Preserve empty JSON schema object maps

src/Providers/OpenAiCompatibleImplementation/AbstractOpenAiCompatibleTextGenerationModel.php

@@ -492,9 +492,15 @@ protected function prepareToolsParam(array $functionDeclarations): array
     {
         $tools = [];
         foreach ($functionDeclarations as $functionDeclaration) {
+            $function = $functionDeclaration->toArray();
+            $parameters = $functionDeclaration->getJsonSerializableParameters();
+            if ($parameters !== null) {
+                $function[FunctionDeclaration::KEY_PARAMETERS] = $parameters;
+            }
+
             $tools[] = [
                 'type' => 'function',
-                'function' => $functionDeclaration->toArray(),
+                'function' => $function,
             ];
         }
 

src/Tools/DTO/FunctionDeclaration.php

@@ -94,6 +94,28 @@ public function getParameters(): ?array
         return $this->parameters;
     }
 
+    /**
+     * Gets the function parameters schema in a JSON-serializable form.
+     *
+     * JSON Schema object-map fields such as properties must encode as JSON
+     * objects even when empty. PHP arrays cannot preserve that distinction
+     * without casting the empty map before serialization.
+     *
+     * @since 0.1.0
+     *
+     * @return array<string, mixed>|\stdClass|null The JSON-serializable parameters schema.
+     */
+    public function getJsonSerializableParameters()
+    {
+        if ($this->parameters === null) {
+            return null;
+        }
+
+        /** @var array<string, mixed>|\stdClass $parameters */
+        $parameters = $this->prepareJsonSchemaObjectMaps($this->parameters, self::KEY_PARAMETERS);
+        return $parameters;
+    }
+
     /**
      * {@inheritDoc}
      *
@@ -143,6 +165,69 @@ public function toArray(): array
         return $data;
     }
 
+    /**
+     * {@inheritDoc}
+     *
+     * @since 0.1.0
+     */
+    #[\ReturnTypeWillChange]
+    public function jsonSerialize()
+    {
+        $data = $this->toArray();
+
+        if ($this->parameters !== null) {
+            $data[self::KEY_PARAMETERS] = $this->getJsonSerializableParameters();
+        }
+
+        return $data;
+    }
+
+    /**
+     * Recursively prepares JSON Schema object-map fields for JSON serialization.
+     *
+     * @since 0.1.0
+     *
+     * @param mixed $value The value to prepare.
+     * @param string|null $key The current JSON Schema key, if available.
+     * @return mixed The prepared value.
+     */
+    private function prepareJsonSchemaObjectMaps($value, ?string $key = null)
+    {
+        if (!is_array($value)) {
+            return $value;
+        }
+
+        if ($value === [] && $this->isJsonSchemaObjectMapKey($key)) {
+            return new \stdClass();
+        }
+
+        foreach ($value as $childKey => $childValue) {
+            $value[$childKey] = $this->prepareJsonSchemaObjectMaps(
+                $childValue,
+                is_string($childKey) ? $childKey : null
+            );
+        }
+
+        return $value;
+    }
+
+    /**
+     * Checks whether the given JSON Schema key represents an object map.
+     *
+     * @since 0.1.0
+     *
+     * @param string|null $key The JSON Schema key.
+     * @return bool True if the key represents an object map, false otherwise.
+     */
+    private function isJsonSchemaObjectMapKey(?string $key): bool
+    {
+        return in_array(
+            $key,
+            [self::KEY_PARAMETERS, 'properties', 'patternProperties', '$defs', 'definitions', 'dependentSchemas'],
+            true
+        );
+    }
+
     /**
      * {@inheritDoc}
      *

tests/unit/Providers/OpenAiCompatibleImplementation/AbstractOpenAiCompatibleTextGenerationModelTest.php

@@ -926,6 +926,35 @@ public function testPrepareToolsParam(): void
         $this->assertEquals($functionDeclaration2->toArray(), $prepared[1]['function']);
     }
 
+    /**
+     * Tests prepareToolsParam() preserves empty JSON Schema object maps.
+     *
+     * @return void
+     */
+    public function testPrepareToolsParamPreservesEmptySchemaObjectMaps(): void
+    {
+        $functionDeclaration = new FunctionDeclaration(
+            'inspect_object',
+            'Inspects an object with optional nested metadata',
+            [
+                'type' => 'object',
+                'properties' => [
+                    'metadata' => [
+                        'type' => 'object',
+                        'properties' => [],
+                    ],
+                ],
+            ]
+        );
+        $model = $this->createModel();
+
+        $prepared = $model->exposePrepareToolsParam([$functionDeclaration]);
+        $json = json_encode($prepared, JSON_THROW_ON_ERROR);
+
+        $this->assertStringContainsString('"properties":{', $json);
+        $this->assertStringNotContainsString('"properties":[]', $json);
+    }
+
     /**
      * Tests prepareResponseFormatParam() with null schema.
      *

tests/unit/Tools/DTO/FunctionDeclarationTest.php

@@ -55,6 +55,7 @@ public function testCreateWithoutParameters(): void
         $this->assertEquals($name, $declaration->getName());
         $this->assertEquals($description, $declaration->getDescription());
         $this->assertNull($declaration->getParameters());
+        $this->assertNull($declaration->getJsonSerializableParameters());
     }
 
     /**
@@ -206,6 +207,33 @@ public function testToArrayWithParameters(): void
         );
     }
 
+    /**
+     * Tests JSON serialization with empty JSON Schema object maps.
+     *
+     * @return void
+     */
+    public function testJsonSerializationPreservesEmptySchemaObjectMaps(): void
+    {
+        $declaration = new FunctionDeclaration(
+            'inspectObject',
+            'Inspects an object with optional nested metadata',
+            [
+                'type' => 'object',
+                'properties' => [
+                    'metadata' => [
+                        'type' => 'object',
+                        'properties' => [],
+                    ],
+                ],
+            ]
+        );
+
+        $json = json_encode($declaration, JSON_THROW_ON_ERROR);
+
+        $this->assertStringContainsString('"properties":{', $json);
+        $this->assertStringNotContainsString('"properties":[]', $json);
+    }
+
     /**
      * Tests array transformation without parameters.
      *