---
title: json_schema
sidebarTitle: json_schema
---
# `fastmcp.utilities.json_schema`
## Functions
### `require_discriminator_property`
```python
require_discriminator_property(schema: dict[str, Any]) -> dict[str, Any]
```
Keep an OpenAPI discriminator's tag mandatory after the keyword is dropped.
Returns a copy of *schema* with ``discriminator.propertyName`` added to each
``anyOf``/``oneOf`` variant's ``required`` list. A Pydantic discriminated
union whose tag has a default omits that tag from ``required``; without this,
an untagged payload passes the generated schema but fails later in the source
model with ``union_tag_not_found``. No-op if there is no string
``propertyName``.
### `dereference_refs`
```python
dereference_refs(schema: dict[str, Any]) -> dict[str, Any]
```
Resolve all $ref references in a JSON schema by inlining definitions.
This function resolves $ref references that point to $defs, replacing them
with the actual definition content while preserving sibling keywords (like
description, default, examples) that Pydantic places alongside $ref.
This is necessary because some MCP clients (e.g., VS Code Copilot) don't
properly handle $ref in tool input schemas.
For self-referencing/circular schemas where full dereferencing is not possible,
this function falls back to resolving only the root-level $ref while preserving
$defs for nested references.
Only local ``$ref`` values (those starting with ``#``) are resolved.
Remote URIs (``http://``, ``file://``, etc.) are stripped before
resolution to prevent SSRF / local-file-inclusion attacks when proxying
schemas from untrusted servers.
**Args:**
- `schema`: JSON schema dict that may contain $ref references
**Returns:**
- A new schema dict with $ref resolved where possible and $defs removed
- when no longer needed
### `resolve_root_ref`
```python
resolve_root_ref(schema: dict[str, Any]) -> dict[str, Any]
```
Resolve $ref at root level to meet MCP spec requirements.
MCP specification requires outputSchema to have "type": "object" at the root level.
When Pydantic generates schemas for self-referential models, it uses $ref at the
root level pointing to $defs. This function resolves such references by inlining
the referenced definition while preserving $defs for nested references.
**Args:**
- `schema`: JSON schema dict that may have $ref at root level
**Returns:**
- A new schema dict with root-level $ref resolved, or the original schema
- if no resolution is needed
### `compress_schema`
```python
compress_schema(schema: dict[str, Any], prune_params: list[str] | None = None, prune_additional_properties: bool = False, prune_titles: bool = False, dereference: bool = False) -> dict[str, Any]
```
Compress and optimize a JSON schema for MCP compatibility.
**Args:**
- `schema`: The schema to compress
- `prune_params`: List of parameter names to remove from properties
- `prune_additional_properties`: Whether to remove additionalProperties\: false.
Defaults to False to maintain MCP client compatibility, as some clients
(e.g., Claude) require additionalProperties\: false for strict validation.
- `prune_titles`: Whether to remove title fields from the schema
- `dereference`: Whether to dereference $ref by inlining definitions.
Defaults to False; dereferencing is typically handled by
middleware at serve-time instead.