ocp/docs/plans/2026-07-13-tui-latency/streaming-spike.md

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 § "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:

{ "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 — 5–7 chunks for a ~600-byte answer, not one per token. Plenty for SSE (delta.content has no minimum size), but do not promise token-by-token output.
• ⚠️ 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.
• Only text blocks fire it (content.map(c => c.type === "text" ? c.text : "")) — thinking blocks are excluded, which is what OCP wants.
• 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

# 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(<transcript>.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:

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 <type>/<subtype>, 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/<pid>.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, 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) that rules TUI mode out for interactive-latency consumers regardless.