diff --git a/app/api/turn/route.test.ts b/app/api/turn/route.test.ts
new file mode 100644
index 0000000..755ab44
--- /dev/null
+++ b/app/api/turn/route.test.ts
@@ -0,0 +1,70 @@
+// Handler-level tests for the manual tick route (Task 5b): POST /api/turn (no body) advances
+// exactly one turn — an empty queue is a FREE SKIP (200, turn +1, nothing else changes, the
+// route never rejects), and a mid-countdown ACTIVE task decrements by exactly one in both the
+// returned snapshot and the persisted row (no resolution, no mission hook — it never hits 0).
+import { mkdtemp, rm } from 'node:fs/promises'
+import { tmpdir } from 'node:os'
+import path from 'node:path'
+import { afterAll, describe, expect, it } from 'vitest'
+import { getGameDb, type GameDb } from '@/lib/db/client'
+import * as schema from '@/lib/db/schema'
+import { addCrew, buildWorldFixture, insertTaskRow } from '@/lib/game/test-fixtures'
+import { POST } from './route'
+
+const dirs: string[] = []
+// The route talks to the app-process singleton (getGameDb), so each test points it at a
+// fresh temp file and uses that very instance for fixtures and assertions — two in-memory
+// sql.js copies of one file would never see each other's writes.
+const g = globalThis as unknown as { __spaceWandererGameDb?: Promise<GameDb> }
+
+async function freshFile(): Promise<{ db: GameDb; file: string }> {
+  const dir = await mkdtemp(path.join(tmpdir(), 'sw-turn-route-test-'))
+  dirs.push(dir)
+  const file = path.join(dir, 'test.db')
+  process.env.SPACE_WANDERER_DB_PATH = file
+  delete g.__spaceWandererGameDb
+  return { db: await getGameDb(), file }
+}
+
+afterAll(async () => {
+  delete process.env.SPACE_WANDERER_DB_PATH
+  delete g.__spaceWandererGameDb
+  for (const d of dirs) await rm(d, { recursive: true, force: true })
+})
+
+// No request body — the handler never reads one.
+const post = () => POST()
+
+describe('POST /api/turn', () => {
+  it('empty queue → free skip: 200, turn +1, nothing else changes', async () => {
+    const { db } = await freshFile()
+    await buildWorldFixture(db)
+    const [before] = await db.db.select().from(schema.saves)
+
+    const res = await post()
+    expect(res.status).toBe(200)
+    const snap = await res.json()
+    expect(snap.turn).toBe(before.turn + 1)
+    expect(await db.db.select().from(schema.tasks)).toHaveLength(0) // the tick creates nothing
+    expect(snap.tasks).toEqual([]) // the composed queue array is empty
+  })
+
+  it('one ACTIVE task → 200, remaining decrements by exactly 1 in the returned snapshot', async () => {
+    const { db } = await freshFile()
+    const f = await buildWorldFixture(db)
+    const [c1] = await addCrew(db, 1)
+    const [before] = await db.db.select().from(schema.saves)
+    // Fixture task: one ACTIVE row, mid-countdown — a direct insert on the queue table
+    // (the 4b save.test pattern: it works with the queue rows directly); it never reaches 0
+    // this tick, so no resolver and no mission hook runs
+    await insertTaskRow(db, { type: 'JUMP', payload: { systemId: f.sys1, hops: 1 }, leadCrewId: c1, helpers: [], duration: 5, remaining: 3, status: 'ACTIVE', queuedAtTurn: before.turn })
+
+    const res = await post()
+    expect(res.status).toBe(200)
+    const snap = await res.json()
+    expect(snap.turn).toBe(before.turn + 1)
+    expect(snap.tasks).toHaveLength(1)
+    expect(snap.tasks[0].remaining).toBe(2) // 3 − 1: exactly one decrement
+    expect((await db.db.select().from(schema.tasks))[0].remaining).toBe(2) // the persisted row matches the snapshot
+  })
+})
diff --git a/app/api/turn/route.ts b/app/api/turn/route.ts
new file mode 100644
index 0000000..02b4fef
--- /dev/null
+++ b/app/api/turn/route.ts
@@ -0,0 +1,21 @@
+import { getGameDb } from '@/lib/db/client'
+import { getWorldState, runTurn } from '@/lib/game/save'
+
+// The manual tick (spec §3): advance exactly one turn — every ACTIVE task ages by one, tasks
+// that reach 0 resolve (the 4a resolvers + their mission hooks, 4b), and PENDING successors
+// auto-start in FIFO order. An empty queue is a FREE SKIP: turn +1 with nothing else changed —
+// the route never rejects a tick, so the UI's NEXT TURN button is never disabled. No request
+// body. Returns the same world snapshot GET /api/world returns, composed with the tick's
+// task array (the snapshot readers do not select the tasks table, so it comes from the TickResult).
+export async function POST() {
+  try {
+    const db = await getGameDb()
+    const tick = await runTurn(db)
+    return Response.json({ ...(await getWorldState(db)), tasks: tick.tasks })
+  } catch (err) {
+    // No domain errors escape runTurn (4b fails the task instead) — anything reaching here is
+    // a non-domain rethrow, i.e. a real bug: the error-mapping pattern, 500 when no status.
+    const e = err as { status?: number; message: string }
+    return Response.json({ error: e.message }, { status: e.status ?? 500 })
+  }
+}
diff --git a/lib/openapi.test.ts b/lib/openapi.test.ts
index d58128e..2fb4f2b 100644
--- a/lib/openapi.test.ts
+++ b/lib/openapi.test.ts
@@ -20,6 +20,7 @@ describe('OpenAPI spec', () => {
       '/api/ship/upgrade',
       '/api/system',
       '/api/tech',
+      '/api/turn',
       '/api/version',
       '/api/world',
     ]);
@@ -171,6 +172,25 @@ describe('OpenAPI spec', () => {
     expect((jumpReq.properties.helperCrewIds as { items: { type: string } }).items.type).toBe('integer');
   });
 
+  it('describes the turn endpoint: POST /api/turn with no body → WorldState + tasks, 500 on internal error', () => {
+    const post = openApiSpec.paths['/api/turn'].post;
+    expect(post).not.toHaveProperty('requestBody');
+    expect(post.responses[200].content['application/json'].schema.$ref).toBe('#/components/schemas/TurnResult');
+    expect(post.responses[500].content['application/json'].schema.$ref).toBe('#/components/schemas/Error');
+
+    // TurnResult extends WorldState with the task queue after the tick.
+    const turnResult = openApiSpec.components.schemas.TurnResult as { allOf: unknown[] };
+    expect((turnResult.allOf[0] as { $ref: string }).$ref).toBe('#/components/schemas/WorldState');
+    const extra = (turnResult.allOf[1] as { properties: Record<string, unknown> }).properties;
+    expect(Object.keys(extra)).toEqual(['tasks']);
+
+    const props = openApiSpec.components.schemas.Task.properties;
+    expect(Object.keys(props)).toEqual(['id', 'saveId', 'type', 'payload', 'leadCrewId', 'helpers', 'duration', 'remaining', 'status', 'queuedAtTurn']);
+    expect((props.type as { enum: string[] }).enum).toEqual(['JUMP', 'EXPLORE', 'SHIP_UPGRADE', 'SHIP_EXPAND', 'SHIP_BUILD', 'RESEARCH']);
+    expect((props.status as { enum: string[] }).enum).toEqual(['ACTIVE', 'PENDING', 'DONE']);
+    expect((props.helpers as { items: { type: string } }).items.type).toBe('integer');
+  });
+
   it('keeps info.version in sync with package.json', () => {
     expect(openApiSpec.info.title).toBe('SpaceWanderer API');
     expect(openApiSpec.info.version).toBe(pkg.version);
diff --git a/lib/openapi.ts b/lib/openapi.ts
index 8f1376d..64eaa8d 100644
--- a/lib/openapi.ts
+++ b/lib/openapi.ts
@@ -51,6 +51,32 @@ export const openApiSpec = {
         },
       },
     },
+    '/api/turn': {
+      post: {
+        summary: 'Advance exactly one turn (manual tick)',
+        description:
+          "Ticks the queue one turn — no request body: every ACTIVE task ages by one, tasks that reach 0 resolve (jump, scan, ship-work or research effects land, mission checks run for what resolved) and PENDING successors auto-start in FIFO order. An empty queue is a FREE SKIP — turn +1 with nothing else changed, always 200 (the UI's NEXT TURN button is never disabled; a task that fails to resolve simply does not land its effect). Returns the same world snapshot as GET /api/world, plus the task queue after the tick.",
+        operationId: 'nextTurn',
+        responses: {
+          200: {
+            description: 'World state after the tick, plus the task queue (every task row: ACTIVE, PENDING, DONE)',
+            content: {
+              'application/json': {
+                schema: { $ref: '#/components/schemas/TurnResult' },
+              },
+            },
+          },
+          500: {
+            description: 'Internal error — no domain failures escape the tick (a failed task is silently dropped, not an error)',
+            content: {
+              'application/json': {
+                schema: { $ref: '#/components/schemas/Error' },
+              },
+            },
+          },
+        },
+      },
+    },
     '/api/jump': {
       post: {
         summary: 'Queue a jump to a solar system within range',
@@ -832,6 +858,42 @@ export const openApiSpec = {
         },
         required: ['systems', 'edges', 'turn'],
       },
+      TurnResult: {
+        allOf: [
+          { $ref: '#/components/schemas/WorldState' },
+          {
+            type: 'object',
+            properties: {
+              tasks: {
+                type: 'array',
+                description:
+                  'Every task row after the tick — ACTIVE (decremented this turn, or auto-started this turn), PENDING (waiting for its team to free) and DONE (resolved this tick or earlier)',
+                items: { $ref: '#/components/schemas/Task' },
+              },
+            },
+            required: ['tasks'],
+          },
+        ],
+      },
+      Task: {
+        type: 'object',
+        properties: {
+          id: { type: 'integer' },
+          saveId: { type: 'integer' },
+          type: { type: 'string', enum: ['JUMP', 'EXPLORE', 'SHIP_UPGRADE', 'SHIP_EXPAND', 'SHIP_BUILD', 'RESEARCH'] },
+          payload: {
+            type: 'string',
+            description: 'JSON per type: JUMP {systemId, hops} · SHIP_UPGRADE/EXPAND {systemId} · SHIP_BUILD {type} · RESEARCH {techId} · EXPLORE {planetId}',
+          },
+          leadCrewId: { type: 'integer', description: 'crew.id of the team lead' },
+          helpers: { type: 'array', description: 'crew.id of the supporting crew (empty when none)', items: { type: 'integer' } },
+          duration: { type: 'integer' },
+          remaining: { type: 'integer' },
+          status: { type: 'string', enum: ['ACTIVE', 'PENDING', 'DONE'] },
+          queuedAtTurn: { type: 'integer' },
+        },
+        required: ['id', 'saveId', 'type', 'payload', 'leadCrewId', 'helpers', 'duration', 'remaining', 'status', 'queuedAtTurn'],
+      },
       JumpRequest: {
         type: 'object',
         properties: {