natemoo-re · Oct 19, 2026 · Oct 9, 2026 · Oct 11, 2026 · Oct 11, 2026 · Oct 16, 2026
diff --git a/.changeset/sync-output-capability.md b/.changeset/sync-output-capability.md
@@ -0,0 +1,26 @@
+---
+"@bomb.sh/tty": minor
+---
+
+Adds synchronized output (DEC mode 2026) driven by the `sync-output` capability.
+
+When `RuntimeCapabilities.syncOutput` is true, every full-screen render is wrapped in Begin/End Synchronized Update (`CSI ? 2026 h` / `CSI ? 2026 l`) so the terminal presents the frame as one atomic repaint without tearing. The capability is raised only on positive evidence, resolved from three channels — a DECRPM 2026 probe reply (`CSI ? 2026 $ p`), an XTGETTCAP `Sync` reply (the terminal's own terminfo-declared synchronization sequence, value discarded in favor of the 2026 form), and detection-time seeding from the `Sync` extended capability in the parsed terminfo entry or the `TMUX` environment variable (tmux consumes mode 2026 from inner applications). Probe replies outrank detection-time evidence in both directions, so a DECRPM 0 or 4 reply turns the capability off even when detection seeded it. Frames render unwrapped when the capability is false or unknown. Zero-diff frames emit nothing whether or not wrapping is active, and line mode is never wrapped.
+
+#### Migration
+
+`Capabilities` gains a required `syncOutput` field. An object literal passed to `createTerm` as `terminfo.capabilities` must include it:
+
+```diff
+  terminfo: {
+    capabilities: {
+      colors: 256,
+      trueColor: false,
+      bce: true,
+      autoMargin: true,
+      xenl: true,
+      altScreen: true,
+      styledUnderline: false,
++     syncOutput: false,
+    },
+  }
+```
diff --git a/Makefile b/Makefile
@@ -14,6 +14,7 @@ EXPORTS = \
   -Wl,--export=__heap_base \
   -Wl,--export=clayterm_size \
   -Wl,--export=init \
+  -Wl,--export=set_capabilities \
   -Wl,--export=reduce \
   -Wl,--export=output \
   -Wl,--export=length \

diff --git a/specs/renderer-spec.md b/specs/renderer-spec.md
@@ -480,6 +480,50 @@ output, Kitty keyboard mode setup, and Kitty graphics emission are deferred to
 their respective follow-up PRs. The renderer continues to emit its existing
 hardcoded ANSI output until one of those specifications is adopted.
 
+### 7.9 Synchronized output (DEC mode 2026)
+
+When the `syncOutput` runtime capability is true, each full-screen frame
+(§8.2.1) is wrapped in Synchronized Output mode 2026 — Begin Synchronized Update
+(`CSI ? 2026 h`) before the frame's first emitted byte and End Synchronized
+Update (`CSI ? 2026 l`) after its last. The terminal buffers everything between
+the pair and presents it as one atomic repaint, so a frame never tears
+mid-update.
+
+**Capability evidence.** `syncOutput` is raised only on positive evidence. Two
+runtime channels feed the `sync-output` `CapabilityEvent` (input-spec §5,
+terminfo-spec §6.3): a DECRPM 2026 reply to the probe (`CSI ? 2026 $ p`) —
+values 1, 2, and 3 (set, reset, permanently set) report support, 0 and 4 report
+none — and an XTGETTCAP `Sync` reply, whose sequence value is discarded in favor
+of the mode 2026 form (terminfo-spec §6.3). Detection may also seed the field
+positively: static `Capabilities` carry `syncOutput`, raised by the `Sync`
+extended capability in the parsed terminfo entry and by the `TMUX` environment
+variable (terminfo-spec §7.1, §7.2). Evidence follows TINV-3 precedence: probe
+replies outrank detect-time seeding in both directions, so a DECRPM 0 or 4 reply
+turns the capability off even when detection seeded it. Without positive
+evidence the field stays false and no frame is wrapped.
+
+**Invalidation and timing.** The renderer reads the capability snapshot at each
+render transaction, so a capability flip takes effect on the next rendered
+frame. No invalidation signal exists, and none is required: a flip between
+frames changes only whether the next frame is wrapped.
+
+**Normative rules:**
+
+- When `syncOutput` is false, the renderer MUST NOT emit `?2026h` or `?2026l`.
+- When `syncOutput` is true, the renderer MUST wrap each full-screen frame in
+  the BSU/ESU pair. Line mode (§8.2.2) is never wrapped: inline output may be
+  interleaved with foreign content, and an unterminated synchronized block would
+  suppress it.
+- A full-screen render whose diff produces no cell changes and no caret
+  transition MUST emit zero bytes, even when `syncOutput` is true. The renderer
+  decides after the diff, so no-op frames carry neither the wrap prefix nor the
+  suffix.
+- Terminals that receive the sequences without supporting the mode ignore it by
+  design; the probe still refines behavior for terminals that report mode 0
+  or 4.
+
+This section discharges the synchronized-output deferral recorded in §7.8.
+
 ---
 
 ## 8. Public Rendering API

diff --git a/specs/terminfo-spec.md b/specs/terminfo-spec.md
@@ -180,18 +180,20 @@ interface Capabilities {
   readonly xenl: boolean;
   readonly altScreen: boolean;
   readonly styledUnderline: boolean;
+  readonly syncOutput: boolean;
 }
 ```
 
-| Field             | Source                                                              |
-| ----------------- | ------------------------------------------------------------------- |
-| `colors`          | `max_colors` from terminfo; 256 at baseline                         |
-| `trueColor`       | `RGB`/`Tc` extended caps; `COLORTERM` env evidence; probe overrides |
-| `bce`             | `bce` boolean cap                                                   |
-| `autoMargin`      | `am` boolean cap                                                    |
-| `xenl`            | `xenl` boolean cap                                                  |
-| `altScreen`       | `smcup` string present                                              |
-| `styledUnderline` | `Su` boolean or `Smulx` string                                      |
+| Field             | Source                                                                  |
+| ----------------- | ----------------------------------------------------------------------- |
+| `colors`          | `max_colors` from terminfo; 256 at baseline                             |
+| `trueColor`       | `RGB`/`Tc` extended caps; `COLORTERM` env evidence; probe overrides     |
+| `bce`             | `bce` boolean cap                                                       |
+| `autoMargin`      | `am` boolean cap                                                        |
+| `xenl`            | `xenl` boolean cap                                                      |
+| `altScreen`       | `smcup` string present                                                  |
+| `styledUnderline` | `Su` boolean or `Smulx` string                                          |
+| `syncOutput`      | `Sync` extended cap; `TMUX` env evidence (§7.2); probe overrides (§6.3) |
 
 Key sequences (`key_*` capabilities) are not stored here. The input parser reads
 them directly from the raw terminfo bytes in `TerminalInfo.keys` at
@@ -252,20 +254,20 @@ type CapabilityEvent =
   };
 ```
 
-Each variant maps to one probe query. The `key` identifies the capability; the
-`value` is the terminal's answer. Capability references per
+Each variant maps to at least one probe query. The `key` identifies the
+capability; the `value` is the terminal's answer. Capability references per
 [terminfo.dev](https://terminfo.dev):
 
-| `key`              | Query                | terminfo.dev slug                                                                               |
-| ------------------ | -------------------- | ----------------------------------------------------------------------------------------------- |
-| `foreground-color` | OSC 10               | [`osc-10-fg-color-query`](https://terminfo.dev/extensions/osc-10-fg-color-query)                |
-| `background-color` | OSC 11               | [`osc-11-bg-color-query`](https://terminfo.dev/extensions/osc-11-bg-color-query)                |
-| `cursor-color`     | OSC 12 or OSC 21     | [`osc-12-cursor-color`](https://terminfo.dev/extensions/osc-12-cursor-color)                    |
-| `colordepth`       | XTGETTCAP `RGB`/`Tc` | [`24-bit-truecolor`](https://terminfo.dev/extensions/24-bit-truecolor)                          |
-| `sync-output`      | DECRPM mode 2026     | [`decset-2026-synchronized-output`](https://terminfo.dev/modes/decset-2026-synchronized-output) |
-| `kitty-keyboard`   | `CSI ? u`            | [`kitty-keyboard-protocol`](https://terminfo.dev/extensions/kitty-keyboard-protocol)            |
-| `kitty-graphics`   | APC `_G…`            | [`kitty-graphics-protocol`](https://terminfo.dev/extensions/kitty-graphics-protocol)            |
-| `pointer-shape`    | OSC 22               | [`osc-22-pointer-shape`](https://terminfo.dev/extensions/osc-22-pointer-shape)                  |
+| `key`              | Query                              | terminfo.dev slug                                                                               |
+| ------------------ | ---------------------------------- | ----------------------------------------------------------------------------------------------- |
+| `foreground-color` | OSC 10                             | [`osc-10-fg-color-query`](https://terminfo.dev/extensions/osc-10-fg-color-query)                |
+| `background-color` | OSC 11                             | [`osc-11-bg-color-query`](https://terminfo.dev/extensions/osc-11-bg-color-query)                |
+| `cursor-color`     | OSC 12 or OSC 21                   | [`osc-12-cursor-color`](https://terminfo.dev/extensions/osc-12-cursor-color)                    |
+| `colordepth`       | XTGETTCAP `RGB`/`Tc`               | [`24-bit-truecolor`](https://terminfo.dev/extensions/24-bit-truecolor)                          |
+| `sync-output`      | DECRPM mode 2026; XTGETTCAP `Sync` | [`decset-2026-synchronized-output`](https://terminfo.dev/modes/decset-2026-synchronized-output) |
+| `kitty-keyboard`   | `CSI ? u`                          | [`kitty-keyboard-protocol`](https://terminfo.dev/extensions/kitty-keyboard-protocol)            |
+| `kitty-graphics`   | APC `_G…`                          | [`kitty-graphics-protocol`](https://terminfo.dev/extensions/kitty-graphics-protocol)            |
+| `pointer-shape`    | OSC 22                             | [`osc-22-pointer-shape`](https://terminfo.dev/extensions/osc-22-pointer-shape)                  |
 
 **Color values.** `Rgb` values in probe responses MUST be recognized in at least
 the `rgb:RR/GG/BB` (1–4 hex digits per channel) and `#`-hash forms.
@@ -278,6 +280,16 @@ D }` where `D` is derived from the static
 preserves TINV-3: the probe response takes precedence over static evidence,
 including in the negative direction.
 
+**XTGETTCAP `Sync`.** A reply carrying a value for `Sync` emits
+`{ key: "sync-output", value: true }`. An invalid-capability reply for `Sync`
+emits `{ key: "sync-output", value: false }`. The value carried by a positive
+reply is the terminal's own synchronization sequence — the iTerm2 DCS form
+(`DCS = 1 s` / `DCS = 2 s`) for some terminals, `CSI ? 2026 h`/`l` for others.
+The value is discarded in v1: only support is recorded, and the renderer emits
+the DEC mode 2026 form (renderer-spec §7.9). A terminal that advertises only the
+DCS form therefore receives 2026-form bytes it ignores; this bounded cost is
+accepted.
+
 **OSC 21.** An OSC 21 reply may supply any subset of `foreground-color`,
 `background-color`, and `cursor-color`. The parser emits one `CapabilityEvent`
 per field the reply carries. OSC 21 arriving alongside OSC 10/11/12 replies does
@@ -331,15 +343,24 @@ With no terminfo bytes, no environment evidence, and no probe responses,
 | `xenl`            | `true`  |
 | `altScreen`       | `true`  |
 | `styledUnderline` | `false` |
+| `syncOutput`      | `false` |
 
 Truecolor is not assumed at baseline. Per TINV-3, it requires positive evidence.
+The same holds for synchronized output: no frame is wrapped until a `Sync`
+extended capability, `TMUX` environment evidence, or a probe response raises it
+(§7.2, §6.3).
 
 ### 7.2 Environment evidence
 
 `detectTerminal()` applies environment evidence after parsing the terminfo
 entry:
 
 - `COLORTERM` equal to `truecolor` or `24bit` sets `trueColor: true`.
+- `TMUX` set to a non-empty value sets `syncOutput: true`. A running tmux
+  consumes DEC mode 2026 from applications running inside it and translates it
+  to the outer terminal's synchronization mechanism (the tmux `Sync` terminfo
+  capability and `sync` terminal feature), so its presence is positive evidence
+  even for tmux releases that do not themselves answer the DECRPM 2026 probe.
 
 Environment evidence outranks the terminfo entry and is outranked by probe
 responses (TINV-3).
@@ -368,7 +389,7 @@ observe only the resulting `Capabilities`.
 - Both storage formats MUST be supported: legacy (magic `0432`, 16-bit numbers)
   and extended number format (magic `01036`, 32-bit numbers).
 - The extended-capability string table MUST be parsed for the user-defined
-  capabilities `RGB`, `Tc`, `Su`, and `Smulx`.
+  capabilities `RGB`, `Tc`, `Su`, `Smulx`, and `Sync`.
 - All reads are bounds-checked against `len`. Out-of-range offsets, truncated
   sections, and odd-length string tables yield the parse failure path (TINV-2).
 - The maximum accepted size is 32 768 bytes, the extended ncurses format limit.
@@ -398,13 +419,14 @@ order:
 | 5  | Pointer shape       | `OSC 22 ; ?__current__ ST`                           | `pointer-shape`       |
 | 6  | Truecolor caps      | `DCS + q 524742 ; 5463 ST` (XTGETTCAP `RGB;Tc`)      | `colordepth`          |
 | 7  | Synchronized output | `CSI ? 2026 $ p` (DECRQM)                            | `sync-output`         |
-| 8  | Kitty keyboard      | `CSI ? u`                                            | `kitty-keyboard`      |
-| 9  | Kitty graphics      | `APC _G i=31,s=1,v=1,a=q,t=d,f=24 ; AAAA ST`         | `kitty-graphics`      |
-| 10 | **Fence:** DA1      | `CSI c`                                              | (internal only)       |
+| 8  | Sync terminfo cap   | `DCS + q 53796e63 ST` (XTGETTCAP `Sync`)             | `sync-output`         |
+| 9  | Kitty keyboard      | `CSI ? u`                                            | `kitty-keyboard`      |
+| 10 | Kitty graphics      | `APC _G i=31,s=1,v=1,a=q,t=d,f=24 ; AAAA ST`         | `kitty-graphics`      |
+| 11 | **Fence:** DA1      | `CSI c`                                              | (internal only)       |
 
 Terminals answer queries in order and ignore queries they do not understand. DA1
 is answered by every terminal, so its response marks the probe complete. Any of
-queries 1–9 not yet answered when the DA1 reply arrives will never be answered,
+queries 1–10 not yet answered when the DA1 reply arrives will never be answered,
 and the renderer keeps their current values.
 
 The batch is safe to emit unconditionally: every query is either answered or

diff --git a/src/clayterm.c b/src/clayterm.c
@@ -87,6 +87,9 @@ struct Clayterm {
   Clay_ErrorData errors[MAX_ERRORS];
   int error_count;
   int animating_count;
+  /* CAP_* bits pushed by the host via set_capabilities(); init() zeroes
+   * them, so the host re-pushes after every re-init */
+  uint32_t caps;
   /* Caret state for hardware-cursor management. The renderer records the
    * first text node carrying a caret declaration per frame, precomputes the
    * caret's byte offset into that node's content, then places the cell as
@@ -116,6 +119,12 @@ struct Clayterm {
  * to 4 (base SMP char) + 8×4 (combining marks) = 36 bytes of cluster text. */
 #define OUT_BYTES_PER_CELL 128
 
+/* Fixed per-frame slack on top of the per-cell budget, for output that wraps
+ * the whole frame rather than scaling with cell count — the BSU/ESU
+ * synchronized-output pair. Keeps the wrap from being dropped on tiny grids
+ * where the per-cell budget alone is tight. */
+#define OUT_FRAME_OVERHEAD 32
+
 /* ── Cell buffer ops ──────────────────────────────────────────────── */
 
 static Cell *cell_at(struct Clayterm *ct, Cell *buf, int x, int y) {
@@ -261,6 +270,19 @@ static void present_cups(struct Clayterm *ct, int row) {
   ct->lastx = -1;
   ct->lasty = -1;
 
+  /* Synchronized Output (DEC mode 2026): the terminal buffers everything
+   * between BSU and ESU and presents it in one atomic repaint, so a frame
+   * never tears mid-update. Unsupported terminals ignore the private mode;
+   * whether we emit the wrap at all is decided by the sync-output
+   * capability. */
+  int sync = ct->caps & RENDER_CAP_SYNC_OUTPUT;
+  int start = ct->out.length;
+  if (sync)
+    buf_str(&ct->out, "\x1b[?2026h");
+  /* Body start: the no-op check below compares against this so the BSU
+   * itself never counts as frame content. */
+  int body = ct->out.length;
+
   for (int y = 0; y < ct->h; y++) {
     for (int x = 0; x < ct->w;) {
       Cell *back = cell_at(ct, ct->back, x, y);
@@ -310,6 +332,14 @@ static void present_cups(struct Clayterm *ct, int row) {
     buf_str(&ct->out, "\x1b[?25l");
   }
   ct->had_caret_last_frame = ct->has_caret;
+
+  if (ct->out.length == body) {
+    /* Nothing was emitted — no cell changes and no caret transition.
+     * Rewind the BSU so a no-op frame still produces zero bytes. */
+    ct->out.length = start;
+  } else if (sync) {
+    buf_str(&ct->out, "\x1b[?2026l"); /* ESU: end synchronized update */
+  }
 }
 
 /**
@@ -624,7 +654,7 @@ int clayterm_size(int w, int h) {
   set_clay_capacity(w, h);
   int cell_count = w * h;
   int cell_bytes = cell_count * (int)sizeof(Cell);
-  int out_bytes = cell_count * OUT_BYTES_PER_CELL;
+  int out_bytes = cell_count * OUT_BYTES_PER_CELL + OUT_FRAME_OVERHEAD;
   int clay_bytes = (int)Clay_MinMemorySize();
   return align8((int)sizeof(struct Clayterm)) + align8(cell_bytes) /* front */
          + align8(cell_bytes)                                      /* back */
@@ -686,7 +716,7 @@ struct Clayterm *init(void *mem, int w, int h) {
   struct Clayterm *ct = (struct Clayterm *)mem;
   int cell_count = w * h;
   int cell_bytes = align8(cell_count * (int)sizeof(Cell));
-  int out_bytes = align8(cell_count * OUT_BYTES_PER_CELL);
+  int out_bytes = align8(cell_count * OUT_BYTES_PER_CELL + OUT_FRAME_OVERHEAD);
   char *base = (char *)mem + align8((int)sizeof(struct Clayterm));
 
   char *clay_mem = base + cell_bytes * 2 + out_bytes;
@@ -701,7 +731,8 @@ struct Clayterm *init(void *mem, int w, int h) {
       .h = h,
       .front = (Cell *)base,
       .back = (Cell *)(base + cell_bytes),
-      .out = {base + cell_bytes * 2, 0, cell_count * OUT_BYTES_PER_CELL},
+      .out = {base + cell_bytes * 2, 0,
+              cell_count * OUT_BYTES_PER_CELL + OUT_FRAME_OVERHEAD},
       .lastfg = 0xffffffff,
       .lastbg = 0xffffffff,
       .lastx = -1,
@@ -716,6 +747,8 @@ struct Clayterm *init(void *mem, int w, int h) {
   return ct;
 }
 
+void set_capabilities(struct Clayterm *ct, uint32_t caps) { ct->caps = caps; }
+
 void reduce(struct Clayterm *ct, uint32_t *buf, int len, int mode, int row,
             float deltaTime) {
   int i = 0;

diff --git a/src/clayterm.h b/src/clayterm.h
@@ -9,9 +9,13 @@
 
 struct Clayterm;
 
+/* Capability bits for set_capabilities(). */
+#define RENDER_CAP_SYNC_OUTPUT (1u << 0)
+
 /* WASM exports */
 int clayterm_size(int w, int h);
 struct Clayterm *init(void *mem, int w, int h);
+void set_capabilities(struct Clayterm *ct, uint32_t caps);
 void reduce(struct Clayterm *ct, uint32_t *buf, int len, int mode, int row,
             float deltaTime);
 char *output(struct Clayterm *ct);