feat(#72): IMP-43 u1~u8 --reuse-from incremental rerun (Step 0/1/2/5/6 reuse + Step 7+ re-execute)

u1 argparse --reuse-from PREV_RUN_ID + post-merge fail-closed guard (rejects
layout/zone_geometry/zone_section/image override axes by name; only
--override-frame is preserved).
u2 src/phase_z2_reuse_snapshot.py — JSON-only Step 6 snapshot with mdx_sha256
integrity key and {value, source_path, upstream_step} provenance per axis
(pickle forbidden per Stage 2 guardrail).
u3 _write_reuse_snapshot at the Step 6 boundary; soft-fails to stderr without
aborting the seed run.
u4 prev_run_dir RO copy of step00/01/02/05/06 + _reuse_snapshot.json into
new run_dir, state rehydration, reuse marker, frame-override application on
restored units, Step 7+ resume.
u4b fail-closed for missing prev_run_dir / missing/corrupt/invalid snapshot /
mdx_sha256 mismatch / accidental new==prev write, with value+path+upstream
diagnostics per axis.
u5 reuse_from Optional[str] threaded through run_phase_z2_mvp1 signature and
CLI dispatch; default None preserves byte-identical pre-IMP-43 behavior.
u6 Front /api/run optional reuseFromRunId forwarding (vite.config.ts +
designAgentApi.ts + run_pipeline_reuse_from.test.ts).
u7a fast CI equivalence (1 mdx × 1 layout × 2 frames); step13 whitelist =
run_id/timestamps/prev_run_id only. u7b 3 layouts × 3 mdx × 32 frames
sweep gated by pytest.mark.sweep (registered in pyproject.toml; default CI
must use -m 'not sweep').
u8 scripts/measure_reuse_savings.py argv-driven A/B/C harness with frame
pin self-discovery + seed-time exclusion; status board §8 TBD anchor
(issue-body 50-70% / 10-20s→3-8s claim explicitly unverified, not mirrored).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Front/client/src/services/designAgentApi.ts

@@ -345,13 +345,25 @@ export interface PipelineOverrides {
 
 export async function runPipeline(
   file: File,
-  overrides?: PipelineOverrides
+  overrides?: PipelineOverrides,
+  // IMP-43 (#72) u6 — optional prev RUN_ID for incremental rerun. When set,
+  // the vite plugin forwards `--reuse-from <PREV_RUN_ID>` to the backend
+  // and the pipeline resumes at Step 7 (Step 0/1/2/5/6 artifacts copied
+  // from the prior run). When omitted / empty, the POST body is
+  // byte-identical to pre-u6 (no reuseFromRunId key → no flag forwarded).
+  reuseFromRunId?: string,
 ): Promise<RunPipelineResult> {
   const content = await file.text();
+  const body: Record<string, unknown> = {
+    filename: file.name,
+    content,
+    overrides,
+  };
+  if (reuseFromRunId) body.reuseFromRunId = reuseFromRunId;
   const res = await fetch("/api/run", {
     method: "POST",
     headers: { "Content-Type": "application/json" },
-    body: JSON.stringify({ filename: file.name, content, overrides }),
+    body: JSON.stringify(body),
   });
   const data = (await res.json()) as RunPipelineResult;
   if (!res.ok && !data.run_id) {

Front/client/tests/run_pipeline_reuse_from.test.ts

@@ -0,0 +1,250 @@
+// IMP-43 (#72) u6 — /api/run reuseFromRunId forwarding coverage.
+//
+// Stage 2 unit scope:
+//   1) Front/client/src/services/designAgentApi.ts `runPipeline`:
+//      • accepts an optional 3rd arg `reuseFromRunId: string`.
+//      • includes `reuseFromRunId` in the POST body when truthy.
+//      • OMITS `reuseFromRunId` from the body when absent / empty / undefined
+//        → byte-identical to the pre-u6 POST contract (absent flag = full
+//        pipeline; backend u1 guard never sees an empty PREV_RUN_ID).
+//      • leaves `filename`, `content`, and `overrides` untouched alongside
+//        the new field (no payload-shape regression).
+//   2) Front/vite.config.ts `/api/run` handler:
+//      • declares `reuseFromRunId?: string` in the payload type so a typed
+//        client cannot send a payload the server silently drops.
+//      • destructures `reuseFromRunId` from `payload` (sibling of
+//        `overrides`, NOT nested under it — the backend u1 post-merge
+//        guard treats reuse as a pipeline mode, not an override).
+//      • forwards `--reuse-from <PREV_RUN_ID>` to spawn cliArgs guarded by
+//        a truthy check (empty string / undefined ⇒ no flag, per Stage 2
+//        contract: invalid CLI args must never reach argparse).
+//      • places the forward block AFTER the `--override-section-assignment`
+//        loop so the spawn argv preserves backend argparse's no-positional-
+//        before-flag expectation and so `--override-frame` (still allowed
+//        by the u1 guard) is positioned ahead of `--reuse-from`.
+//
+// runPipeline is exercised with a duck-typed `File` plus a `vi.stubGlobal`
+// fetch mock — mirrors the user_overrides_service.test.ts pattern. The
+// vite handler is source-sliced (mirrors handle_generate_diag.test.ts)
+// because the handler spawns python and a real /api/run round-trip is
+// out of unit-test scope.
+
+import { afterEach, beforeEach, describe, expect, it, vi, type Mock } from "vitest";
+import { readFileSync } from "node:fs";
+import { resolve } from "node:path";
+import { runPipeline } from "../src/services/designAgentApi";
+
+// ---------------------------------------------------------------------------
+// vite.config.ts source — read once for the handler source-slice assertions.
+// Path: Front/client/tests/ → Front/vite.config.ts (two levels up).
+// ---------------------------------------------------------------------------
+const VITE_CONFIG_PATH = resolve(__dirname, "..", "..", "vite.config.ts");
+const VITE_CONFIG_SOURCE = readFileSync(VITE_CONFIG_PATH, "utf-8");
+
+// ---------------------------------------------------------------------------
+// fetch mock — minimal Response stub mirroring runPipeline's `.ok` + `.json()`
+// + `.status` surface. Same shape as the user_overrides_service.test.ts
+// helper so the two test files stay drift-free.
+// ---------------------------------------------------------------------------
+type MockResponse = {
+  ok: boolean;
+  status: number;
+  json: () => Promise<unknown>;
+};
+
+function mockResponse(body: unknown, ok = true, status = 200): MockResponse {
+  return { ok, status, json: async () => body };
+}
+
+const SUCCESS_BODY = {
+  success: true,
+  run_id: "test_run_id_20260524",
+  exit_code: 0,
+  final_html_exists: true,
+  preview_exists: true,
+  stdout: "",
+  stderr: "",
+};
+
+// Duck-typed File — runPipeline reads only `.name` and `.text()`. Avoids a
+// hard dependency on the global File constructor (varies across node /
+// jsdom / happy-dom test environments).
+function makeFakeFile(name: string, content: string): File {
+  return {
+    name,
+    text: async () => content,
+  } as unknown as File;
+}
+
+let fetchMock: Mock;
+
+beforeEach(() => {
+  fetchMock = vi.fn();
+  vi.stubGlobal("fetch", fetchMock);
+});
+
+afterEach(() => {
+  vi.unstubAllGlobals();
+});
+
+function lastPostBody(): Record<string, unknown> {
+  const lastCall = fetchMock.mock.calls.at(-1);
+  if (!lastCall) throw new Error("fetch was not called");
+  const init = lastCall[1] as RequestInit | undefined;
+  if (!init?.body) throw new Error("fetch was called without a body");
+  return JSON.parse(String(init.body));
+}
+
+// ============================================================================
+// runPipeline (designAgentApi.ts) — forwarding/omission coverage
+// ============================================================================
+
+describe("runPipeline reuseFromRunId forwarding (IMP-43 #72 u6)", () => {
+  it("posts to /api/run via POST with JSON content-type", async () => {
+    fetchMock.mockResolvedValueOnce(mockResponse(SUCCESS_BODY));
+    await runPipeline(makeFakeFile("03.mdx", "# title"));
+    expect(fetchMock).toHaveBeenCalledTimes(1);
+    const [url, init] = fetchMock.mock.calls[0];
+    expect(url).toBe("/api/run");
+    expect((init as RequestInit).method).toBe("POST");
+    expect((init as RequestInit).headers).toMatchObject({
+      "Content-Type": "application/json",
+    });
+  });
+
+  it("includes reuseFromRunId in the POST body when provided", async () => {
+    fetchMock.mockResolvedValueOnce(mockResponse(SUCCESS_BODY));
+    await runPipeline(
+      makeFakeFile("03.mdx", "# title"),
+      undefined,
+      "mdx03_20260524080000",
+    );
+    const body = lastPostBody();
+    expect(body.reuseFromRunId).toBe("mdx03_20260524080000");
+    expect(body.filename).toBe("03.mdx");
+    expect(body.content).toBe("# title");
+  });
+
+  it("omits reuseFromRunId when 3rd arg is undefined (pre-u6 byte-identical)", async () => {
+    fetchMock.mockResolvedValueOnce(mockResponse(SUCCESS_BODY));
+    await runPipeline(makeFakeFile("03.mdx", "# title"));
+    const body = lastPostBody();
+    expect("reuseFromRunId" in body).toBe(false);
+    // Pre-u6 contract: filename/content are the only keys when overrides
+    // is undefined (JSON.stringify drops undefined values; pre-u6 emitted
+    // `JSON.stringify({filename, content, overrides})` with the same
+    // drop-undefined behaviour, so the wire body is byte-identical).
+    expect(Object.keys(body).sort()).toEqual(["content", "filename"]);
+  });
+
+  it("omits reuseFromRunId but keeps overrides when only overrides provided", async () => {
+    fetchMock.mockResolvedValueOnce(mockResponse(SUCCESS_BODY));
+    await runPipeline(makeFakeFile("03.mdx", "# title"), {
+      frames: { "03-1": "frame_07" },
+    });
+    const body = lastPostBody();
+    expect("reuseFromRunId" in body).toBe(false);
+    expect(Object.keys(body).sort()).toEqual([
+      "content",
+      "filename",
+      "overrides",
+    ]);
+    expect(body.overrides).toEqual({ frames: { "03-1": "frame_07" } });
+  });
+
+  it("omits reuseFromRunId when passed an empty string (truthy guard)", async () => {
+    fetchMock.mockResolvedValueOnce(mockResponse(SUCCESS_BODY));
+    await runPipeline(makeFakeFile("03.mdx", "# title"), undefined, "");
+    const body = lastPostBody();
+    expect("reuseFromRunId" in body).toBe(false);
+  });
+
+  it("forwards reuseFromRunId alongside frame overrides (the only u1-permitted combo)", async () => {
+    fetchMock.mockResolvedValueOnce(mockResponse(SUCCESS_BODY));
+    await runPipeline(
+      makeFakeFile("03.mdx", "# title"),
+      { frames: { "03-1+03-2": "frame_07" } },
+      "mdx03_20260524080000",
+    );
+    const body = lastPostBody();
+    expect(body.overrides).toEqual({ frames: { "03-1+03-2": "frame_07" } });
+    expect(body.reuseFromRunId).toBe("mdx03_20260524080000");
+  });
+
+  it("returns the parsed RunPipelineResult on success", async () => {
+    fetchMock.mockResolvedValueOnce(mockResponse(SUCCESS_BODY));
+    const res = await runPipeline(
+      makeFakeFile("03.mdx", "# title"),
+      undefined,
+      "mdx03_20260524080000",
+    );
+    expect(res.success).toBe(true);
+    expect(res.run_id).toBe("test_run_id_20260524");
+  });
+});
+
+// ============================================================================
+// /api/run handler (vite.config.ts) — source-slice forwarding contract
+// ============================================================================
+
+describe("/api/run handler reuseFromRunId source-slice (IMP-43 #72 u6)", () => {
+  it("declares reuseFromRunId?: string on the /api/run payload type", () => {
+    // Payload type at the top of the /api/run handler body. The
+    // optional-string declaration is the single source-of-truth for what
+    // shape the handler accepts; a typed frontend client (u5 saveUserOverrides
+    // sibling pattern) cannot silently send a payload the server drops.
+    expect(VITE_CONFIG_SOURCE).toMatch(/reuseFromRunId\?:\s*string\s*;/);
+  });
+
+  it("destructures reuseFromRunId from payload alongside filename/content/overrides", () => {
+    expect(VITE_CONFIG_SOURCE).toMatch(
+      /const\s*\{\s*filename\s*,\s*content\s*,\s*overrides\s*,\s*reuseFromRunId\s*\}\s*=\s*payload\s*;/,
+    );
+  });
+
+  it("forwards --reuse-from <PREV_RUN_ID> after the override-section-assignment loop", () => {
+    // Stage 2 contract: reuse_from is a pipeline mode, not an override.
+    // The forward block must sit AFTER the last override loop so the spawn
+    // argv preserves the order documented in the u1 backend post-merge
+    // guard (overrides parsed first; reuse_from precondition runs against
+    // the merged overrides view).
+    const reuseFromIdx = VITE_CONFIG_SOURCE.indexOf('"--reuse-from"');
+    const zoneSectionsIdx = VITE_CONFIG_SOURCE.indexOf(
+      '"--override-section-assignment"',
+    );
+    expect(reuseFromIdx).toBeGreaterThan(-1);
+    expect(zoneSectionsIdx).toBeGreaterThan(-1);
+    expect(reuseFromIdx).toBeGreaterThan(zoneSectionsIdx);
+  });
+
+  it("guards the forward with a truthy check on reuseFromRunId", () => {
+    // Empty string / undefined ⇒ no flag pushed (Stage 2 contract: invalid
+    // CLI args must never reach argparse — the backend u1 guard would
+    // fail-closed with `reuse_artifact_missing` on the empty PREV_RUN_ID).
+    const reuseFromIdx = VITE_CONFIG_SOURCE.indexOf('"--reuse-from"');
+    expect(reuseFromIdx).toBeGreaterThan(-1);
+    const preface = VITE_CONFIG_SOURCE.slice(
+      Math.max(0, reuseFromIdx - 200),
+      reuseFromIdx,
+    );
+    expect(preface).toMatch(/if\s*\(\s*reuseFromRunId/);
+    expect(preface).toMatch(/typeof\s+reuseFromRunId\s*===\s*"string"/);
+  });
+
+  it("pushes reuseFromRunId as the --reuse-from argument value (no string interpolation)", () => {
+    // The CLI value must be the raw PREV_RUN_ID — no `=` join, no quoting
+    // (spawn is shell:false). Mirrors the `--override-layout` shape.
+    const reuseFromIdx = VITE_CONFIG_SOURCE.indexOf('"--reuse-from"');
+    expect(reuseFromIdx).toBeGreaterThan(-1);
+    // Window spans both before (`cliArgs.push(`) and after
+    // (`reuseFromRunId)`) the literal so the full push expression is
+    // captured.
+    const window = VITE_CONFIG_SOURCE.slice(
+      Math.max(0, reuseFromIdx - 100),
+      reuseFromIdx + 200,
+    );
+    expect(window).toMatch(
+      /cliArgs\.push\(\s*"--reuse-from"\s*,\s*reuseFromRunId\s*\)/,
+    );
+  });
+});