fix(ai): sanitized Anthropic tool-use arguments with

- Sanitized Anthropic tool-use arguments with toWellFormedDeep on every replay path.
- Updated unsigned-thinking replay tests to assert same-API tool arguments are sanitized.
- Documented the lone-surrogate replay fix in the ai package changelog.

packages/ai/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Fixed Anthropic-origin tool-call arguments bypassing lone-surrogate sanitization on replay: the model itself can emit unpaired surrogate escapes in its own tool-argument JSON (streamed out fine, then rejected with `400 The request body is not valid JSON` on every subsequent request, bricking the session). `tool_use.input` is now always deep-sanitized with `toWellFormed()`; the pass is identity-preserving, so well-formed arguments stay byte-identical and prompt-cache prefixes are unaffected.
+
 ## [15.11.8] - 2026-06-12
 
 ### Breaking Changes

packages/ai/src/providers/anthropic.ts

@@ -3075,13 +3075,12 @@ export function convertAnthropicMessages(
 						type: "tool_use",
 						id: block.id,
 						name: isOAuthToken ? applyClaudeToolPrefix(block.name) : block.name,
-						// Anthropic-origin arguments are guaranteed well-formed (they came
-						// from the API's own JSON); cross-API replays can carry lone
-						// surrogates that Anthropic's strict UTF-8 validation rejects.
-						input:
-							msg.api === "anthropic-messages"
-								? (block.arguments ?? {})
-								: toWellFormedDeep(block.arguments ?? {}),
+						// Always sanitize: the model itself can emit lone-surrogate escapes
+						// in tool-argument JSON (streamed out fine, rejected with a 400 on
+						// replay by Anthropic's strict UTF-8 validation). toWellFormedDeep
+						// is identity-preserving, so well-formed arguments stay
+						// byte-identical and prompt-cache prefixes are unaffected.
+						input: toWellFormedDeep(block.arguments ?? {}),
 					});
 				}
 			}

packages/ai/test/anthropic-unsigned-thinking-replay.test.ts

@@ -101,7 +101,7 @@ describe("Anthropic-compatible unsigned thinking replay (#2005)", () => {
 		expect(blocks[1]).toEqual({ type: "text", text: "Sure." });
 	});
 
-	it("sanitizes lone surrogates in cross-API tool arguments only", () => {
+	it("sanitizes lone surrogates in tool arguments regardless of origin API", () => {
 		const loneSurrogate = "broken \ud83d end";
 		const makeToolCallAssistant = (api: AssistantMessage["api"]): AssistantMessage => ({
 			role: "assistant",
@@ -135,11 +135,12 @@ describe("Anthropic-compatible unsigned thinking replay (#2005)", () => {
 		expect(crossToolUse.input.text).toBe("broken \ufffd end");
 		expect((crossToolUse.input.nested as { parts: string[] }).parts[0]).toBe("broken \ufffd end");
 
-		// Same-API replay stays byte-identical (the args came from Anthropic's own
-		// JSON; rewriting them would destabilize prompt-cache prefixes).
+		// Same-API replay sanitizes too: the model itself can emit lone-surrogate
+		// escapes in its own tool-argument JSON (streamed out fine, 400 on replay).
 		const sameBlocks = assistantWireBlocks([makeUser(), makeToolCallAssistant("anthropic-messages")], makeModel());
 		const sameToolUse = sameBlocks.find(block => block.type === "tool_use") as WireToolUseBlock;
-		expect(sameToolUse.input.text).toBe(loneSurrogate);
+		expect(sameToolUse.input.text).toBe("broken \ufffd end");
+		expect((sameToolUse.input.nested as { parts: string[] }).parts[0]).toBe("broken \ufffd end");
 	});
 
 	it("covers the Xiaomi MiMo Anthropic-compatible reporter configuration without provider allowlists", () => {