[Error] [UriError]: cannot call joinPath on URI without path — workspaceMcpDiscoveryAdapter

[vs-code-engineering]

Error Message

unhandlederror-[UriError]: cannot call joinPath on URI without path

Stack Trace

    at a.joinPath (./src/vs/base/common/uri.ts:362:9)
    at ele.joinPath (./src/vs/base/common/resources.ts:186:13)
    at joinPath (./src/vs/workbench/contrib/mcp/common/discovery/workspaceMcpDiscoveryAdapter.ts:51:21)
    at cye.value (./src/vs/workbench/contrib/mcp/common/discovery/workspaceMcpDiscoveryAdapter.ts:41:9)
    at E._deliver (./src/vs/base/common/event.ts:1354:12)
    at E._deliverQueue (./src/vs/base/common/event.ts:1365:8)
    at E.fire (./src/vs/base/common/event.ts:1389:8)
    at pXt._doUpdateFolders (./src/vs/sessions/services/workspace/browser/workspaceContextService.ts:163:36)

Raw Stack Trace (minified)

Error: [UriError]: cannot call joinPath on URI without path     at a.joinPath (out/vs/sessions/sessions.desktop.main.js:406:59221)     at ele.joinPath (out/vs/sessions/sessions.desktop.main.js:406:68517)     at qBe.watchFolder (out/vs/sessions/sessions.desktop.main.js:3148:53450)     at cye.value (out/vs/sessions/sessions.desktop.main.js:3148:53325)     at E._deliver (out/vs/sessions/sessions.desktop.main.js:405:3066)     at E._deliverQueue (out/vs/sessions/sessions.desktop.main.js:405:3155)     at E.fire (out/vs/sessions/sessions.desktop.main.js:405:3490)     at pXt._doUpdateFolders (out/vs/sessions/sessions.desktop.main.js:4549:2992)

Details

Property	Value
Bucket ID	16422a46-e14b-1fac-d6a5-ea449cc9d5a2
Version	1.115.0-insider
Commit	41dd792b
Last Seen	Tue Apr 14 2026 17:59:52 GMT+0000 (Coordinated Universal Time)
Platforms	Windows, Mac, Linux
Product	Agents

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This issue was automatically created from the VS Code Errors Dashboard

[vs-code-engineering]

🔍 Error Investigation

Summary

CursorWorkspaceMcpDiscoveryAdapter.watchFolder() calls joinPath(folder.uri, '.cursor', 'mcp.json') without checking if the folder's URI has a path component. In the Agents/Sessions environment, workspace folders can be added with URIs that lack a path (e.g., virtual workspace schemes or certain remote URIs), causing URI.joinPath() to throw [UriError]: cannot call joinPath on URI without path. This error has been consistently present since at least v1.114.0-insider (March 27, 2026) across all platforms and affects ~276 users on the latest build alone.

Culprit Commit

No specific culprit commit was identified. The error has been present since the file was first created — the file content at every commit in the error bucket history (spanning v1.114.0 through v1.116.0) is byte-identical. No commit range was provided in the issue body or comments by the regression scan workflow. The bug is a missing guard that has existed since the CursorWorkspaceMcpDiscoveryAdapter class was authored.

Code Flow

sequenceDiagram
    participant WCS as SessionsWorkspaceContextService
    participant Adapter as CursorWorkspaceMcpDiscoveryAdapter
    participant Resources as resources.ts (joinPath)
    participant URI as URI.joinPath

    WCS->>WCS: _doUpdateFolders() — adds folder with path-less URI
    WCS->>Adapter: onDidChangeWorkspaceFolders (e.added)
    Adapter->>Adapter: watchFolder(folder)
    Note over Adapter: ⚠️ Root cause:<br/>No guard for folder.uri.path
    Adapter->>Resources: joinPath(folder.uri, '.cursor', 'mcp.json')
    Resources->>URI: URI.joinPath(uri, ...pathFragment)
    Note over URI: 💥 !uri.path → throws<br/>[UriError]: cannot call<br/>joinPath on URI without path

Loading

Affected Files

File	Role
src/vs/workbench/contrib/mcp/common/discovery/workspaceMcpDiscoveryAdapter.ts	Root cause — watchFolder() (lines 50-76) calls joinPath without guarding folder.uri.path
src/vs/base/common/uri.ts	Crash site — URI.joinPath() (line 361) throws when !uri.path
src/vs/base/common/resources.ts	Data flow — delegates to URI.joinPath
src/vs/sessions/services/workspace/browser/workspaceContextService.ts	Trigger — _doUpdateFolders() fires the workspace change event

Repro Steps

1. Open VS Code Insiders (Agents product)
2. Add a workspace folder that uses a URI scheme without a path component (e.g., a virtual filesystem scheme or a remote URI with only an authority)
3. The CursorWorkspaceMcpDiscoveryAdapter listens to onDidChangeWorkspaceFolders and calls watchFolder() on the new folder
4. joinPath(folder.uri, '.cursor', 'mcp.json') throws because folder.uri.path is empty/undefined

This is non-deterministic in that it depends on what workspace folder URIs are used — it only triggers for URIs without a path component.

Fix Proposal

Fix location: src/vs/workbench/contrib/mcp/common/discovery/workspaceMcpDiscoveryAdapter.ts → watchFolder()

Approach: Add an early return guard at the top of watchFolder() to skip folders whose URI has no path. The .cursor/mcp.json config file only makes sense for filesystem-based workspace folders, so folders with path-less URIs should be silently skipped:

private watchFolder(folder: IWorkspaceFolder) {
    if (!folder.uri.path) {
        return;
    }
    // ...existing code unchanged...
}

This follows the same defensive pattern used by NativeFilesystemMcpDiscovery.setDetails() (line 142 of nativeMcpDiscoveryAbstract.ts), which checks if (!file) { continue; } before calling watchFile.

Recommended Owner

@sandy081 — MCP area owner per triage/working-areas.md (owns MCP UI, MCP Management, MCP Gallery)

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