diff --git a/.changeset/2056-plan-phase-foreign-prefix.md b/.changeset/2056-plan-phase-foreign-prefix.md
new file mode 100644
index 000000000..38f905fec
--- /dev/null
+++ b/.changeset/2056-plan-phase-foreign-prefix.md
@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 2105
+---
+**`init plan-phase` no longer collapses foreign-prefixed task/workstream IDs into numeric phases** — a query like `MEM-01` (where `MEM` is not the configured `project_code`) used to have its prefix stripped and resolve to the unrelated numeric Phase 01; it now reports `phase_found: false` unless a phase directory or roadmap entry literally carries that prefix. The configured `project_code`'s own prefixed phases (e.g. `LKML-01` under `project_code: LKML`) continue to resolve as before. (#2056)
diff --git a/src/init.cts b/src/init.cts
index 4e41febdc..9b791316e 100644
--- a/src/init.cts
+++ b/src/init.cts
@@ -73,7 +73,7 @@ const {
   extractCurrentMilestone,
 } = roadmapParser;
 const { pathExistsInternal, generateSlugInternal, toPosixPath } = coreUtils;
-const { normalizePhaseName, phaseTokenMatches, stripProjectCodePrefix, PHASE_NUMBER_TOKEN_SOURCE } = phaseId;
+const { escapeRegex, normalizePhaseName, phaseTokenMatches, stripProjectCodePrefix, PHASE_NUMBER_TOKEN_SOURCE } = phaseId;
 const { pruneOrphanedWorktrees } = worktreeSafety;
 
 const {
@@ -96,6 +96,44 @@ void stripShippedMilestones;
 // Accept all bold/colon variants of the Requirements header (#2769)
 const REQUIREMENTS_HEADER_RE = /^\*\*Requirements:?\*\*[^\S\n]*:?[^\S\n]*([^\n]*)$/m;
 
+// #2056: normalizePhaseName() strips ANY [A-Z][A-Z0-9_]*- prefix as a project
+// code (e.g. 'MEM-01' → '01'), so a foreign-prefixed workstream/task id would
+// collapse to its numeric suffix and resolve to an unrelated numeric phase via
+// the dir/roadmap fallback. init plan-phase must require EXACT prefixed
+// evidence — a phase dir whose token literally IS the prefixed query, or a
+// roadmap entry literally headed with it — before accepting such a match. The
+// configured project_code's own prefix (e.g. LKML-01 under project_code: LKML)
+// is never foreign and always passes through.
+function parsePhasePrefix(phase: unknown): string | null {
+  const match = String(phase).match(/^([A-Z][A-Z0-9_]*)-(?=\d)/i);
+  return match ? match[1] : null;
+}
+
+function isForeignPrefixedPhaseQuery(phase: unknown, projectCode: unknown): boolean {
+  const prefix = parsePhasePrefix(phase);
+  if (!prefix) return false;
+  const configured = typeof projectCode === 'string' ? projectCode.trim() : '';
+  return !configured || prefix.toUpperCase() !== configured.toUpperCase();
+}
+
+function phaseInfoMatchesExactPrefix(
+  phaseInfo: Record<string, unknown> | null,
+  phase: string,
+): boolean {
+  const num = phaseInfo?.['phase_number'];
+  const numStr = typeof num === 'string' ? num : (typeof num === 'number' ? String(num) : '');
+  return numStr.toUpperCase() === phase.toUpperCase();
+}
+
+function roadmapPhaseMatchesExactPrefix(
+  roadmapPhase: Record<string, unknown> | null,
+  phase: string,
+): boolean {
+  const sectionRaw = roadmapPhase?.['section'];
+  const section = typeof sectionRaw === 'string' ? sectionRaw : '';
+  return new RegExp(`^#{2,4}\\s*Phase\\s+${escapeRegex(phase)}(?:\\b|\\s|:)`, 'i').test(section);
+}
+
 function listPhaseSummaryFiles(phaseDir: string): string[] {
   return (scanPhasePlans(phaseDir) as unknown as Record<string, string[]>)['summaryFiles'];
 }
@@ -430,9 +468,19 @@ function cmdInitPlanPhase(
   }
 
   const config = loadConfig(cwd);
+  const foreignPrefixedPhase = isForeignPrefixedPhaseQuery(phase, config.project_code);
   let phaseInfo = findPhaseInternal(cwd, phase) as unknown as Record<string, unknown> | null;
+  // #2056: a foreign-prefixed query must only match a phase whose token literally
+  // IS the prefixed query (e.g. a real 'MEM-01-*' dir); otherwise the numeric
+  // fallback ('MEM-01' → '01') would resolve to the unrelated numeric phase.
+  if (foreignPrefixedPhase && !phaseInfoMatchesExactPrefix(phaseInfo, phase)) {
+    phaseInfo = null;
+  }
 
-  const roadmapPhase = getRoadmapPhaseInternal(cwd, phase) as unknown as Record<string, unknown> | null;
+  let roadmapPhase = getRoadmapPhaseInternal(cwd, phase) as unknown as Record<string, unknown> | null;
+  if (foreignPrefixedPhase && !roadmapPhaseMatchesExactPrefix(roadmapPhase, phase)) {
+    roadmapPhase = null;
+  }
 
   if (phaseInfo?.['archived'] && roadmapPhase?.['found']) {
     phaseInfo = null;
diff --git a/tests/init.test.cjs b/tests/init.test.cjs
index 2ede0d0e8..99b6412c7 100644
--- a/tests/init.test.cjs
+++ b/tests/init.test.cjs
@@ -90,6 +90,93 @@ describe('init commands', () => {
     assert.strictEqual(output.uat_path, '.planning/phases/03-api/03-UAT.md');
   });
 
+  // #2056: normalizePhaseName() strips ANY [A-Z][A-Z0-9_]*- prefix as a project
+  // code, so a foreign-prefixed workstream/task id like "MEM-01" collapsed to
+  // "01" and resolved to the unrelated numeric Phase 01. init plan-phase must
+  // require exact prefixed evidence (a phase dir/roadmap entry literally
+  // carrying the foreign prefix) before accepting a numeric-fallback match.
+  test('#2056 — init plan-phase does not collapse foreign-prefixed task IDs into numeric phases', () => {
+    seedPhase(tmpDir, '01-stable-baseline-on-main', {
+      '01-CONTEXT.md': '# Phase Context',
+    });
+    fs.writeFileSync(
+      path.join(tmpDir, '.planning', 'ROADMAP.md'),
+      '# Roadmap\n\n### Phase 1: Stable Baseline On Main\n**Goal:** Establish baseline\n**Plans:** 1 plan\n',
+    );
+    fs.writeFileSync(
+      path.join(tmpDir, '.planning', 'config.json'),
+      JSON.stringify({ project_code: 'LKML' }, null, 2),
+    );
+
+    const result = runGsdTools('init plan-phase MEM-01', tmpDir);
+    assert.ok(result.success, `Command failed: ${result.error}`);
+
+    const output = JSON.parse(result.output);
+    assert.strictEqual(output.phase_found, false, 'MEM-01 must NOT resolve to numeric Phase 01');
+    assert.strictEqual(output.phase_dir, null);
+    assert.strictEqual(output.phase_number, null);
+  });
+
+  // #2056 companion: the guard must not reject the configured project_code's
+  // OWN prefixed phases — LKML-01 (project_code = LKML) must still resolve.
+  test('#2056 — init plan-phase still resolves configured project-code-prefixed phases', () => {
+    seedPhase(tmpDir, 'LKML-01-stable-baseline-on-main', {
+      'LKML-01-CONTEXT.md': '# Phase Context',
+    });
+    fs.writeFileSync(
+      path.join(tmpDir, '.planning', 'config.json'),
+      JSON.stringify({ project_code: 'LKML' }, null, 2),
+    );
+
+    const result = runGsdTools('init plan-phase LKML-01', tmpDir);
+    assert.ok(result.success, `Command failed: ${result.error}`);
+
+    const output = JSON.parse(result.output);
+    assert.strictEqual(output.phase_found, true, 'LKML-01 (own project code) must resolve');
+    assert.strictEqual(output.phase_dir, '.planning/phases/LKML-01-stable-baseline-on-main');
+    assert.strictEqual(output.phase_number, 'LKML-01');
+  });
+
+  // #2056 accept-branch: a foreign-prefixed query MUST still resolve when a phase
+  // directory literally carries that prefix (e.g. a real MEM-01-* workstream
+  // phase). Proves the guard accepts exact-prefixed evidence, not just rejects.
+  test('#2056 — init plan-phase resolves a real foreign-prefixed phase dir', () => {
+    seedPhase(tmpDir, 'MEM-01-integration', {
+      'MEM-01-CONTEXT.md': '# Phase Context',
+    });
+    fs.writeFileSync(
+      path.join(tmpDir, '.planning', 'config.json'),
+      JSON.stringify({ project_code: 'LKML' }, null, 2),
+    );
+
+    const result = runGsdTools('init plan-phase MEM-01', tmpDir);
+    assert.ok(result.success, `Command failed: ${result.error}`);
+
+    const output = JSON.parse(result.output);
+    assert.strictEqual(output.phase_found, true, 'a real MEM-01-* dir must resolve under its own prefix');
+    assert.strictEqual(output.phase_dir, '.planning/phases/MEM-01-integration');
+    assert.strictEqual(output.phase_number, 'MEM-01');
+  });
+
+  // #2056 edge: with NO project_code configured, any prefixed query is foreign
+  // and must NOT collapse to a numeric phase. Pins the strict default.
+  test('#2056 — init plan-phase treats prefixed queries as foreign when no project_code is configured', () => {
+    seedPhase(tmpDir, '01-stable-baseline-on-main', {
+      '01-CONTEXT.md': '# Phase Context',
+    });
+    fs.writeFileSync(
+      path.join(tmpDir, '.planning', 'config.json'),
+      JSON.stringify({}, null, 2),
+    );
+
+    const result = runGsdTools('init plan-phase MEM-01', tmpDir);
+    assert.ok(result.success, `Command failed: ${result.error}`);
+
+    const output = JSON.parse(result.output);
+    assert.strictEqual(output.phase_found, false, 'with no project_code, MEM-01 must not resolve to numeric Phase 01');
+    assert.strictEqual(output.phase_number, null);
+  });
+
   test('init plan-phase exposes text_mode from config (defaults false)', () => {
     const result = runGsdTools('init plan-phase 03', tmpDir);
     assert.ok(result.success, `Command failed: ${result.error}`);