Diagnosis: "build agent empty input" after orchestrator output

Status: Diagnosis only — no code change yet. Date: 2026-07-19 Related PR: #818 (fix/preset-tui-slash-command) — same root class as the original /preset fix. Suspected sibling bug reported by user: During superpowers / brainstorm skill conversations, when the orchestrator asks for confirmation or work is interrupted (subagent completes, background task finishes), a build agent turn sometimes appears with an empty user input.

TL;DR

The build agent turn with empty input is the same class of bug as the original /preset issue fixed in #818: a plugin hook calls sessionSdk.promptAsync({ body: { parts: [createInternalAgentTextPart(...)] } }) without specifying an agent field. opencode then resolves the agent via agents.defaultInfo(), which falls back to the built-in build agent whenever default_agent is unset, user-overridden, or not effectively applied. The synthetic: true flag hides the injected text from the TUI, so the user perceives the build turn as having "empty input."

Root cause (causal chain, cross-validated)

Orchestrator enters input-wait. After emitting a confirmation question (skill flow), the assistant turn finishes. opencode's per-session Runner transitions to Idle (packages/opencode/src/effect/runner.ts:115-138, packages/opencode/src/session/run-state.ts:60-63). The session is no longer "busy" from the Runner's perspective.
Plugin hook fires promptAsync with a synthetic part and no agent field. Two call sites in omos do this:
- src/hooks/task-session-manager/index.ts:398-402 — CONTINUATION_NUDGE injection, fires on session.idle / session.status(idle) when the orchestrator session has incomplete todos (matches "subagent completes" / "background task finishes").
- src/interview/service.ts:622, 871, 933, 1007 — interview/skill flow injections (matches "brainstorm skill flow").
opencode does not guard promptAsync against busy/input-wait state. The HTTP handler at packages/opencode/src/server/routes/instance/httpapi/handlers/session.ts:311-329 does not call assertNotBusy and does not consult the Question service. It proceeds straight to promptSvc.prompt. The Runner, being Idle, immediately startRuns the new turn (runner.ts:131-134). No queue, no reject, no cancellation of the pending question.
Agent resolves to build. packages/opencode/src/session/prompt.ts:636-637:
```
const agentName = input.agent
const ag = agentName ? yield* agents.get(agentName) : yield* agents.defaultInfo()
```
When input.agent is omitted, opencode uses agents.defaultInfo() (packages/opencode/src/agent/agent.ts:328-340), which returns the first visible mode: "primary" agent — build (declared first in the agent registry, agent.ts:141-155). omos attempts to set default_agent = "orchestrator" via its config hook (src/index.ts:546-551), but only when default_agent is absent. build is selected whenever:
- config.setDefaultAgent === false (plugin config disables it)
- The user's opencode.json already sets a different default_agent
- The config hook didn't run or didn't apply (SDK/runtime version skew: plugin built against @opencode-ai/sdk v1.4.3, installed runtime v1.18.3)
- The orchestrator agent isn't registered at config-load time
"Empty input" is the synthetic flag's visual effect. synthetic: true only controls TUI visibility. packages/opencode/src/session/message-v2.ts:206 still includes synthetic text parts in model messages (the only filters are !part.ignored and part.text !== ""; there is no synthetic filter when building model messages). The user sees the build agent respond to a turn with no visible user message — perceived as "empty input." (createInternalAgentTextPart appends a \n marker, so the text is non-empty from the LLM's perspective.)
Session agent is durably corrupted. packages/opencode/src/session/prompt.ts:672-689 compares current.agent !== info.agent and, if they differ, calls sessions.setAgentModel({ agent: info.agent, ... }). A single agent-less promptAsync that resolves to build permanently rewrites the session's agent to build — every subsequent turn also routes to build until explicitly reset.

Confirmed-affected call sites

File:line	Trigger	Body omits `agent`?	Gate
`src/hooks/task-session-manager/index.ts:398-402`	`session.idle` / `session.status(idle)` on orchestrator session with incomplete todos	Yes	`continuationConsumed`, `hasInputWait` (3×: lines 282, 362, 386), `isCurrentContinuation`, `isFallbackInProgress`, `backgroundJobBoard.hasTerminalUnreconciled`
`src/interview/service.ts:622`	User submits interview dashboard input	Yes	`sessionBusy` lock, interview active state
`src/interview/service.ts:871`	User submits interview chat	Yes	same
`src/interview/service.ts:933`	User submits interview answer	Yes	same
`src/interview/service.ts:1007`	User submits interview comment	Yes	same
`src/interview/service.ts:504`	Interview URL notification	Yes (but `noReply: true`, non-synthetic text)	none
`src/tools/smartfetch/secondary-model.ts:252`	Smartfetch secondary model query	Yes	none

Correct pattern (for comparison)

src/hooks/foreground-fallback/index.ts:635-639 explicitly includes the agent:

const promptBody = {
  parts: lastUser.parts,
  model: ref,
  ...(agentName ? { agent: agentName } : {}),
};

This is the pattern every promptAsync caller in omos should follow.

Why the `hasInputWait` gate in task-session-manager is not sufficient

The gate exists and works in the common case (task-session-manager/index.ts:282, 362, 386, with tests at index.test.ts:2772-2858, 3013-3048). But:

Documented race window. IDLE_RECONCILE_DELAY_MS = 2_000 (line 54). The comment at lines 49-53 admits: "Completions arriving after the window are still dropped (the race is reduced, not eliminated)." If session.idle fires and the 2s timer expires before question.asked is delivered, and the 3 SDK calls (todo/children/status) in evaluateContinuation all resolve before question.asked arrives, the nudge fires.
Input-wait is not the only trigger. The interview/skill path (src/interview/service.ts) does not consult hasInputWait at all — it injects on user dashboard actions, which can happen while the orchestrator is mid-question.
The gate does not address the missing agent field. Even when the nudge legitimately fires (no input-wait, real incomplete todos), the resulting turn still routes to build if default_agent is unset. The gate prevents some unwanted injections; it does not prevent misrouting when injection happens.

Why this is the same class as the #818 `/preset` fix

#818's original bug: /preset used createInternalAgentTextPart() to trigger an LLM turn that was invisible in the TUI (synthetic: true). The fix moved /preset to pure TUI dialogs (src/tui-preset.ts uses only api.ui.dialog / DialogSelect / DialogPrompt / DialogConfirm — no promptAsync).

This bug: other hooks still use the same createInternalAgentTextPart + promptAsync pattern, and additionally omit the agent field, so the invisible turn routes to build instead of the orchestrator. Same shape: a synthetic part starting an invisible turn. Different symptom: build agent instead of orchestrator.

Fix directions (not implemented — awaiting decision)

Minimal fix

Add agent: 'orchestrator' to the promptAsync body at all four affected call sites:

src/hooks/task-session-manager/index.ts:398-402
src/interview/service.ts:622, 871, 933, 1007

This ensures the continuation nudge and interview injections always route to the orchestrator regardless of opencode's default_agent resolution, eliminating the path to build.

Hardening (optional, larger scope)

Input-wait guard on the interview/skill path. Consult hasInputWait (or an equivalent signal) before injecting in src/interview/service.ts. Do not inject while the orchestrator is waiting for user input.
Post-injection agent assertion. After each promptAsync, assert current.agent was not changed out from under the orchestrator; if it was, restore it via setAgentModel.
Investigate the default_agent application reliability on opencode v1.18.x. The plugin was built against @opencode-ai/sdk v1.4.3; the installed runtime is v1.18.3. The config hook's default_agent = 'orchestrator' mutation may not be applied reliably under this skew. (Note: #799 tracks the package upgrade.)
Shrink or eliminate the IDLE_RECONCILE_DELAY_MS race for sessions that have a pending question.asked / permission.asked.

Evidence index

omos source

Missing agent field (the bug): src/hooks/task-session-manager/index.ts:398-402
Missing agent field (skill flow): src/interview/service.ts:622, 871, 933, 1007
Correct pattern for comparison: src/hooks/foreground-fallback/index.ts:635-639
omos sets default_agent only when absent: src/index.ts:546-551
createInternalAgentTextPart produces synthetic: true: src/utils/internal-initiator.ts:9-21
CONTINUATION_NUDGE is non-empty: src/hooks/task-session-manager/index.ts:56-57
hasInputWait gate (3 checks): src/hooks/task-session-manager/index.ts:282, 362, 386
IDLE_RECONCILE_DELAY_MS race window: src/hooks/task-session-manager/index.ts:54 (admission at lines 49-53)
disableDefaultAgents preserves build and plan: src/cli/config-io.ts:564-600

opencode source (`anomalyco/opencode` @ `dev`)

promptAsync HTTP handler (no busy/input-wait guard): packages/opencode/src/server/routes/instance/httpapi/handlers/session.ts:311-329
prompt HTTP handler (no guard): same file, :295-309
Internal prompt always starts a new turn: packages/opencode/src/session/prompt.ts:1052-1071
Agent selection: input.agent ?? defaultInfo(): packages/opencode/src/session/prompt.ts:636-637
defaultInfo() → default_agent or first visible primary: packages/opencode/src/agent/agent.ts:328-340
build agent definition (default primary, first in registry): packages/opencode/src/agent/agent.ts:141-155
Destructive setAgentModel overwrite on agent change: packages/opencode/src/session/prompt.ts:672-689
Synthetic parts included in model messages: packages/opencode/src/session/message-v2.ts:206
Runner ensureRunning (Idle = run now, no queue): packages/opencode/src/effect/runner.ts:115-138
Runner Idle transition on turn end: packages/opencode/src/session/run-state.ts:60-63
assertNotBusy (NOT used by prompt/promptAsync): packages/opencode/src/session/run-state.ts:71-75

SDK types

default_agent doc: "Falls back to 'build' if not set or invalid": node_modules/@opencode-ai/sdk/dist/v2/gen/types.gen.d.ts:1255-1257
SessionPromptAsyncData.body.agent? is optional: node_modules/@opencode-ai/sdk/dist/v2/gen/types.gen.d.ts:3241-3269
build is a built-in agent: node_modules/@opencode-ai/sdk/dist/v2/gen/types.gen.d.ts:1273-1279

Open questions for the fix

Should the fix be a new PR, or amended into #818? (#818 is currently MERGEABLE / CLEAN / CI green; amending widens its scope and may delay merge.)
Is the interview/skill path expected to always route to the orchestrator, or could it intentionally target a different agent in some flows?
Should we also harden the default_agent application (fix direction #3) as part of this work, or track it separately under #799?

This report is diagnostic only. No code was changed. The fix awaits the user's decision on scope and PR strategy.

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