MCP tools and vscode.lm.registerTool tools silently dropped in Agent mode on Remote SSH host (regression of #317992, still broken in 1.128)

[SylvanBrocard]

Type: Bug

MCP Tools and Extension-Registered Tools Not Injected into Agent Session on Remote SSH (VS Code 1.128 — Regression of #317992)

Type: Bug

---

Description

On VS Code 1.128.0, both MCP server tools and tools registered via vscode.lm.registerTool are silently absent from the Copilot Agent session when running under a Remote SSH extension host. The servers start successfully, tools are discovered, but none are injected into the agent context.

This appears to be a regression — issue #317992 was marked as fixed in 1.126, but the problem persists on 1.128.

---

Affected Tool Sources

Tool Source	Type	Works?
github/github-mcp-server (Docker stdio, remote ext host)	MCP	❌
sehejjain.lsp-mcp-bridge (via vscode.lm.registerTool)	Extension API	❌
context7 MCP (local extension host)	MCP	✅

The common factor for the broken tools is that they are both spawned from / registered in the remote extension host (Remote SSH). The working tool (context7) runs on the local extension host.

---

Steps to Reproduce

1. Connect to a remote machine via Remote SSH
2. Configure an MCP server in workspace/remote settings (e.g. GitHub MCP via Docker stdio):

"github": {
    "command": "docker",
    "args": ["run", "-i", "--rm", "-e", "GITHUB_PERSONAL_ACCESS_TOKEN", "-e", "GITHUB_HOST", "ghcr.io/github/github-mcp-server"],
    "env": {
        "GITHUB_PERSONAL_ACCESS_TOKEN": "<token>",
        "GITHUB_HOST": "https://your-ghe-host"
    },
    "type": "stdio"
}

3. Install an extension that registers tools via vscode.lm.registerTool on the remote (e.g. sehejjain.lsp-mcp-bridge)
4. Open Copilot Chat in Agent mode
5. Observe that neither the MCP tools nor the extension-registered tools appear in the agent session

---

Expected Behaviour

All discovered/registered tools from the remote extension host should be injected into the agent session, as they were prior to 1.126.

---

Actual Behaviour

• MCP server starts correctly, 38 tools discovered (confirmed in output log)
• Extension-registered tools visible in Tool Configuration UI
• Neither set of tools appear in the agent debug tool list
• A locally-hosted MCP server (context7) works correctly in the same session

Server log confirms healthy startup:

[info] Starting server from Remote extension host
[info] Discovered 38 tools
time=... level=INFO msg="session initialized"

Agent debug view shows 54 built-in tools only — no github/* or LSP tools present.

---

Attempted Workarounds

• "github.copilot.chat.virtualTools.threshold": 0 — no effect
• Developer: Reload Window — no effect
• Tools picker shows toolset enabled with checkmark — no effect

---

Environment

• VS Code: 1.128.0 (fc3def6774c76082adf699d366f31a557ce5573f, 2026-07-07)
• Remote OS: Linux x64 6.8.0-90-generic (Intel Xeon, 8 CPUs, 31GB RAM, ~40% VM)
• Local OS: Windows 11 ARM64 (Snapdragon X Elite, 12 CPUs, 31.56GB RAM)
• Remote extension: Remote - SSH 0.124.0
• GitHub Copilot Chat: (see extensions list)
• Relevant extensions on remote: sehejjain.lsp-mcp-bridge 1.0.1, github.vscode-pull-request-github 0.156.0

---

Related Issues

• Regression since VS Code 1.119.1: many Azure DevOps MCP tools no longer surface in Copilot Chat Agent mode #317992 — marked fixed in 1.126, appears incomplete for Remote SSH configuration

VS Code version: Code 1.128.0 (fc3def6, 2026-07-07T15:14:24-07:00)
OS version: Windows_NT arm64 10.0.26100
Modes:
Remote OS version: Linux x64 6.8.0-90-generic

System Info

Item	Value
CPUs	Snapdragon(R) X Elite - X1E78100 - Qualcomm(R) Oryon(TM) CPU (12 x 2976)
GPU Status	2d_canvas: enabled GPU0: VENDOR= 0x4d4f4351, DEVICE=0x36334330 [Qualcomm(R) Adreno(TM) X1-85 GPU], DRIVER_VENDOR=Qualcomm, DRIVER_VERSION=31.0.133.1 ACTIVE GPU1: VENDOR= 0x1414, DEVICE=0x008c [Microsoft Basic Render Driver], DRIVER_VERSION=10.0.26100.8521 Machine model name: Machine model version: direct_rendering_display_compositor: disabled_off_ok gpu_compositing: enabled multiple_raster_threads: enabled_on opengl: enabled_on rasterization: enabled raw_draw: disabled_off_ok skia_graphite: disabled_off trees_in_viz: disabled_off video_decode: enabled video_encode: enabled webgl: enabled webgpu: enabled webnn: disabled_off
Load (avg)	undefined
Memory (System)	31.56GB (12.71GB free)
Process Argv	--skip-sessions-welcome --crash-reporter-id d4b504f0-1e36-4ccf-96e5-6da6c2e2812a --crash-reporter-id d4b504f0-1e36-4ccf-96e5-6da6c2e2812a
Screen Reader	no
VM	0%

Item	Value
Remote	SSH: hu-sbrocard-fra
OS	Linux x64 6.8.0-90-generic
CPUs	Intel(R) Xeon(R) Gold 6240R CPU @ 2.40GHz (8 x 0)
Memory (System)	31.34GB (25.74GB free)
VM	40%

Extensions (54)

Name	Identifier	Author	Version
Mermaid Markdown Syntax Highlighting	bpruitt-goddard.mermaid-markdown-syntax-highlighting	bpruitt-goddard	1.8.1
GitHub Repositories	github.remotehub	GitHub	0.64.0
Jupyter Keymap	ms-toolsai.jupyter-keymap	ms-toolsai	1.1.2
Dev Containers	ms-vscode-remote.remote-containers	ms-vscode-remote	0.463.0
Remote - SSH	ms-vscode-remote.remote-ssh	ms-vscode-remote	0.124.0
Remote - SSH: Editing Configuration Files	ms-vscode-remote.remote-ssh-edit	ms-vscode-remote	0.87.0
WSL	ms-vscode-remote.remote-wsl	ms-vscode-remote	0.104.3
Remote Development	ms-vscode-remote.vscode-remote-extensionpack	ms-vscode-remote	0.26.0
Azure Repos	ms-vscode.azure-repos	ms-vscode	0.40.0
Remote Explorer	ms-vscode.remote-explorer	ms-vscode	0.5.0
Remote Repositories	ms-vscode.remote-repositories	ms-vscode	0.42.0
Remote - Tunnels	ms-vscode.remote-server	ms-vscode	1.5.3
vscode-pdf	tomoki1207.pdf	tomoki1207	1.2.2
Claude Code for VS Code	anthropic.claude-code	Anthropic	2.1.204
ty	astral-sh.ty	astral-sh	2026.60.0
Ruff	charliermarsh.ruff	charliermarsh	2026.56.0
cmake-format	cheshirekow.cmake-format	cheshirekow	0.6.11
Doxygen Documentation Generator	cschlosser.doxdocgen	cschlosser	1.4.0
markdownlint	davidanson.vscode-markdownlint	DavidAnson	0.61.2
GitLens — Git supercharged	eamodio.gitlens	eamodio	18.2.0
Syntax Highlighter	evgeniypeshkov.syntax-highlighter	evgeniypeshkov	0.5.0
GitHub Pull Requests	github.vscode-pull-request-github	GitHub	0.156.0
Prettier SQL VSCode	inferrinizzard.prettier-sql-vscode	inferrinizzard	1.6.0
Format Files	jbockle.jbockle-format-files	jbockle	3.4.0
clangd	llvm-vs-code-extensions.vscode-clangd	llvm-vs-code-extensions	0.6.0
Bash IDE	mads-hartmann.bash-ide-vscode	mads-hartmann	1.43.0
Git Graph	mhutchie.git-graph	mhutchie	1.30.0
Container Tools	ms-azuretools.vscode-containers	ms-azuretools	2.4.5
Python Debugger	ms-python.debugpy	ms-python	2026.6.0
Python	ms-python.python	ms-python	2026.4.0
Pylance	ms-python.vscode-pylance	ms-python	2026.2.1
Python Environments	ms-python.vscode-python-envs	ms-python	1.36.0
Data Wrangler	ms-toolsai.datawrangler	ms-toolsai	1.24.2
Jupyter	ms-toolsai.jupyter	ms-toolsai	2026.6.2026070801
Jupyter Keymap	ms-toolsai.jupyter-keymap	ms-toolsai	1.1.2
Jupyter Notebook Renderers	ms-toolsai.jupyter-renderers	ms-toolsai	1.3.0
Jupyter Cell Tags	ms-toolsai.vscode-jupyter-cell-tags	ms-toolsai	0.1.9
Jupyter Slide Show	ms-toolsai.vscode-jupyter-slideshow	ms-toolsai	0.1.6
CMake Tools	ms-vscode.cmake-tools	ms-vscode	1.23.52
C/C++ DevTools	ms-vscode.cpp-devtools	ms-vscode	0.5.13
C/C++	ms-vscode.cpptools	ms-vscode	1.32.2
C/C++ Extension Pack	ms-vscode.cpptools-extension-pack	ms-vscode	1.5.1
Hex Editor	ms-vscode.hexeditor	ms-vscode	1.11.1
Live Preview	ms-vscode.live-server	ms-vscode	0.4.19
Chat Customizations Evaluations	ms-vscode.vscode-chat-customizations-evaluations	ms-vscode	1.0.8
vscode-just	nefrob.vscode-just-syntax	nefrob	0.10.2
autoDocstring - Python Docstring Generator	njpwerner.autodocstring	njpwerner	0.6.1
Codex – OpenAI’s coding agent	openai.chatgpt	openai	26.5623.141536
YAML	redhat.vscode-yaml	redhat	1.24.0
Language Server tools for Copilot	sehejjain.lsp-mcp-bridge	sehejjain	1.0.1
Even Better TOML	tamasfe.even-better-toml	tamasfe	0.21.2
VS Code Counter	uctakeoff.vscode-counter	uctakeoff	3.7.2
vscode-netron	vincent-templier.vscode-netron	vincent-templier	1.1.7
CodeViz Stat	vizzuhq.code-viz-stat	vizzuhq	0.1.4

A/B Experiments

vsliv368cf:30146710
binariesv615:30325510
nativeloc1:31344060
dwcopilot:31170013
dwoutputs:31242946
copilot_t_ci:31333650
e5gg6876:31282496
pythonrdcb7:31342333
6518g693:31463988
aj953862:31281341
6abeh943:31336334
envsactivate1:31538736
cloudbuttont:31379625
3efgi100_wstrepl:31403338
839jf696:31457053
ddidtcf:31399634
ec5jj548:31422691
cp_cls_t_966_ss:31526232
4je02754:31466945
c3h7c220:31478652
cp_cls_c_1081:31454833
conptydll_true:31498968
ia-use-proxy-models-svc:31452481
e9c30283:31461165
test_treatment2:31471001
46204921:31447328
capi_a_098a3278:31544979
idci7584:31464702
e3e4d672:31494082
ei9d7968:31496641
nes-extended-on:31455476
chat:31457767
8hig5102:31480529
89g7j272:31518289
7e187181:31503455
i2gc6536:31499202
52612955:31516516
ghj88844:31499326
h08i8180_copy:31475368
ddid_c:31478207
hmra_i5g22:31518061
ja75b849:31546597
7df3h592:31512476
cp_cls_t_1082:31535311
logging_enabled_new:31498466
j0d79568:31499440
jb_cp_cls_t_632:31543129
b1ei0813:31506238
748c7209:31512887
32d76977:31512328
ha629193:31508444
cdk-lw-on:31524445
f9bf7985:31543584
31fi7170_t:31522396
a1ije391_t:31540920
g4a0e106:31529594
41836166:31537719
jbcp_cls_pctr_t:31531130
61138546:31518536
cp_intellij_c_nes:31533503
gf14b233:31526830
nes-curr-leftover-1500:31538366
h561g132:31533256
session_8:31532895
ihg5j128:31534457
f412h606:31546039
7g2b5551:31542111
nes-nc-lc-nf:31546584

[SylvanBrocard]

I might be wrong about the symptoms:

• Moving the MCP from the remote host to the local host didn't solve anything.
• However the MCP works fine on local projects, so this still looks like a SSH specific bug.
• This might not actually be a regression of Regression since VS Code 1.119.1: many Azure DevOps MCP tools no longer surface in Copilot Chat Agent mode #317992 , unlike them, all the tools are discovered, visible in picker, checked, but absent from the agent tools list

There is a boundary somewhere that doesn't get crossed between tools discovery and injection.
I can't figure out how to diagnose it.

• Chat Debug view just shows me the list of available tools, from which the Github MCP are missing
• Agent Debug logs tell me about the discovery of skills, agents, and hooks, but not tools
• There is a "Optimizing tools" prompt that runs before the conversation starts. I assume whatever happens there is what prunes the tools list. I can't find a log that would show me what happens in here. I assume it's linked to virtual tools, but I've set github.copilot.chat.virtualTools.threshold to 0, and I don't see a "tools_search" tool in the agent tools list anyway.

Edit: apologies for my previous tone

[SylvanBrocard]

I got a model diagnosis which seems convincing:

Root Cause of Missing MCP/Registered Tools in Agent Session

Confirmed code path

defaultIntentRequestHandler.ts drives the "Optimizing tool selection" step. The relevant sequence (directly verified in the codebase):

// 1. Tools are snapshotted synchronously from toolsService at request time
if (this.toolGrouping) {
    this.toolGrouping.tools = tools;               // setter on existing instance
} else {
    this.toolGrouping = this.toolGroupingService.create(this.options.conversation.sessionId, tools);
    for (const ref of this.options.request.toolReferences) {
        this.toolGrouping.ensureExpanded(ref.name);
    }
}

// 2. compute() fires immediately against that snapshot
const computePromise = this.toolGrouping.compute(this.options.request.prompt, token);

// 3. Progress message shown while it resolves
const timeout = setTimeout(() => {
    outputStream?.progress(l10n.t('Optimizing tool selection'), async () => {
        await computePromise;
    });
});

Inside ToolGrouping

The tools setter only re-groups when tool names change:

// toolGrouping.ts
public set tools(tools: readonly LanguageModelToolInformation[]) {
    if (!arraysEqual(this._tools, tools, (a, b) => a.name === b.name)) {
        this._tools = tools;
        this._didToolsChange = true;
    }
}

_doCompute then snapshots this._tools.slice() at call time. ToolGroupingService wraps this in an LRUCache<string, IToolGrouping> keyed per session — so a stale grouping computed without remote tools can be reused across turns if the key hasn't changed.

The bug

tools at step 1 comes from toolsService.getEnabledTools(), which is synchronous and only reflects tools that have already registered at the moment handleRequest is invoked. MCP servers and vscode.lm.registerTool callers in extension hosts register asynchronously — their tools arrive via onDidChangeTools after the request handler has already fired and computePromise has resolved.

The result: the agent session is built from a tool grouping that silently excludes all late-arriving remote tools.

The correct pattern already exists — in tests only

chatSetupProviders.ts (test scenario) waits explicitly before proceeding:

return Event.toPromise(
    Event.filter(languageModelToolsService.onDidChangeTools, () => {
        for (const tool of languageModelToolsService.getTools()) {
            if (tool.source.type !== 'internal') return true;
        }
        return false;
    })
);

This pattern is absent from the production request handler path.

Suggested fix direction

Before calling toolGrouping.compute(), the handler (or its caller) should wait for onDidChangeTools to fire at least once with a non-internal tool present, with a short timeout fallback to avoid blocking indefinitely. Alternatively, if onDidChangeTools fires during computePromise resolution and the new tool set differs from the snapshot, compute() should be re-run before the result is consumed.

[SylvanBrocard]

Update: Root Cause is in getEnabledTools, not the virtual tools grouping

After further investigation I can narrow this down more precisely.

The actual filter (toolsService.ts)

getEnabledTools in extensions/copilot/src/extension/tools/vscode-node/toolsService.ts
builds a map from request.tools — the tool picker state serialized at the time the
request was created:

const requestToolsByName = new Map(
    Iterable.map(request.tools, ([t, enabled]) => [typeof t === 'string' ? t : t.name, enabled])
);

It then filters every tool through this:

// 0. If explicitly disabled via tool picker → exclude
const toolPickerSelection = requestToolsByName.get(getContributedToolName(tool.name));
if (toolPickerSelection === false) {
    return false;
}

// 3. If neither enabled nor disabled, include ONLY if tagged 'extension_installed_by_tool'
if (toolPickerSelection === undefined && tool.tags.includes('extension_installed_by_tool')) {
    return true;
}
// ...falls through → excluded

The bug

MCP tools that register after request.tools was serialized are absent from
requestToolsByName. Their toolPickerSelection is undefined, they don't carry the
extension_installed_by_tool tag, so they fall through and are silently excluded —
regardless of what the virtual tools threshold is set to.

This also explains why:

• Setting threshold: 0 doesn't help — it maps to Infinity (virtual tools never
  activated) but the filter in getEnabledTools is upstream of the virtual tools path
  entirely, so the tool list passed to the grouper is already incomplete.
• Toggling built-in tools on/off between turns works — those tools are present in
  request.tools from the start and their enabled boolean is respected correctly.
• The dummy-first-message workaround is unreliable — even if onDidChangeTools
  fires before the second turn, the new MCP tools won't be in request.tools for that
  turn's request object unless the tool picker was re-serialized after they registered.

Fix direction

request.tools needs to be populated (or re-reconciled) after the MCP tool registry
has settled, before getEnabledTools consumes it. The simplest consistent fix would be
to treat toolPickerSelection === undefined as include by default (mirroring the
extension_installed_by_tool special case) rather than silently excluding, so that
late-registering tools are not dropped on the floor.