diff --git a/docs/custom-providers.md b/docs/custom-providers.md
index c869459b1..b1bbb8c95 100644
--- a/docs/custom-providers.md
+++ b/docs/custom-providers.md
@@ -366,6 +366,14 @@ Required fields for ACP providers:
 - `label`
 - `command` — the command to spawn the agent process (must support ACP over stdio)
 
+### Generic ACP diagnostics
+
+Paseo diagnostics for `extends: "acp"` providers report the configured command, resolved launcher binary, version output, ACP `initialize`, ACP `session/new`, model count, modes, and final status.
+
+For package-runner commands such as `npx -y @google/gemini-cli --acp`, the version probe keeps the package spec and runs `npx -y @google/gemini-cli --version`. This diagnoses the actual agent package instead of only proving that `npx` exists.
+
+ACP probes use short timeouts and browser-suppression environment variables so agents that enter an auth/browser flow fail as a diagnostic error instead of hanging the provider screen.
+
 ### Example: Google Gemini CLI
 
 [Gemini CLI](https://github.com/google-gemini/gemini-cli) supports ACP via the `--acp` flag.
diff --git a/packages/server/src/server/agent/provider-registry.test.ts b/packages/server/src/server/agent/provider-registry.test.ts
index e07411d9f..14db97fff 100644
--- a/packages/server/src/server/agent/provider-registry.test.ts
+++ b/packages/server/src/server/agent/provider-registry.test.ts
@@ -17,6 +17,8 @@ const mockState = vi.hoisted(() => {
       genericAcp: [] as Array<{
         command: string[];
         env?: Record<string, string>;
+        providerId?: string;
+        label?: string;
       }>,
     },
     isCommandAvailable: vi.fn(async (_command: string) => false),
@@ -241,7 +243,12 @@ vi.mock("./providers/generic-acp-agent.js", () => ({
     readonly provider = "acp";
     readonly runtimeSettings?: unknown;
 
-    constructor(options: { command: string[]; env?: Record<string, string> }) {
+    constructor(options: {
+      command: string[];
+      env?: Record<string, string>;
+      providerId?: string;
+      label?: string;
+    }) {
       this.runtimeSettings = {
         command: {
           mode: "replace",
@@ -252,6 +259,8 @@ vi.mock("./providers/generic-acp-agent.js", () => ({
       mockState.constructorArgs.genericAcp.push({
         command: options.command,
         env: options.env,
+        providerId: options.providerId,
+        label: options.label,
       });
     }
 
@@ -387,12 +396,16 @@ test("new provider extending acp uses GenericACPAgentClient", () => {
       env: {
         ACP_TOKEN: "secret",
       },
+      providerId: "my-agent",
+      label: "My Agent",
     },
     {
       command: ["my-agent", "--acp"],
       env: {
         ACP_TOKEN: "secret",
       },
+      providerId: "my-agent",
+      label: "My Agent",
     },
   ]);
 });
diff --git a/packages/server/src/server/agent/provider-registry.ts b/packages/server/src/server/agent/provider-registry.ts
index 4da1d875a..21e71ea05 100644
--- a/packages/server/src/server/agent/provider-registry.ts
+++ b/packages/server/src/server/agent/provider-registry.ts
@@ -517,6 +517,8 @@ function addDerivedProviders(
             logger,
             command,
             env: override.env,
+            providerId,
+            label: override.label ?? providerId,
           }),
       });
       continue;
diff --git a/packages/server/src/server/agent/providers/acp-agent.ts b/packages/server/src/server/agent/providers/acp-agent.ts
index 20100d1ae..8ca3307a6 100644
--- a/packages/server/src/server/agent/providers/acp-agent.ts
+++ b/packages/server/src/server/agent/providers/acp-agent.ts
@@ -713,7 +713,10 @@ export class ACPAgentClient implements AgentClient {
     }
   }
 
-  protected async spawnProcess(launchEnv?: Record<string, string>): Promise<SpawnedACPProcess> {
+  protected async spawnProcess(
+    launchEnv?: Record<string, string>,
+    options?: { initializeTimeoutMs?: number },
+  ): Promise<SpawnedACPProcess> {
     const { command, args } = await this.resolveLaunchCommand();
     const child = spawnProcess(command, args, {
       cwd: process.cwd(),
@@ -743,14 +746,35 @@ export class ACPAgentClient implements AgentClient {
       { logger: this.logger, provider: this.provider },
     );
     const connection = new ClientSideConnection(() => this.buildProbeClient(), stream);
-    const initialize = await Promise.race([
-      connection.initialize({
-        protocolVersion: PROTOCOL_VERSION,
-        clientCapabilities: ACP_CLIENT_CAPABILITIES,
-        clientInfo: { name: "Paseo", version: "dev" },
-      }),
-      spawnErrorPromise,
-    ]);
+
+    let timeout: ReturnType<typeof setTimeout> | null = null;
+    const initializeTimeoutPromise = options?.initializeTimeoutMs
+      ? new Promise<never>((_, reject) => {
+          timeout = setTimeout(() => {
+            reject(new Error(`ACP initialize timed out after ${options.initializeTimeoutMs}ms`));
+          }, options.initializeTimeoutMs);
+        })
+      : null;
+
+    let initialize: InitializeResponse;
+    try {
+      initialize = await Promise.race([
+        connection.initialize({
+          protocolVersion: PROTOCOL_VERSION,
+          clientCapabilities: ACP_CLIENT_CAPABILITIES,
+          clientInfo: { name: "Paseo", version: "dev" },
+        }),
+        spawnErrorPromise,
+        ...(initializeTimeoutPromise ? [initializeTimeoutPromise] : []),
+      ]);
+    } catch (error) {
+      await terminateChildProcess(child, 2_000);
+      throw error;
+    } finally {
+      if (timeout) {
+        clearTimeout(timeout);
+      }
+    }
 
     return { child, connection, initialize };
   }
@@ -782,8 +806,7 @@ export class ACPAgentClient implements AgentClient {
         // No active session to close here; ignore capability.
       }
     } finally {
-      probe.child.kill("SIGTERM");
-      await waitForChildExit(probe.child, 2_000);
+      await terminateChildProcess(probe.child, 2_000);
     }
   }
 
@@ -2706,3 +2729,14 @@ async function waitForChildExit(
     child.kill("SIGKILL");
   }
 }
+
+async function terminateChildProcess(
+  child: ChildProcessWithoutNullStreams,
+  timeoutMs: number,
+): Promise<void> {
+  child.kill("SIGTERM");
+  child.stdin.destroy();
+  child.stdout.destroy();
+  child.stderr.destroy();
+  await waitForChildExit(child, timeoutMs);
+}
diff --git a/packages/server/src/server/agent/providers/generic-acp-agent.diagnostic.test.ts b/packages/server/src/server/agent/providers/generic-acp-agent.diagnostic.test.ts
new file mode 100644
index 000000000..2632c2d85
--- /dev/null
+++ b/packages/server/src/server/agent/providers/generic-acp-agent.diagnostic.test.ts
@@ -0,0 +1,99 @@
+import { describe, expect, test, vi } from "vitest";
+
+import { createTestLogger } from "../../../test-utils/test-logger.js";
+import { buildVersionProbeCommand, GenericACPAgentClient } from "./generic-acp-agent.js";
+import type { SpawnedACPProcess } from "./acp-agent.js";
+
+describe("GenericACPAgentClient diagnostics", () => {
+  test("probes npx-backed agent packages instead of npx itself", () => {
+    expect(buildVersionProbeCommand(["npx", "-y", "@google/gemini-cli@0.41.1", "--acp"])).toEqual({
+      command: "npx",
+      args: ["-y", "@google/gemini-cli@0.41.1", "--version"],
+    });
+
+    expect(buildVersionProbeCommand(["pnpm", "dlx", "@agent/foo@1.2.3", "--acp"])).toEqual({
+      command: "pnpm",
+      args: ["dlx", "@agent/foo@1.2.3", "--version"],
+    });
+  });
+
+  test("reports command, binary, ACP initialize, session, models, and modes", async () => {
+    class TestGenericACPAgentClient extends GenericACPAgentClient {
+      protected override async spawnProcess(): Promise<SpawnedACPProcess> {
+        return {
+          child: { kill: vi.fn(), exitCode: 0, signalCode: null, once: vi.fn() },
+          initialize: {
+            protocolVersion: 1,
+            agentInfo: { name: "Cursor Agent", version: "2026.05.09" },
+            agentCapabilities: {},
+          },
+          connection: {
+            newSession: vi.fn().mockResolvedValue({
+              sessionId: "session-1",
+              models: {
+                currentModelId: "composer-2[fast=true]",
+                availableModels: [
+                  {
+                    modelId: "composer-2[fast=true]",
+                    name: "Composer 2",
+                  },
+                ],
+              },
+              modes: {
+                currentModeId: "ask",
+                availableModes: [
+                  { id: "agent", name: "Agent" },
+                  { id: "ask", name: "Ask" },
+                ],
+              },
+              configOptions: [],
+            }),
+          },
+        } as SpawnedACPProcess;
+      }
+
+      protected override async closeProbe(): Promise<void> {}
+    }
+
+    const client = new TestGenericACPAgentClient({
+      logger: createTestLogger(),
+      command: [process.execPath, "acp"],
+      providerId: "cursor",
+      label: "Cursor",
+    });
+
+    const { diagnostic } = await client.getDiagnostic();
+
+    expect(diagnostic).toContain("Cursor (ACP)");
+    expect(diagnostic).toContain("Provider ID: cursor");
+    expect(diagnostic).toContain(`Configured command: ${process.execPath} acp`);
+    expect(diagnostic).toContain(`Launcher binary: ${process.execPath}`);
+    expect(diagnostic).toContain(`Version command: ${process.execPath} --version`);
+    expect(diagnostic).toContain("ACP initialize: ok (protocol 1, Cursor Agent 2026.05.09)");
+    expect(diagnostic).toContain("ACP session/new: ok (session-1)");
+    expect(diagnostic).toContain("Models: 1");
+    expect(diagnostic).toContain("Modes: Agent, Ask");
+    expect(diagnostic).toContain("Status: Available");
+  });
+
+  test("reports ACP probe failures instead of falling back to no diagnostic", async () => {
+    class FailingGenericACPAgentClient extends GenericACPAgentClient {
+      protected override async spawnProcess(): Promise<SpawnedACPProcess> {
+        throw new Error("initialize timed out");
+      }
+    }
+
+    const client = new FailingGenericACPAgentClient({
+      logger: createTestLogger(),
+      command: [process.execPath, "acp"],
+      providerId: "cursor",
+      label: "Cursor",
+    });
+
+    const { diagnostic } = await client.getDiagnostic();
+
+    expect(diagnostic).toContain("Cursor (ACP)");
+    expect(diagnostic).toContain("ACP initialize: Error - initialize timed out");
+    expect(diagnostic).toContain("Status: Error (ACP probe failed: initialize timed out)");
+  });
+});
diff --git a/packages/server/src/server/agent/providers/generic-acp-agent.ts b/packages/server/src/server/agent/providers/generic-acp-agent.ts
index 682b16f61..0c2d07a47 100644
--- a/packages/server/src/server/agent/providers/generic-acp-agent.ts
+++ b/packages/server/src/server/agent/providers/generic-acp-agent.ts
@@ -1,16 +1,32 @@
+import { homedir } from "node:os";
 import type { Logger } from "pino";
 
-import { isCommandAvailable } from "../../../utils/executable.js";
-import { ACPAgentClient } from "./acp-agent.js";
+import { findExecutable, isCommandAvailable } from "../../../utils/executable.js";
+import { execCommand } from "../../../utils/spawn.js";
+import { createProviderEnvSpec } from "../provider-launch-config.js";
+import { ACPAgentClient, type SessionStateResponse } from "./acp-agent.js";
+import {
+  formatDiagnosticStatus,
+  formatProviderDiagnostic,
+  formatProviderDiagnosticError,
+  toDiagnosticErrorMessage,
+} from "./diagnostic-utils.js";
+
+const ACP_DIAGNOSTIC_INITIALIZE_TIMEOUT_MS = 8_000;
+const ACP_DIAGNOSTIC_SESSION_TIMEOUT_MS = 8_000;
 
 interface GenericACPAgentClientOptions {
   logger: Logger;
   command: [string, ...string[]];
   env?: Record<string, string>;
+  providerId?: string;
+  label?: string;
 }
 
 export class GenericACPAgentClient extends ACPAgentClient {
   private readonly command: [string, ...string[]];
+  private readonly providerId?: string;
+  private readonly label?: string;
 
   constructor(options: GenericACPAgentClientOptions) {
     super({
@@ -23,6 +39,8 @@ export class GenericACPAgentClient extends ACPAgentClient {
     });
 
     this.command = options.command;
+    this.providerId = options.providerId;
+    this.label = options.label;
   }
 
   protected override async resolveLaunchCommand(): Promise<{ command: string; args: string[] }> {
@@ -35,4 +53,247 @@ export class GenericACPAgentClient extends ACPAgentClient {
   override async isAvailable(): Promise<boolean> {
     return isCommandAvailable(this.command[0]);
   }
+
+  async getDiagnostic(): Promise<{ diagnostic: string }> {
+    const providerName = formatProviderName(this.label, this.providerId);
+
+    try {
+      const resolvedBinary = await findExecutable(this.command[0]);
+      const available = resolvedBinary !== null;
+      const versionProbe = buildVersionProbeCommand(this.command);
+      const probeResult = available
+        ? await this.runDiagnosticACPProbe()
+        : {
+            status: formatDiagnosticStatus(false),
+            initialize: "Not checked",
+            session: "Not checked",
+            models: "Not checked",
+            modes: "Not checked",
+          };
+
+      return {
+        diagnostic: formatProviderDiagnostic(providerName, [
+          { label: "Provider ID", value: this.providerId ?? "unknown" },
+          { label: "Configured command", value: this.command.join(" ") },
+          { label: "Launcher binary", value: resolvedBinary ?? "not found" },
+          {
+            label: "Version command",
+            value: formatCommand(versionProbe.command, versionProbe.args),
+          },
+          {
+            label: "Version",
+            value: resolvedBinary
+              ? await resolveCommandVersion(versionProbe, this.runtimeSettings?.env)
+              : "unknown",
+          },
+          { label: "ACP initialize", value: probeResult.initialize },
+          { label: "ACP session/new", value: probeResult.session },
+          { label: "Models", value: probeResult.models },
+          { label: "Modes", value: probeResult.modes },
+          { label: "Status", value: probeResult.status },
+        ]),
+      };
+    } catch (error) {
+      return {
+        diagnostic: formatProviderDiagnosticError(providerName, error),
+      };
+    }
+  }
+
+  private async runDiagnosticACPProbe(): Promise<ACPDiagnosticProbeResult> {
+    let initializeValue = "Not checked";
+    let sessionValue = "Not checked";
+
+    try {
+      const probe = await this.spawnProcess(
+        {
+          NO_BROWSER: "true",
+          NO_OPEN_BROWSER: "1",
+          GEMINI_CLI_NO_BROWSER: "true",
+          CI: "1",
+        },
+        {
+          initializeTimeoutMs: ACP_DIAGNOSTIC_INITIALIZE_TIMEOUT_MS,
+        },
+      );
+      try {
+        initializeValue = formatInitializeResult(probe.initialize);
+        const response = await withTimeout(
+          probe.connection.newSession({
+            cwd: homedir(),
+            mcpServers: [],
+          }),
+          ACP_DIAGNOSTIC_SESSION_TIMEOUT_MS,
+          "ACP session/new",
+        );
+        sessionValue = response.sessionId ? `ok (${response.sessionId})` : "ok";
+        return {
+          status: formatDiagnosticStatus(true),
+          initialize: initializeValue,
+          session: sessionValue,
+          ...summarizeSessionState(this.transformSessionResponse(response)),
+        };
+      } finally {
+        await this.closeProbe(probe);
+      }
+    } catch (error) {
+      return {
+        status: formatDiagnosticStatus(true, {
+          source: "ACP probe",
+          cause: error,
+        }),
+        initialize: formatProbeError(initializeValue, error),
+        session:
+          initializeValue === "Not checked" ? "Not checked" : formatProbeError(sessionValue, error),
+        models: "Not checked",
+        modes: "Not checked",
+      };
+    }
+  }
+}
+
+interface ACPDiagnosticProbeResult {
+  status: string;
+  initialize: string;
+  session: string;
+  models: string;
+  modes: string;
+}
+
+export interface CommandInvocation {
+  command: string;
+  args: string[];
+}
+
+function formatProviderName(label: string | undefined, providerId: string | undefined): string {
+  if (label) {
+    return `${label} (ACP)`;
+  }
+  if (providerId) {
+    return `${providerId} (ACP)`;
+  }
+  return "Custom ACP";
+}
+
+function formatCommand(command: string, args: string[]): string {
+  return [command, ...args].join(" ");
+}
+
+export function buildVersionProbeCommand(command: [string, ...string[]]): CommandInvocation {
+  const [launcher, ...args] = command;
+  if (isPackageRunner(launcher)) {
+    return {
+      command: launcher,
+      args: [...takePackageRunnerPrefix(args), "--version"],
+    };
+  }
+
+  return {
+    command: launcher,
+    args: ["--version"],
+  };
+}
+
+function isPackageRunner(command: string): boolean {
+  return ["npx", "bunx", "pnpm", "uvx"].includes(command);
+}
+
+function takePackageRunnerPrefix(args: string[]): string[] {
+  if (args.length === 0) {
+    return [];
+  }
+  if (args[0] === "dlx") {
+    return ["dlx", ...takePackageSpecPrefix(args.slice(1))];
+  }
+  return takePackageSpecPrefix(args);
+}
+
+function takePackageSpecPrefix(args: string[]): string[] {
+  const prefix: string[] = [];
+  for (let index = 0; index < args.length; index += 1) {
+    const arg = args[index];
+    prefix.push(arg);
+    if (arg === "--package" || arg === "-p") {
+      if (args[index + 1]) {
+        prefix.push(args[index + 1]);
+        index += 1;
+      }
+      continue;
+    }
+    if (!arg.startsWith("-")) {
+      break;
+    }
+  }
+  return prefix;
+}
+
+function formatInitializeResult(initialize: {
+  protocolVersion: number;
+  agentInfo?: unknown;
+}): string {
+  const agentInfo = isAgentInfo(initialize.agentInfo)
+    ? `${initialize.agentInfo.name}${initialize.agentInfo.version ? ` ${initialize.agentInfo.version}` : ""}`
+    : "ok";
+  return `ok (protocol ${initialize.protocolVersion}, ${agentInfo})`;
+}
+
+function isAgentInfo(value: unknown): value is { name: string; version?: string } {
+  return (
+    typeof value === "object" &&
+    value !== null &&
+    "name" in value &&
+    typeof Reflect.get(value, "name") === "string"
+  );
+}
+
+function summarizeSessionState(
+  response: SessionStateResponse,
+): Pick<ACPDiagnosticProbeResult, "models" | "modes"> {
+  const models = response.models?.availableModels ?? [];
+  const modes = response.modes?.availableModes ?? [];
+  return {
+    models: `${models.length}`,
+    modes:
+      modes.length > 0 ? modes.map((mode) => mode.name || mode.id).join(", ") : "none reported",
+  };
+}
+
+function formatProbeError(currentValue: string, error: unknown): string {
+  if (currentValue !== "Not checked") {
+    return currentValue;
+  }
+  return `Error - ${toDiagnosticErrorMessage(error)}`;
+}
+
+async function resolveCommandVersion(
+  invocation: CommandInvocation,
+  env: Record<string, string> | undefined,
+): Promise<string> {
+  try {
+    const { stdout, stderr } = await execCommand(invocation.command, invocation.args, {
+      ...createProviderEnvSpec({ runtimeSettings: { env } }),
+      timeout: 5_000,
+    });
+    return stdout.trim() || stderr.trim() || "unknown";
+  } catch (error) {
+    return `error: ${toDiagnosticErrorMessage(error)}`;
+  }
+}
+
+async function withTimeout<T>(promise: Promise<T>, timeoutMs: number, label: string): Promise<T> {
+  let timeout: ReturnType<typeof setTimeout> | null = null;
+  try {
+    return await Promise.race([
+      promise,
+      new Promise<never>((_, reject) => {
+        timeout = setTimeout(() => {
+          reject(new Error(`${label} timed out after ${timeoutMs}ms`));
+        }, timeoutMs);
+      }),
+    ]);
+  } finally {
+    if (timeout) {
+      clearTimeout(timeout);
+    }
+  }
 }