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docs/foundation/data/validation/src/utils/coerce.md

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+# Coerce Utilities
+
+## Purpose
+
+The coerce utilities allow schemas to accept a broader range of input types by converting them before validation. They bridge the gap between raw HTTP parameters (always strings), form data, and strongly-typed schema expectations. Each coerce function is pure and never throws — failures return `null`. The higher-order `coerce(schema)` wrapper applies the appropriate coercion strategy for string inputs and falls back to the raw schema if no coercion path works.
+
+## Features
+
+- `coerceString(input)` — always succeeds via `String(input)`
+- `coerceNumber(input)` — via `Number(input)`, returns `null` for NaN
+- `coerceBoolean(input)` — handles `"true"/"false"/"1"/"0"/1/0/true/false`
+- `coerceDate(input)` — converts ISO strings and timestamps to `Date`
+- `coerce(schema)` — wraps any schema with automatic string coercion
+
+## Usage: Basic Example
+
+```typescript
+import { coerce, number } from "@suaveplan/validation";
+
+const ageSchema = coerce(number().int().min(0));
+ageSchema.parse("25"); // 25 (string coerced to number)
+ageSchema.parse(25); // 25 (passthrough)
+```
+
+## Usage: Advanced Example
+
+```typescript
+import { coerce, boolean, date, object } from "@suaveplan/validation";
+
+// Query parameter parsing (all values arrive as strings)
+const QueryParams = object({
+  active: coerce(boolean()),
+  since: coerce(date()),
+  limit: coerce(number().int().max(100)),
+});
+
+QueryParams.parse({
+  active: "true",
+  since: "2024-01-01",
+  limit: "50",
+});
+```
+
+## API Reference
+
+### `coerceString(input: unknown): string`
+
+Calls `String(input)`. Always succeeds; never returns null.
+
+### `coerceNumber(input: unknown): number | null`
+
+Calls `Number(input)`. Returns `null` if the result is `NaN`.
+
+### `coerceBoolean(input: unknown): boolean | null`
+
+Recognises `true/false` (booleans passthrough), `"true"/"1"/1` → `true`, `"false"/"0"/0` → `false`. Returns `null` for unrecognised input.
+
+### `coerceDate(input: unknown): Date | null`
+
+Passes through existing `Date` instances. Converts strings and numbers via `new Date(input)`. Returns `null` if the result is an invalid date.
+
+### `coerce<T>(schema: BaseSchema<unknown, T>): CoercedSchema<T>`
+
+Returns a `CoerceWrapperSchema` that attempts string coercions in order (number → boolean → date) for string inputs, falling back to the raw schema if none succeed.
+
+### `type CoercedSchema<T>`
+
+Alias for `BaseSchema<unknown, T>`.
+
+## Implementation Notes
+
+- `coerce(schema)` clears `ctx.issues` between coercion attempts so that intermediate failures don't pollute the final error list.
+- Coercion order for string inputs is: numeric → boolean → date → raw. The first successful conversion is used.
+- For non-string inputs, `coerce(schema)` delegates directly to the inner schema with no transformation.