diff --git a/src/hooks/runtime-fallback/first-prompt-watchdog.test.ts b/src/hooks/runtime-fallback/first-prompt-watchdog.test.ts
index a3fcb72df..cc187b7ac 100644
--- a/src/hooks/runtime-fallback/first-prompt-watchdog.test.ts
+++ b/src/hooks/runtime-fallback/first-prompt-watchdog.test.ts
@@ -2,11 +2,18 @@ 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"
-import { createFirstPromptWatchdog } from "./first-prompt-watchdog"
+import { createFirstPromptWatchdog, observeEventForWatchdog, type FirstPromptWatchdog } from "./first-prompt-watchdog"
 
-const WATCHDOG_MS = 40
-const SAFE_WAIT_AFTER_FIRE_MS = 120
-const SAFE_WAIT_BEFORE_FIRE_MS = 15
+// Real timers are unavoidable here (bun:test has no built-in fake-timer API),
+// so margins are sized generously to survive a loaded CI runner. Specifically:
+//   - SAFE_WAIT_BEFORE_FIRE_MS must be << WATCHDOG_MS so the cancel call lands
+//     before the timer fires even with significant scheduler delay
+//     (margin: WATCHDOG_MS - SAFE_WAIT_BEFORE_FIRE_MS >= 60ms here).
+//   - SAFE_WAIT_AFTER_FIRE_MS must be >> WATCHDOG_MS so we conclusively
+//     observe whether the timer fired (margin: ~2.5x WATCHDOG_MS).
+const WATCHDOG_MS = 100
+const SAFE_WAIT_BEFORE_FIRE_MS = 40
+const SAFE_WAIT_AFTER_FIRE_MS = 250
 
 function wait(ms: number): Promise<void> {
   return new Promise((resolve) => setTimeout(resolve, ms))
@@ -178,7 +185,7 @@ describe("first-prompt-watchdog", () => {
     watchdog.dispose()
   })
 
-  it("#given a subagent silent past the threshold with no fallback configured #when the watchdog fires #then it logs but does not abort or dispatch (lets PR #3950 quota-abort path handle it later if an error event arrives)", async () => {
+  it("#given a subagent silent past the threshold with no fallback configured #when the watchdog fires #then it logs but does not abort or dispatch (lets the existing error-event paths handle it if one arrives later)", async () => {
     // given
     const sessionID = "session-no-fallback"
     subagentSessions.add(sessionID)
@@ -198,3 +205,135 @@ describe("first-prompt-watchdog", () => {
     watchdog.dispose()
   })
 })
+
+interface RecordedWatchdogCalls {
+  user: Array<{ sessionID: string; model?: string; agent?: string }>
+  progress: string[]
+  terminal: string[]
+}
+
+function createRecordingWatchdog(calls: RecordedWatchdogCalls): FirstPromptWatchdog {
+  return {
+    onUserMessage(sessionID, model, agent) {
+      calls.user.push({ sessionID, model, agent })
+    },
+    onAssistantProgress(sessionID) {
+      calls.progress.push(sessionID)
+    },
+    onSessionTerminal(sessionID) {
+      calls.terminal.push(sessionID)
+    },
+    dispose() {},
+  }
+}
+
+describe("observeEventForWatchdog", () => {
+  const sessionID = "session-observed"
+
+  function freshCalls(): RecordedWatchdogCalls {
+    return { user: [], progress: [], terminal: [] }
+  }
+
+  it("#given a message.updated event with role=user #when observed #then onUserMessage is called with sessionID/model/agent", () => {
+    const calls = freshCalls()
+    observeEventForWatchdog(
+      {
+        type: "message.updated",
+        properties: { info: { sessionID, role: "user", model: "openai/gpt-5.4-mini", agent: "sisyphus-junior" } },
+      },
+      createRecordingWatchdog(calls),
+    )
+    expect(calls.user).toEqual([{ sessionID, model: "openai/gpt-5.4-mini", agent: "sisyphus-junior" }])
+    expect(calls.progress).toEqual([])
+    expect(calls.terminal).toEqual([])
+  })
+
+  it.each([
+    ["text", { type: "text", text: "hello" }],
+    ["reasoning", { type: "reasoning", text: "thinking..." }],
+    ["tool", { type: "tool" }],
+    ["tool_use", { type: "tool_use", id: "t1", name: "Read" }],
+    ["tool_result", { type: "tool_result", tool_use_id: "t1" }],
+    ["tool-call", { type: "tool-call" }],
+    ["step-start", { type: "step-start" }],
+    ["file", { type: "file" }],
+  ])("#given a message.updated assistant event whose only part is type=%s #when observed #then onAssistantProgress is called (model is *working*, not silent)", (_label, part) => {
+    const calls = freshCalls()
+    observeEventForWatchdog(
+      {
+        type: "message.updated",
+        properties: { info: { sessionID, role: "assistant" }, parts: [part] },
+      },
+      createRecordingWatchdog(calls),
+    )
+    expect(calls.progress).toEqual([sessionID])
+  })
+
+  it("#given a message.updated assistant event with parts: [] and no error/finish #when observed #then no progress is signalled (no activity yet)", () => {
+    const calls = freshCalls()
+    observeEventForWatchdog(
+      {
+        type: "message.updated",
+        properties: { info: { sessionID, role: "assistant" }, parts: [] },
+      },
+      createRecordingWatchdog(calls),
+    )
+    expect(calls.progress).toEqual([])
+  })
+
+  it("#given a message.updated assistant event with info.error set #when observed #then onAssistantProgress is called (the existing error-handling path takes over from here)", () => {
+    const calls = freshCalls()
+    observeEventForWatchdog(
+      {
+        type: "message.updated",
+        properties: { info: { sessionID, role: "assistant", error: { name: "RateLimitError", message: "429" } } },
+      },
+      createRecordingWatchdog(calls),
+    )
+    expect(calls.progress).toEqual([sessionID])
+  })
+
+  it("#given a message.updated assistant event with info.finish set #when observed #then onAssistantProgress is called", () => {
+    const calls = freshCalls()
+    observeEventForWatchdog(
+      {
+        type: "message.updated",
+        properties: { info: { sessionID, role: "assistant", finish: "stop" } },
+      },
+      createRecordingWatchdog(calls),
+    )
+    expect(calls.progress).toEqual([sessionID])
+  })
+
+  it.each([["session.idle"], ["session.stop"], ["session.deleted"], ["session.error"]])(
+    "#given a %s event #when observed #then onSessionTerminal is called",
+    (eventType) => {
+      const calls = freshCalls()
+      observeEventForWatchdog(
+        { type: eventType, properties: { sessionID } },
+        createRecordingWatchdog(calls),
+      )
+      expect(calls.terminal).toEqual([sessionID])
+    },
+  )
+
+  it("#given a session.deleted event whose sessionID is carried under properties.info.id #when observed #then onSessionTerminal is still called (matches event-handler shape)", () => {
+    const calls = freshCalls()
+    observeEventForWatchdog(
+      { type: "session.deleted", properties: { info: { id: sessionID } } },
+      createRecordingWatchdog(calls),
+    )
+    expect(calls.terminal).toEqual([sessionID])
+  })
+
+  it("#given an unrelated event type #when observed #then no watchdog method is called", () => {
+    const calls = freshCalls()
+    observeEventForWatchdog(
+      { type: "session.created", properties: { info: { id: sessionID } } },
+      createRecordingWatchdog(calls),
+    )
+    expect(calls.user).toEqual([])
+    expect(calls.progress).toEqual([])
+    expect(calls.terminal).toEqual([])
+  })
+})
diff --git a/src/hooks/runtime-fallback/first-prompt-watchdog.ts b/src/hooks/runtime-fallback/first-prompt-watchdog.ts
index e4fe61e91..fdcbda8b3 100644
--- a/src/hooks/runtime-fallback/first-prompt-watchdog.ts
+++ b/src/hooks/runtime-fallback/first-prompt-watchdog.ts
@@ -20,6 +20,67 @@ export interface FirstPromptWatchdog {
   dispose(): void
 }
 
+const TERMINAL_EVENT_TYPES = new Set([
+  "session.idle",
+  "session.stop",
+  "session.deleted",
+  "session.error",
+])
+
+/**
+ * Translate an OpenCode session event into the appropriate watchdog signal.
+ *
+ * Progress semantics for cancelling the watchdog:
+ *   - assistant `info.error` set: the existing message-update-handler will
+ *     deal with the error path; the watchdog has done its job.
+ *   - assistant `info.finish` set: the response completed.
+ *   - any assistant part with a known type (`text`, `reasoning`, `tool`,
+ *     `tool_use`, `tool_result`, `tool-call`, `step-start`, `file`, ...):
+ *     the model has started responding. A subagent that immediately runs
+ *     tools is *working*, not silent — so any part presence cancels.
+ */
+export function observeEventForWatchdog(
+  event: { type: string; properties?: unknown },
+  watchdog: FirstPromptWatchdog,
+): void {
+  const props = event.properties as Record<string, unknown> | undefined
+  if (!props) return
+
+  if (event.type === "message.updated") {
+    const info = props.info as Record<string, unknown> | undefined
+    const sessionID = info?.sessionID as string | undefined
+    const role = info?.role as string | undefined
+    if (!sessionID || !role) return
+
+    if (role === "user") {
+      const model = info?.model as string | undefined
+      const agent = info?.agent as string | undefined
+      watchdog.onUserMessage(sessionID, model, agent)
+      return
+    }
+
+    if (role === "assistant") {
+      const hasError = info?.error !== undefined
+      const hasFinish = info?.finish !== undefined
+      const eventParts = props.parts as Array<{ type?: string }> | undefined
+      const infoParts = info?.parts as Array<{ type?: string }> | undefined
+      const parts = eventParts ?? infoParts ?? []
+      const hasAnyPart = parts.some((part) => typeof part?.type === "string")
+      if (hasError || hasFinish || hasAnyPart) {
+        watchdog.onAssistantProgress(sessionID)
+      }
+    }
+    return
+  }
+
+  if (TERMINAL_EVENT_TYPES.has(event.type)) {
+    const sessionID =
+      (props.sessionID as string | undefined) ??
+      ((props.info as Record<string, unknown> | undefined)?.id as string | undefined)
+    if (sessionID) watchdog.onSessionTerminal(sessionID)
+  }
+}
+
 export function createFirstPromptWatchdog(
   deps: HookDeps,
   helpers: AutoRetryHelpers,
diff --git a/src/hooks/runtime-fallback/hook.ts b/src/hooks/runtime-fallback/hook.ts
index 1c12a8638..4178a7dca 100644
--- a/src/hooks/runtime-fallback/hook.ts
+++ b/src/hooks/runtime-fallback/hook.ts
@@ -2,7 +2,7 @@ import { createAutoRetryHelpers } from "./auto-retry"
 import { createChatMessageHandler } from "./chat-message-handler"
 import { DEFAULT_CONFIG } from "./constants"
 import { createEventHandler } from "./event-handler"
-import { createFirstPromptWatchdog } from "./first-prompt-watchdog"
+import { createFirstPromptWatchdog, observeEventForWatchdog } from "./first-prompt-watchdog"
 import { createMessageUpdateHandler } from "./message-update-handler"
 import type { HookDeps, RuntimeFallbackHook, RuntimeFallbackInterval, RuntimeFallbackOptions, RuntimeFallbackPluginInput, RuntimeFallbackTimeout } from "./types"
 
@@ -64,55 +64,6 @@ export function createRuntimeFallbackHook(
   const chatMessageHandler = factories.createChatMessageHandler(deps)
   const firstPromptWatchdog = factories.createFirstPromptWatchdog(deps, helpers)
 
-  const TERMINAL_EVENT_TYPES = new Set([
-    "session.idle",
-    "session.stop",
-    "session.deleted",
-    "session.error",
-  ])
-
-  const observeForWatchdog = (event: { type: string; properties?: unknown }): void => {
-    const props = event.properties as Record<string, unknown> | undefined
-    if (!props) return
-
-    if (event.type === "message.updated") {
-      const info = props.info as Record<string, unknown> | undefined
-      const sessionID = info?.sessionID as string | undefined
-      const role = info?.role as string | undefined
-      if (!sessionID || !role) return
-
-      if (role === "user") {
-        const model = info?.model as string | undefined
-        const agent = info?.agent as string | undefined
-        firstPromptWatchdog.onUserMessage(sessionID, model, agent)
-        return
-      }
-
-      if (role === "assistant") {
-        const hasError = info?.error !== undefined
-        const hasFinish = info?.finish !== undefined
-        const eventParts = props.parts as Array<{ type?: string; text?: string }> | undefined
-        const infoParts = info?.parts as Array<{ type?: string; text?: string }> | undefined
-        const parts = eventParts ?? infoParts ?? []
-        const hasContent = parts.some((part) => {
-          if (part.type !== "text" && part.type !== "reasoning") return false
-          return (part.text ?? "").trim().length > 0
-        })
-        if (hasError || hasFinish || hasContent) {
-          firstPromptWatchdog.onAssistantProgress(sessionID)
-        }
-      }
-      return
-    }
-
-    if (TERMINAL_EVENT_TYPES.has(event.type)) {
-      const sessionID =
-        (props.sessionID as string | undefined) ??
-        ((props.info as Record<string, unknown> | undefined)?.id as string | undefined)
-      if (sessionID) firstPromptWatchdog.onSessionTerminal(sessionID)
-    }
-  }
-
   let cleanupInterval: RuntimeFallbackInterval | null = null
   let intervalStarted = false
 
@@ -131,7 +82,7 @@ export function createRuntimeFallbackHook(
     ensureInterval()
 
     if (config.enabled) {
-      observeForWatchdog(event)
+      observeEventForWatchdog(event, firstPromptWatchdog)
     }
 
     if (event.type === "message.updated") {