fix(code-mode): render deep results iteratively

packages/core/tools/src/code-mode.ts

@@ -102,9 +102,94 @@ function jsonNormalizeArgs(value: unknown): { dispatched: unknown; logged: unkno
   return { dispatched: structuredClone(snapshot), logged: structuredClone(snapshot) }
 }
 
+/** Two-space JSON presentation, matching the existing shallow `run_code` text contract. */
+const JSON_INDENT = '  '
+
+/**
+ * ECMAScript caps `JSON.stringify`'s `space` string at ten characters. The
+ * renderer also caps TOTAL indentation there, compacting deeper subtrees, so
+ * formatted output remains linear in the canonical JSON size.
+ */
+const MAX_JSON_INDENT_CHARS = 10
+
+/** A pending fragment in the iterative JSON presentation traversal. */
+type JsonRenderTask =
+  | { kind: 'text'; text: string }
+  | { kind: 'value'; value: JsonValue; depth: number; compact: boolean }
+
+/** Render one non-string JSON root without recursive traversal or unbounded indentation growth. */
+function renderJsonValue(value: Exclude<JsonValue, string>): string {
+  const chunks: string[] = []
+  const tasks: JsonRenderTask[] = [{ kind: 'value', value, depth: 0, compact: false }]
+  for (let task = tasks.pop(); task !== undefined; task = tasks.pop()) {
+    if (task.kind === 'text') {
+      chunks.push(task.text)
+      continue
+    }
+
+    const current = task.value
+    if (current === null || typeof current === 'boolean' || typeof current === 'number') {
+      chunks.push(String(current))
+      continue
+    }
+    if (typeof current === 'string') {
+      chunks.push(JSON.stringify(current))
+      continue
+    }
+
+    const compact = task.compact || (task.depth + 1) * JSON_INDENT.length > MAX_JSON_INDENT_CHARS
+    const childDepth = task.depth + 1
+    if (Array.isArray(current)) {
+      chunks.push('[')
+      if (current.length === 0) {
+        chunks.push(']')
+        continue
+      }
+      tasks.push({ kind: 'text', text: compact ? ']' : `\n${JSON_INDENT.repeat(task.depth)}]` })
+      for (let index = current.length - 1; index >= 0; index--) {
+        const item = current[index]
+        /* v8 ignore next -- canonical JsonValue arrays are dense. */
+        if (item === undefined) throw new Error('cannot render a sparse JSON array')
+        tasks.push({ kind: 'value', value: item, depth: childDepth, compact })
+        tasks.push({
+          kind: 'text',
+          text: compact
+            ? index === 0 ? '' : ','
+            : `${index === 0 ? '\n' : ',\n'}${JSON_INDENT.repeat(childDepth)}`,
+        })
+      }
+      continue
+    }
+
+    const keys = Object.keys(current)
+    chunks.push('{')
+    if (keys.length === 0) {
+      chunks.push('}')
+      continue
+    }
+    tasks.push({ kind: 'text', text: compact ? '}' : `\n${JSON_INDENT.repeat(task.depth)}}` })
+    for (let index = keys.length - 1; index >= 0; index--) {
+      const key = keys[index]
+      /* v8 ignore next -- the loop is bounded by the captured key count. */
+      if (key === undefined) throw new Error('cannot render a missing JSON object key')
+      const item = current[key]
+      /* v8 ignore next -- canonical JsonValue records contain no undefined properties. */
+      if (item === undefined) throw new Error('cannot render an undefined JSON object property')
+      tasks.push({ kind: 'value', value: item, depth: childDepth, compact })
+      tasks.push({
+        kind: 'text',
+        text: compact
+          ? `${index === 0 ? '' : ','}${JSON.stringify(key)}:`
+          : `${index === 0 ? '\n' : ',\n'}${JSON_INDENT.repeat(childDepth)}${JSON.stringify(key)}: `,
+      })
+    }
+  }
+  return chunks.join('')
+}
+
 /** Render one present program completion value for the model-facing result text. */
 function renderValue(value: JsonValue): string {
-  return typeof value === 'string' ? value : JSON.stringify(value, null, 2)
+  return typeof value === 'string' ? value : renderJsonValue(value)
 }
 
 /** The run_code result's `meta` payload (JSON-serializable; `presentResult` narrows it back). */