feat(watcher): forward FS events as workspace/didChangeWatchedFiles

LSP servers maintain their own workspace index built at initialize time and rely on the client to push file-system events. Previously the daemon only synced the single file being queried, so externally created/changed files (codegen, build scripts, git checkout, the agent's own writes from the perspective of other open files) left the server's index stale until manual /lsp-destroy. Each ClientEntry now lazily owns a WorkspaceWatcher (chokidar + picomatch) that translates FS events into workspace/didChangeWatchedFiles batches. Patterns come from the server via client/registerCapability (no speculative watching). Ignores layer a tiny baseline (.git, .DS_Store) over the repo's root .gitignore, with a fallback list for non-git workspaces. Events debounce 50ms quiet / 500ms max wait. Notable: gopls registers absolute-path globs (/abs/root/**/*.go) rather than relative ones, so compileWatchers() matches each event against both relative and absolute path forms. Caught by the integration test; unit regression test added. Rollback: PI_LSP_DISABLE_WATCHERS=1 disables all watcher creation. - src/client.ts: honor register/unregisterCapability for workspace/didChangeWatchedFiles; advertise dynamicRegistration; expose getFileWatchers/onWatchersChanged/sendNotification - src/watcher.ts: new WorkspaceWatcher with layered ignores, debounce+batch, Created+Deleted coalescing, dual-form glob matching - src/daemon.ts: per-entry watcher lifecycle, PI_LSP_DISABLE_WATCHERS, LSP_DEBUG-gated pattern/event logging - test/unit/watcher.test.ts: 11 tests against real chokidar + temp dir - test/integration/watcher-gopls.test.ts: end-to-end against gopls - AGENTS.md: new "Workspace File Watching" section - flake.nix: add go (required by gopls integration test)
2026-05-19 23:43:32 -04:00
parent e143e05758
commit 77876264ee
9 changed files with 869 additions and 5 deletions
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -41,6 +41,38 @@ The daemon tracks opened files per-entry in a `Map<uri, mtimeMs>`. On each reque

 A per-entry `serializer` promise chain prevents concurrent syncs from racing.

+### Workspace File Watching
+
+Each `ClientEntry` lazily owns a `WorkspaceWatcher` (`src/watcher.ts`,
+chokidar + picomatch) that translates filesystem events into
+`workspace/didChangeWatchedFiles` notifications. This keeps the server's
+workspace index fresh when files are created/changed/deleted **outside** of
+LSP tool calls (build scripts, codegen, `git checkout`, the agent's own
+file writes).
+
+Non-obvious bits:
+
+- **Patterns come from the server.** We honor `client/registerCapability`
+  for `workspace/didChangeWatchedFiles` and store the registrations on the
+  `LspClient`. **Don't re-stub those handlers**; they look harmless but
+  break the entire feature. If a server doesn't register, we don't watch.
+- **Servers send mixed pattern forms.** Gopls registers absolute-path
+  globs (`/abs/root/**/*.go`); others send relative (`**/*.ts`) or
+  `RelativePattern` objects. `compileWatchers()` tries both relative and
+  absolute matching against each event so we accept all forms.
+- **Ignore layering.** Always-ignore baseline (`.git/`, `.DS_Store`) +
+  root `.gitignore` parsed via the `ignore` package + a small fallback
+  for non-git workspaces. Nested gitignores aren't supported yet.
+- **Debounce.** 50ms quiet period, capped at 500ms max wait so sustained
+  event streams (branch switches) still flush in bounded time.
+- **Watcher and mtime-sync coexist.** When the agent edits a file we'll
+  emit `didChangeWatchedFiles` *and* the next request's `syncFile` will
+  send a `didChange`. Servers treat the two as orthogonal (workspace
+  index vs. editor buffer) and dedupe internally. This matches VS Code.
+- **Rollback.** `PI_LSP_DISABLE_WATCHERS=1` short-circuits all watcher
+  creation — if something goes wrong in a real workspace, this restores
+  the prior "only the queried file is synced" behavior.
+
 ### Extension vs Daemon Responsibilities

 | Concern | Where |
@@ -60,7 +92,8 @@ cli.ts              — CLI for testing/debugging (daemon-aware or --no-daemon)
 daemon.ts           — Entrypoint that starts the daemon process

 src/
-  client.ts         — LspClient: spawns a language server, JSON-RPC handshake, file sync
+  client.ts         — LspClient: spawns a language server, JSON-RPC handshake, file sync, file-watcher registrations
+  watcher.ts        — WorkspaceWatcher: chokidar + picomatch → workspace/didChangeWatchedFiles batches
  commands.ts       — CLI command dispatcher (maps command names → LSP methods)
  config.ts         — Per-repo `.pi-lsp.json` loader: walk-up + merge with built-ins, mtime cache
  daemonClient.ts   — High-level helpers (daemonRequest, daemonDiagnostics, etc.)
--- a/flake.nix
+++ b/flake.nix
@@ -25,6 +25,7 @@
            typescript-language-server

            # Tests
+            go
            gopls
            pyright
          ];
--- a/package-lock.json
+++ b/package-lock.json
@@ -8,6 +8,9 @@
      "name": "@evan/pi-lsp",
      "version": "0.1.0",
      "dependencies": {
+        "chokidar": "^5.0.0",
+        "ignore": "^7.0.5",
+        "picomatch": "^4.0.4",
        "vscode-jsonrpc": "^8.2.1",
        "vscode-languageserver-protocol": "^3.17.5"
      },
@@ -17,6 +20,7 @@
      "devDependencies": {
        "@mariozechner/pi-coding-agent": "^0.72.0",
        "@types/node": "^22.10.0",
+        "@types/picomatch": "^4.0.3",
        "oxlint": "^1.62.0",
        "tsx": "^4.19.2",
        "typebox": "^1.1.37",
@@ -2789,6 +2793,13 @@
        "undici-types": "~6.21.0"
      }
    },
+    "node_modules/@types/picomatch": {
+      "version": "4.0.3",
+      "resolved": "https://registry.npmjs.org/@types/picomatch/-/picomatch-4.0.3.tgz",
+      "integrity": "sha512-iG0T6+nYJ9FAPmx9SsUlnwcq1ZVRuCXcVEvWnntoPlrOpwtSTKNDC9uVAxTsC3PUvJ+99n4RpAcNgBbHX3JSnQ==",
+      "dev": true,
+      "license": "MIT"
+    },
    "node_modules/@types/retry": {
      "version": "0.12.0",
      "resolved": "https://registry.npmjs.org/@types/retry/-/retry-0.12.0.tgz",
@@ -2967,6 +2978,21 @@
        "url": "https://github.com/chalk/chalk?sponsor=1"
      }
    },
+    "node_modules/chokidar": {
+      "version": "5.0.0",
+      "resolved": "https://registry.npmjs.org/chokidar/-/chokidar-5.0.0.tgz",
+      "integrity": "sha512-TQMmc3w+5AxjpL8iIiwebF73dRDF4fBIieAqGn9RGCWaEVwQ6Fb2cGe31Yns0RRIzii5goJ1Y7xbMwo1TxMplw==",
+      "license": "MIT",
+      "dependencies": {
+        "readdirp": "^5.0.0"
+      },
+      "engines": {
+        "node": ">= 20.19.0"
+      },
+      "funding": {
+        "url": "https://paulmillr.com/funding/"
+      }
+    },
    "node_modules/cli-highlight": {
      "version": "2.1.11",
      "resolved": "https://registry.npmjs.org/cli-highlight/-/cli-highlight-2.1.11.tgz",
@@ -3642,7 +3668,6 @@
      "version": "7.0.5",
      "resolved": "https://registry.npmjs.org/ignore/-/ignore-7.0.5.tgz",
      "integrity": "sha512-Hs59xBNfUIunMFgWAbGX5cq6893IbWg4KnrjbYwX3tx0ztorVgTDA6B2sxf8ejHJ4wz8BqGUMYlnzNBer5NvGg==",
-      "dev": true,
      "license": "MIT",
      "engines": {
        "node": ">= 4"
@@ -4085,6 +4110,18 @@
      "dev": true,
      "license": "MIT"
    },
+    "node_modules/picomatch": {
+      "version": "4.0.4",
+      "resolved": "https://registry.npmjs.org/picomatch/-/picomatch-4.0.4.tgz",
+      "integrity": "sha512-QP88BAKvMam/3NxH6vj2o21R6MjxZUAd6nlwAS/pnGvN9IVLocLHxGYIzFhg6fUQ+5th6P4dv4eW9jX3DSIj7A==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=12"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/jonschlinkert"
+      }
+    },
    "node_modules/proper-lockfile": {
      "version": "4.1.2",
      "resolved": "https://registry.npmjs.org/proper-lockfile/-/proper-lockfile-4.1.2.tgz",
@@ -4180,6 +4217,19 @@
        "once": "^1.3.1"
      }
    },
+    "node_modules/readdirp": {
+      "version": "5.0.0",
+      "resolved": "https://registry.npmjs.org/readdirp/-/readdirp-5.0.0.tgz",
+      "integrity": "sha512-9u/XQ1pvrQtYyMpZe7DXKv2p5CNvyVwzUB6uhLAnQwHMSgKMBR62lc7AHljaeteeHXn11XTAaLLUVZYVZyuRBQ==",
+      "license": "MIT",
+      "engines": {
+        "node": ">= 20.19.0"
+      },
+      "funding": {
+        "type": "individual",
+        "url": "https://paulmillr.com/funding/"
+      }
+    },
    "node_modules/require-directory": {
      "version": "2.1.1",
      "resolved": "https://registry.npmjs.org/require-directory/-/require-directory-2.1.1.tgz",
--- a/package.json
+++ b/package.json
@@ -21,12 +21,16 @@
    "test:integration": "NODE_OPTIONS='--import=tsx' node --test test/integration/**/*.ts"
  },
  "dependencies": {
+    "chokidar": "^5.0.0",
+    "ignore": "^7.0.5",
+    "picomatch": "^4.0.4",
    "vscode-jsonrpc": "^8.2.1",
    "vscode-languageserver-protocol": "^3.17.5"
  },
  "devDependencies": {
    "@mariozechner/pi-coding-agent": "^0.72.0",
    "@types/node": "^22.10.0",
+    "@types/picomatch": "^4.0.3",
    "oxlint": "^1.62.0",
    "tsx": "^4.19.2",
    "typebox": "^1.1.37",
--- a/src/client.ts
+++ b/src/client.ts
@@ -7,8 +7,11 @@ import {
  type MessageConnection,
 } from "vscode-jsonrpc/node.js";
 import type {
+  FileSystemWatcher,
  InitializeParams,
  PublishDiagnosticsParams,
+  Registration,
+  Unregistration,
 } from "vscode-languageserver-protocol";
 import type { ServerConfig } from "./types.ts";
 import { ServerNotFoundError } from "./types.ts";
@@ -40,6 +43,12 @@ export class LspClient {
  // version numbers in didOpen/didChange. We track them so the daemon
  // can resync files via notifyChange after on-disk edits.
  private versions = new Map<string, number>();
+  // File Watcher Registrations - Servers announce which globs they care
+  // about via client/registerCapability("workspace/didChangeWatchedFiles").
+  // Stored by registration id so unregister can remove them; listeners
+  // are notified so the daemon can (re)build a chokidar watcher.
+  private fileWatchers = new Map<string, FileSystemWatcher[]>();
+  private watchersListeners = new Set<() => void>();

  constructor(private readonly server: ServerConfig) {}

@@ -115,8 +124,34 @@ export class LspClient {
        );
      },
    );
-    this.conn.onRequest("client/registerCapability", () => null);
-    this.conn.onRequest("client/unregisterCapability", () => null);
+    this.conn.onRequest(
+      "client/registerCapability",
+      (params: { registrations?: Registration[] }) => {
+        let changed = false;
+        for (const reg of params.registrations ?? []) {
+          if (reg.method !== "workspace/didChangeWatchedFiles") continue;
+          const opts = reg.registerOptions as
+            | { watchers?: FileSystemWatcher[] }
+            | undefined;
+          this.fileWatchers.set(reg.id, opts?.watchers ?? []);
+          changed = true;
+        }
+        if (changed) for (const l of this.watchersListeners) l();
+        return null;
+      },
+    );
+    this.conn.onRequest(
+      "client/unregisterCapability",
+      (params: { unregisterations?: Unregistration[] }) => {
+        let changed = false;
+        for (const unreg of params.unregisterations ?? []) {
+          if (unreg.method !== "workspace/didChangeWatchedFiles") continue;
+          if (this.fileWatchers.delete(unreg.id)) changed = true;
+        }
+        if (changed) for (const l of this.watchersListeners) l();
+        return null;
+      },
+    );

    this.conn.listen();

@@ -135,7 +170,13 @@ export class LspClient {
          publishDiagnostics: {},
          synchronization: { didSave: true },
        },
-        workspace: { workspaceFolders: true, configuration: true },
+        workspace: {
+          workspaceFolders: true,
+          configuration: true,
+          // Dynamic Registration - Required for servers like gopls to send
+          // client/registerCapability("workspace/didChangeWatchedFiles").
+          didChangeWatchedFiles: { dynamicRegistration: true },
+        },
      },
    };
    await this.conn.sendRequest("initialize", {
@@ -217,6 +258,28 @@ export class LspClient {
    return this.conn.sendRequest(method, params) as Promise<R>;
  }

+  // Send Raw LSP Notification - Used by the daemon's workspace watcher to
+  // push workspace/didChangeWatchedFiles batches without going through the
+  // typed command surface.
+  sendNotification(method: string, params: unknown): void {
+    this.conn.sendNotification(method, params);
+  }
+
+  // Get File Watchers - Flat list of all watcher patterns the server has
+  // registered across all registration ids.
+  getFileWatchers(): FileSystemWatcher[] {
+    return Array.from(this.fileWatchers.values()).flat();
+  }
+
+  // On Watchers Changed - Subscribe to register/unregister events for
+  // workspace/didChangeWatchedFiles. Returns an unsubscribe fn.
+  onWatchersChanged(listener: () => void): () => void {
+    this.watchersListeners.add(listener);
+    return () => {
+      this.watchersListeners.delete(listener);
+    };
+  }
+
  // Clear Diagnostics - Drops the cached diagnostics for a URI so callers
  // can force waitForDiagnostics to await a fresh publish after didChange.
  clearDiagnostics(uri: string): void {
--- a/src/daemon.ts
+++ b/src/daemon.ts
@@ -8,6 +8,7 @@ import * as path from "node:path";
 import { LspClient } from "./client.ts";
 import { findRoot, findServerById, pathToUri } from "./root.ts";
 import type { ServerConfig } from "./types.ts";
+import { WorkspaceWatcher, type FileEvent } from "./watcher.ts";
 import {
  logPath,
  socketPath,
@@ -37,6 +38,11 @@ interface ClientEntry {
  idleTimer: NodeJS.Timeout | null;
  ttlMs: number;
  lastUsed: number;
+  // watcher: Lazy - created on first registerCapability for watched files,
+  // disposed on eviction. Null if the server never registered any watchers
+  // (or if PI_LSP_DISABLE_WATCHERS is set).
+  watcher: WorkspaceWatcher | null;
+  unsubscribeWatchers: (() => void) | null;
 }

 const entries = new Map<string, ClientEntry>();
@@ -88,6 +94,8 @@ async function getOrCreateEntry(
    idleTimer: null,
    ttlMs,
    lastUsed: Date.now(),
+    watcher: null,
+    unsubscribeWatchers: null,
  };
  entries.set(key, entry);
  try {
@@ -96,10 +104,52 @@ async function getOrCreateEntry(
    entries.delete(key);
    throw err;
  }
+  attachWatcher(entry);
  bumpIdle(entry);
  return entry;
 }

+// Attach Watcher - Subscribes to the client's watcher-registration events.
+// The first non-empty registration lazily creates the WorkspaceWatcher;
+// subsequent register/unregister calls update its pattern set in place.
+// Honors PI_LSP_DISABLE_WATCHERS for emergency rollback.
+function attachWatcher(entry: ClientEntry): void {
+  if (process.env.PI_LSP_DISABLE_WATCHERS) return;
+  const sync = () => {
+    const patterns = entry.client.getFileWatchers();
+    if (patterns.length === 0 && !entry.watcher) return;
+    if (!entry.watcher) {
+      entry.watcher = new WorkspaceWatcher(entry.rootDir, (events) =>
+        forwardEvents(entry, events),
+      );
+      log(`watcher`, entry.server.id, entry.rootDir, `patterns=${patterns.length}`);
+    }
+    if (process.env.LSP_DEBUG) {
+      log(`watcher patterns`, entry.server.id, JSON.stringify(patterns));
+    }
+    entry.watcher.setPatterns(patterns);
+  };
+  entry.unsubscribeWatchers = entry.client.onWatchersChanged(sync);
+  // Initial Sync - Server may have already sent registerCapability during
+  // initialize before we subscribed.
+  sync();
+}
+
+// Forward Events - Sends a batched workspace/didChangeWatchedFiles to the
+// server. We catch errors so a downed transport doesn't crash the daemon.
+function forwardEvents(entry: ClientEntry, events: FileEvent[]): void {
+  try {
+    if (process.env.LSP_DEBUG) {
+      log(`watcher fire`, entry.server.id, JSON.stringify(events));
+    }
+    entry.client.sendNotification("workspace/didChangeWatchedFiles", {
+      changes: events,
+    });
+  } catch (err) {
+    log(`watcher send failed`, entry.server.id, (err as Error).message);
+  }
+}
+
 // Bump Idle - Resets the idle eviction timer. Called on every request that
 // touches the entry. We log evictions so the daemon's behavior is visible.
 function bumpIdle(entry: ClientEntry) {
@@ -113,6 +163,8 @@ function evict(entry: ClientEntry, reason: string) {
  log(`evict`, entry.key, reason);
  entries.delete(entry.key);
  if (entry.idleTimer) clearTimeout(entry.idleTimer);
+  if (entry.unsubscribeWatchers) entry.unsubscribeWatchers();
+  void entry.watcher?.dispose();
  void entry.client.dispose();
  // Auto Shutdown - If this was the last entry, there's nothing left to
  // manage. Tear down the daemon so it doesn't sit idle forever.
@@ -292,6 +344,8 @@ function shutdownDaemon(reason: string) {
  if (server) server.close();
  for (const entry of entries.values()) {
    if (entry.idleTimer) clearTimeout(entry.idleTimer);
+    if (entry.unsubscribeWatchers) entry.unsubscribeWatchers();
+    void entry.watcher?.dispose();
    void entry.client.dispose();
  }
  entries.clear();
--- a/src/watcher.ts
+++ b/src/watcher.ts
@@ -0,0 +1,265 @@
+// Workspace Watcher - Per-entry filesystem watcher that translates chokidar
+// events into LSP `workspace/didChangeWatchedFiles` notifications. One
+// instance lives on each daemon ClientEntry; it's torn down on eviction.
+//
+// Design:
+//   - Patterns come from the server via `client/registerCapability`. If the
+//     server registers nothing, we don't watch anything (no speculative
+//     watching - wastes inotify slots and risks event storms).
+//   - We honor the repo's `.gitignore` (root level only for v1) plus a tiny
+//     always-ignore baseline so the watcher tracks the same set the LSP
+//     server would naturally care about.
+//   - Events are debounced (50ms quiet period) but force-flushed every 500ms
+//     so sustained event streams (e.g. `git checkout` of a big branch)
+//     can't stall the batch indefinitely.
+import * as fs from "node:fs";
+import * as path from "node:path";
+import chokidar, { type FSWatcher } from "chokidar";
+import ignore, { type Ignore } from "ignore";
+import picomatch from "picomatch";
+import type { FileSystemWatcher } from "vscode-languageserver-protocol";
+import { pathToUri } from "./root.ts";
+
+// LSP Constants - Inlined to avoid pulling the enum into a hot path. See
+// https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#fileChangeType
+const FILE_CHANGE_CREATED = 1;
+const FILE_CHANGE_CHANGED = 2;
+const FILE_CHANGE_DELETED = 3;
+
+// WatchKind - Bitmask the server uses to opt out of specific event kinds.
+// Default (when `kind` is omitted) is 7 = Create | Change | Delete.
+const WATCH_KIND_CREATE = 1;
+const WATCH_KIND_CHANGE = 2;
+const WATCH_KIND_DELETE = 4;
+
+// Debounce Settings - Quiet period before flushing, plus a max wait so a
+// continuous stream of events still gets delivered in bounded time.
+const DEBOUNCE_QUIET_MS = 50;
+const DEBOUNCE_MAX_WAIT_MS = 500;
+
+// Always-Ignore Baseline - Things that are never interesting to an LSP
+// server regardless of gitignore contents. `.git/` is the obvious one
+// (gitignore won't list itself); the rest are OS/editor noise.
+export const BASELINE_IGNORES = [
+  "**/.git/**",
+  "**/.DS_Store",
+  "**/.hg/**",
+  "**/.svn/**",
+];
+
+// Fallback Ignores - Used only when no `.gitignore` exists at rootDir.
+// Conservative list to keep non-git workspaces from blowing up.
+const NO_GITIGNORE_FALLBACK = ["**/node_modules/**", "**/.git/**"];
+
+export interface FileEvent {
+  uri: string;
+  type: 1 | 2 | 3;
+}
+
+// Build Ignore Matcher - Returns a function that takes a path relative to
+// rootDir and returns true if it should be ignored. Reads root .gitignore
+// once at construction; nested gitignores are intentionally not supported
+// in v1 (covers ~95% of repos; add nesting if it becomes a real problem).
+function buildIgnoreMatcher(rootDir: string): (relPath: string) => boolean {
+  const gitignorePath = path.join(rootDir, ".gitignore");
+  let ig: Ignore | null = null;
+  if (fs.existsSync(gitignorePath)) {
+    try {
+      ig = ignore().add(fs.readFileSync(gitignorePath, "utf8"));
+    } catch {
+      // Treat parse failure as no gitignore. The fallback will catch the
+      // worst offenders (node_modules) so we don't watch everything.
+      ig = null;
+    }
+  }
+  // Baseline always applies; gitignore (or fallback) layers on top.
+  const baselineMatcher = picomatch(BASELINE_IGNORES);
+  if (ig) {
+    return (relPath) => {
+      if (baselineMatcher(relPath)) return true;
+      // `ignore` requires posix-style paths and bare relative paths.
+      const posixRel = relPath.split(path.sep).join("/");
+      if (!posixRel || posixRel === ".") return false;
+      return ig!.ignores(posixRel);
+    };
+  }
+  const fallbackMatcher = picomatch(NO_GITIGNORE_FALLBACK);
+  return (relPath) => baselineMatcher(relPath) || fallbackMatcher(relPath);
+}
+
+// Compile Watcher Patterns - Turns the server-supplied FileSystemWatcher[]
+// into a single matcher that returns the matched `kind` bitmask (or 0 if
+// no pattern matched). Returning the kind lets us filter individual event
+// types (Created/Changed/Deleted) per the spec.
+//
+// Servers send patterns in mixed forms: bare relative globs (`**/*.ts`),
+// absolute path globs (gopls sends `/abs/root/**/*.go`), or RelativePattern
+// objects. We try matching the candidate path in both relative and absolute
+// form so we accept any of those without server-specific special casing.
+function compileWatchers(
+  watchers: FileSystemWatcher[],
+  rootDir: string,
+): (relPath: string, absPath: string) => number {
+  const compiled = watchers.map((w) => {
+    const pattern = typeof w.globPattern === "string"
+      ? w.globPattern
+      : resolveRelativePattern(w.globPattern, rootDir);
+    return {
+      match: picomatch(pattern, { dot: true }),
+      kind: w.kind ?? (WATCH_KIND_CREATE | WATCH_KIND_CHANGE | WATCH_KIND_DELETE),
+    };
+  });
+  return (relPath, absPath) => {
+    const posixRel = relPath.split(path.sep).join("/");
+    const posixAbs = absPath.split(path.sep).join("/");
+    let kind = 0;
+    for (const c of compiled) {
+      if (c.match(posixRel) || c.match(posixAbs)) kind |= c.kind;
+    }
+    return kind;
+  };
+}
+
+// Resolve Relative Pattern - LSP 3.17 added RelativePattern with a baseUri
+// that may be a string or WorkspaceFolder. We collapse to a glob relative
+// to rootDir; out-of-root patterns fall back to the raw pattern (won't
+// match anything inside rootDir, which is the safe behavior).
+function resolveRelativePattern(
+  rp: { baseUri: string | { uri: string }; pattern: string },
+  rootDir: string,
+): string {
+  const baseUri = typeof rp.baseUri === "string" ? rp.baseUri : rp.baseUri.uri;
+  if (!baseUri.startsWith("file://")) return rp.pattern;
+  const basePath = decodeURIComponent(baseUri.slice("file://".length));
+  const relBase = path.relative(rootDir, basePath);
+  if (relBase.startsWith("..") || path.isAbsolute(relBase)) return rp.pattern;
+  return relBase ? `${relBase.split(path.sep).join("/")}/${rp.pattern}` : rp.pattern;
+}
+
+export class WorkspaceWatcher {
+  private chokidar: FSWatcher | null = null;
+  private isIgnored: (relPath: string) => boolean;
+  private matchKind: (relPath: string, absPath: string) => number = () => 0;
+  private pending = new Map<string, 1 | 2 | 3>();
+  private quietTimer: NodeJS.Timeout | null = null;
+  private maxWaitTimer: NodeJS.Timeout | null = null;
+  private disposed = false;
+
+  constructor(
+    private readonly rootDir: string,
+    private readonly onEvents: (events: FileEvent[]) => void,
+  ) {
+    this.isIgnored = buildIgnoreMatcher(rootDir);
+  }
+
+  // Set Patterns - Called whenever the server's registered watchers change.
+  // First non-empty call lazily starts chokidar; subsequent calls update
+  // the matcher in place (chokidar already watches everything under
+  // rootDir minus ignores, so we just recompile filtering).
+  setPatterns(watchers: FileSystemWatcher[]): void {
+    if (this.disposed) return;
+    this.matchKind = compileWatchers(watchers, this.rootDir);
+    if (watchers.length === 0) {
+      // Server unregistered everything - stop watching but keep instance
+      // alive so re-registration works without recreating chokidar state.
+      void this.stopChokidar();
+      return;
+    }
+    if (!this.chokidar) this.startChokidar();
+  }
+
+  private startChokidar(): void {
+    this.chokidar = chokidar.watch(this.rootDir, {
+      ignoreInitial: true,
+      followSymlinks: false,
+      // Ignore Function - Called by chokidar for each path; returning true
+      // prevents both watching and event emission. Cheaper than emitting
+      // and filtering afterward.
+      ignored: (absPath: string) => {
+        if (absPath === this.rootDir) return false;
+        const rel = path.relative(this.rootDir, absPath);
+        return this.isIgnored(rel);
+      },
+      // Atomic Writes - Many editors write-via-rename; awaitWriteFinish
+      // coalesces those into a single change event. Conservative thresholds
+      // keep latency low while still suppressing partial-write noise.
+      awaitWriteFinish: { stabilityThreshold: 50, pollInterval: 25 },
+    });
+    this.chokidar.on("add", (p) => this.queue(p, FILE_CHANGE_CREATED));
+    this.chokidar.on("change", (p) => this.queue(p, FILE_CHANGE_CHANGED));
+    this.chokidar.on("unlink", (p) => this.queue(p, FILE_CHANGE_DELETED));
+    // ENOSPC and friends - log but don't crash the entry.
+    this.chokidar.on("error", (err) => {
+      process.stderr.write(`[pi-lsp:watcher] ${this.rootDir}: ${String(err)}\n`);
+    });
+  }
+
+  private async stopChokidar(): Promise<void> {
+    if (!this.chokidar) return;
+    const w = this.chokidar;
+    this.chokidar = null;
+    await w.close();
+  }
+
+  private queue(absPath: string, type: 1 | 2 | 3): void {
+    const rel = path.relative(this.rootDir, absPath);
+    const kind = this.matchKind(rel, absPath);
+    if (kind === 0) return;
+    // Per-Spec Filter - Skip events the server explicitly opted out of.
+    if (type === FILE_CHANGE_CREATED && !(kind & WATCH_KIND_CREATE)) return;
+    if (type === FILE_CHANGE_CHANGED && !(kind & WATCH_KIND_CHANGE)) return;
+    if (type === FILE_CHANGE_DELETED && !(kind & WATCH_KIND_DELETE)) return;
+    const uri = pathToUri(absPath);
+    // Coalesce per URI - Last event wins. The exception is Created followed
+    // by Deleted (and vice versa), which we collapse to "nothing happened"
+    // by deleting the entry; the server doesn't need to learn about a file
+    // that briefly existed.
+    const prev = this.pending.get(uri);
+    if (
+      (prev === FILE_CHANGE_CREATED && type === FILE_CHANGE_DELETED) ||
+      (prev === FILE_CHANGE_DELETED && type === FILE_CHANGE_CREATED)
+    ) {
+      this.pending.delete(uri);
+    } else {
+      this.pending.set(uri, type);
+    }
+    this.scheduleFlush();
+  }
+
+  private scheduleFlush(): void {
+    if (this.quietTimer) clearTimeout(this.quietTimer);
+    this.quietTimer = setTimeout(() => this.flush(), DEBOUNCE_QUIET_MS);
+    if (!this.maxWaitTimer) {
+      this.maxWaitTimer = setTimeout(() => this.flush(), DEBOUNCE_MAX_WAIT_MS);
+    }
+  }
+
+  private flush(): void {
+    if (this.quietTimer) clearTimeout(this.quietTimer);
+    if (this.maxWaitTimer) clearTimeout(this.maxWaitTimer);
+    this.quietTimer = null;
+    this.maxWaitTimer = null;
+    if (this.pending.size === 0) return;
+    const events: FileEvent[] = Array.from(this.pending, ([uri, type]) => ({
+      uri,
+      type,
+    }));
+    this.pending.clear();
+    try {
+      this.onEvents(events);
+    } catch (err) {
+      process.stderr.write(
+        `[pi-lsp:watcher] onEvents threw: ${(err as Error).message}\n`,
+      );
+    }
+  }
+
+  async dispose(): Promise<void> {
+    if (this.disposed) return;
+    this.disposed = true;
+    if (this.quietTimer) clearTimeout(this.quietTimer);
+    if (this.maxWaitTimer) clearTimeout(this.maxWaitTimer);
+    this.pending.clear();
+    await this.stopChokidar();
+  }
+}
--- a/test/integration/watcher-gopls.test.ts
+++ b/test/integration/watcher-gopls.test.ts
@@ -0,0 +1,145 @@
+// Watcher Integration Test — proves the FS-watcher → LSP wiring works
+// end-to-end against gopls: a stale "undefined symbol" diagnostic clears
+// after the missing file is created externally (no LSP query touches it).
+//
+// This is the canonical staleness scenario from `_scratch/plan-fs-watching.md`.
+import { describe, it, before, after } from "node:test";
+import * as assert from "node:assert/strict";
+import * as fs from "node:fs";
+import * as os from "node:os";
+import * as path from "node:path";
+import {
+  setTestSocket,
+  stopTestDaemon,
+  runCliJson,
+  requireServer,
+} from "../helpers.ts";
+
+const skip = requireServer("gopls");
+
+// Poll Until - Runs `fn` repeatedly until `predicate(result)` is true or
+// the timeout elapses. We need this because gopls reanalysis after a
+// workspace/didChangeWatchedFiles is asynchronous; diagnostics arrive on
+// a `textDocument/publishDiagnostics` push that we then re-fetch.
+async function pollUntil<T>(
+  fn: () => Promise<T>,
+  predicate: (v: T) => boolean,
+  timeoutMs: number,
+  intervalMs = 250,
+): Promise<T> {
+  const deadline = Date.now() + timeoutMs;
+  let last: T = await fn();
+  while (Date.now() < deadline) {
+    if (predicate(last)) return last;
+    await new Promise((r) => setTimeout(r, intervalMs));
+    last = await fn();
+  }
+  return last;
+}
+
+interface DiagResult {
+  [serverId: string]: { diagnostics?: { message: string }[] };
+}
+
+describe("watcher: gopls picks up externally-created files", { skip: skip ?? undefined }, () => {
+  let tmpDir: string;
+  let mainFile: string;
+  let helperFile: string;
+  const env = { ...process.env };
+  let cleanup: () => void;
+
+  before(async () => {
+    cleanup = setTestSocket(env);
+    await stopTestDaemon(env);
+
+    tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), "pi-lsp-gopls-watch-"));
+    // Minimal Go Module - go.mod is gopls's primary root marker.
+    fs.writeFileSync(path.join(tmpDir, "go.mod"), "module example.com/wtest\n\ngo 1.21\n");
+    mainFile = path.join(tmpDir, "main.go");
+    helperFile = path.join(tmpDir, "helper.go");
+    // main.go references Helper() which doesn't exist yet \u2014 should produce
+    // an "undefined: Helper" diagnostic.
+    fs.writeFileSync(
+      mainFile,
+      "package main\n\nfunc main() {\n\tHelper()\n}\n",
+    );
+  });
+
+  after(async () => {
+    await stopTestDaemon(env);
+    cleanup();
+    fs.rmSync(tmpDir, { recursive: true, force: true });
+  });
+
+  it("initially reports undefined symbol", async () => {
+    // Poll - gopls's workspace load on a fresh tmp dir takes a beat; the
+    // first diagnostics call can return "No active builds contain ..."
+    // before the real analysis lands. Wait up to 15s for it to settle.
+    const result = await pollUntil(
+      async () =>
+        (await runCliJson(
+          [mainFile, "diagnostics", '{"timeoutMs":3000}'],
+          env,
+        )) as DiagResult,
+      (r) => {
+        const diags = r["gopls"]?.diagnostics ?? [];
+        return diags.some(
+          (d) =>
+            d.message.toLowerCase().includes("undefined") ||
+            d.message.includes("Helper"),
+        );
+      },
+      15000,
+      500,
+    );
+    const diags = result["gopls"]?.diagnostics ?? [];
+    const hasUndefined = diags.some((d) =>
+      d.message.toLowerCase().includes("undefined") || d.message.includes("Helper"),
+    );
+    assert.ok(
+      hasUndefined,
+      `Expected undefined-symbol diagnostic, got: ${JSON.stringify(diags)}`,
+    );
+  });
+
+  it("clears the diagnostic after helper.go is created externally", async () => {
+    // Create The Missing File Without Touching It Via LSP - This is the
+    // whole point: only chokidar + workspace/didChangeWatchedFiles can tell
+    // gopls about helper.go's existence.
+    fs.writeFileSync(
+      helperFile,
+      "package main\n\nfunc Helper() {}\n",
+    );
+
+    // Poll - Allow up to 15s for the watcher to fire, gopls to reanalyze,
+    // and the diagnostic to clear. Re-query the same file (main.go) only.
+    const result = await pollUntil(
+      async () =>
+        (await runCliJson(
+          [mainFile, "diagnostics", '{"timeoutMs":3000}'],
+          env,
+        )) as DiagResult,
+      (r) => {
+        const diags = r["gopls"]?.diagnostics ?? [];
+        return !diags.some(
+          (d) =>
+            d.message.toLowerCase().includes("undefined") ||
+            d.message.includes("Helper"),
+        );
+      },
+      15000,
+      500,
+    );
+
+    const finalDiags = result["gopls"]?.diagnostics ?? [];
+    const stillUndefined = finalDiags.some(
+      (d) =>
+        d.message.toLowerCase().includes("undefined") ||
+        d.message.includes("Helper"),
+    );
+    assert.ok(
+      !stillUndefined,
+      `Expected diagnostic to clear after creating helper.go, still got: ${JSON.stringify(finalDiags)}`,
+    );
+  });
+});
--- a/test/unit/watcher.test.ts
+++ b/test/unit/watcher.test.ts
@@ -0,0 +1,249 @@
+// Watcher Unit Tests — exercises WorkspaceWatcher against a temp dir with
+// real chokidar. We use real FS because mocking it would test the mock,
+// not the actual behavior under inotify.
+import { describe, it, before, after, beforeEach } from "node:test";
+import * as assert from "node:assert/strict";
+import * as fs from "node:fs";
+import * as os from "node:os";
+import * as path from "node:path";
+import { WorkspaceWatcher, type FileEvent } from "../../src/watcher.ts";
+import { pathToUri } from "../../src/root.ts";
+
+// Wait For - Polls a predicate up to timeoutMs. Returns true if it became
+// true, false if it timed out. Used to wait on async chokidar events
+// without arbitrary sleeps.
+async function waitFor(
+  predicate: () => boolean,
+  timeoutMs = 2000,
+): Promise<boolean> {
+  const deadline = Date.now() + timeoutMs;
+  while (Date.now() < deadline) {
+    if (predicate()) return true;
+    await new Promise((r) => setTimeout(r, 25));
+  }
+  return predicate();
+}
+
+// Wait Quiet - Waits for the debounce window to elapse so any pending
+// events flush. Slightly longer than DEBOUNCE_MAX_WAIT_MS.
+const FLUSH_WAIT_MS = 700;
+
+describe("WorkspaceWatcher", () => {
+  let tmpDir: string;
+  let received: FileEvent[][] = [];
+  let watcher: WorkspaceWatcher | null = null;
+
+  before(() => {
+    tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), "pi-lsp-watcher-"));
+  });
+
+  after(() => {
+    fs.rmSync(tmpDir, { recursive: true, force: true });
+  });
+
+  beforeEach(async () => {
+    if (watcher) {
+      await watcher.dispose();
+      watcher = null;
+    }
+    received = [];
+    // Reset Dir - Wipe contents between tests so file lists are predictable.
+    for (const entry of fs.readdirSync(tmpDir)) {
+      fs.rmSync(path.join(tmpDir, entry), { recursive: true, force: true });
+    }
+  });
+
+  it("emits Created event for new file matching pattern", async () => {
+    watcher = new WorkspaceWatcher(tmpDir, (evs) => received.push(evs));
+    watcher.setPatterns([{ globPattern: "**/*.ts" }]);
+    // Settle - chokidar's `ready` is implicit; give it a moment.
+    await new Promise((r) => setTimeout(r, 200));
+
+    fs.writeFileSync(path.join(tmpDir, "foo.ts"), "x");
+
+    const ok = await waitFor(() => received.length > 0);
+    assert.ok(ok, "Expected at least one batch");
+    const all = received.flat();
+    const match = all.find((e) => e.uri === pathToUri(path.join(tmpDir, "foo.ts")));
+    assert.ok(match, `Expected event for foo.ts, got ${JSON.stringify(all)}`);
+    assert.strictEqual(match.type, 1);
+  });
+
+  it("emits Deleted event when file removed", async () => {
+    const file = path.join(tmpDir, "del.ts");
+    fs.writeFileSync(file, "x");
+    watcher = new WorkspaceWatcher(tmpDir, (evs) => received.push(evs));
+    watcher.setPatterns([{ globPattern: "**/*.ts" }]);
+    await new Promise((r) => setTimeout(r, 200));
+
+    fs.unlinkSync(file);
+
+    const ok = await waitFor(() =>
+      received.flat().some((e) => e.type === 3 && e.uri === pathToUri(file)),
+    );
+    assert.ok(ok, `Expected delete event, got ${JSON.stringify(received)}`);
+  });
+
+  it("emits Changed event when file content changes", async () => {
+    const file = path.join(tmpDir, "chg.ts");
+    fs.writeFileSync(file, "x");
+    watcher = new WorkspaceWatcher(tmpDir, (evs) => received.push(evs));
+    watcher.setPatterns([{ globPattern: "**/*.ts" }]);
+    await new Promise((r) => setTimeout(r, 200));
+
+    fs.writeFileSync(file, "y");
+
+    const ok = await waitFor(() =>
+      received.flat().some((e) => e.type === 2 && e.uri === pathToUri(file)),
+    );
+    assert.ok(ok, `Expected change event, got ${JSON.stringify(received)}`);
+  });
+
+  it("skips files not matching the pattern", async () => {
+    watcher = new WorkspaceWatcher(tmpDir, (evs) => received.push(evs));
+    watcher.setPatterns([{ globPattern: "**/*.ts" }]);
+    await new Promise((r) => setTimeout(r, 200));
+
+    fs.writeFileSync(path.join(tmpDir, "ignored.txt"), "x");
+    await new Promise((r) => setTimeout(r, FLUSH_WAIT_MS));
+
+    const all = received.flat();
+    assert.strictEqual(
+      all.length,
+      0,
+      `Expected no events for .txt, got ${JSON.stringify(all)}`,
+    );
+  });
+
+  it("honors .gitignore at root", async () => {
+    fs.writeFileSync(path.join(tmpDir, ".gitignore"), "ignored-dir/\n");
+    fs.mkdirSync(path.join(tmpDir, "ignored-dir"));
+    watcher = new WorkspaceWatcher(tmpDir, (evs) => received.push(evs));
+    watcher.setPatterns([{ globPattern: "**/*.ts" }]);
+    await new Promise((r) => setTimeout(r, 200));
+
+    fs.writeFileSync(path.join(tmpDir, "ignored-dir", "x.ts"), "x");
+    fs.writeFileSync(path.join(tmpDir, "watched.ts"), "x");
+
+    await new Promise((r) => setTimeout(r, FLUSH_WAIT_MS));
+
+    const all = received.flat();
+    const uris = all.map((e) => e.uri);
+    assert.ok(
+      uris.includes(pathToUri(path.join(tmpDir, "watched.ts"))),
+      "watched.ts should fire",
+    );
+    assert.ok(
+      !uris.some((u) => u.includes("ignored-dir")),
+      `ignored-dir/x.ts should be filtered, got ${JSON.stringify(uris)}`,
+    );
+  });
+
+  it("respects WatchKind to filter event types", async () => {
+    watcher = new WorkspaceWatcher(tmpDir, (evs) => received.push(evs));
+    // Kind 1 = Create only - delete events should be suppressed.
+    watcher.setPatterns([{ globPattern: "**/*.ts", kind: 1 }]);
+    await new Promise((r) => setTimeout(r, 200));
+
+    const file = path.join(tmpDir, "only-create.ts");
+    fs.writeFileSync(file, "x");
+    await new Promise((r) => setTimeout(r, FLUSH_WAIT_MS));
+    fs.unlinkSync(file);
+    await new Promise((r) => setTimeout(r, FLUSH_WAIT_MS));
+
+    const all = received.flat();
+    assert.ok(
+      all.some((e) => e.type === 1),
+      "Expected create event",
+    );
+    assert.ok(
+      !all.some((e) => e.type === 3),
+      `Did not expect delete events, got ${JSON.stringify(all)}`,
+    );
+  });
+
+  it("batches multiple rapid events into one onEvents call", async () => {
+    watcher = new WorkspaceWatcher(tmpDir, (evs) => received.push(evs));
+    watcher.setPatterns([{ globPattern: "**/*.ts" }]);
+    await new Promise((r) => setTimeout(r, 200));
+
+    // Write Several Files Quickly - within the 50ms debounce window they
+    // should all land in one batch, capped at 500ms max wait.
+    for (let i = 0; i < 5; i++) {
+      fs.writeFileSync(path.join(tmpDir, `f${i}.ts`), "x");
+    }
+
+    await new Promise((r) => setTimeout(r, FLUSH_WAIT_MS));
+
+    const all = received.flat();
+    assert.strictEqual(all.length, 5, `Expected 5 events, got ${all.length}`);
+    // The batches should be << 5 (debounce coalesces). Allow up to 2 in
+    // case the loop straddled a flush boundary.
+    assert.ok(
+      received.length <= 2,
+      `Expected <=2 batches, got ${received.length}: ${JSON.stringify(received)}`,
+    );
+  });
+
+  it("ignores .git/ even when not in gitignore", async () => {
+    watcher = new WorkspaceWatcher(tmpDir, (evs) => received.push(evs));
+    watcher.setPatterns([{ globPattern: "**/*" }]);
+    await new Promise((r) => setTimeout(r, 200));
+
+    fs.mkdirSync(path.join(tmpDir, ".git"));
+    fs.writeFileSync(path.join(tmpDir, ".git", "HEAD"), "ref: foo");
+
+    await new Promise((r) => setTimeout(r, FLUSH_WAIT_MS));
+
+    const all = received.flat();
+    assert.ok(
+      !all.some((e) => e.uri.includes("/.git/")),
+      `.git contents should be ignored, got ${JSON.stringify(all)}`,
+    );
+  });
+
+  it("stops emitting after dispose", async () => {
+    watcher = new WorkspaceWatcher(tmpDir, (evs) => received.push(evs));
+    watcher.setPatterns([{ globPattern: "**/*.ts" }]);
+    await new Promise((r) => setTimeout(r, 200));
+
+    await watcher.dispose();
+    watcher = null;
+
+    fs.writeFileSync(path.join(tmpDir, "post-dispose.ts"), "x");
+    await new Promise((r) => setTimeout(r, FLUSH_WAIT_MS));
+
+    const all = received.flat();
+    assert.strictEqual(all.length, 0, `Expected no events after dispose`);
+  });
+
+  it("matches absolute-path glob patterns (gopls-style)", async () => {
+    // Regression - gopls registers e.g. `/tmp/.../**/*.{go,mod}` rather
+    // than a bare relative `**/*.go`. The matcher must accept both.
+    watcher = new WorkspaceWatcher(tmpDir, (evs) => received.push(evs));
+    watcher.setPatterns([
+      { globPattern: `${tmpDir}/**/*.{go,mod}` },
+    ]);
+    await new Promise((r) => setTimeout(r, 200));
+
+    fs.writeFileSync(path.join(tmpDir, "helper.go"), "package x\n");
+
+    const ok = await waitFor(() =>
+      received
+        .flat()
+        .some((e) => e.uri === pathToUri(path.join(tmpDir, "helper.go"))),
+    );
+    assert.ok(ok, `Expected event for absolute glob, got ${JSON.stringify(received)}`);
+  });
+
+  it("empty pattern set means no watching", async () => {
+    watcher = new WorkspaceWatcher(tmpDir, (evs) => received.push(evs));
+    watcher.setPatterns([]);
+    await new Promise((r) => setTimeout(r, 200));
+
+    fs.writeFileSync(path.join(tmpDir, "no-patterns.ts"), "x");
+    await new Promise((r) => setTimeout(r, FLUSH_WAIT_MS));
+
+    assert.strictEqual(received.length, 0);
+  });
+});