diff --git a/CHANGELOG.md b/CHANGELOG.md
index 98912f2..a56f1de 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,6 +10,7 @@ Tagged releases are published to npm from GitHub Actions when a **GitHub Release
 
 ### Added
 
+- `UrlGeneratorFn` type alias (same as `UrlGenerator`) and `RegisterBuiltinUrlGeneratorsOptions` with `reinstallBuiltins` on `registerBuiltinUrlGenerators()` to restore default `mailing` / `slack-Cpplang` generators after overrides; README “Custom URL generators” section and tests for custom registration and built-in override.
 - Zod `toolErrorSchema` and exported types `ToolError` / `ToolErrorCode` for parsing MCP tool failures; all tools now return this JSON shape in the text content when `isError` is true.
 - `validateMetadataFilterDetailed()` returns `{ message, field }` for invalid filters; `validateMetadataFilter()` remains a string-only wrapper for backward compatibility.
 - `.coderabbit.yaml` sets the pre-merge **docstring coverage** threshold to **79%** (default **80%**) so marginal documentation-only gaps do not block merges; adjust upward as coverage improves.
diff --git a/README.md b/README.md
index 228eb3e..f4ca377 100644
--- a/README.md
+++ b/README.md
@@ -102,6 +102,34 @@ Run `pinecone-read-only-mcp --help` for CLI equivalents (`--cache-ttl-seconds`,
 
 The server uses **process-global** memory for the suggest-flow gate (`suggest_query_params` context), namespaces cache, URL generator registry, and active configuration. **Stdio MCP (one client per Node process)** matches this model. If you embed `setupServer` behind a multi-tenant HTTP transport, isolate those structures per session yourself or treat the suggest-flow guard as best-effort only.
 
+### Custom URL generators
+
+Namespaces other than `mailing` and `slack-Cpplang` (or different URL rules for any namespace) can use programmatic registration — no fork required.
+
+Import `registerUrlGenerator` and types `UrlGeneratorFn` / `UrlGenerationResult` from `@will-cppa/pinecone-read-only-mcp`. Register **additional** namespaces before tools that emit URLs run (typically right after `setupServer` resolves config). To **replace** the built-in `mailing` or `slack-Cpplang` generators, call `registerUrlGenerator` **after** `setupServer`, because `setupServer` installs the defaults first.
+
+```ts
+import {
+  registerUrlGenerator,
+  setupServer,
+  type UrlGenerationResult,
+  type UrlGeneratorFn,
+} from '@will-cppa/pinecone-read-only-mcp';
+
+const server = await setupServer(config);
+
+const myDocs: UrlGeneratorFn = (metadata): UrlGenerationResult => {
+  const id = typeof metadata.doc_id === 'string' ? metadata.doc_id : null;
+  return id
+    ? { url: `https://docs.example.com/${id}`, method: 'generated.custom' }
+    : { url: null, method: 'unavailable', reason: 'doc_id missing' };
+};
+
+registerUrlGenerator('product-docs', myDocs);
+```
+
+A fuller embedding sample lives in [examples/custom-url-generator.ts](examples/custom-url-generator.ts).
+
 ### Claude Desktop Configuration
 
 Add to your `claude_desktop_config.json`:
diff --git a/src/server.ts b/src/server.ts
index 7badaac..6ed356c 100644
--- a/src/server.ts
+++ b/src/server.ts
@@ -9,7 +9,7 @@
  * - {@link PineconeClient} — hybrid search, count, namespace listing, etc.
  * - {@link resolveConfig} — merge CLI-style overrides with `process.env`.
  * - {@link setPineconeClient} — inject a client instance before `setupServer()`.
- * - {@link registerUrlGenerator} / {@link unregisterUrlGenerator} — extend URL synthesis.
+ * - {@link registerUrlGenerator} / {@link unregisterUrlGenerator} — extend URL synthesis (`UrlGeneratorFn`).
  * - {@link toolErrorSchema} / {@link ToolError} — parse MCP tool failures (`isError: true` JSON bodies).
  * - Built-in `mailing` / `slack-Cpplang` URL generators are registered from {@link setupServer}
  *   via {@link registerBuiltinUrlGenerators}; call it yourself if you use the library without `setupServer`.
@@ -53,7 +53,12 @@ export {
   hasUrlGenerator,
   registerBuiltinUrlGenerators,
 } from './server/url-generation.js';
-export type { UrlGenerationResult, UrlGenerator } from './server/url-generation.js';
+export type {
+  UrlGenerationResult,
+  UrlGenerator,
+  UrlGeneratorFn,
+  RegisterBuiltinUrlGeneratorsOptions,
+} from './server/url-generation.js';
 /** Build {@link ServerConfig} from CLI overrides + environment variables. */
 export { resolveConfig } from './config.js';
 export type { ServerConfig, LogLevel, LogFormat, ConfigOverrides } from './config.js';
diff --git a/src/server/url-generation.test.ts b/src/server/url-generation.test.ts
index 716df20..c6269f9 100644
--- a/src/server/url-generation.test.ts
+++ b/src/server/url-generation.test.ts
@@ -1,5 +1,11 @@
-import { describe, expect, it, beforeAll } from 'vitest';
-import { generateUrlForNamespace, registerBuiltinUrlGenerators } from './url-generation.js';
+import { describe, expect, it, beforeAll, afterEach } from 'vitest';
+import type { UrlGeneratorFn } from './url-generation.js';
+import {
+  generateUrlForNamespace,
+  registerBuiltinUrlGenerators,
+  registerUrlGenerator,
+  unregisterUrlGenerator,
+} from './url-generation.js';
 
 beforeAll(() => {
   registerBuiltinUrlGenerators();
@@ -105,3 +111,35 @@ describe('generateUrlForNamespace', () => {
     expect(r.method).toBe('unavailable');
   });
 });
+
+describe('registerUrlGenerator', () => {
+  const customNs = 'acme-docs';
+
+  afterEach(() => {
+    unregisterUrlGenerator(customNs);
+    registerBuiltinUrlGenerators({ reinstallBuiltins: true });
+  });
+
+  it('registers a custom generator for a new namespace', () => {
+    const fn: UrlGeneratorFn = () => ({
+      url: 'https://example.com/doc/1',
+      method: 'generated.custom',
+    });
+    registerUrlGenerator(customNs, fn);
+    const r = generateUrlForNamespace(customNs, {});
+    expect(r.url).toBe('https://example.com/doc/1');
+    expect(r.method).toBe('generated.custom');
+  });
+
+  it('allows a custom generator to override the mailing built-in', () => {
+    registerUrlGenerator('mailing', () => ({
+      url: 'https://override.example/mailing',
+      method: 'generated.custom',
+    }));
+    const r = generateUrlForNamespace('mailing', {
+      doc_id: 'boost-announce@lists.boost.org/message/O5VYCDZADVDHK5Z5LAYJBHMDOAFQL7P6',
+    });
+    expect(r.url).toBe('https://override.example/mailing');
+    expect(r.method).toBe('generated.custom');
+  });
+});
diff --git a/src/server/url-generation.ts b/src/server/url-generation.ts
index b4e20a1..8acbd11 100644
--- a/src/server/url-generation.ts
+++ b/src/server/url-generation.ts
@@ -28,8 +28,14 @@ export type UrlGenerationResult = {
  */
 export type UrlGenerator = (metadata: Record<string, unknown>) => UrlGenerationResult;
 
+/**
+ * Alias for {@link UrlGenerator} (issue / API naming: `UrlGeneratorFn`).
+ * Use either type when implementing custom URL synthesis.
+ */
+export type UrlGeneratorFn = UrlGenerator;
+
 /** Registry of namespace -> URL generator. Built-ins register via {@link registerBuiltinUrlGenerators}. */
-const urlGenerators = new Map<string, UrlGenerator>();
+const urlGenerators = new Map<string, UrlGeneratorFn>();
 
 /** Return a trimmed non-empty string or null for empty/missing values. */
 function asString(value: unknown): string | null {
@@ -94,12 +100,35 @@ function generatorSlackCpplang(metadata: Record<string, unknown>): UrlGeneration
 
 let builtinGeneratorsRegistered = false;
 
+/** Options for {@link registerBuiltinUrlGenerators}. */
+export type RegisterBuiltinUrlGeneratorsOptions = {
+  /**
+   * When `true`, re-applies the built-in `mailing` and `slack-Cpplang` generators
+   * even if they were already registered or were replaced by {@link registerUrlGenerator}.
+   * Use to restore defaults after an override (e.g. in tests).
+   */
+  reinstallBuiltins?: boolean;
+};
+
 /**
- * Register built-in generators (`mailing`, `slack-Cpplang`). Idempotent.
+ * Register built-in generators (`mailing`, `slack-Cpplang`).
+ *
+ * With no options (or `reinstallBuiltins` omitted / `false`), the call is idempotent:
+ * only the first invocation in the process installs the two built-ins.
+ *
+ * With `{ reinstallBuiltins: true }`, always resets `mailing` and `slack-Cpplang` to
+ * the library implementations (does not remove other custom namespaces).
+ *
  * Invoked from {@link setupServer} so embedders get the same defaults as the CLI;
  * pure library use without calling `setupServer` should register explicitly if needed.
  */
-export function registerBuiltinUrlGenerators(): void {
+export function registerBuiltinUrlGenerators(options?: RegisterBuiltinUrlGeneratorsOptions): void {
+  if (options?.reinstallBuiltins) {
+    urlGenerators.set('mailing', generatorMailing);
+    urlGenerators.set('slack-Cpplang', generatorSlackCpplang);
+    builtinGeneratorsRegistered = true;
+    return;
+  }
   if (builtinGeneratorsRegistered) return;
   urlGenerators.set('mailing', generatorMailing);
   urlGenerators.set('slack-Cpplang', generatorSlackCpplang);
@@ -110,7 +139,7 @@ export function registerBuiltinUrlGenerators(): void {
  * Register a URL generator for a namespace, replacing any existing entry.
  *
  * @param namespace exact namespace name (matches the value returned by `list_namespaces`).
- * @param generator function that turns a record's metadata into a URL.
+ * @param generator function that turns a record's metadata into a URL ({@link UrlGeneratorFn}).
  *
  * @example
  * ```ts
@@ -124,8 +153,15 @@ export function registerBuiltinUrlGenerators(): void {
  * });
  * ```
  */
-export function registerUrlGenerator(namespace: string, generator: UrlGenerator): void {
-  urlGenerators.set(namespace, generator);
+export function registerUrlGenerator(namespace: string, generator: UrlGeneratorFn): void {
+  const normalizedNamespace = namespace.trim();
+  if (normalizedNamespace.length === 0) {
+    throw new TypeError('namespace must be a non-empty string');
+  }
+  if (typeof generator !== 'function') {
+    throw new TypeError('generator must be a function');
+  }
+  urlGenerators.set(normalizedNamespace, generator);
 }
 
 /** Remove a namespace's URL generator. Returns true if a generator was removed. */