Document dict-to-Message prompt migration in upgrade guides#3225
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Both the MCP SDK's bundled FastMCP 1.0 and standalone FastMCP v2 silently coerced raw dicts (
{"role": "user", "content": "..."}) into prompt messages. v3 requires typedMessageobjects or plain strings, and raisesTypeErrorfor dicts. This bit our own integration tests (PrefectHQ/horizon-integration-tests#36), so it'll bite others too.Adds the dict migration path to both upgrade guides (
from-fastmcp-2andfrom-mcp-sdk) with before/after examples and LLM prompt updates.