diff --git a/src/hooks/runtime-fallback/message-update-handler.ts b/src/hooks/runtime-fallback/message-update-handler.ts
index 0c07405a0..7382ebd2e 100644
--- a/src/hooks/runtime-fallback/message-update-handler.ts
+++ b/src/hooks/runtime-fallback/message-update-handler.ts
@@ -8,6 +8,7 @@ import { getFallbackModelsForSession } from "./fallback-models"
 import { resolveFallbackBootstrapModel } from "./fallback-bootstrap-model"
 import { dispatchFallbackRetry } from "./fallback-retry-dispatcher"
 import { hasVisibleAssistantResponse } from "./visible-assistant-response"
+import { subagentSessions } from "../../features/claude-code-session-state"
 
 export { hasVisibleAssistantResponse } from "./visible-assistant-response"
 
@@ -112,6 +113,16 @@ export function createMessageUpdateHandler(deps: HookDeps, helpers: AutoRetryHel
       const fallbackModels = getFallbackModelsForSession(sessionID, resolvedAgent, pluginConfig)
 
       if (fallbackModels.length === 0) {
+        if (
+          subagentSessions.has(sessionID) &&
+          classifyErrorType(error) === "quota_exceeded"
+        ) {
+          log(`[${HOOK_NAME}] Aborting subagent on unrecoverable quota error (no fallback configured)`, {
+            sessionID,
+            model,
+          })
+          await helpers.abortSessionRequest(sessionID, "message.updated.subagent-quota-no-fallback")
+        }
         return
       }
 
diff --git a/src/hooks/runtime-fallback/subagent-quota-abort.test.ts b/src/hooks/runtime-fallback/subagent-quota-abort.test.ts
new file mode 100644
index 000000000..1507cf6f6
--- /dev/null
+++ b/src/hooks/runtime-fallback/subagent-quota-abort.test.ts
@@ -0,0 +1,142 @@
+import { afterEach, beforeEach, describe, expect, it } from "bun:test"
+import type { HookDeps, RuntimeFallbackPluginInput } from "./types"
+import type { AutoRetryHelpers } from "./auto-retry"
+import { subagentSessions } from "../../features/claude-code-session-state"
+
+type MessageUpdateHandlerModule = typeof import("./message-update-handler")
+
+async function importFreshMessageUpdateHandlerModule(): Promise<MessageUpdateHandlerModule> {
+  return import(`./message-update-handler?subagent-quota-${Date.now()}-${Math.random()}`)
+}
+
+function createContext(): RuntimeFallbackPluginInput {
+  return {
+    client: {
+      session: {
+        abort: async () => ({}),
+        messages: async () => ({ data: [] }),
+        promptAsync: async () => ({}),
+      },
+      tui: {
+        showToast: async () => ({}),
+      },
+    },
+    directory: "/test/dir",
+  }
+}
+
+function createDeps(): HookDeps {
+  return {
+    ctx: createContext(),
+    config: {
+      enabled: true,
+      retry_on_errors: [429, 503, 529],
+      max_fallback_attempts: 3,
+      cooldown_seconds: 60,
+      timeout_seconds: 30,
+      notify_on_fallback: false,
+    },
+    options: undefined,
+    pluginConfig: {},
+    sessionStates: new Map(),
+    sessionLastAccess: new Map(),
+    sessionRetryInFlight: new Set(),
+    sessionAwaitingFallbackResult: new Set(),
+    sessionFallbackTimeouts: new Map(),
+    sessionStatusRetryKeys: new Map(),
+  }
+}
+
+function createHelpers(abortCalls: Array<{ sessionID: string; source: string }>): AutoRetryHelpers {
+  return {
+    abortSessionRequest: async (sessionID: string, source: string) => {
+      abortCalls.push({ sessionID, source })
+    },
+    clearSessionFallbackTimeout: () => {},
+    scheduleSessionFallbackTimeout: () => {},
+    autoRetryWithFallback: async () => {},
+    resolveAgentForSessionFromContext: async () => undefined,
+    cleanupStaleSessions: () => {},
+  }
+}
+
+const QUOTA_ERROR = {
+  name: "QuotaExceededError",
+  message: "You exceeded your current quota. Please check your plan and billing details.",
+}
+
+const QUOTA_INFO = {
+  role: "assistant",
+  model: "openai/gpt-5.5",
+  error: QUOTA_ERROR,
+}
+
+describe("createMessageUpdateHandler subagent quota abort", () => {
+  beforeEach(() => {
+    subagentSessions.clear()
+  })
+
+  afterEach(() => {
+    subagentSessions.clear()
+  })
+
+  it("#given a subagent session hits a quota error with no fallback configured #when the assistant error event fires #then the subagent session is aborted so the parent tool call can resolve", async () => {
+    // given
+    const { createMessageUpdateHandler } = await importFreshMessageUpdateHandlerModule()
+    const sessionID = "session-momus-subagent"
+    subagentSessions.add(sessionID)
+    const abortCalls: Array<{ sessionID: string; source: string }> = []
+    const deps = createDeps()
+    const handler = createMessageUpdateHandler(deps, createHelpers(abortCalls))
+
+    // when
+    await handler({ info: { sessionID, ...QUOTA_INFO } })
+
+    // then
+    expect(abortCalls).toEqual([
+      { sessionID, source: "message.updated.subagent-quota-no-fallback" },
+    ])
+  })
+
+  it("#given a non-subagent (user) session hits the same quota error #when the assistant error event fires #then the user session is NOT aborted", async () => {
+    // given
+    const { createMessageUpdateHandler } = await importFreshMessageUpdateHandlerModule()
+    const sessionID = "session-user-foreground"
+    // NOT added to subagentSessions
+    const abortCalls: Array<{ sessionID: string; source: string }> = []
+    const deps = createDeps()
+    const handler = createMessageUpdateHandler(deps, createHelpers(abortCalls))
+
+    // when
+    await handler({ info: { sessionID, ...QUOTA_INFO } })
+
+    // then
+    expect(abortCalls).toEqual([])
+  })
+
+  it("#given a subagent session hits a non-quota retryable error (rate limit) with no fallback configured #when the assistant error event fires #then the subagent is NOT aborted (preserves existing behavior for other error classes)", async () => {
+    // given
+    const { createMessageUpdateHandler } = await importFreshMessageUpdateHandlerModule()
+    const sessionID = "session-rate-limited-subagent"
+    subagentSessions.add(sessionID)
+    const abortCalls: Array<{ sessionID: string; source: string }> = []
+    const deps = createDeps()
+    const handler = createMessageUpdateHandler(deps, createHelpers(abortCalls))
+
+    // when
+    await handler({
+      info: {
+        sessionID,
+        role: "assistant",
+        model: "openai/gpt-5.5",
+        error: {
+          name: "RateLimitError",
+          message: "rate limit exceeded, retrying in 30s",
+        },
+      },
+    })
+
+    // then
+    expect(abortCalls).toEqual([])
+  })
+})