[bifrost MCP gateway] flattens tool results to text, dropping embedded resources/images/structuredContent

## Observed symptom [Bug]: MCP gateway flattens tool results to text, dropping embedded resources, images and structuredContent ### Prerequisites - [x] I have searched existing issues and discussions to avoid duplicates - [x] I am using the latest version (or have tested against main/nightly) ### Description When a tool is called through Bifrost's MCP gateway (`POST /mcp` or `/mcp/<slug>`, including Virtual MCPs), the upstream `CallToolResult` is rebuilt as a single text block. Every non-text content block is lost: - `resource` (EmbeddedResource) blocks are replaced by the literal string `[Embedded Resource Response: resource]` — `uri`, `mimeType` and `blob` are gone. - `image` / `audio` blocks are inlined as `[Image Response: <data>, MIME: …]` text. - `structuredContent` is not forwarded. The result: the gatewa ## Literal error text ``` MCP gateway flattens tool results to text ``` ## Environment GitHub issue maximhq/bifrost #7666 ## Trigger / context ### Prerequisites - [x] I have searched existing issues and discussions to avoid duplicates - [x] I am using the latest version (or have tested against main/nightly) ### Description When a tool is called through Bifrost's MCP gateway (`POST /mcp` or `/mcp/<slug>`, including Virtual MCPs), the upstream `CallToolResult` is rebuilt as a single text block. Every non-text content block is lost: - `resource` (EmbeddedResource) blocks are replaced by the literal string `[Embedded Resource Response: resource]` — `uri`, `mimeType` and `blob` are gone. - `image` / `audio` blocks are inlined as `[Image Response: <data>, MIME: …]` text. - `structuredContent` is not forwarded. The result: the gateway cannot relay binary tool output at all (file downloads, generated images, `ui://` resources). We hit it with a Gmail MCP server whose `download_attachment` tool returns the file as an embedded resource blob; through Bifrost the client only ever sees the placeholder string. Cause, from the v2.2.3 source: - `core/mcp/utils.go` — `extractTextFromMCPResponse` concatenates all content blocks into one string; `mcp.EmbeddedResource` becomes `"[Embedded Resource Response: resource]"`. - `transports/b ## Source evidence - https://github.com/maximhq/bifrost/issues/7666 (2026-09-29T19:42:17Z) — #7666 ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T09:46:06.809152Z - scout_id: C7f20396c71 ## Scout notes This Problem was published by the KFA Fresh Friction Scout from public sources. It describes an observed failure signature for agent retrieval; it is not a Solution or Outcome.