feat(chat): handle credit-gated send responses

src/stores/chat/__tests__/chat-machine.helpers.test.ts

@@ -38,6 +38,12 @@ function makeChatState(overrides: Partial<ChatState> = {}): ChatState {
     pendingReplyCount: 1,
     upgradePromptVisible: false,
     upgradeReason: null,
+    canSendMessage: true,
+    cannotSendReason: null,
+    creditBalance: 0,
+    creditsCharged: 0,
+    requiredCredits: 0,
+    shortfallCredits: 0,
     isLoadingMore: false,
     hasMore: true,
     historyOffset: 0,
@@ -168,6 +174,96 @@ describe("applyHttpSendOutput", () => {
     expect(nextState.upgradePromptVisible).toBe(true);
     expect(nextState.upgradeReason).toBe("weekly_limit");
   });
+
+  it("stores insufficient credit state and removes failed optimistic messages", () => {
+    const context = makeChatState({
+      messages: [
+        {
+          content: "hello",
+          isFromAI: false,
+          date: "2026-06-25",
+        },
+      ],
+    });
+
+    const nextState = applyHttpSendOutput(context, {
+      response: makeResponse({
+        reply: "",
+        messageId: "",
+        canSendMessage: false,
+        cannotSendReason: "insufficient_credits",
+        creditBalance: 0,
+        creditsCharged: 0,
+        requiredCredits: 2,
+        shortfallCredits: 2,
+        lockDetail: {
+          locked: true,
+          showContent: false,
+          showUpgrade: true,
+          reason: "insufficient_credits",
+          hint: "Insufficient credits.",
+          detail: {
+            requiredCredits: 2,
+            currentCredits: 0,
+            shortfallCredits: 2,
+          },
+        },
+      }),
+      reply: null,
+    });
+
+    expect(nextState.messages).toEqual([]);
+    expect(nextState.isReplyingAI).toBe(false);
+    expect(nextState.upgradePromptVisible).toBe(true);
+    expect(nextState.upgradeReason).toBe("insufficient_credits");
+    expect(nextState.canSendMessage).toBe(false);
+    expect(nextState.cannotSendReason).toBe("insufficient_credits");
+    expect(nextState.requiredCredits).toBe(2);
+    expect(nextState.shortfallCredits).toBe(2);
+  });
+
+  it("keeps a valid reply while marking future sends as unavailable", () => {
+    const context = makeChatState({
+      messages: [
+        {
+          content: "hello",
+          isFromAI: false,
+          date: "2026-06-25",
+        },
+      ],
+    });
+    const reply = sendResponseToUiMessage(
+      makeResponse({
+        reply: "This one went through, but you need credits next.",
+        canSendMessage: false,
+        cannotSendReason: "insufficient_credits",
+        creditBalance: 0,
+        requiredCredits: 2,
+        shortfallCredits: 2,
+      }),
+    );
+
+    const nextState = applyHttpSendOutput(context, {
+      response: makeResponse({
+        reply: "This one went through, but you need credits next.",
+        canSendMessage: false,
+        cannotSendReason: "insufficient_credits",
+        creditBalance: 0,
+        requiredCredits: 2,
+        shortfallCredits: 2,
+      }),
+      reply,
+    });
+
+    expect(nextState.messages).toHaveLength(2);
+    expect(nextState.messages?.[1]).toMatchObject({
+      content: "This one went through, but you need credits next.",
+      isFromAI: true,
+    });
+    expect(nextState.upgradePromptVisible).toBe(true);
+    expect(nextState.upgradeReason).toBe("insufficient_credits");
+    expect(nextState.canSendMessage).toBe(false);
+  });
 });
 
 describe("localMessagesToUi", () => {

src/stores/chat/__tests__/chat-machine.transitions.test.ts

@@ -467,6 +467,11 @@ describe("chatMachine transitions", () => {
               limit: 3,
             },
           },
+          canSendMessage: false,
+          cannotSendReason: "insufficient_credits",
+          creditBalance: 0,
+          requiredCredits: 2,
+          shortfallCredits: 2,
         }),
         reply: null,
       },
@@ -474,11 +479,14 @@ describe("chatMachine transitions", () => {
 
     expect(actor.getSnapshot().context.upgradePromptVisible).toBe(true);
     expect(actor.getSnapshot().context.upgradeReason).toBe("weekly_limit");
+    expect(actor.getSnapshot().context.canSendMessage).toBe(false);
 
     actor.send({ type: "ChatPaymentSucceeded" });
 
     expect(actor.getSnapshot().context.upgradePromptVisible).toBe(false);
     expect(actor.getSnapshot().context.upgradeReason).toBeNull();
+    expect(actor.getSnapshot().context.canSendMessage).toBe(true);
+    expect(actor.getSnapshot().context.cannotSendReason).toBeNull();
 
     actor.stop();
   });