feat(cli): Documentation for model configs. (#12967)

Author: joshualitt

docs/cli/generation-settings.md

@@ -0,0 +1,210 @@
+# Advanced Model Configuration
+
+This guide details the Model Configuration system within the Gemini CLI.
+Designed for researchers, AI quality engineers, and advanced users, this system
+provides a rigorous framework for managing generative model hyperparameters and
+behaviors.
+
+> **Warning**: This is a power-user feature. Configuration values are passed
+> directly to the model provider with minimal validation. Incorrect settings
+> (e.g., incompatible parameter combinations) may result in runtime errors from
+> the API.
+
+## 1. System Overview
+
+The Model Configuration system (`ModelConfigService`) enables deterministic
+control over model generation. It decouples the requested model identifier
+(e.g., a CLI flag or agent request) from the underlying API configuration. This
+allows for:
+
+- **Precise Hyperparameter Tuning**: Direct control over `temperature`, `topP`,
+  `thinkingBudget`, and other SDK-level parameters.
+- **Environment-Specific Behavior**: Distinct configurations for different
+  operating contexts (e.g., testing vs. production).
+- **Agent-Scoped Customization**: Applying specific settings only when a
+  particular agent is active.
+
+The system operates on two core primitives: **Aliases** and **Overrides**.
+
+## 2. Configuration Primitives
+
+These settings are located under the `modelConfigs` key in your configuration
+file.
+
+### Aliases (`customAliases`)
+
+Aliases are named, reusable configuration presets. Users should define their own
+aliases (or override system defaults) in the `customAliases` map.
+
+- **Inheritance**: An alias can `extends` another alias (including system
+  defaults like `chat-base`), inheriting its `modelConfig`. Child aliases can
+  overwrite or augment inherited settings.
+- **Abstract Aliases**: An alias is not required to specify a concrete `model`
+  if it serves purely as a base for other aliases.
+
+**Example Hierarchy**:
+
+```json
+"modelConfigs": {
+  "customAliases": {
+    "base": {
+      "modelConfig": {
+        "generateContentConfig": { "temperature": 0.0 }
+      }
+    },
+    "chat-base": {
+      "extends": "base",
+      "modelConfig": {
+        "generateContentConfig": { "temperature": 0.7 }
+      }
+    }
+  }
+}
+```
+
+### Overrides (`overrides`)
+
+Overrides are conditional rules that inject configuration based on the runtime
+context. They are evaluated dynamically for each model request.
+
+- **Match Criteria**: Overrides apply when the request context matches the
+  specified `match` properties.
+  - `model`: Matches the requested model name or alias.
+  - `overrideScope`: Matches the distinct scope of the request (typically the
+    agent name, e.g., `codebaseInvestigator`).
+
+**Example Override**:
+
+```json
+"modelConfigs": {
+  "overrides": [
+    {
+      "match": {
+        "overrideScope": "codebaseInvestigator"
+      },
+      "modelConfig": {
+        "generateContentConfig": { "temperature": 0.1 }
+      }
+    }
+  ]
+}
+```
+
+## 3. Resolution Strategy
+
+The `ModelConfigService` resolves the final configuration through a two-step
+process:
+
+### Step 1: Alias Resolution
+
+The requested model string is looked up in the merged map of system `aliases`
+and user `customAliases`.
+
+1.  If found, the system recursively resolves the `extends` chain.
+2.  Settings are merged from parent to child (child wins).
+3.  This results in a base `ResolvedModelConfig`.
+4.  If not found, the requested string is treated as the raw model name.
+
+### Step 2: Override Application
+
+The system evaluates the `overrides` list against the request context (`model`
+and `overrideScope`).
+
+1.  **Filtering**: All matching overrides are identified.
+2.  **Sorting**: Matches are prioritized by **specificity** (the number of
+    matched keys in the `match` object).
+    - Specific matches (e.g., `model` + `overrideScope`) override broad matches
+      (e.g., `model` only).
+    - Tie-breaking: If specificity is equal, the order of definition in the
+      `overrides` array is preserved (last one wins).
+3.  **Merging**: The configurations from the sorted overrides are merged
+    sequentially onto the base configuration.
+
+## 4. Configuration Reference
+
+The configuration follows the `ModelConfigServiceConfig` interface.
+
+### `ModelConfig` Object
+
+Defines the actual parameters for the model.
+
+| Property                | Type     | Description                                                        |
+| :---------------------- | :------- | :----------------------------------------------------------------- |
+| `model`                 | `string` | The identifier of the model to be called (e.g., `gemini-2.5-pro`). |
+| `generateContentConfig` | `object` | The configuration object passed to the `@google/genai` SDK.        |
+
+### `GenerateContentConfig` (Common Parameters)
+
+Directly maps to the SDK's `GenerateContentConfig`. Common parameters include:
+
+- **`temperature`**: (`number`) Controls output randomness. Lower values (0.0)
+  are deterministic; higher values (>0.7) are creative.
+- **`topP`**: (`number`) Nucleus sampling probability.
+- **`maxOutputTokens`**: (`number`) Limit on generated response length.
+- **`thinkingConfig`**: (`object`) Configuration for models with reasoning
+  capabilities (e.g., `thinkingBudget`, `includeThoughts`).
+
+## 5. Practical Examples
+
+### Defining a Deterministic Baseline
+
+Create an alias for tasks requiring high precision, extending the standard chat
+configuration but enforcing zero temperature.
+
+```json
+"modelConfigs": {
+  "customAliases": {
+    "precise-mode": {
+      "extends": "chat-base",
+      "modelConfig": {
+        "generateContentConfig": {
+          "temperature": 0.0,
+          "topP": 1.0
+        }
+      }
+    }
+  }
+}
+```
+
+### Agent-Specific Parameter Injection
+
+Enforce extended thinking budgets for a specific agent without altering the
+global default, e.g. for the `codebaseInvestigator`.
+
+```json
+"modelConfigs": {
+  "overrides": [
+    {
+      "match": {
+        "overrideScope": "codebaseInvestigator"
+      },
+      "modelConfig": {
+        "generateContentConfig": {
+          "thinkingConfig": { "thinkingBudget": 4096 }
+        }
+      }
+    }
+  ]
+}
+```
+
+### Experimental Model Evaluation
+
+Route traffic for a specific alias to a preview model for A/B testing, without
+changing client code.
+
+```json
+"modelConfigs": {
+  "overrides": [
+    {
+      "match": {
+        "model": "gemini-2.5-pro"
+      },
+      "modelConfig": {
+        "model": "gemini-2.5-pro-experimental-001"
+      }
+    }
+  ]
+}
+```