diff --git a/packages/code-runtime/code-runtime-python/src/protocol.ts b/packages/code-runtime/code-runtime-python/src/protocol.ts
index 29c790eaec..10d13a211c 100644
--- a/packages/code-runtime/code-runtime-python/src/protocol.ts
+++ b/packages/code-runtime/code-runtime-python/src/protocol.ts
@@ -202,7 +202,10 @@ function scalarJson(current: unknown): string {
  * number (non-finite, negative zero) is caught only when the value fits the
  * budget — an over-budget value is rejected regardless, so the distinction is
  * moot. Same JSON-plain precondition and traversal shape as
- * {@link encodeJsonPlain}; per-scalar encoding delegates to `JSON.stringify`.
+ * {@link encodeJsonPlain}; per-scalar byte length is measured through
+ * {@link scalarJson} (matching the encoder, so a beyond-safe-range integer
+ * meters its exact BigInt digits, not `JSON.stringify`'s rounded spelling) and
+ * `JSON.stringify` for strings.
  * @param value - a JSON-plain value (e.g. straight from `JSON.parse`).
  * @param maxBytes - the completion-value budget in bytes.
  * @returns `{ ok: true, bytes }` with the exact serialized size, or
@@ -235,15 +238,22 @@ export function checkDoneValue(value: unknown, maxBytes: number): { ok: true; by
       for (const item of current) stack.push(item)
     } else if (typeof current === 'object' && current !== null) {
       const record = current as Record<string, unknown>
-      // Count own keys WITHOUT Object.entries/Object.keys: either would
-      // allocate one slot (entries: one pair array) per member before the
-      // bound below could run, recreating the spike the bound exists to stop.
+      // Count own keys WITHOUT Object.entries/Object.keys (either allocates one
+      // slot per member up front), AND bail mid-count the instant the minimum
+      // encoding exceeds the budget: braces (+2), each entry a quoted key
+      // (>= 2 bytes) + colon + >= 1-byte value (>= 4 bytes), and a comma per
+      // gap. A forged wide object with millions of keys and a small cap must
+      // fail in O(cap), not walk its whole breadth first. `bytes` still holds
+      // the pre-object total throughout this loop.
       let count = 0
-      for (const key in record) if (Object.hasOwn(record, key)) count += 1
+      for (const key in record) {
+        if (!Object.hasOwn(record, key)) continue
+        count += 1
+        if (bytes + 2 + count * 4 + (count - 1) > maxBytes) return { ok: false, reason: 'over-budget' }
+      }
+      // The loop's final iteration already proved the whole object's lower
+      // bound fits, so no separate post-count check is needed here.
       bytes += 2 + (count > 1 ? count - 1 : 0)
-      // Same pre-enqueue bound: each entry contributes its quoted key (>= 2
-      // bytes), the colon, and a >= 1-byte value.
-      if (bytes + count * 4 > maxBytes) return { ok: false, reason: 'over-budget' }
       for (const key in record) {
         if (!Object.hasOwn(record, key)) continue
         // The same string lower bound, before escaping the key.
diff --git a/packages/code-runtime/code-runtime-python/tests/protocol.spec.ts b/packages/code-runtime/code-runtime-python/tests/protocol.spec.ts
index dc0a01d47d..f0e06af723 100644
--- a/packages/code-runtime/code-runtime-python/tests/protocol.spec.ts
+++ b/packages/code-runtime/code-runtime-python/tests/protocol.spec.ts
@@ -207,6 +207,21 @@ describe('checkDoneValue', () => {
     const wide: Record<string, number> = {}
     for (let i = 0; i < 10; i++) wide[`k${i}`] = i
     expect(checkDoneValue(wide, 12)).toEqual({ ok: false, reason: 'over-budget' })
+    // A forged object with millions of keys and a small cap must reject in
+    // O(cap): the key COUNT loop itself bails once the running minimum encoding
+    // (braces + 4 bytes/entry + commas) crosses the budget, rather than walking
+    // the whole breadth before checking. Observable as a bounded key subset:
+    // build a Proxy whose ownKeys would yield far more than the cap admits and
+    // assert the metered walk never enumerates past it.
+    let enumerated = 0
+    const millionKeys = new Proxy({}, {
+      ownKeys() { return Array.from({ length: 2_000_000 }, (_unused, i) => `k${i}`) },
+      getOwnPropertyDescriptor() { enumerated += 1; return { enumerable: true, configurable: true, value: 0 } },
+    })
+    expect(checkDoneValue(millionKeys, 64)).toEqual({ ok: false, reason: 'over-budget' })
+    // With cap 64, at most ~16 entries (4 bytes each) can fit before the bound
+    // trips, so the walk enumerates far fewer than the 2,000,000 declared keys.
+    expect(enumerated).toBeLessThan(1000)
   })
 
   it('rejects an over-budget string on its length before escaping it', () => {