Cannot start rust-analyzer LSP at project-level

[heaths]

Describe the bug

When configured in .github/lsp.json, /lsp test rust-analyzer does not work with the following content:

{
  "lspServers": {
    "rust-analyzer": {
      "command": "rust-analyzer",
      "args": [
        "--stdio"
      ],
      "fileExtensions": {
        ".rs": "rust"
      },
      "initializationOptions": {
        "cargo": {
          "buildScripts": {
            "enable": true
          },
          "allFeatures": true
        },
        "procMacro": {
          "enable": true
        },
        "checkOnSave": {
          "command": "clippy"
        }
      }
    }
  }
}

I've tried /lsp reload, exiting and resuming the session or even starting a new one. /lsp show was showing it configured, but /lsp test rust-analyzer always erred with:

Server "rust-analyzer" not found. Available: (none)

I tried asking copilot several times. At one point it even moved the file to .copilot/lsp.json, which isn't documented.

It wasn't until it tried adding it to ~/.copilot/lsp-config.json with the following content that rust-analyzer started correctly:

{
  "lspServers": {
    "rust-analyzer": {
      "command": "rust-analyzer",
      "args": ["--stdio"],
      "fileExtensions": {
        ".rs": "rust"
      }
    }
  }
}

rust-analyzer is in my $PATH in ~/.cargo/bin.

Affected version

0.0.405

Steps to reproduce the behavior

1. Ask Copilot to add rust-analyzer to .github/lsp.json. This mostly worked. Initially it didn't add the --stdio argument but did eventually when it tried to figure out what was wrong. See my .github/lsp.json config above for what the final JSON was.
2. Run /lsp reload or /exit and restart (resume or start anew).
3. Run /lsp test rust-analyzer.

Even when which rust-analyzer returns the right path, copilot errs:

Server "rust-analyzer" not found. Available: (none)

Expected behavior

/lsp test rust-analyzer works with .github/lsp.json project-based configuration.

Additional context

• macOS 26.2
• bash 5.3.9
• rust-analyzer 1.95.0-nightly

[heaths]

To note, I also tried starting with --experimental and /experimental on once inside to no avail. /lsp show lists it but it won't start unless configured at the user level.

[FaBrand]

FYI @heaths i tried with your original (longer) config and atleast it started:

(At current latest version 1.0.14)