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ocp/docs/plans/2026-07-13-tui-latency/messagedisplay-deltas.jsonl
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taodengandClaude Fable 5 ddaea4df17 docs: REVERSE the streaming verdict — MessageDisplay hook makes it achievable
The previous commit on this branch concluded TUI streaming was impossible. That was
WRONG, and this corrects it before it could be merged.

The adversarial reviewer commissioned to refute the claim found, on a second pass while
verifying the fold-in, that its OWN first-pass hook enumeration had been truncated by a
400-char grep cap: it reported 21 hook events; the shipped 2.1.207 bundle has 30.
Event #30 is MessageDisplay.

Independently reproduced before acting on it (30 events confirmed via `strings` on the
binary; payload shape `hook_event_name:"MessageDisplay",turn_id,message_id,index,final,
delta`), then live-tested with a MessageDisplay command hook registered via --settings on
a PLAIN INTERACTIVE TUI spawn (no -p, no --bare), claude-sonnet-4-6, --effort low:

  banner: "Sonnet 4.6 with low effort · Claude Max"   ← subscription pool, verified

  7 fires, mid-turn, spread across generation:
    index=0 final=false  '## Mutex\n\n'
    index=1 final=false  'A **mutual exclusion lock** prevents concurrent access to a shar…'
    index=4 final=false  'let counter = 0;\n\nasync function increment() {\n  const release =…'
    index=6 final=true   '```'

  concat(deltas) === T (transcript-authoritative)  ->  TRUE  (579 == 579 bytes)
  T.startsWith(S) at EVERY step                    ->  TRUE  (prefix-stable)
  '## ' / '**' / '```javascript' present in deltas ->  raw markdown SOURCE, not rendered

This satisfies every invariant the previous version declared unobtainable: byte-faithful,
incremental, prefix-stable, no -p, subscription pool. Granularity is block-level (~5-7
chunks/answer), not token-level — which is all an SSE delta.content needs.

Backlog #2 REOPENS and should be built. Implementer caveat recorded: the hook's source
sets forceSyncExecution -> claude BLOCKS on it, so the hook must write and exit
immediately (FIFO/socket), never work inline. Only text blocks fire it (thinking excluded).
ALIGNMENT: consumes claude's OWN hook surface as emitted — forwarding, not inventing
(Class B / ADR 0007; no cli.js citation applies).

Everything still true is kept, and the dead ends are kept as dead ends (they document what
NOT to build): the pane is a rendered view whose source markers are irrecoverable
(capture-pane -e emits IDENTICAL SGR 1 for an H2 and a bold span — a provably non-unique
inverse); the transcript is event-granular; --debug-file carries timing but no payload;
--output-format stream-json requires -p (the metered pool). Also kept: the ~4s (n=1
same-turn) overhead correction, backlog #4's null result with its mechanistic single-user
reason, and the honest value framing — streaming moves the FIRST byte, not the last, so
the complete-answer consumer that motivated this work gains nothing from it.

The wrong conclusion and its refutation are both preserved in the doc. "We checked, it's
impossible" is the most expensive claim to get wrong: it closes a door nobody re-opens.

Docs-only. No code change, no version bump.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VqgWJcjxrjjL9L9SkpZyXR
2026-07-13 16:16:12 +10:00

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{"hook_event_name": "MessageDisplay", "index": 0, "final": false, "delta": "## Mutex\n\n"}
{"hook_event_name": "MessageDisplay", "index": 1, "final": false, "delta": "A **mutual exclusion lock** prevents concurrent access to a shared resource, ensuring only one thread runs the critical section at a time.\n\n"}
{"hook_event_name": "MessageDisplay", "index": 2, "final": false, "delta": "- Acquiring a locked mutex blocks the caller until the current holder releases it.\n"}
{"hook_event_name": "MessageDisplay", "index": 3, "final": false, "delta": "- Failing to release a mutex causes a deadlock, freezing all waiting threads.\n\n```javascript\nconst { Mutex } = require('async-mutex');\n\nconst mutex = new Mutex();\n"}
{"hook_event_name": "MessageDisplay", "index": 4, "final": false, "delta": "let counter = 0;\n\nasync function increment() {\n const release = await mutex.acquire();\n try {\n"}
{"hook_event_name": "MessageDisplay", "index": 5, "final": false, "delta": " counter++; // only one caller here at a time\n } finally {\n release();\n }\n}\n"}
{"hook_event_name": "MessageDisplay", "index": 6, "final": true, "delta": "```"}