DEV Community

Sualeh Fatehi
Sualeh Fatehi

Posted on

Generate MCP Tool Schemas Directly From Java Code

#java #mcp #json #jsonschema

If you are building an MCP server, every tool you expose needs an inputSchema. MCP servers written with Spring AI support often start with a simple data class for tool inputs. Then come changes: a new field, a renamed property, or updated constraints. The JSON schema in the tool registration rarely keeps up - that means clients may send invalid payloads. By generating the schema from the source of truth — the Java type — you remove that drift.

Writing that JSON by hand is repetitive, easy to get wrong. MCP supports only a specific sub-type of the JSON Schema specification. The MCP JSON Schema library keeps the parameter schema and the code in lockstep by generating the MCP-compatible JSON Schema from a Jackson 3 annotated Java class or record.

What you get:

  • Use of Jackson 3 annotations for naming, required fields, and descriptions that carry over into the schema
  • Use of Jakarta Bean Validation to adds meaningful constraints to the schema (for example, @Max,@Min, @Positive, @PositiveOrZero, @Negative, @NegativeOrZero on numbers, or @Size, @NotBlank on strings)
  • Automatic handling of required fields, defaults, enums, and descriptions
  • Output that targets the MCP JSON Schema subset, not the entire JSON Schema specification

How it works

Add a dependency to us.fatehi:mcp-json-schema in Maven or Gradle.

<dependency>
    <groupId>us.fatehi</groupId>
    <artifactId>mcp-json-schema</artifactId>
    <version>1.0.1</version>
</dependency>
Enter fullscreen mode Exit fullscreen mode

Define a parameters type as a Jackson‑annotated record or class and let the library produce the inputSchema JSON. Use annotations to describe intent, and let the library translate that into the MCP schema format.

For example:

import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.annotation.JsonPropertyDescription;
import tools.jackson.databind.PropertyNamingStrategies;
import tools.jackson.databind.annotation.JsonNaming;

@JsonNaming(PropertyNamingStrategies.KebabCaseStrategy.class)
public record SampleParameters(
    @JsonPropertyDescription("Type of database table dependant objects.")
    @JsonProperty(defaultValue = "NONE", required = true)
    DependantObjectType dependantObjectType,

    @JsonPropertyDescription("Table name.")
    String tableName) {

  public enum DependantObjectType 
    { NONE, COLUMNS, INDEXES, FOREIGN_KEYS, TRIGGERS }
}
Enter fullscreen mode Exit fullscreen mode

Next, generate the MCP inputSchema:

import us.fatehi.mcp_json_schema.McpJsonSchemaUtility;

// Provide this value as the tool's input_schema 
// in your Spring AI MCP server implementation
String inputSchemaJson = 
  McpJsonSchemaUtility.inputSchema(SampleParameters.class);
Enter fullscreen mode Exit fullscreen mode

Prefer a JsonNode for programmatic changes? Use:

var schemaNode = 
  McpJsonSchemaUtility.generateJsonSchema(SampleParameters.class);
Enter fullscreen mode Exit fullscreen mode

The source code is available at sualeh/mcp-json-schema

Top comments (0)