diff --git a/.github/scripts/check_ci_results.py b/.github/scripts/check_ci_results.py
new file mode 100644
index 000000000..36d1eed3b
--- /dev/null
+++ b/.github/scripts/check_ci_results.py
@@ -0,0 +1,34 @@
+#!/usr/bin/env python3
+
+"""Fail a terminal CI job unless every serialized dependency succeeded.
+
+Parent workflows pass GitHub's `toJSON(needs)` object through the NEEDS
+environment variable. Treat skipped and cancelled dependencies as failures too:
+for a required fan-in job, only an explicit success is safe to accept.
+"""
+
+import json
+import os
+
+
+def main() -> None:
+    # Keep result policy in one script so blocking-ci and postmerge-ci cannot
+    # drift in how they interpret dependency conclusions.
+    needs = json.loads(os.environ["NEEDS"])
+    failures = sorted(
+        (name, dependency["result"])
+        for name, dependency in needs.items()
+        if dependency["result"] != "success"
+    )
+
+    if failures:
+        print("CI dependencies did not succeed:")
+        for name, result in failures:
+            print(f"{name}: {result}")
+        raise SystemExit(1)
+
+    print("All CI dependencies succeeded.")
+
+
+if __name__ == "__main__":
+    main()
diff --git a/.github/scripts/v8_canary_changes.py b/.github/scripts/v8_canary_changes.py
index 6d9693c60..0acc4ec3f 100644
--- a/.github/scripts/v8_canary_changes.py
+++ b/.github/scripts/v8_canary_changes.py
@@ -1,12 +1,44 @@
 #!/usr/bin/env python3
 
+"""Decide which V8 canary work is needed for a commit range.
+
+The workflow deliberately has no trigger-level path filters because it is both
+directly triggered for pull requests and called by postmerge-ci. Keeping the
+patterns here gives those entrypoints one source of truth; unrelated events
+still run metadata but skip the expensive build matrices.
+"""
+
 import argparse
 import subprocess
 import tomllib
+from fnmatch import fnmatchcase
 from pathlib import Path
 
 
 ROOT = Path(__file__).resolve().parents[2]
+# These patterns replace the old pull_request/push path filters. Include parent
+# workflow changes because they can alter whether the canary is invoked.
+CANARY_PATH_PATTERNS = {
+    ".bazelrc",
+    ".github/actions/setup-bazel-ci/**",
+    ".github/scripts/run_bazel_with_buildbuddy.py",
+    ".github/scripts/rusty_v8_bazel.py",
+    ".github/scripts/rusty_v8_module_bazel.py",
+    ".github/scripts/v8_canary_changes.py",
+    ".github/workflows/postmerge-ci.yml",
+    ".github/workflows/rusty-v8-release.yml",
+    ".github/workflows/v8-canary.yml",
+    "MODULE.bazel",
+    "MODULE.bazel.lock",
+    "codex-rs/Cargo.toml",
+    "patches/BUILD.bazel",
+    "patches/llvm_*.patch",
+    "patches/rules_cc_*.patch",
+    "patches/v8_*.patch",
+    "third_party/v8/**",
+}
+# Windows source builds are a narrower, more expensive subset of the canary.
+# A V8 version change also requires them even when no path below changed.
 WINDOWS_SOURCE_BUILD_PATHS = {
     ".github/scripts/rusty_v8_bazel.py",
     ".github/scripts/rusty_v8_module_bazel.py",
@@ -16,6 +48,30 @@ WINDOWS_SOURCE_BUILD_PATHS = {
 }
 
 
+def matching_canary_paths(changed_files: set[str]) -> set[str]:
+    """Return changed paths that require the general V8 build matrix."""
+    return {
+        path
+        for path in changed_files
+        if any(fnmatchcase(path, pattern) for pattern in CANARY_PATH_PATTERNS)
+    }
+
+
+def canary_required(
+    changed_files: set[str],
+    base_v8_version: str,
+    head_v8_version: str,
+    *,
+    force: bool = False,
+) -> bool:
+    """Return whether the general V8 build matrix should run."""
+    return (
+        force
+        or base_v8_version != head_v8_version
+        or bool(matching_canary_paths(changed_files))
+    )
+
+
 def resolved_v8_version(cargo_lock: bytes) -> str:
     versions = sorted(
         {
@@ -36,6 +92,7 @@ def windows_source_required(
     *,
     force: bool = False,
 ) -> bool:
+    """Return whether Windows must rebuild rusty_v8 from source."""
     return (
         force
         or base_v8_version != head_v8_version
@@ -58,6 +115,8 @@ def merge_base(base: str, head: str, *, root: Path = ROOT) -> str:
 
 
 def changed_files(base: str, head: str, *, root: Path = ROOT) -> set[str]:
+    # Three-dot diff gives PRs merge-base semantics while remaining equivalent
+    # to before/after for ordinary linear pushes to main.
     output = git_output(
         "diff",
         "--name-only",
@@ -79,25 +138,44 @@ def parse_args() -> argparse.Namespace:
 def main() -> None:
     args = parse_args()
     if args.force:
-        required = True
-        reason = "manual workflow dispatch"
+        # workflow_dispatch has no comparison range, and callers use it as a
+        # manual retry path, so it intentionally runs every variant.
+        canary = True
+        canary_reason = "manual workflow dispatch"
+        windows_source = True
+        windows_source_reason = "manual workflow dispatch"
     elif not args.base or not args.head:
         raise SystemExit("--base and --head are required unless --force is set")
     else:
         files = changed_files(args.base, args.head)
         base_version = v8_version_at_revision(merge_base(args.base, args.head))
         head_version = v8_version_at_revision(args.head)
-        required = windows_source_required(files, base_version, head_version)
+
+        matched_canary_paths = sorted(matching_canary_paths(files))
+        canary = canary_required(files, base_version, head_version)
+        windows_source = windows_source_required(files, base_version, head_version)
         if base_version != head_version:
-            reason = f"v8 version changed from {base_version} to {head_version}"
+            canary_reason = (
+                f"v8 version changed from {base_version} to {head_version}"
+            )
+            windows_source_reason = canary_reason
         else:
-            matched_paths = sorted(files & WINDOWS_SOURCE_BUILD_PATHS)
-            reason = (
-                ", ".join(matched_paths) if matched_paths else "no relevant changes"
+            canary_reason = (
+                ", ".join(matched_canary_paths)
+                if matched_canary_paths
+                else "no relevant changes"
+            )
+            matched_windows_paths = sorted(files & WINDOWS_SOURCE_BUILD_PATHS)
+            windows_source_reason = (
+                ", ".join(matched_windows_paths)
+                if matched_windows_paths
+                else "no relevant changes"
             )
 
-    print(f"windows_source_required={str(required).lower()}")
-    print(f"windows_source_reason={reason}")
+    print(f"canary_required={str(canary).lower()}")
+    print(f"canary_reason={canary_reason}")
+    print(f"windows_source_required={str(windows_source).lower()}")
+    print(f"windows_source_reason={windows_source_reason}")
 
 
 if __name__ == "__main__":
diff --git a/.github/workflows/bazel.yml b/.github/workflows/bazel.yml
index 7a6db869e..10dd8670d 100644
--- a/.github/workflows/bazel.yml
+++ b/.github/workflows/bazel.yml
@@ -4,10 +4,7 @@ name: Bazel
 # https://github.com/cerisier/toolchains_llvm_bootstrapped/blob/main/.github/workflows/ci.yaml
 
 on:
-  pull_request: {}
-  push:
-    branches:
-      - main
+  workflow_call:
   workflow_dispatch:
 
 concurrency:
diff --git a/.github/workflows/blob-size-policy.yml b/.github/workflows/blob-size-policy.yml
index ea50e8ee2..fddc51afe 100644
--- a/.github/workflows/blob-size-policy.yml
+++ b/.github/workflows/blob-size-policy.yml
@@ -1,7 +1,7 @@
 name: blob-size-policy
 
 on:
-  pull_request: {}
+  workflow_call:
 
 jobs:
   check:
@@ -14,13 +14,24 @@ jobs:
           fetch-depth: 0
           persist-credentials: false
 
-      - name: Determine PR comparison range
+      - name: Determine comparison range
         id: range
         shell: bash
         run: |
           set -euo pipefail
-          echo "base=${{ github.event.pull_request.base.sha }}" >> "$GITHUB_OUTPUT"
-          echo "head=${{ github.event.pull_request.head.sha }}" >> "$GITHUB_OUTPUT"
+
+          # PRs inspect the proposed diff; main pushes inspect only the commit
+          # range that just landed. Both paths feed the same blob-size checker.
+          if [[ "${{ github.event_name }}" == "pull_request" ]]; then
+            base='${{ github.event.pull_request.base.sha }}'
+            head='${{ github.event.pull_request.head.sha }}'
+          else
+            base='${{ github.event.before }}'
+            head='${{ github.sha }}'
+          fi
+
+          echo "base=$base" >> "$GITHUB_OUTPUT"
+          echo "head=$head" >> "$GITHUB_OUTPUT"
 
       - name: Check changed blob sizes
         env:
diff --git a/.github/workflows/blocking-ci.yml b/.github/workflows/blocking-ci.yml
new file mode 100644
index 000000000..fe66fdd50
--- /dev/null
+++ b/.github/workflows/blocking-ci.yml
@@ -0,0 +1,79 @@
+name: blocking-ci
+
+# This is the single entrypoint for checks that block a PR merge. It also runs
+# after pushes to main so the same check family stays grouped in the Actions UI.
+on:
+  pull_request: {}
+  push:
+    branches: [main]
+
+jobs:
+  # Keep reusable workflow calls alphabetized. The `required` job below is the
+  # version-controlled list that the main-branch ruleset should require.
+  bazel:
+    name: Bazel
+    uses: ./.github/workflows/bazel.yml
+    secrets: inherit
+
+  blob-size-policy:
+    name: Blob size policy
+    uses: ./.github/workflows/blob-size-policy.yml
+    secrets: inherit
+
+  cargo-deny:
+    name: cargo-deny
+    uses: ./.github/workflows/cargo-deny.yml
+    secrets: inherit
+
+  codespell:
+    name: Codespell
+    uses: ./.github/workflows/codespell.yml
+    secrets: inherit
+
+  repo-checks:
+    name: repo-checks
+    uses: ./.github/workflows/repo-checks.yml
+    secrets: inherit
+
+  rust-ci:
+    name: rust-ci
+    uses: ./.github/workflows/rust-ci.yml
+    secrets: inherit
+
+  sdk:
+    name: sdk
+    uses: ./.github/workflows/sdk.yml
+    secrets: inherit
+
+  required:
+    name: CI required
+    # Without `always()`, GitHub skips this job after a failed dependency and a
+    # required check can appear successful instead of reporting the failure.
+    if: ${{ always() }}
+    needs:
+      - bazel
+      - blob-size-policy
+      - cargo-deny
+      - codespell
+      - repo-checks
+      - rust-ci
+      - sdk
+    runs-on: ubuntu-24.04
+    steps:
+      # Keep the helper on the same revision as the caller and child workflows.
+      # CI workflow uploads are restricted, so this repository does not need a
+      # separate trusted-base checkout for the terminal policy step. Using the
+      # PR head also lets the introducing PR exercise a newly added helper.
+      #
+      # During the initial rollout, PR branches created before
+      # check_ci_results.py exists must rebase onto main before this gate can
+      # run.
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
+          persist-credentials: false
+
+      - name: Require successful dependencies
+        env:
+          NEEDS: ${{ toJSON(needs) }}
+        run: python3 .github/scripts/check_ci_results.py
diff --git a/.github/workflows/cargo-deny.yml b/.github/workflows/cargo-deny.yml
index aa37d7cec..be00e1147 100644
--- a/.github/workflows/cargo-deny.yml
+++ b/.github/workflows/cargo-deny.yml
@@ -1,10 +1,7 @@
 name: cargo-deny
 
 on:
-  pull_request:
-  push:
-    branches:
-      - main
+  workflow_call:
 
 # Cargo's libgit2 transport has been flaky when fetching git dependencies with
 # nested submodules. Prefer the system git CLI across every Cargo invocation.
diff --git a/.github/workflows/codespell.yml b/.github/workflows/codespell.yml
index b642a6dbe..a1751c8d2 100644
--- a/.github/workflows/codespell.yml
+++ b/.github/workflows/codespell.yml
@@ -3,10 +3,7 @@
 name: Codespell
 
 on:
-  push:
-    branches: [main]
-  pull_request:
-    branches: [main]
+  workflow_call:
 
 permissions:
   contents: read
diff --git a/.github/workflows/postmerge-ci.yml b/.github/workflows/postmerge-ci.yml
new file mode 100644
index 000000000..ac2b46d1d
--- /dev/null
+++ b/.github/workflows/postmerge-ci.yml
@@ -0,0 +1,40 @@
+name: postmerge-ci
+
+# This is the single entrypoint for main-only CI that is intentionally outside
+# the merge-blocking suite. It keeps the broader postmerge signal in one run.
+on:
+  push:
+    branches: [main]
+
+jobs:
+  # Keep reusable workflow calls alphabetized. Each child retains its own
+  # workflow_dispatch trigger so maintainers can rerun flaky suites directly.
+  rust-ci-full:
+    name: rust-ci-full
+    uses: ./.github/workflows/rust-ci-full.yml
+    secrets: inherit
+
+  v8-canary:
+    name: v8-canary
+    uses: ./.github/workflows/v8-canary.yml
+    secrets: inherit
+
+  results:
+    name: Postmerge CI results
+    needs:
+      - rust-ci-full
+      - v8-canary
+    if: ${{ always() }}
+    runs-on: ubuntu-24.04
+    steps:
+      # Postmerge runs use the pushed main commit, so this helper always comes
+      # from the same revision that defined the parent workflow.
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          ref: ${{ github.sha }}
+          persist-credentials: false
+
+      - name: Require successful dependencies
+        env:
+          NEEDS: ${{ toJSON(needs) }}
+        run: python3 .github/scripts/check_ci_results.py
diff --git a/.github/workflows/ci.yml b/.github/workflows/repo-checks.yml
similarity index 98%
rename from .github/workflows/ci.yml
rename to .github/workflows/repo-checks.yml
index a78ae034e..3a76a202e 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/repo-checks.yml
@@ -1,8 +1,7 @@
-name: ci
+name: repo-checks
 
 on:
-  pull_request: {}
-  push: { branches: [main] }
+  workflow_call:
 
 jobs:
   build-test:
diff --git a/.github/workflows/rust-ci-full.yml b/.github/workflows/rust-ci-full.yml
index 7ad1d4b3a..cb99c8911 100644
--- a/.github/workflows/rust-ci-full.yml
+++ b/.github/workflows/rust-ci-full.yml
@@ -1,8 +1,10 @@
 name: rust-ci-full
 on:
+  workflow_call:
   push:
     branches:
-      - main
+      # Main pushes enter through postmerge-ci. Keep this opt-in branch trigger
+      # for developers who want the full suite before merging.
       - "**full-ci**"
   workflow_dispatch:
 
diff --git a/.github/workflows/rust-ci.yml b/.github/workflows/rust-ci.yml
index f693994d1..cf1afdfe2 100644
--- a/.github/workflows/rust-ci.yml
+++ b/.github/workflows/rust-ci.yml
@@ -1,6 +1,6 @@
 name: rust-ci
 on:
-  pull_request: {}
+  workflow_call:
   workflow_dispatch:
 
 # Cargo's libgit2 transport has been flaky when fetching git dependencies with
diff --git a/.github/workflows/sdk.yml b/.github/workflows/sdk.yml
index dcb940679..41f887471 100644
--- a/.github/workflows/sdk.yml
+++ b/.github/workflows/sdk.yml
@@ -1,9 +1,7 @@
 name: sdk
 
 on:
-  push:
-    branches: [main]
-  pull_request: {}
+  workflow_call:
 
 jobs:
   python-sdk:
diff --git a/.github/workflows/v8-canary.yml b/.github/workflows/v8-canary.yml
index efa06aaad..53d958510 100644
--- a/.github/workflows/v8-canary.yml
+++ b/.github/workflows/v8-canary.yml
@@ -1,44 +1,12 @@
 name: v8-canary
 
+# Do not use trigger-level path filters here. This workflow is also called by
+# postmerge-ci, and GitHub cannot share a path filter between pull_request and
+# workflow_call. v8_canary_changes.py is the single source of truth instead;
+# unrelated events run only the cheap metadata job below.
 on:
-  pull_request:
-    paths:
-      - ".bazelrc"
-      - ".github/actions/setup-bazel-ci/**"
-      - ".github/scripts/run_bazel_with_buildbuddy.py"
-      - ".github/scripts/rusty_v8_bazel.py"
-      - ".github/scripts/rusty_v8_module_bazel.py"
-      - ".github/scripts/v8_canary_changes.py"
-      - ".github/workflows/rusty-v8-release.yml"
-      - ".github/workflows/v8-canary.yml"
-      - "MODULE.bazel"
-      - "MODULE.bazel.lock"
-      - "codex-rs/Cargo.toml"
-      - "patches/BUILD.bazel"
-      - "patches/llvm_*.patch"
-      - "patches/rules_cc_*.patch"
-      - "patches/v8_*.patch"
-      - "third_party/v8/**"
-  push:
-    branches:
-      - main
-    paths:
-      - ".bazelrc"
-      - ".github/actions/setup-bazel-ci/**"
-      - ".github/scripts/run_bazel_with_buildbuddy.py"
-      - ".github/scripts/rusty_v8_bazel.py"
-      - ".github/scripts/rusty_v8_module_bazel.py"
-      - ".github/scripts/v8_canary_changes.py"
-      - ".github/workflows/rusty-v8-release.yml"
-      - ".github/workflows/v8-canary.yml"
-      - "MODULE.bazel"
-      - "MODULE.bazel.lock"
-      - "codex-rs/Cargo.toml"
-      - "patches/BUILD.bazel"
-      - "patches/llvm_*.patch"
-      - "patches/rules_cc_*.patch"
-      - "patches/v8_*.patch"
-      - "third_party/v8/**"
+  workflow_call:
+  pull_request: {}
   workflow_dispatch:
 
 # Cargo's libgit2 transport has been flaky when fetching git dependencies with
@@ -54,6 +22,10 @@ jobs:
   metadata:
     runs-on: ubuntu-latest
     outputs:
+      # A stale PR head can contain the old detector, which does not emit this
+      # output. Missing must mean "run" so older branches cannot silently skip
+      # the expensive V8 coverage while reporting success.
+      canary_required: ${{ steps.changes.outputs.canary_required || 'true' }}
       v8_version: ${{ steps.v8_version.outputs.version }}
       windows_source_required: ${{ steps.changes.outputs.windows_source_required }}
 
@@ -77,7 +49,7 @@ jobs:
           version="$(python3 .github/scripts/rusty_v8_bazel.py resolved-v8-crate-version)"
           echo "version=${version}" >> "$GITHUB_OUTPUT"
 
-      - name: Detect whether Windows source artifacts need rebuilding
+      - name: Detect V8 canary changes
         id: changes
         env:
           BASE_SHA: ${{ github.event_name == 'pull_request' && github.event.pull_request.base.sha || github.event.before }}
@@ -87,6 +59,8 @@ jobs:
         run: |
           set -euo pipefail
 
+          # Manual runs have no meaningful before/after range. Force every V8
+          # variant so workflow_dispatch remains a reliable retry path.
           if [[ "${EVENT_NAME}" == "workflow_dispatch" ]]; then
             output="$(python3 .github/scripts/v8_canary_changes.py --force)"
           else
@@ -104,6 +78,8 @@ jobs:
   build:
     name: Build ${{ matrix.variant }} ${{ matrix.target }}
     needs: metadata
+    # Metadata always runs; only relevant changes pay for the large matrix.
+    if: ${{ needs.metadata.outputs.canary_required == 'true' }}
     runs-on: ${{ matrix.runner }}
     permissions:
       contents: read