diff --git a/docs/reference/contracts/interface-budget-contract.md b/docs/reference/contracts/interface-budget-contract.md
index 2484f1bb86..e8c8bdb2f7 100644
--- a/docs/reference/contracts/interface-budget-contract.md
+++ b/docs/reference/contracts/interface-budget-contract.md
@@ -10,7 +10,7 @@ and size/count budgets.
 | --- | --- | --- | --- | --- | --- | --- |
 | `heartbeat_prompt_json` | heartbeat automation | wake and route one bounded turn | `quota should-run`, `status`, or `review-packet --handoff-only` | `json_chars <= 5400` plus `interface_budget.within_budget=true` | `nested_keys <= 40` | `top_level_keys <= 30` |
 | `review_packet_handoff_only_json` | project-agent handoff | forward the smallest sufficient task packet | full `review-packet` or run-history artifact | `json_chars <= 3000` plus `handoff_interface_budget.within_budget=true` | `nested_keys <= 40` | `top_level_keys <= 18` |
-| `quota_should_run_json` | quota guard | decide whether the selected goal may spend compute | `status`, `history`, or active state | `json_chars <= 20500` | `nested_keys <= 575` | `top_level_keys <= 55` |
+| `quota_should_run_json` | quota guard | decide whether the selected goal may spend compute | `status`, `history`, or active state | `json_chars <= 21000` | `nested_keys <= 575` | `top_level_keys <= 55` |
 | `dashboard_status_json` | operator dashboard | render first-screen operator state | `history`, run artifacts, or project-local adapter output | `json_chars <= 25000` | `nested_keys <= 350` | `top_level_keys <= 27` |
 
 These four budgets measure compact machine payloads. For
@@ -66,6 +66,25 @@ execution authority or change to frozen experiment or promotion criteria.
 已占满旧限、没有 cadence 余量，改为 55 后保留一个键。quota 字符、status 键数、
 夹具数量、采样深度和语义断言不变；不授予执行权限，也不改变冻结的实验或 promotion 标准。
 
+The standard premerge runner adds an isolation directory around this fixture.
+On base `6a92640346051b79d08fb369354180b613c8fa46` and the budget-only candidate,
+quota measures 20,341 characters directly and 20,509 through the native runner,
+with 565 nested and 54 top-level keys. Eight complete fixture-root references
+each gain the runner's 21-character directory component, accounting for all 168
+additional characters. Preserve the original 20,500-ceiling
+failure. Calibrate only this character regression allowance to 21,000, leaving
+491 characters in the runner; keep the 575/55 structure limits and complete
+source paths. The same quota payload with an oversized reason remains rejected.
+No production payload, execution budget, host hard limit or frozen qualification
+result changes; this is not a compaction or efficiency claim.
+
+标准 premerge 的隔离目录会增长同一夹具的完整路径。上述 base 与仅改预算的候选均为
+直接运行 20,341 字符、原生 runner 20,509 字符，565 个嵌套键及 54 个顶层键；
+8 处完整夹具路径各增加 21 字符，解释全部 168 字符差异。
+保留原 20,500 超限失败，仅将字符回归余量调至 21,000，给 runner 留 491 字符；
+575/55 结构限制、完整来源路径及超长 reason 拒绝继续。生产负载、执行预算、宿主
+硬边界和冻结资格结果均不变，不据此声称压缩或效率提升。
+
 The successful thin heartbeat projection contains only `schema_version`,
 `ok`, `goal_id`, optional `agent_id`, `task_body`, and the compact
 `interface_budget`; exact Turn identity and `bootstrap=true` are included only
diff --git a/examples/control_plane/hot-path-interface-budget-smoke.py b/examples/control_plane/hot-path-interface-budget-smoke.py
index 94043b3285..a0f8c3c757 100644
--- a/examples/control_plane/hot-path-interface-budget-smoke.py
+++ b/examples/control_plane/hot-path-interface-budget-smoke.py
@@ -79,7 +79,10 @@
         # 15,419/18,636 characters, 378/513 nested keys, and 52/53 top-level
         # keys. Keep the complete typed envelope rather than dropping source
         # facts that the consumer must inspect before it spends compute.
-        "max_json_chars": 20_500,
+        # Native canary isolation adds a parent directory to every fixture
+        # path: the same payload is 20,341 chars direct / 20,509 in the runner.
+        # Retain complete source paths and 491 chars of runner headroom.
+        "max_json_chars": 21_000,
         # Explicitly importing this same fixture exposes the current canonical
         # read shape on both base/head: 20,287 chars, 564 nested / 54 top-level
         # keys. Preserve source/Task provenance; retain 11 nested and one
@@ -465,6 +468,17 @@ def main() -> int:
             assert_surface("quota_should_run_json", quota_payload),
             assert_surface("dashboard_status_json", status_payload),
         ]
+        oversized_quota = {
+            **quota_payload,
+            "reason": "x" * int(SURFACE_BUDGETS["quota_should_run_json"]["max_json_chars"]),
+        }
+        try:
+            assert_surface("quota_should_run_json", oversized_quota)
+        except AssertionError as error:
+            assert error.args[0][0] == "quota_should_run_json", error
+            assert error.args[0][1] > SURFACE_BUDGETS["quota_should_run_json"]["max_json_chars"], error
+        else:
+            raise AssertionError("quota character budget accepted an oversized payload")
         cadence = build_interface_budget_cadence(
             summaries,
             checked_at="2099-01-01T00:10:00+00:00",