Resumed over-limit session is permanently stuck: prompts fail with "Prompt is too long", auto-compaction never triggers, and "/compact" is a silent no-op

[feiandxs]

Environment

• @zed-industries/claude-code-acp 0.16.2 (spawned via npx -y @zed-industries/claude-code-acp@0.16.2)
• Claude Code CLI 2.1.237, logged in via claude /login
• Node v26.0.0, macOS (arm64)
• Protocol: ACP v1 over stdio, plain JSON-RPC (no editor involved — reproduced with a raw stdio driver)

Summary

A session whose history is already near/over the model context limit (created in terminal Claude Code; ~21 MB jsonl, ~4200 updates replayed on session/load) becomes permanently unusable through the adapter:

1. session/load succeeds and replays history normally.
2. Any session/prompt fails with -32603 Internal error: Prompt is too long. The rejection is near-instant (~200 ms), so it looks like an SDK-side pre-check rather than an API round-trip.
3. SDK-side auto-compaction never triggers on this path — related to the position in Does claude-agent-acp currently trigger SDK-side automatic compaction? #887 ("managed by Claude Code entirely"), but empirically it does not happen for resumed sessions that are already over the limit.
4. Sending /compact" as a prompt (it is advertised in available_commands) returns stop_reason: end_turnin **~40 ms** with **zero**session/updatenotifications — nocompactingstatus, nocompact_boundary, no summary — and the session remains stuck: the next session/prompt` fails with the same error.

In terminal Claude Code the same situation is recoverable (claude --resume <id> + /compact), so the capability exists in the CLI; it is just unreachable through the adapter exactly when it is needed most.

Repro (raw JSON-RPC over stdio)

→ initialize {protocolVersion: 1}
→ session/load {sessionId: <big-session>, cwd: <its cwd>, mcpServers: []}
← ok (history replayed as session/update stream)
→ session/prompt {sessionId, prompt: [{type: "text", text: "hi"}]}
← error -32603 "Internal error: Prompt is too long"   (~200 ms)
→ session/prompt {sessionId, prompt: [{type: "text", text: "/compact"}]}
← result {stopReason: "end_turn"}                      (~40 ms, no notifications at all)
→ session/prompt {sessionId, prompt: [{type: "text", text: "hi"}]}
← error -32603 "Internal error: Prompt is too long"

Expected (any of these would unblock hosts)

1. Auto-compaction runs on the resumed-session path the same way it does in interactive terminal sessions (per Does claude-agent-acp currently trigger SDK-side automatic compaction? #887 this is expected to be Claude Code's job — but it observably doesn't happen here); or
2. /compact" over session/prompt` actually executes compaction (today it silently no-ops in this state — possibly related to Surface the /compact conversation summary over ACP instead of dropping it #873, but here nothing runs at all, rather than the summary being dropped); or
3. At minimum, a structured/typed error (instead of a string inside -32603) so hosts can detect the condition and guide users to recover in the terminal.

Happy to provide the full driver script or additional traces if useful.

Related: #887, #296, #26, #873