From 19a67ef5211996a737d9f9fe608ab2c80f8a6714 Mon Sep 17 00:00:00 2001
From: Theo Browne <me@t3.gg>
Date: Wed, 14 Oct 2026 01:38:38 -0700
Subject: [PATCH] fix(acp): custom models now reach the agent instead of
 failing to open (#19313)

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
---
 .../provider/acp/AcpJsonRpcConnection.test.ts | 37 +++++++++++++++++++
 .../src/server/adapter.ts                     |  4 ++
 .../src/server/AcpSessionRuntime.ts           |  9 +++++
 3 files changed, 50 insertions(+)

diff --git a/apps/server/src/provider/acp/AcpJsonRpcConnection.test.ts b/apps/server/src/provider/acp/AcpJsonRpcConnection.test.ts
index b11fb5baec3a0a391c5e9ca33a3c19fb93237f46..14ff363da47b80f247f6729e597096975784ea37 100644
--- a/apps/server/src/provider/acp/AcpJsonRpcConnection.test.ts
+++ b/apps/server/src/provider/acp/AcpJsonRpcConnection.test.ts
@@ -1752,4 +1752,41 @@ describe("AcpSessionRuntime", () => {
       Effect.ensuring(Effect.sync(() => NodeFS.rmSync(tempDir, { recursive: true, force: true }))),
     );
   });
+
+  it.effect("sends configured custom models the agent does not advertise", () => {
+    const tempDir = NodeFS.mkdtempSync(NodePath.join(NodeOS.tmpdir(), "acp-runtime-"));
+    const requestLogPath = NodePath.join(tempDir, "requests.ndjson");
+    return Effect.gen(function* () {
+      const runtime = yield* AcpSessionRuntime.AcpSessionRuntime;
+      yield* runtime.start();
+
+      yield* runtime.setModel("cline-free/custom-model");
+      const error = yield* runtime.setModel("unconfigured/model").pipe(Effect.flip);
+      expect(error._tag).toBe("AcpRequestError");
+
+      const sentModels = NodeFS.readFileSync(requestLogPath, "utf8")
+        .trim()
+        .split("\n")
+        .filter((line) => line.length > 0)
+        .map((line) => JSON.parse(line) as { method?: string; params?: { value?: unknown } })
+        .filter((message) => message.method === "session/set_config_option")
+        .map((message) => message.params?.value);
+      expect(sentModels).toEqual(["cline-free/custom-model"]);
+    }).pipe(
+      Effect.provide(
+        AcpSessionRuntime.layer({
+          ...mockRuntimeOptions,
+          spawn: {
+            command: mockAgentCommand,
+            args: mockAgentArgs,
+            env: { T3_ACP_REQUEST_LOG_PATH: requestLogPath },
+          },
+          customModels: ["cline-free/custom-model"],
+        }),
+      ),
+      Effect.scoped,
+      Effect.provide(NodeServices.layer),
+      Effect.ensuring(Effect.sync(() => NodeFS.rmSync(tempDir, { recursive: true, force: true }))),
+    );
+  });
 });
diff --git a/packages/provider-acp-registry/src/server/adapter.ts b/packages/provider-acp-registry/src/server/adapter.ts
index 0fa9a83d05c78fa5d57447495f18823e3a9016a5..4cc847a9931eeda19a52e7c29ac74a4fbd8b8bfe 100644
--- a/packages/provider-acp-registry/src/server/adapter.ts
+++ b/packages/provider-acp-registry/src/server/adapter.ts
@@ -9,6 +9,7 @@ import {
   makeCopilotSubagentRouting,
 } from "./copilotAcp.ts";
 import { defaultInstanceIdForDriver, ProviderDriverKind } from "@t3tools/contracts";
+import { readCustomModelEntries } from "@t3tools/shared/model";
 import { AcpRegistrySettings } from "../settings.ts";
 import * as HostProcess from "@t3tools/shared/HostProcess";
 import { resolveSelfInvocation, type SelfInvocation } from "@t3tools/shared/nodeRuntime";
@@ -168,6 +169,9 @@ function makeAcpRegistryRuntime(
                   env: { ...resolved.spawn.env, ...processEnvironment },
                 },
           ...(options.settings.authMethodId ? { authMethodId: options.settings.authMethodId } : {}),
+          customModels: readCustomModelEntries(options.settings.customModels).map(
+            (entry) => entry.slug,
+          ),
         }),
       );
       return yield* Effect.service(AcpSessionRuntime.AcpSessionRuntime).pipe(
diff --git a/packages/provider-acp/src/server/AcpSessionRuntime.ts b/packages/provider-acp/src/server/AcpSessionRuntime.ts
index 3ae4f8da03e650d888003c2663cc0a9be1c79262..25578f151b3634f91beb9f5bef34eb122388106f 100644
--- a/packages/provider-acp/src/server/AcpSessionRuntime.ts
+++ b/packages/provider-acp/src/server/AcpSessionRuntime.ts
@@ -155,6 +155,12 @@ export interface AcpSessionRuntimeOptions {
   readonly onTermination?: (error: EffectAcpErrors.AcpError) => Effect.Effect<void>;
   readonly onOutgoingResponseFailure?: EffectAcpClient.AcpClientOptions["onOutgoingResponseFailure"];
   readonly onOutgoingResponse?: EffectAcpClient.AcpClientOptions["onOutgoingResponse"];
+  /**
+   * Model ids the user configured for this instance. The model config option
+   * accepts them even when the agent does not advertise them, and the agent
+   * validates them instead.
+   */
+  readonly customModels?: ReadonlyArray<string>;
 }
 
 export interface AcpSessionRequestLogEvent {
@@ -1977,6 +1983,9 @@ export const make = (
         if (allowedValues.includes(value)) {
           return;
         }
+        if (configOption.category === "model" && options.customModels?.includes(value)) {
+          return;
+        }
         return yield* new EffectAcpErrors.AcpRequestError({
           code: -32602,
           errorMessage: `Invalid value ${formatConfigOptionValue(value)} for session config option "${configOption.id}": expected one of ${allowedValues.join(", ")}`,
-- 
2.54.0