diff --git a/gitnexus-shared/src/scope-resolution/parsed-file.ts b/gitnexus-shared/src/scope-resolution/parsed-file.ts
index 98a327ad4..01b70f206 100644
--- a/gitnexus-shared/src/scope-resolution/parsed-file.ts
+++ b/gitnexus-shared/src/scope-resolution/parsed-file.ts
@@ -98,4 +98,26 @@ export interface ParsedFile {
    * side effects — the contract default) leave this undefined.
    */
   readonly captureSideChannel?: unknown;
+
+  /**
+   * Per-function control-flow graphs for this file (#2081 M1, PDG/taint
+   * substrate). A DISTINCT field from {@link captureSideChannel} — different
+   * producer, consumer, and lifecycle: the worker builds it from the
+   * tree-sitter AST via `LanguageProvider.cfgVisitor` (only on a `--pdg` run),
+   * and scope-resolution emits BasicBlock nodes + CFG edges from it while the
+   * disk-backed ParsedFile store is still live (it is NOT a capture-time
+   * marker the resolver restores into module maps). Kept separate so a future
+   * change to either channel's shape invalidates independently.
+   *
+   * Shared / ingestion code treats this as opaque (`unknown`) per AGENTS.md.
+   * Concretely it is a `readonly FunctionCfg[]` (see
+   * `core/ingestion/cfg/types.ts`) — plain JSON-serializable data (no AST
+   * refs, no class instances) so it round-trips through the parse cache and
+   * the `parsedfile-store` (whose interning reviver keys on `nodeId`, which
+   * these blocks/edges deliberately lack).
+   *
+   * Optional: `undefined` on non-`--pdg` runs and for languages with no
+   * `cfgVisitor` — the default for every run today.
+   */
+  readonly cfgSideChannel?: unknown;
 }
diff --git a/gitnexus/src/core/ingestion/cfg/collect.ts b/gitnexus/src/core/ingestion/cfg/collect.ts
new file mode 100644
index 000000000..6749e1fe9
--- /dev/null
+++ b/gitnexus/src/core/ingestion/cfg/collect.ts
@@ -0,0 +1,55 @@
+/**
+ * collectFunctionCfgs (issue #2081, M1).
+ *
+ * Walks a parsed file's tree-sitter tree and builds one {@link FunctionCfg} per
+ * CFG-bearing function via the language's {@link CfgVisitor}. Runs IN THE PARSE
+ * WORKER (where the AST lives — KTD1/KTD7); the result rides on
+ * `ParsedFile.cfgSideChannel` across the worker→main boundary.
+ *
+ * Nested functions are enumerated independently — each gets its own CFG, and
+ * appears as an opaque straight-line block in its enclosing function's CFG (the
+ * visitor does not descend into nested function bodies). `maxFunctionLines`
+ * bounds per-function cost: a function whose source span exceeds the cap is
+ * skipped (and counted) rather than walked, so a pathological mega-function
+ * cannot blow up worker time/memory. A cap of `0` means no limit.
+ */
+import type { SyntaxNode } from '../utils/ast-helpers.js';
+import type { CfgVisitor, FunctionCfg } from './types.js';
+
+export interface CollectedCfgs {
+  readonly cfgs: readonly FunctionCfg[];
+  /** Functions skipped for exceeding `maxFunctionLines` (0 ⇒ none skipped). */
+  readonly skipped: number;
+}
+
+export function collectFunctionCfgs(
+  root: SyntaxNode,
+  visitor: CfgVisitor<SyntaxNode>,
+  filePath: string,
+  maxFunctionLines = 0,
+): CollectedCfgs {
+  const cfgs: FunctionCfg[] = [];
+  let skipped = 0;
+  const stack: SyntaxNode[] = [root];
+
+  while (stack.length) {
+    const node = stack.pop() as SyntaxNode;
+    if (visitor.isFunction(node)) {
+      const lines = node.endPosition.row - node.startPosition.row + 1;
+      if (maxFunctionLines > 0 && lines > maxFunctionLines) {
+        skipped++;
+      } else {
+        const cfg = visitor.buildFunctionCfg(node, filePath);
+        if (cfg) cfgs.push(cfg);
+      }
+    }
+    // Descend regardless (a skipped mega-function may still contain small
+    // nested functions that are worth a CFG of their own).
+    for (let i = node.namedChildCount - 1; i >= 0; i--) {
+      const child = node.namedChild(i);
+      if (child) stack.push(child);
+    }
+  }
+
+  return { cfgs, skipped };
+}
diff --git a/gitnexus/src/core/ingestion/cfg/types.ts b/gitnexus/src/core/ingestion/cfg/types.ts
index 4a2603a3c..ea9ac5acd 100644
--- a/gitnexus/src/core/ingestion/cfg/types.ts
+++ b/gitnexus/src/core/ingestion/cfg/types.ts
@@ -61,4 +61,13 @@ export interface FunctionCfg {
  */
 export interface CfgVisitor<TNode = unknown> {
   buildFunctionCfg(fnNode: TNode, filePath: string): FunctionCfg | undefined;
+
+  /**
+   * Whether `node` is a CFG-bearing function this visitor handles. Lets the
+   * worker enumerate functions (and apply the per-function line budget) by a
+   * cheap node-type test, instead of attempting to build a CFG for every AST
+   * node. `buildFunctionCfg` still re-checks, so this is purely an optimization
+   * + the seam the line-budget hooks into.
+   */
+  isFunction(node: TNode): boolean;
 }
diff --git a/gitnexus/src/core/ingestion/cfg/visitors/typescript.ts b/gitnexus/src/core/ingestion/cfg/visitors/typescript.ts
index 78b632158..0f064d1ef 100644
--- a/gitnexus/src/core/ingestion/cfg/visitors/typescript.ts
+++ b/gitnexus/src/core/ingestion/cfg/visitors/typescript.ts
@@ -512,9 +512,14 @@ function buildFunctionCfg(fnNode: SyntaxNode, filePath: string): FunctionCfg | u
   return builder.finish();
 }
 
+/** Whether a node is a TS/JS function this visitor builds a CFG for. */
+function isFunction(node: SyntaxNode): boolean {
+  return TS_FUNCTION_TYPES.has(node.type);
+}
+
 /** The TS/JS CFG visitor (shared by TypeScript and JavaScript). */
 export function createTypeScriptCfgVisitor(): CfgVisitor<SyntaxNode> {
-  return { buildFunctionCfg };
+  return { buildFunctionCfg, isFunction };
 }
 
 export { TS_FUNCTION_TYPES };
diff --git a/gitnexus/src/core/ingestion/language-provider.ts b/gitnexus/src/core/ingestion/language-provider.ts
index b375050fa..6fed8ddf8 100644
--- a/gitnexus/src/core/ingestion/language-provider.ts
+++ b/gitnexus/src/core/ingestion/language-provider.ts
@@ -35,6 +35,7 @@ import type { MethodExtractor } from './method-types.js';
 import type { VariableExtractor } from './variable-types.js';
 import type { ImportResolverFn } from './import-resolvers/types.js';
 import type { SyntaxNode } from './utils/ast-helpers.js';
+import type { CfgVisitor } from './cfg/types.js';
 import type { NodeLabel } from 'gitnexus-shared';
 
 // ── Shared type aliases ────────────────────────────────────────────────────
@@ -332,6 +333,17 @@ interface LanguageProviderConfig {
    */
   readonly collectCaptureSideChannel?: (filePath: string) => unknown;
 
+  /**
+   * Per-language control-flow-graph builder (#2081 M1, PDG/taint substrate).
+   * Invoked IN THE PARSE WORKER (where the AST lives) for each function node,
+   * gated on the `--pdg` opt-in; the resulting per-function CFGs are serialized
+   * onto `ParsedFile.cfgSideChannel` and emitted as BasicBlock nodes + CFG
+   * edges during scope-resolution. `TNode` is `SyntaxNode` for the tree-sitter
+   * languages. Default: undefined (language has no CFG support yet — TS/JS are
+   * the M1 set).
+   */
+  readonly cfgVisitor?: CfgVisitor<SyntaxNode>;
+
   /**
    * Interpret a raw `@import.statement` capture group into a `ParsedImport`.
    * The central finalize algorithm resolves `ParsedImport.targetRaw` to a
diff --git a/gitnexus/src/core/ingestion/languages/typescript.ts b/gitnexus/src/core/ingestion/languages/typescript.ts
index 73b33dfd8..7f41fe731 100644
--- a/gitnexus/src/core/ingestion/languages/typescript.ts
+++ b/gitnexus/src/core/ingestion/languages/typescript.ts
@@ -16,6 +16,7 @@ import {
   javascriptClassConfig,
 } from '../class-extractors/configs/typescript-javascript.js';
 import type { SyntaxNode } from '../utils/ast-helpers.js';
+import { createTypeScriptCfgVisitor } from '../cfg/visitors/typescript.js';
 import { typeConfig as typescriptConfig } from '../type-extractors/typescript.js';
 import { tsExportChecker } from '../export-detection.js';
 import { createImportResolver } from '../import-resolvers/resolver-factory.js';
@@ -351,6 +352,8 @@ export const typescriptProvider = defineLanguage({
   // canonical capture vocabulary in ./typescript/query.ts
   // (TYPESCRIPT_SCOPE_QUERY constant).
   emitScopeCaptures: emitTsScopeCaptures,
+  // CFG/PDG substrate (#2081 M1) — runs in the worker on a --pdg run.
+  cfgVisitor: createTypeScriptCfgVisitor(),
   interpretImport: interpretTsImport,
   interpretTypeBinding: interpretTsTypeBinding,
   bindingScopeFor: tsBindingScopeFor,
@@ -412,6 +415,8 @@ export const javascriptProvider = defineLanguage({
   // JSDoc type bindings) live in ./javascript/captures.ts.
   // See ./javascript/index.ts for the full per-module rationale.
   emitScopeCaptures: emitJsScopeCaptures,
+  // CFG/PDG substrate (#2081 M1) — TS and JS share the same grammar family.
+  cfgVisitor: createTypeScriptCfgVisitor(),
   interpretImport: interpretJsImport,
   interpretTypeBinding: interpretJsTypeBinding,
   bindingScopeFor: jsBindingScopeFor,
diff --git a/gitnexus/src/core/ingestion/pipeline-phases/parse-impl.ts b/gitnexus/src/core/ingestion/pipeline-phases/parse-impl.ts
index 244f178dd..0cc4bbdaa 100644
--- a/gitnexus/src/core/ingestion/pipeline-phases/parse-impl.ts
+++ b/gitnexus/src/core/ingestion/pipeline-phases/parse-impl.ts
@@ -448,6 +448,10 @@ export async function runChunkedParseAndResolve(
         // Initialized below before the chunk loop (same deferred-init pattern
         // as `parsedFileStorePath`); this closure only runs from the loop.
         durableParsedFileStoragePath: durableParsedFileDir,
+        // CFG/PDG opt-in (#2081 M1) — baked into each worker's workerData so the
+        // worker builds + attaches cfgSideChannel. Off by default.
+        pdg: options?.pdg === true,
+        pdgMaxFunctionLines: options?.pdgMaxFunctionLines,
         // Fan each chunk across the whole pool (#worker-idle): without this a
         // chunk smaller than the 8 MB sub-batch cap became a single job on a
         // single worker. Honors an explicit `subBatchMaxBytes` / env override.
@@ -724,7 +728,7 @@ export async function runChunkedParseAndResolve(
           filePath: f.path,
           contentHash: fileContentHash(f.content),
         }));
-        chunkHash = computeChunkHash(entries);
+        chunkHash = computeChunkHash(entries, options?.pdg === true);
       }
 
       const cachedRaw =
diff --git a/gitnexus/src/core/ingestion/pipeline.ts b/gitnexus/src/core/ingestion/pipeline.ts
index 0e610e647..7898b4b18 100644
--- a/gitnexus/src/core/ingestion/pipeline.ts
+++ b/gitnexus/src/core/ingestion/pipeline.ts
@@ -50,6 +50,20 @@ export interface PipelineOptions {
    * to retain those nodes under `skipGraphPhases`.
    */
   skipGraphPhases?: boolean;
+  /**
+   * Build the control-flow-graph / PDG substrate (#2081 M1, opt-in via `--pdg`).
+   * Off by default: workers skip all CFG work and emit no `cfgSideChannel`, and
+   * scope-resolution emits no BasicBlock nodes or CFG edges — so the default
+   * graph is byte-identical to a pre-#2081 run. Folded into the parse-cache key
+   * so a pdg-off warm cache is not reused on a `--pdg` run.
+   */
+  pdg?: boolean;
+  /**
+   * Per-function source-line cap for worker-side CFG construction
+   * (`undefined`/0 ⇒ no cap). Bounds the cost of a pathological mega-function;
+   * over-cap functions are skipped (no CFG emitted for them).
+   */
+  pdgMaxFunctionLines?: number;
   /**
    * Request parsing with the worker pool disabled. The sequential parser was
    * removed — the worker pool is the sole parse path — so setting this now
diff --git a/gitnexus/src/core/ingestion/workers/parse-worker.ts b/gitnexus/src/core/ingestion/workers/parse-worker.ts
index b0f787855..6fe17ebd0 100644
--- a/gitnexus/src/core/ingestion/workers/parse-worker.ts
+++ b/gitnexus/src/core/ingestion/workers/parse-worker.ts
@@ -109,6 +109,7 @@ import {
   persistDurableParsedFileShardSync,
 } from '../../../storage/parsedfile-store.js';
 import { extractLaravelRoutes, type ExtractedRoute } from '../route-extractors/laravel.js';
+import { collectFunctionCfgs } from '../cfg/collect.js';
 
 import { logger } from '../../logger.js';
 export type { ExtractedRoute } from '../route-extractors/laravel.js';
@@ -134,6 +135,18 @@ const DURABLE_PARSED_FILE_STORAGE_PATH: string | undefined = (
 )?.durableParsedFileStoragePath;
 let shardSeq = 0;
 
+// ── PDG/CFG opt-in (#2081 M1) ───────────────────────────────────────────────
+// Read ONCE at worker init from `workerData` (the worker never sees
+// PipelineOptions — config arrives via the pool factory's `workerData`, see
+// KTD7 / U5). When `pdg` is set, the worker builds a per-function control-flow
+// graph from the tree-sitter AST (where it lives) and serializes it onto
+// `ParsedFile.cfgSideChannel`. Off ⇒ no CFG work and no field — the default for
+// every run today. `pdgMaxFunctionLines` bounds per-function CFG cost
+// (0/undefined ⇒ no cap; see collectFunctionCfgs).
+const PDG_ENABLED: boolean = (workerData as { pdg?: boolean } | undefined)?.pdg === true;
+const PDG_MAX_FUNCTION_LINES: number =
+  (workerData as { pdgMaxFunctionLines?: number } | undefined)?.pdgMaxFunctionLines ?? 0;
+
 // ── Bootstrap-stage diagnostics (#1741) ────────────────────────────────────
 // When GITNEXUS_WORKER_BOOTSTRAP=1 (or --verbose sets GITNEXUS_VERBOSE), each
 // worker reports its startup stage timings to stderr — which the pool tees
@@ -1201,9 +1214,26 @@ const processFileGroup = (
       // copy — scopes/defs are carried by reference) to attach the field rather
       // than mutate the frozen object.
       const sideChannel = provider.collectCaptureSideChannel?.(file.path);
-      result.parsedFiles.push(
-        sideChannel !== undefined ? { ...parsedFile, captureSideChannel: sideChannel } : parsedFile,
-      );
+      let withChannels =
+        sideChannel !== undefined ? { ...parsedFile, captureSideChannel: sideChannel } : parsedFile;
+
+      // CFG side-channel (#2081 M1): build the per-function control-flow graph
+      // here, where the tree-sitter AST is still in hand, and attach it as plain
+      // serializable data. Only on a --pdg run and only for languages with a
+      // cfgVisitor (TS/JS in M1). The same disk-store/warm-cache machinery that
+      // carries captureSideChannel carries this — its coherence rests on the
+      // SCHEMA_BUMP + the pdg-folded chunk-hash key (see parse-cache.ts).
+      if (PDG_ENABLED && provider.cfgVisitor) {
+        const { cfgs } = collectFunctionCfgs(
+          tree.rootNode,
+          provider.cfgVisitor,
+          file.path,
+          PDG_MAX_FUNCTION_LINES,
+        );
+        if (cfgs.length) withChannels = { ...withChannels, cfgSideChannel: cfgs };
+      }
+
+      result.parsedFiles.push(withChannels);
     }
 
     // Build per-file type environment + constructor bindings in a single AST walk.
diff --git a/gitnexus/src/core/ingestion/workers/worker-pool.ts b/gitnexus/src/core/ingestion/workers/worker-pool.ts
index 5082453a0..60a233362 100644
--- a/gitnexus/src/core/ingestion/workers/worker-pool.ts
+++ b/gitnexus/src/core/ingestion/workers/worker-pool.ts
@@ -232,6 +232,15 @@ export interface WorkerPoolOptions {
    * `undefined` ⇒ no durable write.
    */
   durableParsedFileStoragePath?: string;
+  /**
+   * CFG/PDG opt-in (#2081 M1). Baked into every spawned worker's `workerData`
+   * (like the store paths above); when `true`, workers build a per-function
+   * control-flow graph from the tree-sitter AST and attach it to
+   * `ParsedFile.cfgSideChannel`. `undefined`/`false` ⇒ no CFG work.
+   */
+  pdg?: boolean;
+  /** Per-function source-line cap for worker-side CFG construction (0 ⇒ no cap). */
+  pdgMaxFunctionLines?: number;
 }
 
 export class WorkerPoolDispatchError extends Error {
@@ -884,9 +893,12 @@ export const createWorkerPool = (
   // signature is unchanged so the zero-arg test factories keep working.
   const parsedFileStoreStoragePath = options?.parsedFileStoreStoragePath;
   const durableParsedFileStoragePath = options?.durableParsedFileStoragePath;
+  // CFG/PDG opt-in (#2081 M1) — carried in workerData alongside the store paths.
+  const pdg = options?.pdg === true;
+  const pdgMaxFunctionLines = options?.pdgMaxFunctionLines;
   const workerStoreData =
-    parsedFileStoreStoragePath || durableParsedFileStoragePath
-      ? { parsedFileStoreStoragePath, durableParsedFileStoragePath }
+    parsedFileStoreStoragePath || durableParsedFileStoragePath || pdg
+      ? { parsedFileStoreStoragePath, durableParsedFileStoragePath, pdg, pdgMaxFunctionLines }
       : undefined;
   const spawnWorker =
     options?.workerFactory ??
diff --git a/gitnexus/src/storage/parse-cache.ts b/gitnexus/src/storage/parse-cache.ts
index c61ade5ef..9adf1414a 100644
--- a/gitnexus/src/storage/parse-cache.ts
+++ b/gitnexus/src/storage/parse-cache.ts
@@ -55,7 +55,7 @@ import type { ParseWorkerResult } from '../core/ingestion/workers/parse-worker.j
 // the main thread (the #1983 OOM). Because the two stores share this version,
 // any future change to the `ParsedFile` serialization shape MUST bump
 // SCHEMA_BUMP so both invalidate in lockstep.
-const SCHEMA_BUMP = 4;
+const SCHEMA_BUMP = 5; // #2081 M1: ParsedFile gained `cfgSideChannel`
 const GITNEXUS_PKG_VERSION = (() => {
   try {
     // package.json sits at gitnexus/package.json — two levels up from
@@ -143,10 +143,16 @@ export const fileContentHash = (content: Buffer | string): string => sha256Hex(c
  */
 export const computeChunkHash = (
   entries: Array<{ filePath: string; contentHash: string }>,
+  pdg = false,
 ): string => {
   const sorted = [...entries].sort((a, b) => (a.filePath < b.filePath ? -1 : 1));
   const joined = sorted.map((e) => `${e.filePath}:${e.contentHash}`).join('\n');
-  return sha256Hex(joined);
+  // Fold the `--pdg` opt-in into the key (#2081 M1) so a chunk cached WITHOUT a
+  // CFG (`cfgSideChannel`) is NOT reused on a `--pdg` run, and vice-versa — the
+  // #2038-class warm-cache trap where an option-blind key silently serves
+  // field-less shards. Only prefixed when `pdg` is on, so the default (pdg-off)
+  // path keeps its existing keys and warm caches survive this change.
+  return sha256Hex(pdg ? `pdg\n${joined}` : joined);
 };
 
 /**
diff --git a/gitnexus/test/integration/cfg/worker-roundtrip.test.ts b/gitnexus/test/integration/cfg/worker-roundtrip.test.ts
new file mode 100644
index 000000000..1a4c870b1
--- /dev/null
+++ b/gitnexus/test/integration/cfg/worker-roundtrip.test.ts
@@ -0,0 +1,119 @@
+import { describe, it, expect } from 'vitest';
+import Parser from 'tree-sitter';
+import TypeScript from 'tree-sitter-typescript';
+import { collectFunctionCfgs } from '../../../src/core/ingestion/cfg/collect.js';
+import { computeChunkHash, mapReplacer, mapReviver } from '../../../src/storage/parse-cache.js';
+import { getProvider } from '../../../src/core/ingestion/languages/index.js';
+import { SupportedLanguages } from '../../../src/config/supported-languages.js';
+import type { CfgVisitor } from '../../../src/core/ingestion/cfg/types.js';
+import type { SyntaxNode } from '../../../src/core/ingestion/utils/ast-helpers.js';
+
+// U3 — the worker→main boundary + cache coherence for the CFG side-channel.
+// These pin the contracts that make the disk-store + warm/durable parse cache
+// carry the CFG intact across the --pdg flag (R3, R4) WITHOUT spinning a real
+// worker pool: the worker simply calls collectFunctionCfgs (tested here) and
+// attaches the result as plain data, and the parse-cache key folds the flag.
+
+function tsRoot(code: string): SyntaxNode {
+  const parser = new Parser();
+  parser.setLanguage(TypeScript.typescript);
+  return parser.parse(code).rootNode;
+}
+
+const tsVisitor = (): CfgVisitor<SyntaxNode> => {
+  const v = getProvider(SupportedLanguages.TypeScript).cfgVisitor;
+  if (!v) throw new Error('typescript provider has no cfgVisitor');
+  return v;
+};
+
+describe('U3 — TS/JS provider exposes a cfgVisitor; others do not (worker gate)', () => {
+  it('TS and JS providers carry a cfgVisitor', () => {
+    expect(getProvider(SupportedLanguages.TypeScript).cfgVisitor).toBeDefined();
+    expect(getProvider(SupportedLanguages.JavaScript).cfgVisitor).toBeDefined();
+  });
+
+  it('a non-CFG language (Python) has no cfgVisitor ⇒ worker emits no cfgSideChannel', () => {
+    // `provider.cfgVisitor &&` short-circuits in the worker → no CFG, no field.
+    expect(getProvider(SupportedLanguages.Python).cfgVisitor).toBeUndefined();
+  });
+});
+
+describe('U3 — collectFunctionCfgs', () => {
+  it('produces one CFG per function with the expected branch edges', () => {
+    const root = tsRoot(`
+      function a(x: number) { if (x) { p(); } else { q(); } }
+      function b() { return 1; }
+    `);
+    const { cfgs, skipped } = collectFunctionCfgs(root, tsVisitor(), 'a.ts');
+    expect(skipped).toBe(0);
+    expect(cfgs).toHaveLength(2);
+    const a = cfgs.find((c) => c.blocks.some((bl) => bl.text.includes('p();')));
+    expect(a).toBeDefined();
+    const kinds = new Set(a!.edges.map((e) => e.kind));
+    expect(kinds.has('cond-true')).toBe(true);
+    expect(kinds.has('cond-false')).toBe(true);
+    // every block belongs to its declaring file
+    for (const c of cfgs) expect(c.filePath).toBe('a.ts');
+  });
+
+  it('a file with no functions yields an empty CFG set (no error)', () => {
+    const { cfgs, skipped } = collectFunctionCfgs(
+      tsRoot(`const x = 1; export {};`),
+      tsVisitor(),
+      'x.ts',
+    );
+    expect(cfgs).toHaveLength(0);
+    expect(skipped).toBe(0);
+  });
+
+  it('maxFunctionLines skips an over-cap function and counts the skip', () => {
+    const big = `function big() {\n${'  step();\n'.repeat(20)}}`;
+    const root = tsRoot(`${big}\nfunction small() { ok(); }`);
+    const { cfgs, skipped } = collectFunctionCfgs(root, tsVisitor(), 'f.ts', 5);
+    expect(skipped).toBe(1); // big() exceeds the 5-line cap
+    // small() is still built
+    expect(cfgs.some((c) => c.blocks.some((bl) => bl.text.includes('ok();')))).toBe(true);
+    expect(cfgs.some((c) => c.blocks.some((bl) => bl.text.includes('step();')))).toBe(false);
+  });
+});
+
+describe('U3 — CFG side-channel JSON round-trip (no AST leakage, no field loss)', () => {
+  it('serialize → JSON → deserialize yields an identical CFG', () => {
+    const root = tsRoot(`function f(xs: number[]) {
+      for (const x of xs) { if (x > 0) { use(x); } else { break; } }
+      done();
+    }`);
+    const { cfgs } = collectFunctionCfgs(root, tsVisitor(), 'rt.ts');
+    expect(cfgs.length).toBeGreaterThan(0);
+    // The worker serializes ParsedFile via mapReplacer; the store revives via
+    // mapReviver. The CFG is plain data, so it must survive byte-for-byte.
+    const round = JSON.parse(JSON.stringify(cfgs, mapReplacer), mapReviver);
+    expect(round).toEqual(cfgs);
+    // No tree-sitter nodes leaked: every value is a primitive/array/plain object.
+    for (const c of round) {
+      for (const b of c.blocks) expect(typeof b.text).toBe('string');
+      for (const e of c.edges) expect(typeof e.from).toBe('number');
+    }
+  });
+});
+
+describe('U3 — parse-cache key folds the --pdg flag (R4, #2038-class guard)', () => {
+  const entries = [
+    { filePath: 'b.ts', contentHash: 'h2' },
+    { filePath: 'a.ts', contentHash: 'h1' },
+  ];
+
+  it('pdg-on and pdg-off produce DIFFERENT chunk keys', () => {
+    expect(computeChunkHash(entries, false)).not.toBe(computeChunkHash(entries, true));
+  });
+
+  it('the same flag value is stable and order-independent', () => {
+    const reordered = [...entries].reverse();
+    expect(computeChunkHash(entries, true)).toBe(computeChunkHash(reordered, true));
+    expect(computeChunkHash(entries, false)).toBe(computeChunkHash(reordered, false));
+  });
+
+  it('default (no flag arg) equals the explicit pdg-off key — warm caches survive the change', () => {
+    expect(computeChunkHash(entries)).toBe(computeChunkHash(entries, false));
+  });
+});