feat: bring simple-mode insert arity diagnostics to parity with advanced

A wrong-count simple-mode insert now shows the friendly per-column arity
message at typing time (instead of a bare "expected `,`/`)`") and is
blocked from dispatch at submit — unifying simple and advanced mode onto
the one ADR-0027 model (structural parse + ERROR diagnostic), where they
had diverged.

Grammar: a simple-mode-only arity gate (dsl_insert_value_list) routes a
wrong-count DSL insert tuple to the type-blind fallback so it matches
structurally and the per-tuple arity diagnostic fires. The gate is gated
to simple mode, so advanced behaviour is unchanged. count_tuple_values
and the target-column selection (insert_target_columns) are now shared
by both grammars.

Diagnostic: dml_insert_arity_diagnostics is mode-aware — advanced Form B
expects all columns; simple Form B/C expects the user-fillable columns
(serial/shortid auto-fill). It counts the DSL Form A role and scans the
keyword-less Form C tuple. New catalog keys name the fillable/auto split
and the all-auto-table case.

Submit: a wrong-count DSL insert now parses Ok + carries the ERROR
diagnostic, so a unified Ok-arm pre-flight (dsl_insert_count_mismatch_notes)
blocks dispatch and teaches; the previous Err-arm note retires.
advanced_alternative_note's gate now reads the validity verdict so it
still fires for the parse-Ok-with-error shape.

Docs: ADR-0036 Amendment 2 (+ README index) and requirements.md H1a.

docs/adr/0036-typed-dml-values-vs-verbatim.md

@@ -422,6 +422,67 @@ So the live highlight catches *numeric-shape* mismatches (`int`/`decimal`/
 `bool`), not malformed dates. The column-type **hint** still shows for
 every type.
 
+## Amendment 2 — Simple-mode arity-diagnostic parity (issue #17, 2026-05-29)
+
+Phase 3b (Amendment 1) introduced, for the **advanced** grammar, an
+*arity-gating tuple lookahead* (`tuple_value_list`, `sql_insert.rs`):
+a wrong-count `VALUES` tuple is routed to the type-blind fallback so it
+**structurally matches** and the §8.1 arity diagnostic
+(`dml_insert_arity_diagnostics`) fires its friendly *"N column(s) … M
+value(s)"* message instead of the engine's raw error.
+
+The **simple-mode DSL** insert (`data.rs` → `column_value_list`) never
+had that gate: its value list is a fixed-length typed `Seq`, so a
+wrong-count tuple **Mismatched** (parse error) and the arity diagnostic
+never ran. Simple-mode learners — the primary audience — got a bare
+`expected `,`/`)`` while advanced mode got the friendly message. Issue
+#1 had papered over this with a *submit-time* teaching note bolted onto
+the parse-error path; that workaround existed only because the
+diagnostic couldn't fire in simple mode.
+
+**This amendment brings the two modes onto one model.**
+
+1. **Grammar (simple-mode only).** A `tuple_value_list`-style gate
+   (`dsl_insert_value_list`, `data.rs`) routes a wrong-count DSL insert
+   tuple to the shared type-blind `FALLBACK_VALUE_LIST`, so it matches
+   and the diagnostic fires. The gate is **gated to `Mode::Simple`**: in
+   advanced mode the DSL insert node stays strict (a non-SQL shape like
+   Form C must not spuriously match — advanced inserts are owned by
+   `sql_insert.rs`), so **advanced behaviour is byte-for-byte
+   unchanged**. `count_tuple_values` + the target-column selection
+   (`insert_target_columns`) are now shared by both grammars so the gate
+   and the typed slots never disagree.
+
+2. **Diagnostic is mode-aware.** `dml_insert_arity_diagnostics` gains a
+   `mode` parameter. The expected count differs by mode **because the
+   modes' auto-fill differs** (the very distinction this ADR preserves):
+   - **advanced Form B** auto-fills nothing → expects **all** columns
+     (unchanged);
+   - **simple Form B/C** auto-fills `serial`/`shortid` (ADR-0018 §3) →
+     expects the **user-fillable** columns.
+   It also now counts the DSL Form A column role (`insert_first_item`)
+   alongside the SQL `insert_column`, and scans the simple-mode Form C
+   tuple (no `values` keyword). New catalog keys
+   `diagnostic.insert_arity_mismatch_form_b_simple` (names the fillable
+   *and* the auto-generated columns) and
+   `diagnostic.insert_arity_mismatch_all_auto` (every column
+   auto-generated).
+
+3. **Unified submit pre-flight.** A wrong-count DSL insert now parses
+   `Ok` (so the typing-time diagnostic can fire), so dispatch is gated
+   by an Ok-arm pre-flight (`dsl_insert_count_mismatch_notes`) that
+   **blocks `ExecuteDsl`** and shows the teaching note — mirroring the
+   advanced Ok-arm pre-flight (`form_b_positional_count_mismatch_note`).
+   The issue #1 Err-arm note retires. The user-facing invalidity is the
+   ADR-0027 validity verdict (the `[ERR]` indicator), which reads the
+   diagnostic; the structural `InputState` is now `Valid` (it parses).
+
+**Scope guard.** This is *arity-diagnostic UX parity only*. It does
+**not** consolidate value-handling, execution, or auto-fill across modes
+— the deliberate mode-distinctness this ADR was narrowed to preserve
+stands. The count difference per mode is a *consequence* of that
+distinctness, not a violation of it.
+
 ## See also
 
 - ADR-0030 §4 / ADR-0033 §10 — the execute-path this ADR **augments**

docs/requirements.md

@@ -540,10 +540,15 @@ handoff-14 cleanup; 449 after B2/C2.)
   names the schema-correct next token (`,` between values, `)`
   after the last) instead of the type-blind close-paren, and so a
   wrong-arity closed tuple surfaces the real parse error rather
-  than a misleading "submit with Enter"). A systematic pass is
-  still pending; simple-mode wrong-count tuples still get a generic
-  expected-token message rather than the friendly arity diagnostic
-  advanced mode shows (issue #17).
+  than a misleading "submit with Enter"; **issue #17 / ADR-0036
+  Amendment 2, 2026-05-29** then brought the §8.1 arity diagnostic
+  to **simple mode** at parity with advanced — a wrong-count DSL
+  insert (Form A/B/C) now fires the friendly *"N value(s) for
+  `col`…"* message at typing time, counted against the user-fillable
+  columns, with `serial`/`shortid` auto-fill named; new keys
+  `diagnostic.insert_arity_mismatch_form_b_simple` /
+  `diagnostic.insert_arity_mismatch_all_auto`). A systematic pass is
+  still pending.
 - [ ] **H2** `hint` provides contextual help for the current
   input or the most recent error.
 - [ ] **H3** `help` provides general reference and per-command

src/app.rs

@@ -1361,6 +1361,30 @@ impl App {
                         source: input.to_string(),
                     }];
                 }
+                // Issue #17: simple-mode (DSL) counterpart. A wrong-count
+                // DSL insert now parses `Ok` (so the typing-time arity
+                // diagnostic can fire), so dispatch is gated here — the
+                // same teaching the old parse-error path showed, now with
+                // the insert reliably blocked from reaching the worker.
+                if let Some(notes) = crate::input_render::dsl_insert_count_mismatch_notes(
+                    input,
+                    &cmd,
+                    &self.schema_cache,
+                ) {
+                    self.push_output(OutputLine {
+                        text: crate::t!("dsl.running", input = input),
+                        kind: OutputKind::Echo,
+                        mode_at_submission: mode,
+                        styled_runs: None,
+                    });
+                    for note in notes {
+                        self.note_error(note);
+                    }
+                    self.note_error(render_usage_block(input));
+                    return vec![Action::JournalFailure {
+                        source: input.to_string(),
+                    }];
+                }
                 self.push_output(OutputLine {
                     text: crate::t!("dsl.running", input = input),
                     kind: OutputKind::Echo,
@@ -1426,19 +1450,13 @@ impl App {
                 {
                     self.note_error(note);
                 }
-                // Issue #1 sub-task 2: append a teaching note when the
-                // Form B `insert into <T> values (…)` line failed
-                // because the user supplied more values than the
-                // non-auto-generated columns expect. The parse error
-                // shows the literal "expected `)`"; the note explains
-                // *why* fewer values are expected and shows the
-                // column-list override path.
-                if mode == Mode::Simple
-                    && let Some(note) =
-                        crate::input_render::form_b_extra_values_note(input, &self.schema_cache)
-                {
-                    self.note_error(note);
-                }
+                // Issue #1 sub-task 2's Form B teaching note used to be
+                // appended here, because a wrong-count Form B insert
+                // failed to parse and landed in this Err arm. As of issue
+                // #17 such tuples parse `Ok` (so the typing-time arity
+                // diagnostic fires) and the teaching + dispatch block now
+                // live in the Ok arm's `dsl_insert_count_mismatch_notes`
+                // pre-flight — a single model shared with advanced mode.
                 // ADR-0021 §2: append the usage block (if a
                 // known command-entry keyword was consumed) or
                 // the available-commands fallback (§5).
@@ -3129,6 +3147,52 @@ mod tests {
         );
     }
 
+    #[test]
+    fn simple_mode_submit_of_form_b_count_mismatch_does_not_dispatch() {
+        // Issue #17 EXECUTION SAFETY. Once simple-mode wrong-count Form B
+        // tuples parse `Ok` (so the typing-time arity diagnostic can
+        // fire), the submit path must still NOT dispatch the insert — a
+        // wrong-count insert would otherwise reach the worker and fail
+        // (or, worse, write the wrong row). The unified Ok-arm pre-flight
+        // must block dispatch exactly as the old parse-error path did.
+        let mut app = App::new();
+        install_customers_schema_two_serials(&mut app);
+        type_str(&mut app, "insert into Customers values ('Oli', 52, 3)");
+        let actions = submit(&mut app);
+        assert!(
+            !actions.iter().any(|a| matches!(a, Action::ExecuteDsl { .. })),
+            "simple-mode Form B count mismatch must NOT dispatch; got: {actions:?}",
+        );
+    }
+
+    #[test]
+    fn simple_mode_submit_of_form_b_undersupply_does_not_dispatch() {
+        // Companion to the above for under-supply.
+        let mut app = App::new();
+        install_customers_schema_two_serials(&mut app);
+        type_str(&mut app, "insert into Customers values ('Oli')");
+        let actions = submit(&mut app);
+        assert!(
+            !actions.iter().any(|a| matches!(a, Action::ExecuteDsl { .. })),
+            "simple-mode Form B under-supply must NOT dispatch; got: {actions:?}",
+        );
+    }
+
+    #[test]
+    fn simple_mode_submit_of_form_a_count_mismatch_does_not_dispatch() {
+        // Form A (explicit column list) wrong count must also not
+        // dispatch — it previously parse-errored; the unified pre-flight
+        // must keep it blocked.
+        let mut app = App::new();
+        install_customers_schema_two_serials(&mut app);
+        type_str(&mut app, "insert into Customers (Name, Age) values ('Oli')");
+        let actions = submit(&mut app);
+        assert!(
+            !actions.iter().any(|a| matches!(a, Action::ExecuteDsl { .. })),
+            "simple-mode Form A count mismatch must NOT dispatch; got: {actions:?}",
+        );
+    }
+
     #[test]
     fn simple_mode_submit_of_sql_construct_appends_advanced_pointer() {
         // ADR-0033 Amendment 3 (+ Amendment 5): submitting a line in

src/dsl/grammar/data.rs

@@ -27,7 +27,10 @@
 use crate::dsl::command::{Command, Expr, RowFilter};
 use crate::dsl::grammar::{
     CommandNode, IdentSource, Node, NumberValidator, ValidationError, Word, expr,
-    shared::{column_value_list, current_column_value},
+    shared::{
+        FALLBACK_VALUE_LIST, column_value_list, count_tuple_values,
+        current_column_value, insert_target_columns,
+    },
     sql_delete, sql_insert, sql_select, sql_update,
 };
 use crate::dsl::walker::context::WalkContext;
@@ -141,12 +144,13 @@ static INSERT_COMMA: Node = Node::Punct(',');
 /// First-paren resolver (ADR-0024 §Phase D Form-C type-awareness).
 /// Peeks the first token after `(` to route to Form A's
 /// column-name list or Form C's typed value list.
-fn insert_first_paren(_ctx: &WalkContext, source: &str, pos: usize) -> Node {
+fn insert_first_paren(ctx: &WalkContext, source: &str, pos: usize) -> Node {
     if first_paren_item_is_value_literal(source, pos) {
-        // Form C — bare value list. `column_value_list` with no
-        // user-listed columns dispatches per non-auto-generated
-        // column, exactly as Form B does.
-        Node::DynamicSubgrammar(column_value_list)
+        // Form C — bare value list. Arity-gated exactly like Form B's
+        // `values (…)`: a correct-count tuple gets the typed per-column
+        // slots; a wrong-count tuple routes to the type-blind fallback
+        // so it still matches and the arity diagnostic fires (issue #17).
+        dsl_insert_value_list(ctx, source, pos)
     } else {
         // Form A (or Form A in progress / empty paren).
         Node::Repeated {
@@ -189,12 +193,51 @@ fn first_paren_item_is_value_literal(source: &str, pos: usize) -> bool {
 
 const INSERT_PAREN_LIST: Node = Node::Lookahead(insert_first_paren);
 
-/// Schema-aware value list: when the walker has a populated
-/// `current_table_columns`, unfolds to a `Seq` of typed slots
-/// per column (`int_slot`, `text_slot`, …). When schemaless,
-/// falls back to the pre-Phase-D `Repeated(VALUE_LITERAL, ',', 1)`
-/// shape (ADR-0024 §Phase D §column_value_list).
-const INSERT_VALUES_LIST: Node = Node::DynamicSubgrammar(column_value_list);
+/// Insert value-list arity gate (issue #17) — the simple-mode DSL
+/// counterpart of the advanced grammar's `tuple_value_list`
+/// (`sql_insert.rs`). Routes a correct-arity tuple to the typed
+/// per-column slots ([`column_value_list`]) and a wrong-arity tuple to
+/// the type-blind [`FALLBACK_VALUE_LIST`], so the wrong-count tuple
+/// still structurally matches and the per-tuple arity diagnostic
+/// (ADR-0033 §8.1, made mode-aware for issue #17) fires its friendly
+/// message instead of a bare "expected `,`/`)`".
+///
+/// Target arity comes from [`insert_target_columns`] — the same source
+/// `column_value_list` uses, so gate and slots never disagree. `None`
+/// (schemaless / unknown table / all-auto-generated) → fallback: either
+/// we can't gate (schemaless) or the all-auto case wants the tuple to
+/// match so the diagnostic can explain it.
+///
+/// **Simple-mode only.** The fallback routing is what lets a wrong-count
+/// tuple structurally match (so the diagnostic fires); that is a
+/// simple-mode behaviour. In advanced mode the DSL insert node must stay
+/// strict — otherwise a non-SQL shape like Form C (`insert into T
+/// (1, 2)`, no `values`) would spuriously match here and be accepted in
+/// advanced mode, where SQL requires `values` and the dedicated SQL
+/// grammar (`sql_insert.rs`) owns inserts. Keeping advanced strict
+/// preserves the pre-#17 advanced behaviour exactly (issue #17).
+fn dsl_insert_value_list(ctx: &WalkContext, source: &str, pos: usize) -> Node {
+    if ctx.mode != crate::mode::Mode::Simple {
+        return Node::DynamicSubgrammar(column_value_list);
+    }
+    let Some(cols) = insert_target_columns(ctx) else {
+        return FALLBACK_VALUE_LIST;
+    };
+    let (count, closed) = count_tuple_values(source, pos);
+    let arity_ok = if closed { count == cols.len() } else { count <= cols.len() };
+    if arity_ok {
+        Node::DynamicSubgrammar(column_value_list)
+    } else {
+        FALLBACK_VALUE_LIST
+    }
+}
+
+/// Schema-aware value list, arity-gated (issue #17): a correct-count
+/// tuple unfolds to a `Seq` of typed slots per column (`int_slot`,
+/// `text_slot`, …); a wrong-count tuple or a schemaless walk falls back
+/// to the type-blind `Repeated(VALUE_LITERAL, ',', 1)` shape (ADR-0024
+/// §Phase D §column_value_list).
+const INSERT_VALUES_LIST: Node = Node::Lookahead(dsl_insert_value_list);
 
 const INSERT_OPTIONAL_VALUES_NODES: &[Node] = &[
     Node::Word(Word::keyword("values")),

src/dsl/grammar/shared.rs

@@ -371,12 +371,116 @@ pub(crate) const FALLBACK_VALUE_LITERAL: Node = Node::Hinted {
     inner: &FALLBACK_VALUE_LITERAL_INNER,
 };
 
-const FALLBACK_VALUE_LIST: Node = Node::Repeated {
+/// The type-blind value list. `pub(crate)` so the insert value-list
+/// arity gate (`data.rs`, issue #17) can route a wrong-count tuple here
+/// — exactly as the advanced grammar's `tuple_value_list` does — so the
+/// tuple still structurally matches and the per-tuple arity diagnostic
+/// (ADR-0033 §8.1) fires instead of a bare "expected `,`/`)`".
+pub(crate) const FALLBACK_VALUE_LIST: Node = Node::Repeated {
     inner: &FALLBACK_VALUE_LITERAL,
     separator: Some(&Node::Punct(',')),
     min: 1,
 };
 
+/// The columns an insert value tuple maps onto (ADR-0024 §Phase D).
+///
+/// Mirrors `db::do_insert`'s `user_cols` logic. `None` when the walker
+/// is schemaless, the table is unknown, or — for Form B/C — every column
+/// is auto-generated (callers fall back to the type-blind value list).
+///
+/// - **Form A** (`user_listed_columns` set): the listed columns, in the
+///   user's order; names the schema doesn't know are dropped.
+/// - **Form B/C** (no column list): the table's non-auto-generated
+///   columns, in declaration order. `serial` / `shortid` are skipped
+///   because the simple-mode dispatch auto-fills them (ADR-0018 §3).
+///
+/// This is the single source of truth shared by [`column_value_list`]
+/// (which builds the typed slots) and the `data.rs` arity gate (which
+/// counts them) so the two never disagree (issue #17).
+pub fn insert_target_columns<'c>(
+    ctx: &'c WalkContext<'_>,
+) -> Option<Vec<&'c TableColumn>> {
+    let table_cols = ctx.current_table_columns.as_ref()?;
+    if table_cols.is_empty() {
+        return None;
+    }
+    let cols: Vec<&TableColumn> = ctx.user_listed_columns.as_ref().map_or_else(
+        || {
+            table_cols
+                .iter()
+                .filter(|c| !matches!(c.user_type, Type::Serial | Type::ShortId))
+                .collect()
+        },
+        |user_listed| {
+            user_listed
+                .iter()
+                .filter_map(|name| {
+                    table_cols
+                        .iter()
+                        .find(|c| c.name.eq_ignore_ascii_case(name))
+                })
+                .collect()
+        },
+    );
+    if cols.is_empty() { None } else { Some(cols) }
+}
+
+/// Count the value positions in a `VALUES`/insert tuple whose contents
+/// begin at `pos` (just past the opening `(`), and whether the tuple is
+/// *closed* (a depth-0 `)` was reached) vs still being typed (scan hit
+/// end-of-input first). Depth-aware: commas nested in a function call /
+/// subquery (paren depth ≥ 1) or inside a string literal are not
+/// separators. Returns `(0, _)` for an empty tuple `()`.
+///
+/// Shared by the advanced grammar's `tuple_value_list` (`sql_insert.rs`)
+/// and the simple-mode DSL insert arity gate (`data.rs`) so both modes
+/// count tuple values identically (issue #17).
+pub(crate) fn count_tuple_values(source: &str, pos: usize) -> (usize, bool) {
+    let bytes = source.as_bytes();
+    let mut i = pos;
+    let mut depth: i32 = 0;
+    let mut commas = 0usize;
+    let mut seen_value = false;
+    let mut closed = false;
+    while i < bytes.len() {
+        match bytes[i] {
+            b'\'' => {
+                // Skip a single-quoted string literal (`''` escape).
+                i += 1;
+                seen_value = true;
+                while i < bytes.len() {
+                    if bytes[i] == b'\'' {
+                        if bytes.get(i + 1) == Some(&b'\'') {
+                            i += 2;
+                            continue;
+                        }
+                        i += 1;
+                        break;
+                    }
+                    i += 1;
+                }
+                continue;
+            }
+            b'(' => {
+                depth += 1;
+                seen_value = true;
+            }
+            b')' => {
+                if depth == 0 {
+                    closed = true;
+                    break; // tuple close
+                }
+                depth -= 1;
+            }
+            b',' if depth == 0 => commas += 1,
+            b if !b.is_ascii_whitespace() => seen_value = true,
+            _ => {}
+        }
+        i += 1;
+    }
+    (if seen_value { commas + 1 } else { 0 }, closed)
+}
+
 /// Value slot keyed on `WalkContext::current_column`.
 ///
 /// Picks the typed slot for the column whose name was most
@@ -399,50 +503,12 @@ pub fn current_column_value(ctx: &WalkContext) -> Node {
 /// `Repeated(VALUE_LITERAL, ',', 1)` shape so existing
 /// callers/tests continue to work.
 pub fn column_value_list(ctx: &WalkContext) -> Node {
-    let Some(table_cols) = ctx.current_table_columns.as_ref() else {
+    // Target columns per the shared insert mapping (Form A = listed,
+    // Form B/C = non-auto-generated). `None` → schemaless / unknown
+    // table / all-auto-generated → the type-blind fallback list.
+    let Some(target_cols) = insert_target_columns(ctx) else {
         return FALLBACK_VALUE_LIST;
     };
-    if table_cols.is_empty() {
-        return FALLBACK_VALUE_LIST;
-    }
-    // Three dispatch shapes (ADR-0024 §Phase D §column_value_list,
-    // matching `db::do_insert`'s user_cols logic):
-    //
-    // 1. Form A — user listed explicit columns
-    //    (`insert into T (col1, col2, …) values (…)`): one slot
-    //    per listed column, in the user's order, types resolved
-    //    from the schema.
-    // 2. Form B — bare values keyword
-    //    (`insert into T values (…)`): one slot per non-auto-
-    //    generated column of T, in declaration order. Serial /
-    //    shortid columns are skipped because the dispatch path
-    //    auto-fills them (ADR-0018 §3).
-    // 3. Schemaless / fallback: the generic value-literal list.
-    let target_cols: Vec<&TableColumn> = ctx.user_listed_columns.as_ref().map_or_else(
-        || {
-            // Form B — exclude auto-generated columns.
-            table_cols
-                .iter()
-                .filter(|c| !matches!(c.user_type, Type::Serial | Type::ShortId))
-                .collect()
-        },
-        |user_listed| {
-            // Form A — resolve each listed name from the schema.
-            // Names the schema doesn't know about silently drop;
-            // the bind-time path catches unknown columns.
-            user_listed
-                .iter()
-                .filter_map(|name| {
-                    table_cols
-                        .iter()
-                        .find(|c| c.name.eq_ignore_ascii_case(name))
-                })
-                .collect()
-        },
-    );
-    if target_cols.is_empty() {
-        return FALLBACK_VALUE_LIST;
-    }
     // Build a Seq of typed slots interleaved with commas. Each
     // slot embeds its column name so the hint resolver can
     // mention the column by name ("for `Email`: Type a quoted

src/dsl/grammar/sql_insert.rs

@@ -14,7 +14,7 @@
 //! sub-phases.
 
 use crate::completion::TableColumn;
-use crate::dsl::grammar::shared::SET_VALUE;
+use crate::dsl::grammar::shared::{SET_VALUE, count_tuple_values};
 use crate::dsl::grammar::sql_expr;
 use crate::dsl::grammar::sql_select::{
     RETURNING_CLAUSE, SQL_SELECT_COMPOUND, WHERE_CLAUSE, reject_internal_table,
@@ -127,57 +127,9 @@ fn target_value_columns(ctx: &WalkContext) -> Vec<TableColumn> {
     )
 }
 
-/// Count the value positions in the `VALUES` tuple whose contents begin
-/// at `pos` (just past the opening `(`), and whether the tuple is
-/// *closed* (a depth-0 `)` was reached) vs still being typed (scan hit
-/// end-of-input first). Depth-aware: commas nested in a function call /
-/// subquery (paren depth ≥ 1) or inside a string literal are not
-/// separators. Returns `(0, _)` for an empty tuple `()`.
-fn count_tuple_values(source: &str, pos: usize) -> (usize, bool) {
-    let bytes = source.as_bytes();
-    let mut i = pos;
-    let mut depth: i32 = 0;
-    let mut commas = 0usize;
-    let mut seen_value = false;
-    let mut closed = false;
-    while i < bytes.len() {
-        match bytes[i] {
-            b'\'' => {
-                // Skip a single-quoted string literal (`''` escape).
-                i += 1;
-                seen_value = true;
-                while i < bytes.len() {
-                    if bytes[i] == b'\'' {
-                        if bytes.get(i + 1) == Some(&b'\'') {
-                            i += 2;
-                            continue;
-                        }
-                        i += 1;
-                        break;
-                    }
-                    i += 1;
-                }
-                continue;
-            }
-            b'(' => {
-                depth += 1;
-                seen_value = true;
-            }
-            b')' => {
-                if depth == 0 {
-                    closed = true;
-                    break; // tuple close
-                }
-                depth -= 1;
-            }
-            b',' if depth == 0 => commas += 1,
-            b if !b.is_ascii_whitespace() => seen_value = true,
-            _ => {}
-        }
-        i += 1;
-    }
-    (if seen_value { commas + 1 } else { 0 }, closed)
-}
+// `count_tuple_values` moved to `grammar::shared` (issue #17) so the
+// simple-mode DSL insert arity gate can share it; the advanced grammar
+// imports it above.
 
 /// Tuple value-list lookahead (ADR-0036 Phase 3b). Gates the typed
 /// per-column path on arity so the typed `Seq` is used only where it

src/dsl/walker/mod.rs

@@ -1465,10 +1465,16 @@ fn dml_auto_column_diagnostics(
 fn dml_insert_arity_diagnostics(
     path: &MatchedPath,
     schema: Option<&crate::completion::SchemaCache>,
+    mode: crate::mode::Mode,
 ) -> Vec<outcome::Diagnostic> {
     use crate::dsl::grammar::IdentSource;
+    use crate::dsl::types::Type;
     use outcome::{Diagnostic, MatchedKind, Severity};
 
+    // Form A column count: the explicit `(col, …)` list. The SQL
+    // (advanced) grammar tags these `insert_column`; the DSL (simple)
+    // grammar tags them `insert_first_item` (issue #17) — count both so
+    // a DSL Form A insert isn't mis-classified as Form B.
     let col_arity = path
         .items
         .iter()
@@ -1477,31 +1483,70 @@ fn dml_insert_arity_diagnostics(
                 it.kind,
                 MatchedKind::Ident {
                     source: IdentSource::Columns,
-                    role: "insert_column",
+                    role: "insert_column" | "insert_first_item",
                 }
             )
         })
         .count();
 
-    // Resolve the expected arity + which message template to use.
-    // Form A: explicit column list → its own length.
-    // Form B: no list → the target table's column count, *if* we know
-    // it. Without a schema or a recognised target the pass goes
-    // silent (the unknown-table case is flagged by the schema-
-    // existence pass instead).
-    let (expected, message_key): (usize, &'static str) = if col_arity > 0 {
-        (col_arity, "diagnostic.insert_arity_mismatch")
-    } else {
-        let Some(schema) = schema else {
-            return Vec::new();
-        };
-        let Some(target) = path.items.iter().find_map(|it| match it.kind {
+    // Is this an INSERT? `into` is insert-exclusive in both grammars
+    // (update uses `set`, delete `from`, show `data`), so it tells the
+    // DSL insert apart from other commands that also tag a `table_name`.
+    let is_insert = path
+        .items
+        .iter()
+        .any(|it| matches!(it.kind, MatchedKind::Word("into")));
+    // Insert target table. The SQL grammar tags it `insert_target_table`
+    // (insert-specific); the DSL grammar reuses the generic `table_name`
+    // role, so only trust that when `into` confirmed an insert (issue
+    // #17).
+    let target_table: Option<&str> = path
+        .items
+        .iter()
+        .find_map(|it| match it.kind {
             MatchedKind::Ident {
                 source: IdentSource::Tables,
                 role: "insert_target_table",
             } => Some(it.text.as_str()),
+            MatchedKind::Ident {
+                source: IdentSource::Tables,
+                role: "table_name",
+            } if is_insert => Some(it.text.as_str()),
             _ => None,
-        }) else {
+        });
+
+    // Resolve the expected arity + a message builder. The builder
+    // captures the per-case args because the message key (and its
+    // placeholders) differ by form and mode.
+    //
+    // - **Form A** (both modes): the listed-column count, mode-neutral.
+    // - **Advanced Form B**: every column needs a value (auto-fills
+    //   nothing, ADR-0036) → the full table count.
+    // - **Simple Form B** (issue #17): the dispatch auto-fills
+    //   serial/shortid (ADR-0018 §3), so only the user-fillable columns
+    //   take values. When some columns are skipped the message names
+    //   both sets; when none are skipped the count equals the table's
+    //   and the plain Form B wording is accurate; when *every* column is
+    //   auto-generated no value belongs at all.
+    type MsgFn<'a> = Box<dyn Fn(usize) -> String + 'a>;
+    let (expected, make_message): (usize, MsgFn) = if col_arity > 0 {
+        (
+            col_arity,
+            Box::new(move |actual| {
+                crate::friendly::translate(
+                    "diagnostic.insert_arity_mismatch",
+                    &[
+                        ("expected", &col_arity as &dyn std::fmt::Display),
+                        ("actual", &actual as &dyn std::fmt::Display),
+                    ],
+                )
+            }),
+        )
+    } else {
+        let Some(schema) = schema else {
+            return Vec::new();
+        };
+        let Some(target) = target_table else {
             return Vec::new();
         };
         let Some(cols) = schema.table_columns.get(target) else {
@@ -1510,35 +1555,123 @@ fn dml_insert_arity_diagnostics(
         if cols.is_empty() {
             return Vec::new();
         }
-        (cols.len(), "diagnostic.insert_arity_mismatch_form_b")
+        let is_auto = |t: Type| matches!(t, Type::Serial | Type::ShortId);
+        let fillable: Vec<&str> = cols
+            .iter()
+            .filter(|c| !is_auto(c.user_type))
+            .map(|c| c.name.as_str())
+            .collect();
+        if mode == crate::mode::Mode::Advanced || fillable.len() == cols.len() {
+            // Advanced Form B (all columns), or simple Form B over a
+            // table with no auto-generated columns — "all N" is accurate.
+            let expected = if mode == crate::mode::Mode::Advanced {
+                cols.len()
+            } else {
+                fillable.len()
+            };
+            (
+                expected,
+                Box::new(move |actual| {
+                    crate::friendly::translate(
+                        "diagnostic.insert_arity_mismatch_form_b",
+                        &[
+                            ("expected", &expected as &dyn std::fmt::Display),
+                            ("actual", &actual as &dyn std::fmt::Display),
+                        ],
+                    )
+                }),
+            )
+        } else if fillable.is_empty() {
+            // Simple Form B, every column auto-generated → no value
+            // belongs at all.
+            let table = target.to_string();
+            (
+                0,
+                Box::new(move |actual| {
+                    crate::friendly::translate(
+                        "diagnostic.insert_arity_mismatch_all_auto",
+                        &[
+                            ("table", &table as &dyn std::fmt::Display),
+                            ("actual", &actual as &dyn std::fmt::Display),
+                        ],
+                    )
+                }),
+            )
+        } else {
+            // Simple Form B with a mix — name the user-fillable and the
+            // auto-generated columns so the learner understands the skip.
+            let expected = fillable.len();
+            let columns = fillable
+                .iter()
+                .map(|n| format!("`{n}`"))
+                .collect::<Vec<_>>()
+                .join(", ");
+            let skipped = cols
+                .iter()
+                .filter(|c| is_auto(c.user_type))
+                .map(|c| format!("`{}`", c.name))
+                .collect::<Vec<_>>()
+                .join(", ");
+            (
+                expected,
+                Box::new(move |actual| {
+                    crate::friendly::translate(
+                        "diagnostic.insert_arity_mismatch_form_b_simple",
+                        &[
+                            ("expected", &expected as &dyn std::fmt::Display),
+                            ("columns", &columns as &dyn std::fmt::Display),
+                            ("skipped", &skipped as &dyn std::fmt::Display),
+                            ("actual", &actual as &dyn std::fmt::Display),
+                        ],
+                    )
+                }),
+            )
+        }
     };
 
-    // Index of the row-source keyword (first VALUES / SELECT / WITH).
-    let Some(kw_idx) = path
+    // Locate the row source to scan. Form A/B and INSERT…SELECT carry a
+    // `values`/`select`/`with` keyword. The DSL Form C (`insert into T
+    // (1, 2)` — bare value list, no `values`) has none: its single value
+    // tuple follows the target table directly, with no column-name list
+    // (col_arity == 0 here), so scan from just after the table and treat
+    // it with the VALUES tuple logic (issue #17).
+    let (kw, tail): (&str, &[outcome::MatchedItem]) = if let Some(kw_idx) = path
         .items
         .iter()
         .position(|it| matches!(it.kind, MatchedKind::Word("values" | "select" | "with")))
-    else {
+    {
+        let MatchedKind::Word(kw) = path.items[kw_idx].kind else {
             return Vec::new();
         };
-    let MatchedKind::Word(kw) = path.items[kw_idx].kind else {
+        (kw, &path.items[kw_idx + 1..])
+    } else if is_insert && mode == crate::mode::Mode::Simple {
+        // DSL Form C (`insert into T (1, 2)` — bare value list, no
+        // `values` keyword) is simple-mode only; advanced SQL always
+        // carries a `values`/`select`/`with` keyword, so this branch
+        // stays out of the advanced path (its arity is handled above).
+        let Some(tbl_idx) = path.items.iter().position(|it| {
+            matches!(
+                it.kind,
+                MatchedKind::Ident {
+                    source: IdentSource::Tables,
+                    role: "insert_target_table" | "table_name",
+                }
+            )
+        }) else {
+            return Vec::new();
+        };
+        ("values", &path.items[tbl_idx + 1..])
+    } else {
         return Vec::new();
     };
 
     let mut diagnostics = Vec::new();
-    let tail = &path.items[kw_idx + 1..];
 
     let emit = |span: (usize, usize), actual: usize, diagnostics: &mut Vec<Diagnostic>| {
         diagnostics.push(Diagnostic {
             severity: Severity::Error,
             span,
-            message: crate::friendly::translate(
-                message_key,
-                &[
-                    ("expected", &expected as &dyn std::fmt::Display),
-                    ("actual", &actual as &dyn std::fmt::Display),
-                ],
-            ),
+            message: make_message(actual),
         });
     };
 
@@ -1600,7 +1733,8 @@ fn dml_insert_arity_diagnostics(
             }
         }
         if proj_arity != expected {
-            emit(anchor.unwrap_or(path.items[kw_idx].span), proj_arity, &mut diagnostics);
+            let fallback = tail.first().map_or((0, 0), |it| it.span);
+            emit(anchor.unwrap_or(fallback), proj_arity, &mut diagnostics);
         }
     }
     diagnostics
@@ -2872,7 +3006,7 @@ fn walk_one_command<'a>(
         // INSERT…SELECT projection. Form A uses the column list's
         // length; Form B uses the schema's column count for the
         // target table.
-        d.extend(dml_insert_arity_diagnostics(&path, ctx.schema));
+        d.extend(dml_insert_arity_diagnostics(&path, ctx.schema, ctx.mode));
         // ADR-0033 §8.3 — WARNING when an INSERT's column list omits
         // a NOT-NULL-no-default (non-auto-gen) column.
         d.extend(dml_not_null_missing_diagnostics(&path, ctx.schema));
@@ -4252,23 +4386,30 @@ mod tests {
 
     #[test]
     fn phase_d_insert_form_b_skips_serial_column() {
-        // Form B: `insert into <T> values (…)` excludes
-        // auto-generated columns from the value list. Supplying
-        // a value for the serial column is a count mismatch.
+        // Form B: `insert into <T> values (…)` excludes auto-generated
+        // columns from the value list. Supplying a value for the serial
+        // column is a count mismatch. As of issue #17 such a wrong-count
+        // tuple parses **structurally** (routed to the type-blind
+        // fallback) so the friendly arity diagnostic can fire — the
+        // mismatch is now reported as an ERROR diagnostic rather than a
+        // bare parse error, matching advanced mode. Dispatch is gated by
+        // the submit pre-flight, not by a parse failure.
         let schema = schema_with(
             "Customers",
             &[("id", Type::Serial), ("Name", Type::Text)],
         );
-        // Two values where Form B expects one (Name only):
-        let err = parse_command_with_schema(
+        // Two values where Form B expects one (Name only): structurally
+        // parses, but the simple-mode arity diagnostic flags it (Form B
+        // expects 1 value for `Name`; `id` is auto-generated).
+        let diags = diag_keys_simple(
             "insert into Customers values (1, 'Alice')",
             &schema,
-        )
-        .expect_err("Form B should reject user-supplied serial");
-        match err {
-            crate::dsl::ParseError::Invalid { .. } => {}
-            other => panic!("expected Invalid, got {other:?}"),
-        }
+        );
+        assert!(
+            diags.iter().any(|d| d.contains("1 value(s)") && d.contains("2 given")),
+            "Form B serial-skip count mismatch must fire the arity \
+             diagnostic (expected 1, got 2); got {diags:?}",
+        );
     }
 
     #[test]
@@ -4703,6 +4844,139 @@ mod tests {
             .collect()
     }
 
+    /// Simple-mode counterpart of [`diag_keys`] — the DSL surface
+    /// (ADR-0003). Issue #17: the arity diagnostic must fire in simple
+    /// mode too, with the user-fillable (serial-skipped) Form B count.
+    fn diag_keys_simple(source: &str, schema: &SchemaCache) -> Vec<String> {
+        let mut ctx = super::context::WalkContext::with_schema(schema);
+        ctx.mode = crate::mode::Mode::Simple;
+        let (result, _cmd) =
+            super::walk(source, super::outcome::WalkBound::EndOfInput, &mut ctx);
+        result.map_or_else(Vec::new, |r| {
+            r.diagnostics.into_iter().map(|d| d.message).collect()
+        })
+    }
+
+    #[test]
+    fn insert_arity_mismatch_simple_form_b_uses_user_fillable_count() {
+        // Issue #17: simple-mode Form B skips serial/shortid (auto-
+        // filled), so `Customers(id serial, Name, Age, SerNo)` expects
+        // 2 values (Name, Age). Three values is a mismatch — and simple
+        // mode must surface the friendly arity diagnostic (as advanced
+        // mode already does), counted against the user-fillable columns,
+        // not the full table.
+        let schema = schema_with(
+            "Customers",
+            &[
+                ("id", Type::Serial),
+                ("Name", Type::Text),
+                ("Age", Type::Int),
+                ("SerNo", Type::Serial),
+            ],
+        );
+        let diags = diag_keys_simple(
+            "insert into Customers values ('Oli', 52, 3)",
+            &schema,
+        );
+        assert!(
+            diags.iter().any(|d| d.contains("2 value(s)") && d.contains("3 given")),
+            "simple Form B must fire arity diagnostic with user-fillable \
+             count (2) and actual (3); got {diags:?}",
+        );
+        // Pedagogical: names the user-fillable and the auto-generated
+        // columns so the learner understands the skip.
+        assert!(
+            diags.iter().any(|d| d.contains("Name")
+                && d.contains("Age")
+                && d.contains("id")
+                && d.contains("SerNo")),
+            "message should name fillable (Name, Age) and auto-gen (id, \
+             SerNo) columns; got {diags:?}",
+        );
+    }
+
+    #[test]
+    fn insert_arity_mismatch_simple_form_a_uses_listed_count() {
+        // Simple Form A: the explicit column list sets the expected
+        // count (mode-neutral). `(Name, Age)` lists 2 columns; one value
+        // is a mismatch. The DSL Form A column role is `insert_first_item`
+        // (issue #17 — the diagnostic counts it as well as the SQL
+        // `insert_column`).
+        let schema = schema_with(
+            "Customers",
+            &[("id", Type::Serial), ("Name", Type::Text), ("Age", Type::Int)],
+        );
+        let diags = diag_keys_simple(
+            "insert into Customers (Name, Age) values ('Oli')",
+            &schema,
+        );
+        assert!(
+            diags.iter().any(|d| d.contains("names 2 column(s)") && d.contains("1 value(s)")),
+            "simple Form A must fire the column-list arity diagnostic; got {diags:?}",
+        );
+    }
+
+    #[test]
+    fn insert_arity_mismatch_simple_form_c_uses_user_fillable_count() {
+        // Simple Form C (`insert into T (vals)`, no `values` keyword)
+        // shares Form B's count semantics — the diagnostic must fire even
+        // though there is no `values` keyword to anchor on (issue #17).
+        let schema = schema_with(
+            "Customers",
+            &[("id", Type::Serial), ("Name", Type::Text), ("Age", Type::Int)],
+        );
+        let diags = diag_keys_simple(
+            "insert into Customers ('Oli', 52, 3)",
+            &schema,
+        );
+        assert!(
+            diags.iter().any(|d| d.contains("2 value(s)") && d.contains("3 given")),
+            "simple Form C must fire the arity diagnostic (expected 2, got 3); got {diags:?}",
+        );
+    }
+
+    #[test]
+    fn insert_arity_mismatch_simple_all_auto_table() {
+        // Edge: every column auto-generated (all serial/shortid). The
+        // user-fillable count is 0, so any supplied value is a mismatch —
+        // the tailored "all auto-generated" message fires (issue #17).
+        let schema = schema_with(
+            "Counters",
+            &[("id", Type::Serial), ("seq", Type::Serial)],
+        );
+        let diags = diag_keys_simple(
+            "insert into Counters values (1)",
+            &schema,
+        );
+        assert!(
+            diags.iter().any(|d| d.contains("all auto-generated")
+                || d.contains("auto-generated, so no values")),
+            "all-auto table must fire the tailored message; got {diags:?}",
+        );
+    }
+
+    #[test]
+    fn insert_arity_advanced_form_b_still_uses_full_column_count() {
+        // Guard: issue #17 made the diagnostic mode-aware, but advanced
+        // Form B must keep the full-column-count semantics (auto-fills
+        // nothing, ADR-0036) — `('Oli', 52, 3)` for a 4-column table
+        // reports "all 4 column(s)", not the simple-mode user-fillable 2.
+        let schema = schema_with(
+            "Customers",
+            &[
+                ("id", Type::Serial),
+                ("Name", Type::Text),
+                ("Age", Type::Int),
+                ("SerNo", Type::Serial),
+            ],
+        );
+        let diags = diag_keys("insert into Customers values ('Oli', 52, 3)", &schema);
+        assert!(
+            diags.iter().any(|d| d.contains("all 4 column(s)") && d.contains("3 value(s)")),
+            "advanced Form B must keep the full-column count (4); got {diags:?}",
+        );
+    }
+
     #[test]
     fn unknown_qualifier_in_qualified_ref_is_error() {
         let schema = two_table_schema();

src/friendly/keys.rs

@@ -51,6 +51,15 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
         "diagnostic.insert_arity_mismatch_form_b",
         &["expected", "actual"],
     ),
+    // ADR-0036 Amendment 1 / issue #17: simple-mode Form B variants.
+    (
+        "diagnostic.insert_arity_mismatch_form_b_simple",
+        &["expected", "columns", "skipped", "actual"],
+    ),
+    (
+        "diagnostic.insert_arity_mismatch_all_auto",
+        &["table", "actual"],
+    ),
     ("diagnostic.not_null_missing", &["column"]),
     ("diagnostic.like_numeric", &["column", "type"]),
     ("diagnostic.projection_alias_misplaced", &["alias", "clause"]),

src/friendly/strings/en-US.yaml

@@ -577,8 +577,16 @@ diagnostic:
   # ADR-0033 §8 — Phase-3 DML diagnostic keys.
   auto_column_overridden: "column `{column}` is auto-generated (`{type}`); providing an explicit value bypasses the auto-counter and may collide with later auto-generated values"
   insert_arity_mismatch: "the column list names {expected} column(s) but {actual} value(s) are given"
-  # ADR-0033 §8.1 / Amendment 5: Form B (no column list) variant.
+  # ADR-0033 §8.1 / Amendment 5: Form B (no column list) variant
+  # (advanced mode — auto-fills nothing, so every column needs a value).
   insert_arity_mismatch_form_b: "with no column list, all {expected} column(s) need a value but {actual} value(s) are given"
+  # ADR-0036 Amendment 1 / issue #17: simple-mode Form B. The DSL
+  # auto-fills serial/shortid columns, so only the user-fillable columns
+  # take values — name both sets so the learner understands the skip.
+  insert_arity_mismatch_form_b_simple: "without a column list, supply {expected} value(s) for {columns} — {skipped} auto-generated; {actual} given"
+  # ADR-0036 Amendment 1 / issue #17: simple-mode Form B where every
+  # column is auto-generated, so the values list takes nothing.
+  insert_arity_mismatch_all_auto: "every column of `{table}` is auto-generated, so no values are needed, but {actual} value(s) are given"
   not_null_missing: "column `{column}` is required (`NOT NULL`, no default); the statement will fail when run"
 
 # Engine-error translations: an engine-rejected SQL statement

src/input_render.rs

@@ -323,10 +323,20 @@ pub fn advanced_alternative_note(
     input: &str,
     cache: &crate::completion::SchemaCache,
 ) -> Option<String> {
-    let definite_dsl_error = matches!(
-        classify_input_with_schema_in_mode(input, cache, Mode::Simple),
-        InputState::DefiniteErrorAt(_)
-    );
+    // The line must be *definitely* invalid in simple mode — a definite
+    // parse error, or (issue #17) a parse that succeeds structurally but
+    // carries a blocking ERROR diagnostic such as a value-count
+    // mismatch. Incomplete input (still being typed) and empty input are
+    // excluded so the pointer doesn't flicker mid-keystroke.
+    let definite_dsl_error = match classify_input_with_schema_in_mode(input, cache, Mode::Simple)
+    {
+        InputState::DefiniteErrorAt(_) => true,
+        InputState::Valid => {
+            crate::dsl::walker::input_verdict_in_mode(input, Some(cache), Mode::Simple)
+                == Some(crate::dsl::walker::outcome::Severity::Error)
+        }
+        InputState::Empty | InputState::IncompleteAtEof => false,
+    };
     if !definite_dsl_error {
         return None;
     }
@@ -363,6 +373,82 @@ pub fn advanced_alternative_note(
 /// without adding pedagogy. The cross-mode pointer wins because it
 /// only fires when switching modes actually works (issue #1 sub-task
 /// 1's gate); when it doesn't fire, this note steps in.
+/// Submit-time pre-flight for a simple-mode (DSL) `Command::Insert`
+/// whose positional value count doesn't match the expected count
+/// (issue #17). Returns the advice line(s) to display when there is a
+/// mismatch — the caller (`dispatch_dsl`) blocks dispatch whenever this
+/// is `Some`, so a wrong-count insert never reaches the worker. `None`
+/// when the command isn't an insert, the table is unknown, or the count
+/// already matches.
+///
+/// This is the simple-mode counterpart of the advanced Ok-arm pre-flight
+/// (`form_b_positional_count_mismatch_note`). Both modes now parse a
+/// wrong-count insert as `Ok` (so the typing-time arity diagnostic can
+/// fire — issue #17), so dispatch is gated here, uniformly, rather than
+/// by a parse error.
+///
+/// Expected count: Form A (explicit `(col, …)`) → the listed count;
+/// Form B/C (no list) → the user-fillable (non-auto-generated) count,
+/// since the dispatch auto-fills serial/shortid (ADR-0018 §3).
+///
+/// Advice selection mirrors the previous Err-arm logic: the cross-mode
+/// pointer wins when the same text is valid in advanced mode; otherwise
+/// Form B/C shows the rich teaching note (names the fillable + auto
+/// columns and the Form-A override) and Form A shows the column-list
+/// arity message.
+#[must_use]
+pub fn dsl_insert_count_mismatch_notes(
+    input: &str,
+    cmd: &crate::dsl::command::Command,
+    cache: &crate::completion::SchemaCache,
+) -> Option<Vec<String>> {
+    use crate::dsl::command::Command;
+    use crate::dsl::types::Type;
+    let Command::Insert {
+        table,
+        columns,
+        values,
+    } = cmd
+    else {
+        return None;
+    };
+    let table_cols = cache.table_columns.get(table)?;
+    let is_auto = |t: Type| matches!(t, Type::Serial | Type::ShortId);
+    let expected = columns.as_ref().map_or_else(
+        || table_cols.iter().filter(|c| !is_auto(c.user_type)).count(),
+        Vec::len,
+    );
+    if values.len() == expected {
+        return None; // counts match — nothing to flag, dispatch proceeds
+    }
+    // Count mismatch → the caller blocks dispatch. Build the advice.
+    // The cross-mode pointer is the single most useful line when the
+    // same text is valid in advanced mode, so it suppresses the rest.
+    if let Some(pointer) = advanced_alternative_note(input, cache) {
+        return Some(vec![pointer]);
+    }
+    let note = if columns.is_some() {
+        // Form A: the column-list arity message.
+        crate::t!(
+            "diagnostic.insert_arity_mismatch",
+            expected = expected,
+            actual = values.len()
+        )
+    } else {
+        // Form B/C: the rich teaching note. Falls back to the all-auto
+        // explanation for a table whose columns are all auto-generated
+        // (the override note doesn't apply there).
+        form_b_extra_values_note(input, cache).unwrap_or_else(|| {
+            crate::t!(
+                "diagnostic.insert_arity_mismatch_all_auto",
+                table = table,
+                actual = values.len()
+            )
+        })
+    };
+    Some(vec![note])
+}
+
 #[must_use]
 pub fn form_b_extra_values_note(
     input: &str,

tests/typing_surface/insert_form_b.rs

@@ -154,16 +154,21 @@ fn form_b_in_progress_without_closing_paren_is_incomplete() {
 #[test]
 fn form_b_with_too_few_values_is_invalid_at_close_paren() {
     let schema = schema_serial_pk();
-    let a = assess_at_end("insert into Customers values ('Alice')", &schema);
-    // Only one value supplied; Form B for Customers needs two.
-    // The grammar's typed slot list expects another `,<value>`
-    // before the `)`. Classify as DefiniteError or Incomplete
-    // (which one depends on whether the closing `)` is past
-    // the missing slot).
-    assert!(
-        !matches!(a.state, InputState::Valid),
-        "input with too-few values must NOT be Valid, got {:?}",
-        a.state,
+    let input = "insert into Customers values ('Alice')";
+    let a = assess_at_end(input, &schema);
+    // Only one value supplied; Form B for Customers needs two. As of
+    // issue #17 a wrong-count tuple parses *structurally* (so the
+    // friendly arity diagnostic can fire) — the user-facing invalidity
+    // is the validity verdict (the `[ERR]` indicator), not the
+    // structural `state`.
+    assert_eq!(
+        rdbms_playground::dsl::walker::input_verdict_in_mode(
+            input,
+            Some(&schema),
+            rdbms_playground::mode::Mode::Simple,
+        ),
+        Some(rdbms_playground::dsl::walker::Severity::Error),
+        "too-few values must light the [ERR] verdict",
     );
     crate::snap!("form_b_too_few_values", a);
 }
@@ -174,14 +179,16 @@ fn form_b_with_extra_value_for_serial_column_is_invalid() {
     // (treating it as the first slot) means an extra value
     // overall — Customers has 3 columns but Form B accepts 2.
     let schema = schema_serial_pk();
-    let a = assess_at_end(
-        "insert into Customers values (1, 'Alice', 'a@b.c')",
-        &schema,
-    );
-    assert!(
-        !matches!(a.state, InputState::Valid),
-        "Form B with a value-for-serial must be invalid, got {:?}",
-        a.state,
+    let input = "insert into Customers values (1, 'Alice', 'a@b.c')";
+    let a = assess_at_end(input, &schema);
+    assert_eq!(
+        rdbms_playground::dsl::walker::input_verdict_in_mode(
+            input,
+            Some(&schema),
+            rdbms_playground::mode::Mode::Simple,
+        ),
+        Some(rdbms_playground::dsl::walker::Severity::Error),
+        "Form B with a value-for-serial must light the [ERR] verdict",
     );
     crate::snap!("form_b_extra_serial_value", a);
 }

tests/typing_surface/insert_form_c.rs

@@ -85,18 +85,21 @@ fn form_c_rejects_number_for_text_column() {
 
 #[test]
 fn form_c_wrong_value_count_is_invalid() {
-    // Customers Form C expects exactly two values (id:serial
-    // skipped). Three values is a count mismatch — caught at
-    // parse time now.
+    // Customers Form C expects exactly two values (id:serial skipped).
+    // Three values is a count mismatch. As of issue #17 it parses
+    // structurally (so the friendly arity diagnostic fires); the
+    // user-facing invalidity is the validity verdict (`[ERR]`).
     let schema = schema_serial_pk();
-    let a = assess_at_end(
-        "insert into Customers ('Alice', 'a@b.c', 'extra')",
-        &schema,
-    );
-    assert!(
-        !matches!(a.state, InputState::Valid),
-        "Form C with too many values must be invalid, got {:?}",
-        a.state,
+    let input = "insert into Customers ('Alice', 'a@b.c', 'extra')";
+    let a = assess_at_end(input, &schema);
+    assert_eq!(
+        rdbms_playground::dsl::walker::input_verdict_in_mode(
+            input,
+            Some(&schema),
+            rdbms_playground::mode::Mode::Simple,
+        ),
+        Some(rdbms_playground::dsl::walker::Severity::Error),
+        "Form C with too many values must light the [ERR] verdict",
     );
     crate::snap!("form_c_wrong_count", a);
 }

tests/typing_surface/snapshots/typing_surface_matrix__typing_surface__insert_form_b__form_b_with_extra_value_for_serial_column_is_invalid@form_b_extra_serial_value.snap

@@ -1,37 +1,20 @@
 ---
 source: tests/typing_surface/insert_form_b.rs
-assertion_line: 186
+assertion_line: 193
 description: "input=\"insert into Customers values (1, 'Alice', 'a@b.c')\" cursor=50"
 expression: "& a"
 ---
 Assessment {
     input: "insert into Customers values (1, 'Alice', 'a@b.c')",
     cursor: 50,
-    state: DefiniteErrorAt(
-        30,
-    ),
+    state: Valid,
     hint: Some(
         Prose(
-            "for `Name`: Type a quoted string (e.g. 'Alice') or null (`id` auto-generated — skipped here; list columns explicitly, e.g. `insert into T (...) values (...)`, to set it.) (trying to write SQL? switch with `mode advanced`, or prefix `:` to run once)",
+            "without a column list, supply 2 value(s) for `Name`, `Email` — `id` auto-generated; 3 given (trying to write SQL? switch with `mode advanced`, or prefix `:` to run once)",
         ),
     ),
-    completion: Some(
-        Completion {
-            replaced_range: (
-                50,
-                50,
-            ),
-            partial_prefix: "",
-            candidates: [
-                Candidate {
-                    text: "null",
-                    kind: Keyword,
-                    mode: Both,
-                },
-            ],
-        },
-    ),
-    parse_result: Err(
-        "Invalid(definite)",
+    completion: None,
+    parse_result: Ok(
+        "Insert",
     ),
 }

tests/typing_surface/snapshots/typing_surface_matrix__typing_surface__insert_form_b__form_b_with_too_few_values_is_invalid_at_close_paren@form_b_too_few_values.snap

@@ -1,22 +1,20 @@
 ---
 source: tests/typing_surface/insert_form_b.rs
-assertion_line: 168
+assertion_line: 173
 description: "input=\"insert into Customers values ('Alice')\" cursor=38"
 expression: "& a"
 ---
 Assessment {
     input: "insert into Customers values ('Alice')",
     cursor: 38,
-    state: DefiniteErrorAt(
-        37,
-    ),
+    state: Valid,
     hint: Some(
         Prose(
-            "after `insert into Customers values ('Alice'`, expected `,` — usage: insert into <Table> [(<col>[, ...])] [values] (<value>[, ...])",
+            "without a column list, supply 2 value(s) for `Name`, `Email` — `id` auto-generated; 1 given",
         ),
     ),
     completion: None,
-    parse_result: Err(
-        "Invalid(definite)",
+    parse_result: Ok(
+        "Insert",
     ),
 }

tests/typing_surface/snapshots/typing_surface_matrix__typing_surface__insert_form_c__form_c_wrong_value_count_is_invalid@form_c_wrong_count.snap

@@ -1,22 +1,35 @@
 ---
 source: tests/typing_surface/insert_form_c.rs
-assertion_line: 101
+assertion_line: 104
 description: "input=\"insert into Customers ('Alice', 'a@b.c', 'extra')\" cursor=49"
 expression: "& a"
 ---
 Assessment {
     input: "insert into Customers ('Alice', 'a@b.c', 'extra')",
     cursor: 49,
-    state: DefiniteErrorAt(
-        39,
-    ),
+    state: Valid,
     hint: Some(
         Prose(
-            "after `insert into Customers ('Alice', 'a@b.c'`, expected `)` — usage: insert into <Table> [(<col>[, ...])] [values] (<value>[, ...])",
+            "without a column list, supply 2 value(s) for `Name`, `Email` — `id` auto-generated; 3 given",
         ),
     ),
-    completion: None,
-    parse_result: Err(
-        "Invalid(definite)",
+    completion: Some(
+        Completion {
+            replaced_range: (
+                49,
+                49,
+            ),
+            partial_prefix: "",
+            candidates: [
+                Candidate {
+                    text: "values",
+                    kind: Keyword,
+                    mode: Both,
+                },
+            ],
+        },
+    ),
+    parse_result: Ok(
+        "Insert",
     ),
 }