> For the complete documentation index, see [llms.txt](https://docs.fastrouter.ai/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.fastrouter.ai/integrations/coding-assistants/aider.md). # Aider What is Aider? [Aider](https://aider.chat/) is an AI pair programming tool that runs in your terminal. It edits code in your local git repository, makes commits, and works across multiple files—driven entirely by natural-language instructions. It connects to any OpenAI-compatible endpoint. By routing Aider through FastRouter, you get: * **100+ models** from OpenAI, Anthropic, Google, xAI, Meta, Groq, Mistral, and more through one endpoint—including coding models like [Grok Code Fast 1](https://fastrouter.ai/models/x-ai/grok-code-fast-1) * **Observability** for every request: cost, tokens, latency, and model selection tracked in real time * **Reliability** through automatic failover across providers, response caching, and intelligent routing * **Governance** with per-key budgets, rate limits, model restrictions, role-based access, and project isolation This guide covers configuring Aider to use FastRouter through its OpenAI-compatible settings. **Prerequisites** * A FastRouter.ai account ([sign up](https://fastrouter.ai)) * Python 3.10 or higher, and a git repository to work in *** #### Quick Start **Step 1: Install Aider** ```bash python -m pip install aider-install aider-install ``` **Step 2: Get Your FastRouter API Key** 1. Sign up or log in at [fastrouter.ai](https://fastrouter.ai) 2. Navigate to your project's **Keys** page 3. Click **Create User Key** 4. Copy the key immediately. FastRouter does not display the key again after creation. **Step 3: Point Aider at FastRouter** Aider uses LiteLLM under the hood. Set FastRouter as the OpenAI-compatible endpoint with two environment variables. **Run these in your terminal—not at Aider's `>` prompt—before launching Aider:** ```bash export OPENAI_API_BASE=https://api.fastrouter.ai/api/v1 export OPENAI_API_KEY=sk-add-your-key-here echo $OPENAI_API_KEY # confirm it prints before continuing ``` > **Note:** Environment variables only last for the current terminal session. For a setup that persists, see Persisting your credentials below. **Step 4: Launch Aider with a FastRouter Model** Prefix the FastRouter model slug with `openai/` so LiteLLM uses the OpenAI-compatible protocol: ```bash cd /path/to/your/git/repo aider --model openai/x-ai/grok-code-fast-1 ``` > **Note:** The `openai/` prefix is **required**. It selects the OpenAI-compatible protocol; the rest (`x-ai/grok-code-fast-1`) is the FastRouter model slug. Without it, LiteLLM cannot determine the provider and fails with `LLM Provider NOT provided`. For an Anthropic model, use `--model openai/anthropic/claude-4.5-sonnet`. If the folder isn't a git repository, Aider offers to create one—answer **Yes** (recommended, so it can track and undo changes), or run with `--no-git` to skip git entirely. On first launch you'll see two harmless warnings: *"Unknown context window size and costs"* and *"Did you mean …"*. Both occur because Aider's local model database doesn't recognize FastRouter slugs. They don't affect anything—silence them with `--no-show-model-warnings`. Aider starts in your repo and is ready for instructions. All requests route through FastRouter, and every request, token count, and cost appears in your [FastRouter Dashboard](https://dashboard.fastrouter.ai/). **Persisting your credentials** Because `export` only lasts for the current terminal session, the recommended setup is a `.env` file in your project root, which Aider loads automatically: ```bash cat > .env <<'EOF' OPENAI_API_BASE=https://api.fastrouter.ai/api/v1 OPENAI_API_KEY=sk-add-your-key-here EOF echo ".env" >> .gitignore # never commit your key ``` With `.env` in place, just run `aider --model openai/x-ai/grok-code-fast-1`—no exports needed. Alternatively, add the two `export` lines to your shell profile (`~/.zshrc` or `~/.bashrc`) to apply them everywhere. > **Security:** If you use a `.env` file, make sure it is git-ignored (the command above does this). Aider auto-ignores `.aider*` but **not** `.env`.
*** #### Use Aider with 100+ Models FastRouter uses the `provider/model-name` format (after LiteLLM's `openai/` protocol prefix). Switch models with the `--model` flag: ```bash # Anthropic Claude aider --model openai/anthropic/claude-4.5-sonnet # OpenAI aider --model openai/openai/gpt-5.2 ``` **Recommended models for Aider:** * `x-ai/grok-code-fast-1` — fast and inexpensive; a good default for routine edits * `anthropic/claude-4.5-sonnet` — highest quality for complex, multi-file changes * `openai/gpt-5.2` — strong all-round alternative A common Aider pattern is a strong "architect" model for planning plus a cheaper "editor" model for the mechanical edits. Both use the same FastRouter key: ```bash aider --model openai/anthropic/claude-4.5-sonnet \ --editor-model openai/x-ai/grok-code-fast-1 ``` For large files, add `--edit-format diff` so Aider sends only the changed lines instead of rewriting whole files. [Explore the full model catalog](https://fastrouter.ai/models) **Automatic Model Selection** Let FastRouter pick the best model for each request based on query complexity, domain, and cost: ```bash aider --model openai/fastrouter/auto ``` [Explore automatic model selection](https://docs.fastrouter.ai/automatic-model-selection) *** #### FAQs **Configuration & Setup** **I get `litellm.AuthenticationError: ... OPENAI_API_KEY ... must be set`. What's wrong?** The environment variables aren't set in the terminal you launched Aider from—usually because you opened a new terminal (variables don't persist across sessions) or typed the `export` lines at Aider's `>` prompt instead of in the shell. Run the two `export` commands in your terminal *before* launching Aider, confirm with `echo $OPENAI_API_KEY`, or use the `.env` file so the key is always available. **I get `litellm.BadRequestError: LLM Provider NOT provided`. What's wrong?** You omitted the `openai/` protocol prefix. The model must be `openai/`—for example, `openai/x-ai/grok-code-fast-1`, not `x-ai/grok-code-fast-1`. **Aider reports an unknown model or context-window warning. Is that a problem?** No. Aider's local model database doesn't have metadata for FastRouter slugs, so it shows *"Unknown context window size and costs"* and *"Did you mean …"* and falls back to sane defaults. Everything works—suppress the warnings with `--no-show-model-warnings`. **Can I use multiple models with the same API key?** Yes. The API key controls access and budget. Switch models with `--model` at any time—no key changes needed. **Can I restrict a key to only use specific models?** Yes. When creating or editing a key, use the **Select Models** setting to limit which models the key can access. FastRouter rejects requests to unauthorized models. **Costs & Budgeting** **Aider can send large repo context. How do I control cost?** Set a budget and rate limit on the key, and use Aider's `/tokens` command to monitor context size. The Dashboard breaks down costs by project, key, model, and tag. **Privacy & Security** **Is my code sent to FastRouter's servers?** FastRouter acts as a pass-through gateway. Requests are routed to the model provider and responses are returned to your client. Content logging can be disabled per key for sensitive workloads. See the **Disable Content Logging** option in key settings. *** #### Next Steps * [Explore the full model catalog](https://fastrouter.ai/models) * [Set up Fallback Models](https://docs.fastrouter.ai/fallback-models) for high availability * [Configure Alerts](https://docs.fastrouter.ai/alerts) for spend and performance monitoring * [Run a Free Audit](https://fastrouter.ai/audit) on your existing LLM traffic to identify savings * [Join the Discord community](https://discord.gg/QfTgEtMyyU) --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://docs.fastrouter.ai/integrations/coding-assistants/aider.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.