[Bug]: Native chat rejects every message when the selected model differs from the agent's current model — OpenCode 2 ACP has no session/set_model (root cause + verified fallback) #32183

Operating system

macOS

Orca version

1.4.237

Details

Summary

In the Chat UI, any conversation whose selected model differs from the OpenCode agent's current model rejects every message immediately with OpenCode didn't accept this message. (rejection.kind = providerRejected). The default model works fine in the same setup. The prompt never reaches the agent or the model provider.

This is the same visible failure as #31475 and #31423, but with the actual root cause identified and a verified fix path.

Root cause

Orca writes the model option via the custom ACP method session/set_model (src/main/acp/acp-session-runtime.ts:224 on current main). The OpenCode v2 line does not implement that method — it returns JSON-RPC -32601 "Method not found": session/set_model. applyAcpMessageOptions (src/main/acp/acp-message-execution-options.ts:38-52) converts any setOption failure into AgentSessionOptionRejectedError, so the message is rejected before session/prompt is ever sent.

session/set_model exists in the OpenCode 1.x ACP implementation (cf. anomalyco/opencode#9941), but the v2 rewrite (latest release v2.1.3) ships session/set_mode and session/set_config_option instead. Verified against both 2.1.2 and 2.1.3 binaries.

Why the default model works: the skip condition at acp-message-execution-options.ts:35 (reported()[key] === value) bypasses the write when the selected model already equals the agent's current model.

Reproduction

1. opencode 2.1.2 (homebrew) with a config default model A and at least one other model B (any custom provider; I used self-hosted models behind an OpenAI-compatible gateway).
2. Open a Chat UI conversation with the OpenCode agent on model A (the config default) → works.
3. New conversation with model B → every message is rejected within ~25 ms with OpenCode didn't accept this message.

Evidence

• agent-session-journal.db: 5 sessions on model B, all 8 dispatches pending → rejected in ~24 ms, reason The agent didn't accept this message., rejection.kind = providerRejected; zero session/prompt reached the agent (provider-side logs show no requests).
• Raw ACP test against opencode acp (2.1.2 and 2.1.3): initialize → session/new → session/set_model {sessionId, modelId: "B"} returns -32601 "Method not found": session/set_model. The same flow with the default model never needs the call and works.
• strings on both binaries: session/set_mode, session/set_config_option present; session/set_model absent.
• opencode run -m B (CLI) and the HTTP API both work — the model itself is healthy.

Verified fix path

OpenCode 2.x does implement model switching via session/set_config_option:

--> session/set_config_option {"configId": "model", "value": "<provider>/<model>"}
<== ok  (session/new's configOptions then reports currentValue = <provider>/<model>)
--> session/prompt {"prompt": "reply with exactly: ok"}
<== "ok"   (usage_update reports the selected model's context window)

Verified against 2.1.2: the switch happens and the prompt runs on the selected model.

Suggested fix

In the ACP model-write path, fall back to session/set_config_option {configId: "model", value} when session/set_model fails with -32601 (Method not found) — or, more generally, prefer the agent's advertised configOptions from session/new when it offers a model config option. The current Pi/OMP fixes (#31863, #31998) address effort-restore rejections but not this path; on main today session/set_model is still called unconditionally (acp-session-runtime.ts:224) and the opencode dialect does not override model writes.

Workarounds

• Set the desired model as the config default in opencode.jsonc (model field) — Orca skips the write when selected == current. (Flips the failure onto the other models, so only useful when one model dominates.)
• Use the opencode CLI/TUI for non-default models.

Related

• [Bug]: Chat UI: prompts rejected immediately ("OpenCode 2 didn't accept this message") when the selected model is a GitHub Copilot model #31475 — same mechanism (GitHub Copilot models), root cause not identified there
• agent-session: first prompt rejected as providerRejected in new OpenCode session (ACP handle still "pending") #31423 — same visible symptom; thread converges on a different variant (leaked provider supervisors)
• [Bug]: OMP native chat rejects every message when the saved model differs from OMP's default and no effort is saved #31995 / [Bug]: Pi native chat rejects every message when Orca's Pi model preference has no effort for it #31849 — same "model-only message rejected" family for OMP/Pi (effort-restore cause), fix PRs fix(acp): treat other models' effort menus as unknown so new chats can seed an effort #31998 / fix(pi): keep model-only messages deliverable #31863