diff --git a/src/resources/extensions/gsd/tests/projection-no-plan-overwrite.test.ts b/src/resources/extensions/gsd/tests/projection-no-plan-overwrite.test.ts
new file mode 100644
index 000000000..e87c3a4ca
--- /dev/null
+++ b/src/resources/extensions/gsd/tests/projection-no-plan-overwrite.test.ts
@@ -0,0 +1,83 @@
+/**
+ * Regression test for #3651 — renderAllProjections must NOT call renderPlanProjection
+ *
+ * renderAllProjections previously called renderPlanProjection inside the slice
+ * loop, which overwrote the authoritative PLAN.md (produced by markdown-renderer.js
+ * in plan-slice/replan-slice tools) with a simplified projection that was missing
+ * key sections (Must-Haves, Verification, Files Likely Touched) and corrupted
+ * multi-line task descriptions.
+ *
+ * The fix removes the renderPlanProjection call from the renderAllProjections
+ * loop. The renderIfMissing recovery path is preserved.
+ */
+
+import { describe, it } from 'node:test'
+import assert from 'node:assert/strict'
+import { readFileSync } from 'node:fs'
+import { resolve } from 'node:path'
+
+// Use process.cwd() based resolution instead of import.meta.url
+// to avoid tsx test runner path resolution issues
+const src = readFileSync(
+  resolve(process.cwd(), 'src', 'resources', 'extensions', 'gsd', 'workflow-projections.ts'),
+  'utf-8',
+)
+
+describe('renderAllProjections must not overwrite PLAN.md (#3651)', () => {
+  it('renderAllProjections function body does NOT invoke renderPlanProjection', () => {
+    // Extract the renderAllProjections function body
+    const fnStart = src.indexOf('export async function renderAllProjections(')
+    assert.ok(fnStart !== -1, 'renderAllProjections function must exist')
+
+    // Find the for-loop over sliceRows inside renderAllProjections
+    const loopStart = src.indexOf('for (const slice of sliceRows)', fnStart)
+    assert.ok(loopStart !== -1, 'slice loop must exist in renderAllProjections')
+
+    // Find the closing of renderAllProjections (next section marker)
+    const fnEnd = src.indexOf('\n// ─── ', fnStart + 1)
+    assert.ok(fnEnd !== -1, 'section delimiter after renderAllProjections must exist')
+
+    const fnBody = src.slice(loopStart, fnEnd)
+
+    // The fix: renderPlanProjection must NOT appear as a function call.
+    // Strip comment lines before checking (comments may mention the function name).
+    const codeOnly = fnBody
+      .split('\n')
+      .filter(line => !line.trim().startsWith('//'))
+      .join('\n')
+
+    const hasPlanCall = /renderPlanProjection\s*\(/.test(codeOnly)
+    assert.equal(
+      hasPlanCall,
+      false,
+      'renderPlanProjection must not be called inside the renderAllProjections slice loop — ' +
+        'authoritative PLAN.md is rendered only by plan-slice/replan-slice tools',
+    )
+  })
+
+  it('renderPlanProjection is still defined (available for regenerateIfMissing)', () => {
+    assert.ok(
+      src.includes('function renderPlanProjection('),
+      'renderPlanProjection function definition must still exist for on-demand recovery',
+    )
+  })
+
+  it('renderAllProjections still renders ROADMAP, SUMMARY, and STATE projections', () => {
+    const fnStart = src.indexOf('export async function renderAllProjections(')
+    const fnEnd = src.indexOf('\n// ─── ', fnStart + 1)
+    const fnBody = src.slice(fnStart, fnEnd)
+
+    assert.ok(
+      fnBody.includes('renderRoadmapProjection('),
+      'renderRoadmapProjection must still be called',
+    )
+    assert.ok(
+      fnBody.includes('renderSummaryProjection('),
+      'renderSummaryProjection must still be called',
+    )
+    assert.ok(
+      fnBody.includes('renderStateProjection('),
+      'renderStateProjection must still be called',
+    )
+  })
+})