fix(background-agent): merge prompt context across compaction gaps

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/features/background-agent/compaction-aware-message-resolver.test.ts

@@ -2,7 +2,15 @@ import { describe, test, expect, beforeEach, afterEach } from "bun:test"
 import { mkdtempSync, writeFileSync, rmSync } from "node:fs"
 import { join } from "node:path"
 import { tmpdir } from "node:os"
-import { isCompactionAgent, findNearestMessageExcludingCompaction } from "./compaction-aware-message-resolver"
+import {
+  isCompactionAgent,
+  findNearestMessageExcludingCompaction,
+  resolvePromptContextFromSessionMessages,
+} from "./compaction-aware-message-resolver"
+import {
+  clearCompactionAgentConfigCheckpoint,
+  setCompactionAgentConfigCheckpoint,
+} from "../../shared/compaction-agent-config-checkpoint"
 
 describe("isCompactionAgent", () => {
   describe("#given agent name variations", () => {
@@ -65,6 +73,7 @@ describe("findNearestMessageExcludingCompaction", () => {
 
   afterEach(() => {
     rmSync(tempDir, { force: true, recursive: true })
+    clearCompactionAgentConfigCheckpoint("ses_checkpoint")
   })
 
   describe("#given directory with messages", () => {
@@ -186,5 +195,65 @@ describe("findNearestMessageExcludingCompaction", () => {
       expect(result).not.toBeNull()
       expect(result?.agent).toBe("newer")
     })
+
+    test("merges partial metadata from multiple recent messages", () => {
+      // given
+      writeFileSync(
+        join(tempDir, "003.json"),
+        JSON.stringify({ model: { providerID: "anthropic", modelID: "claude-opus-4-1" } }),
+      )
+      writeFileSync(join(tempDir, "002.json"), JSON.stringify({ agent: "atlas" }))
+      writeFileSync(join(tempDir, "001.json"), JSON.stringify({ tools: { bash: true } }))
+
+      // when
+      const result = findNearestMessageExcludingCompaction(tempDir)
+
+      // then
+      expect(result).toEqual({
+        agent: "atlas",
+        model: { providerID: "anthropic", modelID: "claude-opus-4-1" },
+        tools: { bash: true },
+      })
+    })
+
+    test("fills missing metadata from compaction checkpoint", () => {
+      // given
+      setCompactionAgentConfigCheckpoint("ses_checkpoint", {
+        agent: "sisyphus",
+        model: { providerID: "openai", modelID: "gpt-5" },
+      })
+      writeFileSync(join(tempDir, "001.json"), JSON.stringify({ tools: { bash: true } }))
+
+      // when
+      const result = findNearestMessageExcludingCompaction(tempDir, "ses_checkpoint")
+
+      // then
+      expect(result).toEqual({
+        agent: "sisyphus",
+        model: { providerID: "openai", modelID: "gpt-5" },
+        tools: { bash: true },
+      })
+    })
+  })
+})
+
+describe("resolvePromptContextFromSessionMessages", () => {
+  test("merges partial prompt context from recent SDK messages", () => {
+    // given
+    const messages = [
+      { info: { agent: "atlas" } },
+      { info: { model: { providerID: "anthropic", modelID: "claude-opus-4-1" } } },
+      { info: { tools: { bash: true } } },
+    ]
+
+    // when
+    const result = resolvePromptContextFromSessionMessages(messages)
+
+    // then
+    expect(result).toEqual({
+      agent: "atlas",
+      model: { providerID: "anthropic", modelID: "claude-opus-4-1" },
+      tools: { bash: true },
+    })
   })
 })

src/features/background-agent/compaction-aware-message-resolver.ts

@@ -1,6 +1,21 @@
 import { readdirSync, readFileSync } from "node:fs"
 import { join } from "node:path"
 import type { StoredMessage } from "../hook-message-injector"
+import { getCompactionAgentConfigCheckpoint } from "../../shared/compaction-agent-config-checkpoint"
+
+type SessionMessage = {
+  info?: {
+    agent?: string
+    model?: {
+      providerID?: string
+      modelID?: string
+      variant?: string
+    }
+    providerID?: string
+    modelID?: string
+    tools?: StoredMessage["tools"]
+  }
+}
 
 export function isCompactionAgent(agent: string | undefined): boolean {
   return agent?.trim().toLowerCase() === "compaction"
@@ -16,42 +31,121 @@ function hasFullAgentAndModel(message: StoredMessage): boolean {
 function hasPartialAgentOrModel(message: StoredMessage): boolean {
   const hasAgent = !!message.agent && !isCompactionAgent(message.agent)
   const hasModel = !!message.model?.providerID && !!message.model?.modelID
-  return hasAgent || hasModel
+  return hasAgent || hasModel || !!message.tools
 }
 
-export function findNearestMessageExcludingCompaction(messageDir: string): StoredMessage | null {
-  try {
-    const files = readdirSync(messageDir)
-      .filter((name) => name.endsWith(".json"))
-      .sort()
-      .reverse()
-
-    for (const file of files) {
-      try {
-        const content = readFileSync(join(messageDir, file), "utf-8")
-        const parsed = JSON.parse(content) as StoredMessage
-        if (hasFullAgentAndModel(parsed)) {
-          return parsed
-        }
-      } catch {
-        continue
-      }
-    }
-
-    for (const file of files) {
-      try {
-        const content = readFileSync(join(messageDir, file), "utf-8")
-        const parsed = JSON.parse(content) as StoredMessage
-        if (hasPartialAgentOrModel(parsed)) {
-          return parsed
-        }
-      } catch {
-        continue
-      }
-    }
-  } catch {
+function convertSessionMessageToStoredMessage(message: SessionMessage): StoredMessage | null {
+  const info = message.info
+  if (!info) {
     return null
   }
 
-  return null
+  const providerID = info.model?.providerID ?? info.providerID
+  const modelID = info.model?.modelID ?? info.modelID
+
+  return {
+    ...(info.agent ? { agent: info.agent } : {}),
+    ...(providerID && modelID
+      ? {
+          model: {
+            providerID,
+            modelID,
+            ...(info.model?.variant ? { variant: info.model.variant } : {}),
+          },
+        }
+      : {}),
+    ...(info.tools ? { tools: info.tools } : {}),
+  }
+}
+
+function mergeStoredMessages(
+  messages: Array<StoredMessage | null>,
+  sessionID?: string,
+): StoredMessage | null {
+  const merged: StoredMessage = {}
+
+  for (const message of messages) {
+    if (!message || isCompactionAgent(message.agent)) {
+      continue
+    }
+
+    if (!merged.agent && message.agent) {
+      merged.agent = message.agent
+    }
+
+    if (!merged.model?.providerID && message.model?.providerID && message.model.modelID) {
+      merged.model = {
+        providerID: message.model.providerID,
+        modelID: message.model.modelID,
+        ...(message.model.variant ? { variant: message.model.variant } : {}),
+      }
+    }
+
+    if (!merged.tools && message.tools) {
+      merged.tools = message.tools
+    }
+
+    if (hasFullAgentAndModel(merged) && merged.tools) {
+      break
+    }
+  }
+
+  const checkpoint = sessionID
+    ? getCompactionAgentConfigCheckpoint(sessionID)
+    : undefined
+
+  if (!merged.agent && checkpoint?.agent) {
+    merged.agent = checkpoint.agent
+  }
+
+  if (!merged.model && checkpoint?.model) {
+    merged.model = {
+      providerID: checkpoint.model.providerID,
+      modelID: checkpoint.model.modelID,
+    }
+  }
+
+  if (!merged.tools && checkpoint?.tools) {
+    merged.tools = checkpoint.tools
+  }
+
+  return hasPartialAgentOrModel(merged) ? merged : null
+}
+
+export function resolvePromptContextFromSessionMessages(
+  messages: SessionMessage[],
+  sessionID?: string,
+): StoredMessage | null {
+  const convertedMessages = messages
+    .map(convertSessionMessageToStoredMessage)
+    .reverse()
+
+  return mergeStoredMessages(convertedMessages, sessionID)
+}
+
+export function findNearestMessageExcludingCompaction(
+  messageDir: string,
+  sessionID?: string,
+): StoredMessage | null {
+  try {
+    const files = readdirSync(messageDir)
+      .filter((name: string) => name.endsWith(".json"))
+      .sort()
+      .reverse()
+
+    const messages: Array<StoredMessage | null> = []
+
+    for (const file of files) {
+      try {
+        const content = readFileSync(join(messageDir, file), "utf-8")
+        messages.push(JSON.parse(content) as StoredMessage)
+      } catch {
+        continue
+      }
+    }
+
+    return mergeStoredMessages(messages, sessionID)
+  } catch {
+    return null
+  }
 }