diff --git a/packages/ai/src/utils/schema/fields.ts b/packages/ai/src/utils/schema/fields.ts
index 41e9aacf1..b25006248 100644
--- a/packages/ai/src/utils/schema/fields.ts
+++ b/packages/ai/src/utils/schema/fields.ts
@@ -154,6 +154,22 @@ export const CLOUD_CODE_ASSIST_TYPE_SPECIFIC_KEYS: Record<string, Record<string,
 	null: {},
 };
 
+/**
+ * Flat set of every type-specific key across all CCA types.
+ * Used to identify sibling keys that need filtering during mixed-type collapse.
+ */
+export const ALL_CCA_TYPE_SPECIFIC_KEYS: Record<string, true> = buildAllCcaTypeSpecificKeys();
+
+function buildAllCcaTypeSpecificKeys(): Record<string, true> {
+	const all: Record<string, true> = {};
+	for (const typeKeys of Object.values(CLOUD_CODE_ASSIST_TYPE_SPECIFIC_KEYS)) {
+		for (const key in typeKeys) {
+			all[key] = true;
+		}
+	}
+	return all;
+}
+
 /**
  * Cloud Code Assist shared schema keys allowed on any type.
  * Used alongside CLOUD_CODE_ASSIST_TYPE_SPECIFIC_KEYS for CCA combiner collapsing.
diff --git a/packages/ai/src/utils/schema/normalize.ts b/packages/ai/src/utils/schema/normalize.ts
index e10a46270..0d2961f26 100644
--- a/packages/ai/src/utils/schema/normalize.ts
+++ b/packages/ai/src/utils/schema/normalize.ts
@@ -11,6 +11,7 @@ import { dereferenceJsonSchema } from "./dereference";
 import { upgradeJsonSchemaTo202012 } from "./draft";
 import { areJsonValuesEqual, mergePropertySchemas } from "./equality";
 import {
+	ALL_CCA_TYPE_SPECIFIC_KEYS,
 	CLOUD_CODE_ASSIST_SHARED_SCHEMA_KEYS,
 	CLOUD_CODE_ASSIST_TYPE_SPECIFIC_KEYS,
 	COMBINATOR_KEYS,
@@ -501,11 +502,26 @@ function collapseMixedTypeCombinerVariants(schema: JsonObject, combiner: "anyOf"
 	if (variantTypes.length < 2 || variantTypes.every(type => type === "object")) {
 		return schema;
 	}
-
 	const nextSchema = copySchemaWithout(schema, combiner);
 	const nonNullTypes = variantTypes.filter(t => t !== "null");
-	nextSchema.type = nonNullTypes[0] ?? variantTypes[0];
-	const chosenTypeAllowedKeys = CLOUD_CODE_ASSIST_TYPE_SPECIFIC_KEYS[nextSchema.type as string] ?? {};
+	const chosenType: string = nonNullTypes[0] ?? variantTypes[0];
+	nextSchema.type = chosenType;
+	const chosenTypeAllowedKeys = CLOUD_CODE_ASSIST_TYPE_SPECIFIC_KEYS[chosenType] ?? {};
+
+	// Strip sibling keys that were copied from the parent and belong to a
+	// different type (e.g. `items` sibling on a now-string-typed schema).
+	for (const key in nextSchema) {
+		if (!Object.hasOwn(nextSchema, key)) continue;
+		if (key === "type") continue;
+		if (
+			Object.hasOwn(ALL_CCA_TYPE_SPECIFIC_KEYS, key) &&
+			!Object.hasOwn(chosenTypeAllowedKeys, key) &&
+			!Object.hasOwn(CLOUD_CODE_ASSIST_SHARED_SCHEMA_KEYS, key)
+		) {
+			delete nextSchema[key];
+		}
+	}
+
 	for (const key in mergedVariantFields) {
 		if (!Object.hasOwn(mergedVariantFields, key)) continue;
 		// Drop type-specific keys that don't belong to the chosen type
diff --git a/packages/ai/test/schema-normalization.test.ts b/packages/ai/test/schema-normalization.test.ts
index b2c23fbc4..22efcb2cd 100644
--- a/packages/ai/test/schema-normalization.test.ts
+++ b/packages/ai/test/schema-normalization.test.ts
@@ -979,6 +979,21 @@ describe("normalizeSchemaForCCA", () => {
 			description: "pr number, url, or branch",
 		});
 	});
+
+	it("strips sibling type-specific keys copied from parent when mixed-type collapse picks opposing type", () => {
+		// Edge case: parent has a sibling `items` outside the anyOf,
+		// and the chosen type is string. The sibling must be stripped.
+		const normalized = normalizeSchemaForCCA({
+			anyOf: [{ type: "string" }, { type: "array", items: { type: "number" } }],
+			items: { type: "string" },
+			description: "pr number, url, or branch",
+		});
+
+		expect(normalized).toEqual({
+			type: "string",
+			description: "pr number, url, or branch",
+		});
+	});
 });
 
 // ---------------------------------------------------------------------------