feat(prdy): add PrdService as the single PRD authority surface (#1275)

- PrdService owns create/read/update/link/import/export; wizard and package
  CLI become prompt layers over the service (no second writer path)
- PRD documents gain a content version and a missions linkage array persisted
  in the YAML authority store (survives restart); linkage writes do not bump
  the content version
- exportMarkdown renders a labeled generated view (id + version + do-not-edit
  header) that no code path reads back; the store loads .yaml/.yml only
- importDocument validates structure with zod before anything else, persists
  valid imports as draft (validity is not approval), and refuses conflicts
  with a typed PrdImportConflictError carrying a proposed successor; the
  original document stays byte-identical until acceptSuccessor
- raw store writers (createPrd/savePrd) are no longer exported from the
  package entry point

packages/prdy/src/cli.ts

@@ -1,6 +1,6 @@
 import { Command } from 'commander';
 
-import { createPrd, listPrds, loadPrd } from './prd.js';
+import { PrdService } from './service.js';
 import { runPrdWizard } from './wizard.js';
 
 interface InitCommandOptions {
@@ -18,6 +18,22 @@ interface ShowCommandOptions {
   readonly id?: string;
 }
 
+interface ImportCommandOptions {
+  readonly project: string;
+  readonly file: string;
+  readonly acceptSuccessor?: boolean;
+}
+
+interface ExportCommandOptions {
+  readonly project: string;
+  readonly id?: string;
+  readonly out?: string;
+}
+
+function serviceFor(project: string): PrdService {
+  return new PrdService({ projectPath: project });
+}
+
 export function buildPrdyCli(): Command {
   const program = new Command();
   program.name('mosaic').description('Mosaic CLI').exitOverride();
@@ -38,11 +54,9 @@ export function buildPrdyCli(): Command {
             template: options.template,
             interactive: true,
           })
-        : await createPrd({
+        : await serviceFor(options.project).create({
             name: options.name,
-            projectPath: options.project,
             template: options.template,
-            interactive: false,
           });
 
       console.log(
@@ -52,6 +66,7 @@ export function buildPrdyCli(): Command {
             id: doc.id,
             title: doc.title,
             status: doc.status,
+            version: doc.version,
             projectPath: doc.projectPath,
           },
           null,
@@ -65,7 +80,7 @@ export function buildPrdyCli(): Command {
     .description('List PRD documents for a project')
     .requiredOption('--project <path>', 'Project path')
     .action(async (options: ListCommandOptions) => {
-      const docs = await listPrds(options.project);
+      const docs = await serviceFor(options.project).list();
       console.log(JSON.stringify(docs, null, 2));
     });
 
@@ -75,20 +90,65 @@ export function buildPrdyCli(): Command {
     .requiredOption('--project <path>', 'Project path')
     .option('--id <id>', 'PRD document id')
     .action(async (options: ShowCommandOptions) => {
-      if (options.id !== undefined) {
+      const doc = await serviceFor(options.project).get(options.id);
-        const docs = await listPrds(options.project);
+      console.log(JSON.stringify(doc, null, 2));
-        const match = docs.find((doc) => doc.id === options.id);
+    });
 
-        if (match === undefined) {
+  prdy
-          throw new Error(`PRD id not found: ${options.id}`);
+    .command('import')
-        }
+    .description('Import a YAML PRD document (validated; conflicts propose a successor)')
+    .requiredOption('--project <path>', 'Project path')
+    .requiredOption('--file <file>', 'Path to YAML PRD document')
+    .option('--accept-successor', 'Accept a conflicted import as the next version')
+    .action(async (options: ImportCommandOptions) => {
+      const service = serviceFor(options.project);
+      const input = { filePath: options.file };
 
-        console.log(JSON.stringify(match, null, 2));
+      if (options.acceptSuccessor) {
+        const successor = await service.acceptSuccessor(input);
+        console.log(
+          JSON.stringify(
+            {
+              ok: true,
+              outcome: 'successor-accepted',
+              id: successor.id,
+              version: successor.version,
+            },
+            null,
+            2,
+          ),
+        );
         return;
       }
 
-      const doc = await loadPrd(options.project);
+      const result = await service.importDocument(input);
-      console.log(JSON.stringify(doc, null, 2));
+      console.log(
+        JSON.stringify(
+          {
+            ok: true,
+            outcome: result.kind,
+            id: result.document.id,
+            version: result.document.version,
+            status: result.document.status,
+          },
+          null,
+          2,
+        ),
+      );
+    });
+
+  prdy
+    .command('export')
+    .description('Render a PRD to a labeled generated-view Markdown file')
+    .requiredOption('--project <path>', 'Project path')
+    .option('--id <id>', 'PRD document id')
+    .option('--out <path>', 'Output path (default docs/prdy/<id>.md)')
+    .action(async (options: ExportCommandOptions) => {
+      const result = await serviceFor(options.project).exportMarkdown({
+        id: options.id,
+        outPath: options.out,
+      });
+      console.log(JSON.stringify({ ok: true, filePath: result.filePath }, null, 2));
     });
 
   return program;

packages/prdy/src/index.ts

@@ -1,12 +1,35 @@
-export { createPrd, loadPrd, savePrd, listPrds } from './prd.js';
+// PrdService is the single authority surface for PRD documents. The raw store
+// writers (createPrd/savePrd) are deliberately NOT exported: every mutation
+// goes through the service so there is no second writer path.
+export { loadPrd, listPrds, parsePrdDocument } from './prd.js';
 export { runPrdWizard } from './wizard.js';
 export { buildPrdyCli, runPrdyCli } from './cli.js';
 export { BUILTIN_PRD_TEMPLATES, resolveTemplate } from './templates.js';
+export {
+  PrdService,
+  PRD_GENERATED_VIEW_LABEL,
+  PrdError,
+  PrdNotFoundError,
+  PrdUpdateError,
+  PrdImportInvalidError,
+  PrdImportConflictError,
+} from './service.js';
 export type {
   PrdStatus,
   PrdTemplate,
   PrdTemplateSection,
   PrdSection,
+  PrdMissionLinkage,
   PrdDocument,
   CreatePrdOptions,
+  PrdServiceOptions,
+  PrdCreateInput,
+  PrdSectionPatch,
+  PrdUpdateInput,
+  PrdLinkMissionInput,
+  PrdPlanForMissionInput,
+  PrdExportInput,
+  PrdExportResult,
+  PrdImportInput,
+  PrdImportResult,
 } from './types.js';

packages/prdy/src/prd.ts

@@ -17,17 +17,49 @@ const prdSectionSchema = z.object({
   fields: z.record(z.string(), z.string()),
 });
 
+const prdMissionLinkageSchema = z.object({
+  missionId: z.string().min(1),
+  missionVersion: z.string().min(1),
+  prdVersion: z.number().int().min(1),
+  requirementIds: z.array(z.string()),
+  linkedAt: z.string().datetime(),
+});
+
 const prdDocumentSchema = z.object({
   id: z.string().min(1),
   title: z.string().min(1),
   status: z.enum(['draft', 'review', 'approved', 'archived']),
   projectPath: z.string().min(1),
   template: z.string().min(1),
+  // Defaults keep documents written by older prdy versions loadable.
+  version: z.number().int().min(1).default(1),
   sections: z.array(prdSectionSchema),
+  missions: z.array(prdMissionLinkageSchema).default([]),
   createdAt: z.string().datetime(),
   updatedAt: z.string().datetime(),
 });
 
+/** YAML timestamp scalars are parsed as Date by some emitters — normalize to ISO strings. */
+function coerceTimestamps(value: unknown): unknown {
+  if (value instanceof Date) {
+    return value.toISOString();
+  }
+  if (Array.isArray(value)) {
+    return value.map(coerceTimestamps);
+  }
+  if (typeof value === 'object' && value !== null) {
+    return Object.fromEntries(
+      Object.entries(value).map(([key, entry]) => [key, coerceTimestamps(entry)]),
+    );
+  }
+  return value;
+}
+
+/** Validate an unknown value as a PRD document (throws zod errors on failure). */
+export function parsePrdDocument(value: unknown): PrdDocument {
+  return prdDocumentSchema.parse(coerceTimestamps(value)) as PrdDocument;
+}
+
 function expandHome(projectPath: string): string {
   if (!projectPath.startsWith('~')) {
     return projectPath;
@@ -74,6 +106,8 @@ function prdDirectory(projectPath: string): string {
   return path.join(projectPath, PRD_DIRECTORY);
 }
 
+export { prdDirectory };
+
 function prdFilePath(projectPath: string, id: string): string {
   return path.join(prdDirectory(projectPath), `${id}.yaml`);
 }
@@ -113,11 +147,13 @@ export async function createPrd(options: CreatePrdOptions): Promise<PrdDocument>
     status: 'draft',
     projectPath: resolvedProjectPath,
     template: template.id,
+    version: 1,
     sections: template.sections.map((section) => ({
       id: section.id,
       title: section.title,
       fields: Object.fromEntries(section.fields.map((field) => [field, ''])),
     })),
+    missions: [],
     createdAt: now,
     updatedAt: now,
   };
@@ -190,7 +226,7 @@ export async function listPrds(projectPath: string): Promise<PrdDocument[]> {
       throw new Error(`Failed to parse PRD file ${filePath}: ${String(error)}`);
     }
 
-    const document = prdDocumentSchema.parse(parsed);
+    const document = parsePrdDocument(parsed);
     documents.push(document);
   }
 

packages/prdy/src/service.spec.ts

@@ -0,0 +1,433 @@
+import { existsSync } from 'node:fs';
+import { mkdtemp, readFile, readdir, writeFile } from 'node:fs/promises';
+import os from 'node:os';
+import path from 'node:path';
+
+import yaml from 'js-yaml';
+import { beforeEach, describe, expect, it } from 'vitest';
+
+import {
+  PRD_GENERATED_VIEW_LABEL,
+  PrdImportConflictError,
+  PrdImportInvalidError,
+  PrdNotFoundError,
+  PrdService,
+  PrdUpdateError,
+} from './index.js';
+import type { PrdDocument } from './index.js';
+
+// ── Helpers ──────────────────────────────────────────────────────────────────
+
+let projectDir: string;
+
+async function makeProject(): Promise<string> {
+  return mkdtemp(path.join(os.tmpdir(), 'prdy-service-'));
+}
+
+function service(): PrdService {
+  return new PrdService({ projectPath: projectDir });
+}
+
+function storeDir(): string {
+  return path.join(projectDir, 'docs', 'prdy');
+}
+
+/** Handcraft a full, schema-valid PRD document for import scenarios. */
+function importFixture(overrides: Partial<PrdDocument> = {}): PrdDocument {
+  return {
+    id: 'imported-prd-20260101-000000',
+    title: 'Imported PRD',
+    status: 'draft',
+    projectPath: '/tmp/elsewhere',
+    template: 'software',
+    version: 1,
+    sections: [
+      { id: 'introduction', title: 'Introduction', fields: { context: '', objective: '' } },
+      {
+        id: 'scope-non-goals',
+        title: 'Scope / Non-Goals',
+        fields: { inScope: '', outOfScope: '' },
+      },
+    ],
+    missions: [],
+    createdAt: '2026-01-01T00:00:00.000Z',
+    updatedAt: '2026-01-01T00:00:00.000Z',
+    ...overrides,
+  };
+}
+
+async function writeImportFile(doc: PrdDocument): Promise<string> {
+  const filePath = path.join(projectDir, `${doc.id}.import.yaml`);
+  await writeFile(filePath, yaml.dump(doc), 'utf8');
+  return filePath;
+}
+
+beforeEach(async () => {
+  projectDir = await makeProject();
+});
+
+// ── Single authority store (AC: prdy path and mission path resolve to the
+//    SAME store under docs/prdy/ with stable ids/versions) ────────────────────
+
+describe('PrdService single authority store', () => {
+  it('persists PRDs from the prdy path and the mission path into the same docs/prdy store', async () => {
+    const direct = await service().create({ name: 'Direct PRD' });
+    const viaMission = await service().planForMission({
+      name: 'Mission PRD',
+      missionId: 'mission-1',
+      missionVersion: '2026-01-01T00:00:00.000Z',
+    });
+
+    const files = await readdir(storeDir());
+    expect(files).toContain(`${direct.id}.yaml`);
+    expect(files).toContain(`${viaMission.id}.yaml`);
+
+    // A fresh service instance (new process equivalent) resolves both.
+    const all = await service().list();
+    expect(all.map((doc) => doc.id).sort()).toEqual([direct.id, viaMission.id].sort());
+
+    // Stable versions: creation is v1; linkage writes do not bump content version.
+    expect((await service().get(direct.id)).version).toBe(1);
+    expect((await service().get(viaMission.id)).version).toBe(1);
+  });
+
+  it('round-trips documents through the store with identity intact', async () => {
+    const created = await service().create({ name: 'Round Trip', template: 'feature' });
+    const fresh = await service().get(created.id);
+
+    expect(fresh).toEqual(created);
+    expect(fresh.id).toBe(created.id);
+    expect(fresh.template).toBe('feature');
+    expect(fresh.status).toBe('draft');
+  });
+
+  it('throws a typed error for unknown ids and empty stores', async () => {
+    await expect(service().get('nope')).rejects.toBeInstanceOf(PrdNotFoundError);
+    await expect(service().get()).rejects.toBeInstanceOf(PrdNotFoundError);
+  });
+});
+
+// ── Mission linkage persistence (AC: linkage survives restart via fresh
+//    service instances) ────────────────────────────────────────────────────────
+
+describe('PrdService mission linkage', () => {
+  it('persists linkage and reads it back from a fresh service instance', async () => {
+    const created = await service().planForMission({
+      name: 'Linked PRD',
+      missionId: 'mission-42',
+      missionVersion: '2026-02-03T04:05:06.000Z',
+      requirementIds: ['FR-1', 'FR-2'],
+    });
+
+    // Fresh instance — nothing in memory from the creating call.
+    const links = await service().listMissionLinks(created.id);
+    expect(links).toHaveLength(1);
+    expect(links[0]).toMatchObject({
+      missionId: 'mission-42',
+      missionVersion: '2026-02-03T04:05:06.000Z',
+      prdVersion: 1,
+      requirementIds: ['FR-1', 'FR-2'],
+    });
+
+    // Linkage is carried in the YAML authority file itself.
+    const raw = await readFile(path.join(storeDir(), `${created.id}.yaml`), 'utf8');
+    const persisted = yaml.load(raw) as PrdDocument;
+    expect(persisted.missions[0]?.missionId).toBe('mission-42');
+    expect(persisted.missions[0]?.requirementIds).toEqual(['FR-1', 'FR-2']);
+  });
+
+  it('refreshes an existing linkage entry in place instead of duplicating', async () => {
+    const created = await service().planForMission({
+      name: 'Relink PRD',
+      missionId: 'mission-7',
+      missionVersion: 'v1',
+    });
+
+    await service().update({
+      id: created.id,
+      sections: [{ id: 'introduction', fields: { objective: 'Ship it' } }],
+    });
+
+    const relinked = await service().linkMission({
+      prdId: created.id,
+      missionId: 'mission-7',
+      missionVersion: 'v2',
+      requirementIds: ['NFR-1'],
+    });
+
+    expect(relinked.missions).toHaveLength(1);
+    expect(relinked.missions[0]).toMatchObject({ missionVersion: 'v2', prdVersion: 2 });
+  });
+
+  it('does not bump the content version when writing linkage', async () => {
+    const created = await service().create({ name: 'Stable Version' });
+    const linked = await service().linkMission({
+      prdId: created.id,
+      missionId: 'm',
+      missionVersion: 'v1',
+    });
+    expect(linked.version).toBe(1);
+  });
+});
+
+// ── Update semantics ──────────────────────────────────────────────────────────
+
+describe('PrdService update', () => {
+  it('applies section patches and bumps the content version', async () => {
+    const created = await service().create({ name: 'Updatable' });
+    const updated = await service().update({
+      id: created.id,
+      sections: [{ id: 'introduction', fields: { context: 'Some context', objective: 'Goal' } }],
+    });
+
+    expect(updated.version).toBe(2);
+    expect(updated.sections[0]?.fields).toMatchObject({
+      context: 'Some context',
+      objective: 'Goal',
+    });
+    expect((await service().get(created.id)).version).toBe(2);
+  });
+
+  it('refuses unknown section ids with a typed error', async () => {
+    const created = await service().create({ name: 'Strict' });
+    await expect(
+      service().update({ id: created.id, sections: [{ id: 'nope', fields: {} }] }),
+    ).rejects.toBeInstanceOf(PrdUpdateError);
+  });
+});
+
+// ── Markdown export is a labeled generated view, never authority ──────────────
+
+describe('PrdService exportMarkdown', () => {
+  it('writes a generated view carrying the label and source identity', async () => {
+    const created = await service().create({ name: 'Exported PRD' });
+    const result = await service().exportMarkdown({ id: created.id });
+
+    expect(result.filePath).toBe(path.join(storeDir(), `${created.id}.md`));
+    expect(result.content).toContain(PRD_GENERATED_VIEW_LABEL);
+    expect(result.content).toContain(`prd-id: ${created.id}`);
+    expect(result.content).toContain('prd-version: 1');
+    expect(result.content).toContain(`source-of-truth: docs/prdy/${created.id}.yaml`);
+  });
+
+  it('reflects the current version after updates', async () => {
+    const created = await service().create({ name: 'Versioned Export' });
+    await service().update({
+      id: created.id,
+      sections: [{ id: 'introduction', fields: { objective: 'v2 goal' } }],
+    });
+    const result = await service().exportMarkdown({ id: created.id });
+    expect(result.content).toContain('prd-version: 2');
+  });
+
+  it('NEGATIVE CONTROL: mutating the exported Markdown cannot change the authority', async () => {
+    const created = await service().create({ name: 'Guarded PRD' });
+    const before = structuredClone(await service().get(created.id));
+
+    const result = await service().exportMarkdown({ id: created.id });
+    await writeFile(
+      result.filePath,
+      `<!-- ${PRD_GENERATED_VIEW_LABEL} -->\n# FAKE\nprd-id: fake-id\nprd-version: 99\n`,
+      'utf8',
+    );
+
+    const after = await service().get(created.id);
+    expect(after).toEqual(before);
+    expect(after.version).toBe(1);
+    expect(after.title).toBe(before.title);
+  });
+
+  it('never parses Markdown files that sit in the store directory', async () => {
+    const created = await service().create({ name: 'Decoy Guard' });
+
+    // A decoy .md file with invalid YAML must be invisible to the store.
+    await writeFile(path.join(storeDir(), 'decoy.md'), 'not: [valid: yaml', 'utf8');
+    // And a decoy .yaml-named Markdown body must not silently validate either.
+    await service().exportMarkdown({ id: created.id });
+
+    const listed = await service().list();
+    expect(listed.map((doc) => doc.id)).toEqual([created.id]);
+    await expect(service().get(created.id)).resolves.toBeTruthy();
+  });
+});
+
+// ── Import: validated, conflict-aware, never silently merging ─────────────────
+
+describe('PrdService importDocument', () => {
+  it('creates a valid import through the service, as draft — validity is not approval', async () => {
+    const filePath = await writeImportFile(importFixture({ status: 'approved' }));
+
+    const result = await service().importDocument({ filePath });
+
+    expect(result.kind).toBe('created');
+    expect(result.document.id).toBe('imported-prd-20260101-000000');
+    expect(result.document.status).toBe('draft'); // structural validity ≠ approval
+    expect(result.document.version).toBe(1);
+
+    const persisted = await service().get('imported-prd-20260101-000000');
+    expect(persisted.status).toBe('draft');
+
+    const files = await readdir(storeDir());
+    expect(files).toContain('imported-prd-20260101-000000.yaml');
+  });
+
+  it('reports identical content as a no-op without writing', async () => {
+    const created = await service().create({ name: 'Existing PRD' });
+    const before = await readFile(path.join(storeDir(), `${created.id}.yaml`), 'utf8');
+
+    const filePath = await writeImportFile(importFixture({ ...created }));
+    const result = await service().importDocument({ filePath });
+
+    expect(result.kind).toBe('identical');
+    const after = await readFile(path.join(storeDir(), `${created.id}.yaml`), 'utf8');
+    expect(after).toBe(before);
+  });
+
+  it('refuses a conflicting import with a typed error, a proposed successor, and untouched bytes', async () => {
+    const existing = await service().create({ name: 'Authority PRD' });
+    await service().linkMission({
+      prdId: existing.id,
+      missionId: 'mission-keep',
+      missionVersion: 'v1',
+      requirementIds: ['FR-0'],
+    });
+    const beforeBytes = await readFile(path.join(storeDir(), `${existing.id}.yaml`), 'utf8');
+
+    const divergent = importFixture({
+      ...existing,
+      title: 'Divergent Title',
+      sections: [
+        {
+          id: 'introduction',
+          title: 'Introduction',
+          fields: { context: 'changed', objective: '' },
+        },
+      ],
+    });
+    const filePath = await writeImportFile(divergent);
+
+    const attempt = service().importDocument({ filePath });
+    let caught: unknown;
+    try {
+      await attempt;
+    } catch (error) {
+      caught = error;
+    }
+    expect(caught).toBeInstanceOf(PrdImportConflictError);
+
+    const error = caught as PrdImportConflictError;
+    expect(error.code).toBe('PRD_IMPORT_CONFLICT');
+    expect(error.existing.id).toBe(existing.id);
+    expect(error.proposal.version).toBe(existing.version + 1); // successor proposal
+    expect(error.proposal.status).toBe('draft');
+
+    // Original authority content untouched on disk.
+    const afterBytes = await readFile(path.join(storeDir(), `${existing.id}.yaml`), 'utf8');
+    expect(afterBytes).toBe(beforeBytes);
+  });
+
+  it('acceptSuccessor persists the proposal explicitly, carrying linkages forward', async () => {
+    const existing = await service().create({ name: 'Successor Base' });
+    await service().linkMission({
+      prdId: existing.id,
+      missionId: 'mission-keep',
+      missionVersion: 'v1',
+    });
+
+    const divergent = importFixture({
+      ...existing,
+      title: 'Accepted Successor Title',
+    });
+    const filePath = await writeImportFile(divergent);
+
+    const successor = await service().acceptSuccessor({ filePath });
+    expect(successor.id).toBe(existing.id);
+    expect(successor.version).toBe(existing.version + 1);
+    expect(successor.title).toBe('Accepted Successor Title');
+    expect(successor.status).toBe('draft');
+    expect(successor.missions.map((m) => m.missionId)).toEqual(['mission-keep']);
+
+    // Persisted for a fresh reader.
+    const fresh = await service().get(existing.id);
+    expect(fresh.version).toBe(2);
+    expect(fresh.title).toBe('Accepted Successor Title');
+  });
+
+  it('refuses structurally-invalid imports with a typed error and creates nothing', async () => {
+    const cases: Array<{ name: string; body: string }> = [
+      { name: 'missing-title.yaml', body: yaml.dump({ id: 'x', status: 'draft' }) },
+      {
+        name: 'bad-status.yaml',
+        body: yaml.dump(importFixture({ status: 'not-a-status' as PrdDocument['status'] })),
+      },
+      {
+        name: 'bad-version.yaml',
+        body: yaml.dump(importFixture({ version: 0 })),
+      },
+      { name: 'not-yaml.yaml', body: '::: not yaml [\n  - {' },
+    ];
+
+    for (const fixture of cases) {
+      const filePath = path.join(projectDir, fixture.name);
+      await writeFile(filePath, fixture.body, 'utf8');
+
+      await expect(service().importDocument({ filePath })).rejects.toBeInstanceOf(
+        PrdImportInvalidError,
+      );
+    }
+
+    // Nothing was created: the authority store does not even exist yet.
+    await expect(readdir(storeDir())).rejects.toMatchObject({ code: 'ENOENT' });
+  });
+
+  it('acceptSuccessor refuses when there is no existing document to succeed', async () => {
+    const filePath = await writeImportFile(importFixture());
+    await expect(service().acceptSuccessor({ filePath })).rejects.toBeInstanceOf(PrdNotFoundError);
+  });
+});
+
+// ── No second writer: no code path reads exported Markdown back into authority ─
+
+describe('no-second-writer invariant (source-level)', () => {
+  // Resolve the package source dir whether vitest runs from the package root
+  // (turbo/pnpm test) or from the worktree root.
+  function resolveSrcDir(): string {
+    const candidates = [path.resolve('src'), path.resolve('packages/prdy/src')];
+    return candidates.find((dir) => existsSync(path.join(dir, 'service.ts'))) ?? candidates[0]!;
+  }
+
+  const srcDir = resolveSrcDir();
+  const sourceFiles = [
+    'cli.ts',
+    'index.ts',
+    'prd.ts',
+    'service.ts',
+    'templates.ts',
+    'types.ts',
+    'wizard.ts',
+  ];
+
+  it('no source file in @mosaicstack/prdy reads a .md file', async () => {
+    for (const file of sourceFiles) {
+      const text = await readFile(path.join(srcDir, file), 'utf8');
+      const readLines = text
+        .split('\n')
+        .map((line) => line.trim())
+        .filter((line) => /readFile|readFileSync|createReadStream/.test(line));
+
+      for (const line of readLines) {
+        expect(line.includes('.md'), `${file} reads a Markdown file: ${line}`).toBe(false);
+      }
+    }
+  });
+
+  it('the mosaic prdy/mission adapters never read a .md file', async () => {
+    const adapterDir = path.resolve(srcDir, '..', '..', 'mosaic', 'src', 'commands');
+    for (const file of ['prdy.ts', 'mission.ts']) {
+      const text = await readFile(path.join(adapterDir, file), 'utf8');
+      expect(text.includes("'.md'") || text.includes('.md`'), `${file} references a .md path`).toBe(
+        false,
+      );
+    }
+  });
+});

packages/prdy/src/service.ts

@@ -0,0 +1,379 @@
+import { promises as fs } from 'node:fs';
+import path from 'node:path';
+
+import yaml from 'js-yaml';
+
+import { createPrd, listPrds, parsePrdDocument, prdDirectory, savePrd } from './prd.js';
+import type {
+  PrdCreateInput,
+  PrdDocument,
+  PrdExportInput,
+  PrdExportResult,
+  PrdImportInput,
+  PrdImportResult,
+  PrdLinkMissionInput,
+  PrdMissionLinkage,
+  PrdPlanForMissionInput,
+  PrdServiceOptions,
+  PrdUpdateInput,
+} from './types.js';
+
+/**
+ * PrdService is the SINGLE authority surface for PRD documents.
+ *
+ * Every mutation path (CLI wizard, `mosaic mission --plan`, import) routes
+ * through this service; the YAML store under `docs/prdy/` is the authority and
+ * exported Markdown is a generated view that no code path reads back.
+ */
+
+// ── Typed errors ───────────────────────────────────────────────────────────────
+
+export class PrdError extends Error {
+  constructor(
+    message: string,
+    readonly code: string,
+  ) {
+    super(message);
+    this.name = 'PrdError';
+  }
+}
+
+export class PrdNotFoundError extends PrdError {
+  constructor(message: string) {
+    super(message, 'PRD_NOT_FOUND');
+    this.name = 'PrdNotFoundError';
+  }
+}
+
+export class PrdUpdateError extends PrdError {
+  constructor(message: string) {
+    super(message, 'PRD_UPDATE_INVALID');
+    this.name = 'PrdUpdateError';
+  }
+}
+
+/** Structural refusal: the import payload failed schema validation. Nothing is written. */
+export class PrdImportInvalidError extends PrdError {
+  constructor(
+    message: string,
+    readonly issues?: string,
+  ) {
+    super(message, 'PRD_IMPORT_INVALID');
+    this.name = 'PrdImportInvalidError';
+  }
+}
+
+/**
+ * Conflict refusal: an existing PRD shares the imported id but the content
+ * diverges. Carries a PROPOSED successor (existing version + 1) that is only
+ * persisted via an explicit {@link PrdService.acceptSuccessor} call — import
+ * never overwrites and never merges.
+ */
+export class PrdImportConflictError extends PrdError {
+  constructor(
+    message: string,
+    readonly existing: PrdDocument,
+    readonly proposal: PrdDocument,
+  ) {
+    super(message, 'PRD_IMPORT_CONFLICT');
+    this.name = 'PrdImportConflictError';
+  }
+}
+
+// ── Service ────────────────────────────────────────────────────────────────────
+
+/** The generated-view label carried by every Markdown export. */
+export const PRD_GENERATED_VIEW_LABEL = 'generated view — do not edit';
+
+export class PrdService {
+  private readonly projectPath: string;
+
+  constructor(options: PrdServiceOptions) {
+    this.projectPath = options.projectPath;
+  }
+
+  /** Create a new PRD (version 1, draft) in the authority store. */
+  async create(input: PrdCreateInput): Promise<PrdDocument> {
+    return createPrd({
+      name: input.name,
+      projectPath: this.projectPath,
+      template: input.template,
+      interactive: false,
+    });
+  }
+
+  /** Read a PRD by id, or the most recently updated one. */
+  async get(id?: string): Promise<PrdDocument> {
+    const documents = await listPrds(this.projectPath);
+
+    if (id === undefined) {
+      const latest = documents[0];
+      if (latest === undefined) {
+        throw new PrdNotFoundError(`No PRD documents found under docs/prdy/ for this project`);
+      }
+      return latest;
+    }
+
+    const match = documents.find((doc) => doc.id === id);
+    if (match === undefined) {
+      throw new PrdNotFoundError(`PRD id not found: ${id}`);
+    }
+    return match;
+  }
+
+  /** List all PRDs in the authority store (most recently updated first). */
+  async list(): Promise<PrdDocument[]> {
+    return listPrds(this.projectPath);
+  }
+
+  /**
+   * Apply section field patches and bump the content version.
+   * Linkage entries are preserved; linkage writes do NOT bump the version.
+   */
+  async update(input: PrdUpdateInput): Promise<PrdDocument> {
+    const doc = await this.get(input.id);
+
+    for (const patch of input.sections) {
+      const section = doc.sections.find((candidate) => candidate.id === patch.id);
+      if (section === undefined) {
+        throw new PrdUpdateError(`Unknown section id: ${patch.id}`);
+      }
+      for (const [field, value] of Object.entries(patch.fields)) {
+        if (!(field in section.fields)) {
+          throw new PrdUpdateError(`Unknown field "${field}" on section "${patch.id}"`);
+        }
+        section.fields[field] = value;
+      }
+    }
+
+    doc.version += 1;
+    doc.updatedAt = new Date().toISOString();
+    await savePrd(doc);
+    return doc;
+  }
+
+  /**
+   * Record (or refresh) a mission ↔ PRD linkage on the PRD document.
+   * Persisted in the YAML authority, so it survives restarts.
+   */
+  async linkMission(input: PrdLinkMissionInput): Promise<PrdDocument> {
+    const doc = await this.get(input.prdId);
+    return this.applyLinkage(doc, input);
+  }
+
+  /** Read back the mission linkages recorded on a PRD. */
+  async listMissionLinks(prdId?: string): Promise<PrdMissionLinkage[]> {
+    const doc = await this.get(prdId);
+    return doc.missions;
+  }
+
+  /**
+   * Mission planning path: create a PRD for a mission AND persist the
+   * mission↔PRD linkage in a single authority write.
+   */
+  async planForMission(input: PrdPlanForMissionInput): Promise<PrdDocument> {
+    const doc = await this.create({ name: input.name, template: input.template });
+    return this.applyLinkage(doc, {
+      prdId: doc.id,
+      missionId: input.missionId,
+      missionVersion: input.missionVersion,
+      requirementIds: input.requirementIds,
+    });
+  }
+
+  /**
+   * Render the PRD to a Markdown GENERATED VIEW.
+   *
+   * The output carries source identity (PRD id + version + generated-view
+   * label). It is written under `docs/prdy/<id>.md` and is NEVER read back:
+   * the authority store only loads `.yaml`/`.yml` files, and no code path in
+   * this package parses the exported Markdown.
+   */
+  async exportMarkdown(input?: PrdExportInput): Promise<PrdExportResult> {
+    const doc = await this.get(input?.id);
+    const content = renderMarkdown(doc);
+    const filePath = input?.outPath ?? path.join(prdDirectory(doc.projectPath), `${doc.id}.md`);
+
+    await fs.mkdir(path.dirname(filePath), { recursive: true });
+    await fs.writeFile(filePath, content, 'utf8');
+    return { filePath, content };
+  }
+
+  /**
+   * Import a YAML PRD document.
+   *
+   * Structural validation (zod) happens BEFORE anything is proposed or
+   * written. A structurally-valid import is persisted as `draft` — validity is
+   * NOT approval. If an existing PRD shares the id with divergent content, a
+   * typed {@link PrdImportConflictError} is thrown carrying a proposed
+   * successor; the original authority document is left byte-identical on disk.
+   */
+  async importDocument(input: PrdImportInput): Promise<PrdImportResult> {
+    const incoming = await this.readImportFile(input.filePath);
+
+    const existing = (await listPrds(this.projectPath)).find((doc) => doc.id === incoming.id);
+    if (existing === undefined) {
+      const document = this.buildImportedDocument(incoming);
+      await savePrd(document);
+      return { kind: 'created', document };
+    }
+
+    if (canonicalCore(existing) === canonicalCore(incoming)) {
+      return { kind: 'identical', document: existing };
+    }
+
+    throw new PrdImportConflictError(
+      `PRD id "${incoming.id}" already exists with divergent content — refusing to overwrite. ` +
+        `Proposed successor: version ${existing.version + 1} (draft). ` +
+        `Accept explicitly with acceptSuccessor().`,
+      existing,
+      this.buildSuccessor(existing, incoming),
+    );
+  }
+
+  /**
+   * Explicitly accept a conflicted import as a successor version of the
+   * existing PRD. Re-validates the source file before writing; the successor
+   * is persisted with status `draft` (acceptance of the import is not approval
+   * of the PRD) and the existing mission linkages are carried forward.
+   */
+  async acceptSuccessor(input: PrdImportInput): Promise<PrdDocument> {
+    const incoming = await this.readImportFile(input.filePath);
+
+    const existing = (await listPrds(this.projectPath)).find((doc) => doc.id === incoming.id);
+    if (existing === undefined) {
+      throw new PrdNotFoundError(
+        `No existing PRD with id "${incoming.id}" — use importDocument to create it`,
+      );
+    }
+
+    const successor = this.buildSuccessor(existing, incoming);
+    await savePrd(successor);
+    return successor;
+  }
+
+  // ── internals ──────────────────────────────────────────────────────────────
+
+  private async applyLinkage(doc: PrdDocument, input: PrdLinkMissionInput): Promise<PrdDocument> {
+    const entry: PrdMissionLinkage = {
+      missionId: input.missionId,
+      missionVersion: input.missionVersion,
+      prdVersion: doc.version,
+      requirementIds: input.requirementIds ?? [],
+      linkedAt: new Date().toISOString(),
+    };
+
+    // One entry per mission: refresh in place if the mission is already linked.
+    const index = doc.missions.findIndex((m) => m.missionId === entry.missionId);
+    if (index === -1) {
+      doc.missions.push(entry);
+    } else {
+      doc.missions[index] = entry;
+    }
+
+    // Linkage is mission-side metadata, not a content revision: bump the
+    // timestamp only so ids/versions stay stable for consumers.
+    doc.updatedAt = new Date().toISOString();
+    await savePrd(doc);
+    return doc;
+  }
+
+  private async readImportFile(filePath: string): Promise<PrdDocument> {
+    let raw: string;
+    try {
+      raw = await fs.readFile(filePath, 'utf8');
+    } catch (error) {
+      throw new PrdImportInvalidError(`Cannot read import file ${filePath}: ${String(error)}`);
+    }
+
+    let parsed: unknown;
+    try {
+      parsed = yaml.load(raw);
+    } catch (error) {
+      throw new PrdImportInvalidError(`Import file is not valid YAML: ${String(error)}`);
+    }
+
+    try {
+      return parsePrdDocument(parsed);
+    } catch (error) {
+      throw new PrdImportInvalidError(
+        `Import file failed PRD schema validation: ${filePath}`,
+        error instanceof Error ? error.message : String(error),
+      );
+    }
+  }
+
+  private buildImportedDocument(incoming: PrdDocument): PrdDocument {
+    const now = new Date().toISOString();
+    return {
+      ...incoming,
+      // The import lands in THIS project's authority store.
+      projectPath: this.projectPath,
+      // A structurally-valid import is not thereby approved.
+      status: 'draft',
+      version: 1,
+      missions: [],
+      createdAt: now,
+      updatedAt: now,
+    };
+  }
+
+  private buildSuccessor(existing: PrdDocument, incoming: PrdDocument): PrdDocument {
+    return {
+      ...incoming,
+      id: existing.id,
+      projectPath: existing.projectPath,
+      status: 'draft',
+      version: existing.version + 1,
+      missions: existing.missions,
+      createdAt: existing.createdAt,
+      updatedAt: new Date().toISOString(),
+    };
+  }
+}
+
+// ── Markdown rendering (generated view) ───────────────────────────────────────
+
+function canonicalCore(doc: PrdDocument): string {
+  return JSON.stringify([doc.title, doc.template, doc.sections]);
+}
+
+function renderMarkdown(doc: PrdDocument): string {
+  const lines: string[] = [
+    '<!--',
+    `${PRD_GENERATED_VIEW_LABEL}`,
+    `source-of-truth: docs/prdy/${doc.id}.yaml (YAML authority)`,
+    `prd-id: ${doc.id}`,
+    `prd-version: ${doc.version}`,
+    `generated-at: ${new Date().toISOString()}`,
+    '-->',
+    '',
+    `# ${doc.title}`,
+    '',
+    `**Status:** ${doc.status} · **Version:** ${doc.version} · **Template:** ${doc.template}`,
+    '',
+  ];
+
+  if (doc.missions.length > 0) {
+    lines.push('## Mission Linkage', '');
+    for (const mission of doc.missions) {
+      const requirements =
+        mission.requirementIds.length > 0 ? mission.requirementIds.join(', ') : 'none selected';
+      lines.push(
+        `- mission \`${mission.missionId}\` @ version \`${mission.missionVersion}\`` +
+          ` (linked at PRD v${mission.prdVersion}) — requirements: ${requirements}`,
+      );
+    }
+    lines.push('');
+  }
+
+  for (const section of doc.sections) {
+    lines.push(`## ${section.title}`, '');
+    for (const [field, value] of Object.entries(section.fields)) {
+      lines.push(`### ${field}`, '', value.trim().length > 0 ? value : '_Not set_.', '');
+    }
+  }
+
+  lines.push('---', '', `_End of generated view for ${doc.id} v${doc.version}._`, '');
+  return lines.join('\n');
+}

packages/prdy/src/types.ts

@@ -19,13 +19,31 @@ export interface PrdSection {
   fields: Record<string, string>;
 }
 
+/**
+ * Mission ↔ PRD linkage recorded on the PRD document (the YAML authority).
+ *
+ * `missionVersion` is the mission-side revision marker available to the CLI
+ * (the gateway exposes `updatedAt` for missions — there is no numeric mission
+ * version yet). `prdVersion` snapshots the PRD content version at link time.
+ */
+export interface PrdMissionLinkage {
+  missionId: string;
+  missionVersion: string;
+  prdVersion: number;
+  requirementIds: string[];
+  linkedAt: string;
+}
+
 export interface PrdDocument {
   id: string;
   title: string;
   status: PrdStatus;
   projectPath: string;
   template: string;
+  /** Content revision counter. Bumped by updates and accepted imports. */
+  version: number;
   sections: PrdSection[];
+  missions: PrdMissionLinkage[];
   createdAt: string;
   updatedAt: string;
 }
@@ -36,3 +54,60 @@ export interface CreatePrdOptions {
   template?: string;
   interactive?: boolean;
 }
+
+// ── PrdService surface (single authority entry point) ─────────────────────────
+
+export interface PrdServiceOptions {
+  projectPath: string;
+}
+
+export interface PrdCreateInput {
+  name: string;
+  template?: string;
+}
+
+export interface PrdSectionPatch {
+  id: string;
+  fields: Record<string, string>;
+}
+
+export interface PrdUpdateInput {
+  /** Defaults to the most recently updated PRD. */
+  id?: string;
+  sections: PrdSectionPatch[];
+}
+
+export interface PrdLinkMissionInput {
+  /** Defaults to the most recently updated PRD. */
+  prdId?: string;
+  missionId: string;
+  missionVersion: string;
+  requirementIds?: string[];
+}
+
+export interface PrdPlanForMissionInput extends PrdLinkMissionInput {
+  name: string;
+  template?: string;
+}
+
+export interface PrdExportInput {
+  /** Defaults to the most recently updated PRD. */
+  id?: string;
+  /** Override the generated-view output path. */
+  outPath?: string;
+}
+
+export interface PrdExportResult {
+  filePath: string;
+  content: string;
+}
+
+/** Discriminated result of a non-conflicting import. */
+export type PrdImportResult =
+  | { kind: 'created'; document: PrdDocument }
+  | { kind: 'identical'; document: PrdDocument };
+
+export interface PrdImportInput {
+  /** Path to a YAML-serialized PRD document (NOT the generated Markdown view). */
+  filePath: string;
+}

packages/prdy/src/wizard.ts

@@ -2,8 +2,8 @@ import path from 'node:path';
 
 import { cancel, intro, isCancel, outro, select, text } from '@clack/prompts';
 
-import { createPrd, savePrd } from './prd.js';
+import { PrdService } from './service.js';
-import type { CreatePrdOptions, PrdDocument } from './types.js';
+import type { CreatePrdOptions, PrdDocument, PrdSectionPatch } from './types.js';
 
 interface WizardAnswers {
   goals: string;
@@ -11,20 +11,41 @@ interface WizardAnswers {
   milestones: string;
 }
 
-function updateSectionField(doc: PrdDocument, sectionKeyword: string, value: string): void {
+/**
-  const section = doc.sections.find((candidate) => candidate.id.includes(sectionKeyword));
+ * Translate wizard answers into section patches using the same keyword
+ * matching the wizard always used (first section whose id contains the
+ * keyword, then first field whose name contains it, else first field).
+ */
+function buildWizardPatches(doc: PrdDocument, answers: WizardAnswers): PrdSectionPatch[] {
+  const bySection = new Map<string, PrdSectionPatch>();
 
-  if (section === undefined) {
+  const add = (keyword: string, value: string): void => {
-    return;
+    const section = doc.sections.find((candidate) => candidate.id.includes(keyword));
-  }
+    if (section === undefined) {
+      return;
+    }
 
-  const fieldName =
+    const fieldName =
-    Object.keys(section.fields).find((field) => field.toLowerCase().includes(sectionKeyword)) ??
+      Object.keys(section.fields).find((field) => field.toLowerCase().includes(keyword)) ??
-    Object.keys(section.fields)[0];
+      Object.keys(section.fields)[0];
 
-  if (fieldName !== undefined) {
+    if (fieldName === undefined || section.fields[fieldName] === value) {
-    section.fields[fieldName] = value;
+      return;
-  }
+    }
+
+    const existing = bySection.get(section.id);
+    if (existing === undefined) {
+      bySection.set(section.id, { id: section.id, fields: { [fieldName]: value } });
+    } else {
+      existing.fields[fieldName] = value;
+    }
+  };
+
+  add('goal', answers.goals);
+  add('constraint', answers.constraints);
+  add('milestone', answers.milestones);
+
+  return [...bySection.values()];
 }
 
 async function promptText(message: string, initialValue = ''): Promise<string> {
@@ -63,15 +84,10 @@ async function promptTemplate(template?: string): Promise<string> {
   return choice;
 }
 
-function applyWizardAnswers(doc: PrdDocument, answers: WizardAnswers): PrdDocument {
+/**
-  updateSectionField(doc, 'goal', answers.goals);
+ * Interactive PRD wizard. All writes go through PrdService — the wizard is a
-  updateSectionField(doc, 'constraint', answers.constraints);
+ * prompt layer, never a second writer path.
-  updateSectionField(doc, 'milestone', answers.milestones);
+ */
-
-  doc.updatedAt = new Date().toISOString();
-  return doc;
-}
-
 export async function runPrdWizard(options: CreatePrdOptions): Promise<PrdDocument> {
   intro('Mosaic PRD wizard');
 
@@ -82,20 +98,15 @@ export async function runPrdWizard(options: CreatePrdOptions): Promise<PrdDocume
   const constraints = await promptText('Key constraints');
   const milestones = await promptText('Planned milestones');
 
-  const doc = await createPrd({
+  const service = new PrdService({ projectPath: options.projectPath });
-    ...options,
+  const doc = await service.create({
     name,
     template,
-    interactive: true,
   });
 
-  const updated = applyWizardAnswers(doc, {
+  const patches = buildWizardPatches(doc, { goals, constraints, milestones });
-    goals,
+  const updated =
-    constraints,
+    patches.length > 0 ? await service.update({ id: doc.id, sections: patches }) : doc;
-    milestones,
-  });
-
-  await savePrd(updated);
 
   outro(`PRD created: ${path.join(updated.projectPath, 'docs', 'prdy', `${updated.id}.yaml`)}`);