diff --git a/src/dsl/walker/mod.rs b/src/dsl/walker/mod.rs
index 2c6c662..1543d36 100644
--- a/src/dsl/walker/mod.rs
+++ b/src/dsl/walker/mod.rs
@@ -1172,6 +1172,138 @@ fn dml_auto_column_diagnostics(
     diagnostics
 }
 
+/// `insert_arity_mismatch` ERROR (ADR-0033 §8.1, sub-phase 3i).
+///
+/// Walker pre-flight when the explicit `(column_name_list)` arity
+/// disagrees with a row's arity — pre-empting the engine's
+/// less-helpful "N values for M columns". Two row-source shapes:
+///
+/// - **VALUES**: each `value_tuple` is checked independently;
+///   **each** offending row emits its own diagnostic on that tuple's
+///   span (a five-row list with three wrong tuples emits three).
+/// - **INSERT … SELECT** (plain, non-`WITH`): the first SELECT leg's
+///   projection arity is compared, anchored on the first projection
+///   item. A `WITH`-prefixed row source is skipped here (the outer
+///   projection isn't the first `select` token) — the engine still
+///   reports it; a false positive would be worse than deferring.
+///
+/// Only fires when an explicit column list is present; the
+/// no-column-list form (arity vs the table's full column count)
+/// is deferred (needs the schema and is outside the 3i exit gate).
+fn dml_insert_arity_diagnostics(path: &MatchedPath) -> Vec<outcome::Diagnostic> {
+    use crate::dsl::grammar::IdentSource;
+    use outcome::{Diagnostic, MatchedKind, Severity};
+
+    let col_arity = path
+        .items
+        .iter()
+        .filter(|it| {
+            matches!(
+                it.kind,
+                MatchedKind::Ident {
+                    source: IdentSource::Columns,
+                    role: "insert_column",
+                }
+            )
+        })
+        .count();
+    if col_arity == 0 {
+        return Vec::new();
+    }
+
+    // Index of the row-source keyword (first VALUES / SELECT / WITH).
+    let Some(kw_idx) = path
+        .items
+        .iter()
+        .position(|it| matches!(it.kind, MatchedKind::Word("values" | "select" | "with")))
+    else {
+        return Vec::new();
+    };
+    let MatchedKind::Word(kw) = path.items[kw_idx].kind 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(
+                "diagnostic.insert_arity_mismatch",
+                &[
+                    ("expected", &col_arity as &dyn std::fmt::Display),
+                    ("actual", &actual as &dyn std::fmt::Display),
+                ],
+            ),
+        });
+    };
+
+    if kw == "values" {
+        // Per-tuple arity: count value exprs (commas at the tuple's
+        // interior depth, +1). Nested parens (calls / subqueries)
+        // sit deeper and their commas are ignored.
+        let mut depth: i32 = 0;
+        let mut tuple_arity = 0usize;
+        let mut tuple_start = 0usize;
+        for it in tail {
+            match it.kind {
+                MatchedKind::Punct('(') => {
+                    if depth == 0 {
+                        tuple_arity = 1;
+                        tuple_start = it.span.0;
+                    }
+                    depth += 1;
+                }
+                MatchedKind::Punct(')') => {
+                    depth -= 1;
+                    if depth == 0 && tuple_arity != col_arity {
+                        emit((tuple_start, it.span.1), tuple_arity, &mut diagnostics);
+                    }
+                }
+                MatchedKind::Punct(',') if depth == 1 => tuple_arity += 1,
+                // A depth-0 keyword ends the VALUES clause — the
+                // tuples are over and what follows (`ON CONFLICT (…)`,
+                // `RETURNING …`) must NOT be mis-read as a value
+                // tuple. Value exprs live inside the tuple parens
+                // (depth ≥ 1), so no legitimate value keyword sits at
+                // depth 0.
+                MatchedKind::Word(_) if depth == 0 => break,
+                _ => {}
+            }
+        }
+    } else if kw == "select" {
+        // First SELECT leg's projection arity: commas at the leg's
+        // depth (0 here — INSERT … SELECT is not parenthesised) up to
+        // the first leg-end / compound keyword. Anchor on the first
+        // projection item.
+        let mut depth: i32 = 0;
+        let mut proj_arity = 1usize;
+        let mut anchor: Option<(usize, usize)> = None;
+        for it in tail {
+            match it.kind {
+                MatchedKind::Punct('(') => depth += 1,
+                MatchedKind::Punct(')') => depth -= 1,
+                MatchedKind::Punct(',') if depth == 0 => proj_arity += 1,
+                MatchedKind::Word(
+                    "from" | "where" | "group" | "having" | "order" | "limit" | "offset"
+                    | "union" | "intersect" | "except" | "on" | "returning",
+                ) if depth == 0 => break,
+                _ => {
+                    if depth == 0 && anchor.is_none() {
+                        anchor = Some(it.span);
+                    }
+                }
+            }
+        }
+        if proj_arity != col_arity {
+            emit(anchor.unwrap_or(path.items[kw_idx].span), proj_arity, &mut diagnostics);
+        }
+    }
+    diagnostics
+}
+
 /// SQL-expression predicate-warning pass (ADR-0032 §11.6 — the
 /// Phase-1 carry-over gap closure).
 ///
@@ -2236,6 +2368,10 @@ fn walk_one_command<'a>(
         // ADR-0033 §8.2 — WARNING when a SQL INSERT lists a
         // serial/shortid (auto-generated) column explicitly.
         d.extend(dml_auto_column_diagnostics(&path, ctx.schema));
+        // ADR-0033 §8.1 — ERROR when a column list's arity disagrees
+        // with a VALUES tuple (per row) or the INSERT…SELECT
+        // projection.
+        d.extend(dml_insert_arity_diagnostics(&path));
         // ADR-0032 §10.3 / §11.2 — diagnostics emitted during
         // the walk by node handlers with direct context the
         // post-walk passes can't reconstruct (primarily the
@@ -4059,6 +4195,89 @@ mod tests {
         );
     }
 
+    #[test]
+    fn insert_arity_mismatch_single_row_fires() {
+        let schema = schema_with("t", &[("a", Type::Int), ("b", Type::Int)]);
+        let diags = diag_keys("sqlinsert into t (a, b) values (1, 2, 3)", &schema);
+        assert!(
+            diags.iter().any(|d| d.contains("value(s) are given")),
+            "3 values for 2 columns should fire; got {diags:?}",
+        );
+    }
+
+    #[test]
+    fn insert_arity_match_is_silent() {
+        let schema = schema_with("t", &[("a", Type::Int), ("b", Type::Int)]);
+        let diags = diag_keys("sqlinsert into t (a, b) values (1, 2)", &schema);
+        assert!(
+            !diags.iter().any(|d| d.contains("value(s) are given")),
+            "matched arity must not fire; got {diags:?}",
+        );
+    }
+
+    #[test]
+    fn insert_arity_mismatch_emits_per_row() {
+        // ADR-0033 §8.1: each offending tuple emits its own
+        // diagnostic; the matched rows stay silent.
+        let schema = schema_with("t", &[("a", Type::Int), ("b", Type::Int)]);
+        let diags = diag_keys(
+            "sqlinsert into t (a, b) values (1, 2), (3, 4, 5), (6), (7, 8)",
+            &schema,
+        );
+        let n = diags.iter().filter(|d| d.contains("value(s) are given")).count();
+        assert_eq!(n, 2, "rows 2 and 3 mismatch, rows 1 and 4 match; got {diags:?}");
+    }
+
+    #[test]
+    fn insert_arity_with_on_conflict_flags_only_the_tuple() {
+        // Regression guard: the VALUES walk must stop at ON CONFLICT —
+        // its `(col)` conflict target is not a value tuple. A real
+        // tuple mismatch still fires; the conflict target never does.
+        let schema = schema_with("t", &[("a", Type::Int), ("b", Type::Int)]);
+        let diags = diag_keys(
+            "sqlinsert into t (a, b) values (1, 2, 3) on conflict (a) do nothing",
+            &schema,
+        );
+        let n = diags.iter().filter(|d| d.contains("value(s) are given")).count();
+        assert_eq!(n, 1, "only the 3-value tuple is flagged, not the conflict target; got {diags:?}");
+    }
+
+    #[test]
+    fn insert_arity_clean_with_on_conflict_is_silent() {
+        // Matched tuple arity + an ON CONFLICT target must be fully
+        // silent (the conflict target isn't counted as a tuple).
+        let schema = schema_with("t", &[("a", Type::Int), ("b", Type::Int)]);
+        let diags = diag_keys(
+            "sqlinsert into t (a, b) values (1, 2) on conflict (a) do update set b = excluded.b",
+            &schema,
+        );
+        assert!(
+            !diags.iter().any(|d| d.contains("value(s) are given")),
+            "no arity diagnostic expected; got {diags:?}",
+        );
+    }
+
+    #[test]
+    fn insert_select_arity_mismatch_fires() {
+        // INSERT … SELECT: 1 listed column vs a 2-item projection.
+        let schema = two_table_schema(); // a(id,name), b(id,total)
+        let diags = diag_keys("sqlinsert into a (id) select id, total from b", &schema);
+        assert!(
+            diags.iter().any(|d| d.contains("value(s) are given")),
+            "2-col projection into 1-col list should fire; got {diags:?}",
+        );
+    }
+
+    #[test]
+    fn insert_select_arity_match_is_silent() {
+        let schema = two_table_schema();
+        let diags = diag_keys("sqlinsert into a (id, name) select id, total from b", &schema);
+        assert!(
+            !diags.iter().any(|d| d.contains("value(s) are given")),
+            "matched projection arity must not fire; got {diags:?}",
+        );
+    }
+
     #[test]
     fn auto_column_overridden_fires_on_explicit_serial() {
         // ADR-0033 §8.2: listing a serial column explicitly warns.
diff --git a/src/friendly/keys.rs b/src/friendly/keys.rs
index 72203c9..0863eab 100644
--- a/src/friendly/keys.rs
+++ b/src/friendly/keys.rs
@@ -44,6 +44,7 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
     ("diagnostic.cte_arity_mismatch", &["cte", "declared", "actual"]),
     ("diagnostic.duplicate_cte", &["name"]),
     ("diagnostic.eq_null", &[]),
+    ("diagnostic.insert_arity_mismatch", &["expected", "actual"]),
     ("diagnostic.like_numeric", &["column", "type"]),
     ("diagnostic.projection_alias_misplaced", &["alias", "clause"]),
     ("diagnostic.type_mismatch", &["column", "type"]),
diff --git a/src/friendly/strings/en-US.yaml b/src/friendly/strings/en-US.yaml
index 6fb47bb..e7a0b06 100644
--- a/src/friendly/strings/en-US.yaml
+++ b/src/friendly/strings/en-US.yaml
@@ -503,6 +503,7 @@ diagnostic:
   duplicate_cte: "duplicate `WITH` table name: `{name}`"
   # 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"
 
 # Engine-error translations: an engine-rejected SQL statement
 # reaches the friendly-error layer (ADR-0019) and these keys