gHashTag · Oct 18, 2026 · Oct 18, 2026 · Oct 18, 2026 · Oct 18, 2026 · Oct 18, 2026
@@ -82,6 +82,50 @@ grep `specs/` for every name it would import. If any of them collides, put the n
 spec that owns those names, under a prefix such as `beat_`. The beat clock (t27#10596, PR #10598)
 lives in `lesson_reel.t27` for this reason.
 
+**Cutting video at a ms time: land on a frame, then measure the runs (t27#10603, PR #10605).**
+
+ffmpeg `xfade` starts on the first frame whose time is at or after `offset`. This is how to get the cut you planned:
+
+- Floor the cut to a base frame: `f = (c - half) * fps / 1000`.
+- Print the offset as `trunc(f * 1000 / fps)` ms. Rounding up misses frame f on 2 frames in 3.
+- Put every input through `fps=30,settb=AVTB` so the offset is not rounded again.
+- Chained `xfade` offsets are absolute reel times when each clip starts at its own frame 0.
+
+To check a graph without looking at pictures:
+
+1. Feed it solid `color=` inputs at `s=2x2`.
+2. Write `-f rawvideo -pix_fmt rgb24`.
+3. Count the runs with `od -An -tu1 -w12 -v | cut -c1-12 | uniq -c`.
+
+The runs are the scene lengths and must equal the spec's frame counts. Expect one extra frame when the offset falls on a whole ms.
+
+Lesson main functions read `std.c.getenv`, so build them on the lab with `zig build-exe -lc`.
+
+**Adding a mode to a graph printer without moving the old ones (t27#10609, PR #10610).**
+
+- Pass the old callers a flag (`graph(..., false)`) instead of copying the printer.
+- Give the input numbers a function with a test that every input is used exactly once, in each layout.
+- Prove the old output did not move: on the lab, build the old spec (from `git show <ref>:path`) and the new one, run both on a grid of lessons, languages and voices, and `cmp -s` every graph. PR #10610 compared 96 graphs and 0 differed.
+- For a large frame, render lossless (`-c:v libx264 -qp 0`) and crop a 2x2 pixel outside every text box (`crop=2:2:x:y,format=rgb24`). The solid runs must still equal the spec's frame counts.
+
+Traps from this tick:
+
+- An untyped array literal in a test (`var a = [_]usize{...}`) becomes a Zig tuple, which cannot be indexed at runtime ("tuple field index must be comptime-known"). Declare `var a : [8]usize = undefined;` and fill it in a loop.
+- `fs-lab.sh "echo $b64a ... ; echo $b64b ..."` with two large files exits 0 and writes nothing. Upload one file per call and compare md5 on both sides.
+- zsh does not word-split `set -- $p`. Write one call per file.
+
+**Keeping overlays off a crossfade (t27#10614, PR #10627).**
+
+- Work in whole frames, not seconds. Give each scene a function for the frames it owns outside its crossfades, clip each overlay to it, and print exactly those frames as `between(n,lo,hi)` terms joined with `+`. A test checks that the printed frames equal the frames `between(t,a,b)` showed before.
+- Drop a piece that has no room; never squeeze it. Name the cost in a test: count what is dropped, assert that each drop is on a crossfade, and assert that every phrase keeps at least one caption.
+- To check the pixels, render lossless from solid-colour clips. Crop a full-width band 40 rows tall over the text, split the raw file per frame, and count the distinct colours per frame with `od -An -tu1 -w3 -v | sort -u | wc -l`. Crossfade frames must show 1. The frames with text are the control: they must be many.
+
+Traps from this tick:
+
+- A minimum-length rule silently drops spoken captions. A short chunk that straddles a crossfade leaves two pieces under the minimum, and the voice speaks words that are never shown. A grid test that says "no overlay on a crossfade" stays green while this happens. Only a count of what was dropped finds it.
+- Lab scripts run under `sh`, which is dash: `$((10#$x))` fails with "expecting EOF". Strip leading zeros with `sed 's/^0*//'` and default with `${i:-0}`.
+- The Mac hook reads the whole Bash command. A heredoc PR body that names the report command gets refused. Write the body with the Write tool and pass `--body-file`.
+
 ## Show a spec (the core: exporter for what it reads, by hand for the rest)
 
 1. Load the spec: `mcp__t27__t27_spec` with `path` and `source: true`, or the file from a

@@ -0,0 +1,28 @@
+{
+  "built_by": "t27c-bootstrap@0.5.2+9448ab4be",
+  "closure": [],
+  "config": "gen=c,rust,verilog,zig;tests=zig",
+  "gen_hash_c": "sha256:5642bc71a3f58fb401c34d2e9260f2190b2335994b58272bfd47bdb2e50cca07",
+  "gen_hash_rust": "sha256:706879bc5d6542378435b0982dbb44e52cc1b9b85b34d7f06149ec7668b5fdd7",
+  "gen_hash_verilog": "sha256:596ced743ed9651f967ac93bae97d97f44b882ea6ed659d4ebc6ec67a16829b8",
+  "gen_hash_zig": "sha256:24befaf4ec44f81b268d7a1f58e3e7312c16da94c50b0442ac76e23e5ea1440a",
+  "module": "skill_t27_reel_beat",
+  "ring": 12,
+  "seal_schema": 2,
+  "sealed_at": "2026-10-18T19:16:57Z",
+  "sealed_by": "t27c-bootstrap@0.5.2",
+  "spec_hash": "sha256:b609e2d632860b22d61a44f12e3d8e3bf11663d080dada9cec88c19ac9e5becd",
+  "spec_path": "specs/skills/t27-reel-beat.t27",
+  "tests": {
+    "failed": 0,
+    "failing": [],
+    "forced": false,
+    "passed": 1,
+    "total": 1,
+    "vacuous": 0
+  },
+  "toolchain": {
+    "test_runner": "sha256:8512b80da533b79289110fca4a65f3336abd7a059ca4b72ea595d5e33ff8ad10",
+    "zig": "0.16.0"
+  }
+}
@@ -31,27 +31,27 @@
     }
   ],
   "config": "gen=c,rust,verilog,zig;tests=zig",
-  "gen_hash_c": "sha256:6a381e722c19647a8f05c723633503d1d931b143538c6183f8832fec577ebeb7",
-  "gen_hash_rust": "sha256:bf7b0432da8ec2e1bcf0f139aa271e9df74986d67c81dc1bb0bcacd19357d1db",
-  "gen_hash_verilog": "sha256:836331bc2320ec6d80ce1580e30f1d3a9cc9fbd89325995f141c090282a68ec7",
-  "gen_hash_zig": "sha256:50a202697595b03abac33b3707f25a239343ae7aa2256b0c43fec31b950cf61b",
+  "gen_hash_c": "sha256:30f4f017bc4dd4c32c1ec195d83f827f10e9d260f30056ebd6f444cf034ca3ea",
+  "gen_hash_rust": "sha256:cd6858c82060106802b3cced271e84811d8ee02b8c4f2ed18ba75cf8068861eb",
+  "gen_hash_verilog": "sha256:d9d8bbee11770df310cf863654c914b318ea03940780b5a9e116cec3f4e33991",
+  "gen_hash_zig": "sha256:f5762a5368a29d6b2978d7ea365deed7c9c7a0d4994814a3d6586c039fdcbf25",
   "module": "lesson_reel",
   "ring": 12,
   "seal_schema": 2,
-  "sealed_at": "2026-10-18T18:33:33Z",
+  "sealed_at": "2026-10-18T19:36:34Z",
   "sealed_by": "t27c-bootstrap@0.5.2",
-  "spec_hash": "sha256:87165b892aaaba0647fe1657669524eca65c31547f11f4e4994abb830c673b0e",
+  "spec_hash": "sha256:b1ca187bbf04f6595be7dbf25b6a656286b07a86c50def9a0605c9ec56f017ec",
   "spec_path": "specs/t27f/lesson_reel.t27",
   "tests": {
     "failed": 0,
     "failing": [],
     "forced": false,
-    "passed": 13,
-    "total": 13,
+    "passed": 25,
+    "total": 25,
     "vacuous": 0
   },
   "toolchain": {
-    "test_runner": "sha256:1982ae948e24d00c5d221ccec12dfce1eb01204b53e37b0f857f93b16d64629d",
+    "test_runner": "sha256:8512b80da533b79289110fca4a65f3336abd7a059ca4b72ea595d5e33ff8ad10",
     "zig": "0.16.0"
   }
 }
@@ -1407,6 +1407,7 @@
   {"path": "specs/skills/t27-measure-corpus.t27", "t27b": "pass"},
   {"path": "specs/skills/t27-phi-loop.t27", "t27b": "pass"},
   {"path": "specs/skills/t27-providers-steward.t27", "t27b": "pass"},
+  {"path": "specs/skills/t27-reel-beat.t27", "t27b": "pass"},
   {"path": "specs/skills/t27-self-host.t27", "t27b": "pass"},
   {"path": "specs/skills/t27-t27f.t27", "t27b": "pass"},
   {"path": "specs/skills/t27-tri-pipeline.t27", "t27b": "pass"},

@@ -0,0 +1,48 @@
+// SPDX-License-Identifier: Apache-2.0
+; specs/skills/t27-reel-beat.t27 -- skill t27/reel-beat
+; Source of truth for the skill card on t27.ai (#/skills?skill=t27/reel-beat).
+; How TRI CLAW cuts a lesson reel on the beat: LR_MODE=4 crossfades the open, main and close
+; scene clips at the beat clock's cuts, on whole frames, and LR_MODE=5 lays mode 3's captions,
+; cards and address over that video. Mode 5's inputs are 0..2 the scene clips, 3..5 the
+; phrases' wavs and 6 the music. The law lives once, in specs/t27f/lesson_reel.t27 (beat_*
+; functions and their tests); this card points at it and copies none. The scene clips are
+; inputs: lesson_reel does not read the content factory's scene plan yet. There is no
+; SKILL.md behind it yet, so the card is spec-only (see "Witness labels" in
+; specs/skills/README.md). Issues gHashTag/t27#10603 and
+; gHashTag/t27#10609, epic gHashTag/t27#10560.
+; The JSON the site serves is generated from this file by the real compiler;
+; nothing below is parsed with a regex.
+; phi^2 + 1/phi^2 = 3 | TRINITY
+
+module skill_t27_reel_beat;
+
+pub const KIND : str = "skill";
+pub const ID : str = "t27/reel-beat";
+pub const NAME : str = "Reel Beat";
+pub const REPO : str = "t27";
+; No SKILL.md exists for this card.
+pub const SOURCE : str = "";
+pub const SUMMARY_EN : str = "Cut a 15 second lesson reel on the beat clock: crossfade the open, main and close scenes at whole-frame cuts that never fall inside a spoken phrase, lay the captions and address on top, and refuse a voice that does not fit";
+pub const COMMAND : str = "/reel-beat";
+; Spec this skill stands on: the beat clock and the graph printer.
+pub const SPECS : [1]str = ["specs/t27f/lesson_reel.t27"];
+pub const TAGS : [4]str = ["social-visual", "reel", "beat", "t27f"];
+pub const ENABLED : bool = true;
+; Default budget; no SKILL.md declares a timeout.
+pub const TIMEOUT_MIN : u16 = 30;
+
+test basic_constants {
+    assert KIND == "skill";
+    assert ID == "t27/reel-beat";
+    assert NAME == "Reel Beat";
+    assert REPO == "t27";
+    assert SOURCE == "";
+    assert COMMAND == "/reel-beat";
+    assert ENABLED == true;
+    assert TIMEOUT_MIN == 30;
+    assert TAGS.len == 4;
+    assert TAGS[0] == "social-visual";
+    assert TAGS[2] == "beat";
+    assert SPECS.len == 1;
+    assert SPECS[0] == "specs/t27f/lesson_reel.t27";
+}