[Feature] Add Fetch tool that saves to file (like Claude Code)

[jdillon]

Problem

The built-in webfetch tool returns entire page content directly into conversation context. For documentation pages (TanStack, MDN, React docs, etc.), this frequently exceeds the 200k token limit:

prompt is too long: 207654 tokens > 200000 maximum

This happens within a single tool response - the page itself is too large. Mid-turn compaction (#6480) can't help because overflow occurs before compaction can trigger.

Proposed Solution

Add a Fetch tool similar to Claude Code's implementation:

1. Downloads content to a temp file (e.g., .opencode/tmp/fetch-{hash}.md)
2. Returns only metadata to context:
   • File path
   • Title
   • Content length
   • First ~500 chars as preview
3. Agent uses read tool to access content in chunks as needed

Claude Code's approach:

• Fetch downloads to temp, returns path + summary
• Agent reads sections as needed using standard file tools
• Large pages never blow context because content lives on disk

Benefits

• Any page size works - content is on disk, not in context
• Agent reads selectively - only loads what's relevant
• Consistent with existing read tool patterns for large files
• Solves a class of problems, not just individual pages

Workaround Question

Is there an existing custom tool or plugin that implements this pattern?

I see custom tools can be created in .opencode/tool/ - has anyone published a fetch-to-file tool that could serve as a workaround until this is built-in? If not, is there a recommended approach for implementing this as a custom tool?

Related Issues

• #4845 (prompt too long)
• #5360, #5478 (duplicates)
• #6480 (mid-turn compaction - helps multi-turn but not single large fetches)