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Issue Content

Problem

When using AsyncPostgresSaver.from_conn_string() with external connection poolers (e.g., Google Cloud SQL connection pooler, PgBouncer in transaction mode), users encounter "prepared statement does not exist" errors:

prepared statement "_pg3_3" does not exist

This occurs because prepare_threshold=0 is hardcoded, which still uses the extended query protocol. Setting prepare_threshold=None resolves the issue.

Proposed Solution

Expose prepare_threshold as an optional parameter:

@classmethod
@asynccontextmanager
async def from_conn_string(
    cls,
    conn_string: str,
    *,
    pipeline: bool = False,
    serde: SerializerProtocol | None = None,
    prepare_threshold: int | None = 0,  # New parameter
) -> AsyncIterator[AsyncPostgresSaver]:

Current Workaround

from psycopg import AsyncConnection
from psycopg.rows import dict_row
from langgraph.checkpoint.postgres import AsyncPostgresSaver

async with await AsyncConnection.connect(
    conn_string, autocommit=True, prepare_threshold=None, row_factory=dict_row
) as conn:
    checkpointer = AsyncPostgresSaver(conn)

Additional Context

This applies to PostgresSaver, ShallowPostgresSaver, and AsyncShallowPostgresSaver as well.