diff --git a/ARCHITECTURE.md b/ARCHITECTURE.md
index 47c6306..c7e7767 100644
--- a/ARCHITECTURE.md
+++ b/ARCHITECTURE.md
@@ -70,7 +70,7 @@ server/                 # webhook server (GitHub App)
 - **incremental processing**: only re-embeds new/changed items. crash-recoverable via sqlite
 - **read-only default**: every GitHub mutation (labels, comments, closes, issue creation) funnels through one `write-gate.ts` gate that defaults to dry-run. CLI writes only under `--apply-labels`; the webhook server writes only when `PRISM_APPLY=1`. `--dry-run` always wins. (Fixes the prior leak where `ensureLabelsExist` created labels even under `--dry-run`, and the server writing unconditionally.)
 - **cross-repo**: config accepts multiple repos, dupe detection works across repo boundaries
-- **canonical selection**: one `selectCanonical()` (src/canonical.ts) picks each cluster's source of truth for the report, the starmap payload, and the live triage bot alike. issue-majority clusters resolve to the earliest report (the original bug); PR-majority to the highest-quality item. fully deterministic - every tie bottoms out at item number - so re-runs name the same canonical
+- **canonical selection**: one `selectCanonical()` (src/canonical.ts) picks each cluster's source of truth for the report, the starmap payload, and the live triage bot alike. issue-majority clusters resolve to the earliest report (the original bug); PR-majority ranks by lifecycle state (merged > open > closed), then a CI veto (a known-red build never outranks a same-state green sibling, before score), then quality score. the veto stops a high-scored PR with failing checks from becoming bestPick over the green fix that actually landed; only `ciStatus === "failure"` demotes, so a not-yet-reported PR is never penalized. fully deterministic - every tie bottoms out at item number - so re-runs name the same canonical
 - **cluster confidence**: clustering is single-linkage (BFS over pairs >= threshold) with a centroid-refinement pass to break chained mega-clusters. because single-linkage can still chain in loosely-related members, each cluster reports both `avgSimilarity` and `minSimilarity` (lowest pairwise). the report/dupes output surfaces min as a confidence tier (high >= 90%, solid >= 80%, loose < 80%) so a low-min "loose" cluster gets eyeballed before anything is closed. avg and min are computed exactly over all pairs (no sampling), so the tier a maintainer sees is reproducible run to run
 
 ## database
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1b87134..e777a80 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,11 @@
 
 all notable changes to pr-prism are documented here.
 
+## [unreleased]
+
+### changed
+- canonical/bestPick selection now vetoes a red build: a PR with failing CI never outranks a same-state sibling with a non-failing build, regardless of quality score. stops a high-scored PR (e.g. one that added a test file) from being named bestPick over the green fix that actually landed, when the added test fails. only `ciStatus === "failure"` demotes, so a PR whose checks have not reported yet is never penalized; state (merged > open > closed) still dominates. starmap `canonical`/`contested` for such clusters shift accordingly (schema unchanged, still v1)
+
 ## [3.0.1] — 2026-07-13
 
 ### fixed
diff --git a/src/__tests__/canonical.test.ts b/src/__tests__/canonical.test.ts
index a071897..d275ee8 100644
--- a/src/__tests__/canonical.test.ts
+++ b/src/__tests__/canonical.test.ts
@@ -220,6 +220,82 @@ describe("selectCanonical state preference (merged PRs are the source of truth)"
   });
 });
 
+describe("selectCanonical CI veto (a red build never becomes bestPick over a green sibling)", () => {
+  it("the odysseus #5207 case: a failing higher-scored PR loses to a passing lower-scored sibling", () => {
+    // Both open at scan time. #5358 scored higher (a test file it added inflated
+    // its quality score) but its CI is red; #5208 is the minimal green fix that
+    // was actually merged. Without the veto, score picks #5358 (the wrong one).
+    const items = [
+      c({ number: 5358, type: "pr", state: "open", score: 0.752, ciStatus: "failure" }),
+      c({ number: 5208, type: "pr", state: "open", score: 0.532, ciStatus: "success" }),
+    ];
+    expect(selectCanonical(items).number).toBe(5208);
+  });
+
+  it("a red build loses to a green sibling no matter how large its score lead", () => {
+    const items = [
+      c({ number: 1, type: "pr", state: "open", score: 0.99, ciStatus: "failure" }),
+      c({ number: 2, type: "pr", state: "open", score: 0.01, ciStatus: "success" }),
+    ];
+    expect(selectCanonical(items).number).toBe(2);
+  });
+
+  it("only failure demotes: pending/unknown/absent CI are never penalized, score still decides", () => {
+    const pending = [
+      c({ number: 1, type: "pr", state: "open", score: 0.9, ciStatus: "pending" }),
+      c({ number: 2, type: "pr", state: "open", score: 0.5, ciStatus: "success" }),
+    ];
+    expect(selectCanonical(pending).number).toBe(1); // pending not demoted -> higher score wins
+    const unknownVsAbsent = [
+      c({ number: 3, type: "pr", state: "open", score: 0.9, ciStatus: "unknown" }),
+      c({ number: 4, type: "pr", state: "open", score: 0.5 }),
+    ];
+    expect(selectCanonical(unknownVsAbsent).number).toBe(3); // neither demoted -> higher score wins
+  });
+
+  it("when every sibling is failing, the CI gate ties and score decides (no bottomless demotion)", () => {
+    const items = [
+      c({ number: 1, type: "pr", state: "open", score: 0.4, ciStatus: "failure" }),
+      c({ number: 2, type: "pr", state: "open", score: 0.9, ciStatus: "failure" }),
+    ];
+    expect(selectCanonical(items).number).toBe(2); // both red -> highest score
+  });
+
+  it("state still dominates CI: a merged PR wins even if its build is red", () => {
+    const items = [
+      c({ number: 1, type: "pr", state: "open", score: 0.9, ciStatus: "success" }),
+      c({ number: 2, type: "pr", state: "merged", score: 0.4, ciStatus: "failure" }),
+    ];
+    expect(selectCanonical(items).number).toBe(2); // merged is in main = source of truth
+  });
+
+  it("the CI gate applies within a state: a green closed PR beats a red closed PR", () => {
+    const items = [
+      c({ number: 1, type: "pr", state: "closed", score: 0.9, ciStatus: "failure" }),
+      c({ number: 2, type: "pr", state: "closed", score: 0.4, ciStatus: "success" }),
+    ];
+    expect(selectCanonical(items).number).toBe(2);
+  });
+
+  it("a pick decided by the CI gate is NOT contested, even when scores are within the tie margin", () => {
+    const items = [
+      c({ number: 1, type: "pr", state: "open", score: 0.52, ciStatus: "failure" }),
+      c({ number: 2, type: "pr", state: "open", score: 0.5, ciStatus: "success" }),
+    ];
+    const d = decideCanonical(items);
+    expect(d.canonical.number).toBe(2); // green wins despite lower score
+    expect(d.contested).toBe(false); // CI decided it, not a coin flip
+  });
+
+  it("issue-majority clusters ignore CI (earliest report stays canonical)", () => {
+    const items = [
+      c({ number: 1, type: "issue", createdAt: "2026-01-01T00:00:00Z", score: 0.1, ciStatus: "failure" }),
+      c({ number: 2, type: "issue", createdAt: "2026-02-01T00:00:00Z", score: 0.9, ciStatus: "success" }),
+    ];
+    expect(selectCanonical(items).number).toBe(1); // earliest report, CI is a PR-mode concept
+  });
+});
+
 describe("selectTracker (tracker substrate: original bug + role-tagged candidates)", () => {
   it("mixed cluster: tracker = earliest issue, PRs are fixes, later issue is a duplicate", () => {
     const items = [
diff --git a/src/canonical.ts b/src/canonical.ts
index f96253f..c60b0a1 100644
--- a/src/canonical.ts
+++ b/src/canonical.ts
@@ -7,8 +7,11 @@
 // Semantics (the cli.ts rule, now shared):
 //   - issue-majority cluster  -> the earliest report is canonical (the original
 //     bug), quality score then item number as tiebreaks.
-//   - PR-majority cluster      -> the highest-quality item is canonical, then
-//     earliest, then most-reviewed, then item number.
+//   - PR-majority cluster      -> ranked in order: lifecycle state (merged >
+//     open > closed), then a CI veto (a known-red build never outranks a
+//     same-state non-failing sibling), then quality score, then earliest, then
+//     most-reviewed, then item number. state and the CI veto both sit BEFORE
+//     score so neither a stale open PR nor a failing one wins on score alone.
 // `mode` lets a caller pin the rule instead of deriving it from the set — the
 // triage bot passes the incoming item's type so its behavior is unchanged.
 
@@ -23,6 +26,8 @@ export interface CanonicalCandidate {
   number: number;
   /** "open" | "closed" | "merged"; absent for callers with no state (triage). */
   state?: string;
+  /** CI rollup; absent for callers with no CI signal (triage). Only "failure" demotes. */
+  ciStatus?: "success" | "failure" | "pending" | "unknown";
 }
 
 function createdMs(item: CanonicalCandidate): number {
@@ -63,11 +68,25 @@ function statePriority(state: string | undefined): number {
   }
 }
 
+/**
+ * Rank a PR by CI outcome: only a KNOWN-red build demotes. A failing PR must not
+ * become bestPick over a same-state green sibling however high its quality score
+ * (the score rewards e.g. having a test file, blind to whether that test passes).
+ * success / pending / unknown / absent all rank equal (1) so a PR whose checks
+ * simply have not reported yet is never penalized, and an all-failing set ties
+ * here and falls through to score unchanged.
+ */
+function ciRank(status: string | undefined): number {
+  return status === "failure" ? 0 : 1;
+}
+
 function isNearTie<T extends CanonicalCandidate>(mode: "issue" | "pr", canonical: T, runnerUp: T): boolean {
   if (mode === "pr") {
     // A different lifecycle state decided the pick (e.g. merged over open) -> a
     // clear winner, not a coin flip. Only a same-state pair is a score near-tie.
     if (statePriority(canonical.state) !== statePriority(runnerUp.state)) return false;
+    // A green build chosen over a red sibling is a decisive reason, not a coin flip.
+    if (ciRank(canonical.ciStatus) !== ciRank(runnerUp.ciStatus)) return false;
     return canonical.score - runnerUp.score < TIE_MARGIN;
   }
   // The pick is on createdAt, so nearness is a time window (a huge score gap
@@ -109,6 +128,10 @@ export function decideCanonical<T extends CanonicalCandidate>(
       // BEFORE score - a merged fix often does not have the top computed score.
       const sp = statePriority(b.state) - statePriority(a.state);
       if (sp !== 0) return sp;
+      // Then veto a known-red build: a same-state green sibling always outranks it,
+      // BEFORE score, so a high quality score cannot promote a failing PR.
+      const cr = ciRank(b.ciStatus) - ciRank(a.ciStatus);
+      if (cr !== 0) return cr;
       if (b.score !== a.score) return b.score - a.score; // then highest score first
       const d = createdMs(a) - createdMs(b);
       if (d !== 0) return d;