fix(tool-sieve): keep mixed prose tool json in strict text mode

internal/js/helpers/stream-tool-sieve/sieve.js

@@ -235,6 +235,17 @@ function consumeToolCapture(state, toolNames) {
     };
   }
 
+  // Strict standalone mode: if the stream already produced meaningful prose,
+  // treat later tool-looking JSON as plain text instead of intercepting.
+  if ((state.recentTextTail || '').trim() !== '' && prefixPart.trim() === '') {
+    return {
+      ready: true,
+      prefix: captured,
+      calls: [],
+      suffix: '',
+    };
+  }
+
   const parsed = parseStandaloneToolCallsDetailed(captured.slice(actualStart, obj.end), toolNames);
   if (!Array.isArray(parsed.calls) || parsed.calls.length === 0) {
     if (parsed.sawToolCallSyntax && parsed.rejectedByPolicy) {
@@ -253,6 +264,18 @@ function consumeToolCapture(state, toolNames) {
     };
   }
 
+  // Strict standalone mode: only intercept when the tool payload stands alone
+  // (allowing only surrounding whitespace). If there is non-whitespace prose
+  // before/after the JSON object, keep everything as normal text.
+  if (prefixPart.trim() !== '' || suffixPart.trim() !== '') {
+    return {
+      ready: true,
+      prefix: captured,
+      calls: [],
+      suffix: '',
+    };
+  }
+
   return {
     ready: true,
     prefix: prefixPart,