diff --git a/changes/unreleased/undefined-operator-parse-record.performance.md b/changes/unreleased/undefined-operator-parse-record.performance.md
new file mode 100644
index 000000000..ef93698d0
--- /dev/null
+++ b/changes/unreleased/undefined-operator-parse-record.performance.md
@@ -0,0 +1 @@
+- The `~` undefined-operator warning now reads the operator sites the parser records instead of walking every node of every document, removing about 8% from load and validation time.
diff --git a/internal/check/passes/undefined_operator.go b/internal/check/passes/undefined_operator.go
index 9f04d4aa4..d43a5c992 100644
--- a/internal/check/passes/undefined_operator.go
+++ b/internal/check/passes/undefined_operator.go
@@ -19,25 +19,22 @@ type UndefinedOperatorPass struct{}
 // later-tier failure never hides the warning.
 func (UndefinedOperatorPass) Level() PassLevel { return LevelSyntax }
 
-// Run walks the parsed tree and warns at each `~` operator expression. It stays
-// a warning in every conformance mode: the specification asks for a warning,
-// not a rejection.
+// Run warns at each `~` operator expression the parser recorded on the root.
+// It stays a warning in every conformance mode: the specification asks for a
+// warning, not a rejection.
 func (UndefinedOperatorPass) Run(ctx *Context, name string, root *ast.RootNamespace) []diag.Diagnostic {
 	if root == nil {
 		return nil
 	}
-	var diags []diag.Diagnostic
-	ast.Inspect(root, func(n ast.Node) bool {
-		if e, ok := n.(*ast.OperatorExpr); ok && e.Operator == ast.OpBitNot {
-			diags = append(diags, diag.Diagnostic{
-				Severity: diag.SeverityWarning,
-				Span:     e.Span(),
-				Message:  msgUndefinedOperator,
-				Code:     codeUndefinedOperator,
-				Source:   "syntax",
-			})
-		}
-		return true
-	})
+	diags := make([]diag.Diagnostic, 0, len(root.UndefinedOperators))
+	for _, e := range root.UndefinedOperators {
+		diags = append(diags, diag.Diagnostic{
+			Severity: diag.SeverityWarning,
+			Span:     e.Span(),
+			Message:  msgUndefinedOperator,
+			Code:     codeUndefinedOperator,
+			Source:   "syntax",
+		})
+	}
 	return diags
 }
diff --git a/internal/syntax/ast/astcodec/codec.go b/internal/syntax/ast/astcodec/codec.go
index 0b6a31c76..820c06dc8 100644
--- a/internal/syntax/ast/astcodec/codec.go
+++ b/internal/syntax/ast/astcodec/codec.go
@@ -166,6 +166,13 @@ func (e *Encoder) ends(ns []*ast.ConnectorEnd) {
 	}
 }
 
+func (e *Encoder) operatorExprs(ns []*ast.OperatorExpr) {
+	e.w.Len(len(ns))
+	for _, n := range ns {
+		e.node(n)
+	}
+}
+
 func (e *Encoder) segments(segs []ast.NameSegment) {
 	e.w.Len(len(segs))
 	for _, s := range segs {
@@ -213,6 +220,7 @@ type Decoder struct {
 	nameSlices pack.Arena[*ast.QualifiedName]
 	regSlices  pack.Arena[*ast.StateRegion]
 	endSlices  pack.Arena[*ast.ConnectorEnd]
+	opSlices   pack.Arena[*ast.OperatorExpr]
 	segArena   pack.Arena[ast.NameSegment]
 	argArena   pack.Arena[ast.NamedArg]
 	paramArena pack.Arena[ast.BodyParam]
@@ -390,6 +398,14 @@ func (d *Decoder) ends() []*ast.ConnectorEnd {
 	return out
 }
 
+func (d *Decoder) operatorExprs() []*ast.OperatorExpr {
+	out := d.opSlices.Take(d.r.Len())
+	for i := range out {
+		out[i] = typed[*ast.OperatorExpr](d)
+	}
+	return out
+}
+
 func (d *Decoder) segments() []ast.NameSegment {
 	return d.segmentsN(d.r.Len())
 }
diff --git a/internal/syntax/ast/astcodec/nodes.go b/internal/syntax/ast/astcodec/nodes.go
index 60303f43a..166d09e98 100644
--- a/internal/syntax/ast/astcodec/nodes.go
+++ b/internal/syntax/ast/astcodec/nodes.go
@@ -1001,6 +1001,7 @@ func (e *Encoder) encodeFields(node ast.Node) {
 	case *ast.RootNamespace:
 		e.base(&n.NodeBase)
 		e.nodes(n.Members)
+		e.operatorExprs(n.UndefinedOperators)
 	case *ast.SelectExpr:
 		e.base(&n.NodeBase)
 		e.node(n.Operand)
@@ -1504,6 +1505,7 @@ func (d *Decoder) decodeFields(node ast.Node) {
 	case *ast.RootNamespace:
 		d.base(&n.NodeBase)
 		n.Members = d.nodes()
+		n.UndefinedOperators = d.operatorExprs()
 	case *ast.SelectExpr:
 		d.base(&n.NodeBase)
 		n.Operand = d.node()
diff --git a/internal/syntax/ast/namespace.go b/internal/syntax/ast/namespace.go
index 6c3fd8133..ed5cb6e16 100644
--- a/internal/syntax/ast/namespace.go
+++ b/internal/syntax/ast/namespace.go
@@ -249,6 +249,9 @@ type Membership struct {
 type RootNamespace struct {
 	NodeBase
 	Members []Node // *Membership | *Import | *Alias | *ErrorNode
+	// UndefinedOperators are the `~` operator expressions of the document in
+	// source order: KerML leaves DataFunctions::'~' undefined, so a tool warns at each.
+	UndefinedOperators []*OperatorExpr
 }
 
 // PrefixMetadata records a `# QualifiedName` metadata annotation reference.
diff --git a/internal/syntax/parser/expr.go b/internal/syntax/parser/expr.go
index cc789a6d3..63eabeb62 100644
--- a/internal/syntax/parser/expr.go
+++ b/internal/syntax/parser/expr.go
@@ -185,9 +185,18 @@ func (p *Parser) parseUnary() ast.Node {
 		return p.parsePrimary()
 	}
 	p.advance() // prefix operator
+	// Reserve the slot before the operand so nested `~~x` records in source order.
+	slot := -1
+	if op == ast.OpBitNot {
+		slot = len(p.undefinedOps)
+		p.undefinedOps = append(p.undefinedOps, nil)
+	}
 	operand := p.parseUnary()
 	e := &ast.OperatorExpr{Operator: op, Operands: []ast.Node{operand}}
 	e.NodeSpan = p.spanFrom(start)
+	if slot >= 0 {
+		p.undefinedOps[slot] = e
+	}
 	return e
 }
 
diff --git a/internal/syntax/parser/parser.go b/internal/syntax/parser/parser.go
index de0f6f5fe..1ea05599a 100644
--- a/internal/syntax/parser/parser.go
+++ b/internal/syntax/parser/parser.go
@@ -36,6 +36,10 @@ type Parser struct {
 	// as a declaration name.
 	Warnings []Diagnostic
 
+	// undefinedOps are the `~` operator expressions the parse built, in source
+	// order; ParseFile hands them to the root.
+	undefinedOps []*ast.OperatorExpr
+
 	// calcBodyDepth counts the calculation bodies being parsed, so a `return`
 	// reached in a statement position inside one is read as the result
 	// parameter it declares rather than as an unknown action keyword.
@@ -117,9 +121,10 @@ func (p *Parser) bodyContext() bodyContext {
 
 // parseCheckpoint captures parser state for backtracking.
 type parseCheckpoint struct {
-	pos           int
-	diagnosticLen int
-	warningLen    int
+	pos            int
+	diagnosticLen  int
+	warningLen     int
+	undefinedOpLen int
 	// triv is a copy of the pending trivia at the checkpoint; trivLogLen is
 	// how much of trivLog was already lexed then.
 	triv        []ast.Trivia
@@ -489,6 +494,7 @@ func (p *Parser) ParseFile() *ast.RootNamespace {
 		}
 	}
 	root.NodeSpan = p.spanFrom(start)
+	root.UndefinedOperators = p.undefinedOps
 	return root
 }
 
@@ -497,13 +503,14 @@ func (p *Parser) ParseFile() *ast.RootNamespace {
 func (p *Parser) checkpoint() parseCheckpoint {
 	p.checkpoints++
 	return parseCheckpoint{
-		pos:           p.pos,
-		diagnosticLen: len(p.Diagnostics),
-		warningLen:    len(p.Warnings),
-		triv:          slices.Clone(p.triv),
-		trivLogLen:    len(p.trivLog),
-		pendingSpan:   p.pendingComment,
-		hadPending:    p.hasPendingComment,
+		pos:            p.pos,
+		diagnosticLen:  len(p.Diagnostics),
+		warningLen:     len(p.Warnings),
+		undefinedOpLen: len(p.undefinedOps),
+		triv:           slices.Clone(p.triv),
+		trivLogLen:     len(p.trivLog),
+		pendingSpan:    p.pendingComment,
+		hadPending:     p.hasPendingComment,
 	}
 }
 
@@ -515,6 +522,7 @@ func (p *Parser) restore(cp parseCheckpoint) {
 	p.pos = cp.pos
 	p.Diagnostics = p.Diagnostics[:cp.diagnosticLen]
 	p.Warnings = p.Warnings[:cp.warningLen]
+	p.undefinedOps = p.undefinedOps[:cp.undefinedOpLen]
 	p.pendingComment = cp.pendingSpan
 	p.hasPendingComment = cp.hadPending
 	p.triv = append(cp.triv, p.trivLog[cp.trivLogLen:]...)
diff --git a/internal/syntax/parser/undefined_operators_test.go b/internal/syntax/parser/undefined_operators_test.go
new file mode 100644
index 000000000..3773d2ab2
--- /dev/null
+++ b/internal/syntax/parser/undefined_operators_test.go
@@ -0,0 +1,70 @@
+package parser
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/Open-MBEE/OpenSysML/internal/syntax/ast"
+	"github.com/Open-MBEE/OpenSysML/internal/syntax/source"
+)
+
+// The parser records every `~` operator expression on the root in source
+// order, wherever the expression sits.
+func TestUndefinedOperatorsRecordsEveryTilde(t *testing.T) {
+	src := "package p {\n" +
+		"attribute a = ~1;\n" +
+		"calc def C { return ~x; }\n" +
+		"attribute b = f(~y, 2);\n" +
+		"}"
+	root := New(source.New("t.sysml", []byte(src))).ParseFile()
+	ops := root.UndefinedOperators
+	if len(ops) != 3 {
+		t.Fatalf("UndefinedOperators len = %d, want 3", len(ops))
+	}
+	for i := 1; i < len(ops); i++ {
+		if ops[i].Span().Offset <= ops[i-1].Span().Offset {
+			t.Fatalf("UndefinedOperators not in source order at %d", i)
+		}
+	}
+	for _, e := range ops {
+		if e.Operator != ast.OpBitNot {
+			t.Fatalf("recorded operator = %v, want OpBitNot", e.Operator)
+		}
+	}
+}
+
+// `~~x` records both the inner and the outer `~` expression.
+func TestUndefinedOperatorsRecordsNestedTildes(t *testing.T) {
+	src := "package p { attribute a = ~~x; }"
+	root := New(source.New("t.sysml", []byte(src))).ParseFile()
+	ops := root.UndefinedOperators
+	if len(ops) != 2 {
+		t.Fatalf("UndefinedOperators len = %d, want 2 for ~~x", len(ops))
+	}
+	outer := strings.Index(src, "~~")
+	if got := []int{ops[0].Span().Offset, ops[1].Span().Offset}; got[0] != outer || got[1] != outer+1 {
+		t.Fatalf("offsets = %v, want outer %d then inner %d", got, outer, outer+1)
+	}
+}
+
+// A checkpoint restore drops the `~` sites the abandoned attempt recorded.
+func TestUndefinedOperatorsFollowsRestore(t *testing.T) {
+	p := newParser("~x + ~y")
+	if p.parseUnary(); len(p.undefinedOps) != 1 {
+		t.Fatalf("after ~x: len = %d, want 1", len(p.undefinedOps))
+	}
+	cp := p.checkpoint()
+	p.advance() // '+'
+	if p.parseUnary(); len(p.undefinedOps) != 2 {
+		t.Fatalf("after ~y: len = %d, want 2", len(p.undefinedOps))
+	}
+	p.restore(cp)
+	p.release()
+	if len(p.undefinedOps) != 1 || p.undefinedOps[0].Span().Offset != 0 {
+		t.Fatalf("after restore: %d ops, want only the ~ at offset 0", len(p.undefinedOps))
+	}
+	p.advance() // '+'
+	if p.parseUnary(); len(p.undefinedOps) != 2 || p.undefinedOps[1].Span().Offset != 5 {
+		t.Fatalf("after reparse: %d ops, want the ~ at offset 5 second", len(p.undefinedOps))
+	}
+}
diff --git a/internal/workspace/libs/stdlib.snapshot b/internal/workspace/libs/stdlib.snapshot
index 3b61e0bc3..78a089a4b 100644
Binary files a/internal/workspace/libs/stdlib.snapshot and b/internal/workspace/libs/stdlib.snapshot differ
diff --git a/tests/parser/testdata/parse/undefined-operator.golden b/tests/parser/testdata/parse/undefined-operator.golden
new file mode 100644
index 000000000..137ae91ec
--- /dev/null
+++ b/tests/parser/testdata/parse/undefined-operator.golden
@@ -0,0 +1,15 @@
+(RootNamespace
+  (Membership visibility="default"
+    (Package name="TildeDemo" library=false standard=false
+      (Membership visibility="default"
+        (Usage kind="attribute" name="magnitude" ref=false direction="none" composite=false derived=false ordered=false nonunique=false
+          (OperatorExpr operator="~"
+            (FeatureReference name="x"))))
+      (Membership visibility="default"
+        (Definition kind="calc" abstract=false variation=false name="Rough"
+          (OperatorExpr operator="+"
+            (OperatorExpr operator="~"
+              (FeatureReference name="y"))
+            (OperatorExpr operator="~"
+              (OperatorExpr operator="~"
+                (FeatureReference name="z")))))))))
\ No newline at end of file
diff --git a/tests/parser/testdata/parse/undefined-operator.sysml b/tests/parser/testdata/parse/undefined-operator.sysml
new file mode 100644
index 000000000..525666210
--- /dev/null
+++ b/tests/parser/testdata/parse/undefined-operator.sysml
@@ -0,0 +1,4 @@
+package TildeDemo {
+	attribute magnitude = ~x;
+	calc def Rough { ~y + ~~z }
+}
diff --git a/tests/parser/undefined_operators_test.go b/tests/parser/undefined_operators_test.go
new file mode 100644
index 000000000..dc93fce76
--- /dev/null
+++ b/tests/parser/undefined_operators_test.go
@@ -0,0 +1,58 @@
+package parser_test
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/Open-MBEE/OpenSysML/internal/syntax/ast"
+	"github.com/Open-MBEE/OpenSysML/internal/syntax/parser"
+	"github.com/Open-MBEE/OpenSysML/internal/syntax/source"
+)
+
+// TestUndefinedOperatorsCoversFixtures checks the parser's `~` record against
+// an independent walk of every parse fixture.
+func TestUndefinedOperatorsCoversFixtures(t *testing.T) {
+	fixtures := filepath.Join("testdata", "parse")
+	entries, err := os.ReadDir(fixtures)
+	if err != nil {
+		t.Fatalf("Failed to read fixtures dir %s: %v", fixtures, err)
+	}
+
+	counted := 0
+	for _, entry := range entries {
+		ext := filepath.Ext(entry.Name())
+		if entry.IsDir() || (ext != ".sysml" && ext != ".kerml") {
+			continue
+		}
+		name := entry.Name()
+		t.Run(name, func(t *testing.T) {
+			content, err := os.ReadFile(filepath.Join(fixtures, name))
+			if err != nil {
+				t.Fatalf("Failed to read fixture %s: %v", name, err)
+			}
+			sf := source.New(name, content)
+			root := parser.New(sf).ParseFile()
+
+			want := 0
+			ast.Inspect(root, func(n ast.Node) bool {
+				if e, ok := n.(*ast.OperatorExpr); ok && e.Operator == ast.OpBitNot {
+					want++
+				}
+				return true
+			})
+			if len(root.UndefinedOperators) != want {
+				t.Errorf("UndefinedOperators len = %d, tree walk counts %d", len(root.UndefinedOperators), want)
+			}
+			for _, e := range root.UndefinedOperators {
+				if e == nil || e.Operator != ast.OpBitNot {
+					t.Errorf("recorded operator is not a `~` expression: %v", e)
+				}
+			}
+			counted += want
+		})
+	}
+	if counted == 0 {
+		t.Error("no fixture exercises a `~` operator; the comparison is vacuous")
+	}
+}