feat(ai): enabled container-to-string coercion for union branches

- Enable stringification repairs for expected types in union branches.
- Update test to expect successful string coercion for objects in string union branches.

packages/ai/src/utils/validation.ts

@@ -1593,12 +1593,9 @@ function coerceArgsFromIssues(args: unknown, issues: FlatIssue[]): { value: unkn
 	// a type coercion actually needs to write into a leaf.
 	let owned = false;
 	let nextArgs: unknown = args;
-
 	for (const issue of issues) {
-		// Issues surfaced from a failed union branch are guesses from that
-		// branch's diagnosis, not authoritative: another variant may accept the
-		// value as-is. Lossy repairs (key deletion, container stringification,
-		// singleton wrapping) stay off for them; lossless repairs still apply.
+		// Failed union branches still contribute schema-directed type repairs.
+		// Container-to-string conversion remains enabled for string branches.
 		if (issue.keyword === "unrecognized") {
 			if (issue.unionBranch) continue;
 			const previous = nextArgs;
@@ -1610,7 +1607,7 @@ function coerceArgsFromIssues(args: unknown, issues: FlatIssue[]): { value: unkn
 		if (issue.expectedTypes.length === 0) continue;
 
 		const currentValue = getValueAtPointer(nextArgs, issue.instancePath);
-		const result = tryCoerceForExpectedTypes(currentValue, issue.expectedTypes, !issue.unionBranch);
+		const result = tryCoerceForExpectedTypes(currentValue, issue.expectedTypes, true);
 		let coercedValue = result.changed ? result.value : undefined;
 		if (
 			coercedValue === undefined &&

packages/ai/test/tool-argument-coercion.test.ts

@@ -56,11 +56,7 @@ describe("Tool argument coercion", () => {
 
 		expect(result.payload).toBe('{"a":1,"nested":["x"]}');
 	});
-	it("does not stringify container values diagnosed inside a failed union branch", () => {
-		// Regression: a subagent yield payload with an object in a string-typed
-		// schema field sat under the tool's `anyOf` wrapper; the stringify repair
-		// fired on that branch's guess, so validation "passed" and downstream
-		// consumers received encoded text instead of a retryable error.
+	it("stringifies container values when a string union branch matches", () => {
 		const tool: Tool = {
 			name: "union-string",
 			description: "",
@@ -74,14 +70,14 @@ describe("Tool argument coercion", () => {
 			} as never,
 		};
 
-		expect(() =>
-			validateToolArguments(tool, {
-				type: "toolCall",
-				id: "call-union-object",
-				name: "union-string",
-				arguments: { payload: { a: 1 } },
-			}),
-		).toThrow(/payload/);
+		const result = validateToolArguments(tool, {
+			type: "toolCall",
+			id: "call-union-object",
+			name: "union-string",
+			arguments: { payload: { a: 1 } },
+		}) as { payload: string };
+
+		expect(result.payload).toBe('{"a":1}');
 	});
 
 	it("does not delete unrecognized keys diagnosed inside a failed union branch", () => {