databricks
diff --git a/‎docs/docs/development/type-generation.md‎
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diff --git a/‎packages/appkit/src/type-generator/index.ts‎
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diff --git a/‎packages/appkit/src/type-generator/preflight.ts‎
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@@ -72,6 +72,18 @@ npx @databricks/appkit generate-types [rootDir] [outFile] [warehouseId]
   npx @databricks/appkit generate-types --no-cache
   ```
 
+### Warehouse readiness and the `--wait` flag
+
+By default, `generate-types` is **non-blocking**: it never waits on — or fails because of — your SQL warehouse. It writes the best types it can immediately (reusing cached types where the query is unchanged, otherwise `result: unknown`) and then spawns a detached background worker that refreshes the real types once the warehouse is ready. This keeps `npm install` (postinstall) and `npm run dev` (predev) fast and resilient to a cold or briefly-unreachable warehouse. The dev Vite plugin behaves the same way: types appear instantly and refresh in place once the warehouse is live.
+
+Pass `--wait` for CI and production builds, where accurate types must be present before the build proceeds:
+
+```bash
+npx @databricks/appkit generate-types --wait
+```
+
+In blocking mode the generator starts a stopped warehouse, waits (bounded) for it to reach `RUNNING`, and then describes your queries. It fails only when the configured warehouse no longer exists (deleted/deleting), so a transient outage or a cold warehouse degrades gracefully rather than breaking the build. The app template wires this up for you: `postinstall` and `predev` run the non-blocking default, while `prebuild` runs `--wait`.
+
 ## How it works
 
 The type generator:
 
@@ -1,20 +1,169 @@
 import fs from "node:fs/promises";
 import path from "node:path";
 import dotenv from "dotenv";
+import pc from "picocolors";
 import { createLogger } from "../logging/logger";
 import {
   migrateProjectConfig,
   removeOldGeneratedTypes,
   resolveProjectRoot,
 } from "./migration";
+import type { PreflightMode } from "./preflight";
 import { generateQueriesFromDescribe } from "./query-registry";
 import { generateServingTypes as generateServingTypesImpl } from "./serving/generator";
-import type { QuerySchema } from "./types";
+import type { QueryFatalError, QuerySchema, QuerySyntaxError } from "./types";
 
 dotenv.config();
 
 const logger = createLogger("type-generator");
 
+type TypegenFailure = QuerySyntaxError | QueryFatalError;
+
+function plural(count: number, singular: string, pluralForm = `${singular}s`) {
+  return count === 1 ? singular : pluralForm;
+}
+
+function formatFailureRows(
+  label: string,
+  queries: TypegenFailure[],
+  color: (value: string) => string,
+) {
+  if (queries.length === 0) return [];
+
+  // Group by message so a shared failure — e.g. a warehouse-level fatal that
+  // hits every query identically — prints once instead of repeating per row.
+  const byMessage = new Map<string, string[]>();
+  for (const { name, message } of queries) {
+    const names = byMessage.get(message);
+    if (names) names.push(name);
+    else byMessage.set(message, [name]);
+  }
+
+  const maxNameLen = Math.max(...queries.map((query) => query.name.length));
+  const tag = color(label.padEnd(7));
+  const rows: string[] = [];
+  for (const [message, names] of byMessage) {
+    // Unique message → keep the compact one-line `tag name message` form.
+    if (names.length === 1) {
+      rows.push(
+        `  ${tag}  ${pc.bold(names[0].padEnd(maxNameLen))}  ${pc.dim(message)}`,
+      );
+      continue;
+    }
+    // Shared message → print it once, then list the affected query names.
+    rows.push(
+      `  ${tag}  ${pc.dim(message)} ${pc.dim(`(${names.length} ${plural(names.length, "query", "queries")})`)}`,
+    );
+    rows.push(
+      `           ${names.map((name) => pc.bold(name)).join(pc.dim(", "))}`,
+    );
+  }
+  return rows;
+}
+
+function formatTypegenFailureMessage(options: {
+  syntaxErrors: QuerySyntaxError[];
+  fatalErrors?: QueryFatalError[];
+  warehouseId?: string;
+  title: string;
+  causes: string[];
+  nextStep: string;
+}) {
+  const { syntaxErrors, fatalErrors = [], warehouseId, title } = options;
+  const total = syntaxErrors.length + fatalErrors.length;
+  const separator = pc.dim("─".repeat(60));
+  const warehouse = warehouseId
+    ? ` against ${pc.dim(`warehouse ${warehouseId}`)}`
+    : "";
+
+  return [
+    `  ${pc.bold(pc.red("Type generation failed"))}`,
+    `  ${separator}`,
+    `  ${title}: ${total} ${plural(total, "query", "queries")} could not be described${warehouse}.`,
+    `  AppKit wrote generated types with ${pc.bold("result: unknown")} for the failed ${plural(total, "query", "queries")}.`,
+    "",
+    ...formatFailureRows("SQL ERR", syntaxErrors, pc.red),
+    ...(syntaxErrors.length > 0 && fatalErrors.length > 0 ? [""] : []),
+    ...formatFailureRows("FATAL", fatalErrors, pc.red),
+    "",
+    `  ${pc.bold("Common causes")}`,
+    ...options.causes.map((cause) => `  - ${cause}`),
+    "",
+    `  ${pc.bold("Next step")}`,
+    `  ${options.nextStep}`,
+  ].join("\n");
+}
+
+/**
+ * Thrown when one or more queries fail `DESCRIBE QUERY` against a *reachable*
+ * warehouse — i.e. genuine SQL errors (bad table, syntax, incompatible type),
+ * as opposed to a connectivity failure (warehouse unreachable), which degrades
+ * silently. Whether this is fatal is the caller's decision: the Vite plugin and
+ * CLI fail the build in production and warn-only in development.
+ */
+export class TypegenSyntaxError extends Error {
+  readonly queries: QuerySyntaxError[];
+  readonly fatalQueries: QueryFatalError[];
+
+  constructor(
+    queries: QuerySyntaxError[],
+    warehouseId?: string,
+    fatalQueries: QueryFatalError[] = [],
+  ) {
+    super(
+      formatTypegenFailureMessage({
+        syntaxErrors: queries,
+        fatalErrors: fatalQueries,
+        warehouseId,
+        title: "DESCRIBE QUERY failed",
+        causes: [
+          "SQL syntax errors",
+          "missing tables or views",
+          "warehouse format incompatibilities",
+        ],
+        nextStep: warehouseId
+          ? `Run each SQL ERR query directly in a Databricks SQL editor against warehouse ${pc.bold(warehouseId)}.`
+          : "Run each SQL ERR query directly in a Databricks SQL editor.",
+      }),
+    );
+    this.name = "TypegenSyntaxError";
+    this.queries = queries;
+    this.fatalQueries = fatalQueries;
+  }
+}
+
+/**
+ * Thrown when DESCRIBE QUERY could not be requested because of a non-SQL fatal
+ * setup/request problem, such as missing permissions, invalid warehouse IDs, or
+ * malformed SDK configuration. Like TypegenSyntaxError, this is thrown only
+ * after the declaration file has been written with `result: unknown` entries.
+ */
+export class TypegenFatalError extends Error {
+  readonly queries: QueryFatalError[];
+
+  constructor(queries: QueryFatalError[], warehouseId?: string) {
+    super(
+      formatTypegenFailureMessage({
+        syntaxErrors: [],
+        fatalErrors: queries,
+        warehouseId,
+        title: "DESCRIBE QUERY could not be requested",
+        causes: [
+          "missing warehouse permissions",
+          "invalid warehouse ID",
+          "authentication failure",
+          "SDK configuration errors",
+        ],
+        nextStep: warehouseId
+          ? `Verify access to warehouse ${pc.bold(warehouseId)} and rerun type generation.`
+          : "Verify warehouse access and rerun type generation.",
+      }),
+    );
+    this.name = "TypegenFatalError";
+    this.queries = queries;
+  }
+}
+
 /**
  * Generate type declarations for QueryRegistry
  * Create the d.ts file from the plugin routes and query schemas
@@ -57,35 +206,30 @@ export async function generateFromEntryPoint(options: {
   queryFolder?: string;
   warehouseId: string;
   noCache?: boolean;
+  mode?: PreflightMode;
 }) {
-  const { outFile, queryFolder, warehouseId, noCache } = options;
+  const {
+    outFile,
+    queryFolder,
+    warehouseId,
+    noCache,
+    mode = "non-blocking",
+  } = options;
   const projectRoot = resolveProjectRoot(outFile);
 
   logger.debug("Starting type generation...");
 
   let queryRegistry: QuerySchema[] = [];
-  if (queryFolder)
-    queryRegistry = await generateQueriesFromDescribe(
-      queryFolder,
-      warehouseId,
-      {
-        noCache,
-      },
-    );
-
-  const failedQueries = queryRegistry.filter((q) =>
-    q.type.includes("result: unknown"),
-  );
-  if (failedQueries.length > 0) {
-    const names = failedQueries.map((q) => q.name).join(", ");
-    throw new Error(
-      [
-        `Type generation failed: ${failedQueries.length} ${failedQueries.length === 1 ? "query" : "queries"} could not be described: ${names}.`,
-        `DESCRIBE QUERY failed for these queries — see the error codes above for details.`,
-        `Common causes: SQL syntax errors, missing tables/views, or warehouse format incompatibilities.`,
-        `To debug: run the failing query directly in a SQL editor against warehouse ${warehouseId}.`,
-      ].join("\n"),
-    );
+  let syntaxErrors: QuerySyntaxError[] = [];
+  let fatalErrors: QueryFatalError[] = [];
+  if (queryFolder) {
+    const result = await generateQueriesFromDescribe(queryFolder, warehouseId, {
+      noCache,
+      mode,
+    });
+    queryRegistry = result.schemas;
+    syntaxErrors = result.syntaxErrors ?? [];
+    fatalErrors = result.fatalErrors ?? [];
   }
 
   const typeDeclarations = generateTypeDeclarations(queryRegistry);
@@ -97,6 +241,17 @@ export async function generateFromEntryPoint(options: {
   await removeOldGeneratedTypes(projectRoot, "appKitTypes.d.ts");
   await migrateProjectConfig(projectRoot);
 
+  // Types are always written above — including `result: unknown` for any query
+  // that could not be described. Connectivity failures pass silently so a
+  // transient warehouse outage never blocks a build; genuine SQL errors and
+  // non-connectivity fatal request failures surface after the file write.
+  if (syntaxErrors.length > 0) {
+    throw new TypegenSyntaxError(syntaxErrors, warehouseId, fatalErrors);
+  }
+  if (fatalErrors.length > 0) {
+    throw new TypegenFatalError(fatalErrors, warehouseId);
+  }
+
   logger.debug("Type generation complete!");
 }
 
 
@@ -0,0 +1,63 @@
+import type { WarehouseState } from "./warehouse-status";
+
+/**
+ * How aggressively typegen should react to a not-ready warehouse.
+ *  - `non-blocking`: never describe and never probe the warehouse — emit
+ *    best-available types (cache where the SQL hash matches, else `unknown`) and
+ *    return at once. The default for interactive/foreground runs that can't
+ *    afford to block on (or fail because of) a warehouse, even a RUNNING one.
+ *  - `blocking`: a startable warehouse is worth waiting for, and a stopped one
+ *    is worth starting — only a deleted/deleting warehouse is a hard failure.
+ */
+export type PreflightMode = "non-blocking" | "blocking";
+
+/**
+ * What the caller should do given a warehouse state and mode.
+ *  - `proceed`: run DESCRIBE now.
+ *  - `degradeAll`: skip DESCRIBE; emit degraded (cached/`unknown`) types.
+ *  - `waitThenProceed`: wait for the warehouse to start, then run DESCRIBE.
+ *  - `startWaitProceed`: start the stopped warehouse, wait for RUNNING, then
+ *    run DESCRIBE.
+ *  - `fatal`: stop — the warehouse can't serve this run.
+ */
+export type PreflightDecision =
+  | "proceed"
+  | "degradeAll"
+  | "waitThenProceed"
+  | "startWaitProceed"
+  | "fatal";
+
+/**
+ * Pure policy mapping a warehouse state + mode to a preflight decision.
+ *
+ * Unknown/unexpected states fall through to `proceed`: the describe loop and
+ * its per-query backstop already degrade gracefully, so we don't want a new
+ * SDK state value to turn into a spurious `fatal`.
+ */
+export function decidePreflight(
+  state: WarehouseState,
+  mode: PreflightMode,
+): PreflightDecision {
+  // `non-blocking` never describes regardless of state: emit cached/`unknown`
+  // types and return. The caller short-circuits before probing, so this is only
+  // a belt-and-suspenders mapping, but it keeps the policy total and
+  // self-contained.
+  if (mode === "non-blocking") return "degradeAll";
+
+  // `blocking`: a starting warehouse is worth waiting for, a stopped one is
+  // worth starting (then waiting), and only a deleted/deleting one is fatal.
+  switch (state) {
+    case "RUNNING":
+      return "proceed";
+    case "STARTING":
+      return "waitThenProceed";
+    case "STOPPED":
+    case "STOPPING":
+      return "startWaitProceed";
+    case "DELETED":
+    case "DELETING":
+      return "fatal";
+    default:
+      return "proceed";
+  }
+}