`mcp_server_name` and `mcp_tool_name` are not populated in Python SDK

[idryzhov]

ToolExecutionStartData class has mcp_server_name and mcp_tool_name fields, but they're not populated when this event is received, even if the tool is MCP tool.

[github-actions]

Investigation Findings

I investigated the Python SDK's handling of ToolExecutionStartData and the mcp_server_name / mcp_tool_name fields.

What I looked at

• python/copilot/generated/session_events.py — the ToolExecutionStartData dataclass and its from_dict deserialization method
• The equivalent implementations in Go (generated_session_events.go), .NET (Generated/SessionEvents.cs), and Node.js (src/generated/session-events.ts)
• Test snapshots in test/snapshots/ for any tool.execution_start events with MCP metadata

What I found

The Python SDK deserialization code is correct — ToolExecutionStartData.from_dict explicitly reads mcpServerName and mcpToolName from the JSON payload:

mcp_server_name = from_union([from_none, from_str], obj.get("mcpServerName"))
mcp_tool_name   = from_union([from_none, from_str], obj.get("mcpToolName"))

These fields will be None only if the CLI runtime does not include them in the tool.execution_start event data. All four SDKs (Python, Go, .NET, Node.js) have the same fields defined and will be equally affected.

Root cause assessment

The schema documents mcpServerName and mcpToolName as fields that should be populated "when the tool is an MCP tool". If those fields are consistently None even when an MCP tool is being invoked, the bug is in the CLI runtime not emitting these fields in the tool.execution_start event — the SDK correctly passes through whatever the runtime sends.

No test snapshots exist that cover tool.execution_start events with MCP tool metadata, so this code path has no regression test coverage.

Label

Marking as bug since the described behavior contradicts the schema documentation. If confirmed, the fix would need to be in the CLI runtime (to emit these fields), and ideally accompanied by a snapshot-based test in this repo to prevent regression.

Generated by Bug Handler for issue #1200 · ● 1.5M · ◷