From 9f61a85d73a7ccc570bfd7ce24786d381d456d4f Mon Sep 17 00:00:00 2001
From: can1357 <me@can.ac>
Date: Sun, 26 Apr 2026 03:44:45 +0200
Subject: [PATCH] fix(ai): removed unsupported array item count constraints
 from Anthropic tool schemas

- Added maxItems to the set of unsupported tool schema fields.
- Removed minItems constraints that are not 0 or 1, as Anthropic only supports these specific values.
- Added test to verify maxItems and non-standard minItems are stripped while minItems: 1 is preserved.
---
 packages/ai/src/providers/anthropic.ts       |  6 ++-
 packages/ai/test/anthropic-alignment.test.ts | 44 ++++++++++++++++++++
 2 files changed, 49 insertions(+), 1 deletion(-)

diff --git a/packages/ai/src/providers/anthropic.ts b/packages/ai/src/providers/anthropic.ts
index 0fa86c4e2..753cb7758 100644
--- a/packages/ai/src/providers/anthropic.ts
+++ b/packages/ai/src/providers/anthropic.ts
@@ -1751,7 +1751,7 @@ export function convertAnthropicMessages(
 	return params;
 }
 
-const ANTHROPIC_UNSUPPORTED_TOOL_SCHEMA_FIELDS = new Set(["patternProperties"]);
+const ANTHROPIC_UNSUPPORTED_TOOL_SCHEMA_FIELDS = new Set(["maxItems", "patternProperties"]);
 const MAX_ANTHROPIC_STRICT_TOOLS = 20;
 const MAX_ANTHROPIC_STRICT_OPTIONAL_PARAMETERS = 24;
 const MAX_ANTHROPIC_STRICT_UNION_PARAMETERS = 16;
@@ -1769,6 +1769,10 @@ function normalizeAnthropicToolSchema(
 		Object.entries(schema).filter(([key]) => !ANTHROPIC_UNSUPPORTED_TOOL_SCHEMA_FIELDS.has(key)),
 	);
 	cache.set(schema, result);
+	const minItems = result.minItems;
+	if (typeof minItems === "number" && minItems !== 0 && minItems !== 1) {
+		delete result.minItems;
+	}
 
 	const type = result.type;
 	const canBeObject =
diff --git a/packages/ai/test/anthropic-alignment.test.ts b/packages/ai/test/anthropic-alignment.test.ts
index 2b7d144c0..90566c3f5 100644
--- a/packages/ai/test/anthropic-alignment.test.ts
+++ b/packages/ai/test/anthropic-alignment.test.ts
@@ -356,6 +356,50 @@ describe("Anthropic request fingerprint alignment", () => {
 		expect(originalNestedSchema).toHaveProperty("patternProperties");
 	});
 
+	it("removes Anthropic-unsupported array item count constraints", async () => {
+		const tools: Tool[] = [
+			{
+				name: "edit_file",
+				description: "edit files",
+				parameters: {
+					type: "object",
+					properties: {
+						sub: {
+							type: "array",
+							items: { type: "string" },
+							minItems: 2,
+							maxItems: 2,
+						},
+						nonEmpty: {
+							type: "array",
+							items: { type: "string" },
+							minItems: 1,
+						},
+					},
+					required: ["sub"],
+				} as unknown as TSchema,
+			},
+		];
+
+		const payload = (await captureAnthropicPayload(ANTHROPIC_MODEL, {
+			systemPrompt: "Stay concise.",
+			messages: [{ role: "user", content: "Hi", timestamp: Date.now() }],
+			tools,
+		})) as {
+			tools?: Array<{
+				input_schema?: {
+					properties?: Record<string, unknown>;
+				};
+			}>;
+		};
+
+		const properties = payload.tools?.[0]?.input_schema?.properties as Record<string, Record<string, unknown>>;
+
+		expect(properties.sub).not.toHaveProperty("minItems");
+		expect(properties.sub).not.toHaveProperty("maxItems");
+		expect(properties.nonEmpty.minItems).toBe(1);
+	});
+
 	it("marks at most twenty Anthropic tools strict", async () => {
 		const tools: Tool[] = Array.from({ length: 25 }, (_, index) => ({
 			name: `tool_${index}`,