fix(ai,agent,coding-agent): normalize null message content at ingestion boundaries (#6343)

The Message types require content to always be present, but untyped JS
extension tools, hand-built histories, and old or hand-edited session
files can violate that contract, crashing rendering, compaction, and
provider request conversion with 'content is not iterable'.

Normalize null/missing content to an empty array at the ingestion
boundaries instead of guarding every consumer:

- transformMessages (choke point before every provider request)
- createToolResultMessage in the agent loop
- session entry loading (message and custom_message entries)
- extension custom messages (sendCustomMessage, before_agent_start)
- message_end extension replacements

fixes #6259, fixes #6276

packages/ai/src/api/transform-messages.ts

@@ -68,7 +68,10 @@ export function transformMessages<TApi extends Api>(
 ): Message[] {
 	// Build a map of original tool call IDs to normalized IDs
 	const toolCallIdMap = new Map<string, string>();
-	const imageAwareMessages = downgradeUnsupportedImages(messages, model);
+	// Normalize null/undefined content from untyped callers (custom tools, hand-built
+	// histories, old session files) so downstream code can rely on the type contract.
+	const normalizedMessages = messages.map((msg) => (msg.content == null ? { ...msg, content: [] } : msg));
+	const imageAwareMessages = downgradeUnsupportedImages(normalizedMessages, model);
 
 	// First pass: transform messages (unsupported image downgrade, thinking blocks, tool call ID normalization)
 	const transformed = imageAwareMessages.map((msg) => {

packages/ai/test/lax-message-content.test.ts

@@ -0,0 +1,67 @@
+/**
+ * The Message types require `content` to always be present, but untyped
+ * callers (custom tools, hand-built histories, old session files) can violate
+ * that contract. `transformMessages` is the choke point before every provider
+ * request and is intentionally lax: it normalizes null/missing content to an
+ * empty array (issues #6259, #6276).
+ */
+
+import { describe, expect, it } from "vitest";
+import { transformMessages } from "../src/api/transform-messages.ts";
+import type { Message, Model } from "../src/types.ts";
+
+// Text-only model so the image downgrade path (replaceImagesWithPlaceholder) runs,
+// which was the primary crash site for null tool result content.
+function makeTextOnlyModel(): Model<"openai-completions"> {
+	return {
+		id: "test-model",
+		name: "Test Model",
+		api: "openai-completions",
+		provider: "openai",
+		baseUrl: "https://example.invalid/v1",
+		reasoning: false,
+		input: ["text"],
+		cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+		contextWindow: 128000,
+		maxTokens: 16000,
+	};
+}
+
+describe("lax message content handling", () => {
+	it("normalizes null/missing content to an empty array instead of crashing", () => {
+		const messages = [
+			{ role: "user", content: null, timestamp: Date.now() },
+			{
+				role: "assistant",
+				content: null,
+				api: "openai-completions",
+				provider: "openai",
+				model: "test-model",
+				usage: {
+					input: 0,
+					output: 0,
+					cacheRead: 0,
+					cacheWrite: 0,
+					totalTokens: 0,
+					cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
+				},
+				stopReason: "stop",
+				timestamp: Date.now(),
+			},
+			{
+				role: "toolResult",
+				toolCallId: "call_1",
+				toolName: "web_search",
+				isError: false,
+				timestamp: Date.now(),
+			},
+		] as unknown as Message[];
+
+		const result = transformMessages(messages, makeTextOnlyModel());
+
+		expect(result).toHaveLength(3);
+		for (const msg of result) {
+			expect(msg.content).toEqual([]);
+		}
+	});
+});