Could not use Proposed API `mcpServerDefinitions` in extension

[Alive-Fish]

Does this issue occur when all extensions are disabled?: Yes/No

Type: Bug

• Use vsce package to pack a VSIX which uses vscode.lm.startMcpGateway.
• Install the above VSIX under a machine and start with code-insiders . --enable-proposed-api=TeamsDevApp.ms-teams-vscode-extension. Followed instructions.
• When use the command which calls vscode.lm.startMcpGateway, it will show the error as below.

Error starting MCP Gateway: Error: Extension 'TeamsDevApp.ms-teams-vscode-extension' CANNOT use API proposal: mcpServerDefinitions. Its package.json#enabledApiProposals-property declares: chatParticipantAdditions, languageModelSystem but NOT mcpServerDefinitions. The missing proposal MUST be added and you must start in extension development mode or use the following command line switch: --enable-proposed-api TeamsDevApp.ms-teams-vscode-extension

Some details:

• Extension has added mcpServerDefinitions under package.json
• Extension change code is here
• Local debugging the extension can normally use vscode.lm.startMcpGateway.

Extension version: 6.4.3
VS Code version: Code - Insiders 1.110.0-insider (f5927e7, 2026-02-28T04:37:38.428Z)
OS version: Windows_NT x64 10.0.26100
Modes:

System Info

Item	Value
CPUs	Intel(R) Xeon(R) Platinum 8370C CPU @ 2.80GHz (16 x 2793)
GPU Status	2d_canvas: unavailable_software GPU0: VENDOR= 0x1414, DEVICE=0x008c [Microsoft Basic Render Driver], DRIVER_VENDOR=Google, DRIVER_VERSION=10.0.26100.7309 GPU1: VENDOR= 0x1414, DEVICE=0x008c [Microsoft Basic Render Driver], DRIVER_VERSION=10.0.26100.7309 GPU2: VENDOR= 0x1414, DEVICE=0x008c [Microsoft Basic Render Driver], DRIVER_VERSION=10.0.26100.7309 Machine model name: Machine model version: direct_rendering_display_compositor: disabled_off_ok gpu_compositing: disabled_software multiple_raster_threads: enabled_on opengl: disabled_off rasterization: disabled_software raw_draw: disabled_off_ok skia_graphite: disabled_off trees_in_viz: disabled_off video_decode: disabled_software video_encode: disabled_software webgl: unavailable_software webgl2: unavailable_software webgpu: unavailable_software webnn: unavailable_software
Load (avg)	undefined
Memory (System)	63.95GB (48.44GB free)
Process Argv	. --enable-proposed-api TeamsDevApp.ms-teams-vscode-extension --crash-reporter-id 78b6575b-2b6e-4744-b445-ae8f018a8fb4
Screen Reader	no
VM	0%

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