diff --git a/packages/astro/src/core/config/config.ts b/packages/astro/src/core/config/config.ts
index 08d48e430c54..bf9b21bbc2f0 100644
--- a/packages/astro/src/core/config/config.ts
+++ b/packages/astro/src/core/config/config.ts
@@ -17,6 +17,11 @@ import { mergeConfig } from './merge.js';
 import { validateConfig } from './validate.js';
 import { loadConfigWithVite } from './vite-load.js';
 
+/**
+ * Resolve the project root directory to an absolute path.
+ * Falls back to `process.cwd()` when no directory is given.
+ * @param cwd A relative or absolute path, or a `file://` URL.
+ */
 export function resolveRoot(cwd?: string | URL): string {
 	if (cwd instanceof URL) {
 		cwd = fileURLToPath(cwd);
diff --git a/packages/astro/src/core/config/merge.ts b/packages/astro/src/core/config/merge.ts
index e9f22a256b99..de7c48215eb6 100644
--- a/packages/astro/src/core/config/merge.ts
+++ b/packages/astro/src/core/config/merge.ts
@@ -80,6 +80,13 @@ function mergeConfigRecursively(
 	return merged;
 }
 
+/**
+ * Deep-merge two Astro config objects. Arrays are concatenated, objects are
+ * recursed, and special keys (`vite`, `server`, `markdown.processor`) receive
+ * bespoke handling so that overrides behave predictably.
+ * @param defaults The base config (typically from the user's config file).
+ * @param overrides Partial overrides (e.g. from integrations or inline config).
+ */
 export function mergeConfig<C extends AstroConfig | AstroInlineConfig>(
 	defaults: C,
 	overrides: DeepPartial<C>,
diff --git a/packages/astro/src/core/config/schemas/base.ts b/packages/astro/src/core/config/schemas/base.ts
index 68aa9e62410e..df4f0268404f 100644
--- a/packages/astro/src/core/config/schemas/base.ts
+++ b/packages/astro/src/core/config/schemas/base.ts
@@ -58,6 +58,7 @@ export type RemarkRehype = ComplexifyWithOmit<_RemarkRehype>;
 /** @lintignore */
 export type Smartypants = ComplexifyWithOmit<_Smartypants>;
 
+/** Default values for every Astro config field, used as fallbacks during validation. */
 export const ASTRO_CONFIG_DEFAULTS = {
 	root: '.',
 	srcDir: './src',
@@ -152,6 +153,7 @@ const smartypantsOptionsSchema: z.ZodType<Smartypants> = z.object({
 	quotes: z.boolean().default(true),
 });
 
+/** Base Zod schema for the full Astro user configuration. Paths are raw strings at this stage; the relative schema applies file-system transforms. */
 export const AstroConfigSchema = z.object({
 	root: z
 		.string()
@@ -610,4 +612,5 @@ export const AstroConfigSchema = z.object({
 		.prefault({}),
 });
 
+/** Inferred TypeScript type produced by parsing `AstroConfigSchema`. */
 export type AstroConfigType = z.infer<typeof AstroConfigSchema>;
diff --git a/packages/astro/src/core/config/schemas/refined.ts b/packages/astro/src/core/config/schemas/refined.ts
index 98ed6968ef4c..db67e91d455f 100644
--- a/packages/astro/src/core/config/schemas/refined.ts
+++ b/packages/astro/src/core/config/schemas/refined.ts
@@ -12,6 +12,12 @@ import {
 	validateRemotePatterns,
 } from './refined-validators.js';
 
+/**
+ * Second-pass validation schema that runs cross-field semantic checks on a
+ * fully-parsed `AstroConfig` (e.g. i18n consistency, asset prefix shape,
+ * font CSS variable format). Used after the base schema and the relative
+ * transforms have both completed.
+ */
 export const AstroConfigRefinedSchema = z.custom<AstroConfig>().superRefine((config, ctx) => {
 	let issues: ConfigValidationIssue[] = [];
 	issues = issues.concat(
diff --git a/packages/astro/src/core/config/schemas/relative.ts b/packages/astro/src/core/config/schemas/relative.ts
index c108b7d3793a..713f17290fa5 100644
--- a/packages/astro/src/core/config/schemas/relative.ts
+++ b/packages/astro/src/core/config/schemas/relative.ts
@@ -13,6 +13,15 @@ function resolveDirAsUrl(dir: string, root: string) {
 	return pathToFileURL(resolvedDir);
 }
 
+/**
+ * Extend the base `AstroConfigSchema` with path transforms that resolve
+ * directory strings (`root`, `srcDir`, `publicDir`, `outDir`, `cacheDir`,
+ * `build.client`, `build.server`) relative to a given filesystem root.
+ * Also normalizes `base` and `image.endpoint.route` according to
+ * `trailingSlash` and handles `server` as a function-or-object union.
+ * @param cmd The running Astro command (e.g. `'dev'`, `'build'`).
+ * @param fileProtocolRoot Absolute filesystem root used for path resolution.
+ */
 export function createRelativeSchema(cmd: string, fileProtocolRoot: string) {
 	let originalBuildClient: string;
 	let originalBuildServer: string;
diff --git a/packages/astro/src/core/config/settings.ts b/packages/astro/src/core/config/settings.ts
index cefb5a332d4c..4f6e83ccac65 100644
--- a/packages/astro/src/core/config/settings.ts
+++ b/packages/astro/src/core/config/settings.ts
@@ -19,6 +19,13 @@ import {
 import { AstroTimer } from './timer.js';
 import { loadTSConfig } from './tsconfig.js';
 
+/**
+ * Create an `AstroSettings` object from a fully-resolved `AstroConfig`,
+ * populating default content-entry types, client directives, and other
+ * runtime bookkeeping without loading a tsconfig.
+ * @param config Resolved Astro configuration.
+ * @param logLevel Logging verbosity for the current session.
+ */
 export function createBaseSettings(
 	config: AstroConfig,
 	logLevel: AstroInlineConfig['logLevel'],
@@ -165,6 +172,13 @@ export function createBaseSettings(
 	};
 }
 
+/**
+ * Create an `AstroSettings` object from a fully-resolved `AstroConfig`,
+ * including tsconfig loading and watch-file registration.
+ * @param config Resolved Astro configuration.
+ * @param logLevel Logging verbosity for the current session.
+ * @param cwd Optional working directory used to locate `tsconfig.json`.
+ */
 export async function createSettings(
 	config: AstroConfig,
 	logLevel: AstroInlineConfig['logLevel'],
diff --git a/packages/astro/src/core/config/tsconfig.ts b/packages/astro/src/core/config/tsconfig.ts
index 0e29495c135e..1bd2a2fa9f80 100644
--- a/packages/astro/src/core/config/tsconfig.ts
+++ b/packages/astro/src/core/config/tsconfig.ts
@@ -4,11 +4,17 @@ import { readTsconfig, type TsconfigResult } from 'get-tsconfig';
 import { parse as parseJsonc, type ParseError } from 'jsonc-parser';
 import type { CompilerOptions, TypeAcquisition } from 'typescript';
 
+/** Default tsconfig that extends Astro's base configuration. */
 export const defaultTSConfig: TSConfig = { extends: 'astro/tsconfigs/base' };
 
+/** Frameworks that require custom TypeScript compiler settings for JSX support. */
 export type frameworkWithTSSettings = 'vue' | 'react' | 'preact' | 'solid-js';
 // The following presets unfortunately cannot be inside the specific integrations, as we need
 // them even in cases where the integrations are not installed
+/**
+ * Per-framework tsconfig presets merged when a framework integration is detected.
+ * Each entry maps to compiler options needed for that framework's JSX transform.
+ */
 export const presets = new Map<frameworkWithTSSettings, TSConfig>([
 	[
 		'vue', // Settings needed for template intellisense when using Volar
@@ -47,6 +53,7 @@ export const presets = new Map<frameworkWithTSSettings, TSConfig>([
 	],
 ]);
 
+/** Result of successfully loading and resolving a tsconfig/jsconfig file. */
 export interface TSConfigLoadedResult {
 	error?: undefined;
 	/** Absolute path of the root tsconfig/jsconfig file that was loaded. */
@@ -62,6 +69,7 @@ export interface TSConfigLoadedResult {
 	sources: string[];
 }
 
+/** Discriminated union of tsconfig load outcomes: success, parse error, or missing file. */
 export type TSConfigResult =
 	| TSConfigLoadedResult
 	| { error: 'invalid-config'; message: string }
@@ -137,6 +145,12 @@ export async function loadTSConfig(root: string | undefined): Promise<TSConfigRe
 	};
 }
 
+/**
+ * Merge framework-specific TypeScript compiler options into an existing tsconfig.
+ * Returns the target unchanged if the framework has no preset.
+ * @param target The tsconfig to augment.
+ * @param framework The framework whose preset should be merged in.
+ */
 export function updateTSConfigForFramework(
 	target: TSConfig,
 	framework: frameworkWithTSSettings,
@@ -190,6 +204,7 @@ type StripEnums<T extends Record<string, any>> = {
 						: any;
 };
 
+/** Subset of `tsconfig.json` fields that Astro reads and manipulates. */
 export interface TSConfig {
 	compilerOptions?: StripEnums<CompilerOptions>;
 	compileOnSave?: boolean;
diff --git a/packages/astro/src/core/config/vite-load.ts b/packages/astro/src/core/config/vite-load.ts
index 91540a09475e..0785a3e620d6 100644
--- a/packages/astro/src/core/config/vite-load.ts
+++ b/packages/astro/src/core/config/vite-load.ts
@@ -12,6 +12,14 @@ interface LoadConfigWithViteOptions {
 	fs: typeof fsType;
 }
 
+/**
+ * Load an Astro config file using Vite's module runner as a fallback when
+ * native Node.js `import()` fails (e.g. for `.ts` files). Closes the
+ * temporary Vite dev server after loading.
+ * @param options.root Project root directory.
+ * @param options.configPath Absolute path to the config file.
+ * @param options.fs Filesystem module to use for file operations.
+ */
 export async function loadConfigWithVite({
 	configPath,
 	fs,