diff --git a/docs/experiment-current-understanding-apparatus.md b/docs/experiment-current-understanding-apparatus.md
index 9658255..5064e50 100644
--- a/docs/experiment-current-understanding-apparatus.md
+++ b/docs/experiment-current-understanding-apparatus.md
@@ -103,4 +103,63 @@ Contents:
 - `situationGraph`: 11 nodes (1 state, 5 observation, 2 metric, 3 unknown), 10 edges, full `reasoningState`, `resolvedNodeIds`, `centralStatement`, `activeUnknownNodeId`
 - `findings`: 9 findings total; all 7 previously verified IDs present (`hciikc`, `ybfsm3`, `k45vyt`, `vhftyr`, `9bgkmw`, `23192q`, `kkh39k`)
 
-Verified: helper parses without invoking live synthesis; fixture immutability confirmed via deep-clone roundtrip.
\ No newline at end of file
+Verified: helper parses without invoking live synthesis; fixture immutability confirmed via deep-clone roundtrip.
+
+## Iteration 6 — Longitudinal Checkpoints (Stage 2)
+
+### New files
+
+| File | Role |
+|------|------|
+| `tests/fixtures/current-understanding-longitudinal-checkpoints.json` | Compact checkpoint manifest referencing original fixture; lists included contribution IDs and finding IDs per checkpoint |
+| `tests/graph/current-understanding-experiment-helper.test.js` (extended) | 15 new longitudinal assertions in `Longitudinal checkpoints — manifest integrity` describe block |
+
+### Checkpoint manifest structure
+
+The manifest contains:
+- `manifestVersion`, `description`, `sourceFixture`, `contaminationAssessment`
+- `contributions[]` — ordered list with timestamps and ordering integers
+- `findingsById` — verbatim copy of each original finding (by ID)
+- `checkpoints[]` — four cumulative states: `{ id, includedContributions[], includedFindings[] }`
+
+### Checkpoint state summary
+
+| Checkpoint | Description | Contribs | Findings | Count |
+|---|---|---|---|---|
+| checkpoint-0 | Baseline (no findings) | none | none | 0 |
+| checkpoint-1 | contrib-0001 | contrib-0001 | 3bw5f0, i5zdcq | 2 |
+| checkpoint-2 | contrib-0001 + contrib-0002 | contrib-0001, contrib-0002 | 7 findings | 7 |
+| checkpoint-3 | All contributions | all three | all 9 original findings | 9 |
+
+### Graph-state contamination analysis
+
+`synthesizeCurrentUnderstanding()` and `buildGraphEvidenceProjection()` project **only**:
+- Nodes with `status: "known"` or `"supported"` (filtered)
+- `centralStatement` (framing only)
+- Findings passed as separate parameter
+
+**Explicitly excluded from prompt:** `resolvedNodeIds`, `activeUnknownNodeId`, `reasoningState`, edges, `currentSummary`, nodes outside known/supported statuses.
+
+**Conclusion: NONE — final-state graph fields do not leak into synthesis input.**
+
+### Compatibility gap
+
+The existing helper consumes a single `{ situationGraph, findings }` fixture via `--file`. The checkpoint manifest is **NOT directly consumable by the existing helper** without bounded extension: the manifest uses `findingsById` lookup keyed by finding ID rather than a flat findings array alongside a pre-filtered graph. For longitudinal comparison experiments, the consuming code must iterate checkpoints and project `{ situationGraph (unchanged), findings[] }` per checkpoint before invoking the helper or synthesis function directly.
+
+### Test assertions added (15 new tests)
+
+1. Manifest has exactly 4 checkpoints
+2. Checkpoint counts are 0, 2, 7, 9
+3. checkpoint-0 has no included findings
+4. checkpoint-1 includes exactly contrib-0001 findings
+5. checkpoint-2 is cumulative of checkpoint-1 plus contrib-0002 findings
+6. checkpoint-3 includes all 9 original findings
+7. All checkpoint findings are verbatim from the original fixture
+8. Contribution ordering matches recorded timestamps
+9. Manifest ordering matches fixture finding order
+10. checkpoint-3 is cumulative of all checkpoints before it
+11. No findings appear before their contribution boundary
+12. Original fixture unchanged (spot check)
+13. Production synthesis contract accepts all checkpoint inputs
+14. Manifest references the source fixture path
+15. Manifest does not fabricate historical graph state
\ No newline at end of file
diff --git a/tests/fixtures/current-understanding-longitudinal-checkpoints.json b/tests/fixtures/current-understanding-longitudinal-checkpoints.json
new file mode 100644
index 0000000..c671e7d
--- /dev/null
+++ b/tests/fixtures/current-understanding-longitudinal-checkpoints.json
@@ -0,0 +1,167 @@
+{
+  "manifestVersion": "1.0.0",
+  "description": "Longitudinal cumulative evidence checkpoints for Current Understanding synthesis comparison experiments.",
+  "sourceFixture": "tests/fixtures/current-understanding-original-investigation.json",
+  "contaminationAssessment": "NONE",
+  "contributions": [
+    {
+      "id": "contrib-0001",
+      "ordering": 1,
+      "timestamp": "2026-10-08T06:50:06.355Z"
+    },
+    {
+      "id": "contrib-0002",
+      "ordering": 2,
+      "timestamp": "2026-10-08T06:55:45.665Z"
+    },
+    {
+      "id": "contrib-0003",
+      "ordering": 3,
+      "timestamp": "2026-10-08T07:01:23.359Z"
+    }
+  ],
+  "checkpoints": [
+    {
+      "id": "checkpoint-0",
+      "includedContributions": [],
+      "includedFindings": []
+    },
+    {
+      "id": "checkpoint-1",
+      "includedContributions": [
+        "contrib-0001"
+      ],
+      "includedFindings": [
+        "finding-3bw5f0",
+        "finding-i5zdcq"
+      ]
+    },
+    {
+      "id": "checkpoint-2",
+      "includedContributions": [
+        "contrib-0001",
+        "contrib-0002"
+      ],
+      "includedFindings": [
+        "finding-3bw5f0",
+        "finding-i5zdcq",
+        "finding-hciikc",
+        "finding-ybfsm3",
+        "finding-k45vyt",
+        "finding-vhftyr",
+        "finding-9bgkmw"
+      ]
+    },
+    {
+      "id": "checkpoint-3",
+      "includedContributions": [
+        "contrib-0001",
+        "contrib-0002",
+        "contrib-0003"
+      ],
+      "includedFindings": [
+        "finding-3bw5f0",
+        "finding-i5zdcq",
+        "finding-hciikc",
+        "finding-ybfsm3",
+        "finding-k45vyt",
+        "finding-vhftyr",
+        "finding-9bgkmw",
+        "finding-23192q",
+        "finding-kkh39k"
+      ]
+    }
+  ],
+  "findingsById": {
+    "finding-3bw5f0": {
+      "id": "finding-3bw5f0",
+      "status": "provisional",
+      "createdAt": "2026-10-08T06:50:06.355Z",
+      "proposition": "One enterprise customer has signed an LOI but no binding contract yet.",
+      "contributionId": "contrib-0001",
+      "userDisposition": null,
+      "sourceObservation": "One enterprise customer has signed an LOI but no binding contract yet.",
+      "originatingTargetNodeId": "nw0dwkz"
+    },
+    "finding-i5zdcq": {
+      "id": "finding-i5zdcq",
+      "status": "provisional",
+      "createdAt": "2026-10-08T06:50:06.355Z",
+      "proposition": "That customer's final contract execution depends on internal pricing approval expected within thirty days.",
+      "contributionId": "contrib-0001",
+      "userDisposition": null,
+      "sourceObservation": "That customer's final contract execution depends on internal pricing approval expected within thirty days.",
+      "originatingTargetNodeId": "nw0dwkz"
+    },
+    "finding-hciikc": {
+      "id": "finding-hciikc",
+      "status": "provisional",
+      "createdAt": "2026-10-08T06:55:45.665Z",
+      "proposition": "Absence of reliable evidence linking a twelve-month delay to changes in customer retention",
+      "contributionId": "contrib-0002",
+      "userDisposition": null,
+      "sourceObservation": "Absence of reliable evidence linking a twelve-month delay to changes in customer retention",
+      "originatingTargetNodeId": "nfl7ee3"
+    },
+    "finding-ybfsm3": {
+      "id": "finding-ybfsm3",
+      "status": "provisional",
+      "createdAt": "2026-10-08T06:55:45.665Z",
+      "proposition": "Failure to identify which prospective customers would remain interested after a delay",
+      "contributionId": "contrib-0002",
+      "userDisposition": null,
+      "sourceObservation": "Failure to identify which prospective customers would remain interested after a delay",
+      "originatingTargetNodeId": "nfl7ee3"
+    },
+    "finding-k45vyt": {
+      "id": "finding-k45vyt",
+      "status": "provisional",
+      "createdAt": "2026-10-08T06:55:45.665Z",
+      "proposition": "Failure to assess whether customer purchasing decisions would shift due to the delay",
+      "contributionId": "contrib-0002",
+      "userDisposition": null,
+      "sourceObservation": "Failure to assess whether customer purchasing decisions would shift due to the delay",
+      "originatingTargetNodeId": "nfl7ee3"
+    },
+    "finding-vhftyr": {
+      "id": "finding-vhftyr",
+      "status": "provisional",
+      "createdAt": "2026-10-08T06:55:45.665Z",
+      "proposition": "Unestablished impact of competitor activity on customer willingness to wait",
+      "contributionId": "contrib-0002",
+      "userDisposition": null,
+      "sourceObservation": "Unestablished impact of competitor activity on customer willingness to wait",
+      "originatingTargetNodeId": "nfl7ee3"
+    },
+    "finding-9bgkmw": {
+      "id": "finding-9bgkmw",
+      "status": "provisional",
+      "createdAt": "2026-10-08T06:55:45.665Z",
+      "proposition": "Unknown financial consequences tied to potential customer loss during the delay",
+      "contributionId": "contrib-0002",
+      "userDisposition": null,
+      "sourceObservation": "Unknown financial consequences tied to potential customer loss during the delay",
+      "originatingTargetNodeId": "nfl7ee3"
+    },
+    "finding-23192q": {
+      "id": "finding-23192q",
+      "status": "provisional",
+      "createdAt": "2026-10-08T07:01:23.359Z",
+      "proposition": "You lack reliable evidence regarding competitors' plans for the next twelve months.",
+      "contributionId": "contrib-0003",
+      "userDisposition": null,
+      "sourceObservation": "You lack reliable evidence regarding competitors' plans for the next twelve months.",
+      "originatingTargetNodeId": "nhgwmv7"
+    },
+    "finding-kkh39k": {
+      "id": "finding-kkh39k",
+      "status": "provisional",
+      "createdAt": "2026-10-08T07:01:23.359Z",
+      "proposition": "You have not established whether competitors will launch similar products, adjust prices, or shift customer expectations.",
+      "contributionId": "contrib-0003",
+      "userDisposition": null,
+      "sourceObservation": "You have not established whether competitors will launch similar products, adjust prices, or shift customer expectations.",
+      "originatingTargetNodeId": "nhgwmv7"
+    }
+  }
+}
\ No newline at end of file
diff --git a/tests/graph/current-understanding-experiment-helper.test.js b/tests/graph/current-understanding-experiment-helper.test.js
index 104ab79..d7a7275 100644
--- a/tests/graph/current-understanding-experiment-helper.test.js
+++ b/tests/graph/current-understanding-experiment-helper.test.js
@@ -611,4 +611,133 @@ describe("CU synthesis experiment helper — --include-output flag", function ()
     expect(JSON.stringify(situationGraph)).toBe(JSON.stringify(graphBefore));
     expect(JSON.stringify(findings)).toBe(JSON.stringify(fixBefore));
   });
+});
+
+// ── Longitudinal checkpoint deterministic verification ──
+
+import longitudinalManifest   from "@/tests/fixtures/current-understanding-longitudinal-checkpoints.json" assert { type: "json" };
+import originalFixture        from "@/tests/fixtures/current-understanding-original-investigation.json"    assert { type: "json" };
+
+describe("Longitudinal checkpoints — manifest integrity", function () {
+
+  it("manifest has exactly 4 checkpoints", async function () {
+    expect(longitudinalManifest.checkpoints.length).toBe(4);
+  });
+
+  it("checkpoint counts are 0, 2, 7, 9", async function () {
+    const expectedCounts = [0, 2, 7, 9];
+    longitudinalManifest.checkpoints.forEach((cp, i) => {
+      expect(cp.includedFindings.length).toBe(expectedCounts[i]);
+    });
+  });
+
+  it("checkpoint-0 has no included findings", async function () {
+    expect(longitudinalManifest.checkpoints[0].includedFindings).toEqual([]);
+  });
+
+  it("checkpoint-1 includes exactly contrib-0001 findings", async function () {
+    const expected = ["finding-3bw5f0", "finding-i5zdcq"];
+    expect(longitudinalManifest.checkpoints[1].includedFindings).toEqual(expected);
+  });
+
+  it("checkpoint-2 is cumulative of checkpoint-1 plus contrib-0002 findings", async function () {
+    const cp2 = new Set(longitudinalManifest.checkpoints[2].includedFindings);
+    const cp1 = longitudinalManifest.checkpoints[1].includedFindings;
+    cp1.forEach(f => expect(cp2.has(f)).toBe(true)); // all checkpoint-1 findings present in cp2
+    expect(cp2.size).toBe(7);
+  });
+
+  it("checkpoint-3 includes all 9 original findings", async function () {
+    const expected = [
+      "finding-3bw5f0", "finding-i5zdcq",
+      "finding-hciikc", "finding-ybfsm3", "finding-k45vyt",
+      "finding-vhftyr", "finding-9bgkmw",
+      "finding-23192q", "finding-kkh39k"
+    ];
+    expect(longitudinalManifest.checkpoints[3].includedFindings).toEqual(expected);
+  });
+
+  it("all checkpoint findings are verbatim from the original fixture", async function () {
+    const originalMap = {};
+    originalFixture.findings.forEach(f => { originalMap[f.id] = f; });
+    longitudinalManifest.checkpoints.forEach(cp => {
+      cp.includedFindings.forEach(fid => {
+        expect(originalMap[fid]).toBeDefined();
+        const manifestFinding = longitudinalManifest.findingsById[fid];
+        expect(manifestFinding).toEqual(originalMap[fid]);
+      });
+    });
+  });
+
+  it("contribution ordering matches recorded timestamps", async function () {
+    const contribs = longitudinalManifest.contributions;
+    for (let i = 1; i < contribs.length; i++) {
+      expect(new Date(contribs[i].timestamp).getTime()).toBeGreaterThan(
+        new Date(contribs[i - 1].timestamp).getTime()
+      );
+    }
+  });
+
+  it("manifest ordering matches fixture finding order", async function () {
+    const originalIds = originalFixture.findings.map(f => f.id);
+    const manifestIds = Object.keys(longitudinalManifest.findingsById);
+    expect(manifestIds).toEqual(originalIds);
+  });
+
+  it("checkpoint-3 is cumulative of all checkpoints before it", async function () {
+    for (let i = 0; i < 3; i++) {
+      const cpSet = new Set(longitudinalManifest.checkpoints[i].includedFindings);
+      const cp3 = longitudinalManifest.checkpoints[3].includedFindings;
+      cpSet.forEach(f => expect(cp3.includes(f)).toBe(true));
+    }
+  });
+
+  it("no findings appear before their contribution boundary", async function () {
+    const contribOrder = {};
+    longitudinalManifest.contributions.forEach(c => { contribOrder[c.id] = c.ordering; });
+    Object.entries(longitudinalManifest.findingsById).forEach(([fid, f]) => {
+      const cpIndex = longitudinalManifest.checkpoints.findIndex(cp =>
+        cp.includedFindings.includes(fid)
+      );
+      // Find which checkpoint first includes this finding
+      expect(cpIndex).toBeGreaterThanOrEqual(contribOrder[f.contributionId] - 1);
+    });
+  });
+
+  it("original fixture is unchanged (spot check)", async function () {
+    expect(originalFixture.findings.length).toBe(9);
+    expect(longitudinalManifest.checkpoints[3].includedFindings.length).toBe(9);
+  });
+
+  it("production synthesis contract accepts checkpoint inputs", async function () {
+    // Verify that the filtered findings from each checkpoint can reach synthesizeCurrentUnderstanding
+    const fake = makeFakeProvider();
+
+    for (const cp of longitudinalManifest.checkpoints) {
+      const graphBefore = deepClone(situationGraph);
+      const cpFindings = cp.includedFindings.map(fid => originalFixture.findings.find(f => f.id === fid));
+
+      await expect(
+        synthesizeCurrentUnderstanding(
+          { situationGraph: situationGraph, findings: cpFindings },
+          { provider: fake }
+        )
+      ).resolves.toBeDefined();
+
+      // Immutability check per checkpoint
+      expect(JSON.stringify(situationGraph)).toBe(JSON.stringify(graphBefore));
+    }
+  });
+
+  it("manifest references the source fixture path", async function () {
+    expect(longitudinalManifest.sourceFixture).toBe(
+      "tests/fixtures/current-understanding-original-investigation.json"
+    );
+  });
+
+  it("manifest does not fabricate historical graph state", async function () {
+    // Manifest contains findings data only, not resolvedNodeIds, activeUnknownNodeId, or reasoningState
+    expect(longitudinalManifest.checkpoints).not.toHaveProperty([0, "resolvedNodeIds"]);
+    expect(longitudinalManifest.contributions).not.toHaveProperty([0, "reasoningState"]);
+  });
 });
\ No newline at end of file