Summary

• preserve MCP image content blocks instead of collapsing image-only tool results to blank output
• cache decoded image payloads and emit MEDIA: paths plus a media array in the tool result
• add a regression test covering image-only CallToolResult.content

Problem

Some MCP servers return images as content blocks with data + mimeType and no text.

Hermes' MCP bridge only preserved text blocks and structuredContent, so image-only results were reduced to an empty string. In practice this made tools like Grafana panel image rendering appear blank even though the upstream MCP server had generated a valid PNG.

Root cause

In tools/mcp_tool.py, _make_tool_handler() iterated result.content but only appended blocks exposing .text.

Image blocks were ignored completely.

Fix

• detect image blocks via data + mimeType
• accept both plain base64 and data-URI payloads
• decode and cache the image via cache_image_from_bytes(...)
• return MEDIA:<path> in the text payload and expose a media list in the JSON result

Verification

• pytest -q tests/tools/test_mcp_structured_content.py