feat(lsp): LSP capability seam, generic stdio provider, and lsp tool

Implements the LSP capability seam RFC as three packages: dsh-lsp (the
ctx.lsp interface — provider registry by branded id + exclusive extension
mapping, per-query order-independent selection, closed request/result
vocabulary, LspError taxonomy), dsh-lsp-local (a generic stdio language-server
provider — Content-Length JSON-RPC framing, per-(provider, workspace) process
single-flight, transient didOpen/query/didClose, an abortable per-instance
queue, UTF-16 negotiation, host-namespace source reads outside ctx.fs, and
bounded shutdown/kill teardown), and dsh-tool-lsp (the model-facing lsp tool —
four operations, one-based UTF-16 cursor conversion, workspace-grouped location
rendering, hover capping, a required session workspace, and a timeout budget).

Why: an agent had text search and file reads but no way to identify a program
symbol — follow an alias, connect an interface to implementations, or read an
inferred type — before changing code. Splitting model contract, seam, and local
subprocess behavior keeps the four semantic queries stable across future remote
or sandbox-native providers without leaking a JSON-RPC escape hatch.

packages/lsp/lsp-local/src/connection.ts

@@ -0,0 +1,240 @@
+/**
+ * A JSON-RPC endpoint over one spawned language server's stdio. Owns id correlation, outbound
+ * requests/notifications, and inbound server→client requests: it answers `workspace/configuration`
+ * from static config, and rejects `workspace/applyEdit` (this host never applies edits or runs
+ * commands). It caps stderr, surfaces framing/decoder failures as a fatal close, and exposes the
+ * child handle so the instance owns process-signal teardown.
+ * @module @deepseek-ai/dsh-lsp-local/connection
+ */
+
+import type { ChildProcessByStdio } from 'node:child_process'
+import { spawn } from 'node:child_process'
+import type { Readable, Writable } from 'node:stream'
+import { encodeMessage, MessageDecoder } from './framing.ts'
+
+/** How to launch the server and answer its config requests. */
+export interface ConnectionSpec {
+  /** The resolved absolute executable path (no shell). */
+  readonly command: string
+  /** Arguments passed to the executable. */
+  readonly args: readonly string[]
+  /** The child's working directory (the canonical workspace). */
+  readonly cwd: string
+  /** The child's environment (credential-scrubbed, with overrides applied). */
+  readonly env: Record<string, string>
+  /** Largest single framed message accepted from the server. */
+  readonly maxMessageBytes: number
+  /** Largest stderr tail retained for diagnostics. */
+  readonly maxStderrBytes: number
+  /** Static answer to every `workspace/configuration` item. */
+  readonly configuration: unknown
+}
+
+interface Pending {
+  resolve: (value: unknown) => void
+  reject: (error: Error) => void
+}
+
+/** A live JSON-RPC endpoint bound to one child process. */
+export class LspConnection {
+  private readonly child: ChildProcessByStdio<Writable, Readable, Readable>
+  private readonly decoder: MessageDecoder
+  private readonly pending = new Map<number, Pending>()
+  private nextId = 1
+  private stderr = ''
+  private closeReason: Error | undefined
+  /** Set once the process has fully exited; the instance awaits it during teardown. */
+  readonly closed: Promise<void>
+
+  /**
+   * @param spec - how to launch the server and answer its config requests.
+   * @param onServerRequest - answers a server→client request; rejects to send an error response.
+   */
+  constructor(
+    private readonly spec: ConnectionSpec,
+    private readonly onServerRequest: (method: string, params: unknown) => Promise<unknown>,
+  ) {
+    this.decoder = new MessageDecoder(spec.maxMessageBytes)
+    this.child = spawn(spec.command, [...spec.args], {
+      cwd: spec.cwd,
+      env: spec.env,
+      stdio: ['pipe', 'pipe', 'pipe'],
+    })
+    this.closed = new Promise<void>((resolve) => {
+      this.child.on('close', () => {
+        const reason = this.closeReason ?? new Error('language server exited')
+        // Record the reason so any request issued AFTER close rejects immediately instead of hanging
+        // (a closed process sends no further responses).
+        this.closeReason = reason
+        this.failAll(reason)
+        resolve()
+      })
+    })
+    this.child.on('error', (error) => { this.fail(error) })
+    // A write to the child's stdin after it exits emits an async 'error'; swallow it so an EPIPE
+    // during teardown does not crash the process. Pending requests fail via the 'close' handler.
+    /* v8 ignore next -- the handler only fires on an async stdin write error during teardown. */
+    this.child.stdin.on('error', () => { /* swallow */ })
+    this.child.stdout.on('data', (chunk: Buffer) => { this.onStdout(chunk) })
+    this.child.stderr.on('data', (chunk: Buffer) => { this.onStderr(chunk) })
+  }
+
+  /** The child's pid, or `-1` when the spawn produced no pid (so signalling is a no-op). */
+  get pid(): number {
+    /* v8 ignore next -- the `-1` fallback only applies to a spawn that produced no pid; defensive. */
+    return this.child.pid ?? -1
+  }
+
+  /** The retained stderr tail, for diagnostics on a failed server. */
+  get stderrTail(): string {
+    return this.stderr
+  }
+
+  /**
+   * Send a request and await its result.
+   * @param method - the JSON-RPC method.
+   * @param params - the request params.
+   * @returns the response result; rejects on an error response, write failure, or close.
+   */
+  request(method: string, params: unknown): Promise<unknown> {
+    const id = this.nextId++
+    const promise = new Promise<unknown>((resolve, reject) => {
+      if (this.closeReason !== undefined) {
+        reject(this.closeReason)
+        return
+      }
+      this.pending.set(id, { resolve, reject })
+      try {
+        this.write({ jsonrpc: '2.0', id, method, params })
+      } catch (error) {
+        /* v8 ignore start -- a stdin write failure surfaces asynchronously via the swallowed
+           'error' listener, so this synchronous catch is a defensive guard. */
+        this.pending.delete(id)
+        reject(asError(error))
+        /* v8 ignore stop */
+      }
+    })
+    // A caller that stops awaiting (e.g. an aborted query) can leave this promise to reject later
+    // when the process closes; a benign no-op handler keeps that from surfacing as an unhandled
+    // rejection. The returned promise still delivers the rejection to the caller's own await/catch.
+    promise.catch(() => {})
+    return promise
+  }
+
+  /**
+   * Send a notification (no id, no response).
+   * @param method - the JSON-RPC method.
+   * @param params - the notification params.
+   */
+  notify(method: string, params: unknown): void {
+    this.write({ jsonrpc: '2.0', method, params })
+  }
+
+  /**
+   * Send a `$/cancelRequest` for an in-flight request id (best-effort; ignores write failure).
+   * @param requestId - the numeric id of the request to cancel.
+   */
+  cancel(requestId: number): void {
+    try {
+      this.write({ jsonrpc: '2.0', method: '$/cancelRequest', params: { id: requestId } })
+    } catch {
+      // The server is already gone or unwritable; the pending request will fail on close.
+    }
+  }
+
+  /**
+   * The id the NEXT `request()` will use, so the instance can pre-arm a cancel.
+   * @returns the numeric id the next request will be assigned.
+   */
+  peekNextId(): number {
+    return this.nextId
+  }
+
+  /** Send SIGTERM to the child (idempotent-safe; a dead child ignores it). */
+  terminate(): void {
+    this.child.kill('SIGTERM')
+  }
+
+  /** Send SIGKILL to the child. */
+  kill(): void {
+    this.child.kill('SIGKILL')
+  }
+
+  private onStdout(chunk: Buffer): void {
+    let messages: unknown[]
+    try {
+      messages = this.decoder.push(chunk)
+    } catch (error) {
+      // A framing/JSON failure corrupts the stream position irrecoverably: fail the instance.
+      this.fail(asError(error))
+      this.child.kill('SIGKILL')
+      return
+    }
+    for (const message of messages) this.dispatch(message)
+  }
+
+  private onStderr(chunk: Buffer): void {
+    if (this.stderr.length >= this.spec.maxStderrBytes) return
+    this.stderr = (this.stderr + chunk.toString('utf8')).slice(0, this.spec.maxStderrBytes)
+  }
+
+  private dispatch(message: unknown): void {
+    if (message === null || typeof message !== 'object') return
+    const frame = message as Record<string, unknown>
+    const id = frame.id
+    const method = frame.method
+    if (typeof method === 'string' && (typeof id === 'number' || typeof id === 'string')) {
+      void this.handleServerRequest(id, method, frame.params)
+      return
+    }
+    if (typeof method === 'string') {
+      // A server→client notification (e.g. diagnostics, logs): ignored by this MVP host.
+      return
+    }
+    if (typeof id === 'number') this.handleResponse(id, frame)
+  }
+
+  private async handleServerRequest(id: number | string, method: string, params: unknown): Promise<void> {
+    try {
+      const result = await this.onServerRequest(method, params)
+      this.write({ jsonrpc: '2.0', id, result })
+    } catch (error) {
+      this.write({ jsonrpc: '2.0', id, error: { code: -32601, message: asError(error).message } })
+    }
+  }
+
+  private handleResponse(id: number, frame: Record<string, unknown>): void {
+    const pending = this.pending.get(id)
+    if (!pending) return
+    this.pending.delete(id)
+    const error = frame.error
+    if (error !== null && typeof error === 'object') {
+      const record = error as Record<string, unknown>
+      pending.reject(new Error(typeof record.message === 'string' ? record.message : 'LSP error response'))
+      return
+    }
+    pending.resolve(frame.result)
+  }
+
+  private write(message: unknown): void {
+    this.child.stdin.write(encodeMessage(message))
+  }
+
+  private fail(error: Error): void {
+    /* v8 ignore next -- the second arm (closeReason already set) needs two fail() calls before close; defensive. */
+    if (this.closeReason === undefined) this.closeReason = error
+    this.failAll(error)
+  }
+
+  private failAll(error: Error): void {
+    const waiting = [...this.pending.values()]
+    this.pending.clear()
+    for (const pending of waiting) pending.reject(error)
+  }
+}
+
+/** Coerce an unknown thrown value to an `Error`. */
+function asError(value: unknown): Error {
+  /* v8 ignore next -- the non-Error branch guards against a non-Error throw, which our paths never produce. */
+  return value instanceof Error ? value : new Error(String(value))
+}

packages/lsp/lsp-local/src/framing.ts

@@ -0,0 +1,99 @@
+/**
+ * LSP base-protocol framing: `Content-Length`-delimited JSON-RPC over a byte stream. The encoder
+ * produces one framed buffer; the decoder buffers incoming bytes and yields complete message bodies,
+ * bounding the header and total message size so a hostile or broken server cannot exhaust memory.
+ * @module @deepseek-ai/dsh-lsp-local/framing
+ */
+
+/** The header/body separator in the LSP base protocol. */
+const HEADER_SEPARATOR = '\r\n\r\n'
+
+/** Cap on the header section so a server that never sends the separator cannot grow the buffer forever. */
+const MAX_HEADER_BYTES = 1 << 16
+
+/**
+ * Encode one JSON-RPC message as a framed LSP buffer (`Content-Length: N\r\n\r\n<utf-8 json>`).
+ * @param message - the JSON-RPC message object to serialize.
+ * @returns the framed bytes ready to write to the server's stdin.
+ */
+export function encodeMessage(message: unknown): Buffer {
+  const body = Buffer.from(JSON.stringify(message), 'utf8')
+  const header = Buffer.from(`Content-Length: ${body.length}\r\n\r\n`, 'ascii')
+  return Buffer.concat([header, body])
+}
+
+/**
+ * A streaming decoder for `Content-Length`-framed JSON-RPC. Feed it stdout chunks; it returns any
+ * whole message bodies that completed. It parses only the `Content-Length` header and ignores other
+ * headers (e.g. `Content-Type`), matching the base protocol.
+ */
+export class MessageDecoder {
+  private buffer: Buffer = Buffer.alloc(0)
+  private readonly maxMessageBytes: number
+
+  /**
+   * @param maxMessageBytes - reject any single framed body larger than this (guards memory).
+   */
+  constructor(maxMessageBytes: number) {
+    this.maxMessageBytes = maxMessageBytes
+  }
+
+  /**
+   * Append a chunk and return every message body that is now complete.
+   * @param chunk - raw bytes from the server's stdout.
+   * @returns the parsed JSON bodies, in arrival order (possibly empty).
+   * @throws Error when a header is malformed or a body exceeds `maxMessageBytes`.
+   */
+  push(chunk: Buffer): unknown[] {
+    this.buffer = this.buffer.length === 0 ? chunk : Buffer.concat([this.buffer, chunk])
+    const messages: unknown[] = []
+    for (;;) {
+      const step = this.next()
+      if (!step.ready) break
+      messages.push(step.message)
+    }
+    return messages
+  }
+
+  /** Parse and consume the next complete message, or report that more bytes are needed. */
+  private next(): { ready: false } | { ready: true; message: unknown } {
+    const separator = this.buffer.indexOf(HEADER_SEPARATOR)
+    if (separator < 0) {
+      if (this.buffer.length > MAX_HEADER_BYTES) {
+        throw new Error(`LSP header exceeded ${MAX_HEADER_BYTES} bytes without a terminator`)
+      }
+      return { ready: false }
+    }
+    const headerText = this.buffer.toString('ascii', 0, separator)
+    const contentLength = parseContentLength(headerText)
+    if (contentLength > this.maxMessageBytes) {
+      throw new Error(`LSP message length ${contentLength} exceeds the ${this.maxMessageBytes}-byte limit`)
+    }
+    const bodyStart = separator + HEADER_SEPARATOR.length
+    const bodyEnd = bodyStart + contentLength
+    if (this.buffer.length < bodyEnd) return { ready: false }
+    const body = this.buffer.toString('utf8', bodyStart, bodyEnd)
+    this.buffer = this.buffer.subarray(bodyEnd)
+    try {
+      return { ready: true, message: JSON.parse(body) }
+    } catch (error) {
+      /* v8 ignore next -- JSON.parse throws a SyntaxError (an Error); the String() fallback is defensive. */
+      throw new Error(`LSP message body was not valid JSON: ${error instanceof Error ? error.message : String(error)}`)
+    }
+  }
+}
+
+/** Read the `Content-Length` header value (case-insensitive), rejecting a missing or non-numeric one. */
+function parseContentLength(headerText: string): number {
+  for (const line of headerText.split('\r\n')) {
+    const colon = line.indexOf(':')
+    if (colon < 0) continue
+    if (line.slice(0, colon).trim().toLowerCase() !== 'content-length') continue
+    const value = Number(line.slice(colon + 1).trim())
+    if (!Number.isInteger(value) || value < 0) {
+      throw new Error(`invalid Content-Length header: ${JSON.stringify(line)}`)
+    }
+    return value
+  }
+  throw new Error(`LSP header block missing Content-Length: ${JSON.stringify(headerText)}`)
+}

packages/lsp/lsp-local/src/host.ts

@@ -0,0 +1,104 @@
+/**
+ * Host-filesystem source access for the local provider, using Node APIs directly in the
+ * subprocess's namespace (never `ctx.fs`): only the LSP result is model-visible, so a query does not
+ * satisfy read-before-write policy and emits no `fs/observed`. Canonicalization derives target
+ * identity from `realpath`, so symlink aliases share a workspace; a source is rejected before server
+ * startup when it is missing, non-regular, non-UTF-8, oversized, or canonically outside the
+ * workspace. External result locations are allowed, but an external path can never become a query
+ * source.
+ * @module @deepseek-ai/dsh-lsp-local/host
+ */
+
+import { readFile, realpath, stat } from 'node:fs/promises'
+import { isAbsolute, resolve as resolvePath, sep } from 'node:path'
+
+/** A validated source: its canonical absolute path and current UTF-8 text. */
+export interface HostSource {
+  /** The canonical (realpath-resolved) absolute path, inside the canonical workspace. */
+  readonly canonicalPath: string
+  /** The file's current text, read as UTF-8. */
+  readonly text: string
+}
+
+/**
+ * Canonicalize a workspace root: it must exist and be a directory. The returned realpath supplies
+ * process cwd, `rootUri`, the sole `workspaceFolders` entry, and pool identity, so symlinked roots
+ * collapse to one instance.
+ * @param workspaceRoot - the caller's workspace root (absolute).
+ * @returns the canonical directory path.
+ * @throws Error when the path is missing or not a directory.
+ */
+export async function canonicalizeWorkspace(workspaceRoot: string): Promise<string> {
+  let canonical: string
+  try {
+    canonical = await realpath(workspaceRoot)
+  } catch (error) {
+    throw new Error(`workspace root "${workspaceRoot}" cannot be resolved: ${messageOf(error)}`)
+  }
+  const info = await stat(canonical)
+  if (!info.isDirectory()) {
+    throw new Error(`workspace root "${workspaceRoot}" is not a directory`)
+  }
+  return canonical
+}
+
+/**
+ * Resolve, canonicalize, validate, and read a query source in one pass. A relative `filePath`
+ * resolves against `canonicalWorkspace`; an absolute one is taken directly. The canonical target
+ * must be a regular UTF-8 file no larger than `maxDocumentBytes`, and must lie inside the canonical
+ * workspace.
+ * @param filePath - the model-supplied source path (relative or absolute).
+ * @param canonicalWorkspace - the already-canonicalized workspace root.
+ * @param maxDocumentBytes - the largest source this host will open.
+ * @returns the canonical path and current UTF-8 text.
+ * @throws Error when the source is missing, non-regular, oversized, non-UTF-8, or out of workspace.
+ */
+export async function readHostSource(
+  filePath: string,
+  canonicalWorkspace: string,
+  maxDocumentBytes: number,
+): Promise<HostSource> {
+  const requested = isAbsolute(filePath) ? filePath : resolvePath(canonicalWorkspace, filePath)
+  let canonicalPath: string
+  try {
+    canonicalPath = await realpath(requested)
+  } catch (error) {
+    throw new Error(`source "${filePath}" cannot be resolved: ${messageOf(error)}`)
+  }
+  if (!isInside(canonicalWorkspace, canonicalPath)) {
+    throw new Error(`source "${filePath}" resolves outside the workspace`)
+  }
+  const info = await stat(canonicalPath)
+  if (!info.isFile()) {
+    throw new Error(`source "${filePath}" is not a regular file`)
+  }
+  if (info.size > maxDocumentBytes) {
+    throw new Error(`source "${filePath}" is ${info.size} bytes, over the ${maxDocumentBytes}-byte limit`)
+  }
+  const buffer = await readFile(canonicalPath)
+  const text = decodeUtf8Strict(buffer, filePath)
+  return { canonicalPath, text }
+}
+
+/** Whether `child` is the workspace itself or a descendant of it (both already canonical). */
+function isInside(workspace: string, child: string): boolean {
+  if (child === workspace) return true
+  /* v8 ignore next -- a canonical non-root workspace never ends with a separator; the guard covers the filesystem root. */
+  const base = workspace.endsWith(sep) ? workspace : workspace + sep
+  return child.startsWith(base)
+}
+
+/** Decode UTF-8 strictly (a replacement char means the source was not valid UTF-8 text). */
+function decodeUtf8Strict(buffer: Buffer, filePath: string): string {
+  const text = buffer.toString('utf8')
+  if (text.includes('<27>')) {
+    throw new Error(`source "${filePath}" is not valid UTF-8 text`)
+  }
+  return text
+}
+
+/** Extract a message from an unknown thrown value without leaking `any`. */
+function messageOf(error: unknown): string {
+  /* v8 ignore next -- Node fs rejections are always Error instances; the String() fallback is defensive. */
+  return error instanceof Error ? error.message : String(error)
+}

packages/lsp/lsp-local/src/index.ts

@@ -0,0 +1,255 @@
+/**
+ * Generic stdio language-server provider for `ctx.lsp`. One plugin instance configures one server
+ * command and its extension→language-id map; load multiple instances for multiple servers. The
+ * provider lazily single-flights one server process per `(provider id, canonical workspace
+ * realpath)`, serves transient-open queries through it, and evicts a crashed process so a later
+ * query can replace it. It reads sources through Node APIs in the host namespace (not `ctx.fs`) and
+ * trusts its configured server — no sandbox confinement.
+ *
+ * Namespace plugin (named exports, no default export). Lifecycle is effect-scoped: disposal
+ * unregisters from `ctx.lsp` and tears down every live server.
+ * @module @deepseek-ai/dsh-lsp-local
+ */
+
+import { accessSync, constants } from 'node:fs'
+import { delimiter, isAbsolute, join } from 'node:path'
+import type { Context } from 'cordis'
+import z from 'schemastery'
+import { LspProviderId } from '@deepseek-ai/dsh-lsp'
+import type {
+  LspProvider,
+  LspProviderQuery,
+  LspQueryResult,
+} from '@deepseek-ai/dsh-lsp'
+// Side-effect type import: declaration-merges `ctx.lsp` onto Context.
+import type {} from '@deepseek-ai/dsh-lsp'
+import { canonicalizeWorkspace } from './host.ts'
+import { LspInstance } from './instance.ts'
+import type { InstanceSpec } from './instance.ts'
+
+export { canonicalizeWorkspace, readHostSource } from './host.ts'
+export { encodeMessage, MessageDecoder } from './framing.ts'
+export {
+  negotiatePositionEncoding,
+  normalizeHover,
+  normalizeLocations,
+  requestMethod,
+  supportsOperation,
+  supportsTransientOpen,
+} from './translate.ts'
+export { LspInstance } from './instance.ts'
+export { LspConnection } from './connection.ts'
+
+/** Cordis plugin name for loader diagnostics. */
+export const name = 'lsp-local'
+
+/** Services required by this plugin. */
+export const inject = ['lsp']
+
+/** Credential-shaped ambient env vars are NOT forwarded to the child by default. */
+const SENSITIVE_ENV_PATTERN = /KEY|SECRET|TOKEN/i
+
+const DEFAULT_MAX_MESSAGE_BYTES = 16_000_000
+const DEFAULT_MAX_STDERR_BYTES = 1_000_000
+const DEFAULT_MAX_DOCUMENT_BYTES = 4_000_000
+const DEFAULT_SHUTDOWN_TIMEOUT_MS = 5_000
+const DEFAULT_KILL_GRACE_MS = 2_000
+
+/** Plugin configuration: one server command plus its extension mapping and host bounds. */
+export interface Config {
+  /** Stable provider id, reserved on `ctx.lsp` with the extensions. */
+  providerId: string
+  /** Executable to spawn (absolute, or resolved on PATH at load). */
+  command: string
+  /** Lowercase leading-dot extension → LSP language id (e.g. `{ '.ts': 'typescript' }`). */
+  extensionToLanguage: Record<string, string>
+  /** Arguments passed to the executable (no shell). Default `[]`. */
+  args?: string[]
+  /** Extra env vars merged on top of the scrubbed ambient env. Default `{}`. */
+  env?: Record<string, string>
+  /** Static `initialize` options forwarded to the server. Default `null`. */
+  initializationOptions?: unknown
+  /** Static answer to every `workspace/configuration` item. Default `null`. */
+  configuration?: unknown
+  /** Largest single framed message accepted from the server (bytes). Default 16000000. */
+  maxMessageBytes?: number
+  /** Largest stderr tail retained for diagnostics (bytes). Default 1000000. */
+  maxStderrBytes?: number
+  /** Largest source file this host will open (bytes). Default 4000000. */
+  maxDocumentBytes?: number
+  /** Graceful `shutdown`/`exit` budget before escalation (ms). Default 5000. */
+  shutdownTimeoutMs?: number
+  /** SIGTERM→SIGKILL grace after graceful shutdown fails (ms). Default 2000. */
+  killGraceMs?: number
+}
+
+/** The resolved config after schemastery fills every default; the provider reads this shape. */
+type ResolvedConfig = Required<Config>
+
+export const Config: z<Config> = z.object({
+  providerId: z.string().required(),
+  command: z.string().required(),
+  args: z.array(String).default([]),
+  env: z.dict(String).default({}),
+  extensionToLanguage: z.dict(String).required(),
+  initializationOptions: z.any().default(null),
+  configuration: z.any().default(null),
+  maxMessageBytes: z.number().default(DEFAULT_MAX_MESSAGE_BYTES),
+  maxStderrBytes: z.number().default(DEFAULT_MAX_STDERR_BYTES),
+  maxDocumentBytes: z.number().default(DEFAULT_MAX_DOCUMENT_BYTES),
+  shutdownTimeoutMs: z.number().default(DEFAULT_SHUTDOWN_TIMEOUT_MS),
+  killGraceMs: z.number().default(DEFAULT_KILL_GRACE_MS),
+})
+
+/**
+ * Register a generic stdio LSP provider. Resolves the executable at load (after credential
+ * scrubbing) and fails before registration when it is unavailable; the process itself launches
+ * lazily on the first matching query.
+ * @param ctx - the plugin context (must inject `lsp`).
+ * @param config - the resolved plugin configuration (schemastery has filled every default).
+ */
+export function apply(ctx: Context, config: Config): void {
+  const resolved = config as ResolvedConfig
+  const childEnv = buildChildEnv(resolved.env)
+  // Resolve the executable eagerly so a misconfigured command fails at load, not on first query.
+  const executable = resolveExecutable(resolved.command, childEnv)
+
+  const provider = new LocalLspProvider(resolved, childEnv, executable)
+  ctx.effect(() => {
+    const dispose = ctx.lsp.registerProvider(provider)
+    return async () => {
+      dispose()
+      await provider.disposeAll()
+    }
+  }, 'lsp-local.registerProvider')
+}
+
+/** A pooled generic provider: one server process per canonical workspace, created on demand. */
+class LocalLspProvider implements LspProvider {
+  readonly id: LspProviderId
+  readonly extensionToLanguage: Readonly<Record<string, string>>
+  /** Single-flight map: canonical workspace realpath → the (pending) instance for it. */
+  private readonly instances = new Map<string, Promise<LspInstance>>()
+  private disposed = false
+
+  constructor(
+    private readonly config: ResolvedConfig,
+    private readonly childEnv: Record<string, string>,
+    private readonly executable: string,
+  ) {
+    this.id = LspProviderId(config.providerId)
+    this.extensionToLanguage = config.extensionToLanguage
+  }
+
+  async query(request: LspProviderQuery, signal?: AbortSignal): Promise<LspQueryResult> {
+    /* v8 ignore next -- the seam unregisters this provider on dispose, so a query never reaches a disposed provider; defensive. */
+    if (this.disposed) throw new Error('lsp-local provider is disposed')
+    const workspace = await canonicalizeWorkspace(request.workspaceRoot)
+    const instance = await this.instanceFor(workspace)
+    try {
+      return await instance.query(request, signal)
+    } finally {
+      // A crashed/closed process must not be reused: drop its slot so the next query starts fresh,
+      // but only if the slot still holds THIS instance (a concurrent replacement must survive).
+      if (instance.dead) {
+        const slot = this.instances.get(workspace)
+        /* v8 ignore next -- the slot-undefined arm needs a concurrent eviction of the same slot; defensive. */
+        if (slot !== undefined && (await settledInstance(slot)) === instance) {
+          this.instances.delete(workspace)
+        }
+      }
+    }
+  }
+
+  /** Single-flight one instance per canonical workspace; a rejected creation clears the slot. */
+  private instanceFor(workspace: string): Promise<LspInstance> {
+    const existing = this.instances.get(workspace)
+    if (existing !== undefined) return existing
+    const created = Promise.resolve().then(() => this.createInstance(workspace))
+    this.instances.set(workspace, created)
+    /* v8 ignore next 3 -- createInstance (the LspInstance constructor) does not throw; spawn failures
+       surface asynchronously through the instance, so this creation-rejection cleanup is defensive. */
+    created.catch(() => {
+      if (this.instances.get(workspace) === created) this.instances.delete(workspace)
+    })
+    return created
+  }
+
+  private createInstance(workspace: string): LspInstance {
+    const spec: InstanceSpec = {
+      command: this.executable,
+      args: this.config.args,
+      cwd: workspace,
+      env: this.childEnv,
+      configuration: this.config.configuration,
+      initializationOptions: this.config.initializationOptions,
+      maxMessageBytes: this.config.maxMessageBytes,
+      maxStderrBytes: this.config.maxStderrBytes,
+      maxDocumentBytes: this.config.maxDocumentBytes,
+      shutdownTimeoutMs: this.config.shutdownTimeoutMs,
+      killGraceMs: this.config.killGraceMs,
+    }
+    return new LspInstance(spec)
+  }
+
+  /** Dispose every live instance and block further queries. */
+  async disposeAll(): Promise<void> {
+    this.disposed = true
+    const pending = [...this.instances.values()]
+    this.instances.clear()
+    await Promise.all(pending.map(async (entry) => {
+      try {
+        const instance = await entry
+        await instance.dispose()
+      } catch {
+        // A never-initialized instance already rejected; nothing to tear down.
+      }
+    }))
+  }
+}
+
+/** Resolve a slot promise to its instance for identity comparison, tolerating a pending rejection. */
+async function settledInstance(slot: Promise<LspInstance>): Promise<LspInstance | undefined> {
+  try {
+    return await slot
+  } catch {
+    /* v8 ignore next -- a slot promise only rejects if createInstance throws, which it never does; defensive. */
+    return undefined
+  }
+}
+
+/** The ambient env minus credential-shaped vars, plus the config's explicit env. */
+function buildChildEnv(extra: Record<string, string>): Record<string, string> {
+  const scrubbed = Object.entries(process.env).filter(
+    ([key, value]) => value !== undefined && !SENSITIVE_ENV_PATTERN.test(key),
+  ) as [string, string][]
+  return { ...Object.fromEntries(scrubbed), ...extra }
+}
+
+/**
+ * Resolve the server executable to an absolute path: an absolute command is verified directly; a
+ * bare command is looked up on the child's PATH. Fails loudly when nothing is executable.
+ */
+function resolveExecutable(command: string, childEnv: Record<string, string>): string {
+  if (isAbsolute(command)) {
+    return command
+  }
+  /* v8 ignore next -- buildChildEnv always sets PATH from the ambient env; the further fallbacks are defensive. */
+  const pathValue = childEnv.PATH ?? process.env.PATH ?? ''
+  for (const dir of pathValue.split(delimiter)) {
+    if (dir === '') continue
+    const candidate = join(dir, command)
+    if (isExecutableSync(candidate)) return candidate
+  }
+  throw new Error(`lsp-local: command "${command}" was not found on PATH`)
+}
+
+/** Synchronous executable check used only at load-time resolution. */
+function isExecutableSync(path: string): boolean {
+  try {
+    accessSync(path, constants.X_OK)
+    return true
+  } catch {
+    return false
+  }
+}

packages/lsp/lsp-local/src/instance.ts

@@ -0,0 +1,293 @@
+/**
+ * One language-server instance: a connection plus the initialize handshake, the serialized abortable
+ * query queue, the transient `didOpen`→request→`didClose` lifecycle, and bounded teardown. One
+ * instance owns one `(provider id, canonical workspace)` process. Queries serialize through a single
+ * queue so a cancellation that fails to stop the server can terminate it without killing unrelated
+ * work; distinct instances run in parallel.
+ * @module @deepseek-ai/dsh-lsp-local/instance
+ */
+
+import { pathToFileURL } from 'node:url'
+import type {
+  LspOperation,
+  LspProviderQuery,
+  LspQueryResult,
+} from '@deepseek-ai/dsh-lsp'
+import { deadline, timeoutOf } from '@deepseek-ai/dsh-timeout'
+import { LspConnection } from './connection.ts'
+import type { ConnectionSpec } from './connection.ts'
+import { readHostSource } from './host.ts'
+import type { WireInitializeResult, WireServerCapabilities } from './protocol.ts'
+import {
+  negotiatePositionEncoding,
+  normalizeHover,
+  normalizeLocations,
+  requestMethod,
+  supportsOperation,
+  supportsTransientOpen,
+} from './translate.ts'
+
+/** Everything an instance needs beyond the connection spec. */
+export interface InstanceSpec extends ConnectionSpec {
+  /** Static `initialize` options forwarded to the server. */
+  readonly initializationOptions: unknown
+  /** Largest source file this host will open (bytes). */
+  readonly maxDocumentBytes: number
+  /** Graceful `shutdown`/`exit` budget before escalation (ms). */
+  readonly shutdownTimeoutMs: number
+  /** SIGTERM→SIGKILL grace after graceful shutdown fails (ms). */
+  readonly killGraceMs: number
+}
+
+/**
+ * A single initialized server process. Not exported as a provider — the provider single-flights and
+ * pools these. `query()` serializes; `dispose()` rejects queued work and tears the process down.
+ */
+export class LspInstance {
+  private readonly connection: LspConnection
+  private capabilities: WireServerCapabilities | undefined
+  /** The serialization tail: each query awaits the prior one, so lifecycles never interleave. */
+  private queue: Promise<unknown> = Promise.resolve()
+  private disposed = false
+  /** Set once the process closes, so the pool can synchronously skip a dead instance. */
+  private processClosed = false
+  /** Populated once `initialize` succeeds; a failed handshake rejects every query. */
+  private readonly ready: Promise<void>
+
+  /**
+   * @param spec - the launch, initialize, and teardown parameters.
+   */
+  constructor(private readonly spec: InstanceSpec) {
+    this.connection = new LspConnection(spec, (method, params) => this.answerServerRequest(method, params))
+    this.ready = this.initialize()
+    // A handshake rejection must not surface as an unhandled rejection before the first query awaits
+    // it; queries attach the real handler.
+    this.ready.catch(() => {})
+    void this.connection.closed.then(() => { this.processClosed = true })
+  }
+
+  /** Synchronous liveness check: true once the process has closed or the instance was disposed. */
+  get dead(): boolean {
+    return this.processClosed || this.disposed
+  }
+
+  /**
+   * Run one query through the serialized queue.
+   * @param request - the resolved provider query.
+   * @param signal - optional cancellation for this query's full lifecycle.
+   * @returns the normalized result.
+   */
+  query(request: LspProviderQuery, signal?: AbortSignal): Promise<LspQueryResult> {
+    const run = this.queue.then(() => this.runQuery(request, signal))
+    // Keep the tail alive regardless of this query's outcome so the next caller still serializes.
+    this.queue = run.then(() => undefined, () => undefined)
+    return run
+  }
+
+  private async initialize(): Promise<void> {
+    const initializeResult = await this.connection.request('initialize', {
+      processId: process.pid,
+      rootUri: pathToFileURL(this.spec.cwd).href,
+      workspaceFolders: [{ uri: pathToFileURL(this.spec.cwd).href, name: 'workspace' }],
+      capabilities: CLIENT_CAPABILITIES,
+      initializationOptions: this.spec.initializationOptions,
+    }) as WireInitializeResult
+    const capabilities = initializeResult.capabilities
+    // An omitted encoding defaults to utf-16; any other value is a protocol error we reject here.
+    negotiatePositionEncoding(capabilities.positionEncoding)
+    this.capabilities = capabilities
+    this.connection.notify('initialized', {})
+  }
+
+  private async runQuery(request: LspProviderQuery, signal?: AbortSignal): Promise<LspQueryResult> {
+    if (this.disposed) throw new Error('LSP instance was disposed')
+    if (signal?.aborted) throw abortError(signal)
+    await this.ready
+    const capabilities = this.capabilities
+    /* v8 ignore next -- `ready` resolves only after capabilities are set, else it rejects above; defensive. */
+    if (capabilities === undefined) throw new Error('LSP instance is not initialized')
+    if (!supportsOperation(capabilities, request.operation)) {
+      throw new Error(`server does not support ${request.operation}`)
+    }
+    if (!supportsTransientOpen(capabilities.textDocumentSync)) {
+      throw new Error('server does not support the transient textDocument/didOpen this host requires')
+    }
+
+    const source = await readHostSource(request.filePath, this.spec.cwd, this.spec.maxDocumentBytes)
+    const uri = pathToFileURL(source.canonicalPath).href
+    let opened = false
+    try {
+      if (signal?.aborted) throw abortError(signal)
+      this.connection.notify('textDocument/didOpen', {
+        textDocument: { uri, languageId: request.languageId, version: 1, text: source.text },
+      })
+      opened = true
+      const payload = await this.sendRequest(request.operation, uri, request.position, signal)
+      return this.normalize(request.operation, payload)
+    } finally {
+      if (opened) {
+        try {
+          this.connection.notify('textDocument/didClose', { textDocument: { uri } })
+        } catch (error) {
+          /* v8 ignore start -- stdin write errors surface asynchronously via the swallowed 'error'
+             listener, so a synchronous didClose write failure is a defensive path. */
+          // A close-write failure does not replace the settled result/error, but the instance can no
+          // longer be trusted: invalidate it and await bounded process termination.
+          this.disposed = true
+          void this.tearDown(error instanceof Error ? error : new Error(String(error)))
+          /* v8 ignore stop */
+        }
+      }
+    }
+  }
+
+  private async sendRequest(
+    operation: LspOperation,
+    uri: string,
+    position: LspProviderQuery['position'],
+    signal?: AbortSignal,
+  ): Promise<unknown> {
+    const params = {
+      textDocument: { uri },
+      position: { line: position.line, character: position.character },
+      // references always includes declarations: the caller gets no flag and impact analysis never
+      // omits the defining site.
+      ...(operation === 'references' ? { context: { includeDeclaration: true } } : {}),
+    }
+    const requestId = this.connection.peekNextId()
+    const send = this.connection.request(requestMethod(operation), params)
+    if (signal === undefined) return send
+    return this.raceAbort(send, requestId, signal)
+  }
+
+  /** Race a pending request against abort; on abort, send `$/cancelRequest` and reject. */
+  private async raceAbort(send: Promise<unknown>, requestId: number, signal: AbortSignal): Promise<unknown> {
+    const abort = new Promise<never>((_, reject) => {
+      const onAbort = (): void => { reject(abortError(signal)) }
+      /* v8 ignore next -- runQuery checks signal.aborted before sending, so it is not yet aborted here; defensive. */
+      if (signal.aborted) { onAbort(); return }
+      signal.addEventListener('abort', onAbort, { once: true })
+      // Remove the abort listener once the request settles either way; the finally-promise inherits
+      // send's rejection, so catch it to avoid an unhandled rejection when abort already won.
+      send.finally(() => { signal.removeEventListener('abort', onAbort) }).catch(() => {})
+    })
+    try {
+      return await Promise.race([send, abort])
+    } catch (error) {
+      if (signal.aborted) this.connection.cancel(requestId)
+      throw error
+    }
+  }
+
+  private normalize(operation: LspOperation, payload: unknown): LspQueryResult {
+    if (operation === 'hover') {
+      return { kind: 'hover', hover: normalizeHover(payload) }
+    }
+    return { kind: 'locations', locations: normalizeLocations(payload) }
+  }
+
+  private answerServerRequest(method: string, params: unknown): Promise<unknown> {
+    if (method === 'workspace/configuration') {
+      // Answer every requested item with the one static configuration value.
+      const record = params as { items?: unknown[] } | null
+      /* v8 ignore next -- a configuration request always carries an items array; the empty fallback is defensive. */
+      const items = Array.isArray(record?.items) ? record.items : []
+      return Promise.resolve(items.map(() => this.spec.configuration))
+    }
+    if (LIFECYCLE_NOOP_METHODS.has(method)) {
+      // Accept lifecycle bookkeeping requests with an empty result; we register nothing dynamic.
+      return Promise.resolve(null)
+    }
+    if (method === 'workspace/applyEdit') {
+      // This host never applies edits or runs commands.
+      return Promise.reject(new Error('workspace/applyEdit is not permitted by this host'))
+    }
+    return Promise.reject(new Error(`unsupported server request: ${method}`))
+  }
+
+  /**
+   * Reject queued work, attempt graceful `shutdown`/`exit`, then escalate SIGTERM→SIGKILL, awaiting
+   * process close so nothing outlives disposal.
+   */
+  async dispose(): Promise<void> {
+    if (this.disposed) {
+      await this.connection.closed
+      return
+    }
+    this.disposed = true
+    await this.tearDown(new Error('LSP instance disposed'))
+  }
+
+  private async tearDown(_reason: Error): Promise<void> {
+    try {
+      using shutdownDeadline = deadline(undefined, this.spec.shutdownTimeoutMs, 'LSP_SHUTDOWN')
+      await this.gracefulShutdown(shutdownDeadline.signal)
+    } catch {
+      // Graceful shutdown failed or timed out: fall through to signal escalation.
+    }
+    await this.forceTerminate()
+  }
+
+  /** Best-effort LSP `shutdown` request then `exit` notification, bounded by `signal`. */
+  private async gracefulShutdown(signal: AbortSignal): Promise<void> {
+    const shutdown = this.connection.request('shutdown', null)
+    await Promise.race([
+      shutdown,
+      new Promise<never>((_, reject) => {
+        /* v8 ignore next -- the shutdown deadline signal is freshly armed and not yet aborted here; defensive. */
+        if (signal.aborted) { reject(abortError(signal)); return }
+        signal.addEventListener('abort', () => { reject(abortError(signal)) }, { once: true })
+      }),
+    ])
+    this.connection.notify('exit', null)
+  }
+
+  /** SIGTERM, wait `killGraceMs` for close, then SIGKILL; await full process close either way. */
+  private async forceTerminate(): Promise<void> {
+    this.connection.terminate()
+    using graceDeadline = deadline(undefined, this.spec.killGraceMs, 'LSP_KILL_GRACE')
+    const closedInTime = await Promise.race([
+      this.connection.closed.then(() => true),
+      new Promise<boolean>((resolve) => {
+        /* v8 ignore next -- the kill-grace deadline signal is freshly armed and not yet aborted here; defensive. */
+        if (graceDeadline.signal.aborted) { resolve(false); return }
+        graceDeadline.signal.addEventListener('abort', () => { resolve(false) }, { once: true })
+      }),
+    ])
+    if (!closedInTime) this.connection.kill()
+    await this.connection.closed
+  }
+}
+
+/** Server→client request methods this host acknowledges with an empty result (no dynamic registration). */
+const LIFECYCLE_NOOP_METHODS = new Set([
+  'window/workDoneProgress/create',
+  'client/registerCapability',
+  'client/unregisterCapability',
+])
+
+/** Build an abort Error carrying the signal's reason (preserving a timeout classification). */
+function abortError(signal: AbortSignal): Error {
+  const timeout = timeoutOf(signal)
+  if (timeout !== undefined) return timeout
+  const reason: unknown = signal.reason
+  if (reason instanceof Error) return reason
+  return new Error('LSP query aborted')
+}
+
+/**
+ * The client capabilities advertised at `initialize`: UTF-16 positions, workspace folders and
+ * configuration, markdown/plaintext hover, and link support for definition/implementation. No
+ * dynamic registration; the server's returned capabilities are authoritative.
+ */
+const CLIENT_CAPABILITIES = {
+  general: { positionEncodings: ['utf-16'] },
+  workspace: { workspaceFolders: true, configuration: true },
+  textDocument: {
+    synchronization: { dynamicRegistration: false },
+    hover: { contentFormat: ['markdown', 'plaintext'] },
+    definition: { linkSupport: true },
+    implementation: { linkSupport: true },
+    references: {},
+  },
+} as const

packages/lsp/lsp-local/src/protocol.ts

@@ -0,0 +1,80 @@
+/**
+ * The subset of LSP wire types this generic host reads and writes: initialize capabilities, the four
+ * request results (`Location`, `LocationLink`, `Hover`), and the `textDocumentSync` shapes used to
+ * decide transient-open support. Types only. Fields absent from a real server payload stay optional;
+ * the translation layer normalizes them into the seam's closed unions.
+ * @module @deepseek-ai/dsh-lsp-local/protocol
+ */
+
+/** A zero-based UTF-16 position on the wire (the protocol's `Position`). */
+export interface WirePosition {
+  readonly line: number
+  readonly character: number
+}
+
+/** A wire range (`Range`). */
+export interface WireRange {
+  readonly start: WirePosition
+  readonly end: WirePosition
+}
+
+/** A `Location`: a document URI plus a range. */
+export interface WireLocation {
+  readonly uri: string
+  readonly range: WireRange
+}
+
+/** A `LocationLink`: the target uri plus the selection range to focus. */
+export interface WireLocationLink {
+  readonly targetUri: string
+  readonly targetSelectionRange: WireRange
+  readonly targetRange?: WireRange
+}
+
+/** A `MarkupContent` hover body (`markdown` or `plaintext`). */
+export interface WireMarkupContent {
+  readonly kind: 'markdown' | 'plaintext'
+  readonly value: string
+}
+
+/** A `MarkedString` object form (`{ language, value }`); the string form is a bare `string`. */
+export interface WireMarkedStringObject {
+  readonly language: string
+  readonly value: string
+}
+
+/** One `MarkedString`: a raw string or a language-tagged code block. */
+export type WireMarkedString = string | WireMarkedStringObject
+
+/** A `Hover`: contents in any of the protocol's three encodings, plus an optional range. */
+export interface WireHover {
+  readonly contents: WireMarkupContent | WireMarkedString | readonly WireMarkedString[]
+  readonly range?: WireRange
+}
+
+/** The legacy enum form of `textDocumentSync` (`0` None, `1` Full, `2` Incremental). */
+export type WireTextDocumentSyncKind = 0 | 1 | 2
+
+/** The options form of `textDocumentSync` (`{ openClose, change }`). */
+export interface WireTextDocumentSyncOptions {
+  readonly openClose?: boolean
+  readonly change?: WireTextDocumentSyncKind
+}
+
+/** A `ServerCapabilities.provider` slot: a boolean or an options object (both mean "supported"). */
+export type WireProviderCapability = boolean | Record<string, unknown> | undefined
+
+/** The `ServerCapabilities` fields this host inspects. */
+export interface WireServerCapabilities {
+  readonly positionEncoding?: string
+  readonly textDocumentSync?: WireTextDocumentSyncKind | WireTextDocumentSyncOptions
+  readonly definitionProvider?: WireProviderCapability
+  readonly referencesProvider?: WireProviderCapability
+  readonly implementationProvider?: WireProviderCapability
+  readonly hoverProvider?: WireProviderCapability
+}
+
+/** The `initialize` result envelope. */
+export interface WireInitializeResult {
+  readonly capabilities: WireServerCapabilities
+}

packages/lsp/lsp-local/src/translate.ts

@@ -0,0 +1,210 @@
+/**
+ * Pure protocol translation for the local host: what the server's capabilities allow, and how its
+ * `Location`/`LocationLink`/`Hover` payloads normalize into the seam's closed result unions. No I/O
+ * or process state — every function here is a pure transform, which the fake-stdio tests pin exactly.
+ * @module @deepseek-ai/dsh-lsp-local/translate
+ */
+
+import type {
+  LspHover,
+  LspLocation,
+  LspOperation,
+  LspRange,
+} from '@deepseek-ai/dsh-lsp'
+import { assertNever } from '@deepseek-ai/dsh-llm'
+import type {
+  WireHover,
+  WireLocation,
+  WireLocationLink,
+  WireMarkedString,
+  WireProviderCapability,
+  WireRange,
+  WireServerCapabilities,
+  WireTextDocumentSyncKind,
+  WireTextDocumentSyncOptions,
+} from './protocol.ts'
+
+/**
+ * The `textDocument/*` request method for each seam operation.
+ * @param operation - the seam operation to map.
+ * @returns the LSP request method name.
+ */
+export function requestMethod(operation: LspOperation): string {
+  switch (operation) {
+    case 'definition': return 'textDocument/definition'
+    case 'references': return 'textDocument/references'
+    case 'implementation': return 'textDocument/implementation'
+    case 'hover': return 'textDocument/hover'
+    /* v8 ignore next -- exhaustive over the closed LspOperation union; unreachable. */
+    default: return assertNever(operation, 'requestMethod')
+  }
+}
+
+/** The `ServerCapabilities` provider field backing each operation. */
+function capabilityValue(capabilities: WireServerCapabilities, operation: LspOperation): WireProviderCapability {
+  switch (operation) {
+    case 'definition': return capabilities.definitionProvider
+    case 'references': return capabilities.referencesProvider
+    case 'implementation': return capabilities.implementationProvider
+    case 'hover': return capabilities.hoverProvider
+    /* v8 ignore next -- exhaustive over the closed LspOperation union; unreachable. */
+    default: return assertNever(operation, 'capabilityValue')
+  }
+}
+
+/** A provider capability is present when the server sent `true` or an options object (not `false`/absent). */
+function supportsCapability(value: WireProviderCapability): boolean {
+  if (value === undefined) return false
+  if (typeof value === 'boolean') return value
+  return true
+}
+
+/**
+ * Whether the server advertises the requested operation.
+ * @param capabilities - the server's `initialize` capabilities.
+ * @param operation - the seam operation to check.
+ * @returns true when the corresponding provider capability is present.
+ */
+export function supportsOperation(capabilities: WireServerCapabilities, operation: LspOperation): boolean {
+  return supportsCapability(capabilityValue(capabilities, operation))
+}
+
+/**
+ * Whether a `textDocumentSync` value permits the transient `didOpen`/`didClose` this host relies on.
+ * @param sync - the server's advertised `textDocumentSync` capability.
+ * @returns true when transient open/close is supported.
+ */
+export function supportsTransientOpen(sync: WireServerCapabilities['textDocumentSync']): boolean {
+  if (sync === undefined) return false
+  if (typeof sync === 'number') return isOpenCloseKind(sync)
+  return sync.openClose === true || (sync.openClose === undefined && changeAllowsOpenClose(sync))
+}
+
+/** Legacy enum: `Full` (1) or `Incremental` (2) imply open/close support; `None` (0) does not. */
+function isOpenCloseKind(kind: WireTextDocumentSyncKind): boolean {
+  return kind === 1 || kind === 2
+}
+
+/** Options without an explicit `openClose` fall back to the legacy `change` enum's implication. */
+function changeAllowsOpenClose(sync: WireTextDocumentSyncOptions): boolean {
+  return sync.change !== undefined && isOpenCloseKind(sync.change)
+}
+
+/**
+ * Normalize the negotiated position encoding. An omitted encoding defaults to `utf-16`; any value
+ * other than `utf-16` is a protocol error this host does not support.
+ * @param encoding - the server's advertised `positionEncoding`, if any.
+ * @returns the string `'utf-16'`.
+ * @throws Error for any non-`utf-16` encoding.
+ */
+export function negotiatePositionEncoding(encoding: string | undefined): 'utf-16' {
+  if (encoding === undefined || encoding === 'utf-16') return 'utf-16'
+  throw new Error(`server negotiated unsupported position encoding "${encoding}"; this host requires utf-16`)
+}
+
+/** Convert a wire range to the seam's range (structurally identical, but re-shaped as `readonly`). */
+function toRange(range: WireRange): LspRange {
+  return {
+    start: { line: range.start.line, character: range.start.character },
+    end: { line: range.end.line, character: range.end.character },
+  }
+}
+
+/** Whether a record is a `LocationLink` (has `targetUri` + `targetSelectionRange`). */
+function isLocationLink(value: Record<string, unknown>): boolean {
+  return typeof value.targetUri === 'string' && isRange(value.targetSelectionRange)
+}
+
+/** Whether a record is a `Location` (has string `uri` + a range). */
+function isLocation(value: Record<string, unknown>): boolean {
+  return typeof value.uri === 'string' && isRange(value.range)
+}
+
+/** Structural range guard used by both location shapes. */
+function isRange(value: unknown): value is WireRange {
+  if (value === null || typeof value !== 'object') return false
+  const range = value as Record<string, unknown>
+  return isPosition(range.start) && isPosition(range.end)
+}
+
+/** Structural position guard. */
+function isPosition(value: unknown): boolean {
+  if (value === null || typeof value !== 'object') return false
+  const position = value as Record<string, unknown>
+  return typeof position.line === 'number' && typeof position.character === 'number'
+}
+
+/**
+ * Normalize a navigation result (`Location`, `Location[]`, `LocationLink[]`, or `null`) to the seam's
+ * locations. `Location` maps directly; `LocationLink` maps `targetUri` + `targetSelectionRange`.
+ * @param payload - the raw `textDocument/definition|references|implementation` result.
+ * @returns the normalized locations (empty for `null`/`[]`).
+ * @throws Error when an element is neither a `Location` nor a `LocationLink`.
+ */
+export function normalizeLocations(payload: unknown): LspLocation[] {
+  if (payload === null || payload === undefined) return []
+  const elements = Array.isArray(payload) ? payload : [payload]
+  const locations: LspLocation[] = []
+  for (const element of elements) {
+    if (element === null || typeof element !== 'object') {
+      throw new Error('LSP navigation result contained a non-object entry')
+    }
+    const record = element as Record<string, unknown>
+    if (isLocationLink(record)) {
+      const link = record as unknown as WireLocationLink
+      locations.push({ uri: link.targetUri, range: toRange(link.targetSelectionRange) })
+    } else if (isLocation(record)) {
+      const location = record as unknown as WireLocation
+      locations.push({ uri: location.uri, range: toRange(location.range) })
+    } else {
+      throw new Error('LSP navigation result contained neither a Location nor a LocationLink')
+    }
+  }
+  return locations
+}
+
+/** Render one `MarkedString` (string form verbatim; object form as a language-tagged fenced block). */
+function renderMarkedString(value: WireMarkedString): string {
+  if (typeof value === 'string') return value
+  return `\`\`\`${value.language}\n${value.value}\n\`\`\``
+}
+
+/**
+ * Normalize a `Hover` (or `null`) to the seam's hover. `MarkupContent` uses its `value`; a string
+ * `MarkedString` is verbatim; a language-tagged `MarkedString` becomes a fenced code block; an array
+ * joins its rendered parts with one blank line. `maxHoverChars` is NOT applied here — the tool caps.
+ * @param payload - the raw `textDocument/hover` result.
+ * @returns the normalized hover, or `null` when there is no content.
+ * @throws Error when the payload is a non-null, non-object, or structurally invalid hover.
+ */
+export function normalizeHover(payload: unknown): LspHover | null {
+  if (payload === null || payload === undefined) return null
+  if (typeof payload !== 'object') throw new Error('LSP hover result was not an object')
+  const hover = payload as unknown as WireHover
+  const contents = renderHoverContents(hover.contents)
+  if (contents === '') return null
+  const range = hover.range
+  return range !== undefined && isRange(range) ? { contents, range: toRange(range) } : { contents }
+}
+
+/** Render the three `Hover.contents` encodings into one string (input is untrusted wire data). */
+function renderHoverContents(contents: unknown): string {
+  if (contents === null || contents === undefined) {
+    throw new Error('LSP hover result had no contents')
+  }
+  if (typeof contents === 'string') return contents
+  if (Array.isArray(contents)) {
+    return contents.map(renderMarkedString).join('\n\n')
+  }
+  if (typeof contents !== 'object') {
+    throw new Error('LSP hover contents were not MarkupContent, MarkedString, or an array')
+  }
+  const record = contents as Record<string, unknown>
+  if (record.kind === 'markdown' || record.kind === 'plaintext') {
+    return typeof record.value === 'string' ? record.value : ''
+  }
+  if (typeof record.language === 'string' && typeof record.value === 'string') {
+    return renderMarkedString({ language: record.language, value: record.value })
+  }
+  throw new Error('LSP hover contents were not MarkupContent, MarkedString, or an array')
+}