This model does not support assistant message prefill / Github Copilot with Opus 4.6

[sbrunecker]

Description

Working with Opus 4.6, opencode frequently stops with the message "This model does not support assistant message prefill. The conversation must end with a user message."

This is from the session export:

    {
      "info": {
        "role": "assistant",
        "time": {
          "created": 1771187182207,
          "completed": 1771187184086
        },
        "error": {
          "name": "APIError",
          "data": {
            "message": "This model does not support assistant message prefill. The conversation must end with a user message.",
            "statusCode": 400,
            "isRetryable": false,
            "responseHeaders": {
              "cache-control": "no-cache",
              "content-security-policy": "default-src 'none'; sandbox",
              "content-type": "text/plain; charset=utf-8",
              "date": "Sun, 15 Feb 2026 20:26:23 GMT",
              "strict-transport-security": "max-age=31536000",
              "transfer-encoding": "chunked",
              "x-content-type-options": "nosniff",
              "x-github-backend": "Kubernetes",
              "x-github-request-id": "EAAC:14EF1C:253883:31B9E3:69922BEC",
              "x-request-id": "00000-ce7fe972-c41c-43ec-b02b-3d2299740ab9"
            },
            "responseBody": "{\"error\":{\"message\":\"This model does not support assistant message prefill. The conversation must end with a user message.\",\"code\":\"invalid_request_body\"}}\n",
            "metadata": {
              "url": "https://api.githubcopilot.com/chat/completions"
            }
          }
        },
        "parentID": "msg_c62f2be7600192uMJUKpSsJ0qr",
        "modelID": "claude-opus-4.6",
        "providerID": "github-copilot",
        "mode": "build",
        "agent": "build",
        "path": {
          "cwd": "/home/stefan/projects/odoo18",
          "root": "/home/stefan/projects/odoo18"
        },
        "cost": 0,
        "tokens": {
          "input": 0,
          "output": 0,
          "reasoning": 0,
          "cache": {
            "read": 0,
            "write": 0
          }
        },
        "id": "msg_c62fb9a7f001Rl3UkWhXmVKLb7",
        "sessionID": "ses_39d0d418cffeIz9g6wg7wg1Caj"
      },
      "parts": []
    }

Plugins

none

OpenCode version

1.2.4

Steps to reproduce

No response

Screenshot and/or share link

No response

Operating System

No response

Terminal

No response

[github-actions]

---

This issue appears to be a duplicate of an existing issue. Please check:

• Github Copilot / Opus 4.6 Error #12337: Github Copilot / Opus 4.6 Error - reports the same "This model does not support assistant message prefill" error with Opus 4.6 on GitHub Copilot (already closed)

[rekram1-node]

Can you share output of opencode debug config

[sbrunecker]

{
  "$schema": "https://opencode.ai/config.json",
  "mcp": {
    "odoo18-postgres-local": {
      "type": "remote",
      "url": "http://localhost:8018/sse"
    },
    "odoo18-postgres-devserver": {
      "type": "remote",
      "url": "http://localhost:8028/sse"
    },
    "chrome-devtools": {
      "type": "local",
      "command": [
        "npx",
        "-y",
        "chrome-devtools-mcp@latest",
        "--executablePath=/usr/bin/google-chrome"
      ]
    }
  },
  "agent": {
    "odoo-migration": {
      "prompt": "...",
      "description": "Primary agent for major Odoo version migrations.",
      "name": "odoo-migration",
      "type": "primary",
      "options": {
        "type": "primary"
      },
      "permission": {}
    },
    "odoo-primary": {
      "prompt": "...",
      "description": "Primary agent for Odoo customizations and migrations.",
      "name": "odoo-primary",
      "type": "primary",
      "options": {
        "type": "primary"
      },
      "permission": {}
    }
  },
  "mode": {},
  "plugin": [],
  "command": {},
  "username": "stefan",
  "keybinds": {
    "leader": "ctrl+x",
    "app_exit": "ctrl+c,ctrl+d,<leader>q",
    "editor_open": "<leader>e",
    "theme_list": "<leader>t",
    "sidebar_toggle": "<leader>b",
    "scrollbar_toggle": "none",
    "username_toggle": "none",
    "status_view": "<leader>s",
    "session_export": "<leader>x",
    "session_new": "<leader>n",
    "session_list": "<leader>l",
    "session_timeline": "<leader>g",
    "session_fork": "none",
    "session_rename": "ctrl+r",
    "session_delete": "ctrl+d",
    "stash_delete": "ctrl+d",
    "model_provider_list": "ctrl+a",
    "model_favorite_toggle": "ctrl+f",
    "session_share": "none",
    "session_unshare": "none",
    "session_interrupt": "escape",
    "session_compact": "<leader>c",
    "messages_page_up": "pageup,ctrl+alt+b",
    "messages_page_down": "pagedown,ctrl+alt+f",
    "messages_line_up": "ctrl+alt+y",
    "messages_line_down": "ctrl+alt+e",
    "messages_half_page_up": "ctrl+alt+u",
    "messages_half_page_down": "ctrl+alt+d",
    "messages_first": "ctrl+g,home",
    "messages_last": "ctrl+alt+g,end",
    "messages_next": "none",
    "messages_previous": "none",
    "messages_last_user": "none",
    "messages_copy": "<leader>y",
    "messages_undo": "<leader>u",
    "messages_redo": "<leader>r",
    "messages_toggle_conceal": "<leader>h",
    "tool_details": "none",
    "model_list": "<leader>m",
    "model_cycle_recent": "f2",
    "model_cycle_recent_reverse": "shift+f2",
    "model_cycle_favorite": "none",
    "model_cycle_favorite_reverse": "none",
    "command_list": "ctrl+p",
    "agent_list": "<leader>a",
    "agent_cycle": "tab",
    "agent_cycle_reverse": "shift+tab",
    "variant_cycle": "ctrl+t",
    "input_clear": "ctrl+c",
    "input_paste": "ctrl+v",
    "input_submit": "return",
    "input_newline": "shift+return,ctrl+return,alt+return,ctrl+j",
    "input_move_left": "left,ctrl+b",
    "input_move_right": "right,ctrl+f",
    "input_move_up": "up",
    "input_move_down": "down",
    "input_select_left": "shift+left",
    "input_select_right": "shift+right",
    "input_select_up": "shift+up",
    "input_select_down": "shift+down",
    "input_line_home": "ctrl+a",
    "input_line_end": "ctrl+e",
    "input_select_line_home": "ctrl+shift+a",
    "input_select_line_end": "ctrl+shift+e",
    "input_visual_line_home": "alt+a",
    "input_visual_line_end": "alt+e",
    "input_select_visual_line_home": "alt+shift+a",
    "input_select_visual_line_end": "alt+shift+e",
    "input_buffer_home": "home",
    "input_buffer_end": "end",
    "input_select_buffer_home": "shift+home",
    "input_select_buffer_end": "shift+end",
    "input_delete_line": "ctrl+shift+d",
    "input_delete_to_line_end": "ctrl+k",
    "input_delete_to_line_start": "ctrl+u",
    "input_backspace": "backspace,shift+backspace",
    "input_delete": "ctrl+d,delete,shift+delete",
    "input_undo": "ctrl+-,super+z",
    "input_redo": "ctrl+.,super+shift+z",
    "input_word_forward": "alt+f,alt+right,ctrl+right",
    "input_word_backward": "alt+b,alt+left,ctrl+left",
    "input_select_word_forward": "alt+shift+f,alt+shift+right",
    "input_select_word_backward": "alt+shift+b,alt+shift+left",
    "input_delete_word_forward": "alt+d,alt+delete,ctrl+delete",
    "input_delete_word_backward": "ctrl+w,ctrl+backspace,alt+backspace",
    "history_previous": "up",
    "history_next": "down",
    "session_child_cycle": "<leader>right",
    "session_child_cycle_reverse": "<leader>left",
    "session_parent": "<leader>up",
    "terminal_suspend": "ctrl+z",
    "terminal_title_toggle": "none",
    "tips_toggle": "<leader>h",
    "display_thinking": "none"
  }
}

[hsuanguo]

I can confirm I have the same issue, with the latest opencode,
It was said that this feature is removed from opus 4.6 https://www.reddit.com/r/LLMDevs/comments/1r0ih5p/opus_removes_lastassistantturn_prefill_you_can_no/

[rekram1-node]

Hm I cant replicate

[rekram1-node]

can u send a session? opencode export > session.json

[Yiximail]

I can reproduce this issue and have identified the root cause: it's web-mode specific.

Key Finding

In opencode web mode, after each assistant response completes, OpenCode automatically sends a second, empty assistant turn (containing only step-start + step-finish, no text content). This phantom message causes the conversation to end with an assistant message, which then triggers the "assistant message prefill" error on the next request.

CLI/TUI mode does NOT have this problem — the same models work perfectly there.

Evidence from SQLite

Session ses_39b0e7c9cffefZN9W4wuV6sIJx, message sequence:

[13:34:35] user      → "测试对话，收到请直接回复 收到"
[13:34:35] assistant → step-start, reasoning, text("收到"), step-finish  ← normal response
[13:34:38] assistant → (empty, no parts)                                 ← phantom message ⚠️

[13:35:20] user      → same prompt (switched to grok model)
[13:35:20] assistant → step-start, text("收到"), step-finish             ← normal response  
[13:35:29] assistant → step-start, step-finish (no text)                 ← phantom message ⚠️

The phantom assistant messages have distinctive characteristics:

• Negative cost: -0.0025166
• Negative input tokens: -25197
• Double cache reads: 50446 (vs normal ~25000)
• Only step-start + step-finish, no text content

Steps to Reproduce

1. opencode web --port 4196
2. Open web UI, send any message
3. After response, check SQLite DB → phantom empty assistant message exists
4. Send another message with a model that doesn't support prefill → 400 error

Tested on v1.2.4 and v1.2.5, both reproduce in web mode only.

[rekram1-node]

can u send the session? Can you show me opencode debug config

[Yiximail]

@rekram1-node Here's the session export and debug config:

Session export (220KB, uploaded as gist):
https://gist.github.com/Yiximail/c1881769ce64ed0ba693858314bd59f1

opencode debug config:

{
  "$schema": "https://opencode.ai/config.json",
  "plugin": [
    "opencode-antigravity-auth@latest",
    "opencode-pty"
  ]
}

(Full provider/model config is auto-generated by the antigravity auth plugin — the models themselves work fine in CLI mode, the issue is only in web mode generating phantom assistant messages.)

Note: The session was exported from SQLite directly since opencode export requires an active TUI session. The format matches the standard export structure with info + parts per message.

[Yiximail]

@rekram1-node Here's the full opencode debug config:

{
  "$schema": "https://opencode.ai/config.json",
  "plugin": [
    "opencode-antigravity-auth@latest",
    "opencode-pty"
  ],
  "provider": {
    "google": {
      "models": {
        "antigravity-gemini-3-pro": {
          "name": "Gemini 3 Pro (Antigravity)",
          "limit": { "context": 1048576, "output": 65535 },
          "modalities": { "input": ["text","image","pdf"], "output": ["text"] },
          "variants": { "low": { "thinkingLevel": "low" }, "high": { "thinkingLevel": "high" } }
        },
        "antigravity-gemini-3-flash": {
          "name": "Gemini 3 Flash (Antigravity)",
          "limit": { "context": 1048576, "output": 65536 },
          "modalities": { "input": ["text","image","pdf"], "output": ["text"] },
          "variants": { "minimal": { "thinkingLevel": "minimal" }, "low": { "thinkingLevel": "low" }, "medium": { "thinkingLevel": "medium" }, "high": { "thinkingLevel": "high" } }
        },
        "antigravity-claude-sonnet-4-5": {
          "name": "Claude Sonnet 4.5 (Antigravity)",
          "limit": { "context": 180000, "output": 64000 },
          "modalities": { "input": ["text","image","pdf"], "output": ["text"] }
        },
        "antigravity-claude-sonnet-4-5-thinking": {
          "name": "Claude Sonnet 4.5 Thinking (Antigravity)",
          "limit": { "context": 180000, "output": 64000 },
          "modalities": { "input": ["text","image","pdf"], "output": ["text"] },
          "variants": { "low": { "thinkingConfig": { "thinkingBudget": 8192 } }, "max": { "thinkingConfig": { "thinkingBudget": 32768 } } }
        },
        "antigravity-claude-opus-4-5-thinking": {
          "name": "Claude Opus 4.5 Thinking (Antigravity)",
          "limit": { "context": 180000, "output": 64000 },
          "modalities": { "input": ["text","image","pdf"], "output": ["text"] },
          "variants": { "low": { "thinkingConfig": { "thinkingBudget": 8192 } }, "max": { "thinkingConfig": { "thinkingBudget": 32768 } } }
        },
        "antigravity-claude-opus-4-6-thinking": {
          "name": "Claude Opus 4.6 Thinking (Antigravity)",
          "limit": { "context": 180000, "output": 64000 },
          "modalities": { "input": ["text","image","pdf"], "output": ["text"] },
          "variants": { "low": { "thinkingConfig": { "thinkingBudget": 8192 } }, "max": { "thinkingConfig": { "thinkingBudget": 32768 } } }
        },
        "gemini-2.5-flash": {
          "name": "Gemini 2.5 Flash (Gemini CLI)",
          "limit": { "context": 1048576, "output": 65536 },
          "modalities": { "input": ["text","image","pdf"], "output": ["text"] }
        },
        "gemini-2.5-pro": {
          "name": "Gemini 2.5 Pro (Gemini CLI)",
          "limit": { "context": 1048576, "output": 65536 },
          "modalities": { "input": ["text","image","pdf"], "output": ["text"] }
        },
        "gemini-3-flash-preview": {
          "name": "Gemini 3 Flash Preview (Gemini CLI)",
          "limit": { "context": 1048576, "output": 65536 },
          "modalities": { "input": ["text","image","pdf"], "output": ["text"] }
        },
        "gemini-3-pro-preview": {
          "name": "Gemini 3 Pro Preview (Gemini CLI)",
          "limit": { "context": 1048576, "output": 65535 },
          "modalities": { "input": ["text","image","pdf"], "output": ["text"] }
        }
      }
    }
  },
  "mcp": {
    "playwright": {
      "type": "local",
      "command": ["npx", "-y", "@playwright/mcp@latest"],
      "enabled": true
    },
    "brave-search": {
      "type": "local",
      "command": ["npx", "-y", "@brave/brave-search-mcp-server", "--transport", "stdio"],
      "environment": { "BRAVE_API_KEY": "[REDACTED]" },
      "enabled": true
    },
    "figma-mcp": {
      "type": "remote",
      "url": "https://mcp.figma.com/mcp",
      "enabled": true
    }
  },
  "permission": {
    "external_directory": {
      "~/Documents/**": "allow",
      "~/.config/opencode/**": "allow"
    }
  }
}

The models are all injected by the opencode-antigravity-auth plugin. The issue reproduces across all of them in web mode.

[rekram1-node]

You are using antigravity models there and I dont see anything weird, I think there is a bug for that specific issue in the antigravity provider perhaps? Because it looks like u sent a message to opus 4.6 on antigravity, it replied, then u sent another and it errored.