[Bug] Compaction summarizer never passes sessionId, so PR #2131's OpenCode headers are absent for that one call — every compaction fails on the OpenCode free tier

[japorto100]

Follow-up to #2072 and #2110, and specifically to the fix in #2131 (merged 2026-09-09). Those
addressed the missing OpenCode session header for provider requests. The compaction summarizer is
still affected, because it never supplies a sessionId for the adapter to use.

What happens

On opencode / muse-spark-1.3-contributor-free (free tier), a session ran 500+ normal turns without
a single provider error. Every compaction attempt failed:

Compaction failed: Turn prefix summarization failed:
Provider rejected the request (MissingSessionID, 400):
Error from provider (Console): OpenCode's free tier can only be used in OpenCode

Reproduced twice, through both entry points:

• await compact.run() from the Python kernel — Requested compaction failed: Summarization failed: …
• /compact from the TUI — Compaction failed: Turn prefix summarization failed: …

Both go through AgentSession._performCompaction, so this is not specific to how compaction is
triggered. Version observed: 0.9.4.

Why

packages/coding-agent/src/core/compaction/compaction.ts builds the summarizer's completion options
without a sessionId:

• generateSummary — line 556-559: { maxTokens, signal, apiKey, headers } (plus reasoning)
• generateTurnPrefixSummary — line 826-829: the same shape

sessionId does not occur anywhere in that file (0 occurrences in 845 lines at HEAD, commit read
2026-09-09).

A normal turn does supply it: Agent.sessionId → createLoopConfig() → sessionId: this.sessionId,
which reaches the same streamSimple/provider path the summarizer uses. The adapters then derive
the header from it, and only from it:

• packages/ai/src/providers/opencode-headers.ts: if (sessionId) merged["x-opencode-session"] = sessionId;
• packages/ai/src/providers/openai-responses.ts, createClient: if (sessionId) { … headers.session_id = sessionId; headers["x-client-request-id"] = sessionId; }

So the summarizer call is the one request in a session's lifetime that carries no session identity,
and #2131 could not change that — it fixed the adapters, and the adapters receive undefined.
The PR's file list confirms it touches only packages/ai/src/providers/* and their tests.

Other providers hide the same gap: the OpenAI-Codex adapter falls back to
options?.sessionId || createCodexRequestId(), so a missing sessionId is invisible there. OpenCode
has no fallback and rejects with 400, which is why this surfaces only on that provider even though
the missing field is provider-independent.

A fix that works, for whatever it is worth

compact() already accepts a SummaryCallRunner (line 707) whose stated purpose is that hosts
"decorate each call with its own request identity", and _performCompaction already uses it to
attach the semantic-edge request id. sessionManager is in scope there, so the session headers can
be added at that same point without changing any function signature:

const primeLabSessionHeaders = (() => {
  try {
    const sid = this.sessionManager?.getSessionId?.();
    return sid ? { session_id: sid, "x-client-request-id": sid } : {};
  } catch { return {}; }
})();

…and spreading that into both call(...) branches of the existing summaryCall.

Applied as a local patch on 0.9.4, the same /compact in the same session that had just failed
completed successfully: Compacted from 137,879 tokens. (Header names there are the pre-#2131 ones;
on current main the equivalent would flow through withOpenCodeHeaders.)

Passing sessionId down into completionOptions in compaction.ts would be the more direct fix and
would let the adapters set whatever each provider needs — including x-opencode-session — but that
is a maintainer call and touches signatures.

Scope and limits

• Observed on one provider and one model. The missing field is provider-independent; the 400 is not.
• I have not verified behaviour on current main at runtime — the source reading above is from
  HEAD, the runtime reproduction is on the released 0.9.4.
• No PR attached: per CONTRIBUTING, unvouched pull requests are closed, so this is a Discussion.
  Happy to test a maintainer's fix against the same reproduction.