fix(coding-agent): allow headers-only model overrides closes #3538

packages/coding-agent/CHANGELOG.md

@@ -13,6 +13,7 @@
 ### Fixed
 
 - Fixed extension session-replacement flows so `ctx.newSession()`, `ctx.fork()`, `ctx.switchSession()`, and imported-session replacements fully rebind before post-switch work runs, added `withSession` replacement callbacks with fresh `ReplacedSessionContext` helpers, and make stale pre-replacement `pi` / `ctx` session-bound accesses throw instead of silently targeting the wrong session ([#2860](https://github.com/badlogic/pi-mono/issues/2860))
+- Fixed `models.json` built-in provider overrides to accept `headers` without requiring `baseUrl`, so request-header-only overrides now load and apply correctly ([#3538](https://github.com/badlogic/pi-mono/issues/3538))
 
 ## [0.68.1] - 2026-04-22
 

packages/coding-agent/src/core/model-registry.ts

@@ -474,10 +474,10 @@ export class ModelRegistry {
 				providerConfig.modelOverrides && Object.keys(providerConfig.modelOverrides).length > 0;
 
 			if (models.length === 0) {
-				// Override-only config: needs baseUrl, compat, modelOverrides, or some combination.
-				if (!providerConfig.baseUrl && !providerConfig.compat && !hasModelOverrides) {
+				// Override-only config: needs baseUrl, headers, compat, modelOverrides, or some combination.
+				if (!providerConfig.baseUrl && !providerConfig.headers && !providerConfig.compat && !hasModelOverrides) {
 					throw new Error(
-						`Provider ${providerName}: must specify "baseUrl", "compat", "modelOverrides", or "models".`,
+						`Provider ${providerName}: must specify "baseUrl", "headers", "compat", "modelOverrides", or "models".`,
 					);
 				}
 			} else if (!isBuiltIn) {

packages/coding-agent/test/model-registry.test.ts

@@ -135,6 +135,28 @@ describe("ModelRegistry", () => {
 			}
 		});
 
+		test("headers-only override resolves at request time", async () => {
+			writeRawModelsJson({
+				anthropic: {
+					headers: {
+						"X-Custom-Header": "custom-value",
+					},
+				},
+			});
+
+			const registry = ModelRegistry.create(authStorage, modelsJsonPath);
+			expect(registry.getError()).toBeUndefined();
+			const anthropicModels = getModelsForProvider(registry, "anthropic");
+
+			for (const model of anthropicModels) {
+				const auth = await registry.getApiKeyAndHeaders(model);
+				expect(auth.ok).toBe(true);
+				if (auth.ok) {
+					expect(auth.headers?.["X-Custom-Header"]).toBe("custom-value");
+				}
+			}
+		});
+
 		test("baseUrl-only override does not affect other providers", () => {
 			writeRawModelsJson({
 				anthropic: overrideConfig("https://my-proxy.example.com/v1"),