Ensure client is only initialized once #758
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Pull Request Overview
This PR ensures that the client initializes only once by adjusting test expectations and modifying client initialization behavior. Key changes include:
- Updating expected text results in several server interaction tests to reflect a single initialization.
- Adding a new test to verify that the initialize method is called only once.
- Introducing a new parameter (“raise_exceptions” in FastMCPTransport and “client_info” in Client) to improve error handling and information flow.
Reviewed Changes
Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.
| File | Description |
|---|---|
| tests/server/test_server_interactions.py | Updated expected response values reflecting single initialization. |
| tests/client/test_client.py | Added a new test ensuring initialize is invoked only once. |
| src/fastmcp/client/transports.py | Introduced raise_exceptions parameter and reworked session connection logic. |
| src/fastmcp/client/client.py | Added client_info parameter and forwarded it within session kwargs. |
Comments suppressed due to low confidence (2)
src/fastmcp/client/transports.py:654
- Consider updating the docstring of FastMCPTransport.init to document the new 'raise_exceptions' parameter and its intended use.
def __init__(self, mcp: FastMCP | FastMCP1Server, raise_exceptions: bool = False):
src/fastmcp/client/client.py:148
- Update the Client.init docstring to include a description for the 'client_info' parameter and explain its purpose.
client_info: mcp.types.Implementation | None = None,
jordicore
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Jul 2, 2025
Ensure client is only initialized once
wkpark
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Oct 13, 2025
In-memory transport is officially supported by FastMCP v2. However, FastMCP v1 already supports in-memory transport (but undocumented) See also: jlowin/fastmcp#758
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In-memory transports were inadvertently sending the initialization request twice - once in the low-level SDK and once in FastMCP. This also explains a confusing request_id increment that we couldn't understand last month!