# Backlog #2 (real streaming): **achievable** — via the `MessageDisplay` hook **Date**: 2026-07-13 **Status**: prereq-spike result. **Streaming IS achievable on the TUI path**, byte-faithfully, on the subscription pool. Three obvious sources are dead ends; a fourth one works. **Scope**: answers the prereq spike that [`README.md`](README.md) § "Backlog #2" demanded *before* any streaming design: > **Prereq spike (do this before designing anything)**: does the transcript JSONL grow *during* a > turn, or only at the end? If only at the end, (a) is dead and you are stuck with (b). The answer: **(a) is dead, (b) is dead — and you are not stuck with either.** The CLI exposes its own streaming interface as a **hook**, which the backlog did not consider. **Measured on**: Mac mini / Claude Code **v2.1.207** / Sonnet 4.6 + Sonnet 5 / Claude Max / real-home mode. Every claim below is reproducible from the commands given. > **Honesty note on how this document was produced.** Its first version concluded the exact opposite — > "streaming is not achievable; the CLI exposes no byte-faithful incremental source" — and was **wrong**. > An adversarial reviewer, commissioned specifically to *refute* it, found `MessageDisplay` on a second > pass; its own first pass had enumerated the hook registry with a truncated grep (it reported 21 > events — there are **30**). Both the wrong conclusion and its refutation are preserved here, because > "we checked, it's impossible" is the most expensive kind of claim to get wrong: it closes a door and > nobody re-opens it. --- ## ✅ The source that works: the `MessageDisplay` hook `claude` fires a **`MessageDisplay`** hook as it renders each block of the assistant's reply. The payload carries the **raw markdown source** of an incremental `delta`, plus a monotonic `index` and a `final` flag: ```json { "hook_event_name": "MessageDisplay", "turn_id": "6cb31d21-…", "message_id": "84ab9832-…", "index": 0, "final": false, "delta": "## Mutex\n\n" } ``` *(payload also carries `session_id`, `transcript_path`, `prompt_id`, `cwd`)* Registered as an ordinary command hook via `--settings` on a **plain interactive TUI spawn** (no `-p`, no `--bare`), `claude-sonnet-4-6`, `--effort low`. Banner verified: `▝▜█████▛▘ Sonnet 4.6 with low effort · Claude Max` — **subscription pool, not metered billing**. One live turn — 7 fires, spread across generation: ``` index=0 final=false len= 10 '## Mutex\n\n' index=1 final=false len= 140 'A **mutual exclusion lock** prevents concurrent access to a shar…' index=2 final=false len= 83 '- Acquiring a locked mutex blocks the caller until the current h…' index=3 final=false len= 163 '- Failing to release a mutex causes a deadlock, freezing all wai…' index=4 final=false len= 96 'let counter = 0;\n\nasync function increment() {\n const release =…' index=5 final=false len= 84 ' counter++; // only one caller here at a time\n } finally {\n …' index=6 final=true len= 3 '```' ``` **Every invariant a proxy needs — all hold:** | requirement | result | |---|---| | **byte-faithful** — deltas are the model's *source*, not the rendered pane | ✅ `## `, `**`, ```` ```javascript ```` all present in the deltas | | **exactness** — `concat(deltas) === T` (the transcript-authoritative text) | ✅ **true**, 579 == 579 bytes | | **prefix-stable** — `T.startsWith(concat(deltas[0..n]))` at every n | ✅ **true at all 7 steps** | | **incremental** — arrives during generation, not at the end | ✅ 7 fires spread across the turn | | **no `-p`** — stays out of the metered `sdk-cli` pool | ✅ plain interactive TUI | | **subscription pool** | ✅ banner `· Claude Max` | This is exactly the contract a streaming design needs: deltas forward straight into SSE `delta.content` chunks, and the transcript's final text `T` stays a cheap end-of-turn assertion (`concat === T`) instead of a reconciliation problem. ### Caveats for the implementer - **Block-level granularity, not token-level** — the hook fires **once per rendered block** (roughly one per paragraph / list item / code block), so the chunk count **scales with answer length**: 7 fires for a ~600-byte answer, **18 for a ~2 KB one**. Plenty for SSE (`delta.content` has no minimum size), but do not promise token-by-token output, and do not hard-code any assumption about chunk count. - **🔴 The sink MUST be keyed by `session_id` — this is live TODAY, not a future concern.** `OCP_TUI_MAX_CONCURRENT` defaults to **2**, so **two `claude` processes already run concurrently**. One hook command writing to one shared sink would **interleave deltas from two different turns into one stream** — request A's client receiving request B's text, the worst failure a proxy can have, and one a single-request test will never surface. The payload carries `session_id` (and `turn_id` / `message_id`), so demux is easy: derive the sink path from `session_id` (`/.jsonl`) and read only your own turn's file. This *also* keeps the design **warm-pool compatible**, because a pre-booted pane's session-id is fixed at boot — one static hook script serves every pane. **Test it with ≥2 concurrent streaming requests carrying distinguishable prompts and assert zero cross-contamination.** - **⚠️ `forceSyncExecution: true` in the hook's source — `claude` BLOCKS on the hook.** A slow hook adds latency to *every* delta. The hook must write and exit immediately (e.g. write to a FIFO / unix socket that OCP reads; never work inline). **Measure the added per-delta latency.** - **Thinking blocks appear to be excluded — but this is NOT yet stress-tested. Verify before shipping.** The exclusion is inferred from `content.map(c => c.type === "text" ? c.text : "")` — but that snippet is from the **`final:true`** call site, not the incremental one. Four live turns (incl. two at `--effort high`) showed no thinking text in any delta and `concat === T` held — **but each transcript's thinking block was empty (`thinking:""`, 0 chars)**, so the exclusion was never actually stressed. **The failure mode is severe**: if thinking deltas *do* fire on some config (Opus, `xhigh`), `concat(deltas) !== T` **and OCP streams the model's private reasoning to the caller**. The end-of-turn `concat === T` assertion would *detect* that but **cannot prevent** it — SSE deltas cannot be un-sent. **Before shipping, run a turn on a model+effort that produces substantive thinking** (a hard reasoning prompt on Opus / `xhigh`) and confirm both (a) no thinking text in any delta and (b) `concat === T` still holds. - OCP already owns the spawn (isolated HOME, its own flags), so injecting `--settings` with a `MessageDisplay` hook sits inside the existing architecture. - **`ALIGNMENT.md`**: this consumes `claude`'s **own** hook surface as emitted — forwarding, not inventing. Not a new endpoint, not a fabricated protocol. (Class B / ADR 0007 — the TUI spawn is OCP-owned; no `cli.js` citation applies.) ### Reproduce in 60 seconds ```bash # hook script: append the payload (arrives on stdin) and exit immediately printf '#!/bin/bash\ncat >> "$MD_LOG"; printf "\\n" >> "$MD_LOG"; exit 0\n' > /tmp/h.sh && chmod +x /tmp/h.sh echo '{"hooks":{"MessageDisplay":[{"hooks":[{"type":"command","command":"MD_LOG=/tmp/deltas.jsonl /tmp/h.sh"}]}]}}' > /tmp/s.json # plain interactive claude in tmux (prefix NOT ocp-tui-*, and never kill-server) tmux new-session -d -s md-probe -x 220 -y 50 \ "claude --model claude-sonnet-4-6 --effort low --session-id $(uuidgen) --settings /tmp/s.json" # …wait for '? for shortcuts', paste a markdown-producing prompt, press Enter… jq -r '"\(.index) \(.final) \(.delta|@json)"' /tmp/deltas.jsonl # incremental raw-markdown deltas # then assert: concat(deltas) == extractLatestAssistantText(.jsonl) ``` --- ## The three dead ends (still worth knowing — they say what NOT to build) ### (a) Incremental transcript reads — **dead: event granularity, not token granularity** The transcript JSONL *does* grow during a turn, but one **whole event at a time**; the assistant's text event is written as **one complete line**, appearing only ~0.3 s before the terminal `turn_duration`. Observed (session `efd5b161`, `turn_duration: 7319 ms`): ``` #6 t+0.0s type=user (the prompt) #15 t+4.7s type=assistant blocks=thinking #16 t+7.0s type=assistant blocks=text ← the ENTIRE answer, in one line #21 t+7.3s type=system subtype=turn_duration ← terminal ``` Cross-checked at **20 ms polling + `fs.watch`** (25× finer): a partial line **never touches disk** — one write, `+1` line, carrying the complete answer. Also forced with the undocumented `CLAUDE_CODE_INCLUDE_PARTIAL_MESSAGES=1`: still 1 assistant event, 0 partials (interactive mode has no stream-json *sink* for it to write to). **The transcript is still needed** — as the terminal-turn signal, as the authoritative `concat === T` check, and as the input to the existing honesty gates (auth-banner detection, `truncated`). It is just not the *streaming* source. ### (b) `tmux capture-pane` diffing — **dead: the pane is a RENDERED view, not the text** The backlog expected to fall back to this, calling it "lossy … (wrapping, scrollback, spinner lines)". The loss is far worse than formatting noise: **the pane does not contain the answer's source bytes at all.** The TUI *renders* markdown, and `capture-pane -p` strips the ANSI that rendering produced. Same turn, same lines: ``` TRANSCRIPT (authoritative T): PANE (capture-pane -p -J -S -500): '## Semaphore' '⏺ Semaphore' ← heading marker gone '' '' 'A **semaphore** is a synchro…' ' A semaphore is a synchro…' ← bold markers gone, indented ``` | token in the answer | in `T` | in the pane's answer region | |---|---|---| | `## ` (ATX heading) | yes | **no** — rendered as `⏺` | | `**` (bold markers) | yes | **no** — rendered to ANSI bold, then stripped by `-p` | | ` ```javascript ` (fence + language) | yes | **no** — fence and language tag both gone | | `- ` (list item) | yes | yes | *(A literal `**` does appear elsewhere in the pane — in the **prompt echo**, because the prompt asked for bold. Not in the answer.)* **`capture-pane -e` (keeping the ANSI) does not rescue it — the inverse is provably non-unique.** With `T` = ``"## Alpha\n\n**bravo**\n\n```javascript\nlet x=1;\n```"``: ``` ⏺\e[39m \e[1mAlpha\n\n\e[0m \e[1mbravo\n\n\e[0m \e[34mlet\e[39m x=\e[32m1\e[39m; ``` `## Alpha` → **SGR 1 (bold)**. `**bravo**` → **SGR 1 (bold)**. *Identical ANSI* — an H2 and a bold span are indistinguishable, never mind `**` vs `__`. The fence and its `javascript` tag are consumed by the syntax highlighter into colours; recovering the tag would mean inverting a highlighter, and `let x=1;` is valid in several languages. So `T.startsWith(paneText)` is **false** — raw and indent-stripped, on essentially every markdown answer. A proxy streaming pane text would be streaming **something the model did not say**. With `MessageDisplay` available there is no reason to go near it. ### (c) `--debug-file` — **dead: it logs stream *timing*, never stream *content*** Worth stating precisely, because a casual check misleads in **both** directions here. The default log level is `debug`, which **suppresses every `verbose` site**. Raise it and per-chunk lines *do* appear, spread across generation: ```bash CLAUDE_CODE_DEBUG_LOG_LEVEL=verbose claude --debug-file /tmp/d.log … ``` ``` 05:51:11.088 [VERBOSE] [shoji-engine] yield stream_event/- ← 16 of these, mid-turn, 05:51:11.537 [VERBOSE] [shoji-engine] yield stream_event/- over ~3.9 s of generation 05:51:15.192 [DEBUG] [shoji-engine] turn 1 end (usage in=575 out=255 api=6736ms stop=end_turn resultLen=857) ``` **But they carry no payload** — the format is `yield /`, a bare presence marker. Run with no category filter (i.e. all categories) at verbose level: `content_block_delta` = **0**, `text_delta` = **0**, `content_block_start` / `message_start` = **0**. The only byte-exact text in the log is the end-of-turn `Stop` hook payload (`"last_assistant_message":"## Title\n\n**alpha bravo charlie**"`) — transcript granularity. The log tells you **when** tokens arrive, never **what** they are. It is also ~2.7 MB per turn. ### Also checked, also not the answer | candidate | outcome | |---|---| | `--output-format stream-json` (the one interface that emits `text_delta`) | **requires `--print`/`-p`** → `cc_entrypoint=sdk-cli` → the **metered** credit pool, which is exactly what TUI mode exists to avoid. Reproduced live. | | `--input-format stream-json` | `Error: --input-format=stream-json requires output-format=stream-json` → same gate. | | `CLAUDE_CODE_INCLUDE_PARTIAL_MESSAGES=1` (undocumented) | No stream-json sink in interactive mode → no partials. Banner stayed `· Claude Max`. | | `sessionMirror` (undocumented) | Gated on `outputFormat === "stream-json"` → the `-p` family. | | `--sdk-url` (hidden) | Forces stream-json + non-interactive → `sdk-cli`. *(inferred from the minified bundle; not banner-tested)* | | `~/.claude/sessions/.json` | Registry metadata only (`{pid, sessionId, cwd, status, version, entrypoint:"cli", kind:"interactive"}`). No assistant text. *(Its `entrypoint:"cli"` incidentally confirms the TUI path stays on the subscription pool.)* | | `~/.claude/history.jsonl` | User prompts only; the answer text is absent. | | Asking the model to emit plain text (so the pane renders faithfully) | Would mean **mutating the caller's prompt** — a correctness violation for a proxy, and still not byte-faithful (wrapping + indent remain). Rejected. | --- ## Value: what streaming actually buys (read before building) Streaming is *possible*. Whether it is *worth it* depends on the consumer, and the honest answer is uncomfortable: - **Streaming never makes the answer arrive sooner. It moves the *first* byte, not the *last*.** The final token lands at the same wall-clock moment either way. - So a consumer that must have the **complete** answer before it can act — e.g. one parsing a structured JSON reply, **which is exactly the 知音 AI use case that motivated this entire investigation** — gains **nothing at all**. Only a **progressively-rendering** consumer (a chat UI) gains. And the number the backlog attached to this item was wrong: - The backlog's "~20 s" was inferred from an external 30–32 s report, **never measured through OCP**. Measured through a real OCP instance (TUI mode, `claude-sonnet-4-6`, ~1850-token prompt, n=5): **median 11.30 s** before [#156](https://github.com/dtzp555-max/ocp/pull/156), **9.55 s** after. - **Same-turn decomposition** (baseline row `i=5`): **11.563 s** wall through OCP vs `turn_duration: 7.319 s` of CLI-internal time on that same turn → **OCP's own overhead ≈ 4.2 s** (n=1, baseline `effort=high` config). *Caveats*: n=1; and `turn_duration` is the CLI's internal duration of an **OCP-driven** turn, not a separate "native" baseline. Do **not** subtract this `effort=high` 7.3 s from the `effort=low` 9.55 s median — a low-effort turn generates faster, so mixing them *understates* the overhead. - So OCP's own overhead is **single-digit seconds**, not ~20 s. The rest of any large number is the model generating a long answer — which streaming hides but does not shorten. **Recommendation**: build it — the contract is clean and the cost is small — but size the expectation honestly. It is a *perceived-latency* feature for progressively-rendering consumers, not a throughput win, and it does not move the **~6 s TTFT floor** ([`README.md`](README.md)) that rules TUI mode out for interactive-latency consumers regardless.