feat(agent): add refusal handling and model fallback

packages/agent/src/adapters/claude/UPSTREAM.md

@@ -38,6 +38,12 @@ Fork of `@anthropic-ai/claude-agent-acp`. Upstream repo: https://github.com/anth
 - `SYSTEM_REMINDER` stripping from Read tool results
 - WebFetch `resourceLink` content enrichment
 - `customTitle` in listSessions (PostHog Code is ahead of upstream here)
+- Refusal support: `Options.fallbackModel` defaults to `FALLBACK_MODEL` in
+  `session/options.ts`; `model_refusal_fallback` system messages emit a
+  `_posthog/status` notification (`refusal_fallback`) in `sdk-to-acp.ts`; a
+  terminal `stop_reason: "refusal"` emits `_posthog/status` (`refusal`) instead
+  of upstream's raw-explanation `agent_message_chunk` (supersedes the v0.42.0
+  "Refusal handling" port and the v0.44.0 `model_refusal_fallback` skip)
 - SettingsManager `PreToolUse` hook for permission rules
 - `ensureLocalSettings` / `clearStatsigCache`
 - `ELECTRON_RUN_AS_NODE` / `ENABLE_TOOL_SEARCH` env vars

packages/agent/src/adapters/claude/claude-agent.ts

@@ -517,6 +517,7 @@ export class ClaudeAcpAgent extends BaseAcpAgent {
     let errored = false;
     let lastAssistantTotalUsage: number | null = null;
     let lastRefusalExplanation: string | null = null;
+    let lastRefusalCategory: string | null = null;
     let lastStreamUsage = {
       input_tokens: 0,
       output_tokens: 0,
@@ -870,15 +871,17 @@ export class ClaudeAcpAgent extends BaseAcpAgent {
             if (
               (message as { stop_reason?: string }).stop_reason === "refusal"
             ) {
-              if (lastRefusalExplanation) {
-                await this.client.sessionUpdate({
-                  sessionId: params.sessionId,
-                  update: {
-                    sessionUpdate: "agent_message_chunk",
-                    content: { type: "text", text: lastRefusalExplanation },
-                  },
-                });
-              }
+              // The API's stop_details.explanation is integrator-facing prose,
+              // so surface the refusal as a structured status row rather than
+              // assistant text.
+              await this.client.extNotification(POSTHOG_NOTIFICATIONS.STATUS, {
+                sessionId: params.sessionId,
+                status: "refusal",
+                ...(lastRefusalExplanation && {
+                  explanation: lastRefusalExplanation,
+                }),
+                ...(lastRefusalCategory && { category: lastRefusalCategory }),
+              });
               return { stopReason: "refusal", usage };
             }
 
@@ -1002,11 +1005,15 @@ export class ClaudeAcpAgent extends BaseAcpAgent {
             if (message.type === "assistant") {
               const inner = message.message as unknown as {
                 stop_reason?: string | null;
-                stop_details?: { explanation?: string | null } | null;
+                stop_details?: {
+                  category?: string | null;
+                  explanation?: string | null;
+                } | null;
               };
               if (inner.stop_reason === "refusal") {
                 lastRefusalExplanation =
                   inner.stop_details?.explanation ?? null;
+                lastRefusalCategory = inner.stop_details?.category ?? null;
               }
             }
 

packages/agent/src/adapters/claude/conversion/sdk-to-acp.test.ts

@@ -4,6 +4,7 @@ import type {
 } from "@agentclientprotocol/sdk";
 import type {
   SDKAssistantMessage,
+  SDKModelRefusalFallbackMessage,
   SDKPartialAssistantMessage,
   SDKUserMessage,
 } from "@anthropic-ai/claude-agent-sdk";
@@ -12,6 +13,7 @@ import { Logger } from "../../../utils/logger";
 import type { Session } from "../types";
 import {
   handleStreamEvent,
+  handleSystemMessage,
   handleUserAssistantMessage,
   type MessageHandlerContext,
   stripMarkerTags,
@@ -64,10 +66,14 @@ describe("stripMarkerTags", () => {
 
 function createHandlerContext() {
   const updates: SessionNotification[] = [];
+  const notifications: Array<{ method: string; params: unknown }> = [];
   const client = {
     sessionUpdate: async (notification: SessionNotification) => {
       updates.push(notification);
     },
+    extNotification: async (method: string, params: unknown) => {
+      notifications.push({ method, params });
+    },
   } as unknown as AgentSideConnection;
   const context: MessageHandlerContext = {
     session: {
@@ -86,7 +92,7 @@ function createHandlerContext() {
       thinkingIds: new Set(),
     },
   };
-  return { context, updates };
+  return { context, updates, notifications };
 }
 
 function streamEvent(
@@ -361,3 +367,67 @@ describe("import replay (no client-side history)", () => {
     expect(chunkTexts(updates, "agent_message_chunk")).toEqual([]);
   });
 });
+
+describe("handleSystemMessage model_refusal_fallback", () => {
+  function refusalFallbackMessage(
+    overrides: Partial<SDKModelRefusalFallbackMessage> = {},
+  ): SDKModelRefusalFallbackMessage {
+    return {
+      type: "system",
+      subtype: "model_refusal_fallback",
+      trigger: "refusal",
+      direction: "retry",
+      original_model: "claude-fable-5",
+      fallback_model: "claude-opus-4-8",
+      request_id: "req_1",
+      api_refusal_category: "cyber",
+      api_refusal_explanation: "This request was declined.",
+      retracted_message_uuids: [],
+      content: "Retried on fallback model",
+      uuid: "00000000-0000-0000-0000-000000000009",
+      session_id: "test-session",
+      ...overrides,
+    };
+  }
+
+  it("emits a refusal_fallback status notification with the model swap", async () => {
+    const { context, updates, notifications } = createHandlerContext();
+
+    await handleSystemMessage(refusalFallbackMessage(), context);
+
+    expect(updates).toEqual([]);
+    expect(notifications).toEqual([
+      {
+        method: "_posthog/status",
+        params: {
+          sessionId: "test-session",
+          status: "refusal_fallback",
+          fromModel: "claude-fable-5",
+          toModel: "claude-opus-4-8",
+          explanation: "This request was declined.",
+        },
+      },
+    ]);
+  });
+
+  it("omits the explanation when the refused response carried none", async () => {
+    const { context, notifications } = createHandlerContext();
+
+    await handleSystemMessage(
+      refusalFallbackMessage({ api_refusal_explanation: null }),
+      context,
+    );
+
+    expect(notifications).toEqual([
+      {
+        method: "_posthog/status",
+        params: {
+          sessionId: "test-session",
+          status: "refusal_fallback",
+          fromModel: "claude-fable-5",
+          toModel: "claude-opus-4-8",
+        },
+      },
+    ]);
+  });
+});

packages/agent/src/adapters/claude/conversion/sdk-to-acp.ts

@@ -796,6 +796,26 @@ export async function handleSystemMessage(
         `Session ${sessionId}: failed to persist history: ${message.error}`,
       );
       break;
+    case "model_refusal_fallback": {
+      logger.info("Refusal retried on fallback model", {
+        sessionId,
+        direction: message.direction,
+        originalModel: message.original_model,
+        fallbackModel: message.fallback_model,
+        category: message.api_refusal_category ?? undefined,
+        requestId: message.request_id ?? undefined,
+      });
+      await client.extNotification(POSTHOG_NOTIFICATIONS.STATUS, {
+        sessionId,
+        status: "refusal_fallback",
+        fromModel: message.original_model,
+        toModel: message.fallback_model,
+        ...(message.api_refusal_explanation && {
+          explanation: message.api_refusal_explanation,
+        }),
+      });
+      break;
+    }
     case "permission_denied": {
       const reason = message.decision_reason ?? message.message;
       await client.sessionUpdate({

packages/agent/src/adapters/claude/session/models.ts

@@ -2,6 +2,10 @@ import type { EffortLevel } from "../types";
 
 export const DEFAULT_MODEL = "opus";
 
+// Refusal/overload rescue target. The SDK rejects fallbackModel === Options.model
+// at spawn, so this must stay distinct from the alias form used for DEFAULT_MODEL.
+export const FALLBACK_MODEL = "claude-opus-4-8";
+
 // Default thinking level when the user hasn't picked one. Adaptive-only models
 // like claude-fable-5 reject the SDK's no-effort `thinking: { type: "disabled" }`
 // shape, so effort-capable models default to high to keep thinking enabled.

packages/agent/src/adapters/claude/session/options.test.ts

@@ -92,6 +92,29 @@ describe("buildSessionOptions", () => {
     expect(options.agents?.["ph-explore"]).toEqual(override);
   });
 
+  it("defaults fallbackModel so refusals and overloads retry on another model", () => {
+    const options = buildSessionOptions(makeParams());
+
+    expect(options.fallbackModel).toBe("claude-opus-4-8");
+    // The SDK throws at spawn when fallbackModel equals Options.model.
+    expect(options.fallbackModel).not.toBe(options.model);
+  });
+
+  it("defaults fallbackModel on resumed sessions", () => {
+    const options = buildSessionOptions({ ...makeParams(), isResume: true });
+
+    expect(options.fallbackModel).toBe("claude-opus-4-8");
+  });
+
+  it("preserves a caller-provided fallbackModel", () => {
+    const options = buildSessionOptions({
+      ...makeParams(),
+      userProvidedOptions: { fallbackModel: "claude-sonnet-5" },
+    });
+
+    expect(options.fallbackModel).toBe("claude-sonnet-5");
+  });
+
   it("threads onEnsureLocalToolsConnected into the signed-commit guard (cloud)", async () => {
     const healSpy = vi.fn().mockResolvedValue(true);
     await runPreToolUseHooks(

packages/agent/src/adapters/claude/session/options.ts

@@ -28,7 +28,7 @@ import type { CodeExecutionMode } from "../tools";
 import type { EffortLevel } from "../types";
 import { APPENDED_INSTRUCTIONS } from "./instructions";
 import { loadUserClaudeJsonMcpServers } from "./mcp-config";
-import { DEFAULT_MODEL } from "./models";
+import { DEFAULT_MODEL, FALLBACK_MODEL } from "./models";
 import type { SettingsManager } from "./settings";
 
 export interface ProcessSpawnedInfo {
@@ -487,6 +487,10 @@ export function buildSessionOptions(params: BuildOptionsParams): Options {
     options.model = DEFAULT_MODEL;
   }
 
+  if (!options.fallbackModel && options.model !== FALLBACK_MODEL) {
+    options.fallbackModel = FALLBACK_MODEL;
+  }
+
   if (params.additionalDirectories) {
     options.additionalDirectories = params.additionalDirectories;
   }

packages/ui/src/features/sessions/components/buildConversationItems.test.ts

@@ -123,6 +123,22 @@ function statusMsg(
   };
 }
 
+function refusalStatusMsg(
+  ts: number,
+  status: "refusal" | "refusal_fallback",
+  fields: { explanation?: string; fromModel?: string; toModel?: string } = {},
+): AcpMessage {
+  return {
+    type: "acp_message",
+    ts,
+    message: {
+      jsonrpc: "2.0",
+      method: "_posthog/status",
+      params: { sessionId: "session-1", status, ...fields },
+    },
+  };
+}
+
 describe("buildConversationItems", () => {
   it("extracts cloud prompt attachments into user messages", () => {
     const uri = makeAttachmentUri("/tmp/hello world.txt");
@@ -222,6 +238,56 @@ describe("buildConversationItems", () => {
     expect(result.isCompacting).toBe(false);
   });
 
+  it("renders a terminal refusal as a status row carrying the explanation", () => {
+    const result = buildConversationItems(
+      [
+        userPromptMsg(1, 1, "hi"),
+        refusalStatusMsg(2, "refusal", {
+          explanation: "This request was declined.",
+        }),
+      ],
+      null,
+    );
+
+    const statusItems = result.items.filter(
+      (i): i is Extract<ConversationItem, { type: "session_update" }> =>
+        i.type === "session_update" && i.update.sessionUpdate === "status",
+    );
+    expect(statusItems.map((i) => i.update)).toEqual([
+      {
+        sessionUpdate: "status",
+        status: "refusal",
+        explanation: "This request was declined.",
+      },
+    ]);
+  });
+
+  it("renders a refusal fallback status row carrying the model swap", () => {
+    const result = buildConversationItems(
+      [
+        userPromptMsg(1, 1, "hi"),
+        refusalStatusMsg(2, "refusal_fallback", {
+          fromModel: "claude-fable-5",
+          toModel: "claude-opus-4-8",
+        }),
+      ],
+      null,
+    );
+
+    const statusItems = result.items.filter(
+      (i): i is Extract<ConversationItem, { type: "session_update" }> =>
+        i.type === "session_update" && i.update.sessionUpdate === "status",
+    );
+    expect(statusItems.map((i) => i.update)).toEqual([
+      {
+        sessionUpdate: "status",
+        status: "refusal_fallback",
+        fromModel: "claude-fable-5",
+        toModel: "claude-opus-4-8",
+      },
+    ]);
+  });
+
   it("marks cloud turns complete from structured turn completion notifications", () => {
     const result = buildConversationItems(
       [userPromptMsg(10, 42, "hello"), turnCompleteMsg(25)],

packages/ui/src/features/sessions/components/buildConversationItems.ts

@@ -535,7 +535,20 @@ function handleNotification(
       status: string;
       isComplete?: boolean;
       error?: string;
+      explanation?: string;
+      fromModel?: string;
+      toModel?: string;
     };
+    if (params.status === "refusal" || params.status === "refusal_fallback") {
+      pushItem(b, {
+        sessionUpdate: "status",
+        status: params.status,
+        explanation: params.explanation,
+        fromModel: params.fromModel,
+        toModel: params.toModel,
+      });
+      return;
+    }
     if (params.status === "compacting") {
       if (params.isComplete) {
         // Successful compaction — flip the existing "Compacting…" status to

packages/ui/src/features/sessions/components/session-update/SessionUpdateView.tsx

@@ -35,6 +35,12 @@ export type RenderItem =
       isComplete?: boolean;
       /** Set when a status ends in failure (e.g. a failed compaction) so the row renders the error. */
       error?: string;
+      /** Refusal statuses: display-only stop_details.explanation from the API. */
+      explanation?: string;
+      /** Refusal fallback: the model that declined the request. */
+      fromModel?: string;
+      /** Refusal fallback: the model that retried the request. */
+      toModel?: string;
     }
   | {
       sessionUpdate: "error";
@@ -125,6 +131,9 @@ export const SessionUpdateView = memo(function SessionUpdateView({
           status={item.status}
           isComplete={item.isComplete}
           error={item.error}
+          explanation={item.explanation}
+          fromModel={item.fromModel}
+          toModel={item.toModel}
         />
       );
     case "error":