fix(cli): preserve session agent for headless prompts

[quanzhuo]

Issue

Fixes #13027

Context

Headless sessions created with an explicit agent lost that selection when the first prompt omitted the agent field. The prompt fell back to the default code agent, so the custom agent's model and permissions were not applied.

This change preserves the agent selected during session creation when a later headless prompt does not explicitly override it.

Implementation

Agent resolution for user messages now falls back to the agent stored on the session when input.agent is absent. Explicit prompt-level agent selections continue to take precedence, and sessions without a selected agent continue to use the configured default agent.

The regression test exercises the public headless HTTP flow by creating a session with a custom test-engineer agent, sending a prompt without an agent, and verifying that both the custom agent and its configured model remain selected.

Screenshots / Video

N/A — this change has no visual impact.

How to Test

Manual/local verification

• Agent: ran git diff --check; it completed successfully.
• Agent: ran bun run script/check-opencode-annotations.ts --worktree; all shared OpenCode changes were correctly annotated.
• Agent: reviewed the HTTP regression test against the session create and prompt route schemas.

Reviewer test steps

1. From packages/opencode/, run bun test ./test/kilocode/headless-session-agent.test.ts.
2. Confirm the test creates a session with agent: "test-engineer" and sends the first prompt without an agent field.
3. Confirm the returned user message uses test-engineer and mock/custom-model rather than the default code agent.
4. Run bun run typecheck from packages/opencode/.

Blocked checks and substitute verification

• Agent: bun test ./test/kilocode/headless-session-agent.test.ts could not start because the current workspace dependencies are incomplete (@opentui/solid/preload is missing). Substitute verification consisted of reviewing the test setup and HTTP route schemas, running git diff --check, and running the OpenCode annotation guard.
• Agent: bun run typecheck from packages/opencode/ could not start because tsgo is missing from the current workspace dependencies. The diff and annotation checks above completed successfully, but the target test and typecheck should be rerun after installing dependencies.

Checklist

• Issue linked above, or exception explained
• Tests/verification described
• Screenshots/video included for visual changes, or marked N/A
• Changeset considered for user-facing changes

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

The previous CRITICAL finding (wrong endpoint path in the regression test) was resolved in 80b54f7: the test now posts to POST /session/:sessionID/message, which is the correct v1 synchronous prompt endpoint (SessionPaths.prompt). Verified the endpoint accepts the noReply + parts payload, returns SessionV1.WithParts ({ info, parts }) where the user message info carries the resolved agent and model, and that with noReply: true no LLM call is made so the unreachable mock provider is never contacted. Session create accepts and persists agent, and GET /session/:sessionID exposes it.

Files Reviewed (3 files)

• .changeset/calm-agents-wait.md
• packages/opencode/src/session/prompt.ts
• packages/opencode/test/kilocode/headless-session-agent.test.ts

Previous Review Summary (commit c1a6c3f)

Current summary above is authoritative. Previous snapshots are kept for context only.

Previous review (commit c1a6c3f)

Status: 1 Issue Found | Recommendation: Address before merge

Overview

Severity	Count
CRITICAL	1
WARNING	0
SUGGESTION	0

The one-line fix in prompt.ts is correct and well-scoped: it falls back to the session's stored agent when input.agent is absent, keeps explicit prompt-level agents taking precedence, and preserves the default-agent path for sessions without one. The changeset is user-facing and the change is properly marked for fork hygiene.

However, the new regression test targets a non-existent route (/session/:id/prompt instead of /session/:id/message), so it will fail once dependencies allow it to run — consistent with the PR note that the test could not be executed locally.

Fix these issues in Kilo Cloud

Issue Details (click to expand)

CRITICAL

File	Line	Issue
packages/opencode/test/kilocode/headless-session-agent.test.ts	52	Test posts to /session/:id/prompt, which does not exist; the v1 prompt endpoint is /session/:id/message. The request hits the UI catch-all and returns 404, failing the test.

Files Reviewed (3 files)

• .changeset/calm-agents-wait.md - no issues
• packages/opencode/src/session/prompt.ts - no issues
• packages/opencode/test/kilocode/headless-session-agent.test.ts - 1 issue

---

_{Reviewed by kimi-k3 · Input: 141.5K · Output: 10.7K · Cached: 1.2M}

_{Review guidance: REVIEW.md from base branch main}

[johnnyeric]

Thanks for pushing this fix!