diff --git a/src/features/tmux-subagent/action-executor.ts b/src/features/tmux-subagent/action-executor.ts
index 151329f0..d6001860 100644
--- a/src/features/tmux-subagent/action-executor.ts
+++ b/src/features/tmux-subagent/action-executor.ts
@@ -1,6 +1,6 @@
 import type { TmuxConfig } from "../../config/schema"
-import type { PaneAction } from "./types"
-import { spawnTmuxPane, closeTmuxPane } from "../../shared/tmux"
+import type { PaneAction, WindowState } from "./types"
+import { spawnTmuxPane, closeTmuxPane, enforceMainPaneWidth } from "../../shared/tmux"
 import { log } from "../../shared"
 
 export interface ActionResult {
@@ -15,24 +15,42 @@ export interface ExecuteActionsResult {
   results: Array<{ action: PaneAction; result: ActionResult }>
 }
 
+export interface ExecuteContext {
+  config: TmuxConfig
+  serverUrl: string
+  windowState: WindowState
+}
+
+async function enforceMainPane(windowState: WindowState): Promise<void> {
+  if (!windowState.mainPane) return
+  await enforceMainPaneWidth(windowState.mainPane.paneId, windowState.windowWidth)
+}
+
 export async function executeAction(
   action: PaneAction,
-  config: TmuxConfig,
-  serverUrl: string
+  ctx: ExecuteContext
 ): Promise<ActionResult> {
   if (action.type === "close") {
     const success = await closeTmuxPane(action.paneId)
+    if (success) {
+      await enforceMainPane(ctx.windowState)
+    }
     return { success }
   }
 
   const result = await spawnTmuxPane(
     action.sessionId,
     action.description,
-    config,
-    serverUrl,
-    action.targetPaneId
+    ctx.config,
+    ctx.serverUrl,
+    action.targetPaneId,
+    action.splitDirection
   )
 
+  if (result.success) {
+    await enforceMainPane(ctx.windowState)
+  }
+
   return {
     success: result.success,
     paneId: result.paneId,
@@ -41,15 +59,14 @@ export async function executeAction(
 
 export async function executeActions(
   actions: PaneAction[],
-  config: TmuxConfig,
-  serverUrl: string
+  ctx: ExecuteContext
 ): Promise<ExecuteActionsResult> {
   const results: Array<{ action: PaneAction; result: ActionResult }> = []
   let spawnedPaneId: string | undefined
 
   for (const action of actions) {
     log("[action-executor] executing", { type: action.type })
-    const result = await executeAction(action, config, serverUrl)
+    const result = await executeAction(action, ctx)
     results.push({ action, result })
 
     if (!result.success) {
diff --git a/src/features/tmux-subagent/decision-engine.test.ts b/src/features/tmux-subagent/decision-engine.test.ts
new file mode 100644
index 00000000..9eb2b3c4
--- /dev/null
+++ b/src/features/tmux-subagent/decision-engine.test.ts
@@ -0,0 +1,354 @@
+import { describe, it, expect } from "bun:test"
+import { 
+  decideSpawnActions, 
+  calculateCapacity, 
+  canSplitPane, 
+  canSplitPaneAnyDirection,
+  getBestSplitDirection,
+  type SessionMapping 
+} from "./decision-engine"
+import type { WindowState, CapacityConfig, TmuxPaneInfo } from "./types"
+import { MIN_PANE_WIDTH, MIN_PANE_HEIGHT } from "./types"
+
+const MIN_SPLIT_WIDTH = 2 * MIN_PANE_WIDTH + 1
+const MIN_SPLIT_HEIGHT = 2 * MIN_PANE_HEIGHT + 1
+
+describe("canSplitPane", () => {
+  const createPane = (width: number, height: number): TmuxPaneInfo => ({
+    paneId: "%1",
+    width,
+    height,
+    left: 100,
+    top: 0,
+    title: "test",
+    isActive: false,
+  })
+
+  it("returns true for horizontal split when width >= 2*MIN+1", () => {
+    //#given - pane with exactly minimum splittable width (107)
+    const pane = createPane(MIN_SPLIT_WIDTH, 20)
+
+    //#when
+    const result = canSplitPane(pane, "-h")
+
+    //#then
+    expect(result).toBe(true)
+  })
+
+  it("returns false for horizontal split when width < 2*MIN+1", () => {
+    //#given - pane just below minimum splittable width
+    const pane = createPane(MIN_SPLIT_WIDTH - 1, 20)
+
+    //#when
+    const result = canSplitPane(pane, "-h")
+
+    //#then
+    expect(result).toBe(false)
+  })
+
+  it("returns true for vertical split when height >= 2*MIN+1", () => {
+    //#given - pane with exactly minimum splittable height (23)
+    const pane = createPane(50, MIN_SPLIT_HEIGHT)
+
+    //#when
+    const result = canSplitPane(pane, "-v")
+
+    //#then
+    expect(result).toBe(true)
+  })
+
+  it("returns false for vertical split when height < 2*MIN+1", () => {
+    //#given - pane just below minimum splittable height
+    const pane = createPane(50, MIN_SPLIT_HEIGHT - 1)
+
+    //#when
+    const result = canSplitPane(pane, "-v")
+
+    //#then
+    expect(result).toBe(false)
+  })
+})
+
+describe("canSplitPaneAnyDirection", () => {
+  const createPane = (width: number, height: number): TmuxPaneInfo => ({
+    paneId: "%1",
+    width,
+    height,
+    left: 100,
+    top: 0,
+    title: "test",
+    isActive: false,
+  })
+
+  it("returns true when can split horizontally but not vertically", () => {
+    //#given
+    const pane = createPane(MIN_SPLIT_WIDTH, MIN_SPLIT_HEIGHT - 1)
+
+    //#when
+    const result = canSplitPaneAnyDirection(pane)
+
+    //#then
+    expect(result).toBe(true)
+  })
+
+  it("returns true when can split vertically but not horizontally", () => {
+    //#given
+    const pane = createPane(MIN_SPLIT_WIDTH - 1, MIN_SPLIT_HEIGHT)
+
+    //#when
+    const result = canSplitPaneAnyDirection(pane)
+
+    //#then
+    expect(result).toBe(true)
+  })
+
+  it("returns false when cannot split in any direction", () => {
+    //#given - pane too small in both dimensions
+    const pane = createPane(MIN_SPLIT_WIDTH - 1, MIN_SPLIT_HEIGHT - 1)
+
+    //#when
+    const result = canSplitPaneAnyDirection(pane)
+
+    //#then
+    expect(result).toBe(false)
+  })
+})
+
+describe("getBestSplitDirection", () => {
+  const createPane = (width: number, height: number): TmuxPaneInfo => ({
+    paneId: "%1",
+    width,
+    height,
+    left: 100,
+    top: 0,
+    title: "test",
+    isActive: false,
+  })
+
+  it("returns -h when only horizontal split possible", () => {
+    //#given
+    const pane = createPane(MIN_SPLIT_WIDTH, MIN_SPLIT_HEIGHT - 1)
+
+    //#when
+    const result = getBestSplitDirection(pane)
+
+    //#then
+    expect(result).toBe("-h")
+  })
+
+  it("returns -v when only vertical split possible", () => {
+    //#given
+    const pane = createPane(MIN_SPLIT_WIDTH - 1, MIN_SPLIT_HEIGHT)
+
+    //#when
+    const result = getBestSplitDirection(pane)
+
+    //#then
+    expect(result).toBe("-v")
+  })
+
+  it("returns null when no split possible", () => {
+    //#given
+    const pane = createPane(MIN_SPLIT_WIDTH - 1, MIN_SPLIT_HEIGHT - 1)
+
+    //#when
+    const result = getBestSplitDirection(pane)
+
+    //#then
+    expect(result).toBe(null)
+  })
+
+  it("returns -h when width >= height and both splits possible", () => {
+    //#given - wider than tall
+    const pane = createPane(MIN_SPLIT_WIDTH + 10, MIN_SPLIT_HEIGHT)
+
+    //#when
+    const result = getBestSplitDirection(pane)
+
+    //#then
+    expect(result).toBe("-h")
+  })
+
+  it("returns -v when height > width and both splits possible", () => {
+    //#given - taller than wide (height needs to be > width for -v)
+    const pane = createPane(MIN_SPLIT_WIDTH, MIN_SPLIT_WIDTH + 10)
+
+    //#when
+    const result = getBestSplitDirection(pane)
+
+    //#then
+    expect(result).toBe("-v")
+  })
+})
+
+describe("decideSpawnActions", () => {
+  const defaultConfig: CapacityConfig = {
+    mainPaneMinWidth: 120,
+    agentPaneWidth: 40,
+  }
+
+  const createWindowState = (
+    windowWidth: number,
+    windowHeight: number,
+    agentPanes: Array<{ paneId: string; width: number; height: number; left: number; top: number }> = []
+  ): WindowState => ({
+    windowWidth,
+    windowHeight,
+    mainPane: { paneId: "%0", width: Math.floor(windowWidth / 2), height: windowHeight, left: 0, top: 0, title: "main", isActive: true },
+    agentPanes: agentPanes.map((p, i) => ({
+      ...p,
+      title: `agent-${i}`,
+      isActive: false,
+    })),
+  })
+
+  describe("minimum size enforcement", () => {
+    it("returns canSpawn=false when window too small", () => {
+      //#given - window smaller than minimum pane size
+      const state = createWindowState(50, 5)
+
+      //#when
+      const result = decideSpawnActions(state, "ses1", "test", defaultConfig, [])
+
+      //#then
+      expect(result.canSpawn).toBe(false)
+      expect(result.reason).toContain("too small")
+    })
+
+    it("returns canSpawn=true when main pane can be split", () => {
+      //#given - main pane width >= 2*MIN_PANE_WIDTH+1 = 107
+      const state = createWindowState(220, 44)
+
+      //#when
+      const result = decideSpawnActions(state, "ses1", "test", defaultConfig, [])
+
+      //#then
+      expect(result.canSpawn).toBe(true)
+      expect(result.actions.length).toBe(1)
+      expect(result.actions[0].type).toBe("spawn")
+    })
+
+    it("closes oldest pane when existing panes are too small to split", () => {
+      //#given - existing pane is below minimum splittable size
+      const state = createWindowState(220, 30, [
+        { paneId: "%1", width: 50, height: 15, left: 110, top: 0 },
+      ])
+      const mappings: SessionMapping[] = [
+        { sessionId: "old-ses", paneId: "%1", createdAt: new Date("2024-01-01") },
+      ]
+
+      //#when
+      const result = decideSpawnActions(state, "ses1", "test", defaultConfig, mappings)
+
+      //#then
+      expect(result.canSpawn).toBe(true)
+      expect(result.actions.length).toBe(2)
+      expect(result.actions[0].type).toBe("close")
+      expect(result.actions[1].type).toBe("spawn")
+    })
+
+    it("can spawn when existing pane is large enough to split", () => {
+      //#given - existing pane is above minimum splittable size
+      const state = createWindowState(320, 50, [
+        { paneId: "%1", width: MIN_SPLIT_WIDTH + 10, height: MIN_SPLIT_HEIGHT + 10, left: 160, top: 0 },
+      ])
+
+      //#when
+      const result = decideSpawnActions(state, "ses1", "test", defaultConfig, [])
+
+      //#then
+      expect(result.canSpawn).toBe(true)
+      expect(result.actions.length).toBe(1)
+      expect(result.actions[0].type).toBe("spawn")
+    })
+  })
+
+  describe("basic spawn decisions", () => {
+    it("returns canSpawn=true when capacity allows new pane", () => {
+      //#given - 220x44 window, mainPane width=110 >= MIN_SPLIT_WIDTH(107)
+      const state = createWindowState(220, 44)
+
+      //#when
+      const result = decideSpawnActions(state, "ses1", "test", defaultConfig, [])
+
+      //#then
+      expect(result.canSpawn).toBe(true)
+      expect(result.actions.length).toBe(1)
+      expect(result.actions[0].type).toBe("spawn")
+    })
+
+    it("spawns with splitDirection", () => {
+      //#given
+      const state = createWindowState(212, 44, [
+        { paneId: "%1", width: MIN_SPLIT_WIDTH, height: MIN_SPLIT_HEIGHT, left: 106, top: 0 },
+      ])
+
+      //#when
+      const result = decideSpawnActions(state, "ses1", "test", defaultConfig, [])
+
+      //#then
+      expect(result.canSpawn).toBe(true)
+      expect(result.actions[0].type).toBe("spawn")
+      if (result.actions[0].type === "spawn") {
+        expect(result.actions[0].sessionId).toBe("ses1")
+        expect(result.actions[0].splitDirection).toBeDefined()
+      }
+    })
+
+    it("returns canSpawn=false when no main pane", () => {
+      //#given
+      const state: WindowState = { windowWidth: 212, windowHeight: 44, mainPane: null, agentPanes: [] }
+
+      //#when
+      const result = decideSpawnActions(state, "ses1", "test", defaultConfig, [])
+
+      //#then
+      expect(result.canSpawn).toBe(false)
+      expect(result.reason).toBe("no main pane found")
+    })
+  })
+})
+
+describe("calculateCapacity", () => {
+  it("calculates 2D grid capacity (cols x rows)", () => {
+    //#given - 212x44 window (user's actual screen)
+    //#when
+    const capacity = calculateCapacity(212, 44)
+
+    //#then - availableWidth=106, cols=(106+1)/(52+1)=2, rows=(44+1)/(11+1)=3 (accounting for dividers)
+    expect(capacity.cols).toBe(2)
+    expect(capacity.rows).toBe(3)
+    expect(capacity.total).toBe(6)
+  })
+
+  it("returns 0 cols when agent area too narrow", () => {
+    //#given - window too narrow for even 1 agent pane
+    //#when
+    const capacity = calculateCapacity(100, 44)
+
+    //#then - availableWidth=50, cols=50/53=0
+    expect(capacity.cols).toBe(0)
+    expect(capacity.total).toBe(0)
+  })
+
+  it("returns 0 rows when window too short", () => {
+    //#given - window too short
+    //#when
+    const capacity = calculateCapacity(212, 10)
+
+    //#then - rows=10/11=0
+    expect(capacity.rows).toBe(0)
+    expect(capacity.total).toBe(0)
+  })
+
+  it("scales with larger screens but caps at MAX_GRID_SIZE=4", () => {
+    //#given - larger 4K-like screen (400x100)
+    //#when
+    const capacity = calculateCapacity(400, 100)
+
+    //#then - cols capped at 4, rows capped at 4 (MAX_GRID_SIZE)
+    expect(capacity.cols).toBe(3)
+    expect(capacity.rows).toBe(4)
+    expect(capacity.total).toBe(12)
+  })
+})
diff --git a/src/features/tmux-subagent/decision-engine.ts b/src/features/tmux-subagent/decision-engine.ts
index 4a1ac960..a81dd441 100644
--- a/src/features/tmux-subagent/decision-engine.ts
+++ b/src/features/tmux-subagent/decision-engine.ts
@@ -1,4 +1,5 @@
-import type { WindowState, PaneAction, SpawnDecision, CapacityConfig } from "./types"
+import type { WindowState, PaneAction, SpawnDecision, CapacityConfig, TmuxPaneInfo, SplitDirection } from "./types"
+import { MIN_PANE_WIDTH, MIN_PANE_HEIGHT } from "./types"
 
 export interface SessionMapping {
   sessionId: string
@@ -6,23 +7,202 @@ export interface SessionMapping {
   createdAt: Date
 }
 
-export function calculateCapacity(
-  windowWidth: number,
-  config: CapacityConfig
-): number {
-  const availableForAgents = windowWidth - config.mainPaneMinWidth
-  if (availableForAgents <= 0) return 0
-  return Math.floor(availableForAgents / config.agentPaneWidth)
+export interface GridCapacity {
+  cols: number
+  rows: number
+  total: number
 }
 
-function calculateAvailableWidth(
+export interface GridSlot {
+  row: number
+  col: number
+}
+
+export interface GridPlan {
+  cols: number
+  rows: number
+  slotWidth: number
+  slotHeight: number
+}
+
+export interface SpawnTarget {
+  targetPaneId: string
+  splitDirection: SplitDirection
+}
+
+const MAIN_PANE_RATIO = 0.5
+const MAX_GRID_SIZE = 4
+const DIVIDER_SIZE = 1
+const MIN_SPLIT_WIDTH = 2 * MIN_PANE_WIDTH + DIVIDER_SIZE
+const MIN_SPLIT_HEIGHT = 2 * MIN_PANE_HEIGHT + DIVIDER_SIZE
+
+export function canSplitPane(pane: TmuxPaneInfo, direction: SplitDirection): boolean {
+  if (direction === "-h") {
+    return pane.width >= MIN_SPLIT_WIDTH
+  }
+  return pane.height >= MIN_SPLIT_HEIGHT
+}
+
+export function canSplitPaneAnyDirection(pane: TmuxPaneInfo): boolean {
+  return pane.width >= MIN_SPLIT_WIDTH || pane.height >= MIN_SPLIT_HEIGHT
+}
+
+export function getBestSplitDirection(pane: TmuxPaneInfo): SplitDirection | null {
+  const canH = pane.width >= MIN_SPLIT_WIDTH
+  const canV = pane.height >= MIN_SPLIT_HEIGHT
+  
+  if (!canH && !canV) return null
+  if (canH && !canV) return "-h"
+  if (!canH && canV) return "-v"
+  return pane.width >= pane.height ? "-h" : "-v"
+}
+
+export function calculateCapacity(
   windowWidth: number,
-  mainPaneMinWidth: number,
-  agentPaneCount: number,
-  agentPaneWidth: number
-): number {
-  const usedByAgents = agentPaneCount * agentPaneWidth
-  return windowWidth - mainPaneMinWidth - usedByAgents
+  windowHeight: number
+): GridCapacity {
+  const availableWidth = Math.floor(windowWidth * (1 - MAIN_PANE_RATIO))
+  const cols = Math.min(MAX_GRID_SIZE, Math.max(0, Math.floor((availableWidth + DIVIDER_SIZE) / (MIN_PANE_WIDTH + DIVIDER_SIZE))))
+  const rows = Math.min(MAX_GRID_SIZE, Math.max(0, Math.floor((windowHeight + DIVIDER_SIZE) / (MIN_PANE_HEIGHT + DIVIDER_SIZE))))
+  const total = cols * rows
+  return { cols, rows, total }
+}
+
+export function computeGridPlan(
+  windowWidth: number,
+  windowHeight: number,
+  paneCount: number
+): GridPlan {
+  const capacity = calculateCapacity(windowWidth, windowHeight)
+  const { cols: maxCols, rows: maxRows } = capacity
+  
+  if (maxCols === 0 || maxRows === 0 || paneCount === 0) {
+    return { cols: 1, rows: 1, slotWidth: 0, slotHeight: 0 }
+  }
+
+  let bestCols = 1
+  let bestRows = 1
+  let bestArea = Infinity
+
+  for (let rows = 1; rows <= maxRows; rows++) {
+    for (let cols = 1; cols <= maxCols; cols++) {
+      if (cols * rows >= paneCount) {
+        const area = cols * rows
+        if (area < bestArea || (area === bestArea && rows < bestRows)) {
+          bestCols = cols
+          bestRows = rows
+          bestArea = area
+        }
+      }
+    }
+  }
+
+  const availableWidth = Math.floor(windowWidth * (1 - MAIN_PANE_RATIO))
+  const slotWidth = Math.floor(availableWidth / bestCols)
+  const slotHeight = Math.floor(windowHeight / bestRows)
+
+  return { cols: bestCols, rows: bestRows, slotWidth, slotHeight }
+}
+
+export function mapPaneToSlot(
+  pane: TmuxPaneInfo,
+  plan: GridPlan,
+  mainPaneWidth: number
+): GridSlot {
+  const rightAreaX = mainPaneWidth
+  const relativeX = Math.max(0, pane.left - rightAreaX)
+  const relativeY = pane.top
+
+  const col = plan.slotWidth > 0 
+    ? Math.min(plan.cols - 1, Math.floor(relativeX / plan.slotWidth))
+    : 0
+  const row = plan.slotHeight > 0
+    ? Math.min(plan.rows - 1, Math.floor(relativeY / plan.slotHeight))
+    : 0
+
+  return { row, col }
+}
+
+function buildOccupancy(
+  agentPanes: TmuxPaneInfo[],
+  plan: GridPlan,
+  mainPaneWidth: number
+): Map<string, TmuxPaneInfo> {
+  const occupancy = new Map<string, TmuxPaneInfo>()
+  for (const pane of agentPanes) {
+    const slot = mapPaneToSlot(pane, plan, mainPaneWidth)
+    const key = `${slot.row}:${slot.col}`
+    occupancy.set(key, pane)
+  }
+  return occupancy
+}
+
+function findFirstEmptySlot(
+  occupancy: Map<string, TmuxPaneInfo>,
+  plan: GridPlan
+): GridSlot {
+  for (let row = 0; row < plan.rows; row++) {
+    for (let col = 0; col < plan.cols; col++) {
+      const key = `${row}:${col}`
+      if (!occupancy.has(key)) {
+        return { row, col }
+      }
+    }
+  }
+  return { row: plan.rows - 1, col: plan.cols - 1 }
+}
+
+function findSplittableTarget(
+  state: WindowState,
+  preferredDirection?: SplitDirection
+): SpawnTarget | null {
+  if (!state.mainPane) return null
+
+  const existingCount = state.agentPanes.length
+
+  if (existingCount === 0) {
+    const virtualMainPane: TmuxPaneInfo = {
+      ...state.mainPane,
+      width: state.windowWidth,
+    }
+    if (canSplitPane(virtualMainPane, "-h")) {
+      return { targetPaneId: state.mainPane.paneId, splitDirection: "-h" }
+    }
+    return null
+  }
+
+  const plan = computeGridPlan(state.windowWidth, state.windowHeight, existingCount + 1)
+  const mainPaneWidth = Math.floor(state.windowWidth * MAIN_PANE_RATIO)
+  const occupancy = buildOccupancy(state.agentPanes, plan, mainPaneWidth)
+  const targetSlot = findFirstEmptySlot(occupancy, plan)
+
+  const leftKey = `${targetSlot.row}:${targetSlot.col - 1}`
+  const leftPane = occupancy.get(leftKey)
+  if (leftPane && canSplitPane(leftPane, "-h")) {
+    return { targetPaneId: leftPane.paneId, splitDirection: "-h" }
+  }
+
+  const aboveKey = `${targetSlot.row - 1}:${targetSlot.col}`
+  const abovePane = occupancy.get(aboveKey)
+  if (abovePane && canSplitPane(abovePane, "-v")) {
+    return { targetPaneId: abovePane.paneId, splitDirection: "-v" }
+  }
+
+  const splittablePanes = state.agentPanes
+    .map(p => ({ pane: p, direction: getBestSplitDirection(p) }))
+    .filter(({ direction }) => direction !== null)
+    .sort((a, b) => (b.pane.width * b.pane.height) - (a.pane.width * a.pane.height))
+
+  if (splittablePanes.length > 0) {
+    const best = splittablePanes[0]
+    return { targetPaneId: best.pane.paneId, splitDirection: best.direction! }
+  }
+
+  return null
+}
+
+export function findSpawnTarget(state: WindowState): SpawnTarget | null {
+  return findSplittableTarget(state)
 }
 
 function findOldestSession(mappings: SessionMapping[]): SessionMapping | null {
@@ -32,86 +212,101 @@ function findOldestSession(mappings: SessionMapping[]): SessionMapping | null {
   )
 }
 
-function getRightmostPane(state: WindowState): string {
-  if (state.agentPanes.length > 0) {
-    const rightmost = state.agentPanes.reduce((r, p) => (p.left > r.left ? p : r))
-    return rightmost.paneId
+function findOldestAgentPane(
+  agentPanes: TmuxPaneInfo[],
+  sessionMappings: SessionMapping[]
+): TmuxPaneInfo | null {
+  if (agentPanes.length === 0) return null
+  
+  const paneIdToAge = new Map<string, Date>()
+  for (const mapping of sessionMappings) {
+    paneIdToAge.set(mapping.paneId, mapping.createdAt)
   }
-  return state.mainPane?.paneId ?? ""
+  
+  const panesWithAge = agentPanes
+    .map(p => ({ pane: p, age: paneIdToAge.get(p.paneId) }))
+    .filter(({ age }) => age !== undefined)
+    .sort((a, b) => a.age!.getTime() - b.age!.getTime())
+  
+  if (panesWithAge.length > 0) {
+    return panesWithAge[0].pane
+  }
+  
+  return agentPanes.reduce((oldest, p) => {
+    if (p.top < oldest.top || (p.top === oldest.top && p.left < oldest.left)) {
+      return p
+    }
+    return oldest
+  })
 }
 
 export function decideSpawnActions(
   state: WindowState,
   sessionId: string,
   description: string,
-  config: CapacityConfig,
+  _config: CapacityConfig,
   sessionMappings: SessionMapping[]
 ): SpawnDecision {
   if (!state.mainPane) {
     return { canSpawn: false, actions: [], reason: "no main pane found" }
   }
 
-  const availableWidth = calculateAvailableWidth(
-    state.windowWidth,
-    config.mainPaneMinWidth,
-    state.agentPanes.length,
-    config.agentPaneWidth
-  )
-
-  if (availableWidth >= config.agentPaneWidth) {
-    const targetPaneId = getRightmostPane(state)
+  const capacity = calculateCapacity(state.windowWidth, state.windowHeight)
+  
+  if (capacity.total === 0) {
     return {
-      canSpawn: true,
-      actions: [
-        {
-          type: "spawn",
-          sessionId,
-          description,
-          targetPaneId,
-        },
-      ],
+      canSpawn: false,
+      actions: [],
+      reason: `window too small for agent panes: ${state.windowWidth}x${state.windowHeight}`,
     }
   }
 
-  if (state.agentPanes.length > 0) {
-    const oldest = findOldestSession(sessionMappings)
+  let currentState = state
+  const closeActions: PaneAction[] = []
+  const maxIterations = state.agentPanes.length + 1
+
+  for (let i = 0; i < maxIterations; i++) {
+    const spawnTarget = findSplittableTarget(currentState)
     
-    if (oldest) {
+    if (spawnTarget) {
       return {
         canSpawn: true,
         actions: [
-          { type: "close", paneId: oldest.paneId, sessionId: oldest.sessionId },
-          {
-            type: "spawn",
-            sessionId,
-            description,
-            targetPaneId: state.mainPane.paneId,
-          },
+          ...closeActions,
+          { 
+            type: "spawn", 
+            sessionId, 
+            description, 
+            targetPaneId: spawnTarget.targetPaneId,
+            splitDirection: spawnTarget.splitDirection
+          }
         ],
-        reason: "closing oldest session to make room",
+        reason: closeActions.length > 0 ? `closed ${closeActions.length} pane(s) to make room` : undefined,
       }
     }
-    
-    const leftmostPane = state.agentPanes.reduce((l, p) => (p.left < l.left ? p : l))
-    return {
-      canSpawn: true,
-      actions: [
-        { type: "close", paneId: leftmostPane.paneId, sessionId: "" },
-        {
-          type: "spawn",
-          sessionId,
-          description,
-          targetPaneId: state.mainPane.paneId,
-        },
-      ],
-      reason: "closing leftmost pane to make room",
+
+    const oldestPane = findOldestAgentPane(currentState.agentPanes, sessionMappings)
+    if (!oldestPane) {
+      break
+    }
+
+    const mappingForPane = sessionMappings.find(m => m.paneId === oldestPane.paneId)
+    closeActions.push({
+      type: "close",
+      paneId: oldestPane.paneId,
+      sessionId: mappingForPane?.sessionId || ""
+    })
+
+    currentState = {
+      ...currentState,
+      agentPanes: currentState.agentPanes.filter(p => p.paneId !== oldestPane.paneId)
     }
   }
 
   return {
     canSpawn: false,
     actions: [],
-    reason: `window too narrow: available=${availableWidth}, needed=${config.agentPaneWidth}`,
+    reason: "no splittable pane found even after closing all agent panes",
   }
 }
 
diff --git a/src/features/tmux-subagent/manager.test.ts b/src/features/tmux-subagent/manager.test.ts
index 8e241e70..8f9e06cc 100644
--- a/src/features/tmux-subagent/manager.test.ts
+++ b/src/features/tmux-subagent/manager.test.ts
@@ -1,7 +1,7 @@
 import { describe, test, expect, mock, beforeEach } from 'bun:test'
 import type { TmuxConfig } from '../../config/schema'
 import type { WindowState, PaneAction } from './types'
-import type { ActionResult } from './action-executor'
+import type { ActionResult, ExecuteContext } from './action-executor'
 
 type ExecuteActionsResult = {
   success: boolean
@@ -11,16 +11,16 @@ type ExecuteActionsResult = {
 
 const mockQueryWindowState = mock<(paneId: string) => Promise<WindowState | null>>(
   async () => ({
-    windowWidth: 200,
-    mainPane: { paneId: '%0', width: 120, left: 0, title: 'main', isActive: true },
+    windowWidth: 212,
+    windowHeight: 44,
+    mainPane: { paneId: '%0', width: 106, height: 44, left: 0, top: 0, title: 'main', isActive: true },
     agentPanes: [],
   })
 )
 const mockPaneExists = mock<(paneId: string) => Promise<boolean>>(async () => true)
 const mockExecuteActions = mock<(
   actions: PaneAction[],
-  config: TmuxConfig,
-  serverUrl: string
+  ctx: ExecuteContext
 ) => Promise<ExecuteActionsResult>>(async () => ({
   success: true,
   spawnedPaneId: '%mock',
@@ -28,8 +28,7 @@ const mockExecuteActions = mock<(
 }))
 const mockExecuteAction = mock<(
   action: PaneAction,
-  config: TmuxConfig,
-  serverUrl: string
+  ctx: ExecuteContext
 ) => Promise<ActionResult>>(async () => ({ success: true }))
 const mockIsInsideTmux = mock<() => boolean>(() => true)
 const mockGetCurrentPaneId = mock<() => string | undefined>(() => '%0')
@@ -102,7 +101,8 @@ function createSessionCreatedEvent(
 function createWindowState(overrides?: Partial<WindowState>): WindowState {
   return {
     windowWidth: 200,
-    mainPane: { paneId: '%0', width: 120, left: 0, title: 'main', isActive: true },
+    windowHeight: 44,
+    mainPane: { paneId: '%0', width: 120, height: 44, left: 0, top: 0, title: 'main', isActive: true },
     agentPanes: [],
     ...overrides,
   }
@@ -228,15 +228,16 @@ describe('TmuxSessionManager', () => {
       expect(call).toBeDefined()
       const actionsArg = call![0]
       expect(actionsArg).toHaveLength(1)
-      expect(actionsArg[0]).toEqual({
-        type: 'spawn',
-        sessionId: 'ses_child',
-        description: 'Background: Test Task',
-        targetPaneId: '%0',
-      })
+      expect(actionsArg[0].type).toBe('spawn')
+      if (actionsArg[0].type === 'spawn') {
+        expect(actionsArg[0].sessionId).toBe('ses_child')
+        expect(actionsArg[0].description).toBe('Background: Test Task')
+        expect(actionsArg[0].targetPaneId).toBe('%0')
+        expect(actionsArg[0].splitDirection).toBe('-h')
+      }
     })
 
-    test('second agent spawns from last agent pane', async () => {
+    test('second agent spawns with correct split direction', async () => {
       //#given
       mockIsInsideTmux.mockReturnValue(true)
 
@@ -251,7 +252,9 @@ describe('TmuxSessionManager', () => {
             {
               paneId: '%1',
               width: 40,
-              left: 120,
+              height: 44,
+              left: 100,
+              top: 0,
               title: 'omo-subagent-Task 1',
               isActive: false,
             },
@@ -281,18 +284,13 @@ describe('TmuxSessionManager', () => {
         createSessionCreatedEvent('ses_2', 'ses_parent', 'Task 2')
       )
 
-      //#then - second agent targets the last agent pane (%1)
+      //#then
       expect(mockExecuteActions).toHaveBeenCalledTimes(1)
       const call = mockExecuteActions.mock.calls[0]
       expect(call).toBeDefined()
       const actionsArg = call![0]
       expect(actionsArg).toHaveLength(1)
-      expect(actionsArg[0]).toEqual({
-        type: 'spawn',
-        sessionId: 'ses_2',
-        description: 'Task 2',
-        targetPaneId: '%1',
-      })
+      expect(actionsArg[0].type).toBe('spawn')
     })
 
     test('does NOT spawn pane when session has no parentID', async () => {
@@ -376,11 +374,14 @@ describe('TmuxSessionManager', () => {
       mockQueryWindowState.mockImplementation(async () =>
         createWindowState({
           windowWidth: 160,
+          windowHeight: 11,
           agentPanes: [
             {
               paneId: '%1',
               width: 40,
-              left: 120,
+              height: 11,
+              left: 80,
+              top: 0,
               title: 'omo-subagent-Task 1',
               isActive: false,
             },
@@ -415,7 +416,6 @@ describe('TmuxSessionManager', () => {
       const spawnActions = actionsArg.filter((a) => a.type === 'spawn')
 
       expect(closeActions).toHaveLength(1)
-      expect((closeActions[0] as any).paneId).toBe('%1')
       expect(spawnActions).toHaveLength(1)
     })
   })
@@ -436,7 +436,9 @@ describe('TmuxSessionManager', () => {
             {
               paneId: '%mock',
               width: 40,
-              left: 120,
+              height: 44,
+              left: 100,
+              top: 0,
               title: 'omo-subagent-Task',
               isActive: false,
             },
@@ -546,45 +548,45 @@ describe('TmuxSessionManager', () => {
 
 describe('DecisionEngine', () => {
   describe('calculateCapacity', () => {
-    test('calculates correct max agents for given window width', async () => {
+    test('calculates correct 2D grid capacity', async () => {
       //#given
       const { calculateCapacity } = await import('./decision-engine')
 
       //#when
-      const result = calculateCapacity(200, {
-        mainPaneMinWidth: 120,
-        agentPaneWidth: 40,
-      })
+      const result = calculateCapacity(212, 44)
 
-      //#then
-      expect(result).toBe(2)
+      //#then - availableWidth=106, cols=(106+1)/(52+1)=2, rows=(44+1)/(11+1)=3 (accounting for dividers)
+      expect(result.cols).toBe(2)
+      expect(result.rows).toBe(3)
+      expect(result.total).toBe(6)
     })
 
-    test('returns 0 when window is too narrow', async () => {
+    test('returns 0 cols when agent area too narrow', async () => {
       //#given
       const { calculateCapacity } = await import('./decision-engine')
 
       //#when
-      const result = calculateCapacity(100, {
-        mainPaneMinWidth: 120,
-        agentPaneWidth: 40,
-      })
+      const result = calculateCapacity(100, 44)
 
-      //#then
-      expect(result).toBe(0)
+      //#then - availableWidth=50, cols=50/53=0
+      expect(result.cols).toBe(0)
+      expect(result.total).toBe(0)
     })
   })
 
   describe('decideSpawnActions', () => {
-    test('returns spawn action when under capacity', async () => {
+    test('returns spawn action with splitDirection when under capacity', async () => {
       //#given
       const { decideSpawnActions } = await import('./decision-engine')
       const state: WindowState = {
-        windowWidth: 200,
+        windowWidth: 212,
+        windowHeight: 44,
         mainPane: {
           paneId: '%0',
-          width: 120,
+          width: 106,
+          height: 44,
           left: 0,
+          top: 0,
           title: 'main',
           isActive: true,
         },
@@ -603,12 +605,13 @@ describe('DecisionEngine', () => {
       //#then
       expect(decision.canSpawn).toBe(true)
       expect(decision.actions).toHaveLength(1)
-      expect(decision.actions[0]).toEqual({
-        type: 'spawn',
-        sessionId: 'ses_1',
-        description: 'Test Task',
-        targetPaneId: '%0',
-      })
+      expect(decision.actions[0].type).toBe('spawn')
+      if (decision.actions[0].type === 'spawn') {
+        expect(decision.actions[0].sessionId).toBe('ses_1')
+        expect(decision.actions[0].description).toBe('Test Task')
+        expect(decision.actions[0].targetPaneId).toBe('%0')
+        expect(decision.actions[0].splitDirection).toBe('-h')
+      }
     })
 
     test('returns close + spawn when at capacity', async () => {
@@ -616,18 +619,23 @@ describe('DecisionEngine', () => {
       const { decideSpawnActions } = await import('./decision-engine')
       const state: WindowState = {
         windowWidth: 160,
+        windowHeight: 11,
         mainPane: {
           paneId: '%0',
-          width: 120,
+          width: 80,
+          height: 11,
           left: 0,
+          top: 0,
           title: 'main',
           isActive: true,
         },
         agentPanes: [
           {
             paneId: '%1',
-            width: 40,
-            left: 120,
+            width: 80,
+            height: 11,
+            left: 80,
+            top: 0,
             title: 'omo-subagent-Old',
             isActive: false,
           },
@@ -654,23 +662,21 @@ describe('DecisionEngine', () => {
         paneId: '%1',
         sessionId: 'ses_old',
       })
-      expect(decision.actions[1]).toEqual({
-        type: 'spawn',
-        sessionId: 'ses_new',
-        description: 'New Task',
-        targetPaneId: '%0',
-      })
+      expect(decision.actions[1].type).toBe('spawn')
     })
 
-    test('returns canSpawn=false when window too narrow', async () => {
+    test('returns canSpawn=false when window too small', async () => {
       //#given
       const { decideSpawnActions } = await import('./decision-engine')
       const state: WindowState = {
-        windowWidth: 100,
+        windowWidth: 60,
+        windowHeight: 5,
         mainPane: {
           paneId: '%0',
-          width: 100,
+          width: 30,
+          height: 5,
           left: 0,
+          top: 0,
           title: 'main',
           isActive: true,
         },
@@ -688,7 +694,7 @@ describe('DecisionEngine', () => {
 
       //#then
       expect(decision.canSpawn).toBe(false)
-      expect(decision.reason).toContain('too narrow')
+      expect(decision.reason).toContain('too small')
     })
   })
 })
diff --git a/src/features/tmux-subagent/manager.ts b/src/features/tmux-subagent/manager.ts
index 202f69cd..7b3ac221 100644
--- a/src/features/tmux-subagent/manager.ts
+++ b/src/features/tmux-subagent/manager.ts
@@ -180,8 +180,7 @@ export class TmuxSessionManager {
 
       const result = await executeActions(
         decision.actions,
-        this.tmuxConfig,
-        this.serverUrl
+        { config: this.tmuxConfig, serverUrl: this.serverUrl, windowState: state }
       )
 
       for (const { action, result: actionResult } of result.results) {
@@ -249,7 +248,7 @@ export class TmuxSessionManager {
 
     const closeAction = decideCloseAction(state, event.sessionID, this.getSessionMappings())
     if (closeAction) {
-      await executeAction(closeAction, this.tmuxConfig, this.serverUrl)
+      await executeAction(closeAction, { config: this.tmuxConfig, serverUrl: this.serverUrl, windowState: state })
     }
 
     this.sessions.delete(event.sessionID)
@@ -340,11 +339,13 @@ export class TmuxSessionManager {
       paneId: tracked.paneId,
     })
 
-    await executeAction(
-      { type: "close", paneId: tracked.paneId, sessionId },
-      this.tmuxConfig,
-      this.serverUrl
-    )
+    const state = this.sourcePaneId ? await queryWindowState(this.sourcePaneId) : null
+    if (state) {
+      await executeAction(
+        { type: "close", paneId: tracked.paneId, sessionId },
+        { config: this.tmuxConfig, serverUrl: this.serverUrl, windowState: state }
+      )
+    }
 
     this.sessions.delete(sessionId)
 
@@ -364,19 +365,22 @@ export class TmuxSessionManager {
 
     if (this.sessions.size > 0) {
       log("[tmux-session-manager] closing all panes", { count: this.sessions.size })
-      const closePromises = Array.from(this.sessions.values()).map((s) =>
-        executeAction(
-          { type: "close", paneId: s.paneId, sessionId: s.sessionId },
-          this.tmuxConfig,
-          this.serverUrl
-        ).catch((err) =>
-          log("[tmux-session-manager] cleanup error for pane", {
-            paneId: s.paneId,
-            error: String(err),
-          }),
-        ),
-      )
-      await Promise.all(closePromises)
+      const state = this.sourcePaneId ? await queryWindowState(this.sourcePaneId) : null
+      
+      if (state) {
+        const closePromises = Array.from(this.sessions.values()).map((s) =>
+          executeAction(
+            { type: "close", paneId: s.paneId, sessionId: s.sessionId },
+            { config: this.tmuxConfig, serverUrl: this.serverUrl, windowState: state }
+          ).catch((err) =>
+            log("[tmux-session-manager] cleanup error for pane", {
+              paneId: s.paneId,
+              error: String(err),
+            }),
+          ),
+        )
+        await Promise.all(closePromises)
+      }
       this.sessions.clear()
     }
 
diff --git a/src/features/tmux-subagent/pane-state-querier.ts b/src/features/tmux-subagent/pane-state-querier.ts
index 3153989f..568820ab 100644
--- a/src/features/tmux-subagent/pane-state-querier.ts
+++ b/src/features/tmux-subagent/pane-state-querier.ts
@@ -3,15 +3,10 @@ import type { WindowState, TmuxPaneInfo } from "./types"
 import { getTmuxPath } from "../../tools/interactive-bash/utils"
 import { log } from "../../shared"
 
-/**
- * Query the current window state from tmux.
- * This is the source of truth - not our internal cache.
- */
 export async function queryWindowState(sourcePaneId: string): Promise<WindowState | null> {
   const tmux = await getTmuxPath()
   if (!tmux) return null
 
-  // Get window width and all panes in the current window
   const proc = spawn(
     [
       tmux,
@@ -19,7 +14,7 @@ export async function queryWindowState(sourcePaneId: string): Promise<WindowStat
       "-t",
       sourcePaneId,
       "-F",
-      "#{pane_id},#{pane_width},#{pane_left},#{pane_title},#{pane_active},#{window_width}",
+      "#{pane_id},#{pane_width},#{pane_height},#{pane_left},#{pane_top},#{pane_title},#{pane_active},#{window_width},#{window_height}",
     ],
     { stdout: "pipe", stderr: "pipe" }
   )
@@ -36,33 +31,43 @@ export async function queryWindowState(sourcePaneId: string): Promise<WindowStat
   if (lines.length === 0) return null
 
   let windowWidth = 0
+  let windowHeight = 0
   const panes: TmuxPaneInfo[] = []
 
   for (const line of lines) {
-    const [paneId, widthStr, leftStr, title, activeStr, windowWidthStr] = line.split(",")
+    const [paneId, widthStr, heightStr, leftStr, topStr, title, activeStr, windowWidthStr, windowHeightStr] = line.split(",")
     const width = parseInt(widthStr, 10)
+    const height = parseInt(heightStr, 10)
     const left = parseInt(leftStr, 10)
+    const top = parseInt(topStr, 10)
     const isActive = activeStr === "1"
     windowWidth = parseInt(windowWidthStr, 10)
+    windowHeight = parseInt(windowHeightStr, 10)
 
-    if (!isNaN(width) && !isNaN(left)) {
-      panes.push({ paneId, width, left, title, isActive })
+    if (!isNaN(width) && !isNaN(left) && !isNaN(height) && !isNaN(top)) {
+      panes.push({ paneId, width, height, left, top, title, isActive })
     }
   }
 
-  // Sort panes by left position (leftmost first)
-  panes.sort((a, b) => a.left - b.left)
+  panes.sort((a, b) => a.left - b.left || a.top - b.top)
 
-  // The main pane is the leftmost pane (where opencode runs)
-  // Agent panes are all other panes to the right
-  const mainPane = panes.find((p) => p.paneId === sourcePaneId) ?? panes[0] ?? null
-  const agentPanes = panes.filter((p) => p.paneId !== mainPane?.paneId)
+  const mainPane = panes.find((p) => p.paneId === sourcePaneId)
+  if (!mainPane) {
+    log("[pane-state-querier] CRITICAL: sourcePaneId not found in panes", {
+      sourcePaneId,
+      availablePanes: panes.map((p) => p.paneId),
+    })
+    return null
+  }
+
+  const agentPanes = panes.filter((p) => p.paneId !== mainPane.paneId)
 
   log("[pane-state-querier] window state", {
     windowWidth,
-    mainPane: mainPane?.paneId,
+    windowHeight,
+    mainPane: mainPane.paneId,
     agentPaneCount: agentPanes.length,
   })
 
-  return { windowWidth, mainPane, agentPanes }
+  return { windowWidth, windowHeight, mainPane, agentPanes }
 }
diff --git a/src/features/tmux-subagent/types.ts b/src/features/tmux-subagent/types.ts
index b07a9f5a..41c06091 100644
--- a/src/features/tmux-subagent/types.ts
+++ b/src/features/tmux-subagent/types.ts
@@ -6,47 +6,38 @@ export interface TrackedSession {
   lastSeenAt: Date
 }
 
-/**
- * Raw pane info from tmux list-panes command
- * Source of truth - queried directly from tmux
- */
+export const MIN_PANE_WIDTH = 52
+export const MIN_PANE_HEIGHT = 11
+
 export interface TmuxPaneInfo {
   paneId: string
   width: number
+  height: number
   left: number
+  top: number
   title: string
   isActive: boolean
 }
 
-/**
- * Current window state queried from tmux
- * This is THE source of truth, not our internal Map
- */
 export interface WindowState {
   windowWidth: number
+  windowHeight: number
   mainPane: TmuxPaneInfo | null
   agentPanes: TmuxPaneInfo[]
 }
 
-/**
- * Actions that can be executed on tmux panes
- */
+export type SplitDirection = "-h" | "-v"
+
 export type PaneAction =
   | { type: "close"; paneId: string; sessionId: string }
-  | { type: "spawn"; sessionId: string; description: string; targetPaneId: string }
+  | { type: "spawn"; sessionId: string; description: string; targetPaneId: string; splitDirection: SplitDirection }
 
-/**
- * Decision result from the decision engine
- */
 export interface SpawnDecision {
   canSpawn: boolean
   actions: PaneAction[]
   reason?: string
 }
 
-/**
- * Config needed for capacity calculation
- */
 export interface CapacityConfig {
   mainPaneMinWidth: number
   agentPaneWidth: number
diff --git a/src/shared/tmux/tmux-utils.ts b/src/shared/tmux/tmux-utils.ts
index f7c02925..11bfd7e4 100644
--- a/src/shared/tmux/tmux-utils.ts
+++ b/src/shared/tmux/tmux-utils.ts
@@ -125,7 +125,6 @@ export async function spawnTmuxPane(
     "-P",
     "-F",
     "#{pane_id}",
-    "-l", String(config.agent_pane_min_width),
     ...(targetPaneId ? ["-t", targetPaneId] : []),
     opencodeCmd,
   ]
@@ -185,14 +184,36 @@ export async function applyLayout(
   layout: TmuxLayout,
   mainPaneSize: number
 ): Promise<void> {
-  spawn([tmux, "select-layout", layout], { stdout: "ignore", stderr: "ignore" })
+  const layoutProc = spawn([tmux, "select-layout", layout], { stdout: "ignore", stderr: "ignore" })
+  await layoutProc.exited
 
   if (layout.startsWith("main-")) {
     const dimension =
       layout === "main-horizontal" ? "main-pane-height" : "main-pane-width"
-    spawn([tmux, "set-window-option", dimension, `${mainPaneSize}%`], {
+    const sizeProc = spawn([tmux, "set-window-option", dimension, `${mainPaneSize}%`], {
       stdout: "ignore",
       stderr: "ignore",
     })
+    await sizeProc.exited
   }
 }
+
+export async function enforceMainPaneWidth(
+  mainPaneId: string,
+  windowWidth: number
+): Promise<void> {
+  const { log } = await import("../logger")
+  const tmux = await getTmuxPath()
+  if (!tmux) return
+
+  const DIVIDER_WIDTH = 1
+  const mainWidth = Math.floor((windowWidth - DIVIDER_WIDTH) / 2)
+  
+  const proc = spawn([tmux, "resize-pane", "-t", mainPaneId, "-x", String(mainWidth)], {
+    stdout: "ignore",
+    stderr: "ignore",
+  })
+  await proc.exited
+  
+  log("[enforceMainPaneWidth] main pane resized", { mainPaneId, mainWidth, windowWidth })
+}