How are messages and parts stored in opencode?

how do they store messages and parts? separately? is message -> UIMessage from the ai sdk?

Now I have a clear picture. Let me summarize what I found:

##Summary: Messages and Parts Storage

Yes, messages and parts are stored separately.

###Storage Structure

The storage uses a file-based system with paths like:

• Messages: ["message", sessionID, messageID] → stored as storage/message/{sessionID}/{messageID}.json
• Parts: ["part", messageID, partID] → stored as storage/part/{messageID}/{partID}.json

###Key Types

From packages/opencode/src/session/message-v2.ts:

1. MessageV2.Info - The message metadata (discriminated union of User and Assistant):

   • User message: role, time, agent, model, tools, etc.
   • Assistant message: role, time, tokens, cost, error, modelID, providerID, etc.

2. MessageV2.Part - A discriminated union of many part types:

   • TextPart, ReasoningPart, FilePart, ToolPart, StepStartPart, StepFinishPart, SnapshotPart, PatchPart, AgentPart, RetryPart, CompactionPart, SubtaskPart

3. MessageV2.WithParts - Combines both:

   export const WithParts = z.object({
     info: Info,
     parts: z.array(Part),
   })
   

###Relation to AI SDK's UIMessage

Yes, MessageV2.Info is converted to UIMessage from the AI SDK, but it's not a direct mapping.

In toModelMessages() (line 436), they:

1. Take an array of WithParts[]
2. Build UIMessage[] objects (from import { type UIMessage } from "ai")
3. Transform their custom part types into the AI SDK's expected format
4. Call convertToModelMessages() from the AI SDK

So their internal Message → Info structure is their own schema, which gets converted to UIMessage when interfacing with the AI SDK for model calls.

###Storage Operations

From packages/opencode/src/session/index.ts:

// Write a message
await Storage.write(["message", msg.sessionID, msg.id], msg)

// Write a part  
await Storage.write(["part", part.messageID, part.id], part)

// Remove a part
await Storage.remove(["part", input.messageID, input.partID])

Parts are retrieved by listing all files in the part directory for a message:

export const parts = fn(Identifier.schema("message"), async (messageID) => {
  const result = [] as MessageV2.Part[]
  for (const item of await Storage.list(["part", messageID])) {
    const read = await Storage.read<MessageV2.Part>(item)
    result.push(read)
  }
  result.sort((a, b) => (a.id > b.id ? 1 : -1))
  return result
})