fix: preserve raw tool result fields (#58)

Author: jakekaplan

.changeset/raw-tool-results.md

@@ -0,0 +1,5 @@
+---
+"@prefecthq/fastmcp-ts": patch
+---
+
+Preserve metadata and extension fields in client tool call results.

src/client/client.ts

@@ -38,6 +38,7 @@ import type {
   Root,
   Tool,
 } from './results.js'
+import { normalizeCallToolResult } from './results.js'
 import type { ClientTransportInput, McpConfig } from './transports.js'
 import { resolveTransport } from './transports.js'
 import type { MultiServerOptions } from './multi-server.js'
@@ -412,11 +413,7 @@ export class Client implements IClient {
     const result = await this._reauthRetry(() =>
       this._sdk().callTool({ name, arguments: args ?? {}, ...this._metaParams() }, sdkOptions),
     )
-    return {
-      content: result.content as ContentBlock[],
-      structuredContent: (result.structuredContent as TData | undefined) ?? null,
-      isError: result.isError === true,
-    }
+    return normalizeCallToolResult<TData>(result)
   }
 
   // -------------------------------------------------------------------------

src/client/multi-server.ts

@@ -9,7 +9,6 @@ import type { CallToolOptions, IClient, RequestOptions } from './interfaces.js'
 import type {
   CallToolResult,
   CompletionResult,
-  ContentBlock,
   GetPromptResult,
   Prompt,
   Resource,
@@ -18,6 +17,7 @@ import type {
   ResourceTemplate,
   Tool,
 } from './results.js'
+import { normalizeCallToolResult } from './results.js'
 import type { McpConfig, McpServerValue } from './transports.js'
 import { resolveEntryTransport } from './transports.js'
 import type { ClientDefaultOptions } from './options.js'
@@ -267,11 +267,7 @@ export class MultiServerClient implements IClient {
       { name: localName, arguments: args ?? {}, ...this._metaParamsFor(serverName) },
       sdkOptions,
     )
-    return {
-      content: result.content as ContentBlock[],
-      structuredContent: (result.structuredContent as TData | undefined) ?? null,
-      isError: result.isError === true,
-    }
+    return normalizeCallToolResult<TData>(result)
   }
 
   // -------------------------------------------------------------------------

src/client/results.ts

@@ -1,4 +1,5 @@
-import type { CreateMessageResult, CreateMessageResultWithTools, ContentBlock } from "@modelcontextprotocol/server";
+import type { CallToolResult as SdkCallToolResult } from '@modelcontextprotocol/client'
+import type { CreateMessageResult, CreateMessageResultWithTools } from "@modelcontextprotocol/server";
 
 export type {
   Tool,
@@ -29,14 +30,24 @@ export type {
 export type AnySamplingResult = CreateMessageResult | CreateMessageResultWithTools
 /**
  * The SDK's CallToolResult with a typed generic for structuredContent.
- * Use TData to get typed access to structured tool output.
+ * The intersection retains its open extension fields while narrowing the two
+ * values FastMCP normalizes.
  */
-export type CallToolResult<TData = unknown> = {
-  content: ContentBlock[]
+export type CallToolResult<TData = unknown> = SdkCallToolResult & {
   structuredContent: TData | null
   isError: boolean
 }
 
+export function normalizeCallToolResult<TData = unknown>(
+  result: SdkCallToolResult,
+): CallToolResult<TData> {
+  return {
+    ...result,
+    structuredContent: (result.structuredContent as TData | undefined) ?? null,
+    isError: result.isError === true,
+  }
+}
+
 export type CompletionResult = {
   values: string[]
   total?: number

tests/client/multi-server.test.ts

@@ -1,6 +1,6 @@
 import { describe, it, expect, vi } from 'vitest'
 import { z } from 'zod/v4'
-import { FastMCP } from 'fastmcp-ts/server'
+import { FastMCP, ToolResult } from 'fastmcp-ts/server'
 import { Client, MultiServerClient } from 'fastmcp-ts/client'
 import type { TextResourceContents } from "@modelcontextprotocol/server";
 
@@ -207,6 +207,24 @@ describe('Client — Multi-server', () => {
       const result = await client.callToolRaw('err_fail')
       expect(result.isError).toBe(true)
     })
+
+    it('callToolRaw() preserves metadata and extension fields', async () => {
+      const mcp = new FastMCP({ name: 'extended', version: '1.0.0' })
+      mcp.tool(
+        { name: 'result', description: 'extended result', input: z.object({}) },
+        () =>
+          new ToolResult({
+            content: [{ type: 'text', text: 'extended' }],
+            _meta: { 'com.example/trace': 'trace-123' },
+            'com.example/result': { display: 'card' },
+          }),
+      )
+      await using client = await MultiServerClient.connect({ mcpServers: { extended: mcp } })
+
+      const result = await client.callToolRaw('extended_result')
+      expect(result._meta).toMatchObject({ 'com.example/trace': 'trace-123' })
+      expect(result['com.example/result']).toEqual({ display: 'card' })
+    })
   })
 
   // -------------------------------------------------------------------------

tests/client/tools.test.ts

@@ -1,6 +1,6 @@
 import { describe, it, expect, vi } from 'vitest'
 import { z } from 'zod/v4'
-import { FastMCP, Image } from 'fastmcp-ts/server'
+import { FastMCP, Image, ToolResult } from 'fastmcp-ts/server'
 import { Client, ToolCallError } from 'fastmcp-ts/client'
 import type { Tool } from 'fastmcp-ts/client'
 
@@ -141,6 +141,31 @@ describe('Client — Tools', () => {
         },
       )
     })
+
+    it('preserves metadata and extension fields', async () => {
+      await withServer(
+        (mcp) => {
+          mcp.tool(
+            { name: 'extended', description: 'a tool', input: z.object({}) },
+            () =>
+              new ToolResult({
+                content: [{ type: 'text', text: 'extended' }],
+                _meta: { 'com.example/trace': 'trace-123' },
+                'com.example/result': { display: 'card' },
+              }),
+          )
+        },
+        async (client) => {
+          const raw = await client.callToolRaw('extended', {})
+          expect(raw._meta).toMatchObject({ 'com.example/trace': 'trace-123' })
+          expect(raw['com.example/result']).toEqual({ display: 'card' })
+
+          const result = await client.callTool('extended', {})
+          expect(result._meta).toMatchObject({ 'com.example/trace': 'trace-123' })
+          expect(result['com.example/result']).toEqual({ display: 'card' })
+        },
+      )
+    })
   })
 })