diff --git a/.claude/rules/testing/built-version-ci.md b/.claude/rules/testing/built-version-ci.md
index d12c832e269..71397efcae8 100644
--- a/.claude/rules/testing/built-version-ci.md
+++ b/.claude/rules/testing/built-version-ci.md
@@ -20,7 +20,7 @@ For a one-line YAML edit, the invariants below and the document map are enough.
 
 Invariants when editing:
 
-- Playwright and ff-matrix legs in `test-smokes-built.yml` must check both the source mode and `github.event.inputs.buckets == ''`.
+- Playwright and ff-matrix legs in `test-smokes-built.yml` must check both `needs.resolve-mode.outputs.mode` and `github.event.inputs.buckets == ''`.
 - `test-ff-matrix.yml` owns the ff-matrix bucket glob.
 - Scheduler jobs in `test-smokes-built.yml` set per-leg OS scope through their `runners:` inputs.
 - Keep the per-call suffix in `test-ff-matrix.yml`'s concurrency group so sibling calls cannot cancel one another.
diff --git a/.github/workflows/test-smokes-built.yml b/.github/workflows/test-smokes-built.yml
index bf409400e81..a32d63c5894 100644
--- a/.github/workflows/test-smokes-built.yml
+++ b/.github/workflows/test-smokes-built.yml
@@ -38,10 +38,46 @@ permissions:
   contents: read
 
 jobs:
+  # Single source of truth for the source mode; every job below gates on its output.
+  # The same expression appears in `run-name` above, where job outputs are not in scope.
+  resolve-mode:
+    name: Resolve source mode
+    runs-on: ubuntu-latest
+    permissions: {}
+    outputs:
+      mode: ${{ steps.m.outputs.mode }}
+    steps:
+      - name: Resolve mode
+        id: m
+        shell: bash
+        env:
+          MODE_INPUT: ${{ (github.event_name == 'workflow_run' && 'nightly') || github.event.inputs.source || 'build' }}
+          UPSTREAM_RUN_ID: ${{ github.event.workflow_run.id }}
+          UPSTREAM_CONCLUSION: ${{ github.event.workflow_run.conclusion }}
+        run: |
+          case "$MODE_INPUT" in
+            build | nightly | release) ;;
+            *)
+              echo "::error::Unexpected source mode '${MODE_INPUT}'. Expected build, nightly, or release."
+              exit 1
+              ;;
+          esac
+          # Records which upstream run triggered this one; the API exposes no such pointer.
+          # The marker token is assembled from a variable so the copy of this script that
+          # GitHub prints in the step's "Run" group cannot itself satisfy the pattern the
+          # post-merge trigger reader matches on - the same reason `grep '[f]oo'` does not
+          # match its own command line.
+          marker="QUARTO-TRIGGER"
+          printf '%s event=%s upstream-run-id=%s upstream-conclusion=%s\n' \
+            "$marker" "$GITHUB_EVENT_NAME" "${UPSTREAM_RUN_ID:-none}" "${UPSTREAM_CONCLUSION:-none}"
+          echo "Resolved source mode: $MODE_INPUT"
+          echo "mode=$MODE_INPUT" >> "$GITHUB_OUTPUT"
+
   # Build the selected ref (the dispatch default).
   build-artifact:
     name: Build quarto dist (linux-amd64)
-    if: (github.event_name == 'workflow_run' && 'nightly' || github.event.inputs.source || 'build') == 'build'
+    if: needs.resolve-mode.outputs.mode == 'build'
+    needs: [resolve-mode]
     runs-on: ubuntu-latest
     outputs:
       sha: ${{ steps.rec.outputs.sha }}
@@ -72,8 +108,8 @@ jobs:
 
   run-smokes-artifact:
     name: Smoke tests against built artifact
-    if: (github.event_name == 'workflow_run' && 'nightly' || github.event.inputs.source || 'build') == 'build'
-    needs: [build-artifact]
+    if: needs.resolve-mode.outputs.mode == 'build'
+    needs: [resolve-mode, build-artifact]
     uses: ./.github/workflows/test-smokes.yml
     with:
       ref: ${{ needs.build-artifact.outputs.sha }}
@@ -85,9 +121,9 @@ jobs:
   run-playwright-artifact:
     name: Playwright tests against built artifact
     if: >-
-      (github.event_name == 'workflow_run' && 'nightly' || github.event.inputs.source || 'build') == 'build'
+      needs.resolve-mode.outputs.mode == 'build'
       && github.event.inputs.buckets == ''
-    needs: [build-artifact]
+    needs: [resolve-mode, build-artifact]
     uses: ./.github/workflows/test-smokes.yml
     with:
       ref: ${{ needs.build-artifact.outputs.sha }}
@@ -99,9 +135,9 @@ jobs:
   run-ff-matrix-artifact:
     name: Feature-format matrix against built artifact
     if: >-
-      (github.event_name == 'workflow_run' && 'nightly' || github.event.inputs.source || 'build') == 'build'
+      needs.resolve-mode.outputs.mode == 'build'
       && github.event.inputs.buckets == ''
-    needs: [build-artifact]
+    needs: [resolve-mode, build-artifact]
     uses: ./.github/workflows/test-ff-matrix.yml
     with:
       ref: ${{ needs.build-artifact.outputs.sha }}
@@ -112,7 +148,8 @@ jobs:
   # Test an existing published release.
   resolve-release:
     name: Resolve release version
-    if: github.event.inputs.source == 'release'
+    if: needs.resolve-mode.outputs.mode == 'release'
+    needs: [resolve-mode]
     runs-on: ubuntu-latest
     outputs:
       version: ${{ steps.r.outputs.version }}
@@ -161,8 +198,8 @@ jobs:
 
   run-smokes-release:
     name: Smoke tests against published release
-    if: github.event.inputs.source == 'release'
-    needs: [resolve-release]
+    if: needs.resolve-mode.outputs.mode == 'release'
+    needs: [resolve-mode, resolve-release]
     uses: ./.github/workflows/test-smokes.yml
     with:
       ref: refs/tags/v${{ needs.resolve-release.outputs.version }}
@@ -173,8 +210,10 @@ jobs:
 
   run-playwright-release:
     name: Playwright tests against published release
-    if: github.event.inputs.source == 'release' && github.event.inputs.buckets == ''
-    needs: [resolve-release]
+    if: >-
+      needs.resolve-mode.outputs.mode == 'release'
+      && github.event.inputs.buckets == ''
+    needs: [resolve-mode, resolve-release]
     uses: ./.github/workflows/test-smokes.yml
     with:
       ref: refs/tags/v${{ needs.resolve-release.outputs.version }}
@@ -185,8 +224,10 @@ jobs:
 
   run-ff-matrix-release:
     name: Feature-format matrix against published release
-    if: github.event.inputs.source == 'release' && github.event.inputs.buckets == ''
-    needs: [resolve-release]
+    if: >-
+      needs.resolve-mode.outputs.mode == 'release'
+      && github.event.inputs.buckets == ''
+    needs: [resolve-mode, resolve-release]
     uses: ./.github/workflows/test-ff-matrix.yml
     with:
       ref: refs/tags/v${{ needs.resolve-release.outputs.version }}
@@ -199,8 +240,9 @@ jobs:
     name: Resolve create-release run
     # Only successful workflow_run builds are testable.
     if: >-
-      (github.event_name == 'workflow_run' && 'nightly' || github.event.inputs.source || 'build') == 'nightly'
+      needs.resolve-mode.outputs.mode == 'nightly'
       && (github.event_name != 'workflow_run' || github.event.workflow_run.conclusion == 'success')
+    needs: [resolve-mode]
     runs-on: ubuntu-latest
     outputs:
       run-id: ${{ steps.r.outputs.run-id }}
@@ -276,9 +318,9 @@ jobs:
   run-smokes-nightly-linux:
     name: Smoke tests against nightly build (linux)
     if: >-
-      (github.event_name == 'workflow_run' && 'nightly' || github.event.inputs.source || 'build') == 'nightly'
+      needs.resolve-mode.outputs.mode == 'nightly'
       && needs.resolve-nightly.outputs.has-linux == 'true'
-    needs: [resolve-nightly]
+    needs: [resolve-mode, resolve-nightly]
     uses: ./.github/workflows/test-smokes.yml
     with:
       # Publish runs add only a version/changelog commit after this SHA.
@@ -292,9 +334,9 @@ jobs:
   run-smokes-nightly-windows:
     name: Smoke tests against nightly build (windows)
     if: >-
-      (github.event_name == 'workflow_run' && 'nightly' || github.event.inputs.source || 'build') == 'nightly'
+      needs.resolve-mode.outputs.mode == 'nightly'
       && needs.resolve-nightly.outputs.has-windows == 'true'
-    needs: [resolve-nightly]
+    needs: [resolve-mode, resolve-nightly]
     uses: ./.github/workflows/test-smokes.yml
     with:
       ref: ${{ needs.resolve-nightly.outputs.sha }}
@@ -308,9 +350,9 @@ jobs:
   run-smokes-nightly-mac:
     name: Smoke tests against nightly build (macOS)
     if: >-
-      (github.event_name == 'workflow_run' && 'nightly' || github.event.inputs.source || 'build') == 'nightly'
+      needs.resolve-mode.outputs.mode == 'nightly'
       && needs.resolve-nightly.outputs.has-mac == 'true'
-    needs: [resolve-nightly]
+    needs: [resolve-mode, resolve-nightly]
     uses: ./.github/workflows/test-smokes.yml
     with:
       ref: ${{ needs.resolve-nightly.outputs.sha }}
@@ -325,10 +367,10 @@ jobs:
   run-playwright-nightly-linux:
     name: Playwright tests against nightly build (linux)
     if: >-
-      (github.event_name == 'workflow_run' && 'nightly' || github.event.inputs.source || 'build') == 'nightly'
+      needs.resolve-mode.outputs.mode == 'nightly'
       && needs.resolve-nightly.outputs.has-linux == 'true'
       && github.event.inputs.buckets == ''
-    needs: [resolve-nightly]
+    needs: [resolve-mode, resolve-nightly]
     uses: ./.github/workflows/test-smokes.yml
     with:
       ref: ${{ needs.resolve-nightly.outputs.sha }}
@@ -341,10 +383,10 @@ jobs:
   run-playwright-nightly-mac:
     name: Playwright tests against nightly build (macOS)
     if: >-
-      (github.event_name == 'workflow_run' && 'nightly' || github.event.inputs.source || 'build') == 'nightly'
+      needs.resolve-mode.outputs.mode == 'nightly'
       && needs.resolve-nightly.outputs.has-mac == 'true'
       && github.event.inputs.buckets == ''
-    needs: [resolve-nightly]
+    needs: [resolve-mode, resolve-nightly]
     uses: ./.github/workflows/test-smokes.yml
     with:
       ref: ${{ needs.resolve-nightly.outputs.sha }}
@@ -357,10 +399,10 @@ jobs:
   run-ff-matrix-nightly-linux:
     name: Feature-format matrix against nightly build (linux)
     if: >-
-      (github.event_name == 'workflow_run' && 'nightly' || github.event.inputs.source || 'build') == 'nightly'
+      needs.resolve-mode.outputs.mode == 'nightly'
       && needs.resolve-nightly.outputs.has-linux == 'true'
       && github.event.inputs.buckets == ''
-    needs: [resolve-nightly]
+    needs: [resolve-mode, resolve-nightly]
     uses: ./.github/workflows/test-ff-matrix.yml
     with:
       ref: ${{ needs.resolve-nightly.outputs.sha }}
@@ -372,10 +414,10 @@ jobs:
   run-ff-matrix-nightly-windows:
     name: Feature-format matrix against nightly build (windows)
     if: >-
-      (github.event_name == 'workflow_run' && 'nightly' || github.event.inputs.source || 'build') == 'nightly'
+      needs.resolve-mode.outputs.mode == 'nightly'
       && needs.resolve-nightly.outputs.has-windows == 'true'
       && github.event.inputs.buckets == ''
-    needs: [resolve-nightly]
+    needs: [resolve-mode, resolve-nightly]
     uses: ./.github/workflows/test-ff-matrix.yml
     with:
       ref: ${{ needs.resolve-nightly.outputs.sha }}
diff --git a/llm-docs/built-version-testing-architecture.md b/llm-docs/built-version-testing-architecture.md
index 2622797bda7..950f6012db2 100644
--- a/llm-docs/built-version-testing-architecture.md
+++ b/llm-docs/built-version-testing-architecture.md
@@ -167,7 +167,7 @@ In practice:
 
 ## Built-mode test legs (scheduler layout)
 
-`test-smokes-built.yml` = the mode **resolvers** (build-artifact / resolve-nightly / resolve-release, unchanged) + a **scheduler**: per-leg caller jobs fanning out to the reusable workflows.
+`test-smokes-built.yml` = a `resolve-mode` job computing the source mode once (`needs.resolve-mode.outputs.mode`, gating all other jobs) + the mode **resolvers** (build-artifact / resolve-nightly / resolve-release) + a **scheduler**: per-leg caller jobs fanning out to the reusable workflows.
 Each source mode schedules three independent legs:
 
 | leg        | goes through                    | bucket                                                  | OS scope                                                                             |