Slack: validate blocks input shape centrally

src/slack/blocks-input.ts

@@ -0,0 +1,41 @@
+import type { Block, KnownBlock } from "@slack/web-api";
+
+const SLACK_MAX_BLOCKS = 50;
+
+function parseBlocksJson(raw: string) {
+  try {
+    return JSON.parse(raw);
+  } catch {
+    throw new Error("blocks must be valid JSON");
+  }
+}
+
+function assertBlocksArray(raw: unknown) {
+  if (!Array.isArray(raw)) {
+    throw new Error("blocks must be an array");
+  }
+  if (raw.length === 0) {
+    throw new Error("blocks must contain at least one block");
+  }
+  if (raw.length > SLACK_MAX_BLOCKS) {
+    throw new Error(`blocks cannot exceed ${SLACK_MAX_BLOCKS} items`);
+  }
+  for (const block of raw) {
+    if (!block || typeof block !== "object" || Array.isArray(block)) {
+      throw new Error("each block must be an object");
+    }
+    const type = (block as { type?: unknown }).type;
+    if (typeof type !== "string" || type.trim().length === 0) {
+      throw new Error("each block must include a non-empty string type");
+    }
+  }
+}
+
+export function parseSlackBlocksInput(raw: unknown): (Block | KnownBlock)[] | undefined {
+  if (raw == null) {
+    return undefined;
+  }
+  const parsed = typeof raw === "string" ? parseBlocksJson(raw) : raw;
+  assertBlocksArray(parsed);
+  return parsed as (Block | KnownBlock)[];
+}