diff --git a/docs/reference/required-work-context.md b/docs/reference/required-work-context.md
index d11909e968..5759fb9b84 100644
--- a/docs/reference/required-work-context.md
+++ b/docs/reference/required-work-context.md
@@ -856,6 +856,11 @@ known pointer does not require another index round trip. Batch within the combin
 output budget, avoid overlapping parent/child selections, and inspect a missing
 pointer's parent before retrying the rejected batch. These are caller guidelines;
 selection validation, required reads and authority checks remain unchanged.
+Selection errors identify the original `--select` argument and, for a failed
+lookup, its token position (both 1-based). Sorting and duplicate elimination do
+not renumber arguments. Inspect the resolved prefix before that token in the
+same capture; malformed pointers need syntax repair first. Diagnostics never
+echo the pointer or task values, and no part of a rejected batch is delivered.
 
 The `context_capture_index_v1` view shares the digest-bound pagination contract.
 An expanded index includes its `list_depth`; `next_command` preserves the depth,
@@ -895,6 +900,10 @@ pointer 从原始摘要绑定的 capture 批量读取详情。提供已有 `--ca
 `--select`，已知路径无须多查一轮目录。按合计输出预算分批，避免同时选择父节点和
 子节点；路径缺失时先查其父节点，再重试被拒绝的批次。这些是调用建议，不改变
 选择校验、必读要求或权限检查。
+选择错误会标明原始 `--select` 参数序号；查找失败时还会标明 token 序号，均从 1
+开始。排序和去重不改变参数编号。在同一 capture 中检查失败 token 前已解析的
+路径前缀；语法错误需先修正 pointer。错误信息不回显路径或任务正文，被拒绝的
+批次不会返回部分内容。
 
 `context_capture_index_v1` 沿用双摘要分页；展开的目录包含 `list_depth`，
 `next_command` 保留深度、路径和两个摘要。更改深度会拒绝旧视图的续读，目录 cursor
diff --git a/loopx/control_plane/quota/context_selection.ts b/loopx/control_plane/quota/context_selection.ts
index 917554f731..379bb01670 100644
--- a/loopx/control_plane/quota/context_selection.ts
+++ b/loopx/control_plane/quota/context_selection.ts
@@ -12,23 +12,30 @@ export function capturedContextSelection(request: JsonObject): JsonObject {
   if (typeof depth !== "number" || !Number.isInteger(depth) || depth < 1 || depth > 8
       || request.list_depth !== undefined && request.list_fields !== true)
     throw new EffectRuntimeRequestError("list-depth requires --list and an integer from 1 to 8");
-  const pointers = [...new Set(selectors as string[])].sort();
+  // Keep caller positions before canonical sorting/deduplication. Diagnostics
+  // identify a failed address without echoing possibly sensitive keys or values.
+  const argumentsByPointer = new Map<string, number>();
+  (selectors as string[]).forEach((pointer, index) => {
+    if (!argumentsByPointer.has(pointer)) argumentsByPointer.set(pointer, index + 1);
+  });
+  const pointers = [...argumentsByPointer.keys()].sort();
   const selections = pointers.map(pointer => {
+    const argument = `--select argument ${argumentsByPointer.get(pointer)}`;
     if (pointer !== "" && !pointer.startsWith("/") || /~(?:[^01]|$)/.test(pointer))
-      throw new EffectRuntimeRequestError("context selection requires RFC 6901 pointers");
+      throw new EffectRuntimeRequestError(`context selection requires RFC 6901 pointers (${argument}; positions are 1-based)`);
     const tokens = pointer === "" ? [] : pointer.slice(1).split("/").map(token => token.replace(/~1/g, "/").replace(/~0/g, "~"));
     const path: (string | number)[] = [];
     let value: unknown = context;
     for (const token of tokens) {
       if (Array.isArray(value)) {
         if (!/^(0|[1-9][0-9]*)$/.test(token) || !Number.isSafeInteger(Number(token)) || Number(token) >= value.length)
-          throw new EffectRuntimeRequestError("context selection array index is missing or invalid");
+          throw new EffectRuntimeRequestError(`context selection array index is missing or invalid (${argument}, token ${path.length + 1}; positions are 1-based)`);
         path.push(Number(token));
         value = value[Number(token)];
       } else if (value !== null && typeof value === "object" && Object.hasOwn(value, token)) {
         path.push(token);
         value = (value as JsonObject)[token];
-      } else throw new EffectRuntimeRequestError("context selection pointer does not exist");
+      } else throw new EffectRuntimeRequestError(`context selection pointer does not exist (${argument}, token ${path.length + 1}; positions are 1-based)`);
     }
     // Return the verified address, not a JS-number-coerced copy of the value.
     // The file adapter retains original JSON values and owns byte serialization.
diff --git a/tests/control_plane/test_context_capture_selection.py b/tests/control_plane/test_context_capture_selection.py
index 942c7361c2..1e17306737 100644
--- a/tests/control_plane/test_context_capture_selection.py
+++ b/tests/control_plane/test_context_capture_selection.py
@@ -263,3 +263,16 @@ def test_cli_rejects_invalid_depth_before_authority(tmp_path, options):
     assert code != 0 and page["ok"] is False
     assert "list-depth" in page["reason"]
     assert not registry.exists() and not runtime.exists()
+
+
+@pytest.mark.parametrize("pointer,token", [("/rows/0/missing", 3), ("/rows/9", 2)])
+def test_cli_missing_batch_pointer_identifies_original_argument_without_source_leak(tmp_path, pointer, token):
+    path, raw, _ = fixture_capture(tmp_path)
+    registry, runtime = tmp_path / "absent.json", tmp_path / "absent-runtime"
+    code, result = _run_cli(registry, runtime, "quota", "context-read",
+        "--capture-file", str(path), "--capture-sha256", hashlib.sha256(raw).hexdigest(),
+        "--select", "/rows", "--select", "/rows", "--select", pointer, cwd=tmp_path)
+    assert code != 0 and result["ok"] is False and "content" not in result
+    assert f"--select argument 3, token {token}" in result["reason"]
+    assert pointer not in result["reason"] and "stop if unavailable" not in json.dumps(result)
+    assert path.read_bytes() == raw and not registry.exists() and not runtime.exists()
diff --git a/tests/control_plane_ts/context_selection.test.ts b/tests/control_plane_ts/context_selection.test.ts
index 891dd0c4c4..c8a82133f2 100644
--- a/tests/control_plane_ts/context_selection.test.ts
+++ b/tests/control_plane_ts/context_selection.test.ts
@@ -55,3 +55,18 @@ test("bounded depth discovers nested addresses in one batch without values", ()
     assert.throws(() => evaluateTurnEnvelope({...request, list_depth}));
   assert.throws(() => evaluateTurnEnvelope({...request, list_fields: false, list_depth: 2}));
 });
+
+
+test("selection failures identify original argument and token without echoing content", () => {
+  const context = {z: true, rows: [{stop: "private-value"}]};
+  for (const [pointer, token] of [["/rows/0/missing", 3], ["/rows/9", 2], ["/rows/01", 2], ["/rows/0/stop/x", 4]] as const) {
+    assert.throws(() => select(context, ["/z", "/z", pointer]), (error: unknown) => {
+      assert.ok(error instanceof Error);
+      assert.match(error.message, new RegExp(`--select argument 3, token ${token}`));
+      assert.ok(!error.message.includes("private-value") && !error.message.includes(pointer));
+      assert.ok(error.message.length < 240);
+      return true;
+    });
+  }
+  assert.throws(() => select(context, ["/z", "secret-".repeat(1000)]), /--select argument 2/);
+});