diff --git a/docs/design/quark-message-content.md b/docs/design/quark-message-content.md
new file mode 100644
index 0000000000..ea6344f6c3
--- /dev/null
+++ b/docs/design/quark-message-content.md
@@ -0,0 +1,94 @@
+# Quark-Owned Message Content Slots
+
+Status: implemented; drive parity pending
+
+## Summary
+
+Assistant message content currently lives as unkeyed arrays inside the
+DataProvider Solid store. Streaming deltas mutate those arrays through
+`produce`, and every consumer that resolves a part re-scans `content`
+(`findLast`, `filter(type)[ordinal]`), so per-delta work scales with message
+size instead of delta size.
+
+This experiment changes only content ownership: each assistant message's parts
+become one Quark keyed collection with stable per-part slots. Streaming events
+already name the slot on the wire — `ordinal` for text and reasoning, `callID`
+for tools — so deltas address slots directly instead of being rediscovered.
+
+Message metadata (finish, usage, retry, error), the event protocol, timeline
+rows, and view components' rendering logic remain unchanged.
+
+The experiment succeeds only if all checks pass:
+
+1. Existing TUI data and row tests preserve their behavior.
+2. Per-delta work is O(1) in message size: one slot publication per delta,
+   zero structural publications, one markdown recompute scoped to the part.
+3. The drive script passes unchanged.
+
+## Ownership Boundary
+
+```
+server events
+    |
+    v
+DataProvider
+    ├── Solid store: session/message metadata (unchanged)
+    └── SessionContent: Keyed<Part> per assistant message   <- this experiment
+              |                        |
+              | slots (structure)      | slot (value channel)
+              v                        v
+       part row resolution      TextPart / ReasoningPart / ToolPart
+```
+
+- Part identity: `text:{ordinal}` and `reasoning:{ordinal}` from event
+  ordinals, tool parts by `callID`. These match the synthetic part IDs the
+  timeline already uses, so `resolvePart`'s positional bridge disappears.
+- Deltas are value-channel publications: `content.modify(key, ...)`. The
+  parts `<For>`/group structure never re-runs on a delta.
+- Part starts are structural insertions; `ended` events publish the
+  authoritative final value into the same slot.
+- Full sync (`message.sync`) seeds each message's collection with `set(parts)`
+  derived from the fetched payload, assigning ordinals per type in order.
+- Revert, session removal, and model-switch refetch drop or reseed collections.
+- Cold consumers (transcript export, copy-last-response, message navigation,
+  permission lookup) read `values()` — plain arrays, same shapes as before.
+
+## Part Layout
+
+Tool identity fields are immutable; streamed fields are mutable. The field
+diff mask means a tool status change skips text-driven work and vice versa.
+
+```ts
+const PartLayout = Layout.keyedUnion({
+  key: Layout.key("partID", Layout.string),
+  tag: "type",
+  variants: {
+    text: Layout.struct({ text: Layout.string }),
+    reasoning: Layout.struct({ text: Layout.string, state: ..., time: ... }),
+    tool: Layout.struct({
+      id: Layout.immutable(Layout.string),
+      name: Layout.immutable(Layout.string),
+      state: ..., time: ..., executed: ..., providerState: ...,
+    }),
+  },
+})
+```
+
+Store values are the client `SessionMessageAssistant*` content shapes plus the
+synthetic `partID`; `values()` strips nothing because consumers already ignore
+unknown fields.
+
+## What Is Not In Scope
+
+- Message metadata stays in the Solid store.
+- No changes to server events or the client package types.
+- No lazy or pull-based consumers; slots publish eagerly as today.
+- Timeline row ownership is the previous experiment and is unchanged.
+
+## Measurement
+
+Instrument with `Keyed.Metrics`: a streaming test drives N deltas into a
+message with M existing parts and asserts slot publications == N and
+structural publications == parts started, independent of M. Markdown
+recompute counts are asserted through the part component render counters in
+the existing test harness.
diff --git a/packages/quark/src/solid.ts b/packages/quark/src/solid.ts
index 2ca1ecf7bd..6be648be09 100644
--- a/packages/quark/src/solid.ts
+++ b/packages/quark/src/solid.ts
@@ -1,10 +1,29 @@
-import { For, from, type Accessor, type JSX } from "solid-js"
+import { createMemo, For, from, type Accessor, type JSX } from "solid-js"
+import type { Keyed } from "./keyed"
 import type { Readable } from "./reactivity"
 
 export function useValue<A>(readable: Readable<A>): Accessor<A> {
   return from(readable, readable())
 }
 
+/**
+ * Reactive accessor for one keyed slot. Tracks structure only until the slot
+ * exists; afterwards value changes flow through the slot alone, so unrelated
+ * structural churn cannot re-render the consumer.
+ */
+export function useSlot<A, Key>(keyed: Keyed.Keyed<A, Key>, key: () => Key): Accessor<A | undefined> {
+  const structure = useValue(keyed.slots)
+  const slot = createMemo(() => {
+    structure()
+    return keyed.get(key())
+  })
+  const value = createMemo(() => {
+    const current = slot()
+    return current ? useValue(current) : undefined
+  })
+  return () => value()?.()
+}
+
 export function KeyedFor<A>(props: {
   readonly each: Accessor<readonly Readable<A>[]>
   readonly fallback?: JSX.Element
diff --git a/packages/tui/src/context/data.tsx b/packages/tui/src/context/data.tsx
index 14990fb933..3dc776beee 100644
--- a/packages/tui/src/context/data.tsx
+++ b/packages/tui/src/context/data.tsx
@@ -19,9 +19,6 @@ import type {
   ReferenceInfo,
   SessionMessageInfo,
   SessionMessageAssistant,
-  SessionMessageAssistantReasoning,
-  SessionMessageAssistantText,
-  SessionMessageAssistantTool,
   SessionInfo,
   SessionPendingInfo,
   ShellInfo,
@@ -33,6 +30,7 @@ import { createStore, produce, reconcile } from "solid-js/store"
 import { createSimpleContext } from "./helper"
 import { useClient } from "./client"
 import { createEffect, createSignal, onCleanup } from "solid-js"
+import { SessionContent } from "../routes/session/content"
 
 export type DataSessionStatus = "idle" | "running"
 
@@ -146,6 +144,9 @@ export const { use: useData, provider: DataProvider } = createSimpleContext({
       directory: process.cwd(),
     })
     const messageIndex = new Map<string, Map<string, number>>()
+    // Assistant content lives in per-message keyed part slots, not the Solid
+    // store: streaming deltas publish one slot instead of reconciling arrays.
+    const content = SessionContent.make()
     const sync = createSync()
 
     function setSessionActive(sessionID: string, status: DataSessionStatus) {
@@ -199,20 +200,6 @@ export const { use: useData, provider: DataProvider } = createSimpleContext({
         const item = messages.findLast((item) => item.type === "compaction" && item.status === "running")
         return item?.type === "compaction" ? item : undefined
       },
-      latestTool(assistant: SessionMessageAssistant | undefined, callID?: string) {
-        return assistant?.content.findLast(
-          (item): item is SessionMessageAssistantTool =>
-            item.type === "tool" && (callID === undefined || item.id === callID),
-        )
-      },
-      latestText(assistant: SessionMessageAssistant | undefined) {
-        return assistant?.content.findLast((item): item is SessionMessageAssistantText => item.type === "text")
-      },
-      latestReasoning(assistant: SessionMessageAssistant | undefined) {
-        return assistant?.content.findLast(
-          (item): item is SessionMessageAssistantReasoning => item.type === "reasoning" && !item.time?.completed,
-        )
-      },
     }
 
     function index(sessionID: string) {
@@ -269,6 +256,7 @@ export const { use: useData, provider: DataProvider } = createSimpleContext({
 
     function removeSession(sessionID: string) {
       messageIndex.delete(sessionID)
+      content.drop(sessionID)
       sync.invalidate(`session:${sessionID}`)
       sync.invalidate(`session.pending:${sessionID}`)
       sync.invalidate(`session.message:${sessionID}`)
@@ -353,6 +341,7 @@ export const { use: useData, provider: DataProvider } = createSimpleContext({
           void client.api.session
             .message({ sessionID: event.data.sessionID, messageID: messageIDFromEvent(event.id) })
             .then((item) => {
+              if (item.type === "assistant") content.seed(event.data.sessionID, item.id, item.content)
               message.update(event.data.sessionID, (draft, index) => {
                 const position = index.get(item.id)
                 if (position === undefined) return message.append(draft, index, item)
@@ -540,143 +529,142 @@ export const { use: useData, provider: DataProvider } = createSimpleContext({
             }
           })
           break
-        case "session.text.started":
-          message.update(event.data.sessionID, (draft, index) => {
-            message.assistant(draft, index, event.data.assistantMessageID)?.content.push({
-              type: "text",
-              text: "",
-            })
-          })
+        case "session.text.started": {
+          const parts = content.ensure(event.data.sessionID, event.data.assistantMessageID)
+          const partID = SessionContent.textID(event.data.ordinal)
+          if (!parts.has(partID)) parts.insert({ type: "text", text: "", partID })
           break
+        }
         case "session.text.delta":
-          message.update(event.data.sessionID, (draft, index) => {
-            const match = message.latestText(message.assistant(draft, index, event.data.assistantMessageID))
-            if (match) match.text += event.data.delta
-          })
+          content
+            .get(event.data.sessionID, event.data.assistantMessageID)
+            ?.modify(SessionContent.textID(event.data.ordinal), (part) =>
+              part.type === "text" ? { ...part, text: part.text + event.data.delta } : part,
+            )
           break
         case "session.text.ended":
-          message.update(event.data.sessionID, (draft, index) => {
-            const match = message.latestText(message.assistant(draft, index, event.data.assistantMessageID))
-            if (match) match.text = event.data.text
-          })
+          content
+            .get(event.data.sessionID, event.data.assistantMessageID)
+            ?.modify(SessionContent.textID(event.data.ordinal), (part) =>
+              part.type === "text" ? { ...part, text: event.data.text } : part,
+            )
           break
-        case "session.tool.input.started":
-          message.update(event.data.sessionID, (draft, index) => {
-            message.assistant(draft, index, event.data.assistantMessageID)?.content.push({
+        case "session.tool.input.started": {
+          const parts = content.ensure(event.data.sessionID, event.data.assistantMessageID)
+          if (!parts.has(event.data.callID))
+            parts.insert({
               type: "tool",
               id: event.data.callID,
               name: event.data.name,
               time: { created: event.created },
               state: { status: "streaming", input: "" },
+              partID: event.data.callID,
             })
-          })
           break
+        }
         case "session.tool.input.delta":
-          message.update(event.data.sessionID, (draft, index) => {
-            const match = message.latestTool(
-              message.assistant(draft, index, event.data.assistantMessageID),
-              event.data.callID,
-            )
-            if (match?.state.status === "streaming") match.state.input += event.data.delta
+          content.get(event.data.sessionID, event.data.assistantMessageID)?.modify(event.data.callID, (part) => {
+            if (part.type !== "tool" || part.state.status !== "streaming") return part
+            return { ...part, state: { ...part.state, input: part.state.input + event.data.delta } }
           })
           break
         case "session.tool.input.ended":
-          message.update(event.data.sessionID, (draft, index) => {
-            const match = message.latestTool(
-              message.assistant(draft, index, event.data.assistantMessageID),
-              event.data.callID,
-            )
-            if (match?.state.status === "streaming") match.state.input = event.data.text
+          content.get(event.data.sessionID, event.data.assistantMessageID)?.modify(event.data.callID, (part) => {
+            if (part.type !== "tool" || part.state.status !== "streaming") return part
+            return { ...part, state: { ...part.state, input: event.data.text } }
           })
           break
         case "session.tool.called":
-          message.update(event.data.sessionID, (draft, index) => {
-            const match = message.latestTool(
-              message.assistant(draft, index, event.data.assistantMessageID),
-              event.data.callID,
-            )
-            if (!match) return
-            match.time.ran = event.created
-            match.executed = event.data.executed
-            match.providerState = event.data.state
-            match.state = { status: "running", input: event.data.input, structured: {}, content: [] }
+          content.get(event.data.sessionID, event.data.assistantMessageID)?.modify(event.data.callID, (part) => {
+            if (part.type !== "tool") return part
+            return {
+              ...part,
+              time: { ...part.time, ran: event.created },
+              executed: event.data.executed,
+              providerState: event.data.state,
+              state: { status: "running", input: event.data.input, structured: {}, content: [] },
+            }
           })
           break
         case "session.tool.progress":
-          message.update(event.data.sessionID, (draft, index) => {
-            const match = message.latestTool(
-              message.assistant(draft, index, event.data.assistantMessageID),
-              event.data.callID,
-            )
-            if (match?.state.status !== "running") return
-            match.state.structured = event.data.structured
-            match.state.content = [...event.data.content]
+          content.get(event.data.sessionID, event.data.assistantMessageID)?.modify(event.data.callID, (part) => {
+            if (part.type !== "tool" || part.state.status !== "running") return part
+            return {
+              ...part,
+              state: { ...part.state, structured: event.data.structured, content: [...event.data.content] },
+            }
           })
           break
         case "session.tool.success":
-          message.update(event.data.sessionID, (draft, index) => {
-            const match = message.latestTool(
-              message.assistant(draft, index, event.data.assistantMessageID),
-              event.data.callID,
-            )
-            if (match?.state.status !== "running") return
-            match.state = {
-              status: "completed",
-              input: match.state.input,
-              structured: event.data.structured,
-              content: [...event.data.content],
-              result: event.data.result,
+          content.get(event.data.sessionID, event.data.assistantMessageID)?.modify(event.data.callID, (part) => {
+            if (part.type !== "tool" || part.state.status !== "running") return part
+            return {
+              ...part,
+              state: {
+                status: "completed",
+                input: part.state.input,
+                structured: event.data.structured,
+                content: [...event.data.content],
+                result: event.data.result,
+              },
+              executed: event.data.executed || part.executed === true,
+              providerResultState: event.data.resultState,
+              time: { ...part.time, completed: event.created },
             }
-            match.executed = event.data.executed || match.executed === true
-            match.providerResultState = event.data.resultState
-            match.time.completed = event.created
           })
           break
         case "session.tool.failed":
-          message.update(event.data.sessionID, (draft, index) => {
-            const match = message.latestTool(
-              message.assistant(draft, index, event.data.assistantMessageID),
-              event.data.callID,
-            )
-            if (!match || (match.state.status !== "streaming" && match.state.status !== "running")) return
-            match.state = {
-              status: "error",
-              error: event.data.error,
-              input: typeof match.state.input === "string" ? {} : match.state.input,
-              structured: match.state.status === "running" ? match.state.structured : {},
-              content: match.state.status === "running" ? match.state.content : [],
-              result: event.data.result,
+          content.get(event.data.sessionID, event.data.assistantMessageID)?.modify(event.data.callID, (part) => {
+            if (part.type !== "tool" || (part.state.status !== "streaming" && part.state.status !== "running"))
+              return part
+            return {
+              ...part,
+              state: {
+                status: "error",
+                error: event.data.error,
+                input: typeof part.state.input === "string" ? {} : part.state.input,
+                structured: part.state.status === "running" ? part.state.structured : {},
+                content: part.state.status === "running" ? part.state.content : [],
+                result: event.data.result,
+              },
+              executed: event.data.executed || part.executed === true,
+              providerResultState: event.data.resultState,
+              time: { ...part.time, completed: event.created },
             }
-            match.executed = event.data.executed || match.executed === true
-            match.providerResultState = event.data.resultState
-            match.time.completed = event.created
           })
           break
-        case "session.reasoning.started":
-          message.update(event.data.sessionID, (draft, index) => {
-            message.assistant(draft, index, event.data.assistantMessageID)?.content.push({
+        case "session.reasoning.started": {
+          const parts = content.ensure(event.data.sessionID, event.data.assistantMessageID)
+          const partID = SessionContent.reasoningID(event.data.ordinal)
+          if (!parts.has(partID))
+            parts.insert({
               type: "reasoning",
               text: "",
               state: event.data.state,
               time: { created: event.created },
+              partID,
             })
-          })
           break
+        }
         case "session.reasoning.delta":
-          message.update(event.data.sessionID, (draft, index) => {
-            const match = message.latestReasoning(message.assistant(draft, index, event.data.assistantMessageID))
-            if (match) match.text += event.data.delta
-          })
+          content
+            .get(event.data.sessionID, event.data.assistantMessageID)
+            ?.modify(SessionContent.reasoningID(event.data.ordinal), (part) =>
+              part.type === "reasoning" ? { ...part, text: part.text + event.data.delta } : part,
+            )
           break
         case "session.reasoning.ended":
-          message.update(event.data.sessionID, (draft, index) => {
-            const match = message.latestReasoning(message.assistant(draft, index, event.data.assistantMessageID))
-            if (match) {
-              match.text = event.data.text
-              match.time = { created: match.time?.created ?? event.created, completed: event.created }
-              if (event.data.state !== undefined) match.state = event.data.state
-            }
-          })
+          content
+            .get(event.data.sessionID, event.data.assistantMessageID)
+            ?.modify(SessionContent.reasoningID(event.data.ordinal), (part) => {
+              if (part.type !== "reasoning") return part
+              return {
+                ...part,
+                text: event.data.text,
+                time: { created: part.time?.created ?? event.created, completed: event.created },
+                state: event.data.state !== undefined ? event.data.state : part.state,
+              }
+            })
           break
         case "session.retry.scheduled":
           message.update(event.data.sessionID, (draft, index) => {
@@ -745,7 +733,10 @@ export const { use: useData, provider: DataProvider } = createSimpleContext({
           message.update(event.data.sessionID, (draft, index) => {
             const position = draft.findIndex((item) => item.id >= event.data.to)
             if (position === -1) return
-            for (const item of draft.splice(position)) index.delete(item.id)
+            for (const item of draft.splice(position)) {
+              index.delete(item.id)
+              content.drop(event.data.sessionID, item.id)
+            }
           })
           break
         case "session.compaction.delta":
@@ -964,9 +955,16 @@ export const { use: useData, provider: DataProvider } = createSimpleContext({
                 await client.api.message.list({ sessionID, limit: 200, order: "desc" })
               ).data.toReversed()
               messageIndex.set(sessionID, new Map(messages.map((message, index) => [message.id, index])))
+              content.drop(sessionID)
+              for (const item of messages) {
+                if (item.type === "assistant") content.seed(sessionID, item.id, item.content)
+              }
               setStore("session", "message", sessionID, reconcile(messages))
             })
           },
+          parts(sessionID: string, messageID: string) {
+            return content.ensure(sessionID, messageID)
+          },
           invalidate(sessionID: string) {
             sync.invalidate(`session.message:${sessionID}`)
           },
diff --git a/packages/tui/src/routes/session/content.ts b/packages/tui/src/routes/session/content.ts
new file mode 100644
index 0000000000..4e4e737da9
--- /dev/null
+++ b/packages/tui/src/routes/session/content.ts
@@ -0,0 +1,91 @@
+import type { SessionMessageAssistant } from "@opencode-ai/client"
+import { Keyed, Layout } from "effect-quark"
+
+/**
+ * Stable per-part reactive slots for assistant message content.
+ *
+ * Streaming events name their part on the wire (text/reasoning by ordinal,
+ * tools by callID); each assistant message owns one keyed collection so a
+ * delta publishes exactly one slot instead of reconciling a content array.
+ * See docs/design/quark-message-content.md.
+ */
+export namespace SessionContent {
+  type ContentPart = SessionMessageAssistant["content"][number]
+  export type Part = ContentPart & { readonly partID: string }
+  export type Parts = Keyed.Keyed<Part, string>
+
+  // Streamed sub-objects are replaced immutably on change, so reference
+  // equality is the correct (and cheapest) field comparator for them.
+  const reference: Layout.Field<unknown> = { equivalent: Object.is }
+
+  const PartLayout = Layout.keyedUnion({
+    key: Layout.key("partID", Layout.string),
+    tag: "type",
+    variants: {
+      text: Layout.struct({ text: Layout.string }),
+      reasoning: Layout.struct({ text: Layout.string, state: reference, time: reference }),
+      tool: Layout.struct({
+        id: Layout.immutable(Layout.string),
+        name: Layout.immutable(Layout.string),
+        executed: reference,
+        providerState: reference,
+        providerResultState: reference,
+        state: reference,
+        time: reference,
+      }),
+    },
+  })
+  // The layout describes the reactive fields of the client content shapes;
+  // the plan is typed against those shapes at this single boundary.
+  const PartPlan = Layout.compile(PartLayout) as unknown as Layout.Plan<Part, string>
+
+  export function textID(ordinal: number) {
+    return `text:${ordinal}`
+  }
+
+  export function reasoningID(ordinal: number) {
+    return `reasoning:${ordinal}`
+  }
+
+  /** Assign the synthetic part IDs used across the TUI to a fetched content array. */
+  export function withPartIDs(content: readonly ContentPart[]): Part[] {
+    const ordinals = { text: 0, reasoning: 0 }
+    return content.map((part) => {
+      if (part.type === "tool") return { ...part, partID: part.id }
+      return { ...part, partID: `${part.type}:${ordinals[part.type]++}` }
+    })
+  }
+
+  export function make(options?: { readonly metrics?: Keyed.Metrics }) {
+    const collections = new Map<string, Parts>()
+    const id = (sessionID: string, messageID: string) => `${sessionID}\u0000${messageID}`
+
+    return {
+      get(sessionID: string, messageID: string) {
+        return collections.get(id(sessionID, messageID))
+      },
+      ensure(sessionID: string, messageID: string) {
+        const key = id(sessionID, messageID)
+        const existing = collections.get(key)
+        if (existing) return existing
+        const created = PartPlan.make([], options)
+        collections.set(key, created)
+        return created
+      },
+      seed(sessionID: string, messageID: string, content: readonly ContentPart[]) {
+        this.ensure(sessionID, messageID).set(withPartIDs(content))
+      },
+      /** Drop one message's collection, or every collection for a session. */
+      drop(sessionID: string, messageID?: string) {
+        if (messageID !== undefined) {
+          collections.delete(id(sessionID, messageID))
+          return
+        }
+        const prefix = `${sessionID}\u0000`
+        for (const key of [...collections.keys()]) {
+          if (key.startsWith(prefix)) collections.delete(key)
+        }
+      },
+    }
+  }
+}
diff --git a/packages/tui/src/routes/session/dialog-message.tsx b/packages/tui/src/routes/session/dialog-message.tsx
index 8935524634..e221619177 100644
--- a/packages/tui/src/routes/session/dialog-message.tsx
+++ b/packages/tui/src/routes/session/dialog-message.tsx
@@ -62,7 +62,9 @@ export function DialogMessage(props: {
               value.type === "user"
                 ? value.text
                 : value.type === "assistant"
-                  ? value.content
+                  ? data.session.message
+                      .parts(props.sessionID, props.messageID)
+                      .values()
                       .filter((content) => content.type === "text")
                       .map((content) => content.text)
                       .join("\n")
diff --git a/packages/tui/src/routes/session/index.tsx b/packages/tui/src/routes/session/index.tsx
index 942e173a84..d5967e239e 100644
--- a/packages/tui/src/routes/session/index.tsx
+++ b/packages/tui/src/routes/session/index.tsx
@@ -13,7 +13,8 @@ import {
   Switch,
   useContext,
 } from "solid-js"
-import { KeyedFor } from "effect-quark/solid"
+import { KeyedFor, useSlot, useValue } from "effect-quark/solid"
+import { SessionContent } from "./content"
 import path from "node:path"
 import { EOL, tmpdir } from "node:os"
 import { mkdir, writeFile } from "node:fs/promises"
@@ -74,7 +75,7 @@ import { Keymap, type KeymapCommand } from "../../context/keymap"
 import { usePathFormatter } from "../../context/path-format"
 import { useLocation } from "../../context/location"
 import { createSessionRows } from "./rows"
-import { messageBoundaryIDs, resolvePart, type PartRef, type SessionRow } from "./timeline"
+import { messageBoundaryIDs, type PartRef, type SessionRow } from "./timeline"
 import { switchLabel } from "../../util/model"
 import { findMessageBoundary, messageNavigationSlack } from "./message-navigation"
 import { stringWidth } from "../../util/string-width"
@@ -312,6 +313,11 @@ export function Session() {
       direction,
       children: scroll.getChildren(),
       messages: messages(),
+      hasText: (messageID) =>
+        data.session.message
+          .parts(route.sessionID, messageID)
+          .values()
+          .some((part) => part.type === "text" && part.text.trim()),
       scrollTop: scroll.scrollTop,
       viewportY: scroll.viewport.y,
       currentID: navigationMessage(),
@@ -681,7 +687,10 @@ export function Session() {
           return
         }
 
-        const textParts = lastAssistantMessage.content.filter((part) => part.type === "text")
+        const textParts = data.session.message
+          .parts(route.sessionID, lastAssistantMessage.id)
+          .values()
+          .filter((part) => part.type === "text")
         if (textParts.length === 0) {
           toast.show({ message: "No text parts found in last assistant message", variant: "error" })
           dialog.clear()
@@ -719,7 +728,9 @@ export function Session() {
         try {
           const sessionData = session()
           if (!sessionData) return
-          const transcript = formatSessionTranscript(sessionData, messages(), showThinking())
+          const transcript = formatSessionTranscript(sessionData, messages(), showThinking(), (messageID) =>
+            data.session.message.parts(route.sessionID, messageID).values(),
+          )
           await clipboard.write?.(transcript)
           toast.show({ message: "Session transcript copied to clipboard!", variant: "success" })
         } catch {
@@ -746,7 +757,9 @@ export function Session() {
 
           const content =
             options.format === "markdown"
-              ? formatSessionTranscript(sessionData, messages(), options.thinking)
+              ? formatSessionTranscript(sessionData, messages(), options.thinking, (messageID) =>
+                  data.session.message.parts(route.sessionID, messageID).values(),
+                )
               : await (async () => {
                   if (options.debug) {
                     const events: { readonly created: number }[] = []
@@ -934,11 +947,15 @@ export function Session() {
                   <SessionRowView
                     row={row()}
                     message={(messageID) => data.session.message.get(route.sessionID, messageID)}
+                    parts={(messageID) => data.session.message.parts(route.sessionID, messageID)}
                     boundaryID={boundaries()[index()]}
                   />
                 )}
               </KeyedFor>
-              <BackgroundToolHint messages={messages()} />
+              <BackgroundToolHint
+                messages={messages()}
+                parts={(messageID) => data.session.message.parts(route.sessionID, messageID)}
+              />
               <Show when={session()?.revert?.messageID}>
                 <RevertMessage
                   count={
@@ -1028,6 +1045,7 @@ export function Session() {
 function SessionRowView(props: {
   row: SessionRow
   message: (messageID: string) => SessionMessageInfo | undefined
+  parts: (messageID: string) => SessionContent.Parts
   boundaryID?: string
 }) {
   return (
@@ -1042,10 +1060,17 @@ function SessionRowView(props: {
           <CompactionQueued />
         </Match>
         <Match when={props.row.type === "part" ? props.row : undefined}>
-          {(row) => <SessionPartView partRef={row().ref} message={props.message} />}
+          {(row) => <SessionPartView partRef={row().ref} message={props.message} parts={props.parts} />}
         </Match>
         <Match when={props.row.type === "group" && props.row.kind === "reasoning" ? props.row : undefined}>
-          {(row) => <SessionReasoningGroupView refs={row().refs} completed={row().completed} message={props.message} />}
+          {(row) => (
+            <SessionReasoningGroupView
+              refs={row().refs}
+              completed={row().completed}
+              message={props.message}
+              parts={props.parts}
+            />
+          )}
         </Match>
         <Match when={props.row.type === "group" && props.row.kind === "exploration" ? props.row : undefined}>
           {(row) => (
@@ -1054,6 +1079,7 @@ function SessionRowView(props: {
               pending={row().pending}
               completed={row().completed}
               message={props.message}
+              parts={props.parts}
             />
           )}
         </Match>
@@ -1073,21 +1099,29 @@ function SessionRowView(props: {
   )
 }
 
-function BackgroundToolHint(props: { messages: SessionMessageInfo[] }) {
+function BackgroundToolHint(props: {
+  messages: SessionMessageInfo[]
+  parts: (messageID: string) => SessionContent.Parts
+}) {
   const { themeV2 } = useTheme()
   const shortcut = Keymap.useShortcut("session.background")
-  const visible = createMemo(() => {
-    const current = props.messages.findLast(
+  const current = createMemo(() =>
+    props.messages.findLast(
       (message): message is SessionMessageAssistant => message.type === "assistant" && !message.time.completed,
-    )
-    return (
-      current?.content.some((part) => {
+    ),
+  )
+  const parts = createMemo(() => {
+    const message = current()
+    return message ? useValue(props.parts(message.id).values) : undefined
+  })
+  const visible = createMemo(
+    () =>
+      parts()?.()?.some((part) => {
         if (part.type !== "tool" || part.state.status !== "running") return false
         const display = toolDisplay(part.name)
         return display === "shell" || display === "subagent"
-      }) ?? false
-    )
-  })
+      }) ?? false,
+  )
   return (
     <Show when={visible() && shortcut()}>
       {(value) => (
@@ -1127,13 +1161,15 @@ function SessionMessageView(props: { message: SessionMessageInfo }) {
   )
 }
 
-function SessionPartView(props: { partRef: PartRef; message: (messageID: string) => SessionMessageInfo | undefined }) {
+function SessionPartView(props: {
+  partRef: PartRef
+  message: (messageID: string) => SessionMessageInfo | undefined
+  parts: (messageID: string) => SessionContent.Parts
+}) {
   const message = createMemo(() => props.message(props.partRef.messageID))
-  const part = createMemo(() => {
-    const item = message()
-    if (item?.type !== "assistant") return
-    return resolvePart(item, props.partRef.partID)
-  })
+  // One reactive slot per part: unrelated deltas in the same message cannot
+  // re-render this row.
+  const part = useSlot(props.parts(props.partRef.messageID), () => props.partRef.partID)
   return (
     <Show when={part()}>
       {(item) => (
@@ -1161,17 +1197,26 @@ function SessionReasoningGroupView(props: {
   refs: readonly PartRef[]
   completed: boolean
   message: (messageID: string) => SessionMessageInfo | undefined
+  parts: (messageID: string) => SessionContent.Parts
 }) {
   const ctx = use()
   const { themeV2, syntax } = useTheme()
   const renderer = useRenderer()
   const [expanded, setExpanded] = createSignal(false)
   const [hover, setHover] = createSignal(false)
+  // Slot accessors are created per ref list; value changes flow through the
+  // individual slots without re-running the accessor construction.
+  const accessors = createMemo(() =>
+    props.refs.map((ref) => ({
+      ref,
+      part: useSlot(props.parts(ref.messageID), () => ref.partID),
+    })),
+  )
   const parts = createMemo(() =>
-    props.refs.flatMap((ref) => {
-      const message = props.message(ref.messageID)
+    accessors().flatMap((entry) => {
+      const message = props.message(entry.ref.messageID)
       if (message?.type !== "assistant") return []
-      const part = resolvePart(message, ref.partID)
+      const part = entry.part()
       if (part?.type !== "reasoning" || !reasoningContent(part)) return []
       return [{ message, part }]
     }),
@@ -1196,7 +1241,11 @@ function SessionReasoningGroupView(props: {
     <Show when={parts().length > 0}>
       <Show
         when={ctx.thinkingMode() === "hide"}
-        fallback={<For each={props.refs}>{(ref) => <SessionPartView partRef={ref} message={props.message} />}</For>}
+        fallback={
+          <For each={props.refs}>
+            {(ref) => <SessionPartView partRef={ref} message={props.message} parts={props.parts} />}
+          </For>
+        }
       >
         <box flexDirection="column" flexShrink={0}>
           <InlineToolRow
@@ -1232,15 +1281,14 @@ function SessionReasoningGroupView(props: {
             <box paddingLeft={3}>
               <For each={props.refs}>
                 {(ref) => {
-                  const message = createMemo(() => {
-                    const item = props.message(ref.messageID)
-                    return item?.type === "assistant" ? item : undefined
-                  })
+                  const slot = useSlot(props.parts(ref.messageID), () => ref.partID)
                   const part = createMemo(() => {
-                    const item = message()
-                    if (!item) return undefined
-                    const part = resolvePart(item, ref.partID)
-                    return part?.type === "reasoning" ? part : undefined
+                    const item = slot()
+                    return item?.type === "reasoning" ? item : undefined
+                  })
+                  const messageCompleted = createMemo(() => {
+                    const item = props.message(ref.messageID)
+                    return item?.type === "assistant" && item.time.completed !== undefined
                   })
                   const content = createMemo(() => {
                     const item = part()
@@ -1258,7 +1306,7 @@ function SessionReasoningGroupView(props: {
                           <code
                             filetype="markdown"
                             drawUnstyledText={false}
-                            streaming={part()?.time?.completed === undefined && message()?.time.completed === undefined}
+                            streaming={part()?.time?.completed === undefined && !messageCompleted()}
                             syntaxStyle={syntax()}
                             content={content()}
                             conceal={ctx.markdownMode() === "rendered"}
@@ -1283,22 +1331,27 @@ function SessionGroupView(props: {
   pending: readonly PartRef[]
   completed: boolean
   message: (messageID: string) => SessionMessageInfo | undefined
+  parts: (messageID: string) => SessionContent.Parts
 }) {
   const { themeV2 } = useTheme()
   const ctx = use()
   const renderer = useRenderer()
   const [expanded, setExpanded] = createSignal(false)
   const [hover, setHover] = createSignal(false)
-  const parts = (refs: readonly PartRef[]) =>
-    refs.flatMap((ref) => {
-      const message = props.message(ref.messageID)
-      if (message?.type !== "assistant") return []
-      const part = resolvePart(message, ref.partID)
+  // Slot accessors per ref list: tool state changes flow through individual
+  // slots; the accessor lists rebuild only when the refs themselves change.
+  const slots = (refs: () => readonly PartRef[]) =>
+    createMemo(() => refs().map((ref) => useSlot(props.parts(ref.messageID), () => ref.partID)))
+  const groupedSlots = slots(() => props.refs)
+  const pendingSlots = slots(() => props.pending)
+  const parts = (accessors: readonly (() => SessionContent.Part | undefined)[]) =>
+    accessors.flatMap((accessor) => {
+      const part = accessor()
       if (part?.type !== "tool") return []
       return [part]
     })
-  const grouped = createMemo(() => parts(props.refs))
-  const pending = createMemo(() => parts(props.pending))
+  const grouped = createMemo(() => parts(groupedSlots()))
+  const pending = createMemo(() => parts(pendingSlots()))
   const label = createMemo(() => {
     const counts = grouped().reduce<Record<string, number>>((result, part) => {
       const tool = toolDisplay(part.name)
@@ -1713,124 +1766,6 @@ function UserMessage(props: { message: SessionMessageUser }) {
   )
 }
 
-function AssistantMessage(props: { message: SessionMessageAssistant; last: boolean }) {
-  const ctx = use()
-  const local = useLocal()
-  const { themeV2 } = useTheme().contextual("elevated")
-  const model = createMemo(
-    () =>
-      ctx
-        .models()
-        .find((model) => model.providerID === props.message.model.providerID && model.id === props.message.model.id)
-        ?.name ?? `${props.message.model.providerID}/${props.message.model.id}`,
-  )
-
-  const final = createMemo(() => {
-    return props.message.finish && !["tool-calls", "unknown"].includes(props.message.finish)
-  })
-
-  const duration = createMemo(() => {
-    if (!final()) return 0
-    if (!props.message.time.completed) return 0
-    return props.message.time.completed - props.message.time.created
-  })
-
-  const exploration = createMemo(() => {
-    const grouped = new Map<string, { first: boolean; parts: SessionMessageAssistantTool[]; active: boolean }>()
-    if (!ctx.groupExploration()) return grouped
-    const runs = props.message.content
-      .map((part) =>
-        part.type === "tool" &&
-        ["read", "glob", "grep"].includes(toolDisplay(part.name)) &&
-        part.state.status !== "streaming"
-          ? part
-          : undefined,
-      )
-      .reduce<SessionMessageAssistantTool[][]>(
-        (runs, part) => {
-          if (part) runs[runs.length - 1].push(part)
-          if (!part && runs[runs.length - 1].length) runs.push([])
-          return runs
-        },
-        [[]],
-      )
-      .filter((run) => run.length > 0)
-    for (const run of runs) {
-      const summary = {
-        parts: run,
-        active: false,
-      }
-      run.forEach((part, index) => grouped.set(part.id, { ...summary, first: index === 0 }))
-    }
-    return grouped
-  })
-
-  return (
-    <>
-      <For each={props.message.content}>
-        {(content, index) => (
-          <Switch>
-            <Match when={content.type === "text"}>
-              <TextPart
-                part={content as SessionMessageAssistantText}
-                last={index() === props.message.content.length - 1}
-              />
-            </Match>
-            <Match when={content.type === "reasoning"}>
-              <ReasoningPart
-                part={content as SessionMessageAssistantReasoning}
-                message={props.message}
-                last={index() === props.message.content.length - 1}
-              />
-            </Match>
-            <Match when={content.type === "tool"}>
-              <Show when={exploration().get((content as SessionMessageAssistantTool).id)?.first !== false}>
-                <Show
-                  when={exploration().get((content as SessionMessageAssistantTool).id)}
-                  fallback={<ToolPart part={content as SessionMessageAssistantTool} />}
-                >
-                  {(summary) => <ExplorationSummary {...summary()} />}
-                </Show>
-              </Show>
-            </Match>
-          </Switch>
-        )}
-      </For>
-      <Show when={props.message.error}>
-        <box
-          border={["left"]}
-          paddingTop={1}
-          paddingBottom={1}
-          paddingLeft={2}
-          backgroundColor={themeV2.background()}
-          customBorderChars={SplitBorder.customBorderChars}
-          borderColor={themeV2.text.feedback.error()}
-        >
-          <text fg={themeV2.text.subdued()}>{errorMessage(props.message.error)}</text>
-        </box>
-      </Show>
-      <AssistantRetry retry={props.message.retry} />
-      <Switch>
-        <Match when={props.last || final() || props.message.error}>
-          <box paddingLeft={3}>
-            <text>
-              <span
-                style={{ fg: props.message.error ? themeV2.text.subdued() : local.agent.color(props.message.agent) }}
-              >
-                {Locale.titlecase(props.message.agent)}
-              </span>
-              <span style={{ fg: themeV2.text.subdued() }}> · {model()}</span>
-              <Show when={duration()}>
-                <span style={{ fg: themeV2.text.subdued() }}> · {Locale.duration(duration())}</span>
-              </Show>
-            </text>
-          </box>
-        </Match>
-      </Switch>
-    </>
-  )
-}
-
 function AssistantRetry(props: { retry: SessionMessageAssistant["retry"] }) {
   const { themeV2 } = useTheme()
   return (
@@ -3035,13 +2970,18 @@ function recordValue(value: unknown): Record<string, unknown> | undefined {
   return value as Record<string, unknown>
 }
 
-function formatSessionTranscript(session: SessionInfo, messages: SessionMessageInfo[], thinking: boolean) {
+function formatSessionTranscript(
+  session: SessionInfo,
+  messages: SessionMessageInfo[],
+  thinking: boolean,
+  partsOf: (messageID: string) => readonly SessionContent.Part[],
+) {
   const body = messages.flatMap((message) => {
     if (message.type === "user") return [`## User\n\n${message.text}`]
     if (message.type === "shell")
       return [`## Shell\n\n\`\`\`\n$ ${message.command}\n${message.output?.output ?? ""}\n\`\`\``]
     if (message.type !== "assistant") return []
-    const content = message.content.flatMap((item) => {
+    const content = partsOf(message.id).flatMap((item) => {
       if (item.type === "text") return [item.text]
       if (item.type === "reasoning") return thinking ? [`_Thinking:_\n\n${item.text}`] : []
       const input = typeof item.state.input === "string" ? item.state.input : JSON.stringify(item.state.input, null, 2)
diff --git a/packages/tui/src/routes/session/message-navigation.ts b/packages/tui/src/routes/session/message-navigation.ts
index 5e75fd5df1..29c2125440 100644
--- a/packages/tui/src/routes/session/message-navigation.ts
+++ b/packages/tui/src/routes/session/message-navigation.ts
@@ -19,6 +19,7 @@ export function findMessageBoundary(input: {
   direction: "next" | "prev"
   children: readonly MessageChild[]
   messages: readonly SessionMessageInfo[]
+  hasText?: (messageID: string) => boolean
   scrollTop: number
   viewportY: number
   currentID?: string
@@ -35,7 +36,9 @@ export function findMessageBoundary(input: {
         return [{ id: child.id, y, top: y }]
       }
       if (input.userOnly || message.type !== "assistant") return []
-      if (!message.content.some((content) => content.type === "text" && content.text.trim())) return []
+      const hasText =
+        input.hasText?.(message.id) ?? message.content.some((content) => content.type === "text" && content.text.trim())
+      if (!hasText) return []
       const y = input.scrollTop + child.y - input.viewportY
       return [{ id: child.id, y, top: Math.max(0, y - 1) }]
     })
diff --git a/packages/tui/src/routes/session/permission.tsx b/packages/tui/src/routes/session/permission.tsx
index 3b3ecf41af..97c3852276 100644
--- a/packages/tui/src/routes/session/permission.tsx
+++ b/packages/tui/src/routes/session/permission.tsx
@@ -146,9 +146,7 @@ export function PermissionPrompt(props: { request: PermissionV2Request; director
   const input = createMemo(() => {
     const tool = props.request.source
     if (!tool) return {}
-    const message = data.session.message.get(props.request.sessionID, tool.messageID)
-    if (message?.type !== "assistant") return {}
-    const part = message.content.find((part) => part.type === "tool" && part.id === tool.callID)
+    const part = data.session.message.parts(props.request.sessionID, tool.messageID).get(tool.callID)?.()
     if (part?.type === "tool" && part.state.status !== "streaming") return part.state.input
     return {}
   })
@@ -167,7 +165,9 @@ export function PermissionPrompt(props: { request: PermissionV2Request; director
               </Match>
               <Match when={true}>
                 <box paddingLeft={1} gap={1}>
-                  <text fg={themeV2.text.subdued()}>This will allow the following patterns until OpenCode is restarted</text>
+                  <text fg={themeV2.text.subdued()}>
+                    This will allow the following patterns until OpenCode is restarted
+                  </text>
                   <box>
                     <For each={props.request.save ?? []}>
                       {(pattern) => (
@@ -643,10 +643,7 @@ function Prompt<const T extends Record<string, string>>(props: {
           ]
         : []),
     ],
-    bindings: [
-      ...(props.escapeKey ? ["app.exit"] : []),
-      ...(props.fullscreen ? ["permission.prompt.fullscreen"] : []),
-    ],
+    bindings: [...(props.escapeKey ? ["app.exit"] : []), ...(props.fullscreen ? ["permission.prompt.fullscreen"] : [])],
   }))
 
   const hint = createMemo(() => (store.expanded ? "minimize" : "fullscreen"))
@@ -703,20 +700,14 @@ function Prompt<const T extends Record<string, string>>(props: {
               <box
                 paddingLeft={1}
                 paddingRight={1}
-                backgroundColor={themeV2.background.action(
-                  option === store.selected ? "focused" : "default",
-                )}
+                backgroundColor={themeV2.background.action(option === store.selected ? "focused" : "default")}
                 onMouseOver={() => setStore("selected", option)}
                 onMouseUp={() => {
                   setStore("selected", option)
                   props.onSelect(option)
                 }}
               >
-                <text
-                  fg={themeV2.text.action(
-                    option === store.selected ? "focused" : "default",
-                  )}
-                >
+                <text fg={themeV2.text.action(option === store.selected ? "focused" : "default")}>
                   {props.options[option]}
                 </text>
               </box>
@@ -726,7 +717,8 @@ function Prompt<const T extends Record<string, string>>(props: {
         <box flexDirection="row" gap={2} flexShrink={0}>
           <Show when={props.fullscreen}>
             <text fg={themeV2.text()}>
-              {shortcuts.get("permission.prompt.fullscreen")} <span style={{ fg: themeV2.text.subdued() }}>{hint()}</span>
+              {shortcuts.get("permission.prompt.fullscreen")}{" "}
+              <span style={{ fg: themeV2.text.subdued() }}>{hint()}</span>
             </text>
           </Show>
           <text fg={themeV2.text()}>
diff --git a/packages/tui/src/routes/session/rows.ts b/packages/tui/src/routes/session/rows.ts
index 0af3239b0d..ec3c42c153 100644
--- a/packages/tui/src/routes/session/rows.ts
+++ b/packages/tui/src/routes/session/rows.ts
@@ -42,7 +42,11 @@ export function createSessionRows(sessionID: Accessor<string>, options?: { reado
     const messages = data.session.message.list(sessionID())
     const inputs = new Set(data.session.input.list(sessionID()))
     const boundary = revertBoundary()
-    const rows = reduceSessionRows(boundary ? messages.filter((message) => message.id < boundary) : messages, inputs)
+    const rows = reduceSessionRows(
+      boundary ? messages.filter((message) => message.id < boundary) : messages,
+      inputs,
+      (message) => data.session.message.parts(sessionID(), message.id).values(),
+    )
     const position = rows.findIndex((row) => row.type === "message" && inputs.has(row.messageID))
     rows.splice(
       position === -1 ? rows.length : position,
diff --git a/packages/tui/src/routes/session/timeline.ts b/packages/tui/src/routes/session/timeline.ts
index be59c6353d..1b70203622 100644
--- a/packages/tui/src/routes/session/timeline.ts
+++ b/packages/tui/src/routes/session/timeline.ts
@@ -1,5 +1,6 @@
 import type { SessionMessageAssistant, SessionMessageInfo } from "@opencode-ai/client"
 import { Keyed, Layout, Transaction } from "effect-quark"
+import { SessionContent } from "./content"
 
 const PartRefLayout = Layout.struct({
   messageID: Layout.string,
@@ -180,7 +181,11 @@ export namespace SessionTimeline {
   }
 }
 
-export function reduceSessionRows(messages: SessionMessageInfo[], inputs = new Set<string>()) {
+export function reduceSessionRows(
+  messages: SessionMessageInfo[],
+  inputs = new Set<string>(),
+  partsOf?: (message: SessionMessageAssistant & { id: string }) => readonly SessionContent.Part[],
+) {
   const isInput = (message: SessionMessageInfo) => inputs.has(message.id)
   const pendingCompactions = messages.filter((message) => message.type === "compaction" && message.status === "running")
   const pending = new Set([...pendingCompactions.map((message) => message.id), ...inputs])
@@ -195,11 +200,10 @@ export function reduceSessionRows(messages: SessionMessageInfo[], inputs = new S
       rows.push(messageRow(message.id))
       return rows
     }
-    const ordinals = { text: 0, reasoning: 0 }
-    message.content.forEach((part) => {
-      const partID = part.type === "tool" ? part.id : `${part.type}:${ordinals[part.type]++}`
+    const parts = partsOf ? partsOf(message) : SessionContent.withPartIDs(message.content)
+    parts.forEach((part) => {
       if ((part.type === "text" || part.type === "reasoning") && !part.text.trim()) return
-      append(rows, { messageID: message.id, partID }, part)
+      append(rows, { messageID: message.id, partID: part.partID }, part)
     })
     if (isTerminalFinish(message.finish) || message.error || message.retry) {
       completePrevious(rows)
@@ -239,15 +243,6 @@ function rowBoundaryMessageID(row: SessionRow, messages: Map<string, SessionMess
   if (message?.type === "assistant") return message.id
 }
 
-export function resolvePart(message: SessionMessageAssistant, partID: string) {
-  const tool = message.content.find((part) => part.type === "tool" && part.id === partID)
-  if (tool) return tool
-  const match = /^(text|reasoning):(\d+)$/.exec(partID)
-  if (!match) return
-  const ordinal = Number(match[2])
-  return message.content.filter((part) => part.type === match[1])[ordinal]
-}
-
 export function isTerminalFinish(finish: string | undefined) {
   return !!finish && !["tool-calls", "unknown"].includes(finish)
 }
diff --git a/packages/tui/test/cli/tui/data.test.tsx b/packages/tui/test/cli/tui/data.test.tsx
index fd39b0f744..a14a842846 100644
--- a/packages/tui/test/cli/tui/data.test.tsx
+++ b/packages/tui/test/cli/tui/data.test.tsx
@@ -883,6 +883,96 @@ test("classifies live tool rows independently of their call ID", async () => {
   }
 })
 
+test("publishes streaming deltas to one part slot without touching siblings", async () => {
+  const events = createEventStream()
+  const sessionID = "session-part-slots"
+  const calls = createFetch((url) => {
+    if (url.pathname === `/api/session/${sessionID}/message`) return json({ data: [], cursor: {} })
+  }, events)
+  let data!: ReturnType<typeof useData>
+  let client!: ReturnType<typeof useClient>
+
+  function Probe() {
+    data = useData()
+    client = useClient()
+    return <box />
+  }
+
+  const app = await testRender(() => (
+    <TestTuiContexts>
+      <ClientProvider api={createApi(calls.fetch)}>
+        <ProjectProvider>
+          <DataProvider>
+            <Probe />
+          </DataProvider>
+        </ProjectProvider>
+      </ClientProvider>
+    </TestTuiContexts>
+  ))
+
+  try {
+    await wait(() => client.connection.status() === "connected")
+    emitEvent(events, {
+      id: "evt_slots_step",
+      created: 1,
+      type: "session.step.started",
+      durable: durable(sessionID),
+      data: {
+        sessionID,
+        assistantMessageID: "message-assistant",
+        agent: "build",
+        model: { id: "model", providerID: "provider" },
+      },
+    })
+    emitEvent(events, {
+      id: "evt_slots_tool",
+      created: 2,
+      type: "session.tool.input.started",
+      durable: durable(sessionID, 1),
+      data: { sessionID, assistantMessageID: "message-assistant", callID: "call-slots", name: "read" },
+    })
+    emitEvent(events, {
+      id: "evt_slots_text",
+      created: 3,
+      type: "session.text.started",
+      durable: durable(sessionID, 2),
+      data: { sessionID, assistantMessageID: "message-assistant", ordinal: 0 },
+    })
+    const parts = data.session.message.parts(sessionID, "message-assistant")
+    await wait(() => parts.get("text:0") !== undefined && parts.get("call-slots") !== undefined)
+
+    const publications = { text: 0, tool: 0, structure: 0 }
+    const disposers = [
+      parts.get("text:0")!.subscribe(() => publications.text++),
+      parts.get("call-slots")!.subscribe(() => publications.tool++),
+      parts.slots.subscribe(() => publications.structure++),
+    ]
+    emitEvent(events, {
+      id: "evt_slots_delta_1",
+      created: 4,
+      type: "session.text.delta",
+      data: { sessionID, assistantMessageID: "message-assistant", ordinal: 0, delta: "one" },
+    })
+    emitEvent(events, {
+      id: "evt_slots_delta_2",
+      created: 5,
+      type: "session.text.delta",
+      data: { sessionID, assistantMessageID: "message-assistant", ordinal: 0, delta: "two" },
+    })
+    await wait(() => {
+      const part = parts.get("text:0")?.()
+      return part?.type === "text" && part.text === "onetwo"
+    })
+
+    // One slot publication per delta; the sibling tool slot and the part
+    // structure never publish, independent of message size.
+    expect(publications).toEqual({ text: 2, tool: 0, structure: 0 })
+    disposers.forEach((dispose) => dispose())
+  } finally {
+    app.renderer.destroy()
+  }
+})
+
 test("does not publish timeline rows for duplicate streaming deltas", async () => {
   const events = createEventStream()
   const sessionID = "session-stream-metrics"
@@ -950,10 +1040,8 @@ test("does not publish timeline rows for duplicate streaming deltas", async () =
       data: { sessionID, assistantMessageID: "message-assistant", ordinal: 0, delta: "two" },
     })
     await wait(() => {
-      const message = data.session.message.get(sessionID, "message-assistant")
-      return (
-        message?.type === "assistant" && message.content.some((part) => part.type === "text" && part.text === "onetwo")
-      )
+      const part = data.session.message.parts(sessionID, "message-assistant").get("text:0")?.()
+      return part?.type === "text" && part.text === "onetwo"
     })
 
     expect(afterFirst).toEqual({ slotPublications: 0, structuralPublications: 1, equivalenceSuppressions: 0 })
@@ -2482,12 +2570,9 @@ test("settles pending tools when a live failure arrives", async () => {
     })
 
     await wait(() => {
-      const assistant = sync.session.message.get("session-1", "msg_explicit_assistant_9")
+      const part = sync.session.message.parts("session-1", "msg_explicit_assistant_9").get("call-1")?.()
       return (
-        assistant?.type === "assistant" &&
-        assistant.content[0]?.type === "tool" &&
-        assistant.content[0].state.status === "running" &&
-        assistant.content[0].state.structured.sessionID === "session-child"
+        part?.type === "tool" && part.state.status === "running" && part.state.structured.sessionID === "session-child"
       )
     })
 
@@ -2507,19 +2592,15 @@ test("settles pending tools when a live failure arrives", async () => {
     })
 
     await wait(() => {
-      const assistant = sync.session.message.get("session-1", "msg_explicit_assistant_9")
-      return (
-        assistant?.type === "assistant" &&
-        assistant.content[0]?.type === "tool" &&
-        assistant.content[0].state.status === "error"
-      )
+      const part = sync.session.message.parts("session-1", "msg_explicit_assistant_9").get("call-1")?.()
+      return part?.type === "tool" && part.state.status === "error"
     })
 
     const assistant = sync.session.message.get("session-1", "msg_explicit_assistant_9")
     expect(assistant?.type).toBe("assistant")
     if (assistant?.type !== "assistant") return
     expect(assistant.id).toBe("msg_explicit_assistant_9")
-    const tool = assistant.content[0]
+    const tool = sync.session.message.parts("session-1", "msg_explicit_assistant_9").get("call-1")?.()
     expect(tool?.type).toBe("tool")
     if (tool?.type !== "tool") return
     expect(tool.state.status).toBe("error")