diff --git a/packages/opencode/src/session/compaction.ts b/packages/opencode/src/session/compaction.ts
index 3ca4f074f9e4..d7b36b17dffa 100644
--- a/packages/opencode/src/session/compaction.ts
+++ b/packages/opencode/src/session/compaction.ts
@@ -119,39 +119,58 @@ function completedCompactions(messages: MessageV2.WithParts[]) {
   })
 }
 
-function buildPrompt(input: { previousSummary?: string; context: string[]; tail?: string }) {
-  const source = input.tail
-    ? "the conversation history above and the serialized recent conversation tail below"
-    : "the conversation history above"
+function buildPrompt(input: { previousSummary?: string; context: string[] }) {
   const anchor = input.previousSummary
     ? [
-        `Update the anchored summary below using ${source}.`,
+        "Update the anchored summary below using the conversation history above.",
         "Preserve still-true details, remove stale details, and merge in the new facts.",
         "<previous-summary>",
         input.previousSummary,
         "</previous-summary>",
       ].join("\n")
-    : `Create a new anchored summary from ${source}.`
-  const tail = input.tail
-    ? [
-        "Fold this serialized recent conversation tail into the summary; it is not provider message history.",
-        "<recent-conversation-tail>",
-        input.tail,
-        "</recent-conversation-tail>",
-      ].join("\n")
-    : undefined
-  return [anchor, ...(tail ? [tail] : []), SUMMARY_TEMPLATE, ...input.context].join("\n\n")
+    : "Create a new anchored summary from the conversation history above."
+  return [anchor, SUMMARY_TEMPLATE, ...input.context].join("\n\n")
 }
 
 const serialize = Effect.fn("SessionCompaction.serialize")(function* (input: {
   messages: MessageV2.WithParts[]
-  model: Provider.Model
 }) {
-  const messages = yield* MessageV2.toModelMessagesEffect(input.messages, input.model, {
-    stripMedia: true,
-    toolOutputMaxChars: TOOL_OUTPUT_MAX_CHARS,
+  const parts = input.messages.flatMap((msg) => {
+    if (msg.info.role === "user") {
+      const content = msg.parts
+        .flatMap((part) => {
+          if (part.type === "text" && !part.ignored && part.text !== "") return [part.text]
+          if (part.type === "file" && MessageV2.isMedia(part.mime)) return [`[Attached ${part.mime}: ${part.filename ?? "file"}]`]
+          return []
+        })
+        .join("\n")
+      return content ? [`[User]: ${content}`] : []
+    }
+    if (msg.info.role === "assistant") {
+      return msg.parts.flatMap((part) => {
+        if (part.type === "reasoning") return part.text ? [`[Assistant thinking]: ${part.text}`] : []
+        if (part.type === "text") return part.text ? [`[Assistant]: ${part.text}`] : []
+        if (part.type !== "tool") return []
+        const input = Object.entries(part.state.input)
+          .map(([key, value]) => `${key}=${JSON.stringify(value)}`)
+          .join(", ")
+        if (part.state.status === "completed") {
+          const output = part.state.time.compacted
+            ? "[Old tool result content cleared]"
+            : part.state.output.length <= TOOL_OUTPUT_MAX_CHARS
+              ? part.state.output
+              : `${part.state.output.slice(0, TOOL_OUTPUT_MAX_CHARS)}\n[Tool output truncated for compaction: omitted ${part.state.output.length - TOOL_OUTPUT_MAX_CHARS} chars]`
+          return [`[Assistant tool call]: ${part.tool}(${input})`, `[Tool result]: ${output}`]
+        }
+        if (part.state.status === "error") {
+          return [`[Assistant tool call]: ${part.tool}(${input})`, `[Tool error]: ${part.state.error}`]
+        }
+        return [`[Assistant tool call]: ${part.tool}(${input})`]
+      })
+    }
+    return []
   })
-  return messages.length ? JSON.stringify(messages, null, 2) : undefined
+  return parts.length ? parts.join("\n\n") : undefined
 })
 
 function preserveRecentBudget(input: { cfg: Config.Info; model: Provider.Model }) {
@@ -167,6 +186,7 @@ function turns(messages: MessageV2.WithParts[]) {
     const msg = messages[i]
     if (msg.info.role !== "user") continue
     if (msg.parts.some((part) => part.type === "compaction")) continue
+    if (msg.parts.some((part) => part.type === "text" && part.metadata?.compaction_tail === true)) continue
     result.push({
       start: i,
       end: messages.length,
@@ -262,7 +282,7 @@ export const layer: Layer.Layer<
       messages: MessageV2.WithParts[]
       model: Provider.Model
     }) {
-      return Token.estimate((yield* serialize(input)) ?? "")
+      return Token.estimate((yield* serialize({ messages: input.messages })) ?? "")
     })
 
     const select = Effect.fn("SessionCompaction.select")(function* (input: {
@@ -425,8 +445,8 @@ export const layer: Layer.Layer<
       )
       const tailMessages = structuredClone(selected.tail)
       yield* plugin.trigger("experimental.chat.messages.transform", {}, { messages: tailMessages })
-      const tail = yield* serialize({ messages: tailMessages, model })
-      const nextPrompt = compacting.prompt ?? buildPrompt({ previousSummary, context: compacting.context, tail })
+      const tail = yield* serialize({ messages: tailMessages })
+      const nextPrompt = compacting.prompt ?? buildPrompt({ previousSummary, context: compacting.context })
       const msgs = structuredClone(selected.head)
       yield* plugin.trigger("experimental.chat.messages.transform", {}, { messages: msgs })
       const modelMessages = yield* MessageV2.toModelMessagesEffect(msgs, model, {
@@ -493,6 +513,37 @@ export const layer: Layer.Layer<
         return "stop"
       }
 
+      if (processor.message.error) return "stop"
+
+      if (tail) {
+        const tailMessage = yield* session.updateMessage({
+          id: MessageID.ascending(),
+          role: "user",
+          sessionID: input.sessionID,
+          time: { created: Date.now() },
+          agent: userMessage.agent,
+          model: userMessage.model,
+          format: userMessage.format,
+          tools: userMessage.tools,
+          system: userMessage.system,
+        })
+        yield* session.updatePart({
+          id: PartID.ascending(),
+          messageID: tailMessage.id,
+          sessionID: input.sessionID,
+          type: "text",
+          metadata: { compaction_tail: true },
+          synthetic: true,
+          text: [
+            "The conversation history before this point was compacted into the summary above.",
+            "The following messages are the latest conversation turns after that summarized history.",
+            "<latest-messages>",
+            tail,
+            "</latest-messages>",
+          ].join("\n\n"),
+        })
+      }
+
       if (result === "continue" && input.auto) {
         if (replay) {
           const original = replay.info
@@ -575,19 +626,19 @@ export const layer: Layer.Layer<
         }
       }
 
-      if (processor.message.error) return "stop"
       if (result === "continue") {
-        const summary = summaryText(
-          (yield* session.messages({ sessionID: input.sessionID })).find((item) => item.info.id === msg.id) ?? {
-            info: msg,
-            parts: [],
-          },
-        )
+        const summaryMessage = (yield* session.messages({ sessionID: input.sessionID })).find(
+          (item) => item.info.id === msg.id,
+        ) ?? {
+          info: msg,
+          parts: [],
+        }
+        const summary = summaryText(summaryMessage) ?? ""
         if (Flag.OPENCODE_EXPERIMENTAL_EVENT_SYSTEM) {
           yield* sync.run(SessionEvent.Compaction.Ended.Sync, {
             sessionID: input.sessionID,
             timestamp: DateTime.makeUnsafe(Date.now()),
-            text: summary ?? "",
+            text: summary,
           })
         }
         yield* bus.publish(Event.Compacted, { sessionID: input.sessionID })
diff --git a/packages/opencode/src/session/prompt.ts b/packages/opencode/src/session/prompt.ts
index 15246dac394d..f7b13d97df73 100644
--- a/packages/opencode/src/session/prompt.ts
+++ b/packages/opencode/src/session/prompt.ts
@@ -1642,7 +1642,12 @@ NOTE: At any point in time through this workflow you should feel free to ask the
           let tasks: (MessageV2.CompactionPart | MessageV2.SubtaskPart)[] = []
           for (let i = msgs.length - 1; i >= 0; i--) {
             const msg = msgs[i]
-            if (!lastUser && msg.info.role === "user") lastUser = msg.info
+            if (
+              !lastUser &&
+              msg.info.role === "user" &&
+              !msg.parts.some((part) => part.type === "text" && part.metadata?.compaction_tail === true)
+            )
+              lastUser = msg.info
             if (!lastAssistant && msg.info.role === "assistant") lastAssistant = msg.info
             if (!lastFinished && msg.info.role === "assistant" && msg.info.finish) lastFinished = msg.info
             if (lastUser && lastFinished) break
diff --git a/packages/opencode/test/session/compaction.test.ts b/packages/opencode/test/session/compaction.test.ts
index c7f349d5cea0..4e16a33540b5 100644
--- a/packages/opencode/test/session/compaction.test.ts
+++ b/packages/opencode/test/session/compaction.test.ts
@@ -282,7 +282,9 @@ function createSummaryCompaction(sessionID: SessionID) {
 function readCompactionPart(sessionID: SessionID) {
   return SessionNs.Service.use((ssn) => ssn.messages({ sessionID })).pipe(
     Effect.map((messages) =>
-      messages.at(-2)?.parts.find((item): item is MessageV2.CompactionPart => item.type === "compaction"),
+      messages
+        .findLast((message) => message.parts.some((item) => item.type === "compaction"))
+        ?.parts.find((item): item is MessageV2.CompactionPart => item.type === "compaction"),
     ),
   )
 }
@@ -1005,7 +1007,7 @@ describe("session.compaction.process", () => {
   )
 
   itCompaction.instance(
-    "serializes retained tail media as text in the summary input",
+    "serializes retained tail media as text in the saved summary",
     () => {
       const stub = llm()
       let captured = ""
@@ -1034,8 +1036,13 @@ describe("session.compaction.process", () => {
         const part = yield* readCompactionPart(session.id)
         expect(part?.type).toBe("compaction")
         expect(part?.tail_start_id).toBeUndefined()
-        expect(captured).toContain("recent image turn")
-        expect(captured).toContain("Attached image/png: big.png")
+        expect(captured).not.toContain("recent image turn")
+        expect(captured).not.toContain("Attached image/png: big.png")
+        const tail = (yield* ssn.messages({ sessionID: session.id })).find(
+          (item) => item.info.role === "user" && item.parts.some((part) => part.type === "text" && part.synthetic),
+        )
+        expect(JSON.stringify(tail?.parts)).toContain("recent image turn")
+        expect(JSON.stringify(tail?.parts)).toContain("Attached image/png: big.png")
       }).pipe(withCompaction({ llm: stub.layer, config: cfg({ tail_turns: 1, preserve_recent_tokens: 100 }) }))
     },
     { git: true },
@@ -1080,12 +1087,17 @@ describe("session.compaction.process", () => {
         expect(part?.type).toBe("compaction")
         expect(part?.tail_start_id).toBeUndefined()
         expect(captured).toContain("zzzz")
-        expect(captured).toContain("keep tail")
+        expect(captured).not.toContain("keep tail")
+        const tail = (yield* ssn.messages({ sessionID: session.id })).find(
+          (item) => item.info.role === "user" && item.parts.some((part) => part.type === "text" && part.synthetic),
+        )
+        expect(JSON.stringify(tail?.parts)).toContain("keep tail")
 
         const filtered = MessageV2.filterCompacted(MessageV2.stream(session.id))
-        expect(filtered.map((msg) => msg.info.id)).toEqual([parent!, expect.any(String)])
+        expect(filtered.map((msg) => msg.info.id)).toEqual([parent!, expect.any(String), expect.any(String)])
         expect(filtered[1]?.info.role).toBe("assistant")
         expect(filtered[1]?.info.role === "assistant" ? filtered[1].info.summary : false).toBe(true)
+        expect(filtered[2]?.info.role).toBe("user")
         expect(filtered.map((msg) => msg.info.id)).not.toContain(large.id)
         expect(filtered.map((msg) => msg.info.id)).not.toContain(keep.id)
       }).pipe(withCompaction({ llm: stub.layer, config: cfg({ tail_turns: 1, preserve_recent_tokens: 100 }) }))
@@ -1112,7 +1124,9 @@ describe("session.compaction.process", () => {
       const last = all.at(-1)
 
       expect(result).toBe("continue")
-      expect(last?.info.role).toBe("assistant")
+      expect(last?.info.role).toBe("user")
+      expect(last?.parts.some((part) => part.type === "text" && part.text.includes("latest-messages"))).toBe(true)
+      expect(all.some((msg) => msg.info.role === "assistant" && msg.info.summary)).toBe(true)
       expect(
         all.some(
           (msg) =>
@@ -1354,7 +1368,7 @@ describe("session.compaction.process", () => {
   )
 
   itCompaction.instance(
-    "summarizes the head while serializing recent tail into summary input",
+    "summarizes the head while appending serialized recent tail to saved summary",
     () => {
       const stub = llm()
       let captured: LLM.StreamInput["messages"] = []
@@ -1386,10 +1400,16 @@ describe("session.compaction.process", () => {
         expect(head).toContain("older context")
         expect(head).not.toContain("keep this turn")
         expect(head).not.toContain("and this one too")
-        expect(prompt).toContain("keep this turn")
-        expect(prompt).toContain("and this one too")
-        expect(prompt).toContain("recent-conversation-tail")
+        expect(prompt).not.toContain("keep this turn")
+        expect(prompt).not.toContain("and this one too")
+        expect(prompt).not.toContain("latest-messages")
         expect(prompt).not.toContain("What did we do so far?")
+        const tail = (yield* ssn.messages({ sessionID: session.id })).find(
+          (item) => item.info.role === "user" && item.parts.some((part) => part.type === "text" && part.synthetic),
+        )
+        expect(JSON.stringify(tail?.parts)).toContain("keep this turn")
+        expect(JSON.stringify(tail?.parts)).toContain("and this one too")
+        expect(JSON.stringify(tail?.parts)).toContain("latest-messages")
       }).pipe(withCompaction({ llm: stub.layer }))
     },
     { git: true },
@@ -1477,6 +1497,44 @@ describe("session.compaction.process", () => {
     }).pipe(withCompaction({ llm: stub.layer, config: cfg({ tail_turns: 2, preserve_recent_tokens: 10_000 }) }))
   })
 
+  itCompaction.instance("summarizes previous synthetic tail on repeated compaction", () => {
+    const stub = llm()
+    let captured = ""
+    stub.push(reply("summary one"))
+    stub.push(reply("summary two", (input) => (captured = JSON.stringify(input.messages))))
+
+    return Effect.gen(function* () {
+      const ssn = yield* SessionNs.Service
+      const session = yield* ssn.create({})
+      yield* createUserMessage(session.id, "older")
+      yield* createUserMessage(session.id, "previous tail")
+      yield* createCompactionMarker(session.id)
+
+      let msgs = yield* ssn.messages({ sessionID: session.id })
+      let parent = msgs.at(-1)?.info.id
+      expect(parent).toBeTruthy()
+      yield* SessionCompaction.use.process({ parentID: parent!, messages: msgs, sessionID: session.id, auto: false })
+
+      yield* createUserMessage(session.id, "new tail")
+      yield* createCompactionMarker(session.id)
+
+      msgs = MessageV2.filterCompacted(MessageV2.stream(session.id))
+      parent = msgs.at(-1)?.info.id
+      expect(parent).toBeTruthy()
+      yield* SessionCompaction.use.process({ parentID: parent!, messages: msgs, sessionID: session.id, auto: false })
+
+      const tails = (yield* ssn.messages({ sessionID: session.id })).filter(
+        (item) => item.info.role === "user" && item.parts.some((part) => part.type === "text" && part.metadata?.compaction_tail === true),
+      )
+      const latestTail = tails.at(-1)
+
+      expect(captured).toContain("previous tail")
+      expect(captured).toContain("latest-messages")
+      expect(JSON.stringify(latestTail?.parts)).toContain("new tail")
+      expect(JSON.stringify(latestTail?.parts)).not.toContain("previous tail")
+    }).pipe(withCompaction({ llm: stub.layer, config: cfg({ tail_turns: 1, preserve_recent_tokens: 10_000 }) }))
+  })
+
   itCompaction.instance(
     "ignores previous summaries when sizing the serialized tail",
     Effect.gen(function* () {