diff --git a/packages/paperclip-runner/src/drivers/acpx/model-verification.test.ts b/packages/paperclip-runner/src/drivers/acpx/model-verification.test.ts
index 6c36e3a0774..d816bf7fe67 100644
--- a/packages/paperclip-runner/src/drivers/acpx/model-verification.test.ts
+++ b/packages/paperclip-runner/src/drivers/acpx/model-verification.test.ts
@@ -2,6 +2,7 @@ import { describe, expect, it, vi } from "vitest";
 
 import { resolveQualifiedAcpxProfile } from "./qualified-profiles.js";
 import { requireVerifiedAcpxModel } from "./model-verification.js";
+import { acpxSidecarErrorCode } from "../../cli/acpx-sidecar-input.js";
 
 describe("ACPX requested model verification", () => {
   it.each(["claude", "codex", "pi", "grok", "cursor", "copilot"] as const)("selects and verifies an unlisted model unchanged with %s", async agent => {
@@ -29,6 +30,33 @@ describe("ACPX requested model verification", () => {
     expect(getStatus).toHaveBeenCalledTimes(1);
   });
 
+  it.each(["claude", "codex", "pi", "grok", "cursor", "copilot"] as const)("names an ACP rejection of an unadvertised model as an unsupported model with %s", async agent => {
+    const model = "chaos-missing-model-abc12345";
+    // claude-agent-acp 0.73.0 answers session/set_config_option for an unknown model with this
+    // JSON-RPC error; ACPX wraps it as below (live probe, 2026-11-10).
+    const acp = Object.assign(new Error("Internal error"), {
+      code: -32603, data: { details: `Invalid value for config option model: ${model}` },
+    });
+    const wrapped = new Error(`Failed session/set_config_option for model "${model}": Invalid value for config option model: ${model} (ACP -32603, adapter reported "Internal error")`, { cause: acp });
+    const getStatus = vi.fn(async () => ({ models: { currentModelId: "default", availableModelIds: ["default", "opus", "sonnet"] } }));
+    const setModel = vi.fn(async () => { throw wrapped; });
+    const failure = await requireVerifiedAcpxModel({ getStatus, setModel }, resolveQualifiedAcpxProfile(agent, model))
+      .then(() => null, (error: unknown) => error as Error & { code?: unknown; retryable?: unknown });
+    expect(failure).toMatchObject({ code: "ACP_MODEL_UNSUPPORTED", retryable: false });
+    expect(failure?.cause).toBe(wrapped);
+    expect(acpxSidecarErrorCode(failure!)).toBe("ACP_MODEL_UNSUPPORTED");
+  });
+
+  it("keeps a transport failure or an advertised model's rejection unchanged", async () => {
+    const acp = Object.assign(new Error("Internal error"), { code: -32603, data: { details: "busy" } });
+    for (const [rejected, available] of [[new Error("socket closed"), ["default"]], [acp, ["default", "listed-model"]]] as const) {
+      const getStatus = vi.fn(async () => ({ models: { currentModelId: "default", availableModelIds: [...available] } }));
+      const setModel = vi.fn(async () => { throw rejected; });
+      await expect(requireVerifiedAcpxModel({ getStatus, setModel }, resolveQualifiedAcpxProfile("claude", "listed-model")))
+        .rejects.toBe(rejected);
+    }
+  });
+
   it("accepts an exact model already reported by the provider", async () => {
     const getStatus = vi.fn(async () => ({
       models: {
diff --git a/packages/paperclip-runner/src/drivers/acpx/model-verification.ts b/packages/paperclip-runner/src/drivers/acpx/model-verification.ts
index 337d2612c77..1b0afdd4a8e 100644
--- a/packages/paperclip-runner/src/drivers/acpx/model-verification.ts
+++ b/packages/paperclip-runner/src/drivers/acpx/model-verification.ts
@@ -38,7 +38,22 @@ export async function requireVerifiedAcpxModel(
       );
     }
     // Catalogs may be incomplete. Let the provider accept or reject the exact ID.
-    await control.setModel(providerModel);
+    try {
+      await control.setModel(providerModel);
+    } catch (error) {
+      // The provider answered the selection of a model it does not advertise with
+      // an ACP error: it rejected that model. Name it as such so the task asks the
+      // user to choose a model. Transport failures and an advertised model's
+      // rejection keep their original error.
+      const advertised = status.models?.availableModelIds ?? [];
+      if (!advertised.includes(providerModel) && acpErrorResponse(error)) {
+        throw Object.assign(
+          new Error(`ACP agent rejected the requested model ${requestedModel}`, { cause: error }),
+          { code: "ACP_MODEL_UNSUPPORTED", retryable: false },
+        );
+      }
+      throw error;
+    }
     status = await control.getStatus();
   }
   if (status.models?.currentModelId !== providerModel) {
@@ -50,6 +65,14 @@ export async function requireVerifiedAcpxModel(
   return normalizeVerifiedModelStatus(status, profile);
 }
 
+/** A JSON-RPC error response from the ACP agent, found on the error or its causes. */
+function acpErrorResponse(error: unknown, depth = 0): boolean {
+  if (!error || typeof error !== "object" || depth > 5) return false;
+  const entry = error as Record<string, unknown>;
+  if (typeof entry.code === "number" && Number.isInteger(entry.code) && typeof entry.message === "string") return true;
+  return ["acp", "cause", "error"].some(key => acpErrorResponse(entry[key], depth + 1));
+}
+
 function acpxModelVerificationError(code: string, message: string): Error {
   return Object.assign(new Error(message), { code });
 }