Server tool results missing from tool-call output field in UIMessage

[flybayer]

TanStack AI version

0.2.0

Framework/Library version

react 18.2.0

Describe the bug and the steps to reproduce it

Summary: Server-side tool results don't update the tool-call part's output field, causing inconsistent UI state between server and client tool execution.

Expected behavior: Both server-executed tools and client-executed tools should have their results visible on the tool-call part's output field, and the state should transition to 'complete'.

Actual behavior:

• Client tools: tool-call part shows state: 'complete' with output populated
• Server tools: tool-call part shows state: 'input-complete' with no output (result only exists in a separate tool-result part)

Root cause: In StreamProcessor, handleToolResultChunk() only creates a tool-result part via updateToolResultPart(), but doesn't update the corresponding tool-call part's output field or state. In contrast, addToolResult() (used for client tools) does both:

// addToolResult() does this (lines 283-300):
let updatedMessages = updateToolCallWithOutput( // Updates tool-call output
 this.messages,
 toolCallId,
 output,
 error ? 'input-complete' : undefined,
 error,
)
updatedMessages = updateToolResultPart(...) // Also creates tool-result part

// handleToolResultChunk() only does this (lines 691-699):
this.messages = updateToolResultPart(...) // Only creates tool-result part
// Missing: updateToolCallWithOutput() call

Steps to reproduce:

1. Define a tool with toolDefinition() that has both inputSchema and outputSchema
2. Create a server-side implementation using .server() with an execute function
3. Create a client-side implementation using .client() with an execute function
4. Call both tools in a chat session
5. Observe the UIMessage.parts - client tool-call parts have output and state: 'complete', server tool-call parts only have state: 'input-complete' with no output

Example to reproduce

import { toolDefinition, chat } from '@tanstack/ai'
import { z } from 'zod'

// Shared tool definition
const myToolDef = toolDefinition({
 name: 'myTool',
 description: 'A test tool',
 inputSchema: z.object({ input: z.string() }),
 outputSchema: z.object({ result: z.string() }),
})

// Server tool - has execute, runs on server
const serverTool = myToolDef.server(async (args) => {
 return { result: `Server processed: ${args.input}` }
})

// Client tool - has execute, runs on client
const clientTool = myToolDef.client(async (args) => {
 return { result: `Client processed: ${args.input}` }
})

// When server tool executes:
// - StreamProcessor receives tool_result chunk
// - handleToolResultChunk() creates tool-result part
// - tool-call part remains at state: 'input-complete', output: undefined ❌

// When client tool executes:
// - StreamProcessor receives tool-input-available chunk
// - Client executes tool and calls addToolResult()
// - addToolResult() updates tool-call part with output AND creates tool-result part
// - tool-call part shows state: 'complete', output: {...} ✅

Screenshots or Videos (Optional)

In the screenshot:

• getCurrentPipelineConfig (client tool): Shows "complete" status with RESULT section visible
• getPipelineSchema (server tool): Shows "input-complete" status with no RESULT section
• validatePipelineConfig (server tool): Shows "input-complete" status with no RESULT section
• applyPipelineConfig (client tool): Shows "complete" status

Suggested Fix

In processor.ts, handleToolResultChunk() should also update the tool-call part similar to addToolResult():

private handleToolResultChunk(
 chunk: Extract<StreamChunk, { type: 'tool_result' }>,
): void {
 const state: ToolResultState = 'complete'

 // Emit legacy handler
 this.handlers.onToolResultStateChange?.(
 chunk.toolCallId,
 chunk.content,
 state,
 )

 // Update UIMessage if we have a current assistant message
 if (this.currentAssistantMessageId) {
 // NEW: Also update the tool-call part's output field
 const output = this.parseToolResultContent(chunk.content)
 this.messages = updateToolCallWithOutput(
 this.messages,
 chunk.toolCallId,
 output,
 )
 
 // Existing: Create tool-result part
 this.messages = updateToolResultPart(
 this.messages,
 this.currentAssistantMessageId,
 chunk.toolCallId,
 chunk.content,
 state,
 )
 this.emitMessagesChange()
 }
}

Do you intend to try to help solve this bug with your own PR?

None

Terms & Code of Conduct

• I agree to follow this project's Code of Conduct
• I understand that if my bug cannot be reliable reproduced in a debuggable environment, it will probably not be fixed and this issue may even be closed.

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🔗 Related PRs

#141 - Add TanStack Start server code example for chat API [merged]

👤 Suggested Assignees

• harlley

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[AlemTuzlak]

@jherr is this a part of your tool hardening fixes?

[flybayer]

this is still an issue for me

[jherr]

Possibly. I'll have a look.

[AlemTuzlak]

@flybayer I know it's been a while but I think I finally fixed the issue you were running into, would you mind confirming?