diff --git a/packages/openapi-generator/src/cli.ts b/packages/openapi-generator/src/cli.ts
index 4decd888..0437538c 100644
--- a/packages/openapi-generator/src/cli.ts
+++ b/packages/openapi-generator/src/cli.ts
@@ -1,6 +1,15 @@
 #!/usr/bin/env node
 
-import { command, run, option, string, flag, boolean, positional } from 'cmd-ts';
+import {
+  command,
+  run,
+  option,
+  string,
+  optional,
+  flag,
+  boolean,
+  positional,
+} from 'cmd-ts';
 import * as E from 'fp-ts/Either';
 import * as fs from 'fs';
 import * as p from 'path';
@@ -11,6 +20,7 @@ import { convertRoutesToOpenAPI } from './openapi';
 import type { Route } from './route';
 import type { Schema } from './ir';
 import { Project } from './project';
+import { KNOWN_IMPORTS } from './knownImports';
 
 const app = command({
   name: 'api-ts',
@@ -57,11 +67,30 @@ const app = command({
       short: 'i',
       defaultValue: () => false,
     }),
+    codecFile: option({
+      type: optional(string),
+      description: 'Custom codec definition file',
+      long: 'codec-file',
+      short: 'c',
+      defaultValue: () => undefined,
+    }),
   },
-  handler: async ({ input, name, version }) => {
+  handler: async ({ input, name, version, codecFile }) => {
     const filePath = p.resolve(input);
 
-    const project = await new Project().parseEntryPoint(filePath);
+    let knownImports = KNOWN_IMPORTS;
+    if (codecFile !== undefined) {
+      const codecFilePath = p.resolve(codecFile);
+      const codecModule = await import(codecFilePath);
+      if (codecModule.default === undefined) {
+        console.error(`Could not find default export in ${codecFilePath}`);
+        process.exit(1);
+      }
+      const customCodecs = codecModule.default(E);
+      knownImports = { ...knownImports, ...customCodecs };
+    }
+
+    const project = await new Project({}, knownImports).parseEntryPoint(filePath);
     if (E.isLeft(project)) {
       console.error(project.left);
       process.exit(1);
diff --git a/packages/openapi-generator/src/codec.ts b/packages/openapi-generator/src/codec.ts
index 9cb46d14..9327208f 100644
--- a/packages/openapi-generator/src/codec.ts
+++ b/packages/openapi-generator/src/codec.ts
@@ -9,7 +9,7 @@ import type { Project } from './project';
 import { findSymbolInitializer, resolveLiteralOrIdentifier } from './resolveInit';
 import type { SourceFile } from './sourceFile';
 
-import { KNOWN_IMPORTS, type KnownCodec } from './knownImports';
+import type { KnownCodec } from './knownImports';
 
 type ResolvedRef = { type: 'ref'; name: string; location: string };
 
@@ -32,7 +32,7 @@ function codecIdentifier(
     } else if (imp.type === 'star') {
       return E.left(`Tried to use star import as codec ${id.value}`);
     }
-    const knownImport = KNOWN_IMPORTS[imp.from]?.[imp.importedName];
+    const knownImport = project.knownImports[imp.from]?.[imp.importedName];
     if (knownImport !== undefined) {
       return E.right({ type: 'codec', schema: knownImport });
     }
@@ -67,7 +67,7 @@ function codecIdentifier(
     }
 
     const name = id.property.value;
-    const knownImport = KNOWN_IMPORTS[objectSym.from]?.[name];
+    const knownImport = project.knownImports[objectSym.from]?.[name];
     if (knownImport !== undefined) {
       return E.right({ type: 'codec', schema: knownImport });
     }
diff --git a/packages/openapi-generator/src/project.ts b/packages/openapi-generator/src/project.ts
index cff295b9..c40f2d61 100644
--- a/packages/openapi-generator/src/project.ts
+++ b/packages/openapi-generator/src/project.ts
@@ -4,15 +4,19 @@ import { promisify } from 'util';
 import * as E from 'fp-ts/Either';
 import resolve from 'resolve';
 
+import { KNOWN_IMPORTS, type KnownCodec } from './knownImports';
 import { parseSource, type SourceFile } from './sourceFile';
 
 const readFile = promisify(fs.readFile);
 
 export class Project {
+  readonly knownImports: Record<string, Record<string, KnownCodec>>;
+
   private files: Record<string, SourceFile>;
 
-  constructor(files: Record<string, SourceFile> = {}) {
+  constructor(files: Record<string, SourceFile> = {}, knownImports = KNOWN_IMPORTS) {
     this.files = files;
+    this.knownImports = knownImports;
   }
 
   add(path: string, sourceFile: SourceFile): void {