fix(ai): encode empty successful tool_result content as empty string

Strict Anthropic-compatible endpoints (Z.AI GLM at api.z.ai/api/anthropic)
reject a whole request when a tool_result block carries content: [] ,
returning 400 code 1213 "The prompt parameter was not received normally".
The official API accepts both shapes, so the empty array only surfaced on
compatible endpoints once a tool returned empty output on a vision-capable
model (text-only models already encode the joined empty string).

Normalize the empty array to "" at encode time, alongside the existing
error-placeholder normalization from #2250.

packages/ai/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Fixed Anthropic-compatible endpoints with strict prompt validation (e.g. Z.AI GLM `api.z.ai/api/anthropic`, which rejects the whole request with `400 code 1213 "The prompt parameter was not received normally"`) failing sessions once a tool returned empty output on a vision-capable model: empty successful `tool_result` blocks now encode as `content: ""` instead of `content: []`, which both the official API and strict compatible endpoints accept.
+
 ## [17.3.5] - 2026-08-16
 
 ### Added

packages/ai/src/providers/anthropic.ts

@@ -3548,6 +3548,12 @@ function buildToolResultBlock(
 		}
 		content = content.filter(block => block.type === "text");
 	}
+	// An empty array is valid for the official API, but strict Anthropic-compatible
+	// endpoints (Z.AI GLM: 400 code 1213 "The prompt parameter was not received
+	// normally") reject it; the empty-string form is accepted by both.
+	if (Array.isArray(content) && content.length === 0) {
+		content = "";
+	}
 	content = ensureErrorToolResultWireContent(content, msg.isError);
 	const block: ContentBlockParam = {
 		type: "tool_result",

packages/ai/test/anthropic-empty-error-tool-result.test.ts

@@ -20,6 +20,13 @@ const anthropicModel: Model<"anthropic-messages"> = buildModel({
 	baseUrl: "https://api.anthropic.com",
 });
 
+const visionModel: Model<"anthropic-messages"> = buildModel({
+	...baseModel,
+	input: ["text", "image"],
+	provider: "anthropic",
+	baseUrl: "https://api.anthropic.com",
+});
+
 const user: UserMessage = {
 	role: "user",
 	content: "run the tool",
@@ -95,4 +102,24 @@ describe("anthropic empty error tool_result encoding", () => {
 		expect(block.is_error).toBe(false);
 		expect(block.content).toBe("");
 	});
+
+	it("encodes empty successful tool results as empty string, not empty array, on image-capable models", () => {
+		// Regression: vision-capable models serialize whitespace-only successful
+		// tool results as `content: []` on the wire. Official Anthropic accepts it,
+		// but strict Anthropic-compatible endpoints (Z.AI GLM, api.z.ai/api/anthropic)
+		// reject the whole request with 400 code 1213 ("The prompt parameter was
+		// not received normally"). `content: ""` is accepted by both.
+		const toolResult: ToolResultMessage = {
+			role: "toolResult",
+			toolCallId: "toolu_empty_error",
+			toolName: "write",
+			content: [{ type: "text", text: "" }],
+			isError: false,
+			timestamp: Date.now(),
+		};
+
+		const block = getToolResultBlock(visionModel, toolResult);
+		expect(block.is_error).toBe(false);
+		expect(block.content).toBe("");
+	});
 });