Bug: relevance_patch_agent saves literal file reference strings instead of erroring

[charliebirch]

Summary

When passing a file path reference like {{FILE:/tmp/prompt_for_patch.txt}} as a value to relevance_patch_agent, the tool saves the literal string {{FILE:/tmp/prompt_for_patch.txt}} as the field value instead of either resolving the file or returning an error.

This silently overwrote a 35,858-character system prompt with a 34-character garbage string, breaking a production agent.

Steps to Reproduce

1. Call relevance_patch_agent with:

   {
     "agent_id": "...",
     "patch": {"system_prompt": "{{FILE:/tmp/prompt_for_patch.txt}}"}
   }

2. The tool returns "patched": true with system_prompt_length: 34
3. The agent's system prompt is now the literal string {{FILE:/tmp/prompt_for_patch.txt}}

Expected Behavior

Either:

• Option A (preferred): Reject the input with an error if the value looks like a file reference template (matches {{FILE:...}} pattern)
• Option B: Document clearly that MCP tool parameters are always literal values with no template resolution

Impact

• Severity: High — silently destroys agent config with no warning
• The tool returns "patched": true which makes it look like success
• Only the system_prompt_length in the response reveals the problem (34 vs expected 35,858)

Suggested Fix

Add input validation: if any string value in patch matches {{FILE:...}} or similar template patterns, return an error explaining that file references are not supported and the actual content must be passed directly.