Closes #98261

What Problem This Solves

Fixes a prompt-cache stability bug in buildAgentSystemPrompt() (src/agents/system-prompt.ts). Two channel/identity-varying sections were emitted into the static prefix, above SYSTEM_PROMPT_CACHE_BOUNDARY:

1. Exec-approval guidance — the /approve instruction differs by channel (CLI/plain /approve vs native approval-card channels like iMessage / Slack / Telegram-with-inline-buttons).
2. ## Authorized Senders — varies by owner identity, and is dropped entirely in minimal (cron/subagent) mode.

Because their content differs by channel/identity, they fork the supposedly byte-identical cacheable prefix very early. On local models with client-managed prefix caching (llama.cpp / MLX / Ollama), this re-prefills the entire system prompt from scratch on any turn whose channel context differs from the previous one — e.g. an interactive turn right after background cron/heartbeat traffic — which the linked issue measured as multi-minute cold prefills.

This is the direct follow-on to #40256, which relocated Messaging/Voice/Reactions below the boundary but missed these two sections.

Why This Change Was Made

Both sections are instructional context — position-independent for correctness — so they were moved into the volatile suffix below SYSTEM_PROMPT_CACHE_BOUNDARY, mirroring the existing treatment of Messaging/Voice/Reactions. The channel-independent /approve rules stay above the boundary, keeping the stable prefix as large as possible.

One behavioral subtlety is preserved: exec-approval guidance previously lived inside the tool_call_style fallback array, so a provider that overrode tool_call_style suppressed it. The relocated emission keeps that gating (it stays suppressed when providerSectionOverrides.tool_call_style is set). This is therefore a pure cache-stability change with no behavioral difference on any channel.

User Impact

Local-model deployments stop paying repeated cold prefills when the channel context changes between turns: the cacheable prefix is now channel/identity-independent, so it stays warm across interactive/cron/heartbeat traffic. No behavioral change for any channel or provider; cloud providers (server-side caching) are unaffected either way. No configuration changes.

Evidence

Real behavior proof (standalone, non-test)

Drove the real buildAgentSystemPrompt from a standalone script (node --import tsx) for two interactive turns on different channels/owners (CLI vs iMessage), and measured the byte-identical prefix they share — i.e. the region a prefix cache can actually reuse. Run on a minimal workspace, so absolute offsets are smaller than the issue's full ~17.8K-token prompt, but the mechanism and direction are identical:

BEFORE — unpatched main:

CLI prompt:       7217 chars; cache boundary at char 6560
iMessage prompt:  7324 chars; cache boundary at char 6653
shared byte-identical prefix across the two channels: 2020 chars
shared prefix reaches the cache boundary (full prefix cacheable): false
first divergence at char 2020:
  CLI : "send the exact /approve command from \"Reply with:\"; do not ask for another code.\nNever exe"
  iMsg: "use native approval card/buttons first. Include a plain /approve command only when the too"

AFTER — this branch (identical inputs):

CLI prompt:       7217 chars; cache boundary at char 6324
iMessage prompt:  7324 chars; cache boundary at char 6324
shared byte-identical prefix across the two channels: 6850 chars
shared prefix reaches the cache boundary (full prefix cacheable): true
first divergence at char 6850:
  CLI : "send the exact /approve command from \"Reply with:\"; do not ask for another code.\n## Author"
  iMsg: "use native approval card/buttons first. Include a plain /approve command only when the too"

The byte-identical cacheable prefix grows from 2,020 → 6,850 chars and now reaches the boundary (false → true); the first cross-channel divergence (the exec-approval line) moves from char 2,020 (above the boundary) to char 6,850 (below it). The two channels' boundaries also converge to the same offset (6560/6653 → 6324/6324), confirming the prefix is byte-identical.

Regression test

vitest run src/agents/system-prompt.test.ts → 91 passed (90 existing + 1 new guard).

New guard keeps channel/identity-varying sections below the cache boundary (#98261) asserts both sections sit below the boundary, and that the above-boundary prefix is byte-identical when only the owner identity changes, and when only the approval-UI capability changes. It fails on main and passes here:

× keeps channel/identity-varying sections below the cache boundary (#98261)
  AssertionError: expected 2020 to be greater than 6653

(the exec-approval guidance is at index 2020, above the boundary at 6653, on main).

Sibling suites also pass: system-prompt.memory.test.ts, bootstrap-budget.test.ts, sanitize-for-prompt.test.ts, heartbeat-system-prompt.test.ts → 36 passed. oxlint and oxfmt --check are clean on both files.

Local note: this machine's disk was too constrained for a full tsgo run; the change reuses existing typed helpers with no signature changes, and CI check-prod-types / check-test-types cover typechecking.