Anthropic provider: empty tool_result content crashes with 'content field is empty' API error

[isc-tdyar]

Bug Description

When a tool call returns an empty string, the Anthropic provider passes content: "" directly into the tool_result content block. The Anthropic API rejects this with:

Error: The content field in the Message object at messages.N is empty. Add a ContentBlock object to the content field and try again.

Location

packages/console/app/src/routes/zen/util/provider/anthropic.ts, in the role === "tool" branch:

if ((m as any).role === "tool") {
  msgsOut.push({
    role: "user",
    content: [
      {
        type: "tool_result",
        tool_use_id: (m as any).tool_call_id,
        content: (m as any).content,  // ← empty string passes through unchecked
        ...cc(),
      },
    ],
  })
}

Reproduction

Any tool that returns an empty string (e.g. a write-only tool, a command with no stdout, or a tool that errors silently) will trigger this. The error identifies the exact message index (e.g. messages.13), making it look like a history corruption issue rather than a missing guard.

Fix

content: (m as any).content || "(no output)",

Or more robustly, filter/normalize before building the block:

const toolContent = (m as any).content
content: (typeof toolContent === "string" && toolContent.length > 0)
  ? toolContent
  : Array.isArray(toolContent) && toolContent.length > 0
  ? toolContent
  : "(no output)",

Environment

• opencode (console/zen provider path)
• Anthropic API (claude-sonnet-4-6, claude-opus-4-6)
• Triggered while running oc-dcp agent loop against a project with tools that produce no stdout