---
title: json_schema
sidebarTitle: json_schema
---

# `fastmcp.utilities.json_schema`

## Functions

### `require_discriminator_property` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/utilities/json_schema.py#L116" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```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` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/utilities/json_schema.py#L147" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```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` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/utilities/json_schema.py#L294" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```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` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/utilities/json_schema.py#L688" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```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.

