diff --git a/CHANGELOG.md b/CHANGELOG.md
index 468c1e9d..d9a18c44 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,11 @@
 
 All notable changes to `mcp/sdk` will be documented in this file.
 
+0.9.1
+-----
+
+* Fix tools with an array shape parameter whose keys are nullable or unions (e.g. `array{name: string, nickname: string|null}`) rejecting valid arguments with `-32602`: a `null` inside the shape no longer turns the parameter into `array|null`, and such keys get their own types instead of `object`.
+
 0.9.0
 -----
 
diff --git a/src/Capability/Discovery/SchemaGenerator.php b/src/Capability/Discovery/SchemaGenerator.php
index 949ed013..e9f02b36 100644
--- a/src/Capability/Discovery/SchemaGenerator.php
+++ b/src/Capability/Discovery/SchemaGenerator.php
@@ -556,7 +556,7 @@ private function parseParametersInfo(\ReflectionMethod|\ReflectionFunction $refl
                 $allowsNull = true;
             } elseif ($hasDefault && null === $defaultValue) {
                 $allowsNull = true;
-            } elseif (str_contains($typeString, 'null') || 'mixed' === strtolower($typeString)) {
+            } elseif (self::hasNullBranch($typeString) || 'mixed' === strtolower($typeString)) {
                 $allowsNull = true;
             }
 
@@ -609,7 +609,7 @@ private function getParameterTypeString(\ReflectionParameter $rp, ?Param $paramT
         // Otherwise, use the DocBlock type if it was valid and non-generic
         if (null !== $docBlockType && !$isDocBlockTypeGeneric) {
             // Consider if DocBlock adds nullability missing from reflection
-            if (false !== stripos($docBlockType, 'null') && $reflectionTypeString && false === stripos($reflectionTypeString, 'null') && !str_ends_with($reflectionTypeString, '|null')) {
+            if (self::hasNullBranch($docBlockType) && $reflectionTypeString && false === stripos($reflectionTypeString, 'null') && !str_ends_with($reflectionTypeString, '|null')) {
                 // If reflection didn't capture null, but docblock did, append |null (if not already mixed)
                 if ('mixed' !== $reflectionTypeString) {
                     return $reflectionTypeString.'|null';
@@ -733,6 +733,17 @@ public static function splitUnion(string $type): array
         return array_values(array_filter($branches, static fn (string $branch): bool => '' !== $branch));
     }
 
+    /**
+     * Whether `null` is one of the top-level alternatives of a type, as in
+     * `string|null`. A `null` inside a generic or a shape, as in
+     * `array{name: string|null}`, belongs to that inner type and does not
+     * make the whole type nullable.
+     */
+    private static function hasNullBranch(string $type): bool
+    {
+        return \in_array('null', array_map(strtolower(...), self::splitUnion($type)), true);
+    }
+
     /**
      * Maps a PHP type string (potentially a union) to an array of JSON Schema type names.
      *
@@ -936,9 +947,16 @@ private function parsePropertyDefinition(string $propDefinition, array &$propert
                     ],
                 ];
             }
-            // Simple type
+            // Simple type, or a union of them such as `string|null` or `?string`
             else {
-                $properties[$propName] = ['type' => $this->mapSimpleTypeToJsonSchema($propType)];
+                if (str_starts_with($propType, '?')) {
+                    $propType = 'null|'.substr($propType, 1);
+                }
+                $types = array_values(array_unique(array_map(
+                    fn (string $branch): string => 'null' === strtolower($branch) ? 'null' : $this->mapSimpleTypeToJsonSchema($branch),
+                    self::splitUnion($propType),
+                )));
+                $properties[$propName] = ['type' => 1 === \count($types) ? $types[0] : $types];
             }
         }
     }
diff --git a/tests/Unit/Capability/Discovery/SchemaGeneratorFixture.php b/tests/Unit/Capability/Discovery/SchemaGeneratorFixture.php
index 613369b2..971d6870 100644
--- a/tests/Unit/Capability/Discovery/SchemaGeneratorFixture.php
+++ b/tests/Unit/Capability/Discovery/SchemaGeneratorFixture.php
@@ -319,6 +319,22 @@ public function nullableTypedArrays(
     ): void {
     }
 
+    /**
+     * A `null` or `|` inside an array shape belongs to one of its keys.
+     *
+     * @param array{name: string, nickname: string|null} $nullUnionMember Shape with a nullable key
+     * @param array{name: string, nickname: ?string}     $nullableMember  Shape with a nullable key
+     * @param array{id: int|string}                      $unionMember     Shape with a union key
+     * @param array{id: int}|null                        $nullableShape   Nullable shape
+     */
+    public function arrayShapesWithUnionMembers(
+        array $nullUnionMember,
+        array $nullableMember,
+        array $unionMember,
+        $nullableShape,
+    ): void {
+    }
+
     // ===== NULLABLE AND OPTIONAL SCENARIOS =====
 
     /**
diff --git a/tests/Unit/Capability/Discovery/SchemaGeneratorTest.php b/tests/Unit/Capability/Discovery/SchemaGeneratorTest.php
index 32817f64..ffd1257d 100644
--- a/tests/Unit/Capability/Discovery/SchemaGeneratorTest.php
+++ b/tests/Unit/Capability/Discovery/SchemaGeneratorTest.php
@@ -293,6 +293,39 @@ public function testRecoversItemsTypeForNullableTypedArrays(): void
         $this->assertEqualsCanonicalizing(['nullableStrings'], $schema['required']);
     }
 
+    public function testKeepsNullableAndUnionKeysOfArrayShapesInsideTheShape(): void
+    {
+        $method = new \ReflectionMethod(SchemaGeneratorFixture::class, 'arrayShapesWithUnionMembers');
+        $schema = $this->schemaGenerator->generate($method);
+        // The `null` of a key must neither make the parameter nullable nor turn it back into a plain array.
+        $this->assertEquals([
+            'type' => 'object',
+            'description' => 'Shape with a nullable key',
+            'properties' => ['name' => ['type' => 'string'], 'nickname' => ['type' => ['string', 'null']]],
+            'required' => ['name', 'nickname'],
+        ], $schema['properties']['nullUnionMember']);
+        $this->assertEquals([
+            'type' => 'object',
+            'description' => 'Shape with a nullable key',
+            'properties' => ['name' => ['type' => 'string'], 'nickname' => ['type' => ['null', 'string']]],
+            'required' => ['name', 'nickname'],
+        ], $schema['properties']['nullableMember']);
+        $this->assertEquals([
+            'type' => 'object',
+            'description' => 'Shape with a union key',
+            'properties' => ['id' => ['type' => ['integer', 'string']]],
+            'required' => ['id'],
+        ], $schema['properties']['unionMember']);
+        // A top-level `|null` still makes the shape nullable.
+        $this->assertEquals([
+            'type' => ['object', 'null'],
+            'description' => 'Nullable shape',
+            'properties' => ['id' => ['type' => 'integer']],
+            'required' => ['id'],
+        ], $schema['properties']['nullableShape']);
+        $this->assertEqualsCanonicalizing(['nullUnionMember', 'nullableMember', 'unionMember', 'nullableShape'], $schema['required']);
+    }
+
     public function testHandlesNullableTypeHintsAndOptionalParameters(): void
     {
         $method = new \ReflectionMethod(SchemaGeneratorFixture::class, 'nullableAndOptional');