[Harness] PostToolUse additionalContext re-serialized between turns, invalidating prompt cache

Bug Description

PostToolUse additionalContext is re-serialized between turns, invalidating the entire prompt cache

Summary

Context returned by a hook as hookSpecificOutput.additionalContext is
delivered to the model in one shape during the turn, and a different shape
from the next turn onwards. The change lands wherever the hook fired, often
far back in history, so it invalidates every cached message after it. On a
long turn this costs a full-context cache write per turn that received hook
context.

Claude Code's own injected reminders (e.g. "The task tools haven't been used
recently…") do not have this problem: they arrive as role: "system"
messages immediately and survive turn boundaries byte-identical. Only the
hook additionalContext path changes shape.

The two representations

During the turn — a <system-reminder>-wrapped text block appended to
the user tool_result message that triggered the hook:

{
  "role": "user",
  "content": [
    { "type": "tool_result", "tool_use_id": "toolu_01SXL5…", "content": "…" },
    { "type": "text",
      "text": "<system-reminder>\nPostToolUse:Bash hook additional context: <GUIDANCE TEXT>\n</system-reminder>" }
  ]
}

From the next turn's first request — the wrapper is stripped and the text
becomes a standalone message inserted after the tool result:

{ "role": "system",
  "content": "PostToolUse:Bash hook additional context: <GUIDANCE TEXT>" }

Diffing the last request of one turn against the first request of the next,
this relocation is the only substantive change in the shared prefix;
everything after it is a one-index shift (verified elementwise), and
system, tools, and context_management are byte-identical.

Impact

Claude Code places a single message-level cache_control breakpoint near the
tail of the message list, so a change this far back drops the entire message
cache; only the system/tools breakpoints survive.

Observed on two sessions (wire capture via a read-only reverse proxy):

| session | request | state of the hook context | cache write | cache read |
|---|---|---|---|---|
| A | turn N, injection | wrapped, at msg 43 | 1,508 | 95,840 |
| A | turn N, last request | wrapped, at msg 43 | 518 | 150,972 |
| A | turn N+1, first request | now role:"system" msg 44 | 140,916 | 6,472 |
| B | turn M, injection | wrapped, at msg 24 | 2,304 | 41,707 |
| B | turn M, second injection | wrapped, at msgs 24 and 45 | 1,635 | 58,213 |
| B | turn M+1, first request | now role:"system" msgs 25, 47, 55 | 57,007 | 8,057 |
| B | turn M+2, first request (no hook context in turn M+1) | — | 2,468 | 65,064 |

The last row is the control: a turn boundary with nothing to convert is
cheap, so the cost is specific to the re-serialization and not to starting a
turn. A third instance in session A cost 87,306 tokens. In session A the read
fell from 143,250 to 6,472 across the boundary.

Reproduction

1. Configure a PostToolUse hook that returns

{"hookSpecificOutput": {"hookEventName": "PostToolUse", "additionalContext": "hello"}}.

1. Start a turn that runs many tool calls, and let the hook fire early in it.
2. Let the turn finish, then send any second prompt.
3. On the second turn's first request, cache_read_input_tokens collapses to

roughly the system + tools prefix and cache_creation_input_tokens is
approximately the whole conversation.

Suggested fix

Emit one representation on both paths — either inject in-flight in the
persisted role: "system" shape, or persist the in-flight
<system-reminder> block as-is. Either way the prefix stops changing and the
cache survives.

(Which shape is correct is a separate question — the wrapped inline form and
the separate system turn presumably read differently to the model. The defect
is the mismatch.)

Secondary, same cause

At the same boundary, the trailing built-in reminder message also changes
from content: [{"type": "text", "text": "…", "cache_control": {…}}] to
content: "…". Harmless today only because it sits at the tail, past the
breakpoint.

Environment

• Claude Code 2.1.220
• macOS 22.6.0 (darwin), zsh
• Model claude-opus-5, 1h ephemeral cache TTL
• Wire captured by a read-only proxy via ANTHROPIC_BASE_URL; nothing is

injected, rewritten or reordered on the wire by the proxy.

Environment Info

• Platform: darwin
• Terminal: tmux
• Version: 2.1.220
• Feedback ID: 73de6cf2-921f-42ac-9b60-595d150a4947

Errors

[]

live turn: {"tool_result": "A1"}, {"text": "<system-reminder>\nPostToolUse:Bash hook additional context: [...]\n</system-reminder>"}
rebuilt:   {"tool_result": "A1\n\n<system-reminder>\nPostToolUse:Bash hook additional context: [...]\n</system-reminder>"}