diff --git a/packages/coding-agent/CHANGELOG.md b/packages/coding-agent/CHANGELOG.md
index 9c2395e3d..76bfeb815 100644
--- a/packages/coding-agent/CHANGELOG.md
+++ b/packages/coding-agent/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Fixed legacy extension plugin validation failing with `Export named 'calculateCost' not found in module '.../legacy-pi-ai-shim.ts'` when the extension imports `calculateCost` (or `modelsAreEqual` / `getBundledProviders`) from `@oh-my-pi/pi-ai`. Those symbols were relocated to `@oh-my-pi/pi-catalog/models` during the catalog split but were never bridged back through the legacy `pi-ai` root shim; the shim now re-exports them alongside the existing `getModel` / `getModels` aliases so plugins written against pre-split pi-ai load again ([#4584](https://github.com/can1357/oh-my-pi/issues/4584)).
+
 ## [16.3.6] - 2026-07-04
 
 ### Changed
diff --git a/packages/coding-agent/src/extensibility/legacy-pi-ai-shim.ts b/packages/coding-agent/src/extensibility/legacy-pi-ai-shim.ts
index 22f484992..e8aa49837 100644
--- a/packages/coding-agent/src/extensibility/legacy-pi-ai-shim.ts
+++ b/packages/coding-agent/src/extensibility/legacy-pi-ai-shim.ts
@@ -19,7 +19,13 @@
  * `types.ts` via the `export *` below — pi-ai still exports both as types,
  * only the runtime `Type` builder and `StringEnum()` helper were removed.
  */
-import { getBundledModel, getBundledModels } from "@oh-my-pi/pi-catalog/models";
+import {
+	calculateCost,
+	getBundledModel,
+	getBundledModels,
+	getBundledProviders,
+	modelsAreEqual,
+} from "@oh-my-pi/pi-catalog/models";
 import { type TSchema, Type } from "./typebox";
 
 export interface StringEnumOptions<T extends string> {
@@ -66,8 +72,15 @@ export function StringEnum<T extends string | number>(
 }
 
 export * from "@oh-my-pi/pi-ai";
-export { Type };
-
-/** Compatibility aliases for renamed catalog functions */
+/**
+ * Compatibility re-exports for catalog symbols that pi-ai historically exposed
+ * from its own barrel prior to the `refactor(catalog)!: split model catalog
+ * from pi-ai` change. Legacy extensions still import these from the pi-ai
+ * root, so the shim bridges them through to their new home in
+ * `@oh-my-pi/pi-catalog/models`. `getModel`/`getModels` are the historical
+ * pi-ai names for `getBundledModel`/`getBundledModels`; the remaining symbols
+ * kept their names across the move.
+ */
+export { calculateCost, getBundledProviders, modelsAreEqual, Type };
 export const getModel = getBundledModel;
 export const getModels = getBundledModels;
diff --git a/packages/coding-agent/test/extensibility/legacy-pi-ai-type-remap.test.ts b/packages/coding-agent/test/extensibility/legacy-pi-ai-type-remap.test.ts
index d27295d68..30661b82d 100644
--- a/packages/coding-agent/test/extensibility/legacy-pi-ai-type-remap.test.ts
+++ b/packages/coding-agent/test/extensibility/legacy-pi-ai-type-remap.test.ts
@@ -3,7 +3,13 @@ import * as fs from "node:fs/promises";
 import * as os from "node:os";
 import * as path from "node:path";
 import * as url from "node:url";
-import { getBundledModel, getBundledModels } from "@oh-my-pi/pi-catalog/models";
+import {
+	calculateCost,
+	getBundledModel,
+	getBundledModels,
+	getBundledProviders,
+	modelsAreEqual,
+} from "@oh-my-pi/pi-catalog/models";
 import {
 	__resetLegacyPiResolutionCache,
 	installLegacyPiSpecifierShim,
@@ -124,6 +130,35 @@ describe("legacy-pi @(scope)/pi-ai root `Type` remap (issue #1437)", () => {
 		expect(loaded.testGetModels).toBe(getBundledModels);
 	});
 
+	it("re-exports calculateCost from @oh-my-pi/pi-catalog/models (issue #4584)", async () => {
+		// `calculateCost` was moved from the `@oh-my-pi/pi-ai` barrel to
+		// `@oh-my-pi/pi-catalog/models` in the catalog split. Legacy extensions
+		// still import it from the pi-ai root, so the shim must bridge it back
+		// to the catalog implementation. The historical regression was a plain
+		// `SyntaxError: Export named 'calculateCost' not found in module
+		// '.../legacy-pi-ai-shim.ts'` at extension-validation time.
+		const loaded = (await loadLegacyPiModule(
+			await writeFixtureExtension(
+				'import { calculateCost } from "@oh-my-pi/pi-ai"; export const probe = calculateCost;',
+			),
+		)) as { probe: unknown };
+		expect(loaded.probe).toBe(calculateCost);
+	});
+
+	it("re-exports modelsAreEqual and getBundledProviders from @oh-my-pi/pi-catalog/models", async () => {
+		const loaded = (await loadLegacyPiModule(
+			await writeFixtureExtension(
+				[
+					'import { modelsAreEqual, getBundledProviders } from "@oh-my-pi/pi-ai";',
+					"export const eq = modelsAreEqual;",
+					"export const providers = getBundledProviders;",
+				].join("\n"),
+			),
+		)) as { eq: unknown; providers: unknown };
+		expect(loaded.eq).toBe(modelsAreEqual);
+		expect(loaded.providers).toBe(getBundledProviders);
+	});
+
 	it("exports StringEnum as a schema builder with options support", async () => {
 		const loaded = (await loadLegacyPiModule(
 			await writeFixtureExtension(