diff --git a/src/Psalm/Internal/Analyzer/Statements/Expression/BinaryOpAnalyzer.php b/src/Psalm/Internal/Analyzer/Statements/Expression/BinaryOpAnalyzer.php
index a65c194f6..15b4199f5 100644
--- a/src/Psalm/Internal/Analyzer/Statements/Expression/BinaryOpAnalyzer.php
+++ b/src/Psalm/Internal/Analyzer/Statements/Expression/BinaryOpAnalyzer.php
@@ -12,6 +12,7 @@ use Psalm\Config;
 use Psalm\Context;
 use Psalm\Issue\FalseOperand;
 use Psalm\Issue\ImplicitToStringCast;
+use Psalm\Issue\ImpureMethodCall;
 use Psalm\Issue\InvalidOperand;
 use Psalm\Issue\MixedOperand;
 use Psalm\Issue\NullOperand;
@@ -925,6 +926,70 @@ class BinaryOpAnalyzer
             $statements_analyzer->node_data->setType($stmt, Type::getBool());
         }
 
+        if ($stmt instanceof PhpParser\Node\Expr\BinaryOp\Equal) {
+            if ($stmt_left_type->hasString() && $stmt_right_type->hasObjectType()) {
+                foreach ($stmt_right_type->getAtomicTypes() as $atomic_type) {
+                    if ($atomic_type instanceof TNamedObject) {
+                        try {
+                            $storage = $codebase->methods->getStorage(
+                                new \Psalm\Internal\MethodIdentifier(
+                                    $atomic_type->value,
+                                    '__tostring'
+                                )
+                            );
+                        } catch (\UnexpectedValueException $e) {
+                            continue;
+                        }
+
+                        if ($context->mutation_free
+                            && !$storage->mutation_free
+                        ) {
+                            if (IssueBuffer::accepts(
+                                new ImpureMethodCall(
+                                    'Cannot call an mutation-free method '
+                                        . $atomic_type->value . '::__toString from a pure context',
+                                    new CodeLocation($source, $stmt)
+                                ),
+                                $statements_analyzer->getSuppressedIssues()
+                            )) {
+                                // fall through
+                            }
+                        }
+                    }
+                }
+            } elseif ($stmt_right_type->hasString() && $stmt_left_type->hasObjectType()) {
+                foreach ($stmt_left_type->getAtomicTypes() as $atomic_type) {
+                    if ($atomic_type instanceof TNamedObject) {
+                        try {
+                            $storage = $codebase->methods->getStorage(
+                                new \Psalm\Internal\MethodIdentifier(
+                                    $atomic_type->value,
+                                    '__tostring'
+                                )
+                            );
+                        } catch (\UnexpectedValueException $e) {
+                            continue;
+                        }
+
+                        if ($context->mutation_free
+                            && !$storage->mutation_free
+                        ) {
+                            if (IssueBuffer::accepts(
+                                new ImpureMethodCall(
+                                    'Cannot call an mutation-free method '
+                                        . $atomic_type->value . '::__toString from a pure context',
+                                    new CodeLocation($statements_analyzer, $stmt)
+                                ),
+                                $statements_analyzer->getSuppressedIssues()
+                            )) {
+                                // fall through
+                            }
+                        }
+                    }
+                }
+            }
+        }
+
         if ($stmt instanceof PhpParser\Node\Expr\BinaryOp\Spaceship) {
             $statements_analyzer->node_data->setType($stmt, Type::getInt());
         }
diff --git a/tests/PureAnnotationTest.php b/tests/PureAnnotationTest.php
index 87056fc12..03539dead 100644
--- a/tests/PureAnnotationTest.php
+++ b/tests/PureAnnotationTest.php
@@ -182,6 +182,23 @@ class PureAnnotationTest extends TestCase
                         return $ar[0] ?? 0;
                     }',
             ],
+            'allowPureToString' => [
+                '<?php
+                    class A {
+                        /** @psalm-pure */
+                        public function __toString() {
+                            return "bar";
+                        }
+                    }
+
+                    /**
+                     * @psalm-pure
+                     */
+                    function foo(string $s, A $a) : string {
+                        if ($a == $s) {}
+                        return $s;
+                    }',
+            ],
         ];
     }
 
@@ -383,6 +400,24 @@ class PureAnnotationTest extends TestCase
                     }',
                 'error_message' => 'ImpureStaticProperty',
             ],
+            'preventImpureToString' => [
+                '<?php
+                    class A {
+                        public function __toString() {
+                            echo "hi";
+                            return "bar";
+                        }
+                    }
+
+                    /**
+                     * @psalm-pure
+                     */
+                    function foo(string $s, A $a) : string {
+                        if ($a == $s) {}
+                        return $s;
+                    }',
+                'error_message' => 'ImpureMethodCall'
+            ],
         ];
     }
 }