feat(chat): paste-to-attach images/PDFs with model capability check

Add multimodal image/PDF input to the chat box via clipboard paste, gated by a
graceful per-model capability check.

UX: a pasted image/PDF inserts an inline token (【image:…】 / 【pdf:…】) into the
draft, so attachments have ORDER relative to typed text and can be referenced
positionally. The token is the only handle — deleting it (atomic Backspace/
Delete, or selection overlap) detaches the file; an input-reconciliation safety
net detaches any attachment whose token is no longer intact. No preview strip.

Capability check: resolveModelCapabilities reads models.dev modalities.input
(new GET /models/capabilities, mirrors /context-limit). The input blocks Send
(no tokens spent) only on a definitive 'no'; unknown capability (catalog offline
/ unmapped provider) stays permissive. Attachments require a fresh turn — Send is
blocked while generating and /chat rejects content mid-turn (409).

Attachments are EPHEMERAL: forwarded to the model for the turn via ordered AI SDK
ImagePart/FilePart content, but never persisted (history keeps the text with
[image]/[pdf] markers). Text-only turns serialize byte-identically to before.

Limits (Anthropic-aligned, enforced at paste + re-validated server-side):
PNG/JPEG/WebP/GIF/PDF; image ≤5MB, PDF ≤32MB, ≤20 attachments, ≤32MB total.

core: UserContentPart types, models/attachments validator, capability resolver,
agent.run+toModelMessages thread ordered content. api: /chat content validation +
passthrough. frontend: attachment-tokens helper, ChatInput paste/token/gating,
per-tab staged attachments, App.svelte capability fetch. +44 tests.

2 files changed, 208 insertions, 3 deletions

diff --git a/packages/core/tests/models/attachments.test.ts b/packages/core/tests/models/attachments.test.ts
new file mode 100644
index 0000000..11a9f82
--- /dev/null
+++ b/packages/core/tests/models/attachments.test.ts
@@ -0,0 +1,136 @@
+import { describe, expect, it } from "vitest";
+import {
+	base64ByteLength,
+	isAcceptedAttachmentMediaType,
+	isImageMediaType,
+	isPdfMediaType,
+	MAX_ATTACHMENTS,
+	MAX_IMAGE_BYTES,
+	MAX_PDF_BYTES,
+	MAX_TOTAL_ATTACHMENT_BYTES,
+	validateUserContent,
+} from "../../src/models/attachments.js";
+import type { UserContentPart } from "../../src/types/index.js";
+
+/** A base64 string that decodes to exactly `bytes` bytes (no padding chars). */
+function base64OfBytes(bytes: number): string {
+	// 4 base64 chars → 3 bytes. Use a multiple of 3 for clean (unpadded) output.
+	const groups = Math.ceil(bytes / 3);
+	return "A".repeat(groups * 4);
+}
+
+function imagePart(data: string, mediaType = "image/png"): UserContentPart {
+	return { type: "attachment", mediaType, data };
+}
+
+describe("media-type predicates", () => {
+	it("classifies image types", () => {
+		expect(isImageMediaType("image/png")).toBe(true);
+		expect(isImageMediaType("image/jpeg")).toBe(true);
+		expect(isImageMediaType("image/webp")).toBe(true);
+		expect(isImageMediaType("image/gif")).toBe(true);
+		expect(isImageMediaType("application/pdf")).toBe(false);
+		expect(isImageMediaType("image/svg+xml")).toBe(false);
+	});
+
+	it("classifies pdf + accepted types", () => {
+		expect(isPdfMediaType("application/pdf")).toBe(true);
+		expect(isPdfMediaType("image/png")).toBe(false);
+		expect(isAcceptedAttachmentMediaType("image/gif")).toBe(true);
+		expect(isAcceptedAttachmentMediaType("application/pdf")).toBe(true);
+		expect(isAcceptedAttachmentMediaType("text/plain")).toBe(false);
+	});
+});
+
+describe("base64ByteLength", () => {
+	it("computes decoded length without padding", () => {
+		// "AAAA" → 3 bytes.
+		expect(base64ByteLength("AAAA")).toBe(3);
+	});
+
+	it("accounts for padding", () => {
+		// "QQ==" → 1 byte ("A").
+		expect(base64ByteLength("QQ==")).toBe(1);
+		// "QUI=" → 2 bytes ("AB").
+		expect(base64ByteLength("QUI=")).toBe(2);
+	});
+
+	it("tolerates a data: URI prefix and whitespace", () => {
+		expect(base64ByteLength("data:image/png;base64,AAAA")).toBe(3);
+		expect(base64ByteLength("AA\nAA")).toBe(3);
+	});
+
+	it("returns 0 for empty input", () => {
+		expect(base64ByteLength("")).toBe(0);
+		expect(base64ByteLength("   ")).toBe(0);
+	});
+});
+
+describe("validateUserContent", () => {
+	it("accepts a small image and ignores text parts", () => {
+		const content: UserContentPart[] = [
+			{ type: "text", text: "hi" },
+			imagePart(base64OfBytes(1024)),
+		];
+		expect(validateUserContent(content)).toEqual({ ok: true, errors: [] });
+	});
+
+	it("accepts an empty / text-only content list", () => {
+		expect(validateUserContent([]).ok).toBe(true);
+		expect(validateUserContent([{ type: "text", text: "no files" }]).ok).toBe(true);
+	});
+
+	it("rejects an unsupported media type", () => {
+		const res = validateUserContent([imagePart(base64OfBytes(10), "image/svg+xml")]);
+		expect(res.ok).toBe(false);
+		expect(res.errors[0]).toMatchObject({ code: "unsupported-type", mediaType: "image/svg+xml" });
+	});
+
+	it("rejects an oversized image but allows a PDF of the same size", () => {
+		const big = base64OfBytes(MAX_IMAGE_BYTES + 3);
+		const imgRes = validateUserContent([imagePart(big, "image/png")]);
+		expect(imgRes.ok).toBe(false);
+		expect(imgRes.errors.some((e) => e.code === "image-too-large")).toBe(true);
+
+		// Same byte size as a PDF is fine (PDF limit is much higher).
+		const pdfRes = validateUserContent([imagePart(big, "application/pdf")]);
+		expect(pdfRes.ok).toBe(true);
+	});
+
+	it("rejects an oversized PDF", () => {
+		const res = validateUserContent([
+			imagePart(base64OfBytes(MAX_PDF_BYTES + 3), "application/pdf"),
+		]);
+		expect(res.ok).toBe(false);
+		expect(res.errors.some((e) => e.code === "pdf-too-large")).toBe(true);
+	});
+
+	it("rejects an empty attachment payload", () => {
+		const res = validateUserContent([imagePart("", "image/png")]);
+		expect(res.ok).toBe(false);
+		expect(res.errors.some((e) => e.code === "empty")).toBe(true);
+	});
+
+	it("rejects too many attachments", () => {
+		const content: UserContentPart[] = Array.from({ length: MAX_ATTACHMENTS + 1 }, () =>
+			imagePart(base64OfBytes(8)),
+		);
+		const res = validateUserContent(content);
+		expect(res.ok).toBe(false);
+		expect(res.errors.some((e) => e.code === "too-many")).toBe(true);
+	});
+
+	it("rejects when the total payload exceeds the request ceiling", () => {
+		// Several individually-legal PDFs that together exceed the total cap.
+		const each = Math.floor(MAX_TOTAL_ATTACHMENT_BYTES / 3);
+		const content: UserContentPart[] = [
+			imagePart(base64OfBytes(each), "application/pdf"),
+			imagePart(base64OfBytes(each), "application/pdf"),
+			imagePart(base64OfBytes(each), "application/pdf"),
+			imagePart(base64OfBytes(each), "application/pdf"),
+		];
+		const res = validateUserContent(content);
+		expect(res.ok).toBe(false);
+		expect(res.errors.some((e) => e.code === "total-too-large")).toBe(true);
+	});
+});
diff --git a/packages/core/tests/models/catalog.test.ts b/packages/core/tests/models/catalog.test.ts
index 51043e6..f4bddc2 100644
--- a/packages/core/tests/models/catalog.test.ts
+++ b/packages/core/tests/models/catalog.test.ts
@@ -4,6 +4,7 @@ import {
 	__resetCatalogCacheForTests,
 	getModelsCatalog,
 	resolveContextLimit,
+	resolveModelCapabilities,
 } from "../../src/models/catalog.js";
 
 const CACHE_PATH = "/tmp/dispatch/models-dev.json";
@@ -13,14 +14,30 @@ const CATALOG = {
 	anthropic: {
 		id: "anthropic",
 		models: {
-			"claude-sonnet-4-5": { limit: { context: 200000, output: 64000 } },
-			"claude-sonnet-4-6": { limit: { context: 1000000, output: 64000 } },
+			"claude-sonnet-4-5": {
+				limit: { context: 200000, output: 64000 },
+				modalities: { input: ["text", "image", "pdf"], output: ["text"] },
+			},
+			"claude-sonnet-4-6": {
+				limit: { context: 1000000, output: 64000 },
+				modalities: { input: ["text", "image", "pdf"], output: ["text"] },
+			},
+			// A text-only model: definitively no image/pdf input.
+			"text-only-model": {
+				limit: { context: 100000, output: 8192 },
+				modalities: { input: ["text"], output: ["text"] },
+			},
+			// An entry predating the modalities field → capability unknown.
+			"legacy-model": { limit: { context: 100000, output: 8192 } },
 		},
 	},
 	opencode: {
 		id: "opencode",
 		models: {
-			"glm-4-6": { limit: { context: 131072, output: 8192 } },
+			"glm-4-6": {
+				limit: { context: 131072, output: 8192 },
+				modalities: { input: ["text", "image"], output: ["text"] },
+			},
 		},
 	},
 };
@@ -156,3 +173,55 @@ describe("getModelsCatalog caching", () => {
 		warn.mockRestore();
 	});
 });
+
+describe("resolveModelCapabilities", () => {
+	it("reports image + pdf for a vision model", async () => {
+		mockFetchOnce(CATALOG);
+		expect(await resolveModelCapabilities("anthropic", "claude-sonnet-4-5")).toEqual({
+			image: true,
+			pdf: true,
+		});
+	});
+
+	it("reports image-only for a model whose modalities omit pdf", async () => {
+		mockFetchOnce(CATALOG);
+		// glm-4-6 lists image but not pdf (resolved via the opencode fallback).
+		expect(await resolveModelCapabilities("opencode-anthropic", "glm-4-6")).toEqual({
+			image: true,
+			pdf: false,
+		});
+	});
+
+	it("reports a definitive no for a text-only model", async () => {
+		mockFetchOnce(CATALOG);
+		expect(await resolveModelCapabilities("anthropic", "text-only-model")).toEqual({
+			image: false,
+			pdf: false,
+		});
+	});
+
+	it("returns null (unknown) for an entry without modalities", async () => {
+		mockFetchOnce(CATALOG);
+		expect(await resolveModelCapabilities("anthropic", "legacy-model")).toBeNull();
+	});
+
+	it("returns null (unknown) for an unknown model id", async () => {
+		mockFetchOnce(CATALOG);
+		expect(await resolveModelCapabilities("anthropic", "no-such-model")).toBeNull();
+	});
+
+	it("returns null for an unsupported provider without hitting the network", async () => {
+		const fetchFn = mockFetchOnce(CATALOG);
+		expect(await resolveModelCapabilities("google", "gemini-2.5-pro")).toBeNull();
+		expect(await resolveModelCapabilities("anthropic", "")).toBeNull();
+		expect(fetchFn).not.toHaveBeenCalled();
+	});
+
+	it("returns null (unknown) when the catalog is offline with no cache", async () => {
+		const fetchFn = vi.fn(() => Promise.reject(new Error("offline")));
+		vi.stubGlobal("fetch", fetchFn);
+		const warn = vi.spyOn(console, "warn").mockImplementation(() => {});
+		expect(await resolveModelCapabilities("anthropic", "claude-sonnet-4-5")).toBeNull();
+		warn.mockRestore();
+	});
+});