`list_mcp_tools` hangs when MCP endpoint returns plain-text error as `text/event-stream`

[51acorsi]

Describe the Bug

Problem

When an AGW MCP endpoint returns 200 text/event-stream with a plain-text body (e.g. Jwt issuer is not configured) instead of a valid SSE data: event, await session.list_tools()
in list_server_tools() blocks indefinitely.

The MCP SSE parser discards lines without a data: prefix — silently, with no error. The ClientSession then waits forever for a JSON-RPC response that will never arrive. The httpx
timeout does not protect against this because the HTTP response itself completes successfully in ~1s.

Since all fragments run concurrently via asyncio.gather, one hanging fragment blocks the entire list_mcp_tools() call until the outer app-level timeout fires (up to 180s).

Proposed fixes

1. Per-fragment timeout (defensive minimum)

result = await asyncio.wait_for(session.list_tools(), timeout=30.0)

2. Race against stream closure (detects the problem immediately)
done, _ = await asyncio.wait(
    [session.list_tools(), stream_closed_event],
    return_when=asyncio.FIRST_COMPLETED
)
# if stream closed before list_tools() responded → raise immediately

3. Validate SSE body before entering ClientSession
first_chunk = await response.aread(64)
if not first_chunk.startswith((b"data:", b"event:")):
    raise McpError(f"Non-SSE response: {first_chunk.decode(errors='replace')}")

Context

- Reproduced on sap.btpn8n:apiResource:ManagedN8nMcpServer:v1, AGW EU12
- The upstream fix (return 4xx or a proper SSE error event) is tracked separately
- This SDK fix is defensive and needed regardless of the upstream fix

### Steps to Reproduce

### Reproduction

Send `initialize` followed by `tools/list` to any MCP endpoint that returns a plain-text error:

```bash
# initialize — works, returns valid SSE
curl -X POST "<mcp-endpoint>" \
  -H "Authorization: Bearer <token>" \
  -H "Content-Type: application/json" \
  -H "Accept: application/json, text/event-stream" \
  -d '{"jsonrpc":"2.0","id":0,"method":"initialize","params":{"protocolVersion":"2025-11-25","capabilities":{},"clientInfo":{"name":"test","version":"1.0"}}}'

# tools/list — returns 200 but body is plain text, not an SSE data: event
curl -X POST "<mcp-endpoint>" \
  -H "Authorization: Bearer <token>" \
  -H "Content-Type: application/json" \
  -H "Accept: application/json, text/event-stream" \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/list","params":{}}'

Actual tools/list response:
HTTP/2 200
content-type: text/event-stream

<plain text error message>

Expected tools/list response:
HTTP/2 200
content-type: text/event-stream

event: message
data: {"jsonrpc":"2.0","id":1,"result":{"tools":[...]}}

The body lacks the data: prefix required by the SSE spec. The MCP SSE parser silently discards it, leaving ClientSession blocked on a queue that will never receive a message.

Expected Behavior

Exception must be thrown, but it should not wait indefinitely for a response.

Screenshots

No response

Used Versions

• Python version via python --version: ...
• SAP Cloud SDK for Python version: ...
• Framework version (if applicable, e.g., Flask, FastAPI): ...

Installed packages via pip list or uv pip list

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Code Examples

# Your code here

Stack Trace

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Log File

Log file

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Affected Development Phase

Getting Started

Impact

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Timeline

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