diff --git a/crates/ledger-cli/tests/documents.rs b/crates/ledger-cli/tests/documents.rs
index 520eabf..4c43781 100644
--- a/crates/ledger-cli/tests/documents.rs
+++ b/crates/ledger-cli/tests/documents.rs
@@ -496,18 +496,18 @@ fn terms_fall_back_in_order() {
     assert_eq!(bill["terms"], "Net 10", "{bill}");
 }
 
-/// `party_id` is the old spelling of `customer`/`vendor` and still names
-/// the same record (until P5); a number that is one record's id and
+/// A customer given by id and by name is the same record (the old
+/// `party_id` spelling is gone, P5); a number that is one record's id and
 /// another's name is refused, never guessed.
 #[test]
-fn party_id_alias_still_works() {
+fn a_customer_by_id_and_by_name_is_one_record() {
     let books = Books::new("documents-alias");
     let mut m = Mcp::spawn(&books);
     let harbor = m.customer("Harbor Coffee");
     let line = json!([{"description": "Work", "amount": "10.00"}]);
-    let by_alias = m.call(
+    let by_id = m.call(
         "books.create_invoice",
-        json!({"party_id": harbor, "date": "2026-02-01", "terms": "Net 30",
+        json!({"customer": (harbor).to_string(), "date": "2026-02-01", "terms": "Net 30",
                "lines": line, "confirmed": true}),
     );
     let by_name = m.call(
@@ -520,8 +520,8 @@ fn party_id_alias_still_works() {
         .iter()
         .map(|e| e["payload"]["customer_id"].as_u64().unwrap())
         .collect();
-    assert_eq!(parties, [harbor, harbor], "one customer, two spellings");
-    assert_eq!(by_alias["total"], by_name["total"]);
+    assert_eq!(parties, [harbor, harbor], "one customer, an id and a name");
+    assert_eq!(by_id["total"], by_name["total"]);
 
     // A customer named "2" that is not customer 2: the number is
     // ambiguous, and is refused rather than guessed.
@@ -730,7 +730,7 @@ fn mark_sent_records_who_and_how() {
             .unwrap();
     let invoice = common::answer_of(&wire.call(
         "books.create_invoice",
-        json!({"party_id": customer, "date": "2026-03-01", "due_date": "2026-03-31",
+        json!({"customer": (customer).to_string(), "date": "2026-03-01", "due_date": "2026-03-31",
                "lines": [{"description": "Design", "amount": "1200.00"}], "confirmed": true}),
     ))
     .unwrap()["id"]
@@ -876,7 +876,7 @@ mod paying_on_terms {
             ok(
                 &mut self.wire,
                 "books.create_invoice",
-                json!({"party_id": self.customer, "date": date, "due_date": due,
+                json!({"customer": (self.customer).to_string(), "date": date, "due_date": due,
                    "lines": [{"description": "Consulting", "amount": amount}],
                    "confirmed": true}),
             )["id"]
@@ -897,7 +897,7 @@ mod paying_on_terms {
             ok(
                 &mut self.wire,
                 "books.create_bill",
-                json!({"party_id": self.vendor, "date": date, "due_date": due,
+                json!({"vendor": (self.vendor).to_string(), "date": date, "due_date": due,
                    "vendor_ref": format!("TH-{}", self.bills),
                    "lines": [{"description": "Paper", "amount": amount, "account": "6999"}],
                    "upload_id": file, "confirmed": true}),
@@ -1258,7 +1258,7 @@ mod bad_debts {
             wire,
             "books.create_invoice",
             json!({
-                "party_id": customer,
+                "customer": (customer).to_string(),
                 "date": "2026-01-05",
                 "due_date": "2026-02-04",
                 "lines": [{ "description": "Design", "amount": "100.00" }],
@@ -1286,10 +1286,10 @@ mod bad_debts {
             &mut wire,
             "books.receive_payment",
             json!({"confirmed": true,
-                "document_id": inv,
+                "invoice": inv,
                 "date": "2026-01-20",
                 "amount": "40.00",
-                "cash_account": "1000",
+                "deposit_to": "1000",
             }),
         );
         // The write-off's evidence, uploaded as an agent uploads it (D-46).
@@ -1361,10 +1361,10 @@ mod bad_debts {
             &mut wire,
             "books.receive_payment",
             json!({"confirmed": true,
-                "document_id": inv,
+                "invoice": inv,
                 "date": "2026-03-02",
                 "amount": "60.00",
-                "cash_account": "1000",
+                "deposit_to": "1000",
             }),
         );
         let doc = ok(
diff --git a/crates/ledger-cli/tests/format_upgrade.rs b/crates/ledger-cli/tests/format_upgrade.rs
index 4a4d513..75ea276 100644
--- a/crates/ledger-cli/tests/format_upgrade.rs
+++ b/crates/ledger-cli/tests/format_upgrade.rs
@@ -767,7 +767,7 @@ fn books_p2_served_cut_over_whole() {
     let invoice = a
         .call(
             "books.create_invoice",
-            json!({"party_id": 1, "date": "2026-01-05", "due_date": "2026-02-04",
+            json!({"customer": (1).to_string(), "date": "2026-01-05", "due_date": "2026-02-04",
                    "lines": [{"description": "Design", "amount": "400.00"}], "confirmed": true}),
         )
         .unwrap()["id"]
@@ -789,7 +789,7 @@ fn books_p2_served_cut_over_whole() {
     let unpaid = a
         .call(
             "books.create_invoice",
-            json!({"party_id": customer, "date": "2026-01-10", "due_date": "2026-02-09",
+            json!({"customer": (customer).to_string(), "date": "2026-01-10", "due_date": "2026-02-09",
                    "lines": [{"description": "Survey", "amount": "300.00"}], "confirmed": true}),
         )
         .unwrap()["id"]
diff --git a/crates/ledger-cli/tests/hostile_args.rs b/crates/ledger-cli/tests/hostile_args.rs
index 1dcfc86..6cc3ca9 100644
--- a/crates/ledger-cli/tests/hostile_args.rs
+++ b/crates/ledger-cli/tests/hostile_args.rs
@@ -1017,13 +1017,13 @@ fn inputs_that_once_panicked_are_refused_or_answered() {
         ),
         (
             "books.create_invoice",
-            json!({"party_id": client, "date": "2026-05-01", "due_date": "2026-06-01",
+            json!({"customer": (client).to_string(), "date": "2026-05-01", "due_date": "2026-06-01",
                    "lines": [{"description": "a", "amount": MAX},
                              {"description": "b", "amount": "1"}], "confirmed": true}),
         ),
         (
             "books.create_bill",
-            json!({"party_id": acme, "date": "2026-05-01", "due_date": "2026-06-01",
+            json!({"vendor": (acme).to_string(), "date": "2026-05-01", "due_date": "2026-06-01",
                    "vendor_ref": "H-1",
                    "lines": [{"description": "a", "amount": MAX, "account": "6000"},
                              {"description": "b", "amount": "1", "account": "6000"}],
@@ -1031,7 +1031,7 @@ fn inputs_that_once_panicked_are_refused_or_answered() {
         ),
         (
             "books.create_invoice",
-            json!({"party_id": client, "date": "2026-05-01", "due_date": "+262142-12-31",
+            json!({"customer": (client).to_string(), "date": "2026-05-01", "due_date": "+262142-12-31",
                    "lines": [{"description": "a", "amount": "1"}], "confirmed": true}),
         ),
         (
diff --git a/crates/ledger-cli/tests/projects_parity.rs b/crates/ledger-cli/tests/projects_parity.rs
index 4328f7a..93261b2 100644
--- a/crates/ledger-cli/tests/projects_parity.rs
+++ b/crates/ledger-cli/tests/projects_parity.rs
@@ -274,7 +274,7 @@ fn invoiced_and_billed_project_figures_are_the_same_in_the_viewer_and_from_the_s
     let bill = server.tool("books.create_bill", bill_args);
     server.tool(
         "books.receive_payment",
-        json!({"confirmed": true, "document_id": inv["id"], "date": "2026-02-20", "amount": "1830.00", "cash_account": "1000"}),
+        json!({"confirmed": true, "invoice": inv["id"], "date": "2026-02-20", "amount": "1830.00", "deposit_to": "1000"}),
     );
     let b = data::load_once(&Source {
         data_dir: data.clone(),
diff --git a/crates/ledger-cli/tests/spec_attachments.rs b/crates/ledger-cli/tests/spec_attachments.rs
index afa85c7..d826849 100644
--- a/crates/ledger-cli/tests/spec_attachments.rs
+++ b/crates/ledger-cli/tests/spec_attachments.rs
@@ -424,7 +424,7 @@ fn a_file_links_to_every_kind_of_subject() {
     let rent_invoice = m.upload(b"rent invoice", "rent.pdf", "application/pdf", "receipt");
     let bill = m.ok(
         "books.create_bill",
-        json!({"party_id": vendor, "date": "2026-02-01", "due_date": "2026-02-28",
+        json!({"vendor": (vendor).to_string(), "date": "2026-02-01", "due_date": "2026-02-28",
                "vendor_ref": "RENT-2",
                "lines": [{"description": "Rent", "amount": "300.00", "account": "6000"}],
                "upload_id": rent_invoice, "confirmed": true}),
@@ -432,7 +432,7 @@ fn a_file_links_to_every_kind_of_subject() {
         .clone();
     let invoice = m.ok(
         "books.create_invoice",
-        json!({"party_id": customer, "date": "2026-02-01", "due_date": "2026-03-01",
+        json!({"customer": (customer).to_string(), "date": "2026-02-01", "due_date": "2026-03-01",
                "lines": [{"description": "Work", "amount": "500.00"}],
                "confirmed": true}),
     )["id"]
diff --git a/crates/ledger-cli/tests/spec_mcp.rs b/crates/ledger-cli/tests/spec_mcp.rs
index 89d31e3..6cf443b 100644
--- a/crates/ledger-cli/tests/spec_mcp.rs
+++ b/crates/ledger-cli/tests/spec_mcp.rs
@@ -911,7 +911,7 @@ fn happy_path(name: &str) -> Mcp {
     // A/R and A/P.
     let inv = m.ok(
         "books.create_invoice",
-        json!({"party_id": customer, "date": "2026-02-01", "due_date": "2026-03-01",
+        json!({"customer": (customer).to_string(), "date": "2026-02-01", "due_date": "2026-03-01",
                "lines": [{"description": "Consulting", "amount": "500.00"}],
                "confirmed": true}),
     )["id"]
@@ -919,7 +919,7 @@ fn happy_path(name: &str) -> Mcp {
         .unwrap();
     m.ok(
         "books.receive_payment",
-        json!({"confirmed": true, "document_id": inv, "date": "2026-02-10", "amount": "200.00", "cash_account": "1000"}),
+        json!({"confirmed": true, "invoice": inv, "date": "2026-02-10", "amount": "200.00", "deposit_to": "1000"}),
     );
     let vendor_invoice = m.upload(
         b"vendor invoice",
@@ -929,7 +929,7 @@ fn happy_path(name: &str) -> Mcp {
     );
     let bill = m.ok(
         "books.create_bill",
-        json!({"party_id": vendor, "date": "2026-02-01", "due_date": "2026-02-28",
+        json!({"vendor": (vendor).to_string(), "date": "2026-02-01", "due_date": "2026-02-28",
                "vendor_ref": "V-1001",
                "lines": [{"description": "Office rent", "amount": "300.00", "account": "6000"}],
                "upload_id": vendor_invoice, "confirmed": true}),
@@ -938,7 +938,7 @@ fn happy_path(name: &str) -> Mcp {
         .unwrap();
     m.ok(
         "books.pay_bill",
-        json!({"confirmed": true, "document_id": bill, "date": "2026-02-05", "amount": "300.00", "cash_account": "1000"}),
+        json!({"confirmed": true, "bill": bill, "date": "2026-02-05", "amount": "300.00", "paid_from": "1000"}),
     );
     m.ok("books.ar_aging", json!({"as_of": "2026-06-30"}));
     m.ok("books.ap_aging", json!({"as_of": "2026-06-30"}));
@@ -946,7 +946,7 @@ fn happy_path(name: &str) -> Mcp {
     // P4b S1: a document voided, and the control accounts' tie-out.
     let dup = m.ok(
         "books.create_invoice",
-        json!({"party_id": customer, "date": "2026-02-02", "due_date": "2026-03-02",
+        json!({"customer": (customer).to_string(), "date": "2026-02-02", "due_date": "2026-03-02",
                "lines": [{"description": "Consulting, entered twice", "amount": "500.00"}],
                "confirmed": true}),
     )["id"]
@@ -972,7 +972,7 @@ fn happy_path(name: &str) -> Mcp {
     // P4b S7: a bad debt, its evidence uploaded and filed with it.
     let bad = m.ok(
         "books.create_invoice",
-        json!({"party_id": customer, "date": "2026-02-03", "due_date": "2026-03-03",
+        json!({"customer": (customer).to_string(), "date": "2026-02-03", "due_date": "2026-03-03",
                "lines": [{"description": "Consulting, never paid", "amount": "400.00"}],
                "confirmed": true}),
     )["id"]
@@ -1685,15 +1685,15 @@ fn run_op(m: &mut Mcp, k: &mut Known, op: &Op) {
             vendor_ref,
         } => (
             "books.create_bill",
-            json!({"party_id": party.resolve(&k.vendors), "date": "2026-03-01",
+            json!({"vendor": (party.resolve(&k.vendors)).to_string(), "date": "2026-03-01",
                    "due_date": "2026-03-31", "vendor_ref": vendor_ref,
                    "lines": [{"description": "thing", "amount": amount, "account": "6000"}],
                    "existing_attachment_id": k.vendor_invoice, "confirmed": true}),
         ),
         Op::PayBill { doc, amount } => (
             "books.pay_bill",
-            json!({"confirmed": true, "document_id": doc.resolve(&k.bills), "date": "2026-03-15", "amount": amount,
-                   "cash_account": "1000"}),
+            json!({"confirmed": true, "bill": doc.resolve(&k.bills), "date": "2026-03-15", "amount": amount,
+                   "paid_from": "1000"}),
         ),
         Op::Payout { deposit } => (
             "books.record_payout",
@@ -1834,7 +1834,7 @@ fn inv3_other_tools_refuse_ungrammatical_amounts() {
         );
         m.refuse(
             "books.create_bill",
-            json!({"party_id": vendor, "date": "2026-03-01", "due_date": "2026-03-31",
+            json!({"vendor": (vendor).to_string(), "date": "2026-03-01", "due_date": "2026-03-31",
                    "lines": [{"description": "x", "amount": bad}]}),
         );
         m.refuse(
@@ -2591,7 +2591,7 @@ fn assigned_roles_steer_invoices() {
         .unwrap();
     m.ok(
         "books.create_invoice",
-        json!({"party_id": customer, "date": "2026-02-01", "due_date": "2026-03-01",
+        json!({"customer": (customer).to_string(), "date": "2026-02-01", "due_date": "2026-03-01",
                "lines": [{"description": "Consulting", "amount": "500.00"}],
                "confirmed": true}),
     );
@@ -2654,7 +2654,7 @@ fn documents_void_and_tie_out_over_the_wire() {
     );
     let inv = m.ok(
         "books.create_invoice",
-        json!({"party_id": customer, "date": "2026-02-01", "due_date": "2026-03-01",
+        json!({"customer": (customer).to_string(), "date": "2026-02-01", "due_date": "2026-03-01",
                "lines": [{"description": "Consulting", "amount": "500.00"}],
                "confirmed": true}),
     )["id"]
@@ -2662,8 +2662,8 @@ fn documents_void_and_tie_out_over_the_wire() {
         .unwrap();
     let paid = m.ok(
         "books.receive_payment",
-        json!({"confirmed": true, "document_id": inv, "date": "2026-02-10", "amount": "200.00",
-               "cash_account": "1000", "instrument": "Harbor checking", "reference": "tr_8812"}),
+        json!({"confirmed": true, "invoice": inv, "date": "2026-02-10", "amount": "200.00",
+               "deposit_to": "1000", "instrument": "Harbor checking", "reference": "tr_8812"}),
     );
     assert_eq!(paid["remaining"], "300.00", "{paid}");
     assert_eq!(paid["status"], "partial", "{paid}");
@@ -2685,7 +2685,7 @@ fn documents_void_and_tie_out_over_the_wire() {
 
     let dup = m.ok(
         "books.create_invoice",
-        json!({"party_id": customer, "date": "2026-02-02", "due_date": "2026-03-02",
+        json!({"customer": (customer).to_string(), "date": "2026-02-02", "due_date": "2026-03-02",
                "lines": [{"description": "Consulting, entered twice", "amount": "500.00"}],
                "confirmed": true}),
     )["id"]
@@ -2753,7 +2753,7 @@ fn documents_list_and_timeline_over_the_wire() {
     let invoice = |m: &mut Mcp, date: &str, due: &str, amount: &str| {
         m.ok(
             "books.create_invoice",
-            json!({"party_id": customer, "date": date, "due_date": due,
+            json!({"customer": (customer).to_string(), "date": date, "due_date": due,
                    "lines": [{"description": "Consulting", "amount": amount}],
                    "confirmed": true}),
         )["id"]
@@ -2764,7 +2764,7 @@ fn documents_list_and_timeline_over_the_wire() {
     let later = invoice(&mut m, "2026-02-05", "2026-04-30", "80.00");
     let pay = m.ok(
         "books.receive_payment",
-        json!({"confirmed": true, "document_id": late, "date": "2026-02-10", "amount": "200.00", "cash_account": "1000"}),
+        json!({"confirmed": true, "invoice": late, "date": "2026-02-10", "amount": "200.00", "deposit_to": "1000"}),
     )["entry_id"]
         .as_u64()
         .unwrap();
diff --git a/crates/ledger-cli/tests/spec_parties.rs b/crates/ledger-cli/tests/spec_parties.rs
index 00e6661..bea7d00 100644
--- a/crates/ledger-cli/tests/spec_parties.rs
+++ b/crates/ledger-cli/tests/spec_parties.rs
@@ -835,15 +835,15 @@ fn party_totals_match_a_hand_computed_year() {
     );
     let bill = m.ok(
         "books.create_bill",
-        json!({"party_id": v, "date": "2026-07-01", "due_date": "2026-07-31",
+        json!({"vendor": (v).to_string(), "date": "2026-07-01", "due_date": "2026-07-31",
                "vendor_ref": "D-1041",
                "lines": [{"description": "Logo", "amount": "800.00", "account": "6000"}],
                "upload_id": logo_invoice, "confirmed": true}),
     );
     let pay = m.ok(
         "books.pay_bill",
-        json!({"confirmed": true, "document_id": bill["id"], "date": "2026-07-15", "amount": "300.00",
-               "cash_account": "1000"}),
+        json!({"confirmed": true, "bill": bill["id"], "date": "2026-07-15", "amount": "300.00",
+               "paid_from": "1000"}),
     )["entry_id"]
         .as_u64()
         .unwrap();
@@ -917,14 +917,14 @@ fn party_totals_for_a_customer_a_member_and_a_prepaid() {
         .unwrap();
     let inv = m.ok(
         "books.create_invoice",
-        json!({"party_id": c, "date": "2026-03-01", "due_date": "2026-03-31",
+        json!({"customer": (c).to_string(), "date": "2026-03-01", "due_date": "2026-03-31",
                "lines": [{"description": "Work", "amount": "1000.00"}],
                "confirmed": true}),
     );
     let pay = m.ok(
         "books.receive_payment",
-        json!({"confirmed": true, "document_id": inv["id"], "date": "2026-03-20", "amount": "600.00",
-               "cash_account": "1000"}),
+        json!({"confirmed": true, "invoice": inv["id"], "date": "2026-03-20", "amount": "600.00",
+               "deposit_to": "1000"}),
     )["entry_id"]
         .as_u64()
         .unwrap();
diff --git a/crates/ledger-cli/tests/spec_projects.rs b/crates/ledger-cli/tests/spec_projects.rs
index 04522bb..26cdc2b 100644
--- a/crates/ledger-cli/tests/spec_projects.rs
+++ b/crates/ledger-cli/tests/spec_projects.rs
@@ -1231,7 +1231,7 @@ fn invoice_and_bill_lines_count_toward_their_projects() {
     );
     let inv = m.ok(
         "books.create_invoice",
-        json!({"party_id": cedar, "date": "2026-03-02", "due_date": "2026-04-01", "lines": [
+        json!({"customer": (cedar).to_string(), "date": "2026-03-02", "due_date": "2026-04-01", "lines": [
             {"description": "Design", "amount": "3000.00", "class": "Zeta site"},
             {"description": "Hosting", "amount": "200.00", "class": harbor.to_string()},
             {"description": "Rush fee", "amount": "100.00"},
@@ -1264,7 +1264,7 @@ fn invoice_and_bill_lines_count_toward_their_projects() {
     );
     let bill = m.ok(
         "books.create_bill",
-        json!({"party_id": dana_vendor, "date": "2026-03-09", "due_date": "2026-04-08",
+        json!({"vendor": (dana_vendor).to_string(), "date": "2026-03-09", "due_date": "2026-04-08",
                "vendor_ref": "DANA-77",
                "lines": [
             {"description": "Illustration", "amount": "1200.00", "account": "6000", "class": "Zeta site"},
@@ -1284,11 +1284,11 @@ fn invoice_and_bill_lines_count_toward_their_projects() {
     // Payments are balance-sheet only: no project.
     let got = m.ok(
         "books.receive_payment",
-        json!({"confirmed": true, "document_id": inv["id"], "date": "2026-03-20", "amount": "3800.00", "cash_account": "1000"}),
+        json!({"confirmed": true, "invoice": inv["id"], "date": "2026-03-20", "amount": "3800.00", "deposit_to": "1000"}),
     );
     let paid = m.ok(
         "books.pay_bill",
-        json!({"confirmed": true, "document_id": bill["id"], "date": "2026-03-30", "amount": "1250.00", "cash_account": "1000"}),
+        json!({"confirmed": true, "bill": bill["id"], "date": "2026-03-30", "amount": "1250.00", "paid_from": "1000"}),
     );
     for e in [&got["entry_id"], &paid["entry_id"]] {
         let e = e.as_u64().unwrap();
@@ -1356,7 +1356,7 @@ fn a_document_naming_an_unknown_project_is_refused_whole() {
     for (tool, party) in [("books.create_invoice", &cedar), ("books.create_bill", &v)] {
         let why = m.refuse(
             tool,
-            json!({"party_id": party, "date": "2026-03-02", "due_date": "2026-04-01", "lines": [
+            json!({"customer": (party).to_string(), "date": "2026-03-02", "due_date": "2026-04-01", "lines": [
                 {"description": "a", "amount": "10.00", "class": "Zeta site"},
                 {"description": "b", "amount": "10.00", "class": "Nowhere"},
             ]}),
@@ -1379,7 +1379,7 @@ fn a_document_for_a_finished_project_says_so() {
     let cedar = m.ok("books.add_customer", json!({"name": "Cedar"}))["id"].clone();
     let out = m.ok(
         "books.create_invoice",
-        json!({"party_id": cedar, "date": "2026-10-05", "due_date": "2026-11-04", "lines": [
+        json!({"customer": (cedar).to_string(), "date": "2026-10-05", "due_date": "2026-11-04", "lines": [
             {"description": "Final", "amount": "10.00", "class": "Harbor"},
             {"description": "Again", "amount": "10.00", "class": "harbor"},
             {"description": "Old", "amount": "10.00", "class": "Dock"},
@@ -1424,7 +1424,7 @@ fn an_invoice_for_an_on_spec_project_asks_about_an_agreement() {
     let v = m.ok("books.add_vendor", json!({"name": "Dana"}))["id"].clone();
     let out = m.ok(
         "books.create_invoice",
-        json!({"party_id": cedar, "date": "2026-10-05", "due_date": "2026-11-04", "lines": [
+        json!({"customer": (cedar).to_string(), "date": "2026-10-05", "due_date": "2026-11-04", "lines": [
             {"description": "Build", "amount": "10.00", "class": "Kestrel"},
             {"description": "More", "amount": "5.00", "class": "kestrel"},
             {"description": "Site", "amount": "7.00", "class": "Zeta"},
@@ -1450,7 +1450,7 @@ fn an_invoice_for_an_on_spec_project_asks_about_an_agreement() {
     );
     let bill = m.ok(
         "books.create_bill",
-        json!({"party_id": v, "date": "2026-10-05", "due_date": "2026-11-04",
+        json!({"vendor": (v).to_string(), "date": "2026-10-05", "due_date": "2026-11-04",
                "vendor_ref": "V-512",
                "lines": [
             {"description": "Icons", "amount": "10.00", "account": "6000", "class": "Kestrel"},
@@ -1475,7 +1475,7 @@ fn books_without_the_feature_refuse_a_tagged_document() {
     for (tool, party) in [("books.create_invoice", &cedar), ("books.create_bill", &v)] {
         let why = m.refuse(
             tool,
-            json!({"party_id": party, "date": "2026-03-02", "due_date": "2026-04-01", "lines": [
+            json!({"customer": (party).to_string(), "date": "2026-03-02", "due_date": "2026-04-01", "lines": [
                 {"description": "a", "amount": "10.00", "class": "Zeta site"},
             ]}),
         );
@@ -1487,7 +1487,7 @@ fn books_without_the_feature_refuse_a_tagged_document() {
     assert_eq!(m.journal(), head, "nothing journaled");
     m.ok(
         "books.create_invoice",
-        json!({"party_id": cedar, "date": "2026-03-02", "due_date": "2026-04-01", "lines": [
+        json!({"customer": (cedar).to_string(), "date": "2026-03-02", "due_date": "2026-04-01", "lines": [
             {"description": "a", "amount": "10.00"},
         ], "confirmed": true}),
     );
diff --git a/crates/ledger-cli/tests/spec_tax_policy.rs b/crates/ledger-cli/tests/spec_tax_policy.rs
index c4e7433..cc9339e 100644
--- a/crates/ledger-cli/tests/spec_tax_policy.rs
+++ b/crates/ledger-cli/tests/spec_tax_policy.rs
@@ -278,7 +278,7 @@ fn require_tax_category_refuses_defect_8() {
     );
     let bill = m.call(
         "books.create_bill",
-        json!({"party_id": vendor["id"], "date": "2026-02-01", "due_date": "2026-03-01",
+        json!({"vendor": (vendor["id"]).to_string(), "date": "2026-02-01", "due_date": "2026-03-01",
                "vendor_ref": "CH-1",
                "lines": [{"description": "hosting", "amount": "42.00", "account": "6150"}],
                "upload_id": first_invoice, "confirmed": true}),
@@ -303,7 +303,7 @@ fn require_tax_category_refuses_defect_8() {
     );
     let again = m.call(
         "books.create_bill",
-        json!({"party_id": vendor["id"], "date": "2026-02-02", "due_date": "2026-03-02",
+        json!({"vendor": (vendor["id"]).to_string(), "date": "2026-02-02", "due_date": "2026-03-02",
                "vendor_ref": "CH-2",
                "lines": [{"description": "hosting", "amount": "1.00", "account": "6150"}],
                "upload_id": second_invoice, "confirmed": true}),
diff --git a/crates/ledger-mcp/src/tools/documents.rs b/crates/ledger-mcp/src/tools/documents.rs
index 56178db..c5cc3ed 100644
--- a/crates/ledger-mcp/src/tools/documents.rs
+++ b/crates/ledger-mcp/src/tools/documents.rs
@@ -455,17 +455,7 @@ impl BooksServer {
     ) -> Result<Json<InvoiceOut>, String> {
         let p = params.0;
         let mut store = self.lock()?;
-        let customer = match (&p.customer, p.party_id) {
-            (Some(c), None) => c.trim().to_string(),
-            (None, Some(id)) => id.to_string(),
-            (Some(c), Some(id)) if c.trim() == id.to_string() => c.trim().to_string(),
-            (Some(_), Some(_)) => {
-                return Err("give the customer once: customer or party_id, not both".into());
-            }
-            (None, None) => {
-                return Err("give customer: a customer id, a party id or a name".into());
-            }
-        };
+        let customer = p.customer.trim().to_string();
         let customer_id = resolve_customer(&store, &customer)?;
         let date = parse_date(&p.date)?;
         let lines = document_lines(&store, p.lines)?;
@@ -606,17 +596,7 @@ impl BooksServer {
     ) -> Result<Json<BillOut>, String> {
         let p = params.0;
         let mut store = self.lock()?;
-        let vendor = match (&p.vendor, p.party_id) {
-            (Some(v), None) => v.trim().to_string(),
-            (None, Some(id)) => id.to_string(),
-            (Some(v), Some(id)) if v.trim() == id.to_string() => v.trim().to_string(),
-            (Some(_), Some(_)) => {
-                return Err("give the vendor once: vendor or party_id, not both".into());
-            }
-            (None, None) => {
-                return Err("give vendor: a vendor id, a party id or a name".into());
-            }
-        };
+        let vendor = p.vendor.trim().to_string();
         let vendor_id = resolve_vendor_arg(&store, &vendor)?;
         let date = parse_date(&p.date)?;
         let received_on = p.received_on.as_deref().map(parse_date).transpose()?;
diff --git a/crates/ledger-mcp/src/tools/settlements.rs b/crates/ledger-mcp/src/tools/settlements.rs
index 5aef5c5..8806f05 100644
--- a/crates/ledger-mcp/src/tools/settlements.rs
+++ b/crates/ledger-mcp/src/tools/settlements.rs
@@ -39,35 +39,17 @@ fn doc_party(ledger: &Ledger, doc: ledger_core::DocRef) -> Result<u64, String> {
     }
 }
 
-/// The one id a payment tool's single form names: `invoice`/`bill`, or
-/// the older `document_id`. Both at once is refused unless they agree.
-fn single_document(primary: Option<u64>, alias: Option<u64>, what: &str) -> Result<u64, String> {
-    match (primary, alias) {
-        (Some(a), Some(b)) if a != b => Err(format!(
-            "pass {what} or document_id, not both: {} and {b} differ",
-            a
-        )),
-        (Some(id), _) | (None, Some(id)) => Ok(id),
-        (None, None) => Err(format!(
-            "pass {what} (or an applications list): see books.documents"
-        )),
-    }
+/// The one id a payment tool's single form names: `invoice` or `bill`.
+fn single_document(primary: Option<u64>, what: &str) -> Result<u64, String> {
+    primary.ok_or_else(|| format!("pass {what} (or an applications list): see books.documents"))
 }
 
-/// The one account a payment tool's single form names: `deposit_to`/
-/// `paid_from`, or the older `cash_account`.
-fn single_account(
-    primary: &Option<String>,
-    alias: &Option<String>,
-    what: &str,
-) -> Result<String, String> {
-    match (primary, alias) {
-        (Some(a), Some(b)) if a != b => Err(format!(
-            "pass {what} or cash_account, not both: {a:?} and {b:?} differ"
-        )),
-        (Some(a), _) | (None, Some(a)) => Ok(a.clone()),
-        (None, None) => Err(format!("pass {what} (or cash_account)")),
-    }
+/// The one account a payment tool's single form names: `deposit_to` or
+/// `paid_from`.
+fn single_account(primary: &Option<String>, what: &str) -> Result<String, String> {
+    primary
+        .clone()
+        .ok_or_else(|| format!("pass {what}: the account the cash moves through"))
 }
 
 /// Every document a payment tool's parameters name, as the core takes
@@ -128,7 +110,6 @@ fn applications(
     }
     let id = single_document(
         if invoices { p.invoice } else { p.bill },
-        p.document_id,
         if invoices { "invoice" } else { "bill" },
     )?;
     let amount = p
@@ -178,7 +159,6 @@ fn settlement(
             } else {
                 &p.paid_from
             },
-            &p.cash_account,
             if invoices { "deposit_to" } else { "paid_from" },
         )?,
     )?;
diff --git a/crates/ledger-mcp/src/types.rs b/crates/ledger-mcp/src/types.rs
index 42642c9..0dc3318 100644
--- a/crates/ledger-mcp/src/types.rs
+++ b/crates/ledger-mcp/src/types.rs
@@ -3265,10 +3265,7 @@ pub struct CreateInvoiceParams {
     pub preview: Option<String>,
     /// The customer: a customer id, its party's id, or its name. A name
     /// that two customers answer to is refused.
-    pub customer: Option<String>,
-    /// The old spelling of `customer` (accepted until P5): a customer id.
-    #[serde(default)]
-    pub party_id: Option<u64>,
+    pub customer: String,
     /// The invoice date, YYYY-MM-DD.
     pub date: String,
     /// Its payment terms ("Net 30", "Net 15 EOM", "2/10 net 30", "Due on
@@ -3300,10 +3297,7 @@ pub struct CreateBillParams {
     pub preview: Option<String>,
     /// The vendor: a vendor id, its party's id, or its name. A name that
     /// two vendors answer to is refused.
-    pub vendor: Option<String>,
-    /// The old spelling of `vendor` (accepted until P5): a vendor id.
-    #[serde(default)]
-    pub party_id: Option<u64>,
+    pub vendor: String,
     /// The bill's date, YYYY-MM-DD.
     pub date: String,
     /// The day the bill was received, when it differs from its date.
@@ -3383,10 +3377,6 @@ pub struct PaymentParams {
     /// preview: nothing posts.
     #[serde(default)]
     pub confirmed: Option<bool>,
-    /// Invoice id (receive_payment) or bill id (pay_bill); the older name
-    /// of `invoice` / `bill`.
-    #[serde(default)]
-    pub document_id: Option<u64>,
     /// The invoice, by id (books.receive_payment).
     #[serde(default)]
     pub invoice: Option<u64>,
@@ -3399,10 +3389,6 @@ pub struct PaymentParams {
     /// applications' amounts are the cash; a discount is beside it.
     #[serde(default)]
     pub amount: Option<String>,
-    /// Cash account id or code; the older name of `deposit_to` /
-    /// `paid_from`.
-    #[serde(default)]
-    pub cash_account: Option<String>,
     /// The account the cash reaches (receive_payment): a bank or card that
     /// holds no role.
     #[serde(default)]
diff --git a/docs/mcp-interface.md b/docs/mcp-interface.md
index 688be57..d91f42b 100644
--- a/docs/mcp-interface.md
+++ b/docs/mcp-interface.md
@@ -2195,8 +2195,8 @@ snapshots; see plan 8.2 S4.
   receivable line tagged with the invoice and the receivable side one line
   per (account, project) part.
   - `customer` is the customer's id, its party's id or its name (a name
-    two customers answer to is refused); `party_id` is the old spelling of
-    the same thing (a customer id), kept until P5.
+    two customers answer to is refused); it is required (the old `party_id`
+    spelling is gone, P5).
   - The due date: `due_date` as given (and it overrides `terms`); else
     `terms` ("Net 30", "Net 15 EOM", "2/10 net 30", "Due on receipt",
     "Due 2026-11-15", rules.md B-39); else the customer's
@@ -2247,8 +2247,8 @@ snapshots; see plan 8.2 S4.
   sha256 }, warnings?, notes? }`.
   Confirmed it posts Dr each line's account / Cr `2000` Accounts Payable,
   the payable line tagged with the bill and one debit line per (account,
-  project) part; `vendor` is a vendor id, its party's id or its name, and
-  `party_id` the old spelling (a vendor id).
+  project) part; `vendor` is a vendor id, its party's id or its name
+  (required; the old `party_id` spelling is gone, P5).
   - `vendor_ref` is the vendor's own invoice number: required in books
     that have declared `documents_v2` (B-40). The same vendor and number
     twice is refused unless `duplicate_reason` says why, which is
@@ -2271,8 +2271,8 @@ snapshots; see plan 8.2 S4.
   - `warnings` names an expense account it created with no tax category;
     `notes` one per finished or abandoned project a line names (none for
     on-spec: a cost of on-spec work is an expense like any other).
-- `books.receive_payment { date, deposit_to | cash_account, instrument?,
-  reference?, invoice | document_id, amount? | applications: [{ invoice,
+- `books.receive_payment { date, deposit_to, instrument?,
+  reference?, invoice, amount? | applications: [{ invoice,
   amount, discount?, discount_reason? }], fee?: { amount, account?,
   vendor? }, paid_in_full_marking?, confirmed?, preview? }` → `{ preview?,
   entry_id, documents: [{ ref, number?, outstanding, status }], deposited,
@@ -2294,11 +2294,11 @@ snapshots; see plan 8.2 S4.
   repeat the single document's figures. An overpayment is refused with the
   excess and the remedy (a plain entry to 2300 Customer Deposits).
   `paid_in_full_marking: true` adds a UCC §3-311 warning and records
-  nothing else. `document_id` and `cash_account` are the older names of
-  `invoice`/`bill` and `deposit_to`/`paid_from`. The books record
+  nothing else. (The older names `document_id` and `cash_account` are
+  gone, P5.) The books record
   payments; they never collect them (D-45).
-- `books.pay_bill { date, paid_from | cash_account, instrument?,
-  reference?, bill | document_id, amount? | applications: [{ bill, amount,
+- `books.pay_bill { date, paid_from, instrument?,
+  reference?, bill, amount? | applications: [{ bill, amount,
   discount?, discount_reason? }], confirmed?, preview? }` → as `receive_payment`, per bill and
   one vendor (B-42): Dr A/P each bill's cash and discount, Cr the payer
   account the cash, Cr purchase discounts the discounts. A discount on a