diff --git a/apps/api/src/lib/collaborators/grants.test.ts b/apps/api/src/lib/collaborators/grants.test.ts
new file mode 100644
index 000000000..f90455113
--- /dev/null
+++ b/apps/api/src/lib/collaborators/grants.test.ts
@@ -0,0 +1,77 @@
+/**
+ * Collaborator grant validation (resource sharing — RFC #190, phase 1).
+ *
+ * These lock the wire contract for sharing a folder/project: the permission
+ * vocabulary must stay identical to `doc_acl` (so nothing here can invent a
+ * permission the precedence function doesn't understand), and "permanent" must
+ * keep meaning NULL rather than an epoch date.
+ */
+import { describe, expect, it } from 'vitest';
+import { GRANTABLE_PERMISSIONS, grantSchema, patchSchema, toExpiryDate } from './grants.js';
+
+const USER = '11111111-2222-3333-4444-555555555555';
+
+describe('GRANTABLE_PERMISSIONS', () => {
+  it('matches the doc_acl vocabulary exactly', () => {
+    expect([...GRANTABLE_PERMISSIONS]).toEqual(['read', 'write', 'admin', 'none']);
+  });
+
+  it('keeps the explicit deny grantable — it is not the same as revoking', () => {
+    expect(grantSchema.safeParse({ user_id: USER, permission: 'none' }).success).toBe(true);
+  });
+});
+
+describe('grantSchema', () => {
+  it('accepts a permanent grant (no expiry)', () => {
+    const r = grantSchema.safeParse({ user_id: USER, permission: 'write' });
+    expect(r.success).toBe(true);
+  });
+
+  it('accepts an expiring grant', () => {
+    const r = grantSchema.safeParse({
+      user_id: USER, permission: 'read', expires_at: '2030-01-01T00:00:00.000Z',
+    });
+    expect(r.success).toBe(true);
+  });
+
+  it('rejects a permission outside the doc_acl vocabulary', () => {
+    // 'comment' is RFC phase 6 — it must not silently pass before the
+    // precedence function understands it.
+    expect(grantSchema.safeParse({ user_id: USER, permission: 'comment' }).success).toBe(false);
+    expect(grantSchema.safeParse({ user_id: USER, permission: 'owner' }).success).toBe(false);
+  });
+
+  it('rejects a non-uuid principal', () => {
+    expect(grantSchema.safeParse({ user_id: 'someone@example.com', permission: 'read' }).success).toBe(false);
+  });
+
+  it('rejects a non-ISO expiry', () => {
+    expect(grantSchema.safeParse({ user_id: USER, permission: 'read', expires_at: 'tomorrow' }).success).toBe(false);
+  });
+});
+
+describe('patchSchema', () => {
+  it('allows changing just the permission', () => {
+    expect(patchSchema.safeParse({ permission: 'admin' }).success).toBe(true);
+  });
+
+  it('allows clearing the expiry (null = permanent)', () => {
+    expect(patchSchema.safeParse({ expires_at: null }).success).toBe(true);
+  });
+
+  it('rejects an empty patch', () => {
+    expect(patchSchema.safeParse({}).success).toBe(false);
+  });
+});
+
+describe('toExpiryDate', () => {
+  it('treats null/undefined as permanent', () => {
+    expect(toExpiryDate(null)).toBeNull();
+    expect(toExpiryDate(undefined)).toBeNull();
+  });
+
+  it('parses an ISO string to the same instant', () => {
+    const iso = '2030-06-01T12:30:00.000Z';
+    expect(toExpiryDate(iso)?.toISOString()).toBe(iso);
+  });
+});
diff --git a/apps/api/src/lib/collaborators/grants.ts b/apps/api/src/lib/collaborators/grants.ts
new file mode 100644
index 000000000..4e870ce08
--- /dev/null
+++ b/apps/api/src/lib/collaborators/grants.ts
@@ -0,0 +1,48 @@
+/**
+ * Collaborator grant validation (resource sharing — RFC #190, phase 1).
+ *
+ * Kept in lib/ rather than the route so it is importable (and unit-testable)
+ * without pulling in the db/config graph — same split as lib/community/handle.ts.
+ *
+ * These mirror `doc_acl` exactly: the permission vocabulary is the storage
+ * vocabulary, so nothing here re-interprets what a grant means. Precedence
+ * (deny-first, best-positive) stays entirely in app_effective_permission().
+ */
+import { z } from 'zod';
+
+/**
+ * Storage-level permissions, unchanged from `doc_acl`.
+ *
+ * `none` is an explicit DENY that beats every positive grant, which is why it is
+ * grantable here and not filtered out — revoking a row and denying are different
+ * intents (a deny also overrides an inherited folder/project grant).
+ *
+ * The RFC's `comment` tier is phase 6; adding it here + to the precedence
+ * function is the only change needed, so this list is the single source of truth.
+ */
+export const GRANTABLE_PERMISSIONS = ['read', 'write', 'admin', 'none'] as const;
+
+export type GrantablePermission = (typeof GRANTABLE_PERMISSIONS)[number];
+
+/** Body for POST …/collaborators — create or re-grant for one user. */
+export const grantSchema = z.object({
+  user_id: z.string().uuid(),
+  permission: z.enum(GRANTABLE_PERMISSIONS),
+  // null / omitted = permanent, matching doc_acl.expires_at semantics.
+  expires_at: z.string().datetime().nullable().optional(),
+});
+
+/** Body for PATCH …/collaborators/:userId — partial update of an existing grant. */
+export const patchSchema = z
+  .object({
+    permission: z.enum(GRANTABLE_PERMISSIONS).optional(),
+    expires_at: z.string().datetime().nullable().optional(),
+  })
+  .refine((v) => v.permission !== undefined || v.expires_at !== undefined, {
+    message: 'Provide at least one of permission or expires_at',
+  });
+
+/** `expires_at` string → Date, treating null/undefined as "permanent". */
+export function toExpiryDate(value: string | null | undefined): Date | null {
+  return value ? new Date(value) : null;
+}
diff --git a/apps/api/src/routes/collaborators.ts b/apps/api/src/routes/collaborators.ts
new file mode 100644
index 000000000..7f0ac73d0
--- /dev/null
+++ b/apps/api/src/routes/collaborators.ts
@@ -0,0 +1,233 @@
+/**
+ * Resource collaborators — per-folder / per-project sharing (RFC #190, phase 1).
+ *
+ *   GET    /api/{folders|projects}/:id/collaborators          list explicit grants
+ *   POST   /api/{folders|projects}/:id/collaborators          grant / re-grant
+ *   PATCH  /api/{folders|projects}/:id/collaborators/:userId  change permission / expiry
+ *   DELETE /api/{folders|projects}/:id/collaborators/:userId  revoke
+ *
+ * Purely additive: it writes the SAME `doc_acl` rows the org access-matrix already
+ * writes (routes/org.ts), for resource types `doc_acl` has always modelled. No
+ * schema change, no change to how permissions resolve — `app_effective_permission`
+ * + `canAccess` (lib/iam.ts) already handle precedence and the folder/project →
+ * doc inheritance, so a grant here automatically covers everything inside.
+ *
+ * Scope of phase 1 (deliberate): the grantee must ALREADY be a member of the
+ * workspace. Inviting an outside email (which needs a pending grant that
+ * materialises on signup, plus the guest-visibility work) is phase 2/3 of the RFC.
+ */
+import { and, eq } from 'drizzle-orm';
+import type { FastifyPluginAsync, FastifyReply, FastifyRequest } from 'fastify';
+import { db } from '../db/index.js';
+import { docAcl, folders, iamAuditLog, projects, users, workspaceMembers } from '../db/schema.js';
+import { withTenant } from '../db/with-tenant.js';
+import { canAccess } from '../lib/iam.js';
+import { grantSchema, patchSchema, toExpiryDate } from '../lib/collaborators/grants.js';
+
+type ResourceType = 'folder' | 'project';
+
+const UUID_RE = /^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/i;
+
+async function audit(
+  workspaceId: string,
+  actorUserId: string,
+  action: string,
+  resourceType: string,
+  resourceId: string,
+  payload: unknown,
+): Promise<void> {
+  await db.insert(iamAuditLog).values({
+    workspaceId, actorUserId, action, resourceType, resourceId, payload: payload as object,
+  });
+}
+
+/** Resource exists in the caller's workspace? Read through RLS so a caller can't
+ *  probe for ids in other tenants — an unknown id and a foreign id both 404. */
+async function resourceExists(
+  tenantId: string,
+  resourceType: ResourceType,
+  resourceId: string,
+): Promise<boolean> {
+  return await withTenant(tenantId, async (tx) => {
+    if (resourceType === 'folder') {
+      const [row] = await tx.select({ id: folders.id }).from(folders)
+        .where(eq(folders.id, resourceId)).limit(1);
+      return Boolean(row);
+    }
+    const [row] = await tx.select({ id: projects.id }).from(projects)
+      .where(eq(projects.id, resourceId)).limit(1);
+    return Boolean(row);
+  });
+}
+
+/**
+ * Shared guard: valid id → resource visible in this tenant → caller holds `admin`
+ * on it. `canAccess` falls through to the workspace role, so workspace owners and
+ * admins manage sharing without needing an explicit grant on every resource.
+ * Returns the tenant/actor on success, or null after replying with the error.
+ */
+async function requireResourceAdmin(
+  req: FastifyRequest,
+  reply: FastifyReply,
+  resourceType: ResourceType,
+  resourceId: string,
+): Promise<{ tenantId: string; actorId: string } | null> {
+  if (!req.auth) { await reply.code(401).send({ error: 'unauthorized' }); return null; }
+  if (!UUID_RE.test(resourceId)) { await reply.code(400).send({ error: 'bad_id' }); return null; }
+
+  const { tenant_id: tenantId, sub: actorId } = req.auth;
+  if (!(await resourceExists(tenantId, resourceType, resourceId))) {
+    await reply.code(404).send({ error: `${resourceType}_not_found` });
+    return null;
+  }
+  if (!(await canAccess(db, actorId, tenantId, resourceType, resourceId, 'admin'))) {
+    await reply.code(403).send({ error: 'requires_admin_on_resource' });
+    return null;
+  }
+  return { tenantId, actorId };
+}
+
+/** Explicit user grants on one resource, joined to the person they belong to. */
+async function listGrants(tenantId: string, resourceType: ResourceType, resourceId: string) {
+  return await db
+    .select({
+      user_id: docAcl.principalId,
+      permission: docAcl.permission,
+      expires_at: docAcl.expiresAt,
+      created_at: docAcl.createdAt,
+      updated_at: docAcl.updatedAt,
+      email: users.email,
+      display_name: users.displayName,
+    })
+    .from(docAcl)
+    .leftJoin(users, eq(users.id, docAcl.principalId))
+    .where(and(
+      eq(docAcl.workspaceId, tenantId),
+      eq(docAcl.resourceType, resourceType),
+      eq(docAcl.resourceId, resourceId),
+      eq(docAcl.principalType, 'user'),
+    ));
+}
+
+function registerFor(app: Parameters<FastifyPluginAsync>[0], resourceType: ResourceType): void {
+  const base = resourceType === 'folder' ? '/api/folders' : '/api/projects';
+
+  // ── List collaborators ──────────────────────────────────────────────────────
+  app.get<{ Params: { id: string } }>(`${base}/:id/collaborators`, async (req, reply) => {
+    const ctx = await requireResourceAdmin(req, reply, resourceType, req.params.id);
+    if (!ctx) return;
+    return reply.send({ collaborators: await listGrants(ctx.tenantId, resourceType, req.params.id) });
+  });
+
+  // ── Grant / re-grant ────────────────────────────────────────────────────────
+  app.post<{ Params: { id: string } }>(`${base}/:id/collaborators`, async (req, reply) => {
+    const ctx = await requireResourceAdmin(req, reply, resourceType, req.params.id);
+    if (!ctx) return;
+
+    const parsed = grantSchema.safeParse(req.body);
+    if (!parsed.success) {
+      return reply.code(400).send({ error: 'validation', issues: parsed.error.issues });
+    }
+    const { user_id: userId, permission, expires_at: expiresAt } = parsed.data;
+
+    // Phase 1 shares with existing members only — an outside email needs the
+    // pending-grant + guest flow (RFC #190 phases 2/3).
+    const [member] = await db
+      .select({ userId: workspaceMembers.userId })
+      .from(workspaceMembers)
+      .where(and(
+        eq(workspaceMembers.workspaceId, ctx.tenantId),
+        eq(workspaceMembers.userId, userId),
+      ))
+      .limit(1);
+    if (!member) {
+      return reply.code(400).send({
+        error: 'not_a_workspace_member',
+        message: 'Invite this person to the workspace first — sharing with an outside email is not available yet.',
+      });
+    }
+
+    await db.insert(docAcl).values({
+      workspaceId: ctx.tenantId,
+      resourceType,
+      resourceId: req.params.id,
+      principalType: 'user',
+      principalId: userId,
+      permission,
+      createdBy: ctx.actorId,
+      expiresAt: toExpiryDate(expiresAt),
+    }).onConflictDoUpdate({
+      target: [docAcl.resourceType, docAcl.resourceId, docAcl.principalType, docAcl.principalId],
+      set: { permission, expiresAt: toExpiryDate(expiresAt), updatedAt: new Date() },
+    });
+
+    await audit(ctx.tenantId, ctx.actorId, 'collaborator.granted', resourceType, req.params.id,
+      { principalType: 'user', principalId: userId, permission, expiresAt: expiresAt ?? null });
+
+    return reply.send({ collaborators: await listGrants(ctx.tenantId, resourceType, req.params.id) });
+  });
+
+  // ── Change permission / expiry ──────────────────────────────────────────────
+  app.patch<{ Params: { id: string; userId: string } }>(
+    `${base}/:id/collaborators/:userId`,
+    async (req, reply) => {
+      const ctx = await requireResourceAdmin(req, reply, resourceType, req.params.id);
+      if (!ctx) return;
+      if (!UUID_RE.test(req.params.userId)) return reply.code(400).send({ error: 'bad_user_id' });
+
+      const parsed = patchSchema.safeParse(req.body);
+      if (!parsed.success) {
+        return reply.code(400).send({ error: 'validation', issues: parsed.error.issues });
+      }
+
+      const set: Record<string, unknown> = { updatedAt: new Date() };
+      if (parsed.data.permission !== undefined) set.permission = parsed.data.permission;
+      if (parsed.data.expires_at !== undefined) {
+        set.expiresAt = toExpiryDate(parsed.data.expires_at);
+      }
+
+      const updated = await db.update(docAcl).set(set).where(and(
+        eq(docAcl.workspaceId, ctx.tenantId),
+        eq(docAcl.resourceType, resourceType),
+        eq(docAcl.resourceId, req.params.id),
+        eq(docAcl.principalType, 'user'),
+        eq(docAcl.principalId, req.params.userId),
+      )).returning({ id: docAcl.id });
+      if (updated.length === 0) return reply.code(404).send({ error: 'grant_not_found' });
+
+      await audit(ctx.tenantId, ctx.actorId, 'collaborator.updated', resourceType, req.params.id,
+        { principalType: 'user', principalId: req.params.userId, ...parsed.data });
+
+      return reply.send({ collaborators: await listGrants(ctx.tenantId, resourceType, req.params.id) });
+    },
+  );
+
+  // ── Revoke ──────────────────────────────────────────────────────────────────
+  app.delete<{ Params: { id: string; userId: string } }>(
+    `${base}/:id/collaborators/:userId`,
+    async (req, reply) => {
+      const ctx = await requireResourceAdmin(req, reply, resourceType, req.params.id);
+      if (!ctx) return;
+      if (!UUID_RE.test(req.params.userId)) return reply.code(400).send({ error: 'bad_user_id' });
+
+      const removed = await db.delete(docAcl).where(and(
+        eq(docAcl.workspaceId, ctx.tenantId),
+        eq(docAcl.resourceType, resourceType),
+        eq(docAcl.resourceId, req.params.id),
+        eq(docAcl.principalType, 'user'),
+        eq(docAcl.principalId, req.params.userId),
+      )).returning({ id: docAcl.id });
+      if (removed.length === 0) return reply.code(404).send({ error: 'grant_not_found' });
+
+      await audit(ctx.tenantId, ctx.actorId, 'collaborator.revoked', resourceType, req.params.id,
+        { principalType: 'user', principalId: req.params.userId });
+
+      return reply.send({ removed: true });
+    },
+  );
+}
+
+export const collaboratorsRoutes: FastifyPluginAsync = async (app) => {
+  registerFor(app, 'folder');
+  registerFor(app, 'project');
+};
diff --git a/apps/api/src/server.ts b/apps/api/src/server.ts
index 29fcb8147..b7a0ecdb0 100644
--- a/apps/api/src/server.ts
+++ b/apps/api/src/server.ts
@@ -24,6 +24,7 @@ import { docsRoutes } from './routes/docs.js';
 import { decisionApprovalsRoutes } from './routes/decision-approvals.js';
 import { searchRoutes } from './routes/search.js';
 import { foldersRoutes } from './routes/folders.js';
+import { collaboratorsRoutes } from './routes/collaborators.js';
 import { mcpTokenRoutes } from './routes/mcp-tokens.js';
 import { flowsRoutes } from './routes/flows.js';
 import { flowPresenceRoutes } from './routes/flows-presence.js';
@@ -181,6 +182,7 @@ await app.register(openApiRoutes);
 await app.register(geminiRoutes);
 await app.register(installRoutes);
   await app.register(projectsRoutes);
+await app.register(collaboratorsRoutes);
 await app.register(documentFilesRoutes);
 await app.register(onlyofficeRoutes);
 // Phase 3 — mount the enterprise (ee) modules if present (dynamic import, so