MCP integration: prompts/get without arguments crashes with 'NoneType' object has no attribute 'items' (mcp v1 server, positional arguments=None)

[godwinbw-egm]

How do you use Sentry?

Sentry Saas (sentry.io)

Version

2.68.0 and 2.68.1 (latest); the code is unchanged on master (692cca25, 2026-09-04)

Steps to Reproduce

Minimal FastMCP (3.4.7, mcp 1.29.x) server with a prompt and sentry_sdk.init active (the MCP integration is auto-enabled):

import sentry_sdk
from fastmcp import FastMCP

sentry_sdk.init(dsn="https://examplePublicKey@o0.ingest.sentry.io/0")
mcp = FastMCP("repro")

@mcp.prompt
def guide() -> str:
    return "hello"

mcp.run(transport="http", host="127.0.0.1", port=8765)

Call prompts/get without arguments (spec-legal — the field is optional), e.g. with the SDK client:

async with streamablehttp_client("http://127.0.0.1:8765/mcp") as (r, w, _):
    async with ClientSession(r, w) as s:
        await s.initialize()
        await s.get_prompt("guide")          # arguments omitted -> fails
        await s.get_prompt("guide", {})      # arguments {}      -> renders

or raw JSON-RPC after initialize / notifications/initialized:

{"jsonrpc":"2.0","id":2,"method":"prompts/get","params":{"name":"guide"}}

Expected Result

The prompt renders (GetPromptResult with one message), exactly as it does when sentry_sdk.init is not called, or when arguments: {} is sent.

Actual Result

{"jsonrpc":"2.0","id":2,"error":{"code":0,"message":"'NoneType' object has no attribute 'items'"}}

Cause, sentry_sdk/integrations/mcp.py (2.68.1):

• The mcp v1 low-level server calls the prompt handler positionally as func(req.params.name, req.params.arguments) (mcp/server/lowlevel/server.py:294), so original_args == (name, None) when the client omitted arguments.
• _extract_handler_data_from_args, handler_type == "prompt" branch (:289-298): arguments = {} … if len(original_args) > 1: arguments = original_args[1] — the None is taken as-is. Only the kwargs branch (elif original_kwargs.get("arguments")) is falsy-guarded.
• _set_span_input_data then does for k, v in arguments.items() (:206) → AttributeError, which the server returns to the client as a JSON-RPC error.

The tool branch has the same shape at :284-285 (arguments = original_args[1]), though tool calls in practice always carry an arguments dict.

Measured 2026-09-04 (fastmcp 3.4.7, mcp 1.29.1, sentry-sdk 2.68.1, Python 3.12):

sentry=False bare     -> OK messages=1
sentry=False empty    -> OK messages=1
sentry=False nonempty -> OK messages=1
sentry=True  bare     -> FAIL mcp.shared.exceptions.McpError: 'NoneType' object has no attribute 'items'
sentry=True  empty    -> OK messages=1
sentry=True  nonempty -> OK messages=1
``` We hit this fleet-wide through an MCP gateway (IBM ContextForge) that forwards `prompts/get` with `arguments` omitted whenever the downstream caller sent none or `{}`; every backend with a `SENTRY_DSN` rejected it.

### Suggested fix

```python
        if len(original_args) > 1:
            arguments = original_args[1] or {}

(and the same or {} on the tool branch's positional read), or a None guard in _set_span_input_data before .items().

[linear-code]

PY-2762

[alexander-alderman-webb]

Thanks will fix this now