diff --git a/app/common/ActionBundle.ts b/app/common/ActionBundle.ts
index eb35f3a6..f98f70b1 100644
--- a/app/common/ActionBundle.ts
+++ b/app/common/ActionBundle.ts
@@ -62,6 +62,16 @@ export interface SandboxActionBundle {
   undo: Array<EnvContent<DocAction>>;   // Inverse actions for all 'stored' actions.
   retValues: any[];                     // Contains retValue for each of userActions.
   rowCount: number;
+  // Mapping of keys (hashes of request args) to all unique requests made in a round of calculation
+  requests?: Record<string, SandboxRequest>;
+}
+
+// Represents a unique call to the Python REQUEST function
+export interface SandboxRequest {
+  url: string;
+  params: Record<string, string> | null;
+  headers: Record<string, string> | null;
+  deps: unknown;  // pass back to the sandbox unchanged in the response
 }
 
 // Local action that's been applied. It now has an actionNum, and includes doc actions packaged
diff --git a/app/common/DocActions.ts b/app/common/DocActions.ts
index 209ce13b..fd75f5ef 100644
--- a/app/common/DocActions.ts
+++ b/app/common/DocActions.ts
@@ -144,6 +144,9 @@ export interface TableRecordValue {
 
 export type UserAction = Array<string|number|object|boolean|null|undefined>;
 
+// Actions that trigger formula calculations in the data engine
+export const CALCULATING_USER_ACTIONS = new Set(['Calculate', 'UpdateCurrentTime', 'RespondToRequests']);
+
 /**
  * Gives a description for an action which involves setting values to a selection.
  * @param {Array} action - The (Bulk)AddRecord/(Bulk)UpdateRecord action to describe.
diff --git a/app/server/lib/ActiveDoc.ts b/app/server/lib/ActiveDoc.ts
index 58d513e8..5310a2c2 100644
--- a/app/server/lib/ActiveDoc.ts
+++ b/app/server/lib/ActiveDoc.ts
@@ -4,7 +4,13 @@
  * change events.
  */
 
-import {getEnvContent, LocalActionBundle, SandboxActionBundle, UserActionBundle} from 'app/common/ActionBundle';
+import {
+  getEnvContent,
+  LocalActionBundle,
+  SandboxActionBundle,
+  SandboxRequest,
+  UserActionBundle
+} from 'app/common/ActionBundle';
 import {ActionGroup, MinimalActionGroup} from 'app/common/ActionGroup';
 import {ActionSummary} from "app/common/ActionSummary";
 import {
@@ -33,12 +39,7 @@ import {
   UserAction
 } from 'app/common/DocActions';
 import {DocData} from 'app/common/DocData';
-import {
-  getDataLimitRatio,
-  getDataLimitStatus,
-  getSeverity,
-  LimitExceededError,
-} from 'app/common/DocLimits';
+import {getDataLimitRatio, getDataLimitStatus, getSeverity, LimitExceededError} from 'app/common/DocLimits';
 import {DocSnapshots} from 'app/common/DocSnapshot';
 import {DocumentSettings} from 'app/common/DocumentSettings';
 import {
@@ -75,6 +76,7 @@ import {GRIST_DOC_SQL, GRIST_DOC_WITH_TABLE1_SQL} from 'app/server/lib/initialDo
 import {ISandbox} from 'app/server/lib/ISandbox';
 import * as log from 'app/server/lib/log';
 import {LogMethods} from "app/server/lib/LogMethods";
+import {DocRequests} from 'app/server/lib/Requests';
 import {shortDesc} from 'app/server/lib/shortDesc';
 import {TableMetadataLoader} from 'app/server/lib/TableMetadataLoader';
 import {DocTriggers} from "app/server/lib/Triggers";
@@ -182,6 +184,7 @@ export class ActiveDoc extends EventEmitter {
 
   private _log = new LogMethods('ActiveDoc ', (s: OptDocSession) => this.getLogMeta(s));
   private _triggers: DocTriggers;
+  private _requests: DocRequests;
   private _dataEngine: Promise<ISandbox>|undefined;
   private _activeDocImport: ActiveDocImport;
   private _onDemandActions: OnDemandActions;
@@ -270,6 +273,7 @@ export class ActiveDoc extends EventEmitter {
     this.docStorage = new DocStorage(docManager.storageManager, docName);
     this.docClients = new DocClients(this);
     this._triggers = new DocTriggers(this);
+    this._requests = new DocRequests(this);
     this._actionHistory = new ActionHistoryImpl(this.docStorage);
     this.docPluginManager = new DocPluginManager(docManager.pluginManager.getPlugins(),
       docManager.pluginManager.appRoot!, this, this._docManager.gristServer);
@@ -1095,7 +1099,7 @@ export class ActiveDoc extends EventEmitter {
       this.dataLimitStatus === "deleteOnly" &&
       !actions.every(action => [
           'RemoveTable', 'RemoveColumn', 'RemoveRecord', 'BulkRemoveRecord',
-          'RemoveViewSection', 'RemoveView', 'ApplyUndoActions',
+          'RemoveViewSection', 'RemoveView', 'ApplyUndoActions', 'RespondToRequests',
         ].includes(action[0] as string))
     ) {
       throw new Error("Document is in delete-only mode");
@@ -1420,6 +1424,10 @@ export class ActiveDoc extends EventEmitter {
       }
       const user = docSession ? await this._granularAccess.getCachedUser(docSession) : undefined;
       sandboxActionBundle = await this._rawPyCall('apply_user_actions', normalActions, user?.toJSON());
+      const {requests} = sandboxActionBundle;
+      if (requests) {
+        this._requests.handleRequestsBatchFromUserActions(requests).catch(e => console.error(e));
+      }
       await this._reportDataEngineMemory();
     } else {
       // Create default SandboxActionBundle to use if the data engine is not called.
@@ -2087,6 +2095,7 @@ export class ActiveDoc extends EventEmitter {
       preferredPythonVersion,
       sandboxOptions: {
         exports: {
+          request: (key: string, args: SandboxRequest) => this._requests.handleSingleRequestWithCache(key, args),
           guessColInfo: (values: Array<string | null>) =>
             guessColInfoWithDocData(values, this.docData!),
           convertFromColumn: (...args: Parameters<ReturnType<typeof convertFromColumn>>) =>
diff --git a/app/server/lib/GranularAccess.ts b/app/server/lib/GranularAccess.ts
index 2ff1febb..fb34afed 100644
--- a/app/server/lib/GranularAccess.ts
+++ b/app/server/lib/GranularAccess.ts
@@ -4,7 +4,13 @@ import { ActionGroup } from 'app/common/ActionGroup';
 import { createEmptyActionSummary } from 'app/common/ActionSummary';
 import { ServerQuery } from 'app/common/ActiveDocAPI';
 import { ApiError } from 'app/common/ApiError';
-import { AddRecord, BulkAddRecord, BulkColValues, BulkRemoveRecord, BulkUpdateRecord } from 'app/common/DocActions';
+import {
+  AddRecord,
+  BulkAddRecord,
+  BulkColValues,
+  BulkRemoveRecord,
+  BulkUpdateRecord,
+} from 'app/common/DocActions';
 import { RemoveRecord, ReplaceTableData, UpdateRecord } from 'app/common/DocActions';
 import { CellValue, ColValues, DocAction, getTableId, isSchemaAction } from 'app/common/DocActions';
 import { TableDataAction, UserAction } from 'app/common/DocActions';
diff --git a/app/server/lib/Requests.ts b/app/server/lib/Requests.ts
new file mode 100644
index 00000000..3835b1ff
--- /dev/null
+++ b/app/server/lib/Requests.ts
@@ -0,0 +1,128 @@
+import {SandboxRequest} from 'app/common/ActionBundle';
+import {ActiveDoc} from 'app/server/lib/ActiveDoc';
+import {makeExceptionalDocSession} from 'app/server/lib/DocSession';
+import {httpEncoding} from 'app/server/lib/httpEncoding';
+import {HttpsProxyAgent} from 'https-proxy-agent';
+import {HttpProxyAgent} from 'http-proxy-agent';
+import fetch from 'node-fetch';
+import * as path from 'path';
+import * as tmp from 'tmp';
+import chunk = require('lodash/chunk');
+import fromPairs = require('lodash/fromPairs');
+import zipObject = require('lodash/zipObject');
+import * as fse from 'fs-extra';
+import * as log from 'app/server/lib/log';
+
+export class DocRequests {
+  // Request responses are briefly cached in files only to handle multiple requests in a formula
+  // and only as long as needed to finish calculating all formulas.
+  // When _numPending reaches 0 again, _cacheDir is deleted.
+  private _numPending: number = 0;
+  private _cacheDir: tmp.SynchrounousResult | null = null;
+
+  constructor(private readonly _activeDoc: ActiveDoc) {}
+
+  public async handleRequestsBatchFromUserActions(requests: Record<string, SandboxRequest>) {
+    const numRequests = Object.keys(requests).length;
+    this._numPending += numRequests;
+    try {
+      // Perform batches of requests in parallel for speed, and hope it doesn't cause rate limiting...
+      for (const keys of chunk(Object.keys(requests), 10)) {
+        const responses: Response[] = await Promise.all(keys.map(async key => {
+          const request = requests[key];
+          const response = await this.handleSingleRequestWithCache(key, request);
+          return {
+            ...response,
+            // Tells the engine which cell(s) made the request and should be recalculated to use the response
+            deps: request.deps,
+          };
+        }));
+        // Tell the sandbox which previous responses we have cached in files.
+        // This lets it know it can immediately and synchronously get those responses again.
+        const cachedRequestKeys = await fse.readdir(this._cacheDir!.name);
+        // Recalculate formulas using this batch of responses.
+        const action = ["RespondToRequests", zipObject(keys, responses), cachedRequestKeys];
+        await this._activeDoc.applyUserActions(makeExceptionalDocSession("system"), [action]);
+      }
+    } finally {
+      this._numPending -= numRequests;
+      if (this._numPending === 0) {
+        log.debug(`Removing DocRequests._cacheDir: ${this._cacheDir!.name}`);
+        this._cacheDir!.removeCallback();
+        this._cacheDir = null;
+      }
+    }
+  }
+
+  public async handleSingleRequestWithCache(key: string, request: SandboxRequest): Promise<Response> {
+    if (!this._cacheDir) {
+      // Use the sync API because otherwise multiple requests being handled at the same time
+      // all reach this point, `await`, and create different dirs.
+      // `unsafeCleanup: true` means the directory can be deleted even if it's not empty, which is what we expect.
+      this._cacheDir = tmp.dirSync({unsafeCleanup: true});
+      log.debug(`Created DocRequests._cacheDir: ${this._cacheDir.name}`);
+    }
+
+    const cachePath = path.resolve(this._cacheDir.name, key);
+    try {
+      const result = await fse.readJSON(cachePath);
+      result.content = Buffer.from(result.content, "base64");
+      return result;
+    } catch {
+      const result = await this._handleSingleRequestRaw(request);
+      const resultForJson = {...result} as any;
+      if ('content' in result) {
+        resultForJson.content = result.content.toString("base64");
+      }
+      fse.writeJSON(cachePath, resultForJson).catch(e => log.warn(`Failed to save response to cache file: ${e}`));
+      return result;
+    }
+  }
+
+  private async _handleSingleRequestRaw(request: SandboxRequest): Promise<Response> {
+    try {
+      if (process.env.GRIST_EXPERIMENTAL_PLUGINS != '1') {
+        throw new Error("REQUEST is not enabled");
+      }
+      const {url, params, headers} = request;
+      const urlObj = new URL(url);
+      log.rawInfo("Handling sandbox request", {host: urlObj.host, docId: this._activeDoc.docName});
+      for (const [param, value] of Object.entries(params || {})) {
+        urlObj.searchParams.append(param, value);
+      }
+      const response = await fetch(urlObj.toString(), {headers: headers || {}, agent: proxyAgent(urlObj)});
+      const content = await response.buffer();
+      const {status, statusText} = response;
+      const encoding = httpEncoding(response.headers.get('content-type'), content);
+      return {
+        content, status, statusText, encoding,
+        headers: fromPairs([...response.headers]),
+      };
+    } catch (e) {
+      return {error: String(e)};
+    }
+  }
+}
+
+interface SuccessfulResponse {
+  content: Buffer;
+  status: number;
+  statusText: string;
+  encoding?: string;
+  headers: Record<string, string>;
+}
+
+interface RequestError {
+  error: string;
+}
+
+type Response = RequestError | SuccessfulResponse;
+
+function proxyAgent(requestUrl: URL) {
+  const proxy = process.env.GRIST_HTTPS_PROXY;
+  if (!proxy) {
+    return undefined;
+  }
+  const ProxyAgent = requestUrl.protocol === "https:" ? HttpsProxyAgent : HttpProxyAgent;
+  return new ProxyAgent(proxy);
+}
diff --git a/app/server/lib/Sharing.ts b/app/server/lib/Sharing.ts
index 5ddd2370..f8383b60 100644
--- a/app/server/lib/Sharing.ts
+++ b/app/server/lib/Sharing.ts
@@ -6,7 +6,7 @@ import {
   LocalActionBundle,
   UserActionBundle
 } from 'app/common/ActionBundle';
-import {DocAction, getNumRows, UserAction} from 'app/common/DocActions';
+import {CALCULATING_USER_ACTIONS, DocAction, getNumRows, UserAction} from 'app/common/DocActions';
 import {allToken} from 'app/common/sharing';
 import * as log from 'app/server/lib/log';
 import {LogMethods} from "app/server/lib/LogMethods";
@@ -215,8 +215,7 @@ export class Sharing {
 
     try {
 
-      const isCalculate = (userActions.length === 1 &&
-                           (userActions[0][0] === 'Calculate' || userActions[0][0] === 'UpdateCurrentTime'));
+      const isCalculate = (userActions.length === 1 && CALCULATING_USER_ACTIONS.has(userActions[0][0] as string));
       // `internal` is true if users shouldn't be able to undo the actions. Applies to:
       // - Calculate/UpdateCurrentTime because it's not considered as performed by a particular client.
       // - Adding attachment metadata when uploading attachments,
diff --git a/app/server/lib/httpEncoding.ts b/app/server/lib/httpEncoding.ts
new file mode 100644
index 00000000..af485ff3
--- /dev/null
+++ b/app/server/lib/httpEncoding.ts
@@ -0,0 +1,43 @@
+// Based on the source code of the Body.textConverted method in node-fetch
+export function httpEncoding(header: string | null, content: Buffer): string | undefined {
+  let res: RegExpExecArray | null = null;
+
+  // header
+  if (header) {
+    res = /charset=([^;]*)/i.exec(header);
+  }
+
+  // no charset in content type, peek at response body for at most 1024 bytes
+  const str = content.slice(0, 1024).toString();
+
+  // html5
+  if (!res && str) {
+    res = /<meta.+?charset=(['"])(.+?)\1/i.exec(str);
+  }
+
+  // html4
+  if (!res && str) {
+    res = /<meta\s+?http-equiv=(['"])content-type\1\s+?content=(['"])(.+?)\2/i.exec(str);
+
+    if (res) {
+      res = /charset=(.*)/i.exec(res.pop()!);
+    }
+  }
+
+  // xml
+  if (!res && str) {
+    res = /<\?xml.+?encoding=(['"])(.+?)\1/i.exec(str);
+  }
+
+  // found charset
+  if (res) {
+    let charset = res.pop();
+
+    // prevent decode issues when sites use incorrect encoding
+    // ref: https://hsivonen.fi/encoding-menu/
+    if (charset === 'gb2312' || charset === 'gbk') {
+      charset = 'gb18030';
+    }
+    return charset;
+  }
+}
diff --git a/buildtools/tsconfig-base.json b/buildtools/tsconfig-base.json
index 7cb62e28..a0bfab3c 100644
--- a/buildtools/tsconfig-base.json
+++ b/buildtools/tsconfig-base.json
@@ -1,5 +1,6 @@
 {
   "compilerOptions": {
+    "allowSyntheticDefaultImports": true,
     "target": "es2016",
     "module": "commonjs",
     "strict": true,
diff --git a/package.json b/package.json
index 78f2473a..8b8940ae 100644
--- a/package.json
+++ b/package.json
@@ -112,6 +112,8 @@
     "grain-rpc": "0.1.7",
     "grainjs": "1.0.2",
     "highlight.js": "9.13.1",
+    "http-proxy-agent": "5.0.0",
+    "https-proxy-agent": "5.0.1",
     "i18n-iso-countries": "6.1.0",
     "image-size": "0.6.3",
     "jquery": "2.2.1",
diff --git a/sandbox/grist/action_obj.py b/sandbox/grist/action_obj.py
index 654329af..1408c125 100644
--- a/sandbox/grist/action_obj.py
+++ b/sandbox/grist/action_obj.py
@@ -27,6 +27,7 @@ class ActionGroup(object):
     self.undo     = []
     self.retValues = []
     self.summary = ActionSummary()
+    self.requests = {}
 
   def flush_calc_changes(self):
     """
diff --git a/sandbox/grist/engine.py b/sandbox/grist/engine.py
index 3b05bd1b..84dac02a 100644
--- a/sandbox/grist/engine.py
+++ b/sandbox/grist/engine.py
@@ -30,6 +30,7 @@ import match_counter
 import objtypes
 from objtypes import strict_equal
 from relation import SingleRowsIdentityRelation
+import sandbox
 import schema
 from schema import RecalcWhen
 import table as table_module
@@ -66,6 +67,16 @@ class OrderError(Exception):
     self.requiring_node = node
     self.requiring_row_id = row_id
 
+
+class RequestingError(Exception):
+  """
+  An exception thrown and handled internally, a bit like OrderError.
+  Indicates that the formula called the REQUEST function and needs to delegate an HTTP request
+  to the NodeJS server.
+  """
+  pass
+
+
 # An item of work to be done by Engine._update
 WorkItem = namedtuple('WorkItem', ('node', 'row_ids', 'locks'))
 
@@ -186,6 +197,7 @@ class Engine(object):
     # What's currently being computed
     self._current_node = None
     self._current_row_id = None
+    self._is_current_node_formula = False  # True for formula columns, False for trigger formulas
 
     # Certain recomputations are triggered by a particular doc action. This keep track of it.
     self._triggering_doc_action = None
@@ -236,6 +248,19 @@ class Engine(object):
 
     self._table_stats = {"meta": [], "user": []}
 
+    #### Attributes used by the REQUEST function:
+    # True when the formula should synchronously call the exported JS method to make the request
+    # immediately instead of reevaluating the formula later. Used when reevaluating a single
+    # formula cell to get an error traceback.
+    self._sync_request = False
+    # dict of string keys to responses, set by the RespondToRequests user action to reevaluate
+    # formulas based on a batch of completed requests.
+    self._request_responses = {}
+    # set of string keys identifying requests that are currently cached in files and can thus
+    # be fetched synchronously via the exported JS method. This allows a single formula to
+    # make multiple different requests without needing to keep all the responses in memory.
+    self._cached_request_keys = set()
+
   @property
   def autocomplete_context(self):
     # See the comment on _autocomplete_context in __init__ above.
@@ -480,7 +505,7 @@ class Engine(object):
     if self._peeking:
       return
 
-    if self._current_node:
+    if self._is_current_node_formula:
       # Add an edge to indicate that the node being computed depends on the node passed in.
       # Note that during evaluation, we only *add* dependencies. We *remove* them by clearing them
       # whenever ALL rows for a node are invalidated (on schema changes and reloads).
@@ -671,6 +696,8 @@ class Engine(object):
     table = self.tables[table_id]
     col = table.get_column(col_id)
     checkpoint = self._get_undo_checkpoint()
+    # Makes calls to REQUEST synchronous, since raising a RequestingError can't work here.
+    self._sync_request = True
     try:
       result = self._recompute_one_cell(table, col, row_id)
       # If the error is gone for a trigger formula
@@ -686,6 +713,7 @@ class Engine(object):
       # It is possible for formula evaluation to have side-effects that produce DocActions (e.g.
       # lookupOrAddDerived() creates those). In case of get_formula_error(), these aren't fully
       # processed (e.g. don't get applied to DocStorage), so it's important to reverse them.
+      self._sync_request = False
       self._undo_to_checkpoint(checkpoint)
 
   def _recompute(self, node, row_ids=None):
@@ -757,9 +785,11 @@ class Engine(object):
     require_rows = sorted(require_rows or [])
 
     previous_current_node = self._current_node
+    previous_is_current_node_formula = self._is_current_node_formula
+    self._current_node = node
     # Prevents dependency creation for non-formula nodes. A non-formula column may include a
     # formula to eval for a newly-added record. Those shouldn't create dependencies.
-    self._current_node = node if col.is_formula() else None
+    self._is_current_node_formula = col.is_formula()
 
     changes = None
     cleaned = []    # this lists row_ids that can be removed from dirty_rows once we are no
@@ -789,11 +819,16 @@ class Engine(object):
           # For common-case formulas, all cells in a column are likely to fail in the same way,
           # so don't bother trying more from this column until we've reordered.
           return
+
+        making_request = False
         try:
           # We figure out if we've hit a cycle here.  If so, we just let _recompute_on_cell
           # know, so it can set the cell value appropriately and do some other bookkeeping.
           cycle = required and (node, row_id) in self._locked_cells
           value = self._recompute_one_cell(table, col, row_id, cycle=cycle, node=node)
+        except RequestingError:
+          making_request = True
+          value = RequestingError
         except OrderError as e:
           if not required:
             # We're out of order, but for a cell we were evaluating opportunistically.
@@ -822,19 +857,24 @@ class Engine(object):
         if column.is_validation_column_name(col.col_id):
           value = (value in (True, None))
 
-        # Convert the value, and if needed, set, and include into the returned action.
-        value = col.convert(value)
-        previous = col.raw_get(row_id)
-        if not strict_equal(value, previous):
-          if not changes:
-            changes = self._changes_map.setdefault(node, [])
-          changes.append((row_id, previous, value))
-          col.set(row_id, value)
+        # When the formula raises a RequestingError, leave the existing value in the cell.
+        # The formula will be evaluated again soon when we have a response.
+        if not making_request:
+          # Convert the value, and if needed, set, and include into the returned action.
+          value = col.convert(value)
+          previous = col.raw_get(row_id)
+          if not strict_equal(value, previous):
+            if not changes:
+              changes = self._changes_map.setdefault(node, [])
+            changes.append((row_id, previous, value))
+            col.set(row_id, value)
+
         exclude.add(row_id)
         cleaned.append(row_id)
         self._recompute_done_counter += 1
     finally:
       self._current_node = previous_current_node
+      self._is_current_node_formula = previous_is_current_node_formula
       # Usually dirty_rows refers to self.recompute_map[node], so this modifies both
       dirty_rows -= cleaned
 
@@ -844,6 +884,43 @@ class Engine(object):
       if not self.recompute_map[node]:
         self.recompute_map.pop(node)
 
+  def _requesting(self, key, args):
+    """
+    Called by the REQUEST function. If we don't have a response already and we can't
+    synchronously get it from the JS side, then note the request to be made in JS asynchronously
+    and raise RequestingError to indicate that the formula
+    should be evaluated again later when we have a response.
+    """
+    # This will make the formula reevaluate periodically with the UpdateCurrentTime action.
+    # This assumes that the response changes with time and having the latest data is ideal.
+    # We will probably want to reconsider this to avoid making unwanted requests,
+    # along with avoiding refreshing the request when the doc is loaded with the Calculate action.
+    self.use_current_time()
+
+    if key in self._request_responses:
+      # This formula is being reevaluated in a RespondToRequests action, and the response is ready.
+      return self._request_responses[key]
+    elif self._sync_request or key in self._cached_request_keys:
+      # Not always ideal, but in this case the best strategy is to make the request immediately
+      # and block while waiting for a response.
+      return sandbox.call_external("request", key, args)
+
+    # We can't get a response to this request now. Note the request so it can be delegated.
+    table_id, column_id = self._current_node
+    (self.out_actions.requests  # `out_actions.requests` is returned by apply_user_actions
+         # Here is where the request arguments are stored if they haven't been already
+         .setdefault(key, args)
+         # While all this stores the cell that made the request so that it can be invalidated later
+         .setdefault("deps", {})
+         .setdefault(table_id, {})
+         .setdefault(column_id, [])
+         .append(self._current_row_id))
+
+    # As with OrderError, note the exception so it gets raised even if the formula catches it
+    self._cell_required_error = RequestingError()
+
+    raise RequestingError()
+
   def _recompute_one_cell(self, table, col, row_id, cycle=False, node=None):
     """
     Recomputes an one formula cell and returns a value.
@@ -1198,6 +1275,11 @@ class Engine(object):
 
     self.out_actions = action_obj.ActionGroup()
     self._user = User(user, self.tables) if user else None
+
+    # These should usually be empty, but may be populated by the RespondToRequests action.
+    self._request_responses = {}
+    self._cached_request_keys = set()
+
     checkpoint = self._get_undo_checkpoint()
     try:
       for user_action in user_actions:
@@ -1252,6 +1334,8 @@ class Engine(object):
     self.out_actions.flush_calc_changes()
     self.out_actions.check_sanity()
     self._user = None
+    self._request_responses = {}
+    self._cached_request_keys = set()
     return self.out_actions
 
   def acl_split(self, action_group):
diff --git a/sandbox/grist/functions/info.py b/sandbox/grist/functions/info.py
index 7b3b9edb..543a8ffc 100644
--- a/sandbox/grist/functions/info.py
+++ b/sandbox/grist/functions/info.py
@@ -3,10 +3,13 @@
 
 from __future__ import absolute_import
 import datetime
+import hashlib
+import json
 import math
 import numbers
 import re
 
+import chardet
 import six
 
 import column
@@ -656,3 +659,91 @@ def is_error(value):
   return ((value is _error_sentinel)
       or isinstance(value, AltText)
       or (isinstance(value, float) and math.isnan(value)))
+
+
+@unimplemented  # exclude from autocomplete while in beta
+def REQUEST(url, params=None, headers=None):
+  # Makes a GET HTTP request with an API similar to `requests.get`.
+  # Actually jumps through hoops internally to make the request asynchronously (usually)
+  # while feeling synchronous to the formula writer.
+
+  # Requests are identified by a string key in various places.
+  # The same arguments should produce the same key so the request is only made once.
+  args = dict(url=url, params=params, headers=headers)
+  args_json = json.dumps(args, sort_keys=True)
+  key = hashlib.sha256(args_json.encode()).hexdigest()
+
+  # This may either return the raw response data or it may raise a special exception
+  # to delegate the request and reevaluate the formula later.
+  response_dict = docmodel.global_docmodel._engine._requesting(key, args)
+
+  if "error" in response_dict:
+    # Indicates a complete failure to make the request, such as a connection problem.
+    # An unsuccessful status code like 404 or 500 doesn't raise this error.
+    raise HTTPError(response_dict["error"])
+
+  return Response(**response_dict)
+
+
+class HTTPError(Exception):
+  pass
+
+
+class Response(object):
+  """
+  Similar to the Response class from the `requests` library.
+  """
+  def __init__(self, content, status, statusText, headers, encoding):
+    self.content = content  # raw bytes
+    self.status_code = status  # e.g. 404
+    self.reason = statusText  # e.g. "Not Found"
+    self.headers = CaseInsensitiveDict(headers)
+    self.encoding = encoding or self.apparent_encoding
+
+  @property
+  def text(self):
+    return self.content.decode(self.encoding)
+
+  def json(self, **kwargs):
+    return json.loads(self.text, **kwargs)
+
+  @property
+  def ok(self):
+    return self.status_code < 400
+
+  def raise_for_status(self):
+    if not self.ok:
+      raise HTTPError("Request failed with status %s" % self.status_code)
+
+  @property
+  def apparent_encoding(self):
+    return chardet.detect(self.content)["encoding"]
+
+  def close(self):
+    pass  # nothing to do
+
+
+class CaseInsensitiveDict(dict):
+  """
+  Similar to dict but treats all keys (which must be strings) case-insensitively,
+  e.g. `d["foo"]` and `d["FOO"]` are equivalent.
+  """
+  def __init__(self, *args, **kwargs):
+    dict.__init__(self, *args, **kwargs)
+    for k in list(self):
+      # Convert key to lowercase
+      self[k] = dict.pop(self, k)
+
+  def update(self, E=None, **F):
+    dict.update(self.__class__(E or {}))
+    dict.update(self.__class__(**F))
+
+
+def _forward_dict_method(name):
+  # Replace method 'name' where the first argument is a key with a version that lowercases the key
+  def method(self, key, *args, **kwargs):
+    return getattr(dict, name)(self, key.lower(), *args, **kwargs)
+  return method
+
+for _name in "__getitem__ __setitem__ __delitem__ __contains__ get setdefault pop has_key".split():
+  setattr(CaseInsensitiveDict, _name, _forward_dict_method(_name))
diff --git a/sandbox/grist/lookup.py b/sandbox/grist/lookup.py
index 6d644146..ebd289fe 100644
--- a/sandbox/grist/lookup.py
+++ b/sandbox/grist/lookup.py
@@ -126,7 +126,7 @@ class BaseLookupMapColumn(column.BaseColumn):
     """
     key = tuple(_extract(val) for val in key)
     engine = self._engine
-    if engine._current_node:
+    if engine._is_current_node_formula:
       rel = self._get_relation(engine._current_node)
       rel._add_lookup(engine._current_row_id, key)
     else:
diff --git a/sandbox/grist/main.py b/sandbox/grist/main.py
index b0604b91..58aa6985 100644
--- a/sandbox/grist/main.py
+++ b/sandbox/grist/main.py
@@ -68,10 +68,13 @@ def run(sandbox):
   @export
   def apply_user_actions(action_reprs, user=None):
     action_group = eng.apply_user_actions([useractions.from_repr(u) for u in action_reprs], user)
-    return dict(
+    result = dict(
       rowCount=eng.count_rows(),
       **eng.acl_split(action_group).to_json_obj()
     )
+    if action_group.requests:
+      result["requests"] = action_group.requests
+    return result
 
   @export
   def fetch_table(table_id, formulas=True, query=None):
diff --git a/sandbox/grist/test_requests.py b/sandbox/grist/test_requests.py
new file mode 100644
index 00000000..4a57c168
--- /dev/null
+++ b/sandbox/grist/test_requests.py
@@ -0,0 +1,136 @@
+# coding=utf-8
+import unittest
+
+import test_engine
+import testutil
+from functions import CaseInsensitiveDict, Response, HTTPError
+
+
+class TestCaseInsensitiveDict(unittest.TestCase):
+  def test_case_insensitive_dict(self):
+    d = CaseInsensitiveDict({"FOO": 1})
+    for key in ["foo", "FOO", "Foo"]:
+      self.assertEqual(d, {"foo": 1})
+      self.assertEqual(list(d), ["foo"])
+      self.assertEqual(d, CaseInsensitiveDict({key: 1}))
+      self.assertIn(key, d)
+      self.assertEqual(d[key], 1)
+      self.assertEqual(d.get(key), 1)
+      self.assertEqual(d.get(key, 2), 1)
+      self.assertEqual(d.get(key + "2", 2), 2)
+      self.assertEqual(d.pop(key), 1)
+      self.assertEqual(d, {})
+      self.assertEqual(d.setdefault(key, 3), 3)
+      self.assertEqual(d, {"foo": 3})
+      self.assertEqual(d.setdefault(key, 4), 3)
+      self.assertEqual(d, {"foo": 3})
+      del d[key]
+      self.assertEqual(d, {})
+      d[key] = 1
+
+
+class TestResponse(unittest.TestCase):
+  def test_ok_response(self):
+    r = Response(b"foo", 200, "OK", {"X-header": "hi"}, None)
+    self.assertEqual(r.content, b"foo")
+    self.assertEqual(r.text, u"foo")
+    self.assertEqual(r.status_code, 200)
+    self.assertEqual(r.ok, True)
+    self.assertEqual(r.reason, "OK")
+    self.assertEqual(r.headers, {"x-header": "hi"})
+    self.assertEqual(r.encoding, "ascii")
+    self.assertEqual(r.apparent_encoding, "ascii")
+    r.raise_for_status()
+    r.close()
+
+  def test_error_response(self):
+    r = Response(b"foo", 500, "Server error", {}, None)
+    self.assertEqual(r.status_code, 500)
+    self.assertEqual(r.ok, False)
+    self.assertEqual(r.reason, "Server error")
+    with self.assertRaises(HTTPError) as cm:
+      r.raise_for_status()
+    self.assertEqual(str(cm.exception), "Request failed with status 500")
+
+  def test_json(self):
+    r = Response(b'{"foo": "bar"}', 200, "OK", {}, None)
+    self.assertEqual(r.json(), {"foo": "bar"})
+
+  def test_encoding_direct(self):
+    r = Response(b"foo", 200, "OK", {}, "some encoding")
+    self.assertEqual(r.encoding, "some encoding")
+    self.assertEqual(r.apparent_encoding, "ascii")
+
+  def test_apparent_encoding(self):
+    text = u"编程"
+    encoding = "utf-8"
+    content = text.encode(encoding)
+    self.assertEqual(content.decode(encoding), text)
+    r = Response(content, 200, "OK", {}, "")
+    self.assertEqual(r.encoding, encoding)
+    self.assertEqual(r.apparent_encoding, encoding)
+    self.assertEqual(r.content, content)
+    self.assertEqual(r.text, text)
+
+
+class TestRequestFunction(test_engine.EngineTestCase):
+  sample = testutil.parse_test_sample({
+    "SCHEMA": [
+      [1, "Table1", [
+        [2, "Request", "Any", True, "$id", "", ""],
+        [3, "Other", "Any", True, "", "", ""],
+      ]],
+    ],
+    "DATA": {
+      "Table1": [
+        ["id"],
+        [1],
+        [2],
+      ],
+    }
+  })
+
+  def test_request_function(self):
+    self.load_sample(self.sample)
+
+    formula = """
+r = REQUEST('my_url', headers={'foo': 'bar'}, params={'b': 1, 'a': 2})
+r.__dict__
+"""
+    out_actions = self.modify_column("Table1", "Request", formula=formula)
+    key = '9d305be9664924aaaf7ebb0bab2e4155d1fa1b9dcde53e417f1a9f9a2c7e09b9'
+    deps = {'Table1': {'Request': [1, 2]}}
+    args = {
+      'url': 'my_url',
+      'headers': {'foo': 'bar'},
+      'params': {'a': 2, 'b': 1},
+      'deps': deps,
+    }
+    self.assertEqual(out_actions.requests, {key: args})
+    self.assertTableData("Table1", cols="subset", data=[
+      ["id", "Request"],
+      [1, 1],
+      [2, 2],
+    ])
+
+    response = {
+      'status': 200,
+      'statusText': 'OK',
+      'content': b'body',
+      'headers': {'h1': 'h2'},
+      'encoding': 'utf16',
+      'deps': deps,
+    }
+    self.apply_user_action(["RespondToRequests", {key: response.copy()}, [key]])
+
+    # Translate names from JS `fetch` API to Python `requests`-style API
+    response["status_code"] = response.pop("status")
+    response["reason"] = response.pop("statusText")
+    # This is sent in the user action but not kept for the response object
+    del response["deps"]
+
+    self.assertTableData("Table1", cols="subset", data=[
+      ["id", "Request"],
+      [1, response],
+      [2, response],
+    ])
diff --git a/sandbox/grist/useractions.py b/sandbox/grist/useractions.py
index 46d8f888..5f25a771 100644
--- a/sandbox/grist/useractions.py
+++ b/sandbox/grist/useractions.py
@@ -9,6 +9,7 @@ import six
 from six.moves import xrange
 
 import acl
+import depend
 import gencode
 from acl_formula import parse_acl_formula_json
 import actions
@@ -331,6 +332,25 @@ class UserActions(object):
     """
     self._engine.update_current_time()
 
+  @useraction
+  def RespondToRequests(self, responses, cached_keys):
+    """
+    Reevaluate formulas which called the REQUEST function using the now available responses.
+    """
+    engine = self._engine
+
+    # The actual raw responses which will be returned to the REQUEST function
+    engine._request_responses = responses
+    # Keys for older requests which are stored in files and can be retrieved synchronously
+    engine._cached_request_keys = set(cached_keys)
+
+    # Invalidate the exact cells which made the exact requests which are being responded to here.
+    for response in six.itervalues(responses):
+      for table_id, table_deps in six.iteritems(response.pop("deps")):
+        for col_id, row_ids in six.iteritems(table_deps):
+          node = depend.Node(table_id, col_id)
+          engine.dep_graph.invalidate_deps(node, row_ids, engine.recompute_map)
+
   #----------------------------------------
   # User actions on records.
   #----------------------------------------
diff --git a/yarn.lock b/yarn.lock
index 65d91a70..8f9fb4ca 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -179,6 +179,11 @@
   resolved "https://registry.yarnpkg.com/@tokenizer/token/-/token-0.1.1.tgz#f0d92c12f87079ddfd1b29f614758b9696bc29e3"
   integrity sha512-XO6INPbZCxdprl+9qa/AAbFFOMzzwqYxpjPgLICrMD6C2FCw6qfJOPcBk6JqqPLSaZ/Qx87qn4rpPmPMwaAK6w==
 
+"@tootallnate/once@2":
+  version "2.0.0"
+  resolved "https://registry.yarnpkg.com/@tootallnate/once/-/once-2.0.0.tgz#f544a148d3ab35801c1f633a7441fd87c2e484bf"
+  integrity sha512-XCuKFP5PS55gnMVu3dty8KPatLqUoy/ZYzDzAGCQ8JNFCkLXzmI7vNHCR+XpbZaMWQK/vQubr7PkYq8g470J/A==
+
 "@types/accept-language-parser@1.5.2":
   version "1.5.2"
   resolved "https://registry.yarnpkg.com/@types/accept-language-parser/-/accept-language-parser-1.5.2.tgz#ea48ed07a3dc9d2ba6666d45c018ad1b5e59d665"
@@ -3647,6 +3652,15 @@ http-errors@~1.8.0:
     statuses ">= 1.5.0 < 2"
     toidentifier "1.0.0"
 
+http-proxy-agent@5.0.0:
+  version "5.0.0"
+  resolved "https://registry.yarnpkg.com/http-proxy-agent/-/http-proxy-agent-5.0.0.tgz#5129800203520d434f142bc78ff3c170800f2b43"
+  integrity sha512-n2hY8YdoRE1i7r6M0w9DIw5GgZN0G25P8zLCRQ8rjXtTU3vsNFBI/vWK/UIeE6g5MUUz6avwAPXmL6Fy9D/90w==
+  dependencies:
+    "@tootallnate/once" "2"
+    agent-base "6"
+    debug "4"
+
 http-signature@~1.2.0:
   version "1.2.0"
   resolved "https://registry.yarnpkg.com/http-signature/-/http-signature-1.2.0.tgz#9aecd925114772f3d95b65a60abb8f7c18fbace1"
@@ -3661,6 +3675,14 @@ https-browserify@^1.0.0:
   resolved "https://registry.yarnpkg.com/https-browserify/-/https-browserify-1.0.0.tgz#ec06c10e0a34c0f2faf199f7fd7fc78fffd03c73"
   integrity sha1-7AbBDgo0wPL68Zn3/X/Hj//QPHM=
 
+https-proxy-agent@5.0.1:
+  version "5.0.1"
+  resolved "https://registry.yarnpkg.com/https-proxy-agent/-/https-proxy-agent-5.0.1.tgz#c59ef224a04fe8b754f3db0063a25ea30d0005d6"
+  integrity sha512-dFcAjpTQFgoLMzC2VwU+C/CbS7uRL0lWmxDITmqm7C+7F0Odmj6s9l6alZc6AELXhrnggM2CeWSXHGOdX2YtwA==
+  dependencies:
+    agent-base "6"
+    debug "4"
+
 https-proxy-agent@5.0.0, https-proxy-agent@^5.0.0:
   version "5.0.0"
   resolved "https://registry.yarnpkg.com/https-proxy-agent/-/https-proxy-agent-5.0.0.tgz#e2a90542abb68a762e0a0850f6c9edadfd8506b2"