fix: synthetic suffix messages break title generation (#15)

[ranxianglei]

Problem

opencode-acp prevents OpenCode's built-in session title generation. Sessions stay on New session - <timestamp> forever. Confirmed still reproducing on 1.5.0 (thanks @kratky-pavel for the detailed repro + logs in #15).

Root cause

This is a different path from the isInternalAgentRequest guard added in #16 (which only handles an internal-agent request flowing through messages.transform).

OpenCode's SessionPrompt.ensureTitle (prompt.ts:182) requires exactly one real user message, else it bails (title gen never scheduled):

const real = (m) => m.info.role === "user" && !m.parts.every((p) => "synthetic" in p && p.synthetic)
if (input.history.filter(real).length !== 1) return

ACP's main-chat message-transform pipeline injects a "Compressed block context…" nudge as a second, standalone user message via createSuffixMessage → createSyntheticUserMessage (lib/messages/inject/inject.ts:54). That function is named createSyntheticUserMessage but never set the synthetic flag on its text part — so ensureTitle counted the nudge as a 2nd real user message → filter(real).length === 2 → precondition failed → title generation was never scheduled (no agent=title / small=true stream appears, because it never starts).

Additionally, title({ history: msgs }) is Effect.forkIn (async), sharing the same msgs array reference that messages.transform (prompt.ts:1574) mutates — a race where the forked title fiber observes the injected 2nd message and bails.

Fix

lib/messages/utils.ts — createSyntheticUserMessage now marks its text part synthetic: true (matching its name and OpenCode's own convention). One-line, targeted; benefits all callers (the compress nudge via createSuffixMessage, and prune.ts compression summaries).

Verified safe against OpenCode source:

• ensureTitle excludes all-synthetic user messages → exactly one real user message → title gen runs.
• MessageV2.toModelMessagesEffect (message-v2.ts:802) includes synthetic text parts (checks only type === "text" && !ignored, no synthetic check) → nudge text still delivered to the LLM.
• TUI already hides synthetic parts → no UI noise.
• ACP's own isSyntheticMessage uses an ID-prefix check (msg_dcp_summary_), unaffected by the synthetic field.

Verification

• npm run typecheck — PASS
• npm test — PASS 487/487 (+1 new contract test replicating ensureTitle's real filter), 0 fail
• npm run build — PASS (325.13 KB)

Devlog: devlog/2026-06-29_synthetic-suffix-fixes-title-gen/ (REQ.md + WORKLOG.md).

Refs #15.