diff --git a/packages/app/src/pages/session/timeline/timeline-row-equality.test.ts b/packages/app/src/pages/session/timeline/timeline-row-equality.test.ts
new file mode 100644
index 000000000000..a9a9f6c37e37
--- /dev/null
+++ b/packages/app/src/pages/session/timeline/timeline-row-equality.test.ts
@@ -0,0 +1,93 @@
+import { describe, expect, test } from "bun:test"
+import { Equal } from "effect"
+import { TimelineRow, type SummaryDiff } from "./timeline-row"
+
+const diff = (file: string, patch: string): SummaryDiff => ({
+  file,
+  patch,
+  additions: 1,
+  deletions: 0,
+  status: "modified",
+})
+
+const rows: TimelineRow.TimelineRow[] = [
+  new TimelineRow.TurnGap({ userMessageID: "msg_1" }),
+  new TimelineRow.TurnGap({ userMessageID: "msg_2" }),
+  new TimelineRow.CommentStrip({ userMessageID: "msg_1" }),
+  new TimelineRow.UserMessage({ userMessageID: "msg_1", anchor: true }),
+  new TimelineRow.UserMessage({ userMessageID: "msg_1", anchor: false }),
+  new TimelineRow.UserMessage({ userMessageID: "msg_2", anchor: true }),
+  new TimelineRow.TurnDivider({ userMessageID: "msg_1", label: "compaction" }),
+  new TimelineRow.TurnDivider({ userMessageID: "msg_1", label: "interrupted" }),
+  new TimelineRow.AssistantPart({
+    userMessageID: "msg_1",
+    previousAssistantPart: false,
+    group: { key: "grp_1", type: "part", ref: { messageID: "msg_1", partID: "part_1" } },
+  }),
+  new TimelineRow.AssistantPart({
+    userMessageID: "msg_1",
+    previousAssistantPart: false,
+    group: { key: "grp_1", type: "part", ref: { messageID: "msg_1", partID: "part_2" } },
+  }),
+  new TimelineRow.AssistantPart({
+    userMessageID: "msg_1",
+    previousAssistantPart: true,
+    group: { key: "grp_1", type: "part", ref: { messageID: "msg_1", partID: "part_1" } },
+  }),
+  new TimelineRow.AssistantPart({
+    userMessageID: "msg_1",
+    previousAssistantPart: false,
+    group: {
+      key: "grp_2",
+      type: "context",
+      refs: [
+        { messageID: "msg_1", partID: "part_1" },
+        { messageID: "msg_1", partID: "part_2" },
+      ],
+    },
+  }),
+  new TimelineRow.AssistantPart({
+    userMessageID: "msg_1",
+    previousAssistantPart: false,
+    group: {
+      key: "grp_2",
+      type: "context",
+      refs: [
+        { messageID: "msg_1", partID: "part_2" },
+        { messageID: "msg_1", partID: "part_1" },
+      ],
+    },
+  }),
+  new TimelineRow.AssistantPart({
+    userMessageID: "msg_1",
+    previousAssistantPart: false,
+    group: { key: "grp_2", type: "context", refs: [{ messageID: "msg_1", partID: "part_1" }] },
+  }),
+  new TimelineRow.Thinking({ userMessageID: "msg_1", reasoningHeading: "Planning" }),
+  new TimelineRow.Thinking({ userMessageID: "msg_1" }),
+  new TimelineRow.Thinking({ userMessageID: "msg_2", reasoningHeading: "Planning" }),
+  new TimelineRow.DiffSummary({ userMessageID: "msg_1", diffs: [diff("a.ts", "one")] }),
+  new TimelineRow.DiffSummary({ userMessageID: "msg_1", diffs: [diff("a.ts", "one")] }),
+  new TimelineRow.DiffSummary({ userMessageID: "msg_1", diffs: [diff("a.ts", "two")] }),
+  new TimelineRow.Error({ userMessageID: "msg_1", text: "boom" }),
+  new TimelineRow.Error({ userMessageID: "msg_1", text: "bang" }),
+  new TimelineRow.Retry({ userMessageID: "msg_1" }),
+  new TimelineRow.Retry({ userMessageID: "msg_2" }),
+]
+
+describe("TimelineRow.equals", () => {
+  test("agrees with Equal.equals for every pair", () => {
+    for (const a of rows) {
+      for (const b of rows) {
+        expect(TimelineRow.equals(a, b)).toBe(Equal.equals(a, b))
+      }
+    }
+  })
+
+  test("is reflexive and key-stable", () => {
+    for (const row of rows) {
+      expect(TimelineRow.equals(row, row)).toBe(true)
+      expect(TimelineRow.key(row)).toBe(TimelineRow.key(row))
+    }
+  })
+})
diff --git a/packages/app/src/pages/session/timeline/timeline-row.ts b/packages/app/src/pages/session/timeline/timeline-row.ts
index 3905254b29b5..7a99e21cee9f 100644
--- a/packages/app/src/pages/session/timeline/timeline-row.ts
+++ b/packages/app/src/pages/session/timeline/timeline-row.ts
@@ -1,5 +1,5 @@
 import type { SnapshotFileDiff } from "@opencode-ai/sdk/v2"
-import type { PartGroup } from "@opencode-ai/session-ui/message-part"
+import { sameGroup, type PartGroup } from "@opencode-ai/session-ui/message-part-groups"
 import { Data, Equal } from "effect"
 
 export type SummaryDiff = SnapshotFileDiff & { file: string }
@@ -75,6 +75,42 @@ export namespace TimelineRow {
   }
 
   export function equals(a: TimelineRow, b: TimelineRow) {
-    return Equal.equals(a, b)
+    if (a === b) return true
+    if (a._tag !== b._tag) return false
+    switch (a._tag) {
+      case "TurnGap":
+      case "CommentStrip":
+        return a.userMessageID === (b as TurnGap | CommentStrip).userMessageID
+      case "UserMessage": {
+        const other = b as UserMessage
+        return a.userMessageID === other.userMessageID && a.anchor === other.anchor
+      }
+      case "TurnDivider": {
+        const other = b as TurnDivider
+        return a.userMessageID === other.userMessageID && a.label === other.label
+      }
+      case "AssistantPart": {
+        const other = b as AssistantPart
+        return (
+          a.userMessageID === other.userMessageID &&
+          a.previousAssistantPart === other.previousAssistantPart &&
+          sameGroup(a.group, other.group)
+        )
+      }
+      case "Thinking": {
+        const other = b as Thinking
+        return a.userMessageID === other.userMessageID && a.reasoningHeading === other.reasoningHeading
+      }
+      case "DiffSummary": {
+        const other = b as DiffSummary
+        return a.userMessageID === other.userMessageID && Equal.equals(a.diffs, other.diffs)
+      }
+      case "Error": {
+        const other = b as Error
+        return a.userMessageID === other.userMessageID && a.text === other.text
+      }
+      case "Retry":
+        return a.userMessageID === (b as Retry).userMessageID
+    }
   }
 }
diff --git a/packages/session-ui/package.json b/packages/session-ui/package.json
index 7db27630da53..d658344c8200 100644
--- a/packages/session-ui/package.json
+++ b/packages/session-ui/package.json
@@ -6,6 +6,7 @@
   "license": "MIT",
   "exports": {
     "./*": "./src/components/*.tsx",
+    "./message-part-groups": "./src/components/message-part-groups.ts",
     "./session-diff": "./src/components/session-diff.ts",
     "./message-file": "./src/components/message-file.ts",
     "./message-part-text": "./src/components/message-part-text.ts",
diff --git a/packages/session-ui/src/components/message-part-groups.ts b/packages/session-ui/src/components/message-part-groups.ts
new file mode 100644
index 000000000000..092d9fe82888
--- /dev/null
+++ b/packages/session-ui/src/components/message-part-groups.ts
@@ -0,0 +1,40 @@
+export type PartRef = {
+  messageID: string
+  partID: string
+}
+
+export type PartGroup =
+  | {
+      key: string
+      type: "part"
+      ref: PartRef
+    }
+  | {
+      key: string
+      type: "context"
+      refs: PartRef[]
+    }
+
+function sameRef(a: PartRef, b: PartRef) {
+  return a.messageID === b.messageID && a.partID === b.partID
+}
+
+export function sameGroup(a: PartGroup, b: PartGroup) {
+  if (a === b) return true
+  if (a.key !== b.key) return false
+  if (a.type !== b.type) return false
+  if (a.type === "part") {
+    if (b.type !== "part") return false
+    return sameRef(a.ref, b.ref)
+  }
+  if (b.type !== "context") return false
+  if (a.refs.length !== b.refs.length) return false
+  return a.refs.every((ref, i) => sameRef(ref, b.refs[i]!))
+}
+
+export function sameGroups(a: readonly PartGroup[] | undefined, b: readonly PartGroup[] | undefined) {
+  if (a === b) return true
+  if (!a || !b) return false
+  if (a.length !== b.length) return false
+  return a.every((item, i) => sameGroup(item, b[i]!))
+}
diff --git a/packages/session-ui/src/components/message-part.tsx b/packages/session-ui/src/components/message-part.tsx
index d2a00b9d44c0..8a4bb1e83dc2 100644
--- a/packages/session-ui/src/components/message-part.tsx
+++ b/packages/session-ui/src/components/message-part.tsx
@@ -62,6 +62,7 @@ import { AnimatedCountList } from "./tool-count-summary"
 import { ToolStatusTitle } from "./tool-status-title"
 import { patchFiles } from "./apply-patch-file"
 import { partDefaultOpen } from "./part-default-open"
+import { sameGroup, sameGroups, type PartGroup, type PartRef } from "./message-part-groups"
 import { animate } from "motion"
 import { attached, inline, kind, typeLabel } from "./message-file"
 import { readPartText } from "./message-part-text"
@@ -619,46 +620,8 @@ function same<T>(a: readonly T[] | undefined, b: readonly T[] | undefined) {
   return a.every((x, i) => x === b[i])
 }
 
-export type PartRef = {
-  messageID: string
-  partID: string
-}
-
-export type PartGroup =
-  | {
-      key: string
-      type: "part"
-      ref: PartRef
-    }
-  | {
-      key: string
-      type: "context"
-      refs: PartRef[]
-    }
-
-function sameRef(a: PartRef, b: PartRef) {
-  return a.messageID === b.messageID && a.partID === b.partID
-}
-
-function sameGroup(a: PartGroup, b: PartGroup) {
-  if (a === b) return true
-  if (a.key !== b.key) return false
-  if (a.type !== b.type) return false
-  if (a.type === "part") {
-    if (b.type !== "part") return false
-    return sameRef(a.ref, b.ref)
-  }
-  if (b.type !== "context") return false
-  if (a.refs.length !== b.refs.length) return false
-  return a.refs.every((ref, i) => sameRef(ref, b.refs[i]!))
-}
-
-export function sameGroups(a: readonly PartGroup[] | undefined, b: readonly PartGroup[] | undefined) {
-  if (a === b) return true
-  if (!a || !b) return false
-  if (a.length !== b.length) return false
-  return a.every((item, i) => sameGroup(item, b[i]!))
-}
+export { sameGroup, sameGroups }
+export type { PartGroup, PartRef }
 
 export function groupParts(parts: { messageID: string; part: PartType }[]) {
   const result: PartGroup[] = []