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

# `fastmcp.utilities.json_schema`

## Functions

### `dereference_refs` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/utilities/json_schema.py#L9" 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.

**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/jlowin/fastmcp/blob/main/src/fastmcp/utilities/json_schema.py#L131" 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/jlowin/fastmcp/blob/main/src/fastmcp/utilities/json_schema.py#L364" 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 = True, prune_titles: bool = False) -> dict[str, Any]
```


Compress and optimize a JSON schema for MCP compatibility.

This function dereferences all $ref entries (inlining definitions) to ensure
compatibility with MCP clients that don't properly handle $ref in schemas
(e.g., VS Code Copilot). It also applies various optimizations to reduce
schema size.

**Args:**
- `schema`: The schema to compress
- `prune_params`: List of parameter names to remove from properties
- `prune_additional_properties`: Whether to remove additionalProperties\: false
- `prune_titles`: Whether to remove title fields from the schema