "a dev/mock env must skip the check: %s", got)
 }
 
-// TestCheckWebhookSignatureKey_Branches pins the non-fatal branches of the
-// webhook signing-key startup check: both configured → silent; key without URL
-// → WARNING (fail-closed availability note); neither → CRITICAL. The fatal
-// key-less-with-URL branch is covered by the subprocess test below (log.Fatalf
-// exits the process).
-func TestCheckWebhookSignatureKey_Branches(t *testing.T) {
+// TestCheckWebhookSignatureKey_NonFatalBranches pins the non-fatal branches of
+// the webhook signing-key startup check: both configured → silent; neither →
+// CRITICAL. The fatal branches — key-less-with-URL and key-without-URL — are
+// covered by the subprocess tests below (log.Fatalf exits the process).
+func TestCheckWebhookSignatureKey_NonFatalBranches(t *testing.T) {
 	t.Setenv("SQUARE_ENVIRONMENT", "production")
 
 	t.Run("both_configured_silent", func(t *testing.T) {
@@ -99,13 +122,6 @@ func TestCheckWebhookSignatureKey_Branches(t *testing.T) {
 		require.Empty(t, got, "both configured must be silent: %s", got)
 	})
 
-	t.Run("key_without_url_warns", func(t *testing.T) {
-		t.Setenv("SQUARE_WEBHOOK_SIGNATURE_KEY", "k")
-		t.Setenv("SQUARE_WEBHOOK_NOTIFICATION_URL", "")
-		got := captureLog(t, checkWebhookSignatureKey)
-		require.Contains(t, got, "SQUARE_WEBHOOK_NOTIFICATION_URL is unset", "key without URL must warn: %s", got)
-	})
-
 	t.Run("neither_configured_critical", func(t *testing.T) {
 		t.Setenv("SQUARE_WEBHOOK_SIGNATURE_KEY", "")
 		t.Setenv("SQUARE_WEBHOOK_NOTIFICATION_URL", "")
@@ -122,23 +138,42 @@ func TestCheckWebhookSignatureKey_Branches(t *testing.T) {
 	})
 }
 
-// TestCheckWebhookSignatureKey_FatalBranch_Exits covers the fail-fast branch —
-// a signing key is REQUIRED when a notification URL is configured, and the
-// startup check calls log.Fatalf (os.Exit). That cannot run in-process, so the
-// test re-executes the test binary with a marker env var and asserts the
-// subprocess exits non-zero with the FATAL message naming the key.
+// TestCheckWebhookSignatureKey_FatalBranch_Exits covers the fail-fast branches
+// — a signing key is REQUIRED when a notification URL is configured, and a
+// configured key with NO URL is equally fatal (the handler would verify HMACs
+// against the public default URL, breaking every genuine event) — both call
+// log.Fatalf (os.Exit). That cannot run in-process, so the test re-executes
+// the test binary with a marker env var and asserts the subprocess exits
+// non-zero with a FATAL message naming the misconfigured variable.
 func TestCheckWebhookSignatureKey_FatalBranch_Exits(t *testing.T) {
 	if os.Getenv("GO_WANT_HELPER_PROCESS") == "1" {
 		t.Setenv("SQUARE_ENVIRONMENT", "production")
-		t.Setenv("SQUARE_WEBHOOK_SIGNATURE_KEY", "")
-		t.Setenv("SQUARE_WEBHOOK_NOTIFICATION_URL", "https://example.com/webhooks")
+		switch os.Getenv("WEBHOOK_BRANCH") {
+		case "KEY_WITHOUT_URL":
+			t.Setenv("SQUARE_WEBHOOK_SIGNATURE_KEY", "k")
+			t.Setenv("SQUARE_WEBHOOK_NOTIFICATION_URL", "")
+		default: // URL_WITHOUT_KEY
+			t.Setenv("SQUARE_WEBHOOK_SIGNATURE_KEY", "")
+			t.Setenv("SQUARE_WEBHOOK_NOTIFICATION_URL", "https://example.com/webhooks")
+		}
 		checkWebhookSignatureKey()
 		return
 	}
-	cmd := exec.Command(os.Args[0], "-test.run=TestCheckWebhookSignatureKey_FatalBranch_Exits")
-	cmd.Env = append(os.Environ(), "GO_WANT_HELPER_PROCESS=1")
-	out, err := cmd.CombinedOutput()
-	require.Error(t, err, "expected the key-less-with-URL branch to exit (log.Fatalf); output: %s", out)
-	require.Contains(t, string(out), "SQUARE_WEBHOOK_SIGNATURE_KEY", "the fatal log must name the missing key: %s", out)
-	require.False(t, strings.Contains(string(out), "unexpected argument"), "the helper must not fail on argument parsing: %s", out)
+	for _, tc := range []struct {
+		name   string
+		branch string
+		wantIn string
+	}{
+		{name: "key_without_url_exits", branch: "KEY_WITHOUT_URL", wantIn: "SQUARE_WEBHOOK_NOTIFICATION_URL"},
+		{name: "url_without_key_exits", branch: "URL_WITHOUT_KEY", wantIn: "SQUARE_WEBHOOK_SIGNATURE_KEY"},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			cmd := exec.Command(os.Args[0], "-test.run=TestCheckWebhookSignatureKey_FatalBranch_Exits")
+			cmd.Env = append(os.Environ(), "GO_WANT_HELPER_PROCESS=1", "WEBHOOK_BRANCH="+tc.branch)
+			out, err := cmd.CombinedOutput()
+			require.Error(t, err, "expected the branch to exit (log.Fatalf); output: %s", out)
+			require.Contains(t, string(out), tc.wantIn, "the fatal log must name the misconfigured variable: %s", out)
+			require.False(t, strings.Contains(string(out), "unexpected argument"), "the helper must not fail on argument parsing: %s", out)
+		})
+	}
 }