diff --git a/src/hooks/compaction-context-injector/session-prompt-config-resolver.ts b/src/hooks/compaction-context-injector/session-prompt-config-resolver.ts
new file mode 100644
index 000000000..8d946ef7e
--- /dev/null
+++ b/src/hooks/compaction-context-injector/session-prompt-config-resolver.ts
@@ -0,0 +1,131 @@
+import { getSessionAgent } from "../../features/claude-code-session-state"
+import type { CompactionAgentConfigCheckpoint } from "../../shared/compaction-agent-config-checkpoint"
+import { log } from "../../shared/logger"
+import { normalizeSDKResponse } from "../../shared/normalize-sdk-response"
+import { normalizePromptTools } from "../../shared/prompt-tools"
+import { getSessionModel } from "../../shared/session-model-state"
+import { getSessionTools } from "../../shared/session-tools-store"
+
+type SessionMessage = {
+  info?: {
+    agent?: string
+    model?: {
+      providerID?: string
+      modelID?: string
+    }
+    providerID?: string
+    modelID?: string
+    tools?: Record<string, boolean | "allow" | "deny" | "ask">
+  }
+}
+
+type ResolverContext = {
+  client: {
+    session: {
+      messages: (input: { path: { id: string } }) => Promise<unknown>
+    }
+  }
+  directory: string
+}
+
+function isCompactionAgent(agent: string | undefined): boolean {
+  return agent?.trim().toLowerCase() === "compaction"
+}
+
+function resolveModel(
+  info: SessionMessage["info"],
+): CompactionAgentConfigCheckpoint["model"] | undefined {
+  const providerID = info?.model?.providerID ?? info?.providerID
+  const modelID = info?.model?.modelID ?? info?.modelID
+
+  if (!providerID || !modelID) {
+    return undefined
+  }
+
+  return { providerID, modelID }
+}
+
+export async function resolveSessionPromptConfig(
+  ctx: ResolverContext,
+  sessionID: string,
+): Promise<CompactionAgentConfigCheckpoint> {
+  const promptConfig: CompactionAgentConfigCheckpoint = {
+    agent: getSessionAgent(sessionID),
+    model: getSessionModel(sessionID),
+    tools: getSessionTools(sessionID),
+  }
+
+  try {
+    const response = await ctx.client.session.messages({ path: { id: sessionID } })
+    const messages = normalizeSDKResponse(response, [] as SessionMessage[], {
+      preferResponseOnMissingData: true,
+    })
+
+    for (let index = messages.length - 1; index >= 0; index--) {
+      const info = messages[index].info
+
+      if (!promptConfig.agent && info?.agent && !isCompactionAgent(info.agent)) {
+        promptConfig.agent = info.agent
+      }
+
+      if (!promptConfig.model) {
+        const model = resolveModel(info)
+        if (model) {
+          promptConfig.model = model
+        }
+      }
+
+      if (!promptConfig.tools) {
+        const tools = normalizePromptTools(info?.tools)
+        if (tools) {
+          promptConfig.tools = tools
+        }
+      }
+
+      if (promptConfig.agent && promptConfig.model && promptConfig.tools) {
+        break
+      }
+    }
+  } catch (error) {
+    log("[compaction-context-injector] Failed to resolve prompt config from messages", {
+      sessionID,
+      directory: ctx.directory,
+      error: String(error),
+    })
+  }
+
+  return promptConfig
+}
+
+export async function resolveLatestSessionPromptConfig(
+  ctx: ResolverContext,
+  sessionID: string,
+): Promise<CompactionAgentConfigCheckpoint> {
+  try {
+    const response = await ctx.client.session.messages({ path: { id: sessionID } })
+    const messages = normalizeSDKResponse(response, [] as SessionMessage[], {
+      preferResponseOnMissingData: true,
+    })
+    const latestInfo = messages.at(-1)?.info
+
+    if (!latestInfo) {
+      return {}
+    }
+
+    const model = resolveModel(latestInfo)
+    const tools = normalizePromptTools(latestInfo.tools)
+
+    return {
+      ...(latestInfo.agent ? { agent: latestInfo.agent } : {}),
+      ...(model ? { model } : {}),
+      ...(tools ? { tools } : {}),
+    }
+  } catch (error) {
+    log("[compaction-context-injector] Failed to resolve latest prompt config", {
+      sessionID,
+      directory: ctx.directory,
+      error: String(error),
+    })
+    return {}
+  }
+}