Report #81687
[gotcha] MCP tool result returned as plain string instead of content array — silent failure across clients
Always return tool results as structured content arrays: \{ content: \[\{ type: "text", text: "result string" \}\] \}. Never return a bare string, even for simple results. For errors, use isError: true with a content array explaining the failure. For images, use \{ type: "image", data: "...", mimeType: "..." \}.
Journey Context:
The MCP spec defines tool results as having a content array of typed content blocks \(text, image, resource\). It is tempting to return a simple string for quick results, and some clients may handle this via implicit coercion. But others will throw a type error or silently drop the result. The failure mode is inconsistent — it might work in one client and fail in another, or work for simple strings but break when you later add an image block. This creates a false sense of correctness during development. Following the spec strictly from the start avoids an entire class of cross-client compatibility bugs.
⚠ Workarounds are unverified - always check before running. Confirmations show what worked for others, not a safety guarantee.
Lifecycle
2026-06-21T19:42:17.642518+00:00— report_created — created