Web UI: external messageID can trigger extra/empty assistant turns (repro with and without proxy) #19284

Description

Summary

When POST /session/{sessionID}/message is called with an externally supplied messageID, OpenCode may create multiple assistant turns for a single user turn, including empty turns (step-start + step-finish, reason stop, no text/tool parts).

This is reproducible with different model providers (Gemini/OpenAI).

Impact

• In Web UI, user may see missing or unstable assistant rendering.
• Session gets extra assistant records with no visible text.
• In severe cases this can become a runaway loop (many assistant turns generated for one user turn).

Web UI vs Terminal

Observed behavior differs by client path:

• Web UI path (problematic): can trigger extra assistant turns (including empty stop turns) for a single user turn.
• Terminal path (stable in normal use): did not reproduce this symptom in regular usage.

Why this likely happens:

• OpenCode session API accepts optional external messageID on POST /session/{sessionID}/message.
• In controlled tests, supplying an external high-lexicographic messageID reproduces runaway/empty turns.
• In normal terminal flow, this external messageID is typically not supplied by user interaction path, so the issue is not triggered.

Why this looks like loop control issue

In OpenCode session/prompt.ts, loop stop condition uses string comparison:

if (lastUser.id < lastAssistant.id) break

When an external user messageID is lexicographically greater than the generated assistant id, this condition fails and loop may continue unexpectedly.

Files of interest (OpenCode)

• packages/opencode/src/session/prompt.ts (loop condition + prompt flow)
• packages/opencode/src/session/schema.ts (message ID schema)
• packages/opencode/src/id/id.ts (ID generation semantics)

Notes

• OpenAI terminal streaming chunk (delta: {}, finish_reason: "stop") is protocol-valid and should not be stripped.
• The issue reproduces independently from a specific upstream provider, which suggests session loop/message-id handling rather than provider protocol incompatibility.
• Any integration can still trigger this if the client path provides problematic external messageID values.

Plugins

No response

OpenCode version

1.2.24

Steps to reproduce

Reproduction (minimal)

1. Create a new session.

2. Send one message without messageID:

   • Expected and observed: 1 user turn -> 1 assistant turn.

3. Send next message with external high lexicographic messageID (example: msg_ffffffffffffCUSTOMIDTESTXYZ):

   • Observed: multiple assistant turns are generated for one user turn, including empty stop turns.

4. Compare with terminal usage (same backend/model route):

   • Observed in normal terminal operation: no runaway empty-turn pattern.
   • This supports a client-path trigger rather than an upstream provider protocol issue.

Control reproduction with direct provider

To eliminate integration-specific influence, run the same external messageID scenario with a direct OpenCode provider (e.g. openai).

Observed result:

• The issue still reproduces (extra assistant turn appears in the same user block).
• Therefore this issue can be triggered without any custom intermediary layer.

Example API sequence

# 1) create session
POST /session

# 2) normal prompt (no messageID)
POST /session/{sessionID}/message
{
  "model": {"providerID": "openai", "modelID": "gpt-5.4"},
  "parts": [{"type": "text", "text": "hello"}]
}

# 3) prompt with external messageID
POST /session/{sessionID}/message
{
  "messageID": "msg_ffffffffffffCUSTOMIDTESTXYZ",
  "model": {"providerID": "openai", "modelID": "gpt-5.4"},
  "parts": [{"type": "text", "text": "hello again"}]
}

Observed data pattern in DB/API

• For the problematic user block:
  • first assistant turn has text,
  • followed by one or more assistant turns with:
    • parts: step-start, step-finish
    • step-finish.reason = "stop"
    • no text part
    • no tool part.

Screenshot and/or share link

No response

Operating System

Alpine Linux v3.23 (VERSION_ID=3.23.3)

Terminal

busybox