Output is truncated at 65536 bytes when piped to another process

[ZoltanLajosKis]

When I pipe the output of an MCP tool call to another process, it gets truncated at 65536 bytes. When the output is a tty or a file, the whole output is received.

❯ mcp-cli call dummy-mcp text 2>/dev/null | wc -c
65536

❯ mcp-cli call dummy-mcp text 2>/dev/null > /tmp/dummy-mcp-text; wc -c /tmp/dummy-mcp-text
200141 /tmp/dummy-mcp-text

Dummy MCP server (FastMCP 3.0.0b1) for testing:

from fastmcp import FastMCP

mcp = FastMCP("dummy-mcp")

@mcp.tool
def text() -> str:
    return "x" * 100000

if __name__ == "__main__":
    mcp.run()