diff --git a/NOW.md b/NOW.md
index e71b104fb5..bee96426c3 100644
--- a/NOW.md
+++ b/NOW.md
@@ -2,6 +2,13 @@
 
 Last updated: 2026-08-08
 
+## typecheck: promote call-arity mismatch from warning to hard error (Closes #1921)
+
+- The arity check existed but only warned, and nothing reads warnings (tri-net's hook greps 'Typecheck OK'; gen paths skip typecheck) -- two tri-net specs shipped wrong-arity calls for months (tri-net#323)
+- Now: error_count += 1, ok = false -- the specs-typecheck gate actually blocks the class
+- Sweep: zero arity violations remain across tri-net's 100+ specs post-#323, so the promotion breaks nothing (27 specs have PRE-EXISTING unrelated typecheck errors, identical under stock t27c)
+- Unit suite 1537/1537; FROZEN_HASH resealed
+
 ## fix(gen-c): [T;N] params, test-block bindings, t27_assert macro (Refs #1919)
 
 - Rust-style `[T; N]` parameter types now lower to `T*` (previously emitted a bare `* name` -- no base type)
diff --git a/bootstrap/src/compiler.rs b/bootstrap/src/compiler.rs
index 6f1cee6cea..b7da330eeb 100644
--- a/bootstrap/src/compiler.rs
+++ b/bootstrap/src/compiler.rs
@@ -12713,7 +12713,12 @@ fn check_expr(node: &Node, symbols: &[SymbolEntry], fns: &[FnEntry], result: &mu
                     .filter(|c| c.kind != NodeKind::Module)
                     .collect();
                 if call_args.len() != fn_entry.params.len() {
-                    result.warnings += 1;
+                    // Arity mismatch is a HARD error: two tri-net specs shipped
+                    // mismatched calls for months because this only warned and
+                    // nothing reads warnings (tri-net#323). Wrong-arity calls
+                    // mangle in every backend.
+                    result.error_count += 1;
+                    result.ok = false;
                     result.errors.push(format!(
                         "function '{}' expects {} args, got {} at line {}",
                         node.name,
diff --git a/bootstrap/stage0/FROZEN_HASH b/bootstrap/stage0/FROZEN_HASH
index 8d524157af..4cef74bcaf 100644
--- a/bootstrap/stage0/FROZEN_HASH
+++ b/bootstrap/stage0/FROZEN_HASH
@@ -1 +1 @@
-409ab06144271d348b7ffdb0332a032c13928984f21c8fc0f838df5ecd6caa28
+8770de8b22dfda24d5c4099b2610c4a5a6efd49e43a19e335f08eeb09f788743
diff --git a/docs/NOW.md b/docs/NOW.md
index c7fec87592..d55e1f0b5c 100644
--- a/docs/NOW.md
+++ b/docs/NOW.md
@@ -1,7 +1,14 @@
-# NOW — fix(gen-c): params, test bindings, assert macro (2026-08-08)
+# NOW — typecheck: arity mismatch is a hard error (2026-08-08)
 
 Last updated: 2026-08-08
 
+## typecheck: promote call-arity mismatch from warning to hard error (Closes #1921)
+
+- The arity check existed but only warned, and nothing reads warnings (tri-net's hook greps 'Typecheck OK'; gen paths skip typecheck) -- two tri-net specs shipped wrong-arity calls for months (tri-net#323)
+- Now: error_count += 1, ok = false -- the specs-typecheck gate actually blocks the class
+- Sweep: zero arity violations remain across tri-net's 100+ specs post-#323, so the promotion breaks nothing (27 specs have PRE-EXISTING unrelated typecheck errors, identical under stock t27c)
+- Unit suite 1537/1537; FROZEN_HASH resealed
+
 ## fix(gen-c): [T;N] params, test-block bindings, t27_assert macro (Refs #1919)
 
 - Rust-style `[T; N]` parameter types now lower to `T*` (previously emitted a bare `* name` -- no base type)