diff --git a/backend/server.py b/backend/server.py
index b13a52b..ba64aa2 100644
--- a/backend/server.py
+++ b/backend/server.py
@@ -28,7 +28,7 @@
 import secrets
 import httpx
 from pydantic import BaseModel, Field
-from typing import List, Optional, Literal
+from typing import Any, List, Optional, Literal
 from datetime import datetime, timezone, timedelta
 
 from llm_provider import run_llm, run_llm_cheap
@@ -148,7 +148,7 @@ async def _check_diagnosis_gate(user_id: str, idea_id: str):
         {"idea_id": idea_id, "user_id": user_id}, {"_id": 0}, sort=[("created_at", -1)]
     )
     if latest_diag:
-        actions = latest_diag.get("this_week_actions", [])
+        actions: List[dict[str, Any]] = list(latest_diag.get("this_week_actions") or [])
         if not any(a.get("done") for a in actions):
             raise HTTPException(
                 status_code=400,
@@ -367,6 +367,13 @@ class CheckinCreate(BaseModel):
     blockers: str = ""
 
 
+class CheckinUpdate(BaseModel):
+    actions_completed: List[str] = []
+    what_changed: str = ""
+    new_learnings: str = ""
+    blockers: str = ""
+
+
 class Action(BaseModel):
     action_id: str
     title: str
@@ -404,10 +411,10 @@ class Checkin(BaseModel):
     checkin_id: str
     idea_id: str
     user_id: str
-    actions_completed: List[str]
     what_changed: str
     new_learnings: str
     blockers: str
+    diagnosis_id: Optional[str] = None
     delta_summary: str = ""  # AI-generated "what changed since last week"
     created_at: datetime
 
@@ -484,6 +491,26 @@ def _serialize_timeline_doc(doc: Optional[dict]) -> Optional[dict]:
     )
 
 
+def _serialize_checkin(doc: dict) -> dict:
+    v = doc.get("created_at")
+    if isinstance(v, str):
+        try:
+            doc["created_at"] = datetime.fromisoformat(v)
+        except Exception:
+            pass
+    return _add_ist_fields(doc, ["created_at"])
+
+
+def _build_action_snapshot(
+    diag: Optional[dict[str, Any]], completed_action_ids: List[str]
+) -> List[dict[str, Any]]:
+    actions: List[dict[str, Any]] = []
+    completed = set(completed_action_ids or [])
+    for action in (diag or {}).get("this_week_actions", []):
+        actions.append({**action, "done": action.get("action_id") in completed})
+    return actions
+
+
 @api.post("/ideas")
 async def create_idea(
     payload: IdeaCreate,
@@ -710,7 +737,11 @@ async def _parse_or_repair_json(raw: str, session_id: str) -> dict:
 
 
 def _build_idea_brief(
-    user: User, idea: dict, prior_diag: Optional[dict], last_checkin: Optional[dict]
+    user: User,
+    idea: dict,
+    prior_diag: Optional[dict],
+    last_checkin: Optional[dict],
+    last_checkin_diag: Optional[dict],
 ) -> str:
     lines = [
         f"FOUNDER: {user.name}",
@@ -733,10 +764,18 @@ def _build_idea_brief(
             f"- Previous key insight: {prior_diag.get('key_insight')}",
         ]
     if last_checkin:
+        completed_actions = []
+        pending_actions = []
+        for action in (last_checkin_diag or {}).get("this_week_actions") or []:
+            if action.get("done"):
+                completed_actions.append(action.get("title"))
+            else:
+                pending_actions.append(action.get("title"))
         lines += [
             "",
             "LAST CHECK-IN:",
-            f"- Actions completed: {last_checkin.get('actions_completed')}",
+            f"- Actions completed: {completed_actions or '(none)'}",
+            f"- Actions not completed: {pending_actions or '(none)'}",
             f"- What changed: {last_checkin.get('what_changed')}",
             f"- New learnings: {last_checkin.get('new_learnings')}",
             f"- Blockers: {last_checkin.get('blockers')}",
@@ -815,7 +854,17 @@ async def _run_diagnosis_job(job_id: str, idea_id: str, user_id: str):
         last_checkin = await db.checkins.find_one(
             {"idea_id": idea_id}, {"_id": 0}, sort=[("created_at", -1)]
         )
-        brief = _build_idea_brief(user, idea, prior, last_checkin)
+        last_checkin_diag = None
+        if last_checkin and last_checkin.get("diagnosis_id"):
+            last_checkin_diag = await db.diagnoses.find_one(
+                {
+                    "idea_id": idea_id,
+                    "user_id": user_id,
+                    "diagnosis_id": last_checkin["diagnosis_id"],
+                },
+                {"_id": 0},
+            )
+        brief = _build_idea_brief(user, idea, prior, last_checkin, last_checkin_diag)
 
         parsed = await asyncio.wait_for(
             _run_diagnosis_llm(brief, f"diag_{idea_id}_{uuid.uuid4().hex[:8]}"),
@@ -910,7 +959,7 @@ async def toggle_action(
     )
     if not diag:
         raise HTTPException(404, "No diagnosis")
-    actions = diag.get("this_week_actions", [])
+    actions: List[dict[str, Any]] = list(diag.get("this_week_actions") or [])
     for a in actions:
         if a.get("action_id") == action_id:
             a["done"] = not a.get("done", False)
@@ -957,16 +1006,27 @@ async def create_checkin(
         streak = 1
 
     checkin_id = f"cki_{uuid.uuid4().hex[:12]}"
+    # find the most recent diagnosis that does NOT yet have a linked checkin
+    latest_diag = None
+    async for d in db.diagnoses.find({"idea_id": idea_id}).sort("created_at", -1):
+        # check if any checkin already links to this diagnosis
+        exists = await db.checkins.find_one(
+            {"diagnosis_id": d.get("diagnosis_id")}, {"_id": 1}
+        )
+        if not exists:
+            latest_diag = d
+            break
     doc = {
         "checkin_id": checkin_id,
         "idea_id": idea_id,
         "user_id": user.user_id,
-        "actions_completed": payload.actions_completed,
         "what_changed": payload.what_changed,
         "new_learnings": payload.new_learnings,
         "blockers": payload.blockers,
         "delta_summary": "",
         "created_at": now.isoformat(),
+        # keep the check-in record self-contained in the checkins collection
+        "diagnosis_id": latest_diag.get("diagnosis_id") if latest_diag else None,
     }
     await db.checkins.insert_one({**doc})
     await db.ideas.update_one(
@@ -979,21 +1039,153 @@ async def create_checkin(
             }
         },
     )
-    # Also mark completed actions as done in latest diagnosis
+    # Also mark completed actions as done in latest diagnosis (if present)
+    if latest_diag:
+        actions = _build_action_snapshot(latest_diag, payload.actions_completed)
+        await db.diagnoses.update_one(
+            {"diagnosis_id": latest_diag["diagnosis_id"]},
+            {"$set": {"this_week_actions": actions}},
+        )
+    return _serialize_timeline_doc({**doc, "streak": streak})
+
+
+@api.get("/ideas/{idea_id}/checkins")
+async def list_checkins(
+    idea_id: str,
+    request: Request,
+    limit: int = 20,
+    before: Optional[str] = None,
+    session_token_cookie: Optional[str] = Cookie(None, alias="session_token"),
+    authorization: Optional[str] = Header(None),
+):
+    user = await get_current_user(request, session_token_cookie, authorization)
+    idea = await db.ideas.find_one(
+        {"idea_id": idea_id, "user_id": user.user_id, "status": {"$ne": "archived"}},
+        {"_id": 0, "idea_id": 1},
+    )
+    if not idea:
+        raise HTTPException(404, "Idea not found")
+
+    limit = max(1, min(limit, 100))
+    query: dict[str, Any] = {"idea_id": idea_id, "user_id": user.user_id}
+    if before:
+        query["created_at"] = {"$lt": before}
+
+    docs = (
+        await db.checkins.find(query, {"_id": 0})
+        .sort("created_at", -1)
+        .to_list(limit + 1)
+    )
+    has_more = len(docs) > limit
+    items = docs[:limit]
+    next_before = items[-1].get("created_at") if has_more and items else None
+
+    enriched_items = []
+    for item in items:
+        enriched = dict(item)
+        if not enriched.get("diagnosis_actions") and item.get("diagnosis_id"):
+            diag = await db.diagnoses.find_one(
+                {"idea_id": idea_id, "user_id": user.user_id, "diagnosis_id": item["diagnosis_id"]},
+                {"_id": 0, "this_week_actions": 1},
+            )
+            enriched["diagnosis_actions"] = diag.get("this_week_actions", []) if diag else []
+        else:
+            enriched["diagnosis_actions"] = enriched.get("diagnosis_actions", [])
+        enriched_items.append(enriched)
+
+    return {
+        "items": [_serialize_checkin(i) for i in enriched_items],
+        "has_more": has_more,
+        "next_before": next_before,
+    }
+
+
+@api.patch("/ideas/{idea_id}/checkins/{checkin_id}")
+async def update_checkin(
+    idea_id: str,
+    checkin_id: str,
+    payload: CheckinUpdate,
+    request: Request,
+    session_token_cookie: Optional[str] = Cookie(None, alias="session_token"),
+    authorization: Optional[str] = Header(None),
+):
+    user = await get_current_user(request, session_token_cookie, authorization)
+    idea = await db.ideas.find_one(
+        {"idea_id": idea_id, "user_id": user.user_id, "status": {"$ne": "archived"}},
+        {"_id": 0, "idea_id": 1},
+    )
+    if not idea:
+        raise HTTPException(404, "Idea not found")
+
+    latest = await db.checkins.find_one(
+        {"idea_id": idea_id, "user_id": user.user_id},
+        {"_id": 0, "checkin_id": 1},
+        sort=[("created_at", -1)],
+    )
+    if not latest:
+        raise HTTPException(404, "Check-in not found")
+    if latest.get("checkin_id") != checkin_id:
+        raise HTTPException(
+            status_code=400,
+            detail="Only the latest check-in can be edited.",
+        )
+
+    existing = await db.checkins.find_one(
+        {"idea_id": idea_id, "user_id": user.user_id, "checkin_id": checkin_id},
+        {"_id": 0},
+    )
+    if not existing:
+        raise HTTPException(404, "Check-in not found")
+
     latest_diag = await db.diagnoses.find_one(
-        {"idea_id": idea_id}, {"_id": 0}, sort=[("created_at", -1)]
+        {"idea_id": idea_id, "user_id": user.user_id},
+        {"_id": 0, "diagnosis_id": 1, "this_week_actions": 1},
+        sort=[("created_at", -1)],
+    )
+    if existing.get("diagnosis_id") != (latest_diag or {}).get("diagnosis_id"):
+        raise HTTPException(
+            status_code=400,
+            detail="Only the check-in for the latest diagnosis can be edited.",
+        )
+
+    updated = {
+        "what_changed": payload.what_changed,
+        "new_learnings": payload.new_learnings,
+        "blockers": payload.blockers,
+    }
+    linked_diag = None
+    if existing.get("diagnosis_id"):
+        linked_diag = await db.diagnoses.find_one(
+            {
+                "idea_id": idea_id,
+                "user_id": user.user_id,
+                "diagnosis_id": existing["diagnosis_id"],
+            },
+            {"_id": 0, "this_week_actions": 1},
+        )
+    if not linked_diag:
+        linked_diag = await db.diagnoses.find_one(
+            {"idea_id": idea_id, "user_id": user.user_id},
+            {"_id": 0, "this_week_actions": 1},
+            sort=[("created_at", -1)],
+        )
+
+    await db.checkins.update_one(
+        {"checkin_id": checkin_id, "idea_id": idea_id, "user_id": user.user_id},
+        {
+            "$set": updated
+        },
     )
+
     if latest_diag:
-        actions = latest_diag.get("this_week_actions", [])
-        completed = set(payload.actions_completed or [])
-        for a in actions:
-            if a.get("action_id") in completed:
-                a["done"] = True
+        actions = _build_action_snapshot(latest_diag, payload.actions_completed)
         await db.diagnoses.update_one(
             {"diagnosis_id": latest_diag["diagnosis_id"]},
             {"$set": {"this_week_actions": actions}},
         )
-    return _serialize_timeline_doc({**doc, "streak": streak})
+
+    out = {**existing, **updated}
+    return _serialize_checkin(out)
 
 
 @api.get("/dashboard/summary")
@@ -1168,7 +1360,7 @@ async def build_shared_diagnosis_content(slug: str) -> dict:
                     "why": a.get("why"),
                     "effort": a.get("effort"),
                 }
-                for a in diag.get("this_week_actions", [])
+                for a in list(diag.get("this_week_actions") or [])
             ],
             "created_at": diag["created_at"],
             "created_at_ist": _to_ist_iso(diag["created_at"]),
diff --git a/frontend/src/lib/api.js b/frontend/src/lib/api.js
index 7d1edb4..a8c360c 100644
--- a/frontend/src/lib/api.js
+++ b/frontend/src/lib/api.js
@@ -101,6 +101,19 @@ export async function createCheckin(idea_id, payload) {
   return data;
 }
 
+export async function listCheckins(idea_id, params = {}) {
+  const { data } = await api.get(`/ideas/${idea_id}/checkins`, { params });
+  return data;
+}
+
+export async function updateCheckin(idea_id, checkin_id, payload) {
+  const { data } = await api.patch(
+    `/ideas/${idea_id}/checkins/${checkin_id}`,
+    payload,
+  );
+  return data;
+}
+
 export async function deleteIdea(idea_id) {
   const { data } = await api.delete(`/ideas/${idea_id}`);
   return data;
diff --git a/frontend/src/pages/IdeaDetail.checkin.test.jsx b/frontend/src/pages/IdeaDetail.checkin.test.jsx
new file mode 100644
index 0000000..10308a4
--- /dev/null
+++ b/frontend/src/pages/IdeaDetail.checkin.test.jsx
@@ -0,0 +1,115 @@
+// Ensure React's act environment is enabled for concurrent rendering in tests
+globalThis.IS_REACT_ACT_ENVIRONMENT = true;
+
+import React from "react";
+import { act } from "react";
+import { createRoot } from "react-dom/client";
+
+jest.mock(
+  "react-router-dom",
+  () => ({
+    useNavigate: () => jest.fn(),
+    useParams: () => ({}),
+    useSearchParams: () => [new URLSearchParams(), jest.fn()],
+  }),
+  { virtual: true },
+);
+
+jest.mock(
+  "@/lib/track",
+  () => ({
+    track: jest.fn(),
+  }),
+  { virtual: true },
+);
+
+const { CheckinDialog } = require("./IdeaDetail");
+
+function renderDialog(props) {
+  const container = document.createElement("div");
+  document.body.appendChild(container);
+  const root = createRoot(container);
+
+  act(() => {
+    root.render(<CheckinDialog {...props} />);
+  });
+
+  return {
+    container,
+    unmount: () => {
+      act(() => {
+        root.unmount();
+      });
+      document.body.removeChild(container);
+    },
+  };
+}
+
+describe("CheckinDialog", () => {
+  const baseProps = {
+    actions: [
+      { action_id: "a1", title: "Call users", done: false },
+      { action_id: "a2", title: "Ship page", done: true },
+    ],
+    diag: {
+      this_week_actions: [
+        { action_id: "a1", done: false },
+        { action_id: "a2", done: true },
+      ],
+    },
+    diagnosisLimits: { unlimited: false, weekly_remaining: 1 },
+    cooldownLeft: 0,
+    latestCheckin: {
+      checkin_id: "cki_latest",
+      actions_completed: ["a1"],
+      what_changed: "Latest change",
+      new_learnings: "Latest learning",
+      blockers: "Latest blocker",
+    },
+    initialCheckin: null,
+    isEditing: false,
+    onClose: () => {},
+    onSubmit: async () => {},
+  };
+
+  test("new check-in starts blank", () => {
+    const view = renderDialog(baseProps);
+
+    const whatChanged = view.container.querySelector(
+      '[data-testid="checkin-changed"]',
+    );
+    // New check-ins should start blank regardless of latestCheckin
+    expect(whatChanged.value).toBe("");
+    view.unmount();
+  });
+
+  test("edit mode hides start mode and uses save label", () => {
+    const view = renderDialog({
+      ...baseProps,
+      isEditing: true,
+      initialCheckin: {
+        checkin_id: "cki_latest",
+        actions_completed: ["a2"],
+        what_changed: "Edited value",
+        new_learnings: "Edited learnings",
+        blockers: "Edited blockers",
+      },
+    });
+
+    // Start mode UI removed; ensure no start-mode button exists
+    expect(
+      view.container.querySelector('[data-testid="checkin-mode-blank"]'),
+    ).toBeNull();
+
+    const submitBtn = view.container.querySelector(
+      '[data-testid="checkin-submit"]',
+    );
+    expect(submitBtn.textContent).toContain("Save changes");
+
+    const whatChanged = view.container.querySelector(
+      '[data-testid="checkin-changed"]',
+    );
+    expect(whatChanged.value).toBe("Edited value");
+    view.unmount();
+  });
+});
diff --git a/frontend/src/pages/IdeaDetail.jsx b/frontend/src/pages/IdeaDetail.jsx
index bb8bfcd..ad08c0a 100644
--- a/frontend/src/pages/IdeaDetail.jsx
+++ b/frontend/src/pages/IdeaDetail.jsx
@@ -2,9 +2,11 @@ import React, { useEffect, useRef, useState } from "react";
 import { useNavigate, useParams, useSearchParams } from "react-router-dom";
 import {
   getIdea,
+  listCheckins,
   diagnose,
   toggleAction,
   createCheckin,
+  updateCheckin,
   deleteIdea,
   waitForDiagnosis,
   createShareLink,
@@ -42,7 +44,7 @@ function formatDuration(seconds = 0) {
     .join(":");
 }
 
-function getRecheckState(diag, diagnosisLimits, cooldownLeft) {
+export function getRecheckState(diag, diagnosisLimits, cooldownLeft) {
   if (diagnosisLimits?.unlimited) {
     return {
       canRecheck: true,
@@ -195,25 +197,21 @@ function ActionCard({ a, idx, onToggle }) {
   );
 }
 
-function CheckinDialog({
+export function CheckinDialog({
   actions,
   diag,
   diagnosisLimits,
   cooldownLeft,
   latestCheckin,
+  initialCheckin,
+  isEditing,
   onClose,
   onSubmit,
 }) {
-  const [completed, setCompleted] = useState(
-    actions.filter((a) => a.done).map((a) => a.action_id),
-  );
-  const [whatChanged, setWhatChanged] = useState(
-    latestCheckin?.what_changed || "",
-  );
-  const [learnings, setLearnings] = useState(
-    latestCheckin?.new_learnings || "",
-  );
-  const [blockers, setBlockers] = useState(latestCheckin?.blockers || "");
+  const [completed, setCompleted] = useState([]);
+  const [whatChanged, setWhatChanged] = useState("");
+  const [learnings, setLearnings] = useState("");
+  const [blockers, setBlockers] = useState("");
   const [busy, setBusy] = useState(false);
   const previewDiag = diag
     ? {
@@ -231,11 +229,21 @@ function CheckinDialog({
   );
 
   useEffect(() => {
-    setCompleted(actions.filter((a) => a.done).map((a) => a.action_id));
-    setWhatChanged(latestCheckin?.what_changed || "");
-    setLearnings(latestCheckin?.new_learnings || "");
-    setBlockers(latestCheckin?.blockers || "");
-  }, [actions, latestCheckin]);
+    // For new check-ins always start blank. When editing, load the initial checkin.
+    const source = isEditing ? initialCheckin : null;
+    const defaultCompleted = actions
+      .filter((a) => a.done)
+      .map((a) => a.action_id);
+    const sourceCompleted = Array.isArray(source?.diagnosis_actions)
+      ? source.diagnosis_actions
+          .filter((action) => action.done)
+          .map((action) => action.action_id)
+      : [];
+    setCompleted(sourceCompleted.length > 0 ? sourceCompleted : defaultCompleted);
+    setWhatChanged(source?.what_changed || "");
+    setLearnings(source?.new_learnings || "");
+    setBlockers(source?.blockers || "");
+  }, [actions, initialCheckin, isEditing]);
 
   const toggle = (id) => {
     setCompleted((c) =>
@@ -275,7 +283,7 @@ function CheckinDialog({
           )}
         </div>
         <h3 className="font-display text-3xl font-semibold tracking-tight mb-2">
-          What actually happened?
+          {isEditing ? "Edit latest check-in" : "What actually happened?"}
         </h3>
         <p className="text-zinc-600 mb-8">
           {previewRecheck.canRecheck
@@ -283,6 +291,8 @@ function CheckinDialog({
             : "Your check-in will still be saved. Re-diagnosis unlocks when the re-diagnosis timer does."}
         </p>
 
+        {/* Removed prefill/start-mode: new check-ins always start blank */}
+
         <div className="mb-6">
           <div className="overline mb-3">actions completed</div>
           <div className="space-y-2">
@@ -364,6 +374,8 @@ function CheckinDialog({
                 <span className="spin inline-block w-3 h-3 border-2 border-white border-t-transparent rounded-full" />{" "}
                 Kicking off…
               </>
+            ) : isEditing ? (
+              "Save changes"
             ) : previewRecheck.canRecheck ? (
               "Submit & re-diagnose ->"
             ) : (
@@ -568,8 +580,14 @@ export default function IdeaDetail() {
   const [search, setSearch] = useSearchParams();
   const navigate = useNavigate();
   const [data, setData] = useState(null);
+  const [checkins, setCheckins] = useState([]);
+  const [checkinsHasMore, setCheckinsHasMore] = useState(false);
+  const [checkinsCursor, setCheckinsCursor] = useState(null);
+  const [checkinsLoadingMore, setCheckinsLoadingMore] = useState(false);
+  const [expandedCheckinId, setExpandedCheckinId] = useState(null);
   const [loading, setLoading] = useState(true);
   const [checkinOpen, setCheckinOpen] = useState(false);
+  const [editingCheckin, setEditingCheckin] = useState(null);
   const [shareOpen, setShareOpen] = useState(false);
   const [diagnosing, setDiagnosing] = useState(false);
   const [error, setError] = useState("");
@@ -580,14 +598,37 @@ export default function IdeaDetail() {
 
   const load = async () => {
     try {
-      const d = await getIdea(idea_id);
+      const [d, checkinPage] = await Promise.all([
+        getIdea(idea_id),
+        listCheckins(idea_id, { limit: 20 }),
+      ]);
       setData(d);
+      setCheckins(checkinPage?.items || []);
+      setExpandedCheckinId(checkinPage?.items?.[0]?.checkin_id || null);
+      setCheckinsHasMore(Boolean(checkinPage?.has_more));
+      setCheckinsCursor(checkinPage?.next_before || null);
       return d;
     } finally {
       setLoading(false);
     }
   };
 
+  const loadMoreCheckins = async () => {
+    if (!checkinsHasMore || checkinsLoadingMore || !checkinsCursor) return;
+    setCheckinsLoadingMore(true);
+    try {
+      const page = await listCheckins(idea_id, {
+        limit: 20,
+        before: checkinsCursor,
+      });
+      setCheckins((prev) => [...prev, ...(page?.items || [])]);
+      setCheckinsHasMore(Boolean(page?.has_more));
+      setCheckinsCursor(page?.next_before || null);
+    } finally {
+      setCheckinsLoadingMore(false);
+    }
+  };
+
   useEffect(() => {
     load().then((d) => {
       if (d?.latest_diagnosis) {
@@ -660,7 +701,11 @@ export default function IdeaDetail() {
 
   const idea = data.idea;
   const diag = data.latest_diagnosis;
-  const latestCheckin = data.checkins?.[0] || null;
+  const latestCheckin = checkins[0] || null;
+  const diagCheckins = checkins.filter((c) => !!c.diagnosis_id);
+  const activeCheckin = diag
+    ? checkins.find((c) => c.diagnosis_id === diag.diagnosis_id) || null
+    : null;
   const style = verdictStyle(diag?.verdict);
   const { canRecheck, label: recheckLabel } = getRecheckState(
     diag,
@@ -687,14 +732,23 @@ export default function IdeaDetail() {
     setError("");
     setNotice("");
     try {
-      await createCheckin(idea_id, payload);
+      if (editingCheckin?.checkin_id) {
+        await updateCheckin(idea_id, editingCheckin.checkin_id, payload);
+      } else {
+        await createCheckin(idea_id, payload);
+      }
     } catch (e) {
       setError(errorMessage(e, "Check-in failed"));
       return;
     }
 
+    setEditingCheckin(null);
     setCheckinOpen(false);
-    setNotice("Weekly check-in saved.");
+    setNotice(
+      editingCheckin
+        ? "Latest check-in updated."
+        : "Weekly check-in saved.",
+    );
 
     try {
       const refreshed = await load();
@@ -705,15 +759,19 @@ export default function IdeaDetail() {
         refreshed?.diagnosis_limits,
         nextCooldownLeft,
       );
-      if (nextState.canRecheck) {
+      if (!editingCheckin && nextState.canRecheck) {
         await runDiagnose();
         return;
       }
       if (nextState.label) {
-        setNotice(`Weekly check-in saved. ${nextState.label}`);
+        setNotice(
+          editingCheckin
+            ? "Latest check-in updated."
+            : `Weekly check-in saved. ${nextState.label}`,
+        );
       }
     } catch (e) {
-      setNotice("Weekly check-in saved.");
+      setNotice(editingCheckin ? "Latest check-in updated." : "Weekly check-in saved.");
       setError(
         errorMessage(e, "Saved your check-in, but couldn't refresh the page"),
       );
@@ -785,15 +843,24 @@ export default function IdeaDetail() {
               <p className="text-lg text-zinc-600">{idea.one_liner}</p>
             </div>
             <div className="flex items-center gap-2 flex-shrink-0 flex-wrap">
-              {diag && !hasPending && (
+              {!hasPending && (
                 <>
                   <button
-                    onClick={() => setCheckinOpen(true)}
+                    onClick={() => {
+                      // Open the check-in attached to the current latest diagnosis.
+                      // If none exists yet, start a new blank check-in for that diagnosis.
+                      setEditingCheckin(activeCheckin);
+                      setCheckinOpen(true);
+                    }}
                     className="btn btn-primary"
                     data-testid="weekly-checkin-btn"
                   >
                     Weekly check-in
                   </button>
+                </>
+              )}
+              {diag && !hasPending && (
+                <>
                   <button
                     onClick={() => setShareOpen(true)}
                     className="btn btn-secondary"
@@ -853,7 +920,7 @@ export default function IdeaDetail() {
         {hasPending && <DiagnosingOverlay />}
 
         {/* Tabs */}
-        {!hasPending && data.diagnoses.length > 0 && (
+        {!hasPending && (data.diagnoses.length > 0 || checkins.length > 0) && (
           <div className="border-b border-zinc-200 mb-8 flex gap-6 no-print">
             <TabBtn
               k="diagnosis"
@@ -862,11 +929,17 @@ export default function IdeaDetail() {
               label="Latest diagnosis"
             />
             <TabBtn
-              k="history"
+              k="evolution"
               cur={tab}
               onClick={setTab}
               label={`Evolution (${data.diagnoses.length})`}
             />
+            <TabBtn
+              k="checkins"
+              cur={tab}
+              onClick={setTab}
+              label={`Check-ins (${diagCheckins.length})`}
+            />
           </div>
         )}
 
@@ -1014,53 +1087,190 @@ export default function IdeaDetail() {
           </div>
         )}
 
-        {!hasPending && tab === "history" && (
-          <div className="space-y-4" data-testid="evolution-list">
-            {data.diagnoses.map((d, i) => {
-              const s = verdictStyle(d.verdict);
-              const date = new Date(d.created_at_ist || d.created_at);
-              return (
-                <div
-                  key={d.diagnosis_id}
-                  className="card p-6"
-                  data-testid={`evolution-${i}`}
-                >
-                  <div className="flex items-center gap-3 mb-3">
-                    <span className="font-mono text-xs text-zinc-500">
-                      #{data.diagnoses.length - i} ·{" "}
-                      {date.toLocaleDateString("en-IN", {
-                        timeZone: "Asia/Kolkata",
-                      })}
-                    </span>
-                    <span className={`badge ${s.badge}`}>{s.label}</span>
-                    <span className="font-mono text-xs text-zinc-500">
-                      FMF {d.fmf_score} · WTP {d.wtp_score} · MOM{" "}
-                      {d.momentum_score}
-                    </span>
+        {!hasPending && tab === "checkins" && (
+          <div className="space-y-5">
+            <section>
+              <div className="overline mb-2">weekly check-ins</div>
+              <div className="space-y-3" data-testid="checkin-history-list">
+                {diagCheckins.length === 0 && (
+                  <div className="card p-5 text-sm text-zinc-600">
+                    No check-ins yet.
                   </div>
-                  <div
-                    className={`font-display text-xl font-medium tracking-tight leading-tight mb-2 ${s.text}`}
+                )}
+                {diagCheckins.map((c, i) => {
+                  const date = new Date(c.created_at_ist || c.created_at);
+                  const isOpen = expandedCheckinId === c.checkin_id;
+                  return (
+                    <div
+                      key={c.checkin_id}
+                      className={`card p-5 transition-colors ${isOpen ? "border-emerald-200 bg-emerald-50/40" : "border-zinc-200 bg-white"}`}
+                      data-testid={`checkin-history-${i}`}
+                    >
+                      <button
+                        type="button"
+                        className="flex w-full items-center justify-between gap-3 text-left"
+                        onClick={() =>
+                          setExpandedCheckinId((current) =>
+                            current === c.checkin_id ? null : c.checkin_id,
+                          )
+                        }
+                        aria-expanded={isOpen}
+                        data-testid={`checkin-toggle-${i}`}
+                      >
+                        <div>
+                          <span className="font-mono text-xs text-zinc-500">
+                            #{diagCheckins.length - i} ·{" "}
+                            {date.toLocaleDateString("en-IN", {
+                              timeZone: "Asia/Kolkata",
+                            })}
+                          </span>
+                        </div>
+                        <div className="flex items-center gap-3 text-zinc-500">
+                          <span className="font-mono text-xs uppercase tracking-[0.15em]">
+                            {isOpen ? "collapse" : "expand"}
+                          </span>
+                          <span className="text-sm leading-none">
+                            {isOpen ? "⌃" : "⌄"}
+                          </span>
+                        </div>
+                      </button>
+                      {isOpen && (
+                        <div className="mt-4 space-y-3 text-sm">
+                          <div className="grid gap-3 sm:grid-cols-3">
+                            <div>
+                              <div className="overline mb-1">what changed</div>
+                              <p className="text-zinc-800 whitespace-pre-wrap leading-relaxed">
+                                {c.what_changed || "-"}
+                              </p>
+                            </div>
+                            <div>
+                              <div className="overline mb-1">learnings</div>
+                              <p className="text-zinc-800 whitespace-pre-wrap leading-relaxed">
+                                {c.new_learnings || "-"}
+                              </p>
+                            </div>
+                            <div>
+                              <div className="overline mb-1">blockers</div>
+                              <p className="text-zinc-800 whitespace-pre-wrap leading-relaxed">
+                                {c.blockers || "-"}
+                              </p>
+                            </div>
+                          </div>
+                          {Array.isArray(c.diagnosis_actions) && c.diagnosis_actions.length > 0 && (
+                            <div>
+                              <div className="overline mb-1">actions</div>
+                              <div className="space-y-2">
+                                {c.diagnosis_actions.map((action) => (
+                                  <div
+                                    key={action.action_id}
+                                    className={`rounded-lg border px-4 py-3 shadow-sm   ${action.done ? "bg-emerald-50/30 border-emerald-200" : "bg-red-50/30 border-red-200"}`}
+                                  >
+                                    <div className="flex items-start justify-between gap-4">
+                                      <div className="min-w-0 flex-1">
+                                        <div className="text-sm font-medium text-zinc-900 leading-snug">
+                                          {action.title}
+                                        </div>
+                                        {action.why && (
+                                          <div className="mt-1 text-xs text-zinc-500 leading-relaxed">
+                                            {action.why}
+                                          </div>
+                                        )}
+                                      </div>
+                                      <span
+                                        className={`inline-flex flex-shrink-0 items-center rounded-full px-3 py-1 text-[10px] font-mono uppercase tracking-[0.15em] ${action.done ? "bg-emerald-100 text-emerald-700" : "bg-red-100 text-red-700"}`}
+                                      >
+                                        {action.done ? "complete" : "incomplete"}
+                                      </span>
+                                    </div>
+                                  </div>
+                                ))}
+                              </div>
+                            </div>
+                          )}
+                        </div>
+                      )}
+                    </div>
+                  );
+                })}
+              </div>
+              {checkinsHasMore && (
+                <div className="mt-4">
+                  <button
+                    className="btn btn-secondary"
+                    onClick={loadMoreCheckins}
+                    disabled={checkinsLoadingMore}
+                    data-testid="checkin-history-load-more"
                   >
-                    {d.verdict_headline}
-                  </div>
-                  <p className="text-sm text-zinc-600 leading-relaxed font-serif italic">
-                    {d.key_insight}
-                  </p>
+                    {checkinsLoadingMore ? "Loading..." : "Load older check-ins"}
+                  </button>
                 </div>
-              );
-            })}
+              )}
+            </section>
+          </div>
+        )}
+
+        {!hasPending && tab === "evolution" && (
+          <div className="space-y-8" data-testid="evolution-list">
+            <section>
+              <div className="overline mb-3">diagnosis evolution</div>
+              <div className="space-y-4">
+                {data.diagnoses.map((d, i) => {
+                  const s = verdictStyle(d.verdict);
+                  const date = new Date(d.created_at_ist || d.created_at);
+                  return (
+                    <div
+                      key={d.diagnosis_id}
+                      className="card p-6"
+                      data-testid={`evolution-${i}`}
+                    >
+                      <div className="flex items-center gap-3 mb-3">
+                        <span className="font-mono text-xs text-zinc-500">
+                          #{data.diagnoses.length - i} ·{" "}
+                          {date.toLocaleDateString("en-IN", {
+                            timeZone: "Asia/Kolkata",
+                          })}
+                        </span>
+                        <span className={`badge ${s.badge}`}>{s.label}</span>
+                        <span className="font-mono text-xs text-zinc-500">
+                          FMF {d.fmf_score} · WTP {d.wtp_score} · MOM{" "}
+                          {d.momentum_score}
+                        </span>
+                      </div>
+                      <div
+                        className={`font-display text-xl font-medium tracking-tight leading-tight mb-2 ${s.text}`}
+                      >
+                        {d.verdict_headline}
+                      </div>
+                      <p className="text-sm text-zinc-600 leading-relaxed font-serif italic">
+                        {d.key_insight}
+                      </p>
+                    </div>
+                  );
+                })}
+                {data.diagnoses.length === 0 && (
+                  <div className="card p-6 text-sm text-zinc-600">
+                    No diagnoses yet.
+                  </div>
+                )}
+              </div>
+            </section>
           </div>
         )}
       </div>
 
-      {checkinOpen && diag && (
+      {checkinOpen && (
         <CheckinDialog
-          actions={diag.this_week_actions || []}
+          actions={diag?.this_week_actions || []}
           diag={diag}
           diagnosisLimits={data.diagnosis_limits}
           cooldownLeft={cooldownLeft}
           latestCheckin={latestCheckin}
-          onClose={() => setCheckinOpen(false)}
+          initialCheckin={editingCheckin}
+          isEditing={Boolean(editingCheckin)}
+          onClose={() => {
+            setCheckinOpen(false);
+            setEditingCheckin(null);
+          }}
           onSubmit={onCheckinSubmit}
         />
       )}