docs(user): guide gateway request-compatibility switches

The Models page has no field for `compat`, and the symptom it addresses —
a gateway holding a working key at a reachable address while refusing
every request — reads as a credential or connectivity problem. Give it
the same treatment `input` already has: name the symptom, show the two
switches that account for most of it, and state the resolution order.

Refs #2646

docs/user/guide/providers.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write docs/user/guide/providers.md
-providers.md: 099f434ec4602aa402239e83c708d81fcadd7732
-providers.zh.md: 367c90b525ad628b3cd86b2d22045c25064e88a1
+providers.md: 969ca13c0b288c8c0c313318a7a3162d7c4b361e
+providers.zh.md: 51079896ca6f38e30a65d7e73170a4a7e7b15569

docs/user/guide/providers.md

@@ -79,6 +79,40 @@ Every list must name at least one modality except a model's own, where an empty
 
 Both fields state a claim about your endpoint rather than checking it. A model that declares images its endpoint does not serve is not caught here; the provider rejects the request instead.
 
+### Request compatibility
+
+A gateway can hold a working key at a reachable address and still refuse every request. pi-ai decides the shape of a request — which role carries the system prompt, which field caps the output, how a thinking level travels — from the endpoint's URL, and an address it does not recognize is addressed as though it were OpenAI itself. Most OpenAI-compatible gateways refuse at least one thing OpenAI accepts.
+
+Two account for most of it. A model that declares reasoning has its system prompt sent as `role: "developer"`, which many gateways reject outright, and the output cap is sent as `max_completion_tokens`, which a server that only knows `max_tokens` refuses. The form has no field for either; correct them on the route in `$DSH_HOME/settings.yaml`:
+
+```yaml
+llm-pi-ai:
+  providers:
+    my-gateway:
+      apiKeyEnv: GATEWAY_API_KEY
+      api: openai-completions
+      baseURL: https://gateway.example/v1
+      compat:
+        supportsDeveloperRole: false
+        maxTokensField: max_tokens
+      models:
+        - id: my-model
+```
+
+A route's `compat` is the default for its models, and a model's own wins field by field, so one model can be corrected without restating the route:
+
+```yaml
+      models:
+        - id: my-model
+        - id: my-reasoner
+          compat:
+            thinkingFormat: deepseek
+```
+
+What neither sets keeps the installed catalog's value for that model, and what the catalog does not describe falls to pi-ai's detection. Give every switch you name a value: a key left empty (`supportsDeveloperRole:`) is refused rather than ignored, because an empty value would erase what the catalog knows while saying nothing in its place. A name no protocol accepts is refused too, and the message lists the ones that are available.
+
+Each switch belongs to the protocols that declare it, so a switch valid on one `api` may be refused on another — the message names what that protocol does offer. Like `input` above, a switch states a claim about your endpoint rather than checking it: setting one your gateway does not actually need simply sends a different request.
+
 ## Select a model
 
 Configured providers appear in the model picker. Selecting a model also makes it the default for new sessions. A session that has already sent a request retains the model recorded in its own log.
@@ -90,6 +124,9 @@ If a saved default names a provider that was deleted, the composer displays **Se
 - **`MISSING_CREDENTIAL`** — Store the provider key through the Models page or supply the referenced environment variable.
 - **`UNKNOWN_MODEL`** — Select a configured model or add the missing model to the custom provider.
 - **Fetching available models returns 401** — Check the key. Model discovery calls the OpenAI-compatible `GET /models` endpoint; enter models manually for endpoints that do not provide it.
+- **The gateway refuses every request although the key and URL are right** — Its request shape differs from OpenAI's. Start with `compat.supportsDeveloperRole: false` and `compat.maxTokensField: max_tokens` on the route.
+- **Only reasoning models fail** — pi-ai sends their system prompt as the `developer` role, which the gateway rejects. Set `compat.supportsDeveloperRole: false`.
+- **A compat switch is refused as having no value** — A key written with nothing after the colon. Give it a value, or remove the key to keep the installed catalog's.
 - **An image is refused before sending** — The model declares no image modality. Give a custom provider's model `input: [text, image]`; DeepSeek's own chat-completions route is text-only and cannot be configured otherwise.
 - **The provider rejects a request carrying an image** — The model declares images its endpoint does not actually serve. Remove `image` from whichever list granted it — the model's `input`, or the route's `defaultInput` — then start a new session: the attached image stays in the session log, so the same request repeats until the session moves off it.
 

docs/user/guide/providers.zh.md

@@ -79,6 +79,40 @@ llm-pi-ai:
 
 这两个字段都是对你端点的断言，而不是对它的检查。声明了端点并不提供的图片能力的模型不会在这里被拦下，改由提供方拒绝该请求。
 
+### 请求兼容性
+
+网关可能持有可用的密钥、地址也通得到，却仍然拒绝每一个请求。pi-ai 依据端点的 URL 决定请求的形状——系统提示词由哪个角色承载、输出上限写在哪个字段、思考级别如何传输——而对于它无法识别的地址，会当作 OpenAI 本身来对待。多数 OpenAI 兼容网关至少会拒绝 OpenAI 所接受的某一样东西。
+
+其中两样占了绝大多数。声明了推理能力的模型，其系统提示词会以 `role: "developer"` 发出，很多网关直接拒绝；输出上限则写作 `max_completion_tokens`，只认 `max_tokens` 的服务端会拒绝。表单里没有这两个字段；请在 `$DSH_HOME/settings.yaml` 的路由上更正：
+
+```yaml
+llm-pi-ai:
+  providers:
+    my-gateway:
+      apiKeyEnv: GATEWAY_API_KEY
+      api: openai-completions
+      baseURL: https://gateway.example/v1
+      compat:
+        supportsDeveloperRole: false
+        maxTokensField: max_tokens
+      models:
+        - id: my-model
+```
+
+路由的 `compat` 是其模型的默认值，模型自身的则逐字段胜出，因此更正某一个模型无需重述整条路由：
+
+```yaml
+      models:
+        - id: my-model
+        - id: my-reasoner
+          compat:
+            thinkingFormat: deepseek
+```
+
+两者都未设置的字段，沿用已安装 catalog 为该模型记录的值；catalog 也未描述的，落到 pi-ai 的检测。凡是写下的开关都要给值：冒号后留空的键（`supportsDeveloperRole:`）会被拒绝而不是被忽略，因为空值会抹掉 catalog 已知的信息，却又没有给出任何替代。任何协议都不接受的名字同样会被拒绝，报错会列出可用的那些。
+
+每个开关归属于声明了它的那些协议，因此在某个 `api` 上合法的开关，在另一个上可能被拒绝——报错会点名该协议实际提供哪些。与上面的 `input` 一样，开关陈述的是关于你的端点的一个断言，而不是对它的检查：设置一个网关其实并不需要的开关，只是发出一个不同的请求而已。
+
 ## 选择模型
 
 已配置的提供方会出现在模型选择器中。选择模型也会将其设为新会话的默认值。已发送过请求的会话会保留自身日志中记录的模型。
@@ -90,6 +124,9 @@ llm-pi-ai:
 - **`MISSING_CREDENTIAL`**：通过模型页存储提供方密钥，或提供被引用的环境变量。
 - **`UNKNOWN_MODEL`**：选择已配置的模型，或向自定义提供方添加缺失的模型。
 - **获取可用模型返回 401**：检查密钥。模型发现会调用 OpenAI 兼容的 `GET /models` 端点；对于不提供该端点的服务，请手动输入模型。
+- **密钥与地址都正确，网关却拒绝每一个请求**：它的请求形状与 OpenAI 不同。先在路由上设 `compat.supportsDeveloperRole: false` 与 `compat.maxTokensField: max_tokens`。
+- **只有推理模型失败**：pi-ai 把它们的系统提示词以 `developer` 角色发出，而网关拒绝该角色。设 `compat.supportsDeveloperRole: false`。
+- **某个 compat 开关因没有值而被拒绝**：冒号后什么都没写。给它一个值，或删掉该键以沿用已安装 catalog 的值。
 - **图片在发送前被拒绝**：该模型未声明图片模态。请给自定义提供方的模型加上 `input: [text, image]`；DeepSeek 自身的 chat-completions 路由是纯文本的，且无法通过配置改变。
 - **提供方拒绝了带图片的请求**：该模型声明了其端点实际并不提供的图片能力。请从授予它图片能力的那个列表中移除 `image`——可能是模型的 `input`，也可能是路由的 `defaultInput`——然后开启新会话：附加的图片会留在会话日志里，因此在会话离开它之前，同一个请求会不断重复。