feat(server): add diagnosticsOnly flag for lint-only servers

Add diagnosticsOnly?: boolean to ServerConfig. When set, the server is
excluded from pickServer() (hover/definition/references/completion/
documentSymbol) but still included in pickDiagnosticServers() for
lsp_diagnostics and auto-check.

Mark oxlint as diagnosticsOnly: true — it now contributes diagnostics
alongside typescript-language-server without interfering with navigation
or completion tools.

cli.ts

@@ -2,7 +2,8 @@
 import * as path from "node:path";
 import { startClientForFile } from "./src/client.ts";
 import { isLspCommand, listCommands, runCommand } from "./src/commands.ts";
-import { pickServer } from "./src/root.ts";
+import { pickServer, isServerAvailable } from "./src/root.ts";
+import { servers } from "./server.ts";
 import {
   daemonDiagnostics,
   daemonRequest,
@@ -104,14 +105,20 @@ async function runViaDaemon(
   const filePath = path.resolve(fileArg);
   let result: unknown;
   if (cmdArg === "diagnostics") {
-    result = await daemonDiagnostics(filePath);
+    // Pick All Available Servers For Diagnostics
+    const ext = path.extname(filePath).replace(/^\./, "");
+    const serverIds = servers
+      .filter((s) => s.match.includes(ext) && isServerAvailable(s))
+      .map((s) => s.id);
+    result = await daemonDiagnostics(filePath, serverIds);
   } else if (cmdArg in methodMap) {
+    const server = pickServer(filePath);
     // References Default - Match commands.ts: include declaration unless
     // caller explicitly overrode `context`.
     if (cmdArg === "references" && !("context" in params)) {
       params.context = { includeDeclaration: true };
     }
-    result = await daemonRequest(filePath, methodMap[cmdArg], params);
+    result = await daemonRequest(filePath, server.id, methodMap[cmdArg], params);
   } else {
     process.stderr.write(
       `Unknown command "${cmdArg}". Known: ${listCommands().join(", ")}\n`,

index.ts

@@ -12,7 +12,7 @@ import {
   daemonRequest,
   daemonStatus,
 } from "./src/daemonClient.ts";
-import { pickServer } from "./src/root.ts";
+import { pickServer, isServerAvailable } from "./src/root.ts";
 import { servers } from "./server.ts";
 import {
   ServerNotFoundError,
@@ -201,17 +201,8 @@ function formatDocumentSymbols(result: unknown): string {
 }
 
 // Format Diagnostics - Turn diagnostic messages into readable text.
-function formatDiagnostics(result: unknown, limit = 20): string {
-  if (
-    !result ||
-    typeof result !== "object" ||
-    !("diagnostics" in result)
-  ) {
-    return "(no diagnostics)";
-  }
-  const diags = (result as any).diagnostics;
-  if (!Array.isArray(diags) || diags.length === 0) return "(no diagnostics)";
-
+// Format Single Server Diagnostics - Renders one server's diagnostics list.
+function formatServerDiagnostics(diags: any[], limit: number): string {
   const severityNames: Record<number, string> = {
     1: "Error",
     2: "Warning",
@@ -236,11 +227,45 @@ function formatDiagnostics(result: unknown, limit = 20): string {
     .join("\n");
 
   if (diags.length > limit) {
-    return `${formatted}\n\n... and ${diags.length - limit} more diagnostics (showing first ${limit})`;
+    return `${formatted}\n\n... and ${diags.length - limit} more (showing first ${limit})`;
   }
   return formatted;
 }
 
+// Format Diagnostics - Handles the grouped result map from the daemon:
+// { [serverId]: { uri, diagnostics[] } }. Single-server results omit the
+// header to avoid noise.
+function formatDiagnostics(result: unknown, limit = 20): string {
+  if (!result || typeof result !== "object") return "(no diagnostics)";
+
+  const grouped = result as Record<string, any>;
+  const serverIds = Object.keys(grouped);
+  if (serverIds.length === 0) return "(no diagnostics)";
+
+  // Collect Servers With Diagnostics
+  const sections: { id: string; diags: any[] }[] = [];
+  for (const id of serverIds) {
+    const entry = grouped[id];
+    const diags = entry?.diagnostics;
+    if (Array.isArray(diags) && diags.length > 0) {
+      sections.push({ id, diags });
+    }
+  }
+
+  if (sections.length === 0) return "(no diagnostics)";
+
+  // Single Server - Skip header for brevity.
+  if (sections.length === 1) {
+    return formatServerDiagnostics(sections[0].diags, limit);
+  }
+
+  // Multiple Servers - Group with headers.
+  const perServer = Math.max(5, Math.floor(limit / sections.length));
+  return sections
+    .map((s) => `## ${s.id}\n${formatServerDiagnostics(s.diags, perServer)}`)
+    .join("\n\n");
+}
+
 // Is Expected Error - Returns true if the error is an expected condition
 // (unsupported file type or missing server binary) that should be
 // suppressed rather than surfaced to the user.
@@ -281,7 +306,7 @@ async function runLsp(
         `LSP server "${server.id}" is disabled. Use /lsp-enable ${server.id} to re-enable.`,
       );
     }
-    return await daemonRequest(filePath, method, params);
+    return await daemonRequest(filePath, server.id, method, params);
   } catch (error) {
     if (isExpectedError(error)) {
       return undefined;
@@ -300,19 +325,26 @@ async function runLsp(
   }
 }
 
-// Run LSP Diagnostics - Diagnostics arrive as a notification, so the
-// daemon has a dedicated op that waits for the next publish. Expected
-// errors (unsupported file type, missing binary) are suppressed.
+// Pick Diagnostic Servers - Returns all available, non-disabled servers
+// matching the file's extension. Used for fan-out diagnostics.
+function pickDiagnosticServers(filePath: string): string[] {
+  const ext = path.extname(filePath).replace(/^\./, "");
+  return servers
+    .filter((s) => s.match.includes(ext) && isServerAvailable(s) && !disabledServers.has(s.id))
+    .map((s) => s.id);
+}
+
+// Run LSP Diagnostics - Fans out to all matching servers in a single
+// daemon call. Returns the grouped result map or undefined if no servers.
 async function runDiagnostics(filePath: string): Promise<unknown> {
   try {
-    return await daemonDiagnostics(filePath, 1500);
+    const serverIds = pickDiagnosticServers(filePath);
+    if (serverIds.length === 0) return undefined;
+    return await daemonDiagnostics(filePath, serverIds, 1500);
   } catch (error) {
     if (isExpectedError(error)) {
       return undefined;
     }
-    // Daemon-wrapped errors (plain Error with expected message) are also
-    // expected — the daemon catches pickServer() throws and returns them
-    // as string error messages.
     if (
       error instanceof Error &&
       (error.message.includes("No LSP server registered") ||
@@ -492,9 +524,12 @@ export default function (pi: ExtensionAPI) {
 
     try {
       const absolutePath = path.resolve(ctx.cwd, filePath);
-      // daemonDiagnostics triggers getOrCreateEntry + syncFile in the daemon.
-      // We don't await it — just fire and forget so the server starts warming up.
-      void daemonDiagnostics(absolutePath).catch(() => {});
+      // Warm Diagnostic Servers - Fire-and-forget so servers are ready by
+      // the time an LSP tool is called.
+      const serverIds = pickDiagnosticServers(absolutePath);
+      if (serverIds.length > 0) {
+        void daemonDiagnostics(absolutePath, serverIds).catch(() => {});
+      }
     } catch {
       // Silently ignore — unsupported file type, missing binary, etc.
     }

server.ts

@@ -64,5 +64,6 @@ export const servers: ServerConfig[] = [
     args: ["--lsp"],
     rootMarkers: [".oxlintrc.json", "oxlint.config.json"],
     languageId: "typescript",
+    diagnosticsOnly: true,
   },
 ];

src/client.ts

@@ -10,35 +10,10 @@ import type {
   InitializeParams,
   PublishDiagnosticsParams,
 } from "vscode-languageserver-protocol";
-import * as path from "node:path";
 import type { ServerConfig } from "./types.ts";
 import { ServerNotFoundError } from "./types.ts";
 import { findRoot, pathToUri, uriToPath } from "./root.ts";
-
-// Is On PATH - Returns true if `cmd` resolves to an executable via the
-// supplied PATH. Absolute/relative paths are checked directly.
-function isOnPath(cmd: string, env: NodeJS.ProcessEnv): boolean {
-  const isExec = (p: string) => {
-    try {
-      fs.accessSync(p, fs.constants.X_OK);
-      return true;
-    } catch {
-      return false;
-    }
-  };
-  if (cmd.includes(path.sep)) return isExec(cmd);
-  const exts =
-    process.platform === "win32"
-      ? (env.PATHEXT ?? ".EXE;.CMD;.BAT").split(";")
-      : [""];
-  for (const dir of (env.PATH ?? "").split(path.delimiter)) {
-    if (!dir) continue;
-    for (const ext of exts) {
-      if (isExec(path.join(dir, cmd + ext))) return true;
-    }
-  }
-  return false;
-}
+import { isOnPath } from "./util.ts";
 
 // LspClient - Thin wrapper that spawns a language server, performs the
 // initialize handshake, auto-opens a file, and exposes sendRequest so the

src/daemon.ts

@@ -6,8 +6,9 @@ import * as fs from "node:fs";
 import * as net from "node:net";
 import * as path from "node:path";
 import { LspClient } from "./client.ts";
-import { findRoot, pickServer, pathToUri } from "./root.ts";
+import { findRoot, pathToUri } from "./root.ts";
 import type { ServerConfig } from "./types.ts";
+import { servers } from "../server.ts";
 import {
   logPath,
   socketPath,
@@ -50,14 +51,25 @@ function log(...args: unknown[]) {
   );
 }
 
-// Get Or Create Entry - Looks up the cached client for a file, spawning a
-// fresh LspClient if needed. The returned entry is guaranteed to have its
-// `ready` promise resolved before the caller uses it.
+// Find Server By ID - Looks up a ServerConfig from the registry by ID.
+// Throws if the ID is not registered.
+function findServerById(serverId: string): ServerConfig {
+  const server = servers.find((s) => s.id === serverId);
+  if (!server) {
+    throw new Error(`Unknown server ID: "${serverId}". Registered: ${servers.map((s) => s.id).join(", ")}`);
+  }
+  return server;
+}
+
+// Get Or Create Entry - Looks up the cached client for a server+file,
+// spawning a fresh LspClient if needed. The returned entry is guaranteed
+// to have its `ready` promise resolved before the caller uses it.
 async function getOrCreateEntry(
   filePath: string,
+  serverId: string,
   launch: LaunchContext,
 ): Promise<ClientEntry> {
-  const server = pickServer(filePath);
+  const server = findServerById(serverId);
   const rootDir = findRoot(filePath, server.rootMarkers);
   const key = `${server.id}::${rootDir}`;
   const existing = entries.get(key);
@@ -161,7 +173,7 @@ async function handle(req: DaemonRequest): Promise<DaemonResponse> {
     switch (req.op) {
       case "request": {
         const filePath = path.resolve(req.file);
-        const entry = await getOrCreateEntry(filePath, req.launch);
+        const entry = await getOrCreateEntry(filePath, req.serverId, req.launch);
         const { uri } = await syncFile(entry, filePath);
         bumpIdle(entry);
         const result = await entry.client.sendRequest(
@@ -172,15 +184,27 @@ async function handle(req: DaemonRequest): Promise<DaemonResponse> {
       }
       case "diagnostics": {
         const filePath = path.resolve(req.file);
-        const entry = await getOrCreateEntry(filePath, req.launch);
-        const { uri, changed } = await syncFile(entry, filePath);
-        bumpIdle(entry);
-        if (changed) entry.client.clearDiagnostics(uri);
-        const result = await entry.client.waitForDiagnostics(
-          uri,
-          req.timeoutMs ?? 1500,
+        const timeoutMs = req.timeoutMs ?? 1500;
+
+        // Fan-Out - Run diagnostics against all requested servers in
+        // parallel. Individual failures are captured, not thrown.
+        const results: Record<string, unknown> = {};
+        const settled = await Promise.allSettled(
+          req.serverIds.map(async (serverId) => {
+            const entry = await getOrCreateEntry(filePath, serverId, req.launch);
+            const { uri, changed } = await syncFile(entry, filePath);
+            bumpIdle(entry);
+            if (changed) entry.client.clearDiagnostics(uri);
+            const diag = await entry.client.waitForDiagnostics(uri, timeoutMs);
+            return { serverId, diag };
+          }),
         );
-        return { id: req.id, ok: true, result };
+        for (const outcome of settled) {
+          if (outcome.status === "fulfilled") {
+            results[outcome.value.serverId] = outcome.value.diag;
+          }
+        }
+        return { id: req.id, ok: true, result: results };
       }
       case "status": {
         const result = {

src/daemonClient.ts

@@ -22,6 +22,7 @@ function unwrap(resp: DaemonResponse): unknown {
 // daemon injects textDocument.uri from `file`, so callers omit it.
 export async function daemonRequest(
   file: string,
+  serverId: string,
   method: string,
   params: Record<string, unknown>,
 ): Promise<unknown> {
@@ -29,6 +30,7 @@ export async function daemonRequest(
     await sendOnce({
       op: "request",
       file,
+      serverId,
       method,
       params,
       launch: buildLaunchContext(),
@@ -38,14 +40,18 @@ export async function daemonRequest(
 
 // Wait For Diagnostics - Diagnostics arrive as a notification, not a
 // response, so the daemon has a dedicated op that awaits the next publish.
+// Accepts an array of server IDs; daemon fans out in parallel and returns
+// a grouped map: { [serverId]: { uri, diagnostics[] } }.
 export async function daemonDiagnostics(
   file: string,
+  serverIds: string[],
   timeoutMs = 1500,
 ): Promise<unknown> {
   return unwrap(
     await sendOnce({
       op: "diagnostics",
       file,
+      serverIds,
       timeoutMs,
       launch: buildLaunchContext(),
     }),

src/daemonProtocol.ts

@@ -38,6 +38,7 @@ export type DaemonRequest =
       id: number;
       op: "request";
       file: string;
+      serverId: string;
       method: string;
       params: Record<string, unknown>;
       launch: LaunchContext;
@@ -46,6 +47,7 @@ export type DaemonRequest =
       id: number;
       op: "diagnostics";
       file: string;
+      serverIds: string[];
       timeoutMs?: number;
       launch: LaunchContext;
     }
@@ -57,6 +59,7 @@ export type DaemonRequestWithoutId =
   | {
       op: "request";
       file: string;
+      serverId: string;
       method: string;
       params: Record<string, unknown>;
       launch: LaunchContext;
@@ -64,6 +67,7 @@ export type DaemonRequestWithoutId =
   | {
       op: "diagnostics";
       file: string;
+      serverIds: string[];
       timeoutMs?: number;
       launch: LaunchContext;
     }

src/root.ts

@@ -4,6 +4,7 @@ import { pathToFileURL, fileURLToPath } from "node:url";
 import { servers, globalRootMarkers } from "../server.ts";
 import type { ServerConfig } from "./types.ts";
 import { UnsupportedExtensionError } from "./types.ts";
+import { isOnPath } from "./util.ts";
 
 // Resolve File URI To Local Path
 export function uriToPath(uri: string): string {
@@ -15,11 +16,24 @@ export function pathToUri(p: string): string {
   return pathToFileURL(path.resolve(p)).toString();
 }
 
+// Server Availability Cache - Checked once per process lifetime per server.
+// Avoids repeated filesystem lookups on every tool call.
+const serverAvailability = new Map<string, boolean>();
+
+// Is Server Available - Returns true if the server binary is on PATH.
+// Result is cached for the lifetime of this process.
+export function isServerAvailable(server: ServerConfig): boolean {
+  if (serverAvailability.has(server.id)) return serverAvailability.get(server.id)!;
+  const available = isOnPath(server.command, process.env);
+  serverAvailability.set(server.id, available);
+  return available;
+}
+
 // Pick Server By File Extension - match[] entries are matched against the
-// file's extension (no dot). First server in the registry wins.
+// file's extension (no dot). First available, non-diagnosticsOnly server wins.
 export function pickServer(filePath: string): ServerConfig {
   const ext = path.extname(filePath).replace(/^\./, "");
-  const hit = servers.find((s) => s.match.includes(ext));
+  const hit = servers.find((s) => s.match.includes(ext) && !s.diagnosticsOnly && isServerAvailable(s));
   if (!hit) {
     throw new UnsupportedExtensionError(`.${ext}`);
   }

src/types.ts

@@ -34,6 +34,10 @@ export interface ServerConfig {
   // Idle TTL - Daemon keeps one server alive per (id, rootDir) and evicts
   // it after this many ms of inactivity. Defaults to 5 minutes.
   idleTtlMs?: number;
+  // Diagnostics Only - When true, this server is excluded from
+  // hover/definition/references/completion/documentSymbol but included
+  // in lsp_diagnostics and auto-check.
+  diagnosticsOnly?: boolean;
 }
 
 // Supported high-level commands exposed via the CLI. Extend this union

src/util.ts

@@ -0,0 +1,27 @@
+import * as fs from "node:fs";
+import * as path from "node:path";
+
+// Is On PATH - Returns true if `cmd` resolves to an executable via the
+// supplied PATH. Absolute/relative paths are checked directly.
+export function isOnPath(cmd: string, env: NodeJS.ProcessEnv): boolean {
+  const isExec = (p: string) => {
+    try {
+      fs.accessSync(p, fs.constants.X_OK);
+      return true;
+    } catch {
+      return false;
+    }
+  };
+  if (cmd.includes(path.sep)) return isExec(cmd);
+  const exts =
+    process.platform === "win32"
+      ? (env.PATHEXT ?? ".EXE;.CMD;.BAT").split(";")
+      : [""];
+  for (const dir of (env.PATH ?? "").split(path.delimiter)) {
+    if (!dir) continue;
+    for (const ext of exts) {
+      if (isExec(path.join(dir, cmd + ext))) return true;
+    }
+  }
+  return false;
+}