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Expose get_session_id callback #2486

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Expose get_session_id callback #2486

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09aadc5
[feat] expose get_session_id callback
mathewjhan Nov 25, 2025
ec5546d
[test] add test for session id callback
mathewjhan Nov 25, 2025
407614f
[fix] add test for uninitialized case and default to None
mathewjhan Nov 26, 2025
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9 changes: 8 additions & 1 deletion src/fastmcp/client/transports.py
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Original file line number Diff line number Diff line change
Expand Up @@ -253,6 +253,7 @@ def __init__(
if isinstance(sse_read_timeout, int | float):
sse_read_timeout = datetime.timedelta(seconds=float(sse_read_timeout))
self.sse_read_timeout = sse_read_timeout
self.get_session_id_cb = None

def _set_auth(self, auth: httpx.Auth | Literal["oauth"] | str | None):
if auth == "oauth":
Expand Down Expand Up @@ -287,12 +288,18 @@ async def connect_session(
auth=self.auth,
**client_kwargs,
) as transport:
read_stream, write_stream, _ = transport
read_stream, write_stream, get_session_id = transport
self.get_session_id_cb = get_session_id
async with ClientSession(
read_stream, write_stream, **session_kwargs
) as session:
yield session

def get_session_id(self) -> str | None:
if self.get_session_id_cb:
return self.get_session_id_cb()
return None

Comment on lines +298 to +302
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@coderabbitai coderabbitai bot Nov 25, 2025

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⚠️ Potential issue | 🟠 Major

Guard against missing get_session_id_cb to avoid AttributeError before connection

get_session_id assumes self.get_session_id_cb always exists. If a caller invokes get_session_id() before any connect_session() has run, this will raise AttributeError instead of cleanly returning None, which contradicts the intended “id or None” API.

Initialize the callback on construction and check it explicitly:

@@ class StreamableHttpTransport(ClientTransport):
     def __init__(
         self,
         url: str | AnyUrl,
         headers: dict[str, str] | None = None,
         auth: httpx.Auth | Literal["oauth"] | str | None = None,
         sse_read_timeout: datetime.timedelta | float | int | None = None,
         httpx_client_factory: McpHttpClientFactory | None = None,
     ):
@@
         if isinstance(sse_read_timeout, int | float):
             sse_read_timeout = datetime.timedelta(seconds=float(sse_read_timeout))
         self.sse_read_timeout = sse_read_timeout
+        # Session id callback provided by underlying streamablehttp_client transport
+        self.get_session_id_cb = None
@@
     def get_session_id(self) -> str | None:
-        if self.get_session_id_cb:
-            return self.get_session_id_cb()
-        return None
+        """
+        Return the current MCP session id, or None if no session has been established yet.
+        """
+        cb = self.get_session_id_cb
+        if cb is None:
+            return None
+        return cb()

If you want stricter typing, you can additionally annotate the attribute in __init__:

from typing import Callable  # at top of file

self.get_session_id_cb: Callable[[], str | None] | None = None

This keeps the method safe to call at any time while preserving the desired str | None return contract.

🤖 Prompt for AI Agents 
In src/fastmcp/client/transports.py around lines 297 to 301, get_session_id()
assumes self.get_session_id_cb exists and can raise AttributeError if called
before connect_session(); initialize self.get_session_id_cb = None in __init__
(and optionally annotate it as Callable[[], str | None] | None after importing
Callable) and change get_session_id() to explicitly check "if
self.get_session_id_cb is not None:" before calling it, returning None otherwise
so the method always returns str | None without raising.

def __repr__(self) -> str:
return f"<StreamableHttpTransport(url='{self.url}')>"

Expand Down
9 changes: 9 additions & 0 deletions tests/client/test_streamable_http.py
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Original file line number Diff line number Diff line change
Expand Up @@ -175,6 +175,15 @@ async def test_http_headers(streamable_http_server: str):
assert json_result["x-demo-header"] == "ABC"


async def test_session_id_callback(streamable_http_server: str):
"""Test getting mcp-session-id from the transport."""
transport = StreamableHttpTransport(streamable_http_server)
assert transport.get_session_id() is None
async with Client(transport=transport):
session_id = transport.get_session_id()
assert session_id is not None


@pytest.mark.parametrize("streamable_http_server", [True, False], indirect=True)
async def test_greet_with_progress_tool(streamable_http_server: str):
"""Test calling the greet tool."""
Expand Down
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