diff --git a/packages/agent/src/adapters/claude/claude-agent.slash-command.test.ts b/packages/agent/src/adapters/claude/claude-agent.slash-command.test.ts
new file mode 100644
index 000000000..1c4bb0fd3
--- /dev/null
+++ b/packages/agent/src/adapters/claude/claude-agent.slash-command.test.ts
@@ -0,0 +1,153 @@
+import type { AgentSideConnection } from "@agentclientprotocol/sdk";
+import type { SDKMessage } from "@anthropic-ai/claude-agent-sdk";
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import { createMockQuery, type MockQuery } from "../../test/mocks/claude-sdk";
+import { Pushable } from "../../utils/streams";
+
+vi.mock("@anthropic-ai/claude-agent-sdk", () => ({
+  query: vi.fn(),
+}));
+
+vi.mock("./mcp/tool-metadata", () => ({
+  fetchMcpToolMetadata: vi.fn().mockResolvedValue(undefined),
+  getConnectedMcpServerNames: vi.fn().mockReturnValue([]),
+  setMcpToolApprovalStates: vi.fn(),
+  isMcpToolReadOnly: vi.fn().mockReturnValue(false),
+  getMcpToolMetadata: vi.fn().mockReturnValue(undefined),
+  getMcpToolApprovalState: vi.fn().mockReturnValue(undefined),
+}));
+
+const { ClaudeAcpAgent } = await import("./claude-agent");
+type Agent = InstanceType<typeof ClaudeAcpAgent>;
+
+interface ClientMocks {
+  sessionUpdate: ReturnType<typeof vi.fn>;
+  extNotification: ReturnType<typeof vi.fn>;
+}
+
+function makeAgent(): { agent: Agent; client: ClientMocks } {
+  const client: ClientMocks = {
+    sessionUpdate: vi.fn().mockResolvedValue(undefined),
+    extNotification: vi.fn().mockResolvedValue(undefined),
+  };
+  const agent = new ClaudeAcpAgent(client as unknown as AgentSideConnection);
+  return { agent, client };
+}
+
+function installFakeSession(agent: Agent, sessionId: string): MockQuery {
+  const query = createMockQuery();
+  const input = new Pushable();
+  const abortController = new AbortController();
+
+  const session = {
+    query,
+    queryOptions: { sessionId, cwd: "/tmp/repo", abortController },
+    input,
+    cancelled: false,
+    interruptReason: undefined,
+    settingsManager: { dispose: vi.fn(), getRepoRoot: () => "/tmp/repo" },
+    permissionMode: "default" as const,
+    abortController,
+    accumulatedUsage: {
+      inputTokens: 0,
+      outputTokens: 0,
+      cachedReadTokens: 0,
+      cachedWriteTokens: 0,
+    },
+    configOptions: [],
+    promptRunning: false,
+    pendingMessages: new Map(),
+    nextPendingOrder: 0,
+    cwd: "/tmp/repo",
+    notificationHistory: [] as unknown[],
+    taskRunId: "run-1",
+    lastContextWindowSize: 200_000,
+    modelId: "claude-sonnet-4-6",
+  };
+
+  (agent as unknown as { session: typeof session }).session = session;
+  (agent as unknown as { sessionId: string }).sessionId = sessionId;
+
+  return query;
+}
+
+function findUnsupportedChunkText(
+  calls: ClientMocks["sessionUpdate"]["mock"]["calls"],
+): string | undefined {
+  const match = calls.find(([call]) => {
+    const update = (
+      call as {
+        update?: { sessionUpdate?: string; content?: { text?: string } };
+      }
+    ).update;
+    return (
+      update?.sessionUpdate === "agent_message_chunk" &&
+      update?.content?.text?.toLowerCase().includes("unsupported")
+    );
+  });
+  return (match?.[0] as { update: { content: { text: string } } } | undefined)
+    ?.update.content.text;
+}
+
+describe("ClaudeAcpAgent.prompt — early idle handling", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  const cases = [
+    {
+      label: "unsupported slash command surfaces error and ends turn",
+      sessionId: "s-slash",
+      prompt: "/plugin install slack",
+      expectsUnsupportedChunk: true,
+      commandInMessage: "/plugin",
+    },
+    {
+      label: "non-slash prompt with early idle is silently skipped",
+      sessionId: "s-regular",
+      prompt: "hello",
+      expectsUnsupportedChunk: false,
+      commandInMessage: null,
+    },
+  ] as const;
+
+  it.each(cases)("$label", async (tc) => {
+    const { agent, client } = makeAgent();
+    const query = installFakeSession(agent, tc.sessionId);
+
+    const promptPromise = agent.prompt({
+      sessionId: tc.sessionId,
+      prompt: [{ type: "text", text: tc.prompt }],
+    });
+
+    // Let the prompt loop start awaiting the first SDK message.
+    await new Promise((resolve) => setImmediate(resolve));
+
+    query._mockHelpers.sendMessage({
+      type: "system",
+      subtype: "session_state_changed",
+      state: "idle",
+    } as unknown as SDKMessage);
+    query._mockHelpers.complete();
+
+    if (tc.expectsUnsupportedChunk) {
+      const result = await promptPromise;
+      expect(result.stopReason).toBe("end_turn");
+
+      const text = findUnsupportedChunkText(client.sessionUpdate.mock.calls);
+      expect(text).toBeDefined();
+      if (tc.commandInMessage) {
+        expect(text).toContain(tc.commandInMessage);
+      }
+    } else {
+      // No unsupported chunk; loop falls through to the existing
+      // "Session did not end in result" failure path.
+      await expect(promptPromise).rejects.toThrow(
+        /Session did not end in result/,
+      );
+      expect(
+        findUnsupportedChunkText(client.sessionUpdate.mock.calls),
+      ).toBeUndefined();
+    }
+  });
+});
diff --git a/packages/agent/src/adapters/claude/claude-agent.ts b/packages/agent/src/adapters/claude/claude-agent.ts
index def341f56..1ff657452 100644
--- a/packages/agent/src/adapters/claude/claude-agent.ts
+++ b/packages/agent/src/adapters/claude/claude-agent.ts
@@ -496,6 +496,28 @@ export class ClaudeAcpAgent extends BaseAcpAgent {
               (message as Record<string, unknown>).state === "idle"
             ) {
               if (!promptReplayed) {
+                // The SDK consumed a slash command we do not handle locally
+                // and produced no output (e.g. /plugin in a non-interactive
+                // context). Without this branch we would loop forever waiting
+                // for an echo that never comes; surface a clear error instead.
+                if (commandMatch) {
+                  const cmd = commandMatch[1];
+                  this.logger.warn(
+                    "Slash command produced no output; treating as unsupported",
+                    { sessionId: params.sessionId, command: cmd },
+                  );
+                  await this.client.sessionUpdate({
+                    sessionId: params.sessionId,
+                    update: {
+                      sessionUpdate: "agent_message_chunk",
+                      content: {
+                        type: "text",
+                        text: `Unsupported slash command: \`${cmd}\`. PostHog Code does not implement this command.`,
+                      },
+                    },
+                  });
+                  return { stopReason: "end_turn" };
+                }
                 this.logger.debug("Skipping idle state before prompt replay", {
                   sessionId: params.sessionId,
                 });