@@ -5,6 +5,12 @@
 Writes to $GITHUB_OUTPUT when set, otherwise stdout, so it can be run locally
 to see exactly what a workflow run would get. tomllib is stdlib from 3.11, and
 GitHub runners are newer than that, so this needs no dependency.
+
+The emitted matrix holds only the flavors this Python can actually build: the
+right architecture, and a pylock file present in the cycle directory. A flavor
+the cycle declares but has no lockfile for -- `whl` today -- costs nothing, so
+it can stay declared until it comes back. Paths are relative to the working
+directory, which in CI is the checkout root.
 """
 import json
 import os
@@ -13,6 +19,27 @@
 from pathlib import Path
 
 
+def buildable_flavors(cfg: dict, requested: str, ver2: str) -> list[dict]:
+    """Flavors with the right architecture and a lockfile on disk.
+
+    Same two conditions the build job used to re-test in PowerShell, one per
+    matrix leg. Deciding here instead means a leg with nothing to do never
+    starts a runner, and a cycle with no lockfiles at all fails loudly rather
+    than going green having built nothing.
+    """
+    arch = "64F" if requested.endswith("F") else "64"
+    cycle_dir = Path(cfg["cycle_dir"])
+    level = cfg.get("release_level", "")
+    tag = ver2.replace(".", "_")
+    kept = []
+    for flavor in cfg["flavors"]:
+        if str(flavor.get("WINPYARCHDET", "")) != arch:
+            continue
+        if (cycle_dir / f"pylock.64-{tag}{flavor['name']}{level}.toml").is_file():
+            kept.append(flavor)
+    return kept
+
+
 def build_config(cfg: dict, requested: str) -> dict:
     pythons = cfg["pythons"]
     if requested not in pythons:
@@ -27,6 +54,14 @@ def build_config(cfg: dict, requested: str) -> dict:
     if short not in entry["src"]:
         raise SystemExit(f"{requested}: '{short}' not found in src {entry['src']}")
 
+    flavors = buildable_flavors(cfg, requested, entry["ver2"])
+    if not flavors:
+        raise SystemExit(
+            f"{requested}: no pylock.64-{entry['ver2'].replace('.', '_')}<flavor>"
+            f"{cfg.get('release_level', '')}.toml under {cfg['cycle_dir']}; "
+            "commit the lockfiles for this cycle before dispatching"
+        )
+
     return {
         "ver2": entry["ver2"],
         "src": entry["src"],
@@ -36,7 +71,7 @@ def build_config(cfg: dict, requested: str) -> dict:
         "pandoc_source": cfg["pandoc"]["source"],
         "pandoc_sha256": cfg["pandoc"]["sha256"],
         # consumed by the build job as strategy.matrix via fromJSON
-        "matrix": json.dumps({"flavor": cfg["flavors"]}, separators=(",", ":")),
+        "matrix": json.dumps({"flavor": flavors}, separators=(",", ":")),
     }
 
 
 
@@ -10,7 +10,7 @@ on:
       cycle:
         description: 'Build cycle (file name in cycles/, without .toml)'
         required: true
-        default: '2026_03'
+        default: '2026_04'
         type: string
       python_versionf:
         description: 'Python version to build; must exist in that cycle file'
 
@@ -0,0 +1,72 @@
+# WinPython 2026-04 build cycle.
+#
+# Everything that changes from one cycle to the next lives here, so a new cycle
+# is a new file rather than a copy of the workflow.
+
+cycle_dir     = "winpython/portable/cycle_2026_04"
+release_level = ""
+
+[pandoc]
+source = "https://github.com/jgm/pandoc/releases/download/3.1.9/pandoc-3.1.9-windows-x86_64.zip"
+sha256 = "11eb6dbe5286c9e5edb0cca4412e7d99ec6578ec04158b0b7fe11f7fd96688e5"
+
+# The python-build-standalone tarball for each version offered by the workflow.
+# ver2 is the WinPython 4-part version; its first 3 parts must appear in src.
+
+[pythons."3.13"]
+ver2 = "3.13.15.1"
+src  = "https://github.com/astral-sh/python-build-standalone/releases/download/20260825/cpython-3.13.15+20260825-x86_64-pc-windows-msvc-install_only_stripped.tar.gz"
+sha  = "c1dc1e267f2a81493ce6e94837263f648f1eb6d0df73a1492469c1fed025ce8f"
+
+[pythons."3.14"]
+ver2 = "3.14.7.1"
+src  = "https://github.com/astral-sh/python-build-standalone/releases/download/20260825/cpython-3.14.7+20260825-x86_64-pc-windows-msvc-install_only_stripped.tar.gz"
+sha  = "dec3faa3445d9efc4979e6c74a839be0d1039877367db32ee9c5f6bc91fae3c8"
+
+[pythons."3.14F"]
+ver2 = "3.14.7.1"
+src  = "https://github.com/astral-sh/python-build-standalone/releases/download/20260825/cpython-3.14.7+20260825-x86_64-pc-windows-msvc-freethreaded-install_only_stripped.tar.gz"
+sha  = "485cf42bdd3e49389e754a9cd1dd65e974f9934c883c478f752024a674fc1056"
+
+[pythons."3.15"]
+ver2 = "3.15.0.5"
+src  = "https://github.com/astral-sh/python-build-standalone/releases/download/20260825/cpython-3.15.0rc1+20260825-x86_64-pc-windows-msvc-install_only_stripped.tar.gz"
+sha  = "3c8fa74403a891aeb6e6116154882ef17ad891b6f12aa52f9dda6d29400bedd6"
+
+[pythons."3.15F"]
+ver2 = "3.15.0.5"
+src  = "https://github.com/astral-sh/python-build-standalone/releases/download/20260825/cpython-3.15.0rc1+20260825-x86_64-pc-windows-msvc-freethreaded-install_only_stripped.tar.gz"
+sha  = "36fd8350ebaf8a577f311099a6683870d0d83f23085b7087159dd53ee03e28cf"
+
+# Flavors become the job matrix. 2026-02 renamed "free" to "dotf", so this
+# varies per cycle too and belongs in the cycle file.
+
+[[flavors]]
+name         = "dot"
+PANDOC       = "0"
+WINPYARCHDET = "64"
+formats      = { zip = true, "7z" = false, exe = true }
+
+[[flavors]]
+name         = "slim"
+PANDOC       = "1"
+WINPYARCHDET = "64"
+formats      = { zip = false, "7z" = true, exe = true }
+
+[[flavors]]
+name         = "whl"
+PANDOC       = "0"
+WINPYARCHDET = "64"
+formats      = { zip = false, "7z" = true, exe = false }
+
+[[flavors]]
+name         = "dotf"
+PANDOC       = "0"
+WINPYARCHDET = "64F"
+formats      = { zip = true, "7z" = false, exe = true }
+
+[[flavors]]
+name         = "slimf"
+PANDOC       = "1"
+WINPYARCHDET = "64F"
+formats      = { zip = false, "7z" = true, exe = true }