diff --git a/gitnexus/src/core/run-analyze.ts b/gitnexus/src/core/run-analyze.ts
index 7910dde60..769125ad5 100644
--- a/gitnexus/src/core/run-analyze.ts
+++ b/gitnexus/src/core/run-analyze.ts
@@ -415,6 +415,14 @@ export const resolvePdgConfig = (options: PdgOptions): RepoMeta['pdg'] =>
         // outlive the model that produced them — ANY model-content change
         // ships as a new digest and repopulates the taint edges.
         taintModelVersion,
+        // #2201 review R3: reaching-defs solver identity. The SSA-sparse rewrite
+        // computes full facts for deep-loop functions the dense worklist used to
+        // truncate to empty, so an existing `--pdg` index carries stale-truncated
+        // REACHING_DEF rows. Absent on any pre-#2201 stamp → the key-union
+        // pdgModeMismatch trips on the first upgraded run and forces the full
+        // writeback that recomputes the fuller coverage (no `--force` needed).
+        // Bump this tag on any future change to which facts the solver emits.
+        reachingDefSolver: 'ssa-sparse-v1',
       }
     : undefined;
 
diff --git a/gitnexus/src/storage/repo-manager.ts b/gitnexus/src/storage/repo-manager.ts
index b03db9fc5..9e39a6cc0 100644
--- a/gitnexus/src/storage/repo-manager.ts
+++ b/gitnexus/src/storage/repo-manager.ts
@@ -194,6 +194,19 @@ export interface RepoMeta {
      * without `--force`. Optional: absent on pre-M3 stamps.
      */
     taintModelVersion?: string;
+    /**
+     * Identity of the reaching-definitions solver the persisted REACHING_DEF
+     * rows were produced under (#2201 review R3). The SSA-sparse rewrite computes
+     * FULL facts for deep-loop functions the old dense worklist truncated to
+     * empty (the blocks×64 ceiling no longer fires) — but an existing `--pdg`
+     * index built under the old solver carries those truncated rows. ABSENT on
+     * any pre-#2201 stamp, so that absence trips `pdgModeMismatch` on the first
+     * upgraded run and forces the full writeback that recomputes the now-fuller
+     * REACHING_DEF coverage without `--force`. Bump the tag on any future change
+     * that alters which facts the solver emits. Optional for that upgrade reason;
+     * resolved (always present) on every post-#2201 write.
+     */
+    reachingDefSolver?: string;
   };
 }
 
diff --git a/gitnexus/test/unit/pdg-mode-flip.test.ts b/gitnexus/test/unit/pdg-mode-flip.test.ts
index 5e8dbe2f9..585747eb7 100644
--- a/gitnexus/test/unit/pdg-mode-flip.test.ts
+++ b/gitnexus/test/unit/pdg-mode-flip.test.ts
@@ -176,6 +176,42 @@ describe('pdgModeMismatch — pre-M5→M5 CDG-cap stamp upgrade (#2085 M5, pure)
   });
 });
 
+describe('pdgModeMismatch — pre-#2201→SSA reaching-defs solver upgrade (#2201 review R3, pure)', () => {
+  it('resolvePdgConfig stamps the reaching-defs solver identity', async () => {
+    const { resolvePdgConfig } = await import('../../src/core/run-analyze.js');
+    const stamp = resolvePdgConfig({ pdg: true });
+    expect(stamp?.reachingDefSolver).toBe('ssa-sparse-v1');
+  });
+
+  it('a pre-#2201 stamp (no solver key) mismatches the SSA request — upgrade recomputes truncated deep-loop facts', async () => {
+    const { pdgModeMismatch } = await import('../../src/core/run-analyze.js');
+    // What a pre-#2201 (M5-era) run wrote: every cap + model digest, but NO
+    // reachingDefSolver. The key-union comparator sees 'ssa-sparse-v1' !==
+    // undefined and trips the full writeback that recomputes the now-fuller
+    // REACHING_DEF coverage — the deep-loop functions the dense worklist
+    // truncated to empty at the blocks×64 ceiling now compute full facts.
+    const m5Stamp = {
+      maxFunctionLines: 2000,
+      maxEdgesPerFunction: 5000,
+      maxReachingDefEdgesPerFunction: 4000,
+      maxCdgEdgesPerFunction: 5000,
+      maxTaintFindingsPerFunction: 200,
+      maxTaintHops: 32,
+      maxInterprocFindings: 2000,
+      maxInterprocHops: 32,
+      maxInterprocEdges: 1000,
+      taintModelVersion,
+    };
+    expect(pdgModeMismatch(m5Stamp, { pdg: true })).toBe(true);
+  });
+
+  it('an identical post-#2201 stamp compares equal (no spurious re-analysis churn)', async () => {
+    const { pdgModeMismatch, resolvePdgConfig } = await import('../../src/core/run-analyze.js');
+    const stamp = resolvePdgConfig({ pdg: true });
+    expect(pdgModeMismatch(stamp, { pdg: true })).toBe(false);
+  });
+});
+
 describe('detect_changes BasicBlock exclusion (#2082 U7)', () => {
   it('the symbol-overlap id-prefix filter excludes exactly the BasicBlock rows', async () => {
     const repo = await setupMiniRepo();
@@ -258,6 +294,7 @@ describe('runFullAnalysis — pdg-mode flip (#2099 F1)', () => {
         maxInterprocHops: 32,
         maxInterprocEdges: 1000,
         taintModelVersion,
+        reachingDefSolver: 'ssa-sparse-v1',
       });
       expect(stamped!.incrementalInProgress).toBeUndefined(); // cleared on success
 
@@ -314,6 +351,7 @@ describe('runFullAnalysis — pdg-mode flip (#2099 F1)', () => {
         maxInterprocHops: 32,
         maxInterprocEdges: 1000,
         taintModelVersion,
+        reachingDefSolver: 'ssa-sparse-v1',
       });
       // The CFG layer survives a rebuild under a tighter edge cap (blocks are
       // never capped, only edges).
diff --git a/gitnexus/test/unit/run-analyze.test.ts b/gitnexus/test/unit/run-analyze.test.ts
index d89114360..b317d6824 100644
--- a/gitnexus/test/unit/run-analyze.test.ts
+++ b/gitnexus/test/unit/run-analyze.test.ts
@@ -349,6 +349,10 @@ describe('pdgModeMismatch / resolvePdgConfig (#2099 F1)', () => {
     // Content digest, not a tunable cap — pinned via the exported constant
     // (its VALUE changes whenever the built-in model changes, by design).
     taintModelVersion,
+    // Solver identity, not a tunable cap — always stamped on a pdg-on run
+    // (#2201 review R3). Bumps when the reaching-defs solver's emitted facts
+    // change; absence on a pre-#2201 stamp forces a re-analysis.
+    reachingDefSolver: 'ssa-sparse-v1',
   };
 
   it('resolvePdgConfig: pdg-off run resolves to undefined (the meta field is omitted)', async () => {
@@ -384,6 +388,7 @@ describe('pdgModeMismatch / resolvePdgConfig (#2099 F1)', () => {
       maxInterprocHops: 0,
       maxInterprocEdges: 0,
       taintModelVersion, // not a cap — always stamped on a pdg-on run
+      reachingDefSolver: 'ssa-sparse-v1', // solver identity — always stamped (#2201 R3)
     });
   });