docs: add Opper provider page

aider/website/docs/llms/opper.md

@@ -0,0 +1,55 @@
+---
+parent: Connecting to LLMs
+nav_order: 500
+---
+
+# Opper
+
+Aider can connect to models served by [Opper](https://opper.ai),
+an AI gateway that provides 700+ models behind one OpenAI-compatible API
+with a single key, with EU or US routing per model route.
+You'll need an Opper API key from [platform.opper.ai](https://platform.opper.ai).
+
+First, install aider:
+
+{% include install.md %}
+
+Then configure Opper as an OpenAI compatible endpoint:
+
+```
+# Mac/Linux:
+export OPENAI_API_BASE=https://api.opper.ai/v3/compat
+export OPENAI_API_KEY=<key>
+
+# Windows:
+setx OPENAI_API_BASE https://api.opper.ai/v3/compat
+setx OPENAI_API_KEY <key>
+# ... restart shell after setx commands
+```
+
+Start working with aider and Opper on your codebase:
+
+```bash
+# Change directory into your codebase
+cd /to/your/project
+
+# Opper model ids are <provider>/<model-name>.
+# Prefix them with openai/ so aider uses its OpenAI compatible client:
+aider --model openai/anthropic/claude-opus-5
+
+aider --model openai/aws/gpt-5.6-sol
+
+aider --model openai/alibaba:global/deepseek-v4-pro
+```
+
+The leading `openai/` tells aider to send the request to your
+`OPENAI_API_BASE`; the rest of the string, e.g. `anthropic/claude-opus-5`,
+is sent to Opper as the model id.
+
+The full list of model ids is served at
+`https://api.opper.ai/v3/compat/models` (authenticated with your key).
+See [Opper's aider setup page](https://opper.ai/apps/aider) for more details.
+
+See the [model warnings](warnings.html)
+section for information on warnings which will occur
+when working with models that aider is not familiar with.