diff --git a/packages/pi-coding-agent/src/core/model-registry-env-fallback.test.ts b/packages/pi-coding-agent/src/core/model-registry-env-fallback.test.ts
new file mode 100644
index 000000000..fb48db7af
--- /dev/null
+++ b/packages/pi-coding-agent/src/core/model-registry-env-fallback.test.ts
@@ -0,0 +1,59 @@
+import assert from "node:assert/strict";
+import { describe, it } from "node:test";
+import type { AuthStorage } from "./auth-storage.js";
+import { ModelRegistry } from "./model-registry.js";
+
+function createRegistryWithCapturedResolver() {
+	let capturedResolver: ((provider: string) => string | undefined) | undefined;
+	const authStorage = {
+		setFallbackResolver: (resolver: (provider: string) => string | undefined) => {
+			capturedResolver = resolver;
+		},
+		onCredentialChange: () => {},
+		getOAuthProviders: () => [],
+		get: () => undefined,
+		hasAuth: () => false,
+		getApiKey: async () => undefined,
+	} as unknown as AuthStorage;
+
+	new ModelRegistry(authStorage, undefined);
+	assert.ok(capturedResolver, "ModelRegistry should register a fallback resolver");
+	return capturedResolver!;
+}
+
+describe("ModelRegistry env fallback resolver (#3782)", () => {
+	it("falls back to built-in provider env vars when models.json has no custom key", () => {
+		const prev = process.env.MINIMAX_API_KEY;
+		process.env.MINIMAX_API_KEY = "minimax-env-test-key";
+
+		try {
+			const resolver = createRegistryWithCapturedResolver();
+			assert.equal(
+				resolver("minimax"),
+				"minimax-env-test-key",
+				"fallback resolver should return built-in provider env keys",
+			);
+		} finally {
+			if (prev === undefined) {
+				delete process.env.MINIMAX_API_KEY;
+			} else {
+				process.env.MINIMAX_API_KEY = prev;
+			}
+		}
+	});
+
+	it("still returns undefined when no custom or built-in env key exists", () => {
+		const prev = process.env.MINIMAX_API_KEY;
+		delete process.env.MINIMAX_API_KEY;
+
+		try {
+			const resolver = createRegistryWithCapturedResolver();
+			assert.equal(resolver("minimax"), undefined);
+			assert.equal(resolver("totally-unknown-provider"), undefined);
+		} finally {
+			if (prev !== undefined) {
+				process.env.MINIMAX_API_KEY = prev;
+			}
+		}
+	});
+});
diff --git a/packages/pi-coding-agent/src/core/model-registry.ts b/packages/pi-coding-agent/src/core/model-registry.ts
index 762e459cc..0041a9680 100644
--- a/packages/pi-coding-agent/src/core/model-registry.ts
+++ b/packages/pi-coding-agent/src/core/model-registry.ts
@@ -8,6 +8,7 @@ import {
 	type AssistantMessageEventStream,
 	type Context,
 	getApiProvider,
+	getEnvApiKey,
 	getModels,
 	getProviders,
 	type KnownProvider,
@@ -245,7 +246,7 @@ export class ModelRegistry {
 			if (keyConfig) {
 				return resolveConfigValue(keyConfig);
 			}
-			return undefined;
+			return getEnvApiKey(provider);
 		});
 
 		// Refresh models when credentials change (e.g., OAuth token refresh with new model limits)