diff --git a/.changeset/tidy-zebras-hum.md b/.changeset/tidy-zebras-hum.md
new file mode 100644
index 000000000..53dcfd099
--- /dev/null
+++ b/.changeset/tidy-zebras-hum.md
@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 2262
+---
+**Phase-directory resolution fails loud on cross-project collisions** — when two unrelated GSD projects share a `.planning/phases/` tree, a bare phase number silently resolved to the first `0N-*` directory found. The fix detects multiple matches and surfaces an `ambiguous_matches` result. (#2237)
diff --git a/src/init.cts b/src/init.cts
index 772d2fa71..a4548a8a1 100644
--- a/src/init.cts
+++ b/src/init.cts
@@ -979,6 +979,22 @@ function cmdInitPhaseOp(cwd: string, phase: string, raw: boolean): void {
   const config = loadConfig(cwd);
   let phaseInfo = guardedFindPhase(cwd, phase, config.project_code);
 
+  // #2237: surface ambiguous phase-directory collisions instead of silently
+  // taking the first match when unrelated projects share a .planning/phases/ tree.
+  if (phaseInfo?.['ambiguous_matches']) {
+    const matches = phaseInfo['ambiguous_matches'] as string[];
+    const result: Record<string, unknown> = {
+      phase_found: false,
+      phase_dir: null,
+      phase_number: null,
+      phase_name: null,
+      ambiguous_matches: matches,
+      warning: `Phase ${phase} is ambiguous: ${matches.length} directories match (${matches.map((m: string) => `"${m}"`).join(', ')}). Set a distinct project_code in .planning/config.json to scope resolution.`,
+    };
+    output(withProjectRoot(cwd, result), raw);
+    return;
+  }
+
   if (phaseInfo?.['archived']) {
     const roadmapPhase = guardedGetRoadmapPhase(cwd, phase, config.project_code);
     if (roadmapPhase?.['found']) {
diff --git a/src/phase-locator.cts b/src/phase-locator.cts
index e0104b329..d33b03822 100644
--- a/src/phase-locator.cts
+++ b/src/phase-locator.cts
@@ -44,6 +44,7 @@ interface PhaseSearchResult {
   has_verification: boolean;
   has_reviews: boolean;
   archived?: string;
+  ambiguous_matches?: string[];
 }
 
 interface ArchivedPhaseDir {
@@ -58,8 +59,31 @@ interface ArchivedPhaseDir {
 function searchPhaseInDir(baseDir: string, relBase: string, normalized: string): PhaseSearchResult | null {
   try {
     const dirs = readSubdirectories(baseDir, true);
-    const match = dirs.find(d => phaseTokenMatches(d, normalized));
-    if (!match) return null;
+    const matches = dirs.filter(d => phaseTokenMatches(d, normalized));
+    if (matches.length === 0) return null;
+
+    // #2237: fail loud when multiple directories match the same bare phase
+    // number — this happens when unrelated projects share a .planning/phases/
+    // tree. Silently taking the first match risks cross-project file writes.
+    if (matches.length > 1) {
+      return {
+        found: false,
+        directory: '',
+        phase_number: normalized,
+        phase_name: null,
+        phase_slug: null,
+        plans: [],
+        summaries: [],
+        incomplete_plans: [],
+        has_research: false,
+        has_context: false,
+        has_verification: false,
+        has_reviews: false,
+        ambiguous_matches: matches,
+      };
+    }
+
+    const match = matches[0];
 
     const phaseToken = extractPhaseToken(match);
     const phaseNumber = phaseToken || normalized;
diff --git a/src/phase.cts b/src/phase.cts
index e5f98be9d..e1f089068 100644
--- a/src/phase.cts
+++ b/src/phase.cts
@@ -416,8 +416,20 @@ function cmdFindPhase(cwd: string, phase: string, raw: boolean): void {
         .map((e) => e.name)
         .sort((a, b) => comparePhaseNum(a, b));
 
-      const match = dirs.find((d) => phaseTokenMatches(d, normalized));
-      if (!match) continue;
+      // #2237: fail loud when multiple directories match the same bare phase
+      // number — prevents cross-project file writes when unrelated projects
+      // share a .planning/phases/ tree.
+      const matches = dirs.filter((d) => phaseTokenMatches(d, normalized));
+      if (matches.length === 0) continue;
+      if (matches.length > 1) {
+        output({
+          ...notFound,
+          ambiguous_matches: matches,
+          warning: `Phase ${normalized} is ambiguous: ${matches.length} directories match (${matches.map(m => `"${m}"`).join(', ')}). Set a distinct project_code in .planning/config.json to scope resolution.`,
+        }, raw, '');
+        return;
+      }
+      const match = matches[0];
 
       const dirMatch =
         match.match(
diff --git a/tests/phase-locator.test.cjs b/tests/phase-locator.test.cjs
index 125fe06a0..5a64f43ee 100644
--- a/tests/phase-locator.test.cjs
+++ b/tests/phase-locator.test.cjs
@@ -411,3 +411,45 @@ describe('searchPhaseInDir: direct filesystem search', () => {
     assert.strictEqual(result, null);
   });
 });
+
+// ─── #2237: ambiguous phase-directory collision ─────────────────────────────
+describe('#2237: ambiguous phase-directory collision', () => {
+  let tmpDir;
+  afterEach(() => { if (tmpDir) { cleanup(tmpDir); tmpDir = null; } });
+
+  test('searchPhaseInDir returns ambiguous_matches when two dirs share phase number', () => {
+    tmpDir = createTempProject('gsd-pl-collision-');
+    const phasesDir = path.join(tmpDir, '.planning', 'phases');
+    // Two unrelated projects with same phase number
+    fs.mkdirSync(path.join(phasesDir, '04-alpha-feature'), { recursive: true });
+    fs.mkdirSync(path.join(phasesDir, '04-beta-feature'), { recursive: true });
+    const result = phaseLocator.searchPhaseInDir(phasesDir, '.planning/phases', '04');
+    assert.ok(result, 'should return a result (not null)');
+    assert.strictEqual(result.found, false, 'ambiguous result must not be "found"');
+    assert.ok(result.ambiguous_matches, 'must have ambiguous_matches');
+    assert.strictEqual(result.ambiguous_matches.length, 2);
+    assert.ok(result.ambiguous_matches.includes('04-alpha-feature'));
+    assert.ok(result.ambiguous_matches.includes('04-beta-feature'));
+  });
+
+  test('findPhaseInternal propagates ambiguous result', () => {
+    tmpDir = createTempProject('gsd-pl-collision-');
+    const phasesDir = path.join(tmpDir, '.planning', 'phases');
+    fs.mkdirSync(path.join(phasesDir, '05-gamma'), { recursive: true });
+    fs.mkdirSync(path.join(phasesDir, '05-delta'), { recursive: true });
+    const result = phaseLocator.findPhaseInternal(tmpDir, '5');
+    assert.ok(result, 'should return a result');
+    assert.strictEqual(result.found, false);
+    assert.ok(result.ambiguous_matches?.length >= 2);
+  });
+
+  test('single match still resolves normally (no false positive)', () => {
+    tmpDir = createTempProject('gsd-pl-collision-');
+    const phasesDir = path.join(tmpDir, '.planning', 'phases');
+    fs.mkdirSync(path.join(phasesDir, '03-only-phase'), { recursive: true });
+    const result = phaseLocator.searchPhaseInDir(phasesDir, '.planning/phases', '03');
+    assert.ok(result);
+    assert.strictEqual(result.found, true);
+    assert.ok(!result.ambiguous_matches, 'single match must not set ambiguous_matches');
+  });
+});