From 4428942b846ab06afc1941b8c73f8cfe0f3c717c Mon Sep 17 00:00:00 2001
From: roboomp <omp@can.ac>
Date: Wed, 10 Jun 2026 20:18:39 +0000
Subject: [PATCH] fix(ai): preserved constrained nullable unions

Only flatten optional anyOf schemas when the union wrapper has no sibling constraints that would exclude null.

Added a strict-schema regression for constrained anyOf properties so null remains an outer branch.
---
 packages/ai/src/utils/schema/normalize.ts   |  9 ++++++++-
 packages/ai/test/schema-strict-mode.test.ts | 19 +++++++++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/packages/ai/src/utils/schema/normalize.ts b/packages/ai/src/utils/schema/normalize.ts
index ac223b11b..26d90e12b 100644
--- a/packages/ai/src/utils/schema/normalize.ts
+++ b/packages/ai/src/utils/schema/normalize.ts
@@ -1437,6 +1437,13 @@ export function sanitizeSchemaForStrictMode(
 	return sanitized;
 }
 
+function canAppendNullVariantToAnyOf(schema: Record<string, unknown>): boolean {
+	for (const key in schema) {
+		if (key !== "anyOf" && key !== "description") return false;
+	}
+	return true;
+}
+
 /**
  * Recursively enforces JSON Schema constraints required by OpenAI/Codex strict mode:
  *   - `additionalProperties: false` on every object node
@@ -1505,7 +1512,7 @@ function enforceStrictSchemaBody(
 					strictProperties[key] = processed;
 					continue;
 				}
-				if (isJsonObject(processed) && Array.isArray(processed.anyOf)) {
+				if (isJsonObject(processed) && Array.isArray(processed.anyOf) && canAppendNullVariantToAnyOf(processed)) {
 					strictProperties[key] = { ...processed, anyOf: [...processed.anyOf, { type: "null" }] };
 					continue;
 				}
diff --git a/packages/ai/test/schema-strict-mode.test.ts b/packages/ai/test/schema-strict-mode.test.ts
index 6450c30b2..34909a84c 100644
--- a/packages/ai/test/schema-strict-mode.test.ts
+++ b/packages/ai/test/schema-strict-mode.test.ts
@@ -456,6 +456,25 @@ describe("enforceStrictSchema", () => {
 		expect(branches.some(branch => Array.isArray(branch.anyOf))).toBe(false);
 	});
 
+	it("wraps constrained optional anyOf schemas so null is not blocked by sibling constraints", () => {
+		const strict = enforceStrictSchema({
+			type: "object",
+			properties: {
+				choice: {
+					type: "string",
+					anyOf: [{ enum: ["a"] }, { enum: ["b"] }],
+				},
+			},
+		});
+		const properties = strict.properties as Record<string, Record<string, unknown>>;
+		const branches = properties.choice.anyOf as Array<Record<string, unknown>>;
+
+		expect(branches).toHaveLength(2);
+		expect(branches[0].type).toBe("string");
+		expect(Array.isArray(branches[0].anyOf)).toBe(true);
+		expect(branches[1]).toEqual({ type: "null" });
+	});
+
 	it("never emits undefined as a schema type", () => {
 		const schema = zodToWireSchema(
 			z.object({