diff --git a/src/hooks/ralph-loop/index.test.ts b/src/hooks/ralph-loop/index.test.ts
index 885ecc2..cf48ec9 100644
--- a/src/hooks/ralph-loop/index.test.ts
+++ b/src/hooks/ralph-loop/index.test.ts
@@ -10,6 +10,8 @@ describe("ralph-loop", () => {
   const TEST_DIR = join(tmpdir(), "ralph-loop-test-" + Date.now())
   let promptCalls: Array<{ sessionID: string; text: string }>
   let toastCalls: Array<{ title: string; message: string; variant: string }>
+  let messagesCalls: Array<{ sessionID: string }>
+  let mockSessionMessages: Array<{ info?: { role?: string }; parts?: Array<{ type: string; text?: string }> }>
 
   function createMockPluginInput() {
     return {
@@ -22,6 +24,10 @@ describe("ralph-loop", () => {
             })
             return {}
           },
+          messages: async (opts: { path: { id: string } }) => {
+            messagesCalls.push({ sessionID: opts.path.id })
+            return { data: mockSessionMessages }
+          },
         },
         tui: {
           showToast: async (opts: { body: { title: string; message: string; variant: string } }) => {
@@ -35,12 +41,14 @@ describe("ralph-loop", () => {
         },
       },
       directory: TEST_DIR,
-    } as Parameters<typeof createRalphLoopHook>[0]
+    } as unknown as Parameters<typeof createRalphLoopHook>[0]
   }
 
   beforeEach(() => {
     promptCalls = []
     toastCalls = []
+    messagesCalls = []
+    mockSessionMessages = []
 
     if (!existsSync(TEST_DIR)) {
       mkdirSync(TEST_DIR, { recursive: true })
@@ -351,6 +359,35 @@ describe("ralph-loop", () => {
       expect(hook.getState()).toBeNull()
     })
 
+    test("should detect completion promise via session messages API", async () => {
+      // #given - active loop with assistant message containing completion promise
+      mockSessionMessages = [
+        { info: { role: "user" }, parts: [{ type: "text", text: "Build something" }] },
+        { info: { role: "assistant" }, parts: [{ type: "text", text: "I have completed the task. <promise>API_DONE</promise>" }] },
+      ]
+      const hook = createRalphLoopHook(createMockPluginInput(), {
+        getTranscriptPath: () => join(TEST_DIR, "nonexistent.jsonl"),
+      })
+      hook.startLoop("session-123", "Build something", { completionPromise: "API_DONE" })
+
+      // #when - session goes idle
+      await hook.event({
+        event: {
+          type: "session.idle",
+          properties: { sessionID: "session-123" },
+        },
+      })
+
+      // #then - loop completed via API detection, no continuation
+      expect(promptCalls.length).toBe(0)
+      expect(toastCalls.some((t) => t.title === "Ralph Loop Complete!")).toBe(true)
+      expect(hook.getState()).toBeNull()
+
+      // #then - messages API was called with correct session ID
+      expect(messagesCalls.length).toBe(1)
+      expect(messagesCalls[0].sessionID).toBe("session-123")
+    })
+
     test("should handle multiple iterations correctly", async () => {
       // #given - active loop
       const hook = createRalphLoopHook(createMockPluginInput())
diff --git a/src/hooks/ralph-loop/index.ts b/src/hooks/ralph-loop/index.ts
index 80da2d0..dc1a500 100644
--- a/src/hooks/ralph-loop/index.ts
+++ b/src/hooks/ralph-loop/index.ts
@@ -18,6 +18,17 @@ interface SessionState {
   isRecovering?: boolean
 }
 
+interface OpenCodeSessionMessage {
+  info?: {
+    role?: string
+  }
+  parts?: Array<{
+    type: string
+    text?: string
+    [key: string]: unknown
+  }>
+}
+
 const CONTINUATION_PROMPT = `[RALPH LOOP - ITERATION {{ITERATION}}/{{MAX}}]
 
 Your previous attempt did not output the completion promise. Continue working on the task.
@@ -81,6 +92,41 @@ export function createRalphLoopHook(
     return str.replace(/[.*+?^${}()|[\]\\]/g, "\\$&")
   }
 
+  async function detectCompletionInSessionMessages(
+    sessionID: string,
+    promise: string
+  ): Promise<boolean> {
+    try {
+      const response = await ctx.client.session.messages({
+        path: { id: sessionID },
+        query: { directory: ctx.directory },
+      })
+
+      const messages = (response as { data?: unknown[] }).data ?? []
+
+      if (!Array.isArray(messages)) return false
+
+      const pattern = new RegExp(`<promise>\\s*${escapeRegex(promise)}\\s*</promise>`, "is")
+
+      for (const msg of messages as OpenCodeSessionMessage[]) {
+        if (msg.info?.role !== "assistant") continue
+
+        for (const part of msg.parts || []) {
+          if (part.type === "text" && part.text) {
+            if (pattern.test(part.text)) {
+              return true
+            }
+          }
+        }
+      }
+
+      return false
+    } catch (err) {
+      log(`[${HOOK_NAME}] Failed to fetch session messages`, { sessionID, error: String(err) })
+      return false
+    }
+  }
+
   const startLoop = (
     sessionID: string,
     prompt: string,
@@ -151,14 +197,20 @@ export function createRalphLoopHook(
         return
       }
 
-      // Generate transcript path from sessionID - OpenCode doesn't pass it in event properties
-      const transcriptPath = getTranscriptPath(sessionID)
+      const completionDetectedViaApi = await detectCompletionInSessionMessages(
+        sessionID,
+        state.completion_promise
+      )
 
-      if (detectCompletionPromise(transcriptPath, state.completion_promise)) {
+      const transcriptPath = getTranscriptPath(sessionID)
+      const completionDetectedViaTranscript = detectCompletionPromise(transcriptPath, state.completion_promise)
+
+      if (completionDetectedViaApi || completionDetectedViaTranscript) {
         log(`[${HOOK_NAME}] Completion detected!`, {
           sessionID,
           iteration: state.iteration,
           promise: state.completion_promise,
+          detectedVia: completionDetectedViaApi ? "session_messages_api" : "transcript_file",
         })
         clearState(ctx.directory, stateDir)