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diff --git a/packages/core/src/models/attachments.ts b/packages/core/src/models/attachments.ts
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+// Validation + limits for multimodal user attachments (images / PDFs).
+//
+// Kept dependency-free (no DB / `bun:sqlite` import) so both the API layer
+// (`/chat` request validation) and any future caller can share the exact same
+// allowlist and size/count ceilings. The limits mirror Anthropic's documented
+// vision/PDF API constraints (the only image-capable providers Dispatch maps),
+// so a request that passes here won't be rejected by the provider for size.
+
+import type { UserAttachmentPart, UserContentPart } from "../types/index.js";
+
+/** Accepted image media types. */
+export const ACCEPTED_IMAGE_MEDIA_TYPES = [
+	"image/png",
+	"image/jpeg",
+	"image/webp",
+	"image/gif",
+] as const;
+
+/** Accepted document media types. */
+export const ACCEPTED_PDF_MEDIA_TYPE = "application/pdf";
+
+/** Every media type we accept as an attachment. */
+export const ACCEPTED_ATTACHMENT_MEDIA_TYPES = [
+	...ACCEPTED_IMAGE_MEDIA_TYPES,
+	ACCEPTED_PDF_MEDIA_TYPE,
+] as const;
+
+/** Per-image byte ceiling (Anthropic: 5 MB/image). */
+export const MAX_IMAGE_BYTES = 5 * 1024 * 1024;
+
+/** Per-PDF byte ceiling (Anthropic: 32 MB/PDF). */
+export const MAX_PDF_BYTES = 32 * 1024 * 1024;
+
+/** Max attachments per message (Anthropic: 20 images/request). */
+export const MAX_ATTACHMENTS = 20;
+
+/**
+ * Total attachment payload ceiling for a single request (decoded bytes). Bounds
+ * the overall request size even when each individual file is within its limit.
+ */
+export const MAX_TOTAL_ATTACHMENT_BYTES = 32 * 1024 * 1024;
+
+/** Whether a media type is an accepted image type. */
+export function isImageMediaType(mediaType: string): boolean {
+	return (ACCEPTED_IMAGE_MEDIA_TYPES as readonly string[]).includes(mediaType);
+}
+
+/** Whether a media type is the accepted PDF type. */
+export function isPdfMediaType(mediaType: string): boolean {
+	return mediaType === ACCEPTED_PDF_MEDIA_TYPE;
+}
+
+/** Whether a media type is an accepted attachment type at all. */
+export function isAcceptedAttachmentMediaType(mediaType: string): boolean {
+	return (ACCEPTED_ATTACHMENT_MEDIA_TYPES as readonly string[]).includes(mediaType);
+}
+
+/**
+ * Decoded byte length of a base64 string, computed WITHOUT allocating the
+ * decoded buffer. Tolerates an optional `data:<mediaType>;base64,` prefix and
+ * any embedded whitespace/newlines. Returns 0 for an empty/whitespace string.
+ */
+export function base64ByteLength(b64: string): number {
+	// Strip a data-URI prefix if present.
+	const comma = b64.indexOf(",");
+	const body = b64.startsWith("data:") && comma !== -1 ? b64.slice(comma + 1) : b64;
+	let len = 0;
+	let pad = 0;
+	for (let i = 0; i < body.length; i++) {
+		const ch = body.charCodeAt(i);
+		// Skip whitespace (space, \t, \n, \r).
+		if (ch === 32 || ch === 9 || ch === 10 || ch === 13) continue;
+		len++;
+		if (body[i] === "=") pad++;
+	}
+	if (len === 0) return 0;
+	// 4 base64 chars → 3 bytes, minus padding.
+	return Math.floor((len * 3) / 4) - pad;
+}
+
+export type AttachmentValidationError =
+	| { code: "unsupported-type"; mediaType: string }
+	| { code: "image-too-large"; mediaType: string; bytes: number; limit: number }
+	| { code: "pdf-too-large"; bytes: number; limit: number }
+	| { code: "too-many"; count: number; limit: number }
+	| { code: "total-too-large"; bytes: number; limit: number }
+	| { code: "empty"; mediaType: string };
+
+export interface AttachmentValidationResult {
+	ok: boolean;
+	errors: AttachmentValidationError[];
+}
+
+/** Extract just the attachment parts from a mixed content list. */
+function attachmentsOf(content: UserContentPart[]): UserAttachmentPart[] {
+	return content.filter((p): p is UserAttachmentPart => p.type === "attachment");
+}
+
+/**
+ * Validate the attachments in a multimodal user content list against the
+ * media-type allowlist and the size/count ceilings. Pure: never throws,
+ * collects every violation so the caller can report them all at once.
+ *
+ * Text parts are ignored (always valid). An empty content list is valid (it's
+ * just a text-only message expressed as parts).
+ */
+export function validateUserContent(content: UserContentPart[]): AttachmentValidationResult {
+	const errors: AttachmentValidationError[] = [];
+	const attachments = attachmentsOf(content);
+
+	if (attachments.length > MAX_ATTACHMENTS) {
+		errors.push({ code: "too-many", count: attachments.length, limit: MAX_ATTACHMENTS });
+	}
+
+	let total = 0;
+	for (const att of attachments) {
+		if (!isAcceptedAttachmentMediaType(att.mediaType)) {
+			errors.push({ code: "unsupported-type", mediaType: att.mediaType });
+			continue;
+		}
+		const bytes = base64ByteLength(att.data);
+		total += bytes;
+		if (bytes === 0) {
+			errors.push({ code: "empty", mediaType: att.mediaType });
+			continue;
+		}
+		if (isPdfMediaType(att.mediaType)) {
+			if (bytes > MAX_PDF_BYTES) {
+				errors.push({ code: "pdf-too-large", bytes, limit: MAX_PDF_BYTES });
+			}
+		} else if (bytes > MAX_IMAGE_BYTES) {
+			errors.push({
+				code: "image-too-large",
+				mediaType: att.mediaType,
+				bytes,
+				limit: MAX_IMAGE_BYTES,
+			});
+		}
+	}
+
+	if (total > MAX_TOTAL_ATTACHMENT_BYTES) {
+		errors.push({ code: "total-too-large", bytes: total, limit: MAX_TOTAL_ATTACHMENT_BYTES });
+	}
+
+	return { ok: errors.length === 0, errors };
+}
+
+/** Convenience: does the content list contain at least one attachment? */
+export function hasAttachments(content: UserContentPart[] | undefined | null): boolean {
+	return !!content && content.some((p) => p.type === "attachment");
+}