diff --git a/ql/src/queries/style/ToStringInQueryLogic.ql b/ql/src/queries/style/ToStringInQueryLogic.ql
new file mode 100644
index 00000000000..93a4b816475
--- /dev/null
+++ b/ql/src/queries/style/ToStringInQueryLogic.ql
@@ -0,0 +1,344 @@
+/**
+ * @name Using 'toString' in query logic
+ * @description A query should not depend on the output of 'toString'.
+ * @kind problem
+ * @problem.severity error
+ * @id ql/to-string-in-logic
+ * @precision medium
+ * @tags maintainability
+ */
+
+import ql
+import codeql_ql.ast.internal.Builtins
+
+class ToString extends Predicate {
+  ToString() { this.getName() = "toString" }
+}
+
+class ToStringCall extends Call {
+  ToStringCall() { this.getTarget() instanceof ToString }
+}
+
+class NodesPredicate extends Predicate {
+  NodesPredicate() { this.getName() = "nodes" }
+}
+
+class EdgesPredicate extends Predicate {
+  EdgesPredicate() { this.getName() = "edges" }
+}
+
+class RegexpReplaceAll extends BuiltinPredicate {
+  RegexpReplaceAll() { this.getName() = "regexpReplaceAll" }
+}
+
+class RegexpReplaceAllCall extends MemberCall {
+  RegexpReplaceAllCall() { this.getTarget() instanceof RegexpReplaceAll }
+}
+
+module DataFlow {
+  private newtype TNode =
+    TExprNode(Expr e) or
+    TOutParameterNode(Parameter p) or
+    TArgumentOutNode(VarAccess acc, Call call, int pos) {
+      acc.(Argument).getCall() = call and acc.(Argument).getPosition() = pos
+    } or
+    TParameterNode(Parameter p)
+
+  /** An argument to a call. */
+  class Argument extends Expr {
+    Call call;
+    int pos;
+
+    Argument() { call.getArgument(pos) = this }
+
+    /** Gets the call that has this argument. */
+    Call getCall() { result = call }
+
+    /** Gets the position of this argument. */
+    int getPosition() { result = pos }
+  }
+
+  private newtype TParameter =
+    TThisParameter(ClassPredicate p) or
+    TResultParameter(Predicate p) { exists(p.getReturnType()) } or
+    TVariableParameter(VarDecl v, int i, Predicate pred) { pred.getParameter(i) = v }
+
+  abstract class Parameter extends TParameter {
+    string toString() { this.hasName(result) }
+
+    abstract predicate hasName(string name);
+
+    abstract int getIndex();
+
+    abstract Predicate getPredicate();
+  }
+
+  class ThisParameter extends Parameter, TThisParameter {
+    ClassPredicate p;
+
+    ThisParameter() { this = TThisParameter(p) }
+
+    override predicate hasName(string name) { name = "this" }
+
+    override int getIndex() { result = -1 }
+
+    override Predicate getPredicate() { result = p }
+  }
+
+  class ResultParameter extends Parameter, TResultParameter {
+    Predicate p;
+
+    ResultParameter() { this = TResultParameter(p) }
+
+    override predicate hasName(string name) { name = "result" }
+
+    override int getIndex() { result = -2 }
+
+    override Predicate getPredicate() { result = p }
+  }
+
+  class VariableParameter extends Parameter, TVariableParameter {
+    VarDecl v;
+    int i;
+    Predicate p;
+
+    VariableParameter() { this = TVariableParameter(v, i, p) }
+
+    override predicate hasName(string name) { name = v.getName() }
+
+    override int getIndex() { result = i }
+
+    override Predicate getPredicate() { result = p }
+  }
+
+  class Node extends TNode {
+    /** Gets the predicate to which this node belongs. */
+    Predicate getPredicate() { none() } // overridden in subclasses
+
+    Expr asExpr() { result = this.(ExprNode).getExpr() }
+
+    /** Gets a textual representation of this element. */
+    string toString() { none() } // overridden by subclasses
+  }
+
+  /**
+   * An expression, viewed as a node in a data flow graph.
+   */
+  class ExprNode extends Node, TExprNode {
+    Expr expr;
+
+    ExprNode() { this = TExprNode(expr) }
+
+    override string toString() { result = expr.toString() }
+
+    /** Gets the expression corresponding to this node. */
+    Expr getExpr() { result = expr }
+  }
+
+  class ParameterNode extends Node, TParameterNode {
+    Parameter p;
+
+    ParameterNode() { this = TParameterNode(p) }
+
+    override string toString() { result = p.toString() }
+  }
+
+  newtype TReturnKind =
+    TNormalReturnKind() or
+    TParameterOutKind(int i) { any(Parameter p).getIndex() = i }
+
+  /** A data flow node that represents the output of a call at the call site. */
+  abstract class OutNode extends Node {
+    /** Gets the underlying call. */
+    abstract Call getCall();
+  }
+
+  class ArgumentOutNode extends Node, TArgumentOutNode, OutNode {
+    VarAccess acc;
+    Call call;
+    int pos;
+
+    ArgumentOutNode() { this = TArgumentOutNode(acc, call, pos) }
+
+    VarAccess getVarAccess() { result = acc }
+
+    override string toString() { result = acc.toString() + " [out]" }
+
+    override Call getCall() { result = call }
+
+    int getIndex() { result = pos }
+  }
+
+  class OutParameterNode extends Node, TOutParameterNode {
+    Parameter p;
+
+    OutParameterNode() { this = TOutParameterNode(p) }
+
+    Parameter getParameter() { result = p }
+
+    override string toString() { result = p.toString() }
+  }
+
+  AstNode getParentOfExpr(Expr e) { result = e.getParent() }
+
+  Formula getEnclosing(Expr e) { result = getParentOfExpr+(e) }
+
+  Formula enlargeScopeStep(Formula f) { result.(Conjunction).getAnOperand() = f }
+
+  Formula enlargeScope(Formula f) {
+    result = enlargeScopeStep*(f) and not exists(enlargeScopeStep(result))
+  }
+
+  predicate varaccesValue(VarAccess va, VarDecl v, Formula scope) {
+    va.getDeclaration() = v and
+    scope = enlargeScope(getEnclosing(va))
+  }
+
+  predicate thisValue(ThisAccess ta, Formula scope) { scope = enlargeScope(getEnclosing(ta)) }
+
+  predicate resultValue(ResultAccess ra, Formula scope) { scope = enlargeScope(getEnclosing(ra)) }
+
+  Formula getParentFormula(Formula f) { f.getParent() = result }
+
+  predicate valueStep(Expr e1, Expr e2) {
+    exists(VarDecl v, Formula scope |
+      varaccesValue(e1, v, scope) and
+      varaccesValue(e2, v, scope)
+    )
+    or
+    exists(VarDecl v, Formula f, Select sel |
+      getParentFormula*(f) = sel.getWhere() and
+      varaccesValue(e1, v, f) and
+      sel.getExpr(_) = e2
+    )
+    or
+    exists(Formula scope |
+      thisValue(e1, scope) and
+      thisValue(e2, scope)
+      or
+      resultValue(e1, scope) and
+      resultValue(e2, scope)
+    )
+    or
+    exists(InlineCast c |
+      e1 = c and e2 = c.getBase()
+      or
+      e2 = c and e1 = c.getBase()
+    )
+    or
+    exists(ComparisonFormula eq |
+      eq.getSymbol() = "=" and
+      eq.getAnOperand() = e1 and
+      eq.getAnOperand() = e2 and
+      e1 != e2
+    )
+  }
+
+  predicate paramStep(Expr e1, Parameter p2) {
+    exists(VarDecl v |
+      p2 = TVariableParameter(v, _, _) and
+      varaccesValue(e1, v, _)
+    )
+    or
+    exists(Formula scope |
+      p2 = TThisParameter(scope.getEnclosingPredicate()) and
+      thisValue(e1, scope)
+      or
+      p2 = TResultParameter(scope.getEnclosingPredicate()) and
+      resultValue(e1, scope)
+    )
+  }
+
+  predicate additionalLocalStep(Node nodeFrom, Node nodeTo) {
+    nodeFrom.asExpr().getType() instanceof StringClass and
+    nodeFrom.asExpr().getType() instanceof StringClass and
+    exists(BinOpExpr binop |
+      nodeFrom.asExpr() = binop.getAnOperand() and
+      nodeTo.asExpr() = binop
+    )
+    or
+    nodeTo.asExpr().(RegexpReplaceAllCall).getBase() = nodeFrom.asExpr()
+  }
+
+  predicate localStep(Node nodeFrom, Node nodeTo) {
+    valueStep(nodeFrom.asExpr(), nodeTo.asExpr())
+    or
+    paramStep(nodeFrom.asExpr(), nodeTo.(OutParameterNode).getParameter())
+    or
+    valueStep(nodeFrom.(ArgumentOutNode).getVarAccess(), nodeTo.asExpr())
+    or
+    additionalLocalStep(nodeFrom, nodeTo)
+  }
+
+  predicate step(Node nodeFrom, Node nodeTo) {
+    // Local flow
+    localStep(nodeFrom, nodeTo)
+    or
+    // Flow out of functions
+    exists(Call call, Parameter p, OutParameterNode outParam, ArgumentOutNode outArg |
+      outParam = nodeFrom and
+      outArg = nodeTo
+    |
+      p = outParam.getParameter() and
+      p.getPredicate() = call.getTarget() and
+      outArg.getCall() = call and
+      outArg.getIndex() = p.getIndex()
+    )
+  }
+
+  predicate flowsFromSource(Node node) {
+    isSource(node.asExpr())
+    or
+    exists(Node mid | flowsFromSource(mid) | step(mid, node))
+  }
+
+  predicate flowsToSink(Node node) {
+    flowsFromSource(node) and isSink(node)
+    or
+    exists(Node mid | flowsToSink(mid) | step(node, mid))
+  }
+
+  predicate isSink(Node sink) {
+    sink.asExpr() = any(Select s).getExpr(_)
+    or
+    sink.getPredicate() instanceof NodesPredicate
+    or
+    sink.getPredicate() instanceof EdgesPredicate
+  }
+
+  predicate isSource(ToStringCall toString) {
+    not toString.getEnclosingPredicate() instanceof ToString and
+    not toString.getEnclosingPredicate() instanceof NodesPredicate and
+    not toString.getEnclosingPredicate() instanceof EdgesPredicate
+  }
+}
+
+predicate flowsToSelect(Expr e) {
+  exists(DataFlow::Node source |
+    source.asExpr() = e and
+    DataFlow::flowsToSink(source)
+  )
+}
+
+from ToStringCall call
+where
+  // It's not part of a toString call
+  DataFlow::isSource(call) and
+  // The call doesn't flow to a select
+  not flowsToSelect(call) and
+  // It's in a query
+  call.getLocation().getFile().getBaseName().matches("%.ql") and
+  // ... and not in a test
+  not (
+    call.getLocation()
+        .getFile()
+        .getAbsolutePath()
+        .toLowerCase()
+        .matches(["%test%", "%consistency%", "%meta%"])
+    or
+    call.getLocation()
+        .getFile()
+        .getAbsolutePath()
+        .regexpMatch(".*/(test|examples|ql-training|recorded-call-graph-metrics)/.*")
+  )
+select call, "Query logic depends on implementation of 'toString'."