diff --git a/CHANGELOG.md b/CHANGELOG.md
index 89464c4..3453ca8 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- `strictPrefix` option on `createId()` and `createSortableId()`. When `true`, the prefix must be 1-63 ASCII letters or digits, otherwise a `TypeError` names the first offending character. Off by default, so existing behavior is unchanged.
 - `entropyBits(options?)`, `sizeForEntropy(bits, alphabet?)` and `collisionProbability(count, options?)` compute the entropy and birthday-collision probability of an ID configuration. `collisionProbability` works in log space with `Math.expm1`, so realistic inputs do not round to 0.
 
 ## [1.1.2] - 2026-10-09
diff --git a/README.md b/README.md
index 70eae47..5c7def5 100644
--- a/README.md
+++ b/README.md
@@ -57,6 +57,20 @@ const eventId = createSortableId({ alphabet: BASE32_CROCKFORD });
 eventId("evt"); // => "evt_00VQ5A1K0MBJGJFX6PWYY6WKY" — no 0/O or 1/l confusion
 ```
 
+## Strict prefixes
+
+By default any non-empty prefix without the separator is accepted. Pass
+`strictPrefix: true` to `createId` or `createSortableId` to require 1-63 ASCII
+letters or digits, so a stray space or typo can't end up in your database:
+
+```ts
+import { createId } from "prefid";
+
+const id = createId({ strictPrefix: true });
+id("user"); // => "user_a8Kd0f2bQ1nR7pZ3xW4mT6y"
+id("us er"); // throws TypeError: prefid: prefix "us er" contains an invalid character " " at index 2.
+```
+
 ## Install
 
 ```bash
diff --git a/src/generators/generate.ts b/src/generators/generate.ts
index 8d3823c..d181b9d 100644
--- a/src/generators/generate.ts
+++ b/src/generators/generate.ts
@@ -14,6 +14,7 @@ export function createId<S extends string = "_">(
   const size = defaults.size ?? DEFAULT_SIZE;
   const separator = defaults.separator ?? DEFAULT_SEPARATOR;
   const alphabet = defaults.alphabet ?? DEFAULT_ALPHABET;
+  const strictPrefix = defaults.strictPrefix ?? false;
 
   if (!Number.isInteger(size) || size < 1 || size > MAX_SIZE) {
     throw new RangeError(
@@ -30,7 +31,7 @@ export function createId<S extends string = "_">(
   }
 
   return function id<P extends string>(prefix: P): PrefixedId<P, S> {
-    assertValidPrefix(prefix, separator);
+    assertValidPrefix(prefix, separator, strictPrefix);
     return `${prefix}${separator}${randomString(alphabet, size)}` as PrefixedId<
       P,
       S
diff --git a/src/generators/sortable.ts b/src/generators/sortable.ts
index eb119de..bb516ea 100644
--- a/src/generators/sortable.ts
+++ b/src/generators/sortable.ts
@@ -16,6 +16,7 @@ export interface SortableIdOptions {
   randomSize?: number;
   timestampSize?: number;
   monotonic?: boolean;
+  strictPrefix?: boolean;
   now?: () => number;
 }
 
@@ -106,6 +107,7 @@ export function createSortableId<S extends string = "_">(
   const separator = options.separator ?? DEFAULT_SEPARATOR;
   const alphabet = options.alphabet ?? DEFAULT_ALPHABET;
   const monotonic = options.monotonic ?? true;
+  const strictPrefix = options.strictPrefix ?? false;
   const clock = options.now ?? Date.now;
   const randomSize = options.randomSize ?? DEFAULT_SORTABLE_RANDOM_SIZE;
 
@@ -131,7 +133,7 @@ export function createSortableId<S extends string = "_">(
   let lastRandom: number[] = [];
 
   return function sortableId<P extends string>(prefix: P): PrefixedId<P, S> {
-    assertValidPrefix(prefix, separator);
+    assertValidPrefix(prefix, separator, strictPrefix);
 
     const reading = clock();
     if (typeof reading !== "number" || !Number.isFinite(reading)) {
diff --git a/src/internal/prefix.ts b/src/internal/prefix.ts
index 6c49c8b..7332b5e 100644
--- a/src/internal/prefix.ts
+++ b/src/internal/prefix.ts
@@ -1,4 +1,11 @@
-export function assertValidPrefix(prefix: string, separator: string): void {
+const STRICT_MAX_LENGTH = 63;
+const NON_ALPHANUMERIC = /[^A-Za-z0-9]/;
+
+export function assertValidPrefix(
+  prefix: string,
+  separator: string,
+  strict: boolean,
+): void {
   if (typeof prefix !== "string" || prefix.length === 0) {
     throw new TypeError("prefid: prefix must be a non-empty string.");
   }
@@ -7,4 +14,20 @@ export function assertValidPrefix(prefix: string, separator: string): void {
       `prefid: prefix "${prefix}" must not contain the separator "${separator}".`,
     );
   }
+  if (strict) {
+    const bad = NON_ALPHANUMERIC.exec(prefix);
+    if (bad) {
+      const char = String.fromCodePoint(
+        prefix.codePointAt(bad.index) as number,
+      );
+      throw new TypeError(
+        `prefid: prefix "${prefix}" contains an invalid character "${char}" at index ${bad.index}.`,
+      );
+    }
+    if (prefix.length > STRICT_MAX_LENGTH) {
+      throw new TypeError(
+        `prefid: prefix is ${prefix.length} characters long; strict mode allows at most ${STRICT_MAX_LENGTH}.`,
+      );
+    }
+  }
 }
diff --git a/src/types/index.ts b/src/types/index.ts
index d3ac1b7..747089b 100644
--- a/src/types/index.ts
+++ b/src/types/index.ts
@@ -7,6 +7,7 @@ export interface IdOptions {
   size?: number;
   separator?: string;
   alphabet?: string;
+  strictPrefix?: boolean;
 }
 
 export type IdGenerator<S extends string = "_"> = <P extends string>(
diff --git a/test/generate.test.ts b/test/generate.test.ts
index c83f306..4e64ccb 100644
--- a/test/generate.test.ts
+++ b/test/generate.test.ts
@@ -85,3 +85,41 @@ describe("createId()", () => {
     expect(() => createId({ separator: "" })).toThrow(TypeError);
   });
 });
+
+describe("strictPrefix", () => {
+  const strict = createId({ strictPrefix: true });
+
+  it("accepts alphanumeric prefixes", () => {
+    expect(strict("user2").startsWith("user2_")).toBe(true);
+  });
+
+  it("rejects a space and names the character and index", () => {
+    expect(() => strict("us er")).toThrow(
+      'prefid: prefix "us er" contains an invalid character " " at index 2.',
+    );
+  });
+
+  it("rejects punctuation", () => {
+    expect(() => strict("us-er")).toThrow(TypeError);
+  });
+
+  it("rejects non-ASCII characters", () => {
+    expect(() => strict("usér")).toThrow(/invalid character "é" at index 2/);
+  });
+
+  it("names a whole emoji, not half of it", () => {
+    expect(() => strict("ab😀")).toThrow(/invalid character "😀" at index 2/);
+  });
+
+  it("allows 63 characters and rejects 64", () => {
+    expect(() => strict("a".repeat(63))).not.toThrow();
+    expect(() => strict("a".repeat(64))).toThrow(
+      "prefid: prefix is 64 characters long; strict mode allows at most 63.",
+    );
+  });
+
+  it("leaves default mode unchanged", () => {
+    expect(id("us er!ü").startsWith("us er!ü_")).toBe(true);
+    expect(createId()("a".repeat(64)).startsWith("a".repeat(64))).toBe(true);
+  });
+});
diff --git a/test/sortable.test.ts b/test/sortable.test.ts
index f29c47d..e168310 100644
--- a/test/sortable.test.ts
+++ b/test/sortable.test.ts
@@ -375,3 +375,20 @@ describe("getDate()", () => {
     expect(getDate(at(MAX_DATE_MS))).toBeUndefined();
   });
 });
+
+describe("createSortableId strictPrefix", () => {
+  const strict = createSortableId({ strictPrefix: true });
+
+  it("accepts alphanumeric prefixes", () => {
+    expect(strict("evt").startsWith("evt_")).toBe(true);
+  });
+
+  it("rejects invalid characters and over-long prefixes", () => {
+    expect(() => strict("e vt")).toThrow(/invalid character " " at index 1/);
+    expect(() => strict("a".repeat(64))).toThrow(TypeError);
+  });
+
+  it("leaves default mode unchanged", () => {
+    expect(createSortableId()("e vt").startsWith("e vt_")).toBe(true);
+  });
+});