fix(core): improve JSON get_format_instructions using Opik Agent Optimizer (#33718)

Author: vincentkoc

libs/core/langchain_core/output_parsers/format_instructions.py

@@ -1,11 +1,16 @@
 """Format instructions."""
 
-JSON_FORMAT_INSTRUCTIONS = """The output should be formatted as a JSON instance that conforms to the JSON schema below.
+JSON_FORMAT_INSTRUCTIONS = """STRICT OUTPUT FORMAT:
+- Return only the JSON value that conforms to the schema. Do not include any additional text, explanations, headings, or separators.
+- Do not wrap the JSON in Markdown or code fences (no ``` or ```json).
+- Do not prepend or append any text (e.g., do not write "Here is the JSON:").
+- The response must be a single top-level JSON value exactly as required by the schema (object/array/etc.), with no trailing commas or comments.
 
-As an example, for the schema {{"properties": {{"foo": {{"title": "Foo", "description": "a list of strings", "type": "array", "items": {{"type": "string"}}}}}}, "required": ["foo"]}}
-the object {{"foo": ["bar", "baz"]}} is a well-formatted instance of the schema. The object {{"properties": {{"foo": ["bar", "baz"]}}}} is not well-formatted.
+The output should be formatted as a JSON instance that conforms to the JSON schema below.
 
-Here is the output schema:
+As an example, for the schema {{"properties": {{"foo": {{"title": "Foo", "description": "a list of strings", "type": "array", "items": {{"type": "string"}}}}}}, "required": ["foo"]}} the object {{"foo": ["bar", "baz"]}} is a well-formatted instance of the schema. The object {{"properties": {{"foo": ["bar", "baz"]}}}} is not well-formatted.
+
+Here is the output schema (shown in a code block for readability only — do not include any backticks or Markdown in your output):
 ```
 {schema}
 ```"""  # noqa: E501