fix: repair truncated/trailing-extra tool call arguments from streaming

Some models (e.g. Qwen3 via vLLM) occasionally emit extra closing
braces at the end of streamed tool-call arguments, producing invalid
JSON like `{...}}}`. When replayed in conversation history, vLLM's
chat template calls json.loads() on the arguments and returns HTTP 400
"Extra data". repairToolCallArguments() strips the trailing garbage by
finding the last balanced close of the top-level JSON value.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Lellansin

src/session.ts

@@ -80,6 +80,53 @@ export function stripModelContextSizeSuffix(model: string): string {
   return model.replace(CONTEXT_SIZE_SUFFIX_RE, "").trimEnd();
 }
 
+function repairToolCallArguments(args: string): string {
+  try {
+    JSON.parse(args);
+    return args;
+  } catch (e) {
+    if (e instanceof SyntaxError && e.message.includes("JSON") && "position" in (e as NodeJS.ErrnoException)) {
+      // Node's JSON.parse error doesn't expose position directly; use a scan instead.
+    }
+    // Find the longest valid JSON prefix by scanning for the end of the top-level value.
+    let depth = 0;
+    let inString = false;
+    let escaped = false;
+    let validEnd = -1;
+    for (let i = 0; i < args.length; i++) {
+      const ch = args[i];
+      if (escaped) {
+        escaped = false;
+        continue;
+      }
+      if (ch === "\\") {
+        escaped = true;
+        continue;
+      }
+      if (ch === '"') {
+        inString = !inString;
+        continue;
+      }
+      if (inString) continue;
+      if (ch === "{" || ch === "[") depth++;
+      else if (ch === "}" || ch === "]") {
+        depth--;
+        if (depth === 0) validEnd = i;
+      }
+    }
+    if (validEnd >= 0 && validEnd < args.length - 1) {
+      const candidate = args.slice(0, validEnd + 1);
+      try {
+        JSON.parse(candidate);
+        return candidate;
+      } catch {
+        // fall through
+      }
+    }
+    return args;
+  }
+}
+
 function parseContextSizeSuffix(model: string): number | undefined {
   const match = CONTEXT_SIZE_SUFFIX_RE.exec(model);
   if (!match) {
@@ -2239,14 +2286,21 @@ ${agentInstructions}
 
       const record = toolCall as Record<string, unknown>;
       const id = typeof record.id === "string" ? record.id.trim() : "";
-      if (id) {
-        return toolCall;
+      const withId = id ? record : { ...record, id: this.generateToolCallId() };
+
+      const fn = withId.function;
+      if (fn && typeof fn === "object" && !Array.isArray(fn)) {
+        const fnRecord = fn as Record<string, unknown>;
+        const args = fnRecord.arguments;
+        if (typeof args === "string") {
+          const fixedArgs = repairToolCallArguments(args);
+          if (fixedArgs !== args) {
+            return { ...withId, function: { ...fnRecord, arguments: fixedArgs } };
+          }
+        }
       }
 
-      return {
-        ...record,
-        id: this.generateToolCallId(),
-      };
+      return withId;
     });
   }