@@ -191,6 +191,19 @@ not a total-turn ceiling.
   `"at-capacity"` (the server is at `MAX_CONCURRENT`, which `/api/health`
   reports as `maxConcurrent`)
 
+**A 409 is terminal for that message, and it is not a failure.** The turn is
+never handed to the engine, the `›` line is never written, and nothing reaches
+the persisted record — a refused send cannot be half-applied, and cannot be
+one the engine ran while the transcript lost. There is no send queue: "refused,
+try again when the turn ends" is the whole contract.
+
+`error` is the user-facing sentence (the composer renders it verbatim) and
+`reason` is the stable machine-readable key; branch on `reason`. For
+`cid-busy` and `session-busy` that sentence states the conversation is already
+running a turn and the message was not delivered, rather than repeating the
+internal detail — which reads "another window" and is wrong for the common
+case of the same tab sending again a moment later.
+
 ### `POST /api/stop`
 
 Cancel the current run. Tries `session/cancel` via acp (the cancel
@@ -1973,9 +1986,14 @@ actually read for each layer, so an operator can confirm which file
 the live config came from.
 
 Layered resolution: `MCODE_WEBUI_MODELS_CONFIG` env → cwd `models.json`
-→ user-level `~/.mcode-webui/providers.json` (the PUT write target).
-Same-id provider deep merge; models dedupe by id with the higher layer
-winning.
+→ the engine's `<engine data dir>/config.yaml` under `custom_provider`
+(the PUT write target). Same-id provider deep merge; models dedupe by id
+with the higher layer winning.
+
+The env and cwd layers are deployment-owned and are never written by
+any handler. The third layer used to be a webui file of its own
+(`~/.mcode-webui/providers.json`); it is now the engine's own provider
+store, and that file is **deprecated** — see "Provider storage" below.
 
 **Response 200**
 ```json
@@ -2014,26 +2032,70 @@ winning.
 }
 ```
 
+- `sources.user` and `userPath` still name the **deprecated**
+  `~/.mcode-webui/providers.json`. The fields did not change and the
+  values did not either: both are documented as "the files this server
+  resolved", and an operator diagnosing a missing provider still needs
+  to be told what to look at. What changed is the answer — the file is
+  read only until the migration completes, and is never written again.
+  The live catalogue is the engine store; `GET /api/models` reads it
+  there too.
 - `auth.apiKeyMasked` is the only apiKey shape returned by any route
   in this surface. A test (and `scripts/check-docs-alignment.mjs`)
   pins the rule: the plaintext key MUST NEVER appear in any
   `/api/providers*` response, regardless of which layer held it.
+**Path forms are reported as the server resolved them, and nothing is
+re-resolved.** `sources.cwd` is `<process.cwd()>/models.json`, and
+`process.cwd()` is the kernel-reported working directory — on macOS
+that is the fully-resolved form, so a server started under `/var`
+reports `/private/var/...`. That is the correct answer to "which file
+did you read", and the write side uses the same resolver, so the file
+the response names is the file the `PUT` will land in. `sources.user`
+and `userPath` come straight from `MCODE_WEBUI_DATA_DIR` and are
+reported exactly as configured.
+
 - `sources.env` is `null` when `MCODE_WEBUI_MODELS_CONFIG` is unset;
   `sources.cwd` is omitted from the layer set in that case (the env
   override is the cwd file).
 
 ### `PUT /api/providers`
 
-Validate-and-persist a v2 provider config to the user-level file
-(`~/.mcode-webui/providers.json`, the file written by this handler).
-The env / cwd layers are deployment-owned and never written here.
-
-The handler atomically writes via rename (no half-written file on
-disk), reloads the layer set on the next call, and broadcasts an
-SSE `providers.updated` named event with the masked payload so
-every connected client refreshes its catalogue without polling.
-`/api/models` picks up the change on the next request — no restart
-required.
+Validate-and-persist a v2 provider config to the **engine's provider
+store** — `<engine data dir>/config.yaml` under `custom_provider`,
+written with mode `0600`. The env / cwd layers are deployment-owned and
+never written here, and neither is the deprecated
+`~/.mcode-webui/providers.json`.
+
+The handler performs **one** write: a temporary file plus a single
+`rename` of the whole document. There is no second file to fall out of
+step, so a request either lands completely or changes nothing — a
+concurrent reader always sees a whole catalogue, never a mixture, and
+never a partially written YAML document. The layer set is re-read on
+the next call, and the handler broadcasts an SSE `providers.updated`
+named event with the masked payload so every connected client refreshes
+its catalogue without polling. `/api/models` picks up the change on
+the next request — no restart required.
+
+**Capability gate.** The two write endpoints (`PUT /api/providers`
+and `POST /api/providers/preset/:id/enable`) declare
+`authCredentials` and gate **hard** on their sub-item
+(`updateUserModelProvider` / `createUserModelProvider`). A provider
+that declares the sub-item absent answers
+`501 {ok:false, code:"engine_capability_not_supported", …}` rather than
+acknowledging a configuration the engine will never read. On the
+default `acp` transport no provider is registered yet, so the gate
+reports `unregistered-transport` and the write proceeds. The three read
+endpoints declare the same capability and gate **soft** — they report
+degradation and keep serving.
+
+**Legacy migration.** While the engine store carries no migration
+marker, the deprecated `providers.json` is still the authority: webui
+folds it into the store, losslessly, on the next read, and stamps the
+marker on success — after which the file is never read again. A failed
+migration (an unparseable `config.yaml`, a write that could not
+complete) leaves the store untouched and the old format readable, and
+the next read retries. Field-by-field equivalence is pinned by
+`packages/webui/test/lib/engine/provider-migration.test.js`.
 
 **Request**
 ```json
@@ -2059,15 +2121,27 @@ required.
 {
   "ok": true,
   "providers": [ /* masked view, same shape as GET */ ],
-  "path": "/home/you/.mcode-webui/providers.json"
+  "path": "/home/you/.minimax/config.yaml",
+  "engineSync": { "ok": true, "written": true, "keys": ["openai_compat"] }
 }
 ```
 
+- `path` is the file this handler wrote: the engine's `config.yaml`.
+  It used to be `~/.mcode-webui/providers.json`.
+- `engineSync` reports the store write itself. `written: false` means
+  the document would have come out unchanged (a no-op PUT does not
+  re-chmod a file an operator just hand-edited). It is `ok: true`
+  whenever the store accepted the write.
 - `400 BAD_BODY` — invalid provider shape, unknown protocol, or
   validation failure (each error carries a human-readable `error`
   string with the offending field).
-- `500 WRITE_FAILED` — disk I/O failure (the in-memory state did
-  not change; the operator should retry).
+- `500 WRITE_FAILED` — the store refused or could not perform the
+  write. Two causes, and the second is the one that matters: a
+  `config.yaml` that does not parse is **refused, never
+  overwritten**, because rewriting it would destroy every engine
+  setting the store does not own. In both cases the previous
+  document is intact, the next `GET` returns the catalogue the client
+  already had, and the operator can retry.
 
 ### `POST /api/providers/test`
 
 
@@ -175,6 +175,16 @@ stdin。
   `"session-busy"`（另一个客户端正在跑这个会话）或 `"at-capacity"`
   （服务端已达 `MAX_CONCURRENT`，即 `/api/health` 里报的 `maxConcurrent`）
 
+**409 对这条消息是终态，而且它不是失败。** 这个回合不会交给引擎，`›` 行
+不会写入，落库记录里也不会有任何东西——被拒的发送不可能被半途应用，也不可能
+出现「引擎跑了、转录却丢了」的情形。这里没有发送队列，契约就是「被拒，回合
+结束后再发」。
+
+`error` 是面向用户的句子（composer 逐字渲染它），`reason` 是稳定的机读键；
+按 `reason` 分支。`cid-busy` 与 `session-busy` 的句子说明「本会话正在跑一个
+回合，这条消息未送达」，而不是复述内部 detail——后者写的是「另一个窗口」，
+对「同一个标签页隔一会儿再发一次」这个常见情形是错的。
+
 ### `POST /api/stop`
 
 取消当前运行。先尝试通过 acp 调用 `session/cancel`
@@ -1812,8 +1822,14 @@ Multipart 文件上传。保存到 `MCODE_WEBUI_UPLOAD_DIR` 并返回
 现网配置来自哪个文件。
 
 分层解析顺序：`MCODE_WEBUI_MODELS_CONFIG` 环境变量 → cwd 下的
-`models.json` → 用户级 `~/.mcode-webui/providers.json`（PUT 的写入
-目标）。同 id 的 provider 做深合并；模型按 id 去重，高层胜出。
+`models.json` → 引擎的 `<引擎数据目录>/config.yaml` 里的
+`custom_provider` 节点（PUT 的写入目标）。同 id 的 provider 做深
+合并；模型按 id 去重，高层胜出。
+
+env 与 cwd 两层由部署方拥有，任何 handler 都不写。第三层过去是
+webui 自己的文件（`~/.mcode-webui/providers.json`），现在是引擎
+自己的 provider 存储；那个文件已**废弃**，详见下文 `PUT /api/providers`
+一节。
 
 **响应 200**
 ```json
@@ -1856,14 +1872,30 @@ Multipart 文件上传。保存到 `MCODE_WEBUI_UPLOAD_DIR` 并返回
   与 `scripts/check-docs-alignment.mjs` 一起把这条规则钉死：无论
   密钥来自哪一层，明文 key 都绝不允许出现在任何
   `/api/providers*` 响应中。
+- `sources.user` 与 `userPath` 仍然指向**已废弃**的
+  `~/.mcode-webui/providers.json`。字段没变，取值也没变：两者的
+  文档语义都是「服务端解析了哪些文件」，运维排查 provider 缺失时
+  仍然需要知道该看哪里。变的是答案——该文件只在迁移完成前被读取，
+  此后不再被写入。真正的目录在引擎存储里，`GET /api/models` 也
+  是从那里读的。
+**路径按服务端解析出的形态上报，不做二次解析。** `sources.cwd` 是
+`<process.cwd()>/models.json`，而 `process.cwd()` 是内核返回的工作
+目录——在 macOS 上它是完全解析后的形态，因此从 `/var` 下启动的服务会
+上报 `/private/var/...`。这正是「你到底读了哪个文件」的正确答案；
+写入侧用的是同一个解析器，所以响应里指名的文件就是 `PUT` 会落到的
+文件。`sources.user` 与 `userPath` 直接来自 `MCODE_WEBUI_DATA_DIR`，
+按配置原样上报。
+
 - `MCODE_WEBUI_MODELS_CONFIG` 未设置时 `sources.env` 为 `null`；
   此时 `sources.cwd` 也从层级集合中省略（环境变量覆盖的就是 cwd
   那个文件）。
 
 ### `PUT /api/providers`
 
-校验并持久化一份 v2 provider 配置到用户级文件
-（`~/.mcode-webui/providers.json`，即本 handler 写入的文件）。
+校验并持久化一份 v2 provider 配置到**引擎的 provider 存储**——
+`<引擎数据目录>/config.yaml` 的 `custom_provider` 节点，文件权限
+`0600`。env / cwd 两层由部署方拥有，本 handler 不写；已废弃的
+`~/.mcode-webui/providers.json` 同样不写。
 env / cwd 两层归部署方所有，永远不在这里被写。
 
 handler 通过 rename 原子写入（磁盘上不会出现半写文件），下一次
@@ -1895,14 +1927,40 @@ handler 通过 rename 原子写入（磁盘上不会出现半写文件），下
 {
   "ok": true,
   "providers": [ /* 掩码视图，形态与 GET 相同 */ ],
-  "path": "/home/you/.mcode-webui/providers.json"
+  "path": "/home/you/.minimax/config.yaml",
+  "engineSync": { "ok": true, "written": true, "keys": ["openai_compat"] }
 }
 ```
 
+- `path` 是本 handler 实际写入的文件：引擎的 `config.yaml`。它
+  过去是 `~/.mcode-webui/providers.json`。
+- `engineSync` 报告这次存储写入本身。`written: false` 表示文档
+  内容不会变化——空转的 PUT 不会去重设运维刚手工编辑过的文件权限。
+  只要存储接受了写入，它就是 `ok: true`。
+
+**能力门控。** 两个写端点（`PUT /api/providers` 与
+`POST /api/providers/preset/:id/enable`）声明 `authCredentials`
+能力，并对自己的子项（`updateUserModelProvider` /
+`createUserModelProvider`）做**硬**门控。声明缺失该子项的 provider
+会得到 `501 {ok:false, code:"engine_capability_not_supported", …}`，
+而不是确认一份引擎永远不会读取的配置。在默认的 `acp` 传输下尚未
+注册任何 provider，门控报告 `unregistered-transport`，写入照常进行。
+三个读端点声明同一能力，做**软**门控——只报告降级，继续服务。
+
+**存量迁移。** 引擎存储里没有迁移标记时，已废弃的
+`providers.json` 仍然是权威来源：webui 会在每次读取时尝试把它
+无损折叠进存储，成功后写入标记，该文件此后再不被读取。迁移失败
+（引擎配置无法解析、写入失败）时存储保持原样，旧格式继续可读，
+下一次读取会重试。目录字段逐项等价由
+`packages/webui/test/lib/engine/provider-migration.test.js` 钉死。
+
 - `400 BAD_BODY` —— provider 形态非法、协议未知，或校验失败
   （每条错误都带一条可读的 `error` 文本，指出出问题的字段）。
-- `500 WRITE_FAILED` —— 磁盘 I/O 失败（内存中的状态没有变化；
-  运维应重试）。
+- `500 WRITE_FAILED` —— 存储拒绝或未能完成写入。两种成因，其
+  中第二种才是重点：无法解析的 `config.yaml` 会被**拒绝，绝不覆盖**，
+  因为覆盖会连带毁掉存储并不拥有的全部引擎配置。两种情况下前一份
+  文档都保持完整，随后的 `GET` 返回客户端原本就有的目录，运维可以
+  直接重试。
 
 ### `POST /api/providers/test`
 
@@ -2018,8 +2076,11 @@ SSE 事件，让每个已连接客户端刷新目录。下一次 `/api/models`
 ```
 
 - `400 UNKNOWN_PRESET` —— `:id` 不是已知模板。
-- `500 WRITE_FAILED` —— 磁盘 I/O 失败（内存中的状态没有变化；
-  运维应重试）。
+- `500 WRITE_FAILED` —— 存储拒绝或未能完成写入。两种成因，其
+  中第二种才是重点：无法解析的 `config.yaml` 会被**拒绝，绝不覆盖**，
+  因为覆盖会连带毁掉存储并不拥有的全部引擎配置。两种情况下前一份
+  文档都保持完整，随后的 `GET` 返回客户端原本就有的目录，运维可以
+  直接重试。
 
 ---
 
 
@@ -608,7 +608,7 @@ is what lets it be re-exported from `engine/index.js` at all.
 static imports, because `routes/model.js` already imported all four
 **before** M3-B4 and the server's boot cost is therefore exactly what it
 was. They reach `@mavis/shared/local-runtime-paths` (via `lib/config.js`)
-and `js-yaml` (via `engine-provider-sync.js`), so the module is deliberately
+and `js-yaml` (via `engine/provider-store.js`), so the module is deliberately
 **not** re-exported from `engine/index.js`: making the shared facade — the
 one import site the whole server shares, and the one `routes/plugins.js`
 must stay light through — heavier than it has ever been would buy nothing.
 
@@ -556,7 +556,7 @@ handler 层测试因此保持封闭。
 `lib/engine-catalogue.js`、`lib/models.js`、`lib/providers-config.js`——
 是静态 import，因为 M3-B4 之前 `routes/model.js` 就静态 import 了这四个，
 所以 server 的启动成本分文未增。但它们会经 `lib/config.js` 抵达
-`@mavis/shared/local-runtime-paths`、经 `engine-provider-sync.js` 抵达
+`@mavis/shared/local-runtime-paths`、经 `engine/provider-store.js` 抵达
 `js-yaml`，所以这个模块**刻意没有**从 `engine/index.js` 转发导出：让
 共享门面——整个 server 唯一的共享 import 站点，也是
 `routes/plugins.js` 必须保持轻量的那个——比它历来更重，换不来任何东西。