MCP tools crash: TypeError on value.output.output in processor.ts (AI SDK 6.0.74 breaking change)

[kil-penguin]

Bug Description

All MCP tool calls crash with:

TypeError: undefined is not an object (evaluating 'output.output.toLowerCase')

This affects every MCP tool (Slack, fetch, grep_app, etc.) — 100% reproducible.

Root Cause

AI SDK v6.0.74 (released Feb 6, 2026) introduced createToolModelOutput() in stream-text.ts (line 1270) which changed the tool-result output structure:

Before (what OpenCode expects):

value.output = {
  output: string,
  metadata: Record<string, any>,
  title: string,
  attachments?: FilePart[]
}

After (what AI SDK now returns):

value.output = {
  type: 'text' | 'json' | 'error-text' | 'error-json' | 'execution-denied' | 'content',
  value: string | JSONValue,
  providerOptions?: ProviderOptions
}

Crash Location

packages/opencode/src/session/processor.ts — tool-result case:

case "tool-result": {
  const match = toolcalls[value.toolCallId]
  if (match && match.state.status === "running") {
    await Session.updatePart({
      ...match,
      state: {
        status: "completed",
        input: value.input ?? match.state.input,
        output: value.output.output,       // ← undefined.toLowerCase() crashes here
        metadata: value.output.metadata,   // ← undefined
        title: value.output.title,         // ← undefined
        attachments: value.output.attachments,
      },
    })
  }
}

Full Data Flow

1. MCP tool execute (mcp/index.ts → convertMcpTool()) returns MCP SDK CallToolResult: { content: [{type: "text", text: "..."}], isError? }
2. AI SDK execute-tool-call.ts wraps it as { type: 'tool-result', output: <raw result> }
3. AI SDK stream-text.ts (line 1270) now passes output through createToolModelOutput() → transforms to { type: 'text', value: '...' }
4. OpenCode processor.ts tries to access .output, .metadata, .title on the new structure → crash

Why It Wasn't Caught

• OpenCode's built-in tools return { output, title, metadata } from their execute functions
• createToolModelOutput() wraps ALL tool outputs (including MCP) into the new ToolResultOutput shape
• The processor was never updated to handle the new shape

Suggested Fix

Option A: Convert MCP result in mcp/index.ts (recommended)

// In convertMcpTool(), wrap the execute return:
execute: async (args: unknown) => {
  const result = await client.callTool(...)
  const textContent = result.content
    ?.filter((c: any) => c.type === "text")
    .map((c: any) => c.text)
    .join("\n") ?? ""
  return {
    output: textContent,
    title: mcpTool.name,
    metadata: {},
  }
},

Option B: Handle new output shape in processor.ts

case "tool-result": {
  const match = toolcalls[value.toolCallId]
  if (match && match.state.status === "running") {
    const raw = value.output ?? {}
    // Handle both old format { output, metadata, title } and new AI SDK format { type, value }
    const output = raw.output ?? raw.value ?? (typeof raw === "string" ? raw : JSON.stringify(raw))
    const metadata = raw.metadata ?? {}
    const title = raw.title ?? match.tool
    await Session.updatePart({
      ...match,
      state: {
        status: "completed",
        input: value.input ?? match.state.input,
        output,
        metadata,
        title,
        time: { start: match.state.time.start, end: Date.now() },
        attachments: raw.attachments,
      },
    })
  }
}

Environment

• OpenCode: v1.1.56
• AI SDK: 6.0.74 (from pnpm catalog)
• @modelcontextprotocol/sdk: 1.25.2
• OS: macOS (darwin)

Reproduction

1. Configure any MCP server in opencode.json
2. Call any MCP tool
3. Crash occurs immediately on tool result processing

[github-actions]

---

This issue appears to be a duplicate of #12987, which reports the identical error with MCP tools:

• Kubernetes MCP server tools fail with TypeError: output.output.toLowerCase #12987: Kubernetes MCP server tools fail with TypeError: output.output.toLowerCase

Both issues report the same root cause: the AI SDK v6.0.74 breaking change in how tool output is structured. Your issue (#13042) provides more comprehensive root cause analysis and suggested fixes, which will be valuable for resolving both.

Please consider reviewing #12987 to see if the context there adds any additional details, and the issue assignee (rekram1-node) should be aware of both reports.

[ahmed-rezk-dev]

I have the same issue

[vendys-sungwan]

same + 1

[Dis2017]

+1

[nus-rick]

+1

[Ctrl30]

+1

[My-Walker]

+1

[ystgd07]

+1

[rekram1-node]

This is an oh my opencode bug not opencode