Fixing eglot-jl world‑age errors by syncing LSP environments

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Summary

Eglot-jl was unable to index imported Julia packages because the language server was started in an isolated environment that did not have the packages pre‑loaded, leading to a world‑age mismatch (UndefVarError: Graphs not defined). The root cause is the way the LanguageServer’s LoadingBay resolves modules when the server’s project differs from the user’s active environment.

Root Cause

  • The eglot-jl-language-server-project variable pointed to ~/.julia/environments/lsp-isolated, a clean environment containing only LanguageServer.jl and SymbolServer.jl.
  • Your source file’s Project.toml declared a dependency on Graphs.jl, but that package was never loaded into the server’s environment.
  • Julia’s world age mechanism detected that Graphs was defined in a newer world (44989) than the one the server was running in (39203), causing the UndefVarError.
  • The LanguageServer tried to import Graphs in Main.SymbolServer.LoadingBay without success, aborting the indexing phase.

Why This Happens in Real Systems

  • Isolated LSP environments are common for reproducibility, but they must mirror the project’s dependency graph.
  • The world‑age guard protects against loading code compiled against a newer world, which is triggered when the server’s precompiled cache is out‑of‑sync with the active project.
  • Eglot-jl does not automatically activate the project of the opened file; it uses the path set in eglot-jl-language-server-project unless overridden.

Real-World Impact

  • Missing completions and hover information for any package not present in the isolated environment.
  • False negatives in static analysis tools that rely on SymbolServer indexing.
  • Developer frustration due to invisible errors that appear only in the LSP logs.
  • Potential CI pipeline failures if the same isolated configuration is reused for automated checks.

Example or Code (if necessary and relevant)

using Pkg
Pkg.activate("~/myproject")
Pkg.instantiate()

How Senior Engineers Fix It

  • Align the LSP project with the source project: 
    (setq eglot-jl-language-server-project (projectile-project-root))
  • Pre‑load required packages in the isolated environment: 
    using Pkg
Pkg.activate("~/.julia/environments/lsp-isolated")
Pkg.add(["Graphs", "MetaGraphsNext"])
  • Clear stale precompilation caches to avoid world‑age mismatches: 
    using Pkg
Pkg.precompile()
  • Enable automatic project activation by adding to init.el: 
    (add-hook 'julia-mode-hook
          (lambda ()
            (let ((proj (projectile-project-root)))
              (when proj
                (setq-local eglot-jl-language-server-project proj)))))
  • Upgrade SymbolServer/LanguageServer to the latest compatible versions, as newer releases improve handling of world‑age issues.

Why Juniors Miss It

  • They often assume the LSP automatically picks up the current project, overlooking the explicit eglot-jl-language-server-project setting.
  • Debugging world‑age errors requires familiarity with Julia’s compilation model, which is rarely covered in entry‑level tutorials.
  • Junior developers may ignore LSP logs, focusing only on editor UI symptoms rather than the underlying server error messages.

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