diff --git a/.changeset/prebuilt-simplify-followup.md b/.changeset/prebuilt-simplify-followup.md
new file mode 100644
index 000000000..b43cee35a
--- /dev/null
+++ b/.changeset/prebuilt-simplify-followup.md
@@ -0,0 +1,5 @@
+---
+"agent-bundle": patch
+---
+
+Simplify prebuilt-payload internals (post-#71 follow-up): dev-server preparations no longer pay the AB4750 payload-freshness mtime walk for commands that discard it, the payload-declaration parse and innermost-payload ownership rule are shared across discovery, normalization, and validation instead of being open-coded per module, and `PreparedProject.snapshotSource` is required so artifact re-snapshots always observe the payload roots the prepared identity hashed.
diff --git a/packages/agent-bundle/src/build/build.ts b/packages/agent-bundle/src/build/build.ts
index 2d6be736b..a2a14c653 100644
--- a/packages/agent-bundle/src/build/build.ts
+++ b/packages/agent-bundle/src/build/build.ts
@@ -75,12 +75,15 @@ interface StagedTarget extends PlannedTarget {
   readonly root: string;
 }
 
-const prebuiltReferenceExists = (
-  model: NormalizedPlugin,
-  artifactPath: string,
-): boolean => (model.payloads ?? []).some((payload) =>
-  artifactPath.startsWith(`${payload.name}/`) &&
-  payload.files.some((file) => `${payload.name}/${file.relativePath}` === artifactPath));
+const prebuiltArtifactPaths = (model: NormalizedPlugin): ReadonlySet<string> =>
+  new Set((model.payloads ?? []).flatMap((payload) =>
+    payload.files.map((file) => `${payload.name}/${file.relativePath}`)));
+
+const missingPrebuiltDiagnostic = (subject: string, artifactPath: string): Diagnostic => ({
+  code: 'AB4748',
+  message: `${subject} ${JSON.stringify(artifactPath)} is not present in its declared payload; run the project's own build before "agent-bundle build".`,
+  severity: 'error',
+});
 
 /**
  * AB4747-AB4749: an artifact build packages prebuilt payloads exactly as
@@ -112,28 +115,21 @@ const prebuiltPayloadDiagnostics = (
       });
     }
   }
+  const artifactPaths = prebuiltArtifactPaths(model);
   const tokenPrefix = `${pathTokens.pluginRoot}/`;
   for (const server of model.mcpServers) {
     if (server.provenance.kind !== 'prebuilt') continue;
     const entry = server.args?.[0];
     if (typeof entry !== 'string' || !entry.startsWith(tokenPrefix)) continue;
     const artifactPath = entry.slice(tokenPrefix.length);
-    if (!prebuiltReferenceExists(model, artifactPath)) {
-      diagnostics.push({
-        code: 'AB4748',
-        message: `MCP server ${JSON.stringify(server.name)} prebuilt entry ${JSON.stringify(artifactPath)} is not present in its declared payload; run the project's own build before "agent-bundle build".`,
-        severity: 'error',
-      });
+    if (!artifactPaths.has(artifactPath)) {
+      diagnostics.push(missingPrebuiltDiagnostic(`MCP server ${JSON.stringify(server.name)} prebuilt entry`, artifactPath));
     }
   }
   for (const hook of model.hooks) {
     if (hook.prebuiltPath === undefined) continue;
-    if (!prebuiltReferenceExists(model, hook.prebuiltPath)) {
-      diagnostics.push({
-        code: 'AB4748',
-        message: `Hook ${JSON.stringify(hook.name)} prebuilt handler ${JSON.stringify(hook.prebuiltPath)} is not present in its declared payload; run the project's own build before "agent-bundle build".`,
-        severity: 'error',
-      });
+    if (!artifactPaths.has(hook.prebuiltPath)) {
+      diagnostics.push(missingPrebuiltDiagnostic(`Hook ${JSON.stringify(hook.name)} prebuilt handler`, hook.prebuiltPath));
     }
   }
   return diagnostics;
diff --git a/packages/agent-bundle/src/config/discover.ts b/packages/agent-bundle/src/config/discover.ts
index 6058f5c32..26d4f6395 100644
--- a/packages/agent-bundle/src/config/discover.ts
+++ b/packages/agent-bundle/src/config/discover.ts
@@ -111,6 +111,29 @@ const discoverAssets = async (
   })));
 };
 
+/** The declared source path of a payload declaration, or undefined when the declaration is not string-or-`{source}` shaped. */
+export const payloadDeclarationEntry = (declaration: unknown): string | undefined => {
+  const entry = typeof declaration === 'string'
+    ? declaration
+    : isRecord(declaration) ? declaration.source : undefined;
+  return typeof entry === 'string' && entry.trim().length > 0 ? entry : undefined;
+};
+
+/**
+ * The absolute, project-contained source directory of one well-shaped payload
+ * declaration. Malformed or escaping declarations return undefined — source
+ * validation reports those (AB4740-AB4742).
+ */
+export const payloadDeclarationSource = (
+  projectRoot: string,
+  declaration: unknown,
+): string | undefined => {
+  const entry = payloadDeclarationEntry(declaration);
+  if (entry === undefined) return undefined;
+  const source = resolve(projectRoot, entry);
+  return isInside(projectRoot, source) ? source : undefined;
+};
+
 /**
  * The absolute source directories of well-shaped payload declarations.
  * Source snapshots use this to include payload files in the project
@@ -124,10 +147,8 @@ export const configuredPayloadRoots = (
   if (configured === undefined || !isRecord(configured)) return [];
   const roots: string[] = [];
   for (const declaration of Object.values(configured)) {
-    const entry = typeof declaration === 'string' ? declaration : declaration?.source;
-    if (typeof entry !== 'string' || entry.trim().length === 0) continue;
-    const source = resolve(projectRoot, entry);
-    if (isInside(projectRoot, source)) roots.push(source);
+    const source = payloadDeclarationSource(projectRoot, declaration);
+    if (source !== undefined) roots.push(source);
   }
   return [...new Set(roots)].sort((left, right) => left.localeCompare(right));
 };
@@ -146,10 +167,8 @@ const discoverPayloads = async (
   if (configured === undefined || !isRecord(configured)) return [];
   const payloads: DiscoveredPayload[] = [];
   for (const [name, declaration] of Object.entries(configured).sort(([left], [right]) => left.localeCompare(right))) {
-    const entry = typeof declaration === 'string' ? declaration : declaration?.source;
-    if (typeof entry !== 'string' || entry.trim().length === 0) continue;
-    const source = resolve(projectRoot, entry);
-    if (!isInside(projectRoot, source)) continue;
+    const source = payloadDeclarationSource(projectRoot, declaration);
+    if (source === undefined) continue;
     let stats;
     try {
       stats = await stat(source);
@@ -160,12 +179,13 @@ const discoverPayloads = async (
       payloads.push({ files: [], name, source });
       continue;
     }
-    const matches = (await fastGlob('**', { ...assetGlobOptions, cwd: source })).sort((left, right) => left.localeCompare(right));
+    const matches = (await fastGlob('**', { ...assetGlobOptions, cwd: source, stats: true }))
+      .sort((left, right) => left.path.localeCompare(right.path));
     payloads.push({
-      files: await Promise.all(matches.map(async (file) => ({
-        bytes: (await stat(file)).size,
-        relativePath: relative(source, file).replaceAll('\\', '/'),
-        source: file,
+      files: await Promise.all(matches.map(async (match) => ({
+        bytes: (match.stats ?? await stat(match.path)).size,
+        relativePath: relative(source, match.path).replaceAll('\\', '/'),
+        source: match.path,
       }))),
       name,
       source,
diff --git a/packages/agent-bundle/src/config/normalize.ts b/packages/agent-bundle/src/config/normalize.ts
index 1c0db6ad9..9108f7b6c 100644
--- a/packages/agent-bundle/src/config/normalize.ts
+++ b/packages/agent-bundle/src/config/normalize.ts
@@ -42,7 +42,7 @@ import type {
   NormalizedSkill,
   SourceProvenance,
 } from '../core/types.ts';
-import type { DiscoveredProject } from './discover.ts';
+import { type DiscoveredProject, payloadDeclarationSource } from './discover.ts';
 import type { LoadedConfig } from './load.ts';
 
 const unique = (values: readonly string[]): string[] => [...new Set(values)];
@@ -230,20 +230,33 @@ const normalizePayloads = (
   const discoveredByName = new Map((discovered.payloads ?? []).map((payload) => [payload.name, payload]));
   const payloads: NormalizedPayload[] = [];
   for (const [name, declaration] of Object.entries(configured).sort(([left], [right]) => left.localeCompare(right))) {
-    const entry = typeof declaration === 'string' ? declaration : declaration.source;
-    if (typeof entry !== 'string' || entry.trim().length === 0) continue;
+    const source = payloadDeclarationSource(loaded.context.projectRoot, declaration);
+    if (source === undefined) continue;
     payloads.push({
       files: (discoveredByName.get(name)?.files ?? []).map((file) => ({ ...file })),
       id: `payload:${name}`,
       name,
       provenance: { kind: 'prebuilt', sourcePath: loaded.configPath },
-      source: resolve(loaded.context.projectRoot, entry),
+      source,
       targets: sortedUnique(typeof declaration === 'string' ? targetNames : (declaration.targets ?? targetNames)),
     });
   }
   return payloads;
 };
 
+/** The innermost declared payload whose source directory contains the file. */
+export const owningPayload = <Payload extends { readonly source: string }>(
+  payloads: readonly Payload[],
+  source: string,
+): Payload | undefined => {
+  let best: Payload | undefined;
+  for (const payload of payloads) {
+    if (!isInside(payload.source, source)) continue;
+    if (best === undefined || payload.source.length > best.source.length) best = payload;
+  }
+  return best;
+};
+
 /**
  * The artifact-relative stable path of a prebuilt file: its declaring payload
  * destination plus the file's payload-relative path. Falls back to the
@@ -255,14 +268,10 @@ export const prebuiltArtifactPath = (
   root: string,
   source: string,
 ): string => {
-  let best: NormalizedPayload | undefined;
-  for (const payload of payloads) {
-    if (!isInside(payload.source, source)) continue;
-    if (best === undefined || payload.source.length > best.source.length) best = payload;
-  }
-  return best === undefined
+  const payload = owningPayload(payloads, source);
+  return payload === undefined
     ? relative(root, source).replaceAll('\\', '/')
-    : `${best.name}/${relative(best.source, source).replaceAll('\\', '/')}`;
+    : `${payload.name}/${relative(payload.source, source).replaceAll('\\', '/')}`;
 };
 
 const isHookEntryList = (
@@ -306,9 +315,8 @@ const normalizeHook = (
   const source = resolve(root, prebuilt ? handlerInput.prebuilt : handlerInput);
   const handler = relative(root, source).replaceAll('\\', '/');
   const prebuiltPath = prebuilt ? prebuiltArtifactPath(payloads, root, source) : undefined;
-  const args = prebuilt && entry.args !== undefined
-    ? entry.args.filter((argument): argument is string => typeof argument === 'string')
-    : undefined;
+  // Non-string arguments are a validation error (AB4746); normalization trusts the declared type.
+  const args = prebuilt ? entry.args : undefined;
   const tools = sortedUnique(entry.tools ?? []).filter(
     (tool): tool is CanonicalHookTool => knownHookTools.has(tool as CanonicalHookTool),
   );
@@ -510,9 +518,10 @@ const normalizeMcpApps = (
 
   for (const [serverName, rawServer] of Object.entries(configured).sort(([left], [right]) => left.localeCompare(right))) {
     const server = serverByName.get(serverName);
+    if (server === undefined || rawServer.apps === undefined) continue;
     // Apps require a local server entry: a compiled source entry, or a prebuilt one.
-    const prebuilt = isPrebuiltEntryInput(rawServer.entry);
-    if (server === undefined || (server.source === undefined && !prebuilt) || rawServer.apps === undefined) continue;
+    const prebuilt = server.provenance.kind === 'prebuilt';
+    if (server.source === undefined && !prebuilt) continue;
     for (const [name, app] of Object.entries(rawServer.apps).sort(([left], [right]) => left.localeCompare(right))) {
       const declaration = app as AgentBundleMcpApp;
       apps.push({
diff --git a/packages/agent-bundle/src/config/validate.ts b/packages/agent-bundle/src/config/validate.ts
index 447245cf7..b21489d84 100644
--- a/packages/agent-bundle/src/config/validate.ts
+++ b/packages/agent-bundle/src/config/validate.ts
@@ -28,9 +28,10 @@ import {
   conventionalCliEntrySource,
   conventionalIndexEntrySource,
   conventionalMcpEntrySource,
+  owningPayload,
   reservedPayloadDestinations,
 } from './normalize.ts';
-import type { DiscoveredProject } from './discover.ts';
+import { type DiscoveredProject, payloadDeclarationEntry, payloadDeclarationSource } from './discover.ts';
 import type { LoadedConfig } from './load.ts';
 import type { SkillDocument } from './skill.ts';
 import { referencedResources } from './skill-references.ts';
@@ -90,9 +91,10 @@ const validateHooks = (
       const handler = entry.handler;
       const prebuilt = isPrebuiltEntryInput(handler);
       if (prebuilt) {
-        const hookTargets = Array.isArray(entry.targets) && entry.targets.every(nonemptyString)
-          ? entry.targets
-          : selectedTargets.filter((target) => registry.supports(target, 'hooks'));
+        const hookTargets = declaredTargetsOr(
+          entry.targets,
+          selectedTargets.filter((target) => registry.supports(target, 'hooks')),
+        );
         diagnostics.push(...validatePrebuiltReference(
           `Hook ${event}`,
           handler,
@@ -243,6 +245,13 @@ const isProtocolJsonValue = (value: unknown, ancestors = new Set<object>()): boo
 const nonemptyString = (value: unknown): value is string =>
   typeof value === 'string' && value.trim().length > 0;
 
+/** A declared targets restriction when it is a well-shaped string array, otherwise the fallback selection. */
+const declaredTargetsOr = (
+  targets: unknown,
+  fallback: readonly string[],
+): readonly string[] =>
+  Array.isArray(targets) && targets.every(nonemptyString) ? targets : fallback;
+
 const validateStringList = (
   value: unknown,
   label: string,
@@ -675,9 +684,7 @@ const validateMcpServer = (
 
   if (entry !== undefined || conventionalEntry !== undefined) {
     if (isPrebuiltEntryInput(entry)) {
-      const serverTargets = Array.isArray(server.targets) && server.targets.every(nonemptyString)
-        ? server.targets
-        : selectedTargetNamesFor(loaded, registry);
+      const serverTargets = declaredTargetsOr(server.targets, selectedTargetNamesFor(loaded, registry));
       diagnostics.push(...validatePrebuiltReference(
         `MCP server ${JSON.stringify(name)}`,
         entry,
@@ -1023,17 +1030,13 @@ const declaredPayloads = (
   const selectedTargets = selectedTargetNamesFor(loaded, registry);
   const payloads: DeclaredPayload[] = [];
   for (const [name, declaration] of Object.entries(configured)) {
-    const entry = typeof declaration === 'string'
-      ? declaration
-      : isRecord(declaration) ? declaration.source : undefined;
-    if (!nonemptyString(entry)) continue;
-    const source = resolve(loaded.context.projectRoot, entry);
-    if (!isInside(loaded.context.projectRoot, source)) continue;
+    const source = payloadDeclarationSource(loaded.context.projectRoot, declaration);
+    if (source === undefined) continue;
     const targets = typeof declaration === 'string' ? undefined : declaration.targets;
     payloads.push({
       name,
       source,
-      targets: Array.isArray(targets) && targets.every(nonemptyString) ? targets : selectedTargets,
+      targets: declaredTargetsOr(targets, selectedTargets),
     });
   }
   return payloads;
@@ -1112,6 +1115,7 @@ const payloadTargetDiagnostics = (
 const validatePayload = (
   loaded: LoadedConfig,
   registry: NormalizationTargetRegistry,
+  freshness: boolean,
 ): Diagnostic[] => {
   const configured = loaded.config.payload;
   if (configured === undefined) return [];
@@ -1128,10 +1132,8 @@ const validatePayload = (
         loaded.configPath,
       ));
     }
-    const entry = typeof declaration === 'string'
-      ? declaration
-      : isRecord(declaration) ? declaration.source : undefined;
-    if (!nonemptyString(entry)) {
+    const entry = payloadDeclarationEntry(declaration);
+    if (entry === undefined) {
       diagnostics.push(sourceDiagnostic(
         'AB4740',
         `Payload ${JSON.stringify(name)} must be a source directory path or an object with a source path.`,
@@ -1142,8 +1144,8 @@ const validatePayload = (
     if (typeof declaration !== 'string') {
       diagnostics.push(...payloadTargetDiagnostics(name, declaration.targets, loaded, registry));
     }
-    const source = resolve(loaded.context.projectRoot, entry);
-    if (!isInside(loaded.context.projectRoot, source)) {
+    const source = payloadDeclarationSource(loaded.context.projectRoot, declaration);
+    if (source === undefined) {
       diagnostics.push(sourceDiagnostic(
         'AB4742',
         `Payload ${JSON.stringify(name)} source must resolve inside the project root.`,
@@ -1191,7 +1193,9 @@ const validatePayload = (
       ));
     }
   }
-  const existing = sources.filter((payload) => existsSync(payload.source));
+  // The freshness nudge walks every project and payload file's mtime, so
+  // flows that discard non-error source diagnostics skip it entirely.
+  const existing = freshness ? sources.filter((payload) => existsSync(payload.source)) : [];
   if (existing.length > 0) {
     const newestSource = newestFileMtime(
       loaded.context.projectRoot,
@@ -1237,9 +1241,7 @@ const validatePrebuiltReference = (
   if (!isInside(loaded.context.projectRoot, source)) {
     return [sourceDiagnostic('AB4744', `${label} prebuilt entry must resolve inside the project root.`, loaded.configPath)];
   }
-  const payload = payloads
-    .filter((candidate) => isInside(candidate.source, source))
-    .sort((left, right) => right.source.length - left.source.length)[0];
+  const payload = owningPayload(payloads, source);
   if (payload === undefined) {
     diagnostics.push(sourceDiagnostic(
       'AB4744',
@@ -1321,10 +1323,20 @@ const validateTools = (loaded: LoadedConfig): Diagnostic[] => {
   return diagnostics;
 };
 
+export interface ValidateSourceOptions {
+  /**
+   * Compute the AB4750 payload-freshness nudge, a full-project mtime walk.
+   * Defaults to true; flows that discard non-error source diagnostics pass
+   * false to skip the walk.
+   */
+  readonly payloadFreshness?: boolean;
+}
+
 export const validateSource = (
   loaded: LoadedConfig,
   discovered: DiscoveredProject,
   registry: NormalizationTargetRegistry,
+  options?: ValidateSourceOptions,
 ): Diagnostic[] => {
   const diagnostics: Diagnostic[] = [];
   const plugin = loaded.config.plugin as unknown;
@@ -1379,7 +1391,7 @@ export const validateSource = (
   diagnostics.push(...validateHooks(loaded, registry, payloads));
   diagnostics.push(...validateLib(loaded));
   diagnostics.push(...validateMcp(loaded, registry, payloads));
-  diagnostics.push(...validatePayload(loaded, registry));
+  diagnostics.push(...validatePayload(loaded, registry, options?.payloadFreshness !== false));
   diagnostics.push(...validateRuntime(loaded));
   diagnostics.push(...validateScripts(loaded, registry));
   diagnostics.push(...validateTools(loaded));
diff --git a/packages/agent-bundle/src/dev/artifacts/artifact-service.ts b/packages/agent-bundle/src/dev/artifacts/artifact-service.ts
index 619ffe2e2..149835940 100644
--- a/packages/agent-bundle/src/dev/artifacts/artifact-service.ts
+++ b/packages/agent-bundle/src/dev/artifacts/artifact-service.ts
@@ -25,7 +25,7 @@ import {
   type NativePlaygroundCatalogPublicationOptions,
   type NativePlaygroundCatalogPublicationReceipt,
 } from '../playground/native-playground-service.ts';
-import { snapshotProjectSource, type PreparedProject } from '../project-service.ts';
+import type { PreparedProject } from '../project-service.ts';
 import { freezeArtifactEpoch, type ArtifactEpoch, type DiagnosticSummary } from '../types.ts';
 
 export interface SucceededArtifactEpochResult {
@@ -228,11 +228,7 @@ export class ArtifactService {
       buildDiagnostics = freezeDiagnostics([...prepared.diagnostics, ...validationDiagnostics]);
       if (hasErrors(buildDiagnostics)) throw new DiagnosticError(buildDiagnostics);
 
-      const currentSource = await (prepared.snapshotSource ?? (() => snapshotProjectSource(
-        prepared.root,
-        prepared.configPath,
-        prepared.outputRoots,
-      )))();
+      const currentSource = await prepared.snapshotSource();
       if (!sameInputs(projectContext.sourceInputs, currentSource.inputs)) {
         throw new DiagnosticError([projectSourceChangedDiagnostic(prepared.configPath)]);
       }
diff --git a/packages/agent-bundle/src/dev/project-service.ts b/packages/agent-bundle/src/dev/project-service.ts
index 759bce7f1..5b1482151 100644
--- a/packages/agent-bundle/src/dev/project-service.ts
+++ b/packages/agent-bundle/src/dev/project-service.ts
@@ -59,7 +59,12 @@ export interface PreparedProject {
   readonly projectContext?: ProjectContext;
   readonly registry: TargetRegistry;
   readonly root: string;
-  readonly snapshotSource?: () => Promise<ProjectSourceSnapshot>;
+  /**
+   * Re-snapshots the project with the same output and payload roots the
+   * prepared identity hashed; a divergent re-snapshot would make
+   * payload-bearing projects always appear drifted to epoch publication.
+   */
+  readonly snapshotSource: () => Promise<ProjectSourceSnapshot>;
   readonly source: SourceStatus;
   /** The consumer bundler escape hatch, passed through for build lowering. */
   readonly tools?: AgentBundleToolsConfig;
@@ -523,12 +528,12 @@ const preparedProject = (
   registry: TargetRegistry,
   root: string,
   source: SourceStatus,
+  snapshotSource: () => Promise<ProjectSourceSnapshot>,
   model?: NormalizedPlugin,
   devRuntime?: DevRuntimePreparedProject,
   devRuntimeDiagnostic?: Diagnostic,
   devAgentApiEnabled?: boolean,
   tools?: AgentBundleToolsConfig,
-  snapshotSource?: () => Promise<ProjectSourceSnapshot>,
 ): PreparedProject => Object.freeze({
   configPath,
   ...(devAgentApiEnabled === true ? { devAgentApiEnabled } : {}),
@@ -540,7 +545,7 @@ const preparedProject = (
   ...(projectContext === undefined ? {} : { projectContext }),
   registry,
   root,
-  ...(snapshotSource === undefined ? {} : { snapshotSource }),
+  snapshotSource,
   source,
   ...(tools === undefined ? {} : { tools }),
 });
@@ -587,6 +592,9 @@ const invalidPreparedProject = (options: {
     options.registry,
     options.root,
     sourceStatus(options.diagnostics, snapshot.revision),
+    // A failed preparation never resolved its payload roots; the re-snapshot
+    // observes the same source tree the failure snapshot did.
+    () => snapshotProjectSource(options.root, options.configPath, options.outputRoots),
   );
 };
 
@@ -712,7 +720,11 @@ export class ProjectService {
     const supplementalMetadataFailure = runtimeMetadata.changed;
     let sourceDiagnostics: readonly Diagnostic[];
     try {
-      sourceDiagnostics = freezeDiagnostics(validateSource(preparedLoaded, discovered, registry));
+      // The AB4750 freshness nudge only surfaces through `validate`; other
+      // commands skip its full-project mtime walk.
+      sourceDiagnostics = freezeDiagnostics(validateSource(preparedLoaded, discovered, registry, {
+        payloadFreshness: command === 'validate',
+      }));
     } catch {
       return failedPreparation(
         'AB7001',
@@ -722,10 +734,12 @@ export class ProjectService {
         snapshot,
       );
     }
+    const snapshotSource = (): Promise<ProjectSourceSnapshot> =>
+      snapshotProjectSource(root, loaded.configPath, outputRoots, payloadRoots);
     if (hasErrors(sourceDiagnostics)) {
       const source = sourceStatus(sourceDiagnostics, snapshot.revision);
       log(this.#options.logger, 'project.invalid-source', { diagnostics: sourceDiagnostics.length, root });
-      return preparedProject(loaded.configPath, snapshot, sourceDiagnostics, outputRoots, undefined, registry, root, source);
+      return preparedProject(loaded.configPath, snapshot, sourceDiagnostics, outputRoots, undefined, registry, root, source, snapshotSource);
     }
 
     let model: NormalizedPlugin;
@@ -834,15 +848,12 @@ export class ProjectService {
       registry,
       root,
       source,
+      snapshotSource,
       model,
       devRuntime,
       devRuntimeDiagnostic,
       devAgentApiEnabled,
       tools,
-      // Re-snapshots must observe the same payload roots the prepared
-      // identity hashed, or payload-bearing projects would always appear
-      // drifted to epoch publication.
-      () => snapshotProjectSource(root, loaded.configPath, outputRoots, payloadRoots),
     );
   }
 }
diff --git a/packages/agent-bundle/tests/dev-package-build-service.test.ts b/packages/agent-bundle/tests/dev-package-build-service.test.ts
index 9d4e07d21..4a4b18e28 100644
--- a/packages/agent-bundle/tests/dev-package-build-service.test.ts
+++ b/packages/agent-bundle/tests/dev-package-build-service.test.ts
@@ -41,6 +41,7 @@ const prepared = (options: {
   outputRoots: [],
   registry: undefined as never,
   root: options.root ?? '/project',
+  snapshotSource: async () => ({ inputs: [], revision: 'test-revision' }),
   source: { diagnostics: [], state: 'ready' },
   ...(options.tools === undefined ? {} : { tools: options.tools }),
 });
diff --git a/packages/agent-bundle/tests/prebuilt-payload.test.ts b/packages/agent-bundle/tests/prebuilt-payload.test.ts
index c3da56656..6edfacafd 100644
--- a/packages/agent-bundle/tests/prebuilt-payload.test.ts
+++ b/packages/agent-bundle/tests/prebuilt-payload.test.ts
@@ -1,5 +1,4 @@
-import { mkdir, mkdtemp, readFile, rm, writeFile } from 'node:fs/promises';
-import { tmpdir } from 'node:os';
+import { readFile } from 'node:fs/promises';
 import { join } from 'node:path';
 
 import { expect, it } from '@rstest/core';
@@ -7,6 +6,7 @@ import { expect, it } from '@rstest/core';
 import { build, validate } from '../src/api.ts';
 import { DiagnosticError } from '../src/core/diagnostics.ts';
 import { parseArtifactManifest } from '../src/build/manifest.ts';
+import { createProjectFixture, removeProjectFixture } from './helpers/project-fixture.ts';
 
 const configSource = (options: { readonly payload?: string; readonly hooks?: string; readonly mcp?: string }): string => [
   'export default {',
@@ -34,18 +34,13 @@ const standardHooksBlock = [
 ].join('\n');
 
 /** A payload fixture whose runtime tree is deliberately not framework-shaped. */
-const writePayloadFiles = async (root: string): Promise<void> => {
-  await mkdir(join(root, 'built', 'runtime', 'mcp'), { recursive: true });
-  await mkdir(join(root, 'built', 'runtime', 'chunks'), { recursive: true });
-  await mkdir(join(root, 'built', 'app'), { recursive: true });
-  await Promise.all([
-    // A bare-specifier import would fail the generated-module graph
-    // validation (AB6005); prebuilt payloads are exempt by design.
-    writeFile(join(root, 'built', 'runtime', 'mcp', 'server.js'), 'import express from "express";\nexport default express;\n'),
-    writeFile(join(root, 'built', 'runtime', 'chunks', '417.js'), 'module.exports = require("./418.js");\n'),
-    writeFile(join(root, 'built', 'runtime', 'hook.js'), 'process.stdout.write("{}");\n'),
-    writeFile(join(root, 'built', 'app', 'index.html'), '<html><body>widget</body></html>\n'),
-  ]);
+const payloadFiles: Readonly<Record<string, string>> = {
+  'built/app/index.html': '<html><body>widget</body></html>\n',
+  'built/runtime/chunks/417.js': 'module.exports = require("./418.js");\n',
+  'built/runtime/hook.js': 'process.stdout.write("{}");\n',
+  // A bare-specifier import would fail the generated-module graph
+  // validation (AB6005); prebuilt payloads are exempt by design.
+  'built/runtime/mcp/server.js': 'import express from "express";\nexport default express;\n',
 };
 
 const createProject = async (options: {
@@ -53,10 +48,16 @@ const createProject = async (options: {
   readonly hooks?: string;
   readonly mcp?: string;
   readonly withPayloadFiles?: boolean;
+  readonly files?: Readonly<Record<string, string>>;
 } = {}): Promise<string> => {
-  const root = await mkdtemp(join(tmpdir(), 'agent-bundle-prebuilt-'));
-  await writeFile(join(root, 'agent-bundle.config.ts'), configSource(options));
-  if (options.withPayloadFiles !== false) await writePayloadFiles(root);
+  const { root } = await createProjectFixture({
+    config: configSource(options),
+    files: {
+      ...(options.withPayloadFiles !== false ? payloadFiles : {}),
+      ...options.files,
+    },
+    prefix: 'agent-bundle-prebuilt-',
+  });
   return root;
 };
 
@@ -142,7 +143,7 @@ it('packages prebuilt payloads at stable paths and lowers prebuilt entries throu
     const revalidated = await validate({ artifact: join(root, 'out'), root });
     expect(revalidated.diagnostics.filter((diagnostic) => diagnostic.severity === 'error')).toEqual([]);
   } finally {
-    await rm(root, { force: true, recursive: true });
+    await removeProjectFixture(root);
   }
 });
 
@@ -171,7 +172,7 @@ it('validates an argument-less prebuilt hook without demanding a wrapper index e
     const revalidated = await validate({ artifact: join(root, 'out'), root });
     expect(revalidated.diagnostics.filter((diagnostic) => diagnostic.severity === 'error')).toEqual([]);
   } finally {
-    await rm(root, { force: true, recursive: true });
+    await removeProjectFixture(root);
   }
 });
 
@@ -197,9 +198,8 @@ it('reports the prebuilt payload source diagnostics', async () => {
       "    runtime: { source: './built/runtime', targets: ['claude'] },",
       '  },',
     ].join('\n'),
+    files: { 'src/hook.ts': 'export default () => undefined;\n' },
   });
-  await mkdir(join(root, 'src'), { recursive: true });
-  await writeFile(join(root, 'src', 'hook.ts'), 'export default () => undefined;\n');
   try {
     const result = await validate({ root });
     const codes = result.diagnostics.map((diagnostic) => [diagnostic.code, diagnostic.severity] as const);
@@ -215,7 +215,7 @@ it('reports the prebuilt payload source diagnostics', async () => {
     // Hook arguments: rejected on compiled handlers and on unsafe values.
     expect(result.diagnostics.filter((diagnostic) => diagnostic.code === 'AB4746').length).toBe(2);
   } finally {
-    await rm(root, { force: true, recursive: true });
+    await removeProjectFixture(root);
   }
 });
 
@@ -229,7 +229,7 @@ it('refuses to build while a payload is empty, a prebuilt entry is absent, or th
       diagnostics: [expect.objectContaining({ code: 'AB4747' })],
     });
   } finally {
-    await rm(emptyPayloadRoot, { force: true, recursive: true });
+    await removeProjectFixture(emptyPayloadRoot);
   }
 
   const missingEntryRoot = await createProject({
@@ -245,7 +245,7 @@ it('refuses to build while a payload is empty, a prebuilt entry is absent, or th
       diagnostics: [expect.objectContaining({ code: 'AB4748' })],
     });
   } finally {
-    await rm(missingEntryRoot, { force: true, recursive: true });
+    await removeProjectFixture(missingEntryRoot);
   }
 
   const overlapRoot = await createProject({ payload: standardPayloadBlock });
@@ -254,6 +254,6 @@ it('refuses to build while a payload is empty, a prebuilt entry is absent, or th
       diagnostics: expect.arrayContaining([expect.objectContaining({ code: 'AB4749' })]),
     });
   } finally {
-    await rm(overlapRoot, { force: true, recursive: true });
+    await removeProjectFixture(overlapRoot);
   }
 });