[Python] Streaming MCP tool (FastMCP streamable-http). Providing client with generated pieces...

I'm trying to create an MCP server on the basis of streamable-HTTP version of MCP. My tool has to provide client with some pieces of info (primitives, objects, whatever). But I cannot build stream of these objects from server to MCP client.

This is my minimalistic MCP tool:

from mcp.server.fastmcp import FastMCP, Context
from pydantic import Field

@mcp.tool()
async def number_stream(count: int, delay: int, ctx: Context):
for i in range(count):
await ctx.info(f"increment: {i}")
await asyncio.sleep(delay)
mcp.run(transport="streamable-http")


It works fine, it sends log events to client this looks like this as a tcp-flow:

POST /mcp/ HTTP/1.1
host: localhost:8000
connection: keep-alive
accept: application/json, text/event-stream
content-type: application/json
accept-language: *
sec-fetch-mode: cors
user-agent: node
accept-encoding: gzip, deflate
content-length: 142

{"jsonrpc":"2.0","id":3,"method":"tools/call","params":{"_meta":{"progressToken":3},"name":"number_stream","arguments":{"count":3,"delay":2}}}
HTTP/1.1 200 OK
date: Wed, 25 Jun 2025 09:45:16 GMT
server: uvicorn
cache-control: no-cache, no-transform
connection: keep-alive
content-type: text/event-stream
x-accel-buffering: no
Transfer-Encoding: chunked
event: message
data: {"method":"notifications/message","params":{"level":"info","data":"increment: 0"},"jsonrpc":"2.0"}
event: message
data: {"method":"notifications/message","params":{"level":"info","data":"increment: 1"},"jsonrpc":"2.0"}
event: message
data: {"method":"notifications/message","params":{"level":"info","data":"increment: 2"},"jsonrpc":"2.0"}
event: message
data: {"jsonrpc":"2.0","id":3,"result":{"content":[],"isError":false}}


But I don't see how I could send sequentially generated objects (ints in this case, but imagine that instead of simple ints those are POJOs) in a natural streaming way. OK, I suppose I could serialize objects into json strings one by one and put them into info-message's data field. But it does not look neat and nice. And I don't see any methods for Context class that are intended to make such stream of generated objects from MCP server.

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