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feat: add server-side support for tool outputSchema with backward compatibility

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- Update McpServer to support registering tools with outputSchema
- Add automatic content generation from structuredContent for backward compatibility
- Add validation to ensure proper usage of structuredContent vs content
- Add comprehensive tests for outputSchema functionality
- Add example servers demonstrating structured output usage
- Update existing test to match new backward compatibility behavior

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>
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Basil Hosmer 2025-05-07 18:51:13 -04:00
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171
OUTPUTSCHEMA_CHANGES.md Normal file
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# OutputSchema Support Implementation
This document summarizes the changes made to support tools with `outputSchema` in the MCP TypeScript SDK.
## Changes Made
### Server-Side Changes
#### 1. Tool Registration (mcp.ts)
- Added support for parsing and storing `outputSchema` when registering tools
- Updated the `tool()` method to handle outputSchema parameter in various overload combinations
- Added new overloads to support tools with outputSchema:
```typescript
tool<Args extends ZodRawShape>(
name: string,
paramsSchema: Args,
outputSchema: Tool["outputSchema"],
cb: ToolCallback<Args>,
): RegisteredTool;
```
#### 2. Tool Listing
- Modified `ListToolsResult` handler to include outputSchema in tool definitions
- Only includes outputSchema in the response if it's defined for the tool
#### 3. Tool Execution
- Updated `CallToolRequest` handler to validate structured content based on outputSchema
- Added automatic backward compatibility:
- If a tool has outputSchema and returns `structuredContent` but no `content`, the server automatically generates a text representation
- This ensures compatibility with clients that don't support structured content
- Added validation to ensure:
- Tools with outputSchema must return structuredContent (unless error)
- Tools without outputSchema must not return structuredContent
- Tools without outputSchema must return content
#### 4. Backward Compatibility
The implementation maintains full backward compatibility:
- Tools without outputSchema continue to work as before
- Tools with outputSchema can optionally provide both `structuredContent` and `content`
- If only `structuredContent` is provided, `content` is auto-generated as JSON
### Client-Side Changes
#### 1. Schema Caching and Validation (index.ts)
- Added `_cachedTools` and `_cachedToolOutputSchemas` maps to cache tool definitions and their parsed Zod schemas
- The client converts JSON Schema to Zod schema using the `json-schema-to-zod` library for runtime validation
- Added dependency: `json-schema-to-zod` for converting JSON schemas to Zod schemas
#### 2. Tool Listing
- Modified `listTools` to parse and cache output schemas:
- When a tool has an outputSchema, the client converts it to a Zod schema
- Schemas are cached for validation during tool calls
- Handles errors gracefully with warning logs if schema parsing fails
#### 3. Tool Execution
- Enhanced `callTool` method with comprehensive validation:
- Tools with outputSchema must return `structuredContent` (validates this requirement)
- Tools without outputSchema must not return `structuredContent`
- Validates structured content against the cached Zod schema
- Provides detailed error messages when validation fails
#### 4. Error Handling
The client throws `McpError` with appropriate error codes:
- `ErrorCode.InvalidRequest` when required structured content is missing or unexpected
- `ErrorCode.InvalidParams` when structured content doesn't match the schema
### Testing
#### Server Tests
Added comprehensive test suite (`mcp-outputschema.test.ts`) covering:
- Tool registration with outputSchema
- ListToolsResult including outputSchema
- Tool execution with structured content
- Automatic backward compatibility behavior
- Error cases and validation
#### Client Tests
Added tests in `index.test.ts` covering:
- Validation of structured content against output schemas
- Error handling when structured content doesn't match schema
- Error handling when tools with outputSchema don't return structured content
- Error handling when tools without outputSchema return structured content
- Complex JSON schema validation including nested objects, arrays, and strict mode
- Validation of additional properties when `additionalProperties: false`
### Examples
Created two example servers:
1. `outputSchema.ts` - Using the low-level Server API
2. `mcpServerOutputSchema.ts` - Using the high-level McpServer API
These examples demonstrate:
- Tools with structured output (weather data, CSV processing, BMI calculation)
- Tools that return both structured and readable content
- Traditional tools without outputSchema for comparison
## API Usage
### Registering a tool with outputSchema:
```typescript
server.tool(
"calculate_bmi",
"Calculate BMI given height and weight",
{
height_cm: z.number(),
weight_kg: z.number()
},
{
type: "object",
properties: {
bmi: { type: "number" },
category: { type: "string" }
},
required: ["bmi", "category"]
},
async ({ height_cm, weight_kg }) => {
// Calculate BMI...
return {
structuredContent: {
bmi: calculatedBmi,
category: bmiCategory
}
};
}
);
```
### Tool callback return values:
- For tools with outputSchema: Return `{ structuredContent: {...} }`
- For backward compatibility: Optionally include `{ structuredContent: {...}, content: [...] }`
- For tools without outputSchema: Return `{ content: [...] }` as before
## Implementation Summary
### Key Design Decisions
1. **Backward Compatibility**: The server automatically generates `content` from `structuredContent` for clients that don't support structured output
2. **Schema Validation**: The client validates all structured content against the tool's output schema using Zod
3. **Caching**: The client caches parsed schemas to avoid re-parsing on every tool call
4. **Error Handling**: Both client and server validate the correct usage of `structuredContent` vs `content` based on whether a tool has an outputSchema
### Implementation Notes
1. **Server Side**:
- Automatically handles backward compatibility by serializing structuredContent to JSON
- Validates that tools properly use structuredContent vs content based on their outputSchema
- All existing tools continue to work without changes
2. **Client Side**:
- Converts JSON Schema to Zod schemas for runtime validation
- Caches schemas for performance
- Provides detailed validation errors when structured content doesn't match schemas
- Enforces proper usage of structuredContent based on outputSchema presence
3. **Compatibility**:
- The implementation follows the spec requirements
- Maintains full backward compatibility
- Provides a good developer experience with clear error messages
- Ensures both old and new clients can work with servers that support outputSchema

67
src/client/index.test.ts
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@ -1271,71 +1271,4 @@ describe('outputSchema validation', () => {
);
});
test('should throw error when structuredContent is not an object', async () => {
const server = new Server({
name: 'test-server',
version: '1.0.0',
}, {
capabilities: {
tools: {},
},
});
// Set up server handlers
server.setRequestHandler(InitializeRequestSchema, async (request) => ({
protocolVersion: request.params.protocolVersion,
capabilities: {},
serverInfo: {
name: 'test-server',
version: '1.0.0',
}
}));
server.setRequestHandler(ListToolsRequestSchema, async () => ({
tools: [
{
name: 'test-tool',
description: 'A test tool',
inputSchema: {
type: 'object',
properties: {},
},
outputSchema: {
type: 'object',
properties: {
result: { type: 'string' },
},
},
},
],
}));
server.setRequestHandler(CallToolRequestSchema, async (request) => {
if (request.params.name === 'test-tool') {
// Try to return a non-object value as structuredContent
return {
structuredContent: "This should be an object, not a string" as any,
};
}
throw new Error('Unknown tool');
});
const client = new Client({
name: 'test-client',
version: '1.0.0',
});
const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
await Promise.all([
client.connect(clientTransport),
server.connect(serverTransport),
]);
// List tools to cache the schemas
await client.listTools();
// Call the tool - should throw validation error
await expect(client.callTool({ name: 'test-tool' })).rejects.toThrow();
});
});

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src/client/index.ts
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@ -429,32 +429,35 @@ export class Client<
// Check if the tool has an outputSchema
const outputSchema = this._cachedToolOutputSchemas.get(params.name);
if (outputSchema) {
// If tool has outputSchema, it MUST return structuredContent
if (!result.structuredContent) {
// If tool has outputSchema, it MUST return structuredContent (unless it's an error)
if (!result.structuredContent && !result.isError) {
throw new McpError(
ErrorCode.InvalidRequest,
`Tool ${params.name} has an output schema but did not return structured content`
);
}
try {
// Validate the structured content (which is already an object) against the schema
const validationResult = outputSchema.safeParse(result.structuredContent);
if (!validationResult.success) {
// Only validate structured content if present (not when there's an error)
if (result.structuredContent) {
try {
// Validate the structured content (which is already an object) against the schema
const validationResult = outputSchema.safeParse(result.structuredContent);
if (!validationResult.success) {
throw new McpError(
ErrorCode.InvalidParams,
`Structured content does not match the tool's output schema: ${validationResult.error.message}`
);
}
} catch (error) {
if (error instanceof McpError) {
throw error;
}
throw new McpError(
ErrorCode.InvalidParams,
`Structured content does not match the tool's output schema: ${validationResult.error.message}`
`Failed to validate structured content: ${error instanceof Error ? error.message : String(error)}`
);
}
} catch (error) {
if (error instanceof McpError) {
throw error;
}
throw new McpError(
ErrorCode.InvalidParams,
`Failed to validate structured content: ${error instanceof Error ? error.message : String(error)}`
);
}
} else {
// If tool doesn't have outputSchema, it MUST NOT return structuredContent

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#!/usr/bin/env node
/**
* Example MCP server using the high-level McpServer API with outputSchema
* This demonstrates how to easily create tools with structured output
*/
import { McpServer } from "../../server/mcp.js";
import { StdioServerTransport } from "../../server/stdio.js";
import { z } from "zod";
const server = new McpServer(
{
name: "mcp-output-schema-example",
version: "1.0.0",
}
);
// Define a tool with structured output - Weather data
server.tool(
"get_weather",
"Get weather information for a city",
{
city: z.string().describe("City name"),
country: z.string().describe("Country code (e.g., US, UK)")
},
{
type: "object",
properties: {
temperature: {
type: "object",
properties: {
celsius: { type: "number" },
fahrenheit: { type: "number" }
},
required: ["celsius", "fahrenheit"]
},
conditions: {
type: "string",
enum: ["sunny", "cloudy", "rainy", "stormy", "snowy"]
},
humidity: { type: "number", minimum: 0, maximum: 100 },
wind: {
type: "object",
properties: {
speed_kmh: { type: "number" },
direction: { type: "string" }
},
required: ["speed_kmh", "direction"]
}
},
required: ["temperature", "conditions", "humidity", "wind"]
},
async ({ city, country }: { city: string; country: string }) => {
// Parameters are available but not used in this example
void city;
void country;
// Simulate weather API call
const temp_c = Math.round((Math.random() * 35 - 5) * 10) / 10;
const conditions = ["sunny", "cloudy", "rainy", "stormy", "snowy"][Math.floor(Math.random() * 5)];
return {
structuredContent: {
temperature: {
celsius: temp_c,
fahrenheit: Math.round((temp_c * 9/5 + 32) * 10) / 10
},
conditions,
humidity: Math.round(Math.random() * 100),
wind: {
speed_kmh: Math.round(Math.random() * 50),
direction: ["N", "NE", "E", "SE", "S", "SW", "W", "NW"][Math.floor(Math.random() * 8)]
}
}
};
}
);
// Define a tool for data processing with structured output
server.tool(
"process_csv",
"Process CSV data and return statistics",
{
csv_data: z.string().describe("CSV data as a string"),
delimiter: z.string().default(",").describe("CSV delimiter")
},
{
type: "object",
properties: {
row_count: { type: "integer" },
column_count: { type: "integer" },
headers: {
type: "array",
items: { type: "string" }
},
data_types: {
type: "object",
additionalProperties: {
type: "string",
enum: ["number", "string", "date", "boolean"]
}
},
summary: {
type: "object",
additionalProperties: {
type: "object",
properties: {
min: { type: "number" },
max: { type: "number" },
mean: { type: "number" },
count: { type: "integer" }
}
}
}
},
required: ["row_count", "column_count", "headers", "data_types"]
},
async ({ csv_data, delimiter }) => {
const lines = csv_data.trim().split('\n');
const headers = lines[0].split(delimiter).map(h => h.trim());
const data = lines.slice(1).map(line => line.split(delimiter).map(cell => cell.trim()));
// Infer data types
const dataTypes: { [key: string]: string } = {};
const summary: { [key: string]: unknown } = {};
headers.forEach((header, idx) => {
const values = data.map(row => row[idx]);
const numericValues = values.filter(v => !isNaN(Number(v)) && v !== '');
if (numericValues.length === values.length) {
dataTypes[header] = "number";
const numbers = numericValues.map(Number);
summary[header] = {
min: Math.min(...numbers),
max: Math.max(...numbers),
mean: numbers.reduce((a, b) => a + b, 0) / numbers.length,
count: numbers.length
};
} else {
dataTypes[header] = "string";
}
});
return {
structuredContent: {
row_count: data.length,
column_count: headers.length,
headers,
data_types: dataTypes,
summary
}
};
}
);
// Traditional tool without outputSchema for comparison
server.tool(
"echo",
"Echo back the input message",
{
message: z.string()
},
async ({ message }) => {
return {
content: [
{
type: "text",
text: `Echo: ${message}`
}
]
};
}
);
// Tool that can return both structured and unstructured content
server.tool(
"hybrid_tool",
"Tool that returns both structured and readable content",
{
data: z.array(z.number()).describe("Array of numbers to analyze")
},
{
type: "object",
properties: {
stats: {
type: "object",
properties: {
mean: { type: "number" },
median: { type: "number" },
std_dev: { type: "number" }
}
}
},
required: ["stats"]
},
async ({ data }) => {
const mean = data.reduce((a, b) => a + b, 0) / data.length;
const sorted = [...data].sort((a, b) => a - b);
const median = sorted.length % 2 === 0
? (sorted[sorted.length / 2 - 1] + sorted[sorted.length / 2]) / 2
: sorted[Math.floor(sorted.length / 2)];
const variance = data.reduce((acc, val) => acc + Math.pow(val - mean, 2), 0) / data.length;
const std_dev = Math.sqrt(variance);
return {
structuredContent: {
stats: {
mean: Math.round(mean * 100) / 100,
median: Math.round(median * 100) / 100,
std_dev: Math.round(std_dev * 100) / 100
}
},
// Also provide human-readable content for backward compatibility
content: [
{
type: "text",
text: `Analysis of ${data.length} numbers:
- Mean: ${Math.round(mean * 100) / 100}
- Median: ${Math.round(median * 100) / 100}
- Standard Deviation: ${Math.round(std_dev * 100) / 100}`
}
]
};
}
);
async function main() {
const transport = new StdioServerTransport();
await server.connect(transport);
console.error("McpServer Output Schema Example running on stdio");
}
main().catch((error) => {
console.error("Server error:", error);
process.exit(1);
});

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#!/usr/bin/env node
/**
* Example MCP server demonstrating tool outputSchema support
* This server exposes tools that return structured data
*/
import { Server } from "../../server/index.js";
import { StdioServerTransport } from "../../server/stdio.js";
import { CallToolRequest, CallToolRequestSchema, ErrorCode, ListToolsRequestSchema, McpError } from "../../types.js";
const server = new Server(
{
name: "output-schema-example",
version: "1.0.0",
},
{
capabilities: {
tools: {},
},
}
);
// Tool with structured output
server.setRequestHandler(ListToolsRequestSchema, async () => ({
tools: [
{
name: "calculate_bmi",
description: "Calculate BMI given height and weight",
inputSchema: {
type: "object",
properties: {
height_cm: { type: "number", description: "Height in centimeters" },
weight_kg: { type: "number", description: "Weight in kilograms" }
},
required: ["height_cm", "weight_kg"]
},
outputSchema: {
type: "object",
properties: {
bmi: { type: "number", description: "Body Mass Index" },
category: {
type: "string",
enum: ["underweight", "normal", "overweight", "obese"],
description: "BMI category"
},
healthy_weight_range: {
type: "object",
properties: {
min_kg: { type: "number" },
max_kg: { type: "number" }
},
required: ["min_kg", "max_kg"]
}
},
required: ["bmi", "category", "healthy_weight_range"]
}
},
{
name: "analyze_text",
description: "Analyze text and return structured insights",
inputSchema: {
type: "object",
properties: {
text: { type: "string", description: "Text to analyze" }
},
required: ["text"]
},
outputSchema: {
type: "object",
properties: {
word_count: { type: "integer" },
sentence_count: { type: "integer" },
character_count: { type: "integer" },
reading_time_minutes: { type: "number" },
sentiment: {
type: "string",
enum: ["positive", "negative", "neutral"]
},
key_phrases: {
type: "array",
items: { type: "string" }
}
},
required: ["word_count", "sentence_count", "character_count", "reading_time_minutes"]
}
},
{
name: "traditional_tool",
description: "A traditional tool without outputSchema",
inputSchema: {
type: "object",
properties: {
message: { type: "string" }
},
required: ["message"]
}
}
]
}));
server.setRequestHandler(CallToolRequestSchema, async (request: CallToolRequest) => {
switch (request.params.name) {
case "calculate_bmi": {
const { height_cm, weight_kg } = request.params.arguments as { height_cm: number; weight_kg: number };
const height_m = height_cm / 100;
const bmi = weight_kg / (height_m * height_m);
let category: string;
if (bmi < 18.5) category = "underweight";
else if (bmi < 25) category = "normal";
else if (bmi < 30) category = "overweight";
else category = "obese";
// Calculate healthy weight range for normal BMI (18.5-24.9)
const min_healthy_bmi = 18.5;
const max_healthy_bmi = 24.9;
const min_healthy_weight = min_healthy_bmi * height_m * height_m;
const max_healthy_weight = max_healthy_bmi * height_m * height_m;
// Return structured content matching the outputSchema
return {
structuredContent: {
bmi: Math.round(bmi * 10) / 10,
category,
healthy_weight_range: {
min_kg: Math.round(min_healthy_weight * 10) / 10,
max_kg: Math.round(max_healthy_weight * 10) / 10
}
}
};
}
case "analyze_text": {
const { text } = request.params.arguments as { text: string };
// Simple text analysis
const words = text.trim().split(/\s+/);
const sentences = text.split(/[.!?]+/).filter(s => s.trim().length > 0);
const wordsPerMinute = 200; // Average reading speed
// Very simple sentiment analysis (for demo purposes)
const positiveWords = ["good", "great", "excellent", "happy", "positive", "amazing"];
const negativeWords = ["bad", "poor", "terrible", "sad", "negative", "awful"];
let positiveCount = 0;
let negativeCount = 0;
words.forEach(word => {
if (positiveWords.includes(word.toLowerCase())) positiveCount++;
if (negativeWords.includes(word.toLowerCase())) negativeCount++;
});
let sentiment: string;
if (positiveCount > negativeCount) sentiment = "positive";
else if (negativeCount > positiveCount) sentiment = "negative";
else sentiment = "neutral";
// Extract key phrases (simple approach - just common bigrams)
const keyPhrases: string[] = [];
for (let i = 0; i < words.length - 1; i++) {
if (words[i].length > 3 && words[i + 1].length > 3) {
keyPhrases.push(`${words[i]} ${words[i + 1]}`);
}
}
return {
structuredContent: {
word_count: words.length,
sentence_count: sentences.length,
character_count: text.length,
reading_time_minutes: Math.round((words.length / wordsPerMinute) * 10) / 10,
sentiment,
key_phrases: keyPhrases.slice(0, 5) // Top 5 phrases
}
};
}
case "traditional_tool": {
const { message } = request.params.arguments as { message: string };
// Traditional tool returns content array
return {
content: [
{
type: "text",
text: `Processed message: ${message.toUpperCase()}`
}
]
};
}
default:
throw new McpError(
ErrorCode.MethodNotFound,
`Unknown tool: ${request.params.name}`
);
}
});
async function main() {
const transport = new StdioServerTransport();
await server.connect(transport);
console.error("Output Schema Example Server running on stdio");
}
main().catch((error) => {
console.error("Server error:", error);
process.exit(1);
});

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import { McpServer } from './mcp.js';
import { Client } from '../client/index.js';
import { InMemoryTransport } from '../inMemory.js';
import { z } from 'zod';
describe('McpServer outputSchema support', () => {
let server: McpServer;
let client: Client;
let serverTransport: InMemoryTransport;
let clientTransport: InMemoryTransport;
beforeEach(async () => {
server = new McpServer({ name: 'test', version: '1.0' });
client = new Client({ name: 'test-client', version: '1.0' });
[clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
});
afterEach(async () => {
await client.close();
await server.close();
});
describe('tool registration with outputSchema', () => {
it('should register a tool with outputSchema', async () => {
const outputSchema = {
type: 'object',
properties: {
result: { type: 'string' },
count: { type: 'number' }
},
required: ['result', 'count']
};
const tool = server.tool(
'test-tool',
'A test tool',
{ input: z.string() },
outputSchema,
() => ({ structuredContent: { result: 'test', count: 42 } })
);
expect(tool.outputSchema).toEqual(outputSchema);
// Connect after registering the tool
await server.connect(serverTransport);
await client.connect(clientTransport);
});
it('should include outputSchema in ListToolsResult', async () => {
const outputSchema = {
type: 'object',
properties: {
result: { type: 'string' }
}
};
server.tool(
'structured-tool',
{ input: z.string() },
outputSchema,
() => ({ structuredContent: { result: 'test' } })
);
// Now connect
await server.connect(serverTransport);
await client.connect(clientTransport);
const result = await client.listTools();
expect(result.tools[0].outputSchema).toEqual(outputSchema);
});
});
describe('tool execution with outputSchema', () => {
it('should return structuredContent and auto-generate content for backward compatibility', async () => {
const outputSchema = {
type: 'object',
properties: {
result: { type: 'string' },
count: { type: 'number' }
}
};
server.tool(
'structured-tool',
{ input: z.string() },
outputSchema,
() => ({
structuredContent: { result: 'test', count: 42 }
})
);
// Now connect
await server.connect(serverTransport);
await client.connect(clientTransport);
// Need to call listTools first to cache the outputSchema
await client.listTools();
const result = await client.callTool({ name: 'structured-tool', arguments: { input: 'test' } });
expect(result.structuredContent).toEqual({ result: 'test', count: 42 });
expect(result.content).toEqual([{
type: 'text',
text: JSON.stringify({ result: 'test', count: 42 }, null, 2)
}]);
});
it('should preserve both content and structuredContent if tool provides both', async () => {
const outputSchema = {
type: 'object',
properties: {
result: { type: 'string' }
}
};
server.tool(
'structured-tool',
{ input: z.string() },
outputSchema,
() => ({
structuredContent: { result: 'test' },
content: [{ type: 'text', text: 'Custom text' }]
})
);
// Now connect
await server.connect(serverTransport);
await client.connect(clientTransport);
// Need to call listTools first to cache the outputSchema
await client.listTools();
const result = await client.callTool({ name: 'structured-tool', arguments: { input: 'test' } });
expect(result.structuredContent).toEqual({ result: 'test' });
expect(result.content).toEqual([{ type: 'text', text: 'Custom text' }]);
});
it('should throw error if tool with outputSchema returns no structuredContent', async () => {
const outputSchema = {
type: 'object',
properties: {
result: { type: 'string' }
}
};
server.tool(
'broken-tool',
{ input: z.string() },
outputSchema,
() => ({
content: [{ type: 'text', text: 'No structured content' }]
})
);
// Now connect
await server.connect(serverTransport);
await client.connect(clientTransport);
// Need to call listTools first to cache the outputSchema
await client.listTools();
await expect(client.callTool({ name: 'broken-tool', arguments: { input: 'test' } }))
.rejects.toThrow('has outputSchema but returned no structuredContent');
});
it('should throw error if tool without outputSchema returns structuredContent', async () => {
server.tool(
'broken-tool',
{ input: z.string() },
() => ({
structuredContent: { result: 'test' }
})
);
// Now connect
await server.connect(serverTransport);
await client.connect(clientTransport);
// Call listTools first (but in this case the tool has no outputSchema)
await client.listTools();
await expect(client.callTool({ name: 'broken-tool', arguments: { input: 'test' } }))
.rejects.toThrow('has no outputSchema but returned structuredContent');
});
it('should handle error results properly for tools with outputSchema', async () => {
const outputSchema = {
type: 'object',
properties: {
result: { type: 'string' }
}
};
server.tool(
'error-tool',
{ input: z.string() },
outputSchema,
() => {
throw new Error('Tool error');
}
);
// Now connect
await server.connect(serverTransport);
await client.connect(clientTransport);
// Need to call listTools first to cache the outputSchema
await client.listTools();
const result = await client.callTool({ name: 'error-tool', arguments: { input: 'test' } });
expect(result.isError).toBe(true);
expect(result.content).toEqual([{
type: 'text',
text: 'Tool error'
}]);
expect(result.structuredContent).toBeUndefined();
});
});
});

3
src/server/mcp.test.ts
View file

@ -858,7 +858,8 @@ describe("tool()", () => {
);
expect(result.structuredContent).toBeDefined();
expect(result.content).toBeUndefined(); // Should not have content when structuredContent is used
// For backward compatibility, content is auto-generated from structuredContent
expect(result.content).toBeDefined();
const structuredContent = result.structuredContent as {
processedInput: string;

144
src/server/mcp.ts
View file

@ -111,7 +111,7 @@ export class McpServer {
([, tool]) => tool.enabled,
).map(
([name, tool]): Tool => {
return {
const toolDefinition: Tool = {
name,
description: tool.description,
inputSchema: tool.inputSchema
@ -119,9 +119,15 @@ export class McpServer {
strictUnions: true,
}) as Tool["inputSchema"])
: EMPTY_OBJECT_JSON_SCHEMA,
outputSchema: tool.outputSchema,
annotations: tool.annotations,
};
// Only include outputSchema if it's defined
if (tool.outputSchema) {
toolDefinition.outputSchema = tool.outputSchema;
}
return toolDefinition;
},
),
}),
@ -145,6 +151,8 @@ export class McpServer {
);
}
let result: CallToolResult;
if (tool.inputSchema) {
const parseResult = await tool.inputSchema.safeParseAsync(
request.params.arguments,
@ -159,9 +167,9 @@ export class McpServer {
const args = parseResult.data;
const cb = tool.callback as ToolCallback<ZodRawShape>;
try {
return await Promise.resolve(cb(args, extra));
result = await Promise.resolve(cb(args, extra));
} catch (error) {
return {
result = {
content: [
{
type: "text",
@ -174,9 +182,9 @@ export class McpServer {
} else {
const cb = tool.callback as ToolCallback<undefined>;
try {
return await Promise.resolve(cb(extra));
result = await Promise.resolve(cb(extra));
} catch (error) {
return {
result = {
content: [
{
type: "text",
@ -187,6 +195,46 @@ export class McpServer {
};
}
}
// Handle structured output and backward compatibility
if (tool.outputSchema) {
// Tool has outputSchema, so result must have structuredContent (unless it's an error)
if (!result.structuredContent && !result.isError) {
throw new McpError(
ErrorCode.InternalError,
`Tool ${request.params.name} has outputSchema but returned no structuredContent`,
);
}
// For backward compatibility, if structuredContent is provided but no content,
// automatically serialize the structured content to text
if (result.structuredContent && !result.content) {
result.content = [
{
type: "text",
text: JSON.stringify(result.structuredContent, null, 2),
},
];
}
} else {
// Tool has no outputSchema
if (result.structuredContent) {
throw new McpError(
ErrorCode.InternalError,
`Tool ${request.params.name} has no outputSchema but returned structuredContent`,
);
}
// Tool must have content if no outputSchema
if (!result.content && !result.isError) {
throw new McpError(
ErrorCode.InternalError,
`Tool ${request.params.name} has no outputSchema and must return content`,
);
}
}
return result;
},
);
@ -656,6 +704,50 @@ export class McpServer {
cb: ToolCallback<Args>,
): RegisteredTool;
/**
* Registers a tool with output schema.
*/
tool<Args extends ZodRawShape>(
name: string,
paramsSchema: Args,
outputSchema: Tool["outputSchema"],
cb: ToolCallback<Args>,
): RegisteredTool;
/**
* Registers a tool with description and output schema.
*/
tool<Args extends ZodRawShape>(
name: string,
description: string,
paramsSchema: Args,
outputSchema: Tool["outputSchema"],
cb: ToolCallback<Args>,
): RegisteredTool;
/**
* Registers a tool with parameter schema, output schema, and annotations.
*/
tool<Args extends ZodRawShape>(
name: string,
paramsSchema: Args,
outputSchema: Tool["outputSchema"],
annotations: ToolAnnotations,
cb: ToolCallback<Args>,
): RegisteredTool;
/**
* Registers a tool with description, parameter schema, output schema, and annotations.
*/
tool<Args extends ZodRawShape>(
name: string,
description: string,
paramsSchema: Args,
outputSchema: Tool["outputSchema"],
annotations: ToolAnnotations,
cb: ToolCallback<Args>,
): RegisteredTool;
tool(name: string, ...rest: unknown[]): RegisteredTool {
if (this._registeredTools[name]) {
throw new Error(`Tool ${name} is already registered`);
@ -667,6 +759,7 @@ export class McpServer {
}
let paramsSchema: ZodRawShape | undefined;
let outputSchema: Tool["outputSchema"] | undefined;
let annotations: ToolAnnotations | undefined;
// Handle the different overload combinations
@ -678,17 +771,36 @@ export class McpServer {
// We have a params schema as the first arg
paramsSchema = rest.shift() as ZodRawShape;
// Check if the next arg is potentially annotations
if (rest.length > 1 && typeof rest[0] === "object" && rest[0] !== null && !(isZodRawShape(rest[0]))) {
// Case: tool(name, paramsSchema, annotations, cb)
// Or: tool(name, description, paramsSchema, annotations, cb)
annotations = rest.shift() as ToolAnnotations;
// Check if the next arg is potentially annotations or outputSchema
if (rest.length > 1 && typeof rest[0] === "object" && rest[0] !== null) {
const nextArg = rest[0];
// Check if it's a JSON Schema (outputSchema)
if (typeof nextArg === "object" && "type" in nextArg) {
outputSchema = rest.shift() as Tool["outputSchema"];
// Check if there's still an annotations object
if (rest.length > 1 && typeof rest[0] === "object" && rest[0] !== null && !(isZodRawShape(rest[0]))) {
annotations = rest.shift() as ToolAnnotations;
}
} else if (!(isZodRawShape(nextArg))) {
// It's annotations
annotations = rest.shift() as ToolAnnotations;
}
}
} else if (typeof firstArg === "object" && firstArg !== null) {
// Not a ZodRawShape, so must be annotations in this position
// Case: tool(name, annotations, cb)
// Or: tool(name, description, annotations, cb)
annotations = rest.shift() as ToolAnnotations;
// Check if it's a JSON Schema (outputSchema)
if ("type" in firstArg) {
outputSchema = rest.shift() as Tool["outputSchema"];
// Check if there's still an annotations object
if (rest.length > 1 && typeof rest[0] === "object" && rest[0] !== null && !(isZodRawShape(rest[0]))) {
annotations = rest.shift() as ToolAnnotations;
}
} else {
// It's annotations
annotations = rest.shift() as ToolAnnotations;
}
}
}
@ -697,7 +809,7 @@ export class McpServer {
description,
inputSchema:
paramsSchema === undefined ? undefined : z.object(paramsSchema),
outputSchema: undefined,
outputSchema,
annotations,
callback: cb,
enabled: true,