feat(ai): enabled literal null string preservation in JSON parsing

- Removed `coerceNullStrings` function and automatic null-string coercion from JSON parsing, changing behavior to preserve literal `"null"` strings.
- Updated `parseStreamingJson` to return parsed JSON without null-string transformation.

packages/ai/CHANGELOG.md

@@ -1,6 +1,9 @@
 # Changelog
 
 ## [Unreleased]
+### Breaking Changes
+
+- Removed `coerceNullStrings` function and its automatic null-string coercion behavior from JSON parsing
 
 ## [13.19.0] - 2026-04-05
 
@@ -1975,4 +1978,4 @@ _Dedicated to Peter's shoulder ([@steipete](https://twitter.com/steipete))_
 
 ## [0.9.4] - 2025-11-26
 
-Initial release with multi-provider LLM support.
+Initial release with multi-provider LLM support.

packages/ai/src/utils/json-parse.ts

@@ -14,39 +14,15 @@ export function parseStreamingJson<T = any>(partialJson: string | undefined): T
 
 	// Try standard parsing first (fastest for complete JSON)
 	try {
-		return coerceNullStrings(JSON.parse(partialJson) as T);
+		return JSON.parse(partialJson) as T;
 	} catch {
 		// Try partial-json for incomplete JSON
 		try {
 			const result = partialParse(partialJson);
-			return coerceNullStrings((result ?? {}) as T);
+			return (result ?? {}) as T;
 		} catch {
 			// If all parsing fails, return empty object
 			return {} as T;
 		}
 	}
 }
-
-/**
- * Recursively replace the literal string `"null"` with `null` in parsed
- * tool-call arguments. Many models serialize optional fields as the four
- * character string `null` instead of the JSON keyword `null`, which
- * downstream code sees as a truthy non-empty string.
- */
-export function coerceNullStrings<T>(value: T): T {
-	if (value === null || value === undefined) return value;
-	if (typeof value === "string") {
-		return (value === "null" ? null : value) as T;
-	}
-	if (Array.isArray(value)) {
-		return value.map(coerceNullStrings) as T;
-	}
-	if (typeof value === "object") {
-		const out: Record<string, unknown> = {};
-		for (const [k, v] of Object.entries(value as Record<string, unknown>)) {
-			out[k] = coerceNullStrings(v);
-		}
-		return out as T;
-	}
-	return value;
-}