diff --git a/src/hooks/ralph-loop/completion-promise-detector.test.ts b/src/hooks/ralph-loop/completion-promise-detector.test.ts
index 814684068..d59151a7b 100644
--- a/src/hooks/ralph-loop/completion-promise-detector.test.ts
+++ b/src/hooks/ralph-loop/completion-promise-detector.test.ts
@@ -58,6 +58,29 @@ describe("detectCompletionInSessionMessages", () => {
       // #then
       expect(detected).toBe(true)
     })
+
+    test("#when sinceMessageIndex equals current message count #then should NOT rescan old DONE", async () => {
+      // #given
+      const messages = [
+        {
+          info: { role: "assistant" },
+          parts: [{ type: "text", text: "Old completion <promise>DONE</promise>" }],
+        },
+      ]
+      const ctx = createPluginInput(messages)
+
+      // #when
+      const detected = await detectCompletionInSessionMessages(ctx, {
+        sessionID: "session-123",
+        promise: "DONE",
+        apiTimeoutMs: 1000,
+        directory: "/tmp",
+        sinceMessageIndex: messages.length,
+      })
+
+      // #then
+      expect(detected).toBe(false)
+    })
   })
 
   describe("#given no sinceMessageIndex (backward compat)", () => {
diff --git a/src/hooks/ralph-loop/completion-promise-detector.ts b/src/hooks/ralph-loop/completion-promise-detector.ts
index 65718e67e..6c915a2c3 100644
--- a/src/hooks/ralph-loop/completion-promise-detector.ts
+++ b/src/hooks/ralph-loop/completion-promise-detector.ts
@@ -130,8 +130,8 @@ export async function detectCompletionInSessionMessages(
 				: []
 
 		const scopedMessages =
-			typeof options.sinceMessageIndex === "number" && options.sinceMessageIndex >= 0 && options.sinceMessageIndex < messageArray.length
-				? messageArray.slice(options.sinceMessageIndex)
+			typeof options.sinceMessageIndex === "number" && options.sinceMessageIndex >= 0
+				? messageArray.slice(Math.min(options.sinceMessageIndex, messageArray.length))
 				: messageArray
 
 		const assistantMessages = (scopedMessages as OpenCodeSessionMessage[]).filter((msg) => msg.info?.role === "assistant")
diff --git a/src/hooks/ralph-loop/index.test.ts b/src/hooks/ralph-loop/index.test.ts
index 172363473..d131744b7 100644
--- a/src/hooks/ralph-loop/index.test.ts
+++ b/src/hooks/ralph-loop/index.test.ts
@@ -288,7 +288,7 @@ describe("ralph-loop", () => {
       await hook.event({
         event: {
           type: "session.idle",
-          properties: { sessionID: "session-123" },
+          properties: { sessionID: "session-123", synthetic: true },
         },
       })
 
@@ -304,6 +304,62 @@ describe("ralph-loop", () => {
       expect(state?.iteration).toBe(2)
     })
 
+    test("#given synthetic and real idle arrive back-to-back #then only one continuation is injected for the same iteration", async () => {
+      // given
+      const hook = createRalphLoopHook(createMockPluginInput(), { idleSettleMs: 0 })
+      hook.startLoop("session-123", "Build a feature", { maxIterations: 10 })
+
+      // when
+      await hook.event({
+        event: {
+          type: "session.idle",
+          properties: { sessionID: "session-123", synthetic: true },
+        },
+      })
+      await hook.event({
+        event: {
+          type: "session.idle",
+          properties: { sessionID: "session-123" },
+        },
+      })
+
+      // then
+      expect(promptCalls.length).toBe(1)
+      expect(promptCalls[0].sessionID).toBe("session-123")
+      expect(hook.getState()?.iteration).toBe(2)
+    })
+
+    test("#given new activity after an idle continuation #when session idles again #then next iteration can continue", async () => {
+      // given
+      const hook = createRalphLoopHook(createMockPluginInput(), { idleSettleMs: 0 })
+      hook.startLoop("session-123", "Build a feature", { maxIterations: 10 })
+
+      await hook.event({
+        event: {
+          type: "session.idle",
+          properties: { sessionID: "session-123" },
+        },
+      })
+
+      // when
+      await hook.event({
+        event: {
+          type: "message.part.updated",
+          properties: { sessionID: "session-123" },
+        },
+      })
+      await hook.event({
+        event: {
+          type: "session.idle",
+          properties: { sessionID: "session-123" },
+        },
+      })
+
+      // then
+      expect(promptCalls.length).toBe(2)
+      expect(hook.getState()?.iteration).toBe(3)
+    })
+
     test("should inject continuation when idle event carries session id in info", async () => {
       // given - active loop state and nested session event shape
       const hook = createRalphLoopHook(createMockPluginInput())
diff --git a/src/hooks/ralph-loop/ralph-loop-event-handler.ts b/src/hooks/ralph-loop/ralph-loop-event-handler.ts
index c64cf2ef0..ddbef7fe6 100644
--- a/src/hooks/ralph-loop/ralph-loop-event-handler.ts
+++ b/src/hooks/ralph-loop/ralph-loop-event-handler.ts
@@ -12,6 +12,8 @@ import { continueIteration } from "./iteration-continuation"
 import { handlePendingVerification } from "./pending-verification-handler"
 import { handleDeletedLoopSession, handleErroredLoopSession } from "./session-event-handler"
 
+const RAPID_IDLE_DEDUP_MS = 500
+
 type LoopStateController = {
 	getState: () => RalphLoopState | null
 	clear: () => boolean
@@ -62,6 +64,10 @@ function getRuntimeRetryActivitySessionID(
 	return undefined
 }
 
+function isSyntheticIdle(props: Record<string, unknown> | undefined): boolean {
+	return props?.synthetic === true
+}
+
 function isAbortError(error: unknown): boolean {
 	return typeof error === "object"
 		&& error !== null
@@ -183,17 +189,20 @@ export function createRalphLoopEventHandler(
 ) {
 	const inFlightSessions = new Set<string>()
 	const runtimeErrorRetriedSessions = new Map<string, number>()
+	const recentHandledSyntheticIdleAt = new Map<string, number>()
 
 	return async ({ event }: { event: { type: string; properties?: unknown } }): Promise<void> => {
 		const props = event.properties as Record<string, unknown> | undefined
 		const runtimeRetryActivitySessionID = getRuntimeRetryActivitySessionID(event.type, props)
 		if (runtimeRetryActivitySessionID) {
 			runtimeErrorRetriedSessions.delete(runtimeRetryActivitySessionID)
+			recentHandledSyntheticIdleAt.delete(runtimeRetryActivitySessionID)
 		}
 
 		if (event.type === "session.idle") {
 			const sessionID = resolveSessionEventID(props)
 			if (!sessionID) return
+			const syntheticIdle = isSyntheticIdle(props)
 
 			if (inFlightSessions.has(sessionID)) {
 				log(`[${HOOK_NAME}] Skipped: handler in flight`, { sessionID })
@@ -241,6 +250,17 @@ export function createRalphLoopEventHandler(
 					return
 				}
 
+				const lastHandledSyntheticIdleAt = recentHandledSyntheticIdleAt.get(sessionID)
+				const now = Date.now()
+				if (!syntheticIdle && lastHandledSyntheticIdleAt !== undefined && now - lastHandledSyntheticIdleAt < RAPID_IDLE_DEDUP_MS) {
+					recentHandledSyntheticIdleAt.delete(sessionID)
+					log(`[${HOOK_NAME}] Skipped: duplicate real idle after synthetic idle`, { sessionID })
+					return
+				}
+				if (syntheticIdle) {
+					recentHandledSyntheticIdleAt.set(sessionID, now)
+				}
+
 				if (await handleCompletionIfDetected(ctx, options, {
 					sessionID,
 					state,
diff --git a/src/plugin/session-status-normalizer.test.ts b/src/plugin/session-status-normalizer.test.ts
index cfb99ec6d..043a85ce1 100644
--- a/src/plugin/session-status-normalizer.test.ts
+++ b/src/plugin/session-status-normalizer.test.ts
@@ -25,6 +25,7 @@ describe("normalizeSessionStatusToIdle", () => {
 				type: "session.idle",
 				properties: {
 					sessionID: "ses_abc123",
+					synthetic: true,
 				},
 			},
 		})
diff --git a/src/plugin/session-status-normalizer.ts b/src/plugin/session-status-normalizer.ts
index 6089bf5c8..79c3bdb85 100644
--- a/src/plugin/session-status-normalizer.ts
+++ b/src/plugin/session-status-normalizer.ts
@@ -18,7 +18,7 @@ export function normalizeSessionStatusToIdle(input: EventInput): EventInput | nu
 	return {
 		event: {
 			type: "session.idle",
-			properties: { sessionID },
+			properties: { sessionID, synthetic: true },
 		},
 	}
 }