Add Fifth LSP server with document sync, parsing, diagnostics, and basic hover/completion stubs #196

Copilot · 2026-01-21T05:02:29Z

Introduces a Fifth Language Server Protocol executable with stdio wiring, document synchronization, parsing, and basic user-facing features. The server now reparses on open/change/save, publishes syntax diagnostics, offers a placeholder hover (line echo), and provides keyword-only completion as a safe default.

Changes Made

Added LSP server entrypoint with cancellation handling and DI setup.
Implemented document sync backed by parsing that caches documents and publishes syntax diagnostics.
Added placeholder hover that echoes the current line and a minimal keyword completion provider.
Retained smoke test coverage for the language server assembly.

Testing

Not run in this update (not requested).

Original prompt

This section details on the original issue you should resolve

<issue_title>Implement Language Server Protocol (LSP) for IDE Integration</issue_title>
<issue_description># Implement Language Server Protocol (LSP) for IDE Integration

Labels: arch-review, ide-support, lsp, critical
Priority: P0
Severity: CRITICAL
Epic: Architectural Improvements Q2 2026

Problem Summary

The compiler has no Language Server Protocol implementation, preventing integration with modern editors (VS Code, Neovim, Emacs, etc.). This severely limits the language's adoption potential and developer experience.

Current State

No LSP-related code in codebase
No language server executable
Only basic VS Code configuration
No incremental compilation (required for LSP)
No real-time diagnostics
No IDE features (autocomplete, go-to-definition, etc.)

Impact

Adoption Barrier

Developers expect IDE features from modern languages
Competing languages (Rust, TypeScript, Swift) all have excellent LSP support
No Fifth language support for popular editors
Makes language feel "unfinished" or "hobby project"

Development Velocity

Contributors cannot efficiently work on Fifth code
No tooling to support language feature development
Testing requires full compilation cycles
Debugging is manual and time-consuming

Feature Gap

Cannot implement standard IDE features:

Hover information (type info, documentation)
Signature help (function parameter hints)
Code completion (context-aware suggestions)
Go to definition/implementation
Find all references
Rename symbol
Code actions/quick fixes
Semantic tokens (syntax highlighting)
Document/workspace symbols
Document formatting

Requirements

Core LSP Server

Implement LSP server as separate executable
Support stdio communication protocol
Handle standard LSP lifecycle (initialize, initialized, shutdown)
Implement core capabilities negotiation

Document Management

Track open documents in workspace
Synchronize document changes (didOpen, didChange, didClose)
Parse documents incrementally
Maintain AST cache per document

Essential Features (MVP)

Diagnostics (textDocument/publishDiagnostics)
- Real-time syntax errors
- Real-time semantic errors
- Error recovery support
Hover (textDocument/hover)
- Type information
- Function signatures
- Symbol documentation
Completion (textDocument/completion)
- Context-aware suggestions
- Keyword completion
- Symbol completion
- Function completion with signatures
Go to Definition (textDocument/definition)
- Navigate to symbol definition
- Support cross-file navigation

Advanced Features (Post-MVP)

Find References (textDocument/references)
Rename Symbol (textDocument/rename)
Document Symbols (textDocument/documentSymbol)
Code Actions (textDocument/codeAction)
Semantic Tokens (textDocument/semanticTokens)
Signature Help (textDocument/signatureHelp)

Architecture

Project Structure

src/language-server/
├── FifthLanguageServer.csproj
├── Program.cs                    # Entry point
├── LanguageServer.cs             # Main server class
├── Handlers/
│   ├── TextDocumentHandler.cs   # Document sync
│   ├── DiagnosticHandler.cs     # Error checking
│   ├── CompletionHandler.cs     # Code completion
│   ├── HoverHandler.cs          # Hover info
│   └── DefinitionHandler.cs     # Go to definition
├── Services/
│   ├── WorkspaceService.cs      # Workspace management
│   ├── DocumentService.cs       # Document tracking
│   ├── ParsingService.cs        # Incremental parsing
│   ├── DiagnosticService.cs     # Error collection
│   ├── CompletionService.cs     # Completion logic
│   └── SymbolService.cs         # Symbol queries
└── Protocol/
    └── LSPTypes.cs              # LSP protocol types

Key Components

WorkspaceService:

Manages open documents
Tracks project structure
Handles workspace-wide operations

DocumentService:

Synchronizes document state
Manages document versions
Caches parsed ASTs

ParsingService:

Incremental parsing with error recovery
AST caching and invalidation
Background parsing for diagnostics

SymbolService:

Symbol resolution using enhanced symbol table
Cross-file symbol queries
Supports "find references" and "go to definition"

Implementation Plan

Phase 1: Infrastructure (Weeks 1-4)

Create language-server project
Add OmniSharp LSP library dependency
Implement basic server lifecycle
Set up stdio communication
Add VS Code extension configuration

Phase 2: Document Synchronization (Weeks 5-8)

Implement document tracking
Handle didOpen/didChange/didClose
Set up incremental parsing
Add document AST caching

Phase 3: Core Features (Weeks 9-12)

Diagnostics:
- Real-time syntax error reporting
- Semantic error reporting
- Error recovery...

Fixes Implement Language Server Protocol (LSP) for IDE Integration #195

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

Co-authored-by: aabs <157775+aabs@users.noreply.github.com>

aabs · 2026-01-21T05:48:36Z

@copilot continue with the work

Copilot · 2026-01-21T06:09:53Z

@copilot continue with the work

Implemented the requested updates: added DI wiring and cancellation handling, registered document sync/hover/completion handlers, fixed handler implementations to compile against OmniSharp 0.19.9, and ensured the language server builds alongside the smoke test.

Co-authored-by: aabs <157775+aabs@users.noreply.github.com>

aabs · 2026-01-21T06:18:29Z

@copilot continue with the implementation

Copilot · 2026-01-21T06:19:42Z

@copilot continue with the implementation

Will continue extending the LSP implementation now (handlers, capabilities, and document/diagnostic wiring).

aabs · 2026-01-21T06:20:45Z

@copilot continue

Copilot · 2026-01-21T06:21:38Z

@copilot continue

Continuing the implementation work now.

aabs

@copilot please now implement the next phases in the issue's implementation plan (2.3 and 2.4 for parser integration)

Copilot · 2026-01-21T22:30:02Z

@copilot please now implement the next phases in the issue's implementation plan (2.3 and 2.4 for parser integration)

Implemented phases 2.3/2.4 parser integration: added parsing and document services with AST/diagnostic caching, wired document sync to reparse on open/change/save and publish diagnostics, and added a smoke test for parsing diagnostics (commit e3b1b12).

…lus smoke tests Co-authored-by: aabs <157775+aabs@users.noreply.github.com>

aabs · 2026-01-21T22:33:01Z

@copilot Proceed with the next stage

Copilot · 2026-01-21T22:33:59Z

@copilot Proceed with the next stage

Continuing to the next stage now.

aabs · 2026-01-21T22:43:17Z

@copilot Now implement phase 3 from the implementation plan in the issue (core features)

aabs · 2026-01-21T22:51:52Z

@copilot continue with the work

Copilot · 2026-01-21T22:53:07Z

@copilot continue with the work

Continuing with the core feature implementation now.

…workspace files, including unopened files, and update acceptance criteria and tasks accordingly.

- Upgraded Microsoft.Extensions.* packages to version 8.0.0 in language server and smoke test projects. - Added StderrLoggerProvider for logging to standard error. - Implemented DefinitionHandler for resolving definitions in the language server. - Created HandlerResults utility for handling empty definition responses. - Added smoke tests for document service diagnostics, definition resolution, and completion handling. - Introduced Getting Started guide for running the Fifth Language Server in VS Code Insiders.

… configuration

001-lsp-server

Copilot

Pull request overview

This PR introduces a Fifth Language Server Protocol (LSP) implementation to enable IDE integration. The server provides stdio-based communication, document synchronization, real-time diagnostics (syntax and semantic), hover information, symbol-based completion, and go-to-definition across workspace files.

Changes:

Added LSP server executable with OmniSharp.Extensions.LanguageServer integration
Implemented document management with in-memory parsing and AST caching
Added workspace-wide symbol indexing that parses both open and unopened files
Created VS Code extension client with TypeScript configuration

Reviewed changes

Copilot reviewed 34 out of 37 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
src/language-server/Program.cs	LSP server entry point with DI and handler registration
src/language-server/Parsing/ParsingService.cs	Parser integration with syntax and semantic diagnostic extraction
src/language-server/DocumentService.cs	Document state management and parse result caching
src/language-server/SymbolService.cs	Workspace symbol indexing and definition resolution
src/language-server/Handlers/*.cs	LSP protocol handlers for sync, hover, completion, and definition
src/vscode-client/*	VS Code extension for connecting to the language server
test/language-server-smoke/*.cs	Smoke tests for LSP functionality
specs/001-lsp-server/*.md	Feature specifications and planning documents
fifthlang.sln	Solution file updates for new projects

Files not reviewed (1)

src/vscode-client/package-lock.json: Language not supported

Copilot · 2026-01-23T04:16:50Z

src/language-server/Parsing/ParsingService.cs

+        {
+            tree = parser.fifth();
+        }
+        catch


The catch-all exception handler on line 30 swallows all exceptions without logging them. While the comment states "Syntax errors are captured by the listener," this will also suppress parser crashes, runtime errors, or other unexpected exceptions that could indicate bugs. Consider at least logging unexpected exceptions before swallowing them, or being more specific about which exceptions are expected.

Suggested change

catch

catch (RecognitionException)

Copilot · 2026-01-23T04:16:51Z

src/language-server/SymbolService.cs

+            catch
+            {


The catch-all exception handler on line 124 silently suppresses all parsing errors for workspace files. This makes it impossible to diagnose issues when files fail to parse. At minimum, failed parses should be logged with the file path and exception details so that developers can understand why symbols are missing from the workspace index.

Suggested change

catch

{

catch (Exception ex)

{

Console.Error.WriteLine($"[SymbolService] Failed to parse workspace file '{file}': {ex}");

Copilot · 2026-01-23T04:16:51Z

test/language-server-smoke/SmokeTests.cs

+using Microsoft.Extensions.Logging.Abstractions;
+using OmniSharp.Extensions.LanguageServer.Protocol;
+using OmniSharp.Extensions.LanguageServer.Protocol.Models;
+using Xunit;


The test project uses xUnit (imported on line 7) rather than TUnit, which is inconsistent with the project's stated testing framework preference. According to the coding guidelines and other test projects in the codebase, TUnit is the preferred test framework. Consider migrating these tests to TUnit for consistency.

Copilot · 2026-01-23T04:16:51Z

src/language-server/SymbolService.cs

+        }
+
+        return Directory.EnumerateFiles(workspaceRoot, "*.5th", SearchOption.AllDirectories)
+            .Where(path => !WorkspaceIgnoreSegments.Any(segment => path.Split(Path.DirectorySeparatorChar).Contains(segment)));


The path filtering logic on line 294 uses Contains() to check if any path segment matches ignore patterns, but this will incorrectly filter paths that contain these strings as substrings. For example, a file in "my-bin-utils/" would be filtered because "bin" appears as a substring. Use Path.GetFileName on each directory segment or ensure exact segment matching to avoid false positives.

Suggested change

.Where(path => !WorkspaceIgnoreSegments.Any(segment => path.Split(Path.DirectorySeparatorChar).Contains(segment)));

.Where(path =>

{

var segments = path.Split(

new[] { Path.DirectorySeparatorChar, Path.AltDirectorySeparatorChar },

StringSplitOptions.RemoveEmptyEntries);

return !WorkspaceIgnoreSegments.Any(ignore =>

segments.Any(segment =>

string.Equals(segment, ignore, StringComparison.OrdinalIgnoreCase)));

});

Copilot · 2026-01-23T04:16:51Z

src/language-server/SymbolService.cs

+            .Select(p => $"{p.Name}: {p.TypeName.Value ?? p.TypeName.ToString()}")
+            .ToList();
+        var returnTypeName = returnType.Name.Value ?? returnType.Name.ToString();
+        var signature = $"{name}({string.Join(", ", parameterList)})";
+        if (!string.IsNullOrWhiteSpace(returnTypeName))
+        {
+            signature += $": {returnTypeName}";
+        }


Default 'ToString()': TypeName inherits 'ToString()' from 'Object', and so is not suitable for printing.

Suggested change

.Select(p => $"{p.Name}: {p.TypeName.Value ?? p.TypeName.ToString()}")

.ToList();

var returnTypeName = returnType.Name.Value ?? returnType.Name.ToString();

var signature = $"{name}({string.Join(", ", parameterList)})";

if (!string.IsNullOrWhiteSpace(returnTypeName))

{

signature += $": {returnTypeName}";

}

.Select(p =>

{

var typeName = p.TypeName.Value;

if (string.IsNullOrWhiteSpace(typeName))

{

typeName = "unknown";

}

return $"{p.Name}: {typeName}";

})

.ToList();

var returnTypeName = returnType.Name.Value;

if (string.IsNullOrWhiteSpace(returnTypeName))

{

returnTypeName = "unknown";

}

var signature = $"{name}({string.Join(", ", parameterList)})";

if (!string.IsNullOrWhiteSpace(returnTypeName))

{

signature += $": {returnTypeName}";

}

Copilot · 2026-01-23T04:16:52Z

src/language-server/SymbolService.cs

+            .Select(p => $"{p.Name}: {p.TypeName.Value ?? p.TypeName.ToString()}")
+            .ToList();
+        var returnTypeName = returnType.Name.Value ?? returnType.Name.ToString();


Default 'ToString()': TypeName inherits 'ToString()' from 'Object', and so is not suitable for printing.

Suggested change

.Select(p => $"{p.Name}: {p.TypeName.Value ?? p.TypeName.ToString()}")

.ToList();

var returnTypeName = returnType.Name.Value ?? returnType.Name.ToString();

.Select(p => $"{p.Name}: {p.TypeName.Value ?? string.Empty}")

.ToList();

var returnTypeName = returnType.Name.Value ?? string.Empty;

Initial plan

332c1ad

Copilot AI assigned Copilot and aabs Jan 21, 2026

Copilot started work on behalf of aabs January 21, 2026 05:02 View session

Add minimal Fifth LSP executable and smoke test scaffold

165af8b

Co-authored-by: aabs <157775+aabs@users.noreply.github.com>

Copilot AI changed the title ~~[WIP] Implement Language Server Protocol for IDE integration~~ Add minimal Fifth LSP executable and smoke test scaffold Jan 21, 2026

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Add minimal Fifth LSP server with handler stubs and smoke test

cd014f7

Co-authored-by: aabs <157775+aabs@users.noreply.github.com>

Copilot AI changed the title ~~Add minimal Fifth LSP executable and smoke test scaffold~~ Add minimal Fifth LSP server with handler stubs and smoke test Jan 21, 2026

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Copilot AI changed the title ~~Add minimal Fifth LSP server with handler stubs and smoke test~~ Add minimal Fifth LSP server with document sync, handler stubs, and smoke test Jan 21, 2026

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Add Fifth LSP server with document sync, parsing, and handler stubs p…

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…lus smoke tests Co-authored-by: aabs <157775+aabs@users.noreply.github.com>

Copilot AI changed the title ~~Add minimal Fifth LSP server with document sync, handler stubs, and smoke test~~ Add Fifth LSP server with document sync, parsing, and handler stubs plus smoke tests Jan 21, 2026

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Co-authored-by: aabs <157775+aabs@users.noreply.github.com>

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Copilot AI changed the title ~~Add Fifth LSP server with document sync, parsing, and handler stubs plus smoke tests~~ Add Fifth LSP server with document sync, parsing, diagnostics, and basic hover/completion stubs Jan 21, 2026

Copilot finished work on behalf of aabs January 21, 2026 22:53

aabs and others added 5 commits January 23, 2026 10:17

spec complete

add8239

Enhance LSP server documentation to clarify symbol resolution across …

8d8edd3

…workspace files, including unopened files, and update acceptance criteria and tasks accordingly.

feat: Add Fifth Language Client for VS Code with language support and…

fcd8e79

… configuration

Merge pull request #197 from aabs:001-lsp-server

baac48e

001-lsp-server

aabs marked this pull request as ready for review January 23, 2026 04:12

Copilot AI review requested due to automatic review settings January 23, 2026 04:12

aabs merged commit 6c3838c into master Jan 23, 2026
5 checks passed

aabs deleted the copilot/implement-lsp-for-ide-integration branch January 23, 2026 04:12

Copilot started reviewing on behalf of aabs January 23, 2026 04:12 View session

Copilot AI reviewed Jan 23, 2026

View reviewed changes

-            catch
-            {
+            catch (Exception ex)
+            {
+                Console.Error.WriteLine($"[SymbolService] Failed to parse workspace file '{file}': {ex}");

-            .Where(path => !WorkspaceIgnoreSegments.Any(segment => path.Split(Path.DirectorySeparatorChar).Contains(segment)));
+            .Where(path =>
+            {
+                var segments = path.Split(
+                    new[] { Path.DirectorySeparatorChar, Path.AltDirectorySeparatorChar },
+                    StringSplitOptions.RemoveEmptyEntries);
+                return !WorkspaceIgnoreSegments.Any(ignore =>
+                    segments.Any(segment =>
+                        string.Equals(segment, ignore, StringComparison.OrdinalIgnoreCase)));
+            });

Add Fifth LSP server with document sync, parsing, diagnostics, and basic hover/completion stubs #196

Add Fifth LSP server with document sync, parsing, diagnostics, and basic hover/completion stubs #196

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Changes Made

Testing

Problem Summary

Current State

Impact

Adoption Barrier

Development Velocity

Feature Gap

Requirements

Core LSP Server

Document Management

Essential Features (MVP)

Advanced Features (Post-MVP)

Architecture

Project Structure

Key Components

Implementation Plan

Phase 1: Infrastructure (Weeks 1-4)

Phase 2: Document Synchronization (Weeks 5-8)

Phase 3: Core Features (Weeks 9-12)

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