diff --git a/.changeset/spec-dts-heap-ceiling-fits-container.md b/.changeset/spec-dts-heap-ceiling-fits-container.md
new file mode 100644
index 0000000000..5465ac7136
--- /dev/null
+++ b/.changeset/spec-dts-heap-ceiling-fits-container.md
@@ -0,0 +1,37 @@
+---
+"@objectstack/spec": patch
+---
+
+build(spec): bound the DTS pass's heap ceiling to something the build container actually has (#12677)
+
+The DTS pass ran under `NODE_OPTIONS="--max-old-space-size=12288"`. A heap
+ceiling is a promise to V8 that the memory exists: below it, V8 defers major GCs
+and lets the resident set grow. A ceiling **above** the container's memory does
+not permit a bigger build — it converts a recoverable JS heap error into a
+kernel SIGKILL, because the process meets the container limit long before V8
+considers the ceiling reached, and `exit 137` carries no diagnostic.
+
+That is what took the docs site down: every `objectstack.ai` production deploy
+since 2026-08-25 died with `@objectstack/spec:build` exit 137. Vercel builds the
+docs app with `turbo run build --filter=@objectstack/docs` inside one
+fixed-memory container, and this package is that app's only workspace
+dependency — so this pass met the container limit alone.
+
+The ceiling is now `6144`. Measured on this package's DTS pass inside a cgroup
+capped at 8192 MB, as peak anonymous RSS of the whole process tree:
+
+| ceiling | result | peak RSS | wall |
+|---|---|---|---|
+| 12288 (before) | ok, ~0.9 GB spare | 7290 MB | 132s |
+| 6144 (after) | ok, ~2.3 GB spare | 5794 MB | 134s |
+| 5120 | ok | 5328 MB | 148s |
+| 4096 | `ERR_WORKER_OUT_OF_MEMORY` | — | 113s |
+
+No output change: every completing ceiling emitted a byte-identical declaration
+tree (122 files, compared as one sha256 over all of them). The number buys
+headroom and costs GC time, not build time.
+
+Should the live type graph ever outgrow `6144`, the pass now fails **loud**
+with `ERR_WORKER_OUT_OF_MEMORY` instead of being killed silently. Raising the
+number past what the build container has would trade that diagnostic back for
+`exit 137`.
diff --git a/packages/spec/package.json b/packages/spec/package.json
index 7e5cc35e5f..29d964fde7 100644
--- a/packages/spec/package.json
+++ b/packages/spec/package.json
@@ -243,7 +243,7 @@
     "spec-changes.json"
   ],
   "scripts": {
-    "build": "pnpm gen:schema && pnpm gen:openapi && tsup && if [ -z \"$OS_SKIP_DTS\" ]; then NODE_OPTIONS=\"--max-old-space-size=12288\" BUILD_DTS=true tsup; fi && node ../../scripts/check-dts-emitted.mjs && node ../../scripts/check-dev-prereqs.mjs --stamp",
+    "build": "pnpm gen:schema && pnpm gen:openapi && tsup && if [ -z \"$OS_SKIP_DTS\" ]; then NODE_OPTIONS=\"--max-old-space-size=6144\" BUILD_DTS=true tsup; fi && node ../../scripts/check-dts-emitted.mjs && node ../../scripts/check-dev-prereqs.mjs --stamp",
     "dev": "tsc --watch",
     "clean": "rm -rf dist",
     "gen:schema": "OS_EAGER_SCHEMAS=1 tsx scripts/build-schemas.ts",
diff --git a/packages/spec/tsup.config.ts b/packages/spec/tsup.config.ts
index cccb2293c2..031bda72de 100644
--- a/packages/spec/tsup.config.ts
+++ b/packages/spec/tsup.config.ts
@@ -138,7 +138,48 @@ const swapServerOnlyGrammarArm: Plugin = {
   },
 };
 
-// Generate DTS separately to avoid memory issues
+/**
+ * Generate DTS separately to avoid memory issues — this pass is by far the
+ * heaviest thing this package's build does, and `package.json` runs it under an
+ * explicit `--max-old-space-size` for a reason worth stating where the next
+ * author will look.
+ *
+ * ⚠️ THE HEAP CEILING MUST FIT THE SMALLEST CONTAINER THIS BUILD RUNS IN.
+ * `--max-old-space-size` is a promise to V8 that the memory is there: below it,
+ * V8 defers major GCs and lets the resident set grow. A ceiling ABOVE the
+ * container's memory therefore does not "allow a big build", it converts a
+ * recoverable JS heap error into a kernel SIGKILL — the process is killed at
+ * the container limit long before V8 ever considers the ceiling reached, and
+ * exit 137 carries no diagnostic at all.
+ *
+ * That is not hypothetical: at a 12288 ceiling this pass was killed on every
+ * Vercel docs deploy for two days (`@objectstack/spec:build` exit 137), because
+ * the docs site is built by `turbo run build --filter=@objectstack/docs` inside
+ * one fixed-memory build container and this package is its only workspace
+ * dependency — so this pass meets the container's limit ALONE, with no
+ * parallelism to cap.
+ *
+ * Measured on this package's DTS pass, inside a cgroup capped at 8192 MB
+ * (peak anonymous RSS of the whole process tree):
+ *
+ *     ceiling   result                        peak RSS   wall
+ *     12288     ok, but only ~0.9 GB spare     7290 MB    132s
+ *      6144     ok                             5794 MB    134s
+ *      5120     ok                             5328 MB    148s
+ *      4096     ERR_WORKER_OUT_OF_MEMORY          —       113s
+ *
+ * 6144 is chosen as the largest ceiling whose WORST case still fits: V8 cannot
+ * exceed it, and the pass's non-heap overhead measured ~250 MB, so the bound is
+ * ~6.4 GB inside an 8 GB container. Every completing ceiling emitted a
+ * byte-identical declaration tree (122 files, one sha256 over all of them), so
+ * this number buys headroom and costs nothing but GC time.
+ *
+ * If this pass starts failing with `ERR_WORKER_OUT_OF_MEMORY`, the live type
+ * graph has outgrown 6144 — that is a loud, actionable failure and the point of
+ * the ceiling. ⛔ Do not "fix" it by raising the number past what the build
+ * container has; that trades this error back for the silent exit 137. Shrink
+ * the graph, or split the pass across entries.
+ */
 const isDts = process.env.BUILD_DTS === 'true';
 
 const mainConfig: Options = {