feat(tmux-subagent): add replace action to prevent mass eviction

- Add column-based splittable calculation (getColumnCount, getColumnWidth)
- New decision tree: splittable → split, k=1 eviction → close+spawn, else → replace
- Add 'replace' action type using tmux respawn-pane (preserves layout)
- Replace oldest pane in-place instead of closing all panes when unsplittable
- Prevents scenario where all agent panes get closed leaving only 1

src/agents/utils.ts

@@ -10,7 +10,7 @@ import { createMetisAgent } from "./metis"
 import { createAtlasAgent } from "./atlas"
 import { createMomusAgent } from "./momus"
 import type { AvailableAgent, AvailableCategory, AvailableSkill } from "./dynamic-agent-prompt-builder"
-import { deepMerge, fetchAvailableModels, resolveModelWithFallback, AGENT_MODEL_REQUIREMENTS, findCaseInsensitive, includesCaseInsensitive } from "../shared"
+import { deepMerge, fetchAvailableModels, resolveModelWithFallback, AGENT_MODEL_REQUIREMENTS, findCaseInsensitive, includesCaseInsensitive, readConnectedProvidersCache } from "../shared"
 import { DEFAULT_CATEGORIES, CATEGORY_DESCRIPTIONS } from "../tools/delegate-task/constants"
 import { resolveMultipleSkills } from "../features/opencode-skill-loader/skill-content"
 import { createBuiltinSkills } from "../features/builtin-skills"
@@ -155,8 +155,10 @@ export async function createBuiltinAgents(
     throw new Error("createBuiltinAgents requires systemDefaultModel")
   }
 
-  // Fetch available models at plugin init
-  const availableModels = client ? await fetchAvailableModels(client) : new Set<string>()
+  const connectedProviders = readConnectedProvidersCache()
+  const availableModels = client 
+    ? await fetchAvailableModels(client, { connectedProviders: connectedProviders ?? undefined }) 
+    : new Set<string>()
 
   const result: Record<string, AgentConfig> = {}
   const availableAgents: AvailableAgent[] = []

src/features/tmux-subagent/action-executor.ts

@@ -1,6 +1,6 @@
 import type { TmuxConfig } from "../../config/schema"
 import type { PaneAction, WindowState } from "./types"
-import { spawnTmuxPane, closeTmuxPane, enforceMainPaneWidth } from "../../shared/tmux"
+import { spawnTmuxPane, closeTmuxPane, enforceMainPaneWidth, replaceTmuxPane } from "../../shared/tmux"
 import { log } from "../../shared"
 
 export interface ActionResult {
@@ -38,6 +38,20 @@ export async function executeAction(
     return { success }
   }
 
+  if (action.type === "replace") {
+    const result = await replaceTmuxPane(
+      action.paneId,
+      action.newSessionId,
+      action.description,
+      ctx.config,
+      ctx.serverUrl
+    )
+    return {
+      success: result.success,
+      paneId: result.paneId,
+    }
+  }
+
   const result = await spawnTmuxPane(
     action.sessionId,
     action.description,
@@ -74,7 +88,7 @@ export async function executeActions(
       return { success: false, results }
     }
 
-    if (action.type === "spawn" && result.paneId) {
+    if ((action.type === "spawn" || action.type === "replace") && result.paneId) {
       spawnedPaneId = result.paneId
     }
   }

src/features/tmux-subagent/decision-engine.ts

@@ -31,11 +31,38 @@ export interface SpawnTarget {
 }
 
 const MAIN_PANE_RATIO = 0.5
+const MAX_COLS = 2
+const MAX_ROWS = 3
 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 getColumnCount(paneCount: number): number {
+  if (paneCount <= 0) return 1
+  return Math.min(MAX_COLS, Math.max(1, Math.ceil(paneCount / MAX_ROWS)))
+}
+
+export function getColumnWidth(agentAreaWidth: number, paneCount: number): number {
+  const cols = getColumnCount(paneCount)
+  const dividersWidth = (cols - 1) * DIVIDER_SIZE
+  return Math.floor((agentAreaWidth - dividersWidth) / cols)
+}
+
+export function isSplittableAtCount(agentAreaWidth: number, paneCount: number): boolean {
+  const columnWidth = getColumnWidth(agentAreaWidth, paneCount)
+  return columnWidth >= MIN_SPLIT_WIDTH
+}
+
+export function findMinimalEvictions(agentAreaWidth: number, currentCount: number): number | null {
+  for (let k = 1; k <= currentCount; k++) {
+    if (isSplittableAtCount(agentAreaWidth, currentCount - k)) {
+      return k
+    }
+  }
+  return null
+}
+
 export function canSplitPane(pane: TmuxPaneInfo, direction: SplitDirection): boolean {
   if (direction === "-h") {
     return pane.width >= MIN_SPLIT_WIDTH
@@ -251,9 +278,10 @@ export function decideSpawnActions(
     return { canSpawn: false, actions: [], reason: "no main pane found" }
   }
 
-  const capacity = calculateCapacity(state.windowWidth, state.windowHeight)
-  
-  if (capacity.total === 0) {
+  const agentAreaWidth = Math.floor(state.windowWidth * (1 - MAIN_PANE_RATIO))
+  const currentCount = state.agentPanes.length
+
+  if (agentAreaWidth < MIN_PANE_WIDTH) {
     return {
       canSpawn: false,
       actions: [],
@@ -261,52 +289,85 @@ export function decideSpawnActions(
     }
   }
 
-  let currentState = state
-  const closeActions: PaneAction[] = []
-  const maxIterations = state.agentPanes.length + 1
+  const oldestPane = findOldestAgentPane(state.agentPanes, sessionMappings)
+  const oldestMapping = oldestPane 
+    ? sessionMappings.find(m => m.paneId === oldestPane.paneId)
+    : null
 
-  for (let i = 0; i < maxIterations; i++) {
-    const spawnTarget = findSplittableTarget(currentState)
-    
+  if (currentCount === 0) {
+    const virtualMainPane: TmuxPaneInfo = { ...state.mainPane, width: state.windowWidth }
+    if (canSplitPane(virtualMainPane, "-h")) {
+      return {
+        canSpawn: true,
+        actions: [{
+          type: "spawn",
+          sessionId,
+          description,
+          targetPaneId: state.mainPane.paneId,
+          splitDirection: "-h"
+        }]
+      }
+    }
+    return { canSpawn: false, actions: [], reason: "mainPane too small to split" }
+  }
+
+  if (isSplittableAtCount(agentAreaWidth, currentCount)) {
+    const spawnTarget = findSplittableTarget(state)
     if (spawnTarget) {
       return {
         canSpawn: true,
-        actions: [
-          ...closeActions,
-          { 
-            type: "spawn", 
-            sessionId, 
-            description, 
-            targetPaneId: spawnTarget.targetPaneId,
-            splitDirection: spawnTarget.splitDirection
-          }
-        ],
-        reason: closeActions.length > 0 ? `closed ${closeActions.length} pane(s) to make room` : undefined,
+        actions: [{
+          type: "spawn",
+          sessionId,
+          description,
+          targetPaneId: spawnTarget.targetPaneId,
+          splitDirection: spawnTarget.splitDirection
+        }]
       }
     }
+  }
 
-    const oldestPane = findOldestAgentPane(currentState.agentPanes, sessionMappings)
-    if (!oldestPane) {
-      break
+  const minEvictions = findMinimalEvictions(agentAreaWidth, currentCount)
+
+  if (minEvictions === 1 && oldestPane) {
+    return {
+      canSpawn: true,
+      actions: [
+        {
+          type: "close",
+          paneId: oldestPane.paneId,
+          sessionId: oldestMapping?.sessionId || ""
+        },
+        {
+          type: "spawn",
+          sessionId,
+          description,
+          targetPaneId: state.mainPane.paneId,
+          splitDirection: "-h"
+        }
+      ],
+      reason: "closed 1 pane to make room for split"
     }
+  }
 
-    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)
+  if (oldestPane) {
+    return {
+      canSpawn: true,
+      actions: [{
+        type: "replace",
+        paneId: oldestPane.paneId,
+        oldSessionId: oldestMapping?.sessionId || "",
+        newSessionId: sessionId,
+        description
+      }],
+      reason: "replaced oldest pane (no split possible)"
     }
   }
 
   return {
     canSpawn: false,
     actions: [],
-    reason: "no splittable pane found even after closing all agent panes",
+    reason: "no pane available to replace"
   }
 }
 

src/features/tmux-subagent/manager.test.ts

@@ -100,9 +100,9 @@ function createSessionCreatedEvent(
 
 function createWindowState(overrides?: Partial<WindowState>): WindowState {
   return {
-    windowWidth: 200,
+    windowWidth: 220,
     windowHeight: 44,
-    mainPane: { paneId: '%0', width: 120, height: 44, left: 0, top: 0, title: 'main', isActive: true },
+    mainPane: { paneId: '%0', width: 110, height: 44, left: 0, top: 0, title: 'main', isActive: true },
     agentPanes: [],
     ...overrides,
   }
@@ -368,8 +368,8 @@ describe('TmuxSessionManager', () => {
       expect(mockExecuteActions).toHaveBeenCalledTimes(0)
     })
 
-    test('closes oldest agent when at max capacity', async () => {
-      //#given
+    test('replaces oldest agent when unsplittable (small window)', async () => {
+      //#given - small window where split is not possible
       mockIsInsideTmux.mockReturnValue(true)
       mockQueryWindowState.mockImplementation(async () =>
         createWindowState({
@@ -405,18 +405,13 @@ describe('TmuxSessionManager', () => {
         createSessionCreatedEvent('ses_new', 'ses_parent', 'New Task')
       )
 
-      //#then
+      //#then - with small window, replace action is used instead of close+spawn
       expect(mockExecuteActions).toHaveBeenCalledTimes(1)
       const call = mockExecuteActions.mock.calls[0]
       expect(call).toBeDefined()
       const actionsArg = call![0]
-      expect(actionsArg.length).toBeGreaterThanOrEqual(1)
-
-      const closeActions = actionsArg.filter((a) => a.type === 'close')
-      const spawnActions = actionsArg.filter((a) => a.type === 'spawn')
-
-      expect(closeActions).toHaveLength(1)
-      expect(spawnActions).toHaveLength(1)
+      expect(actionsArg).toHaveLength(1)
+      expect(actionsArg[0].type).toBe('replace')
     })
   })
 
@@ -614,8 +609,8 @@ describe('DecisionEngine', () => {
       }
     })
 
-    test('returns close + spawn when at capacity', async () => {
-      //#given
+    test('returns replace when split not possible', async () => {
+      //#given - small window where split is never possible
       const { decideSpawnActions } = await import('./decision-engine')
       const state: WindowState = {
         windowWidth: 160,
@@ -654,15 +649,10 @@ describe('DecisionEngine', () => {
         sessionMappings
       )
 
-      //#then
+      //#then - agent area (80) < MIN_SPLIT_WIDTH (105), so replace is used
       expect(decision.canSpawn).toBe(true)
-      expect(decision.actions).toHaveLength(2)
-      expect(decision.actions[0]).toEqual({
-        type: 'close',
-        paneId: '%1',
-        sessionId: 'ses_old',
-      })
-      expect(decision.actions[1].type).toBe('spawn')
+      expect(decision.actions).toHaveLength(1)
+      expect(decision.actions[0].type).toBe('replace')
     })
 
     test('returns canSpawn=false when window too small', async () => {

src/features/tmux-subagent/manager.ts

@@ -166,11 +166,11 @@ export class TmuxSessionManager {
         canSpawn: decision.canSpawn,
         reason: decision.reason,
         actionCount: decision.actions.length,
-        actions: decision.actions.map((a) =>
-          a.type === "close"
-            ? { type: "close", paneId: a.paneId }
-            : { type: "spawn", sessionId: a.sessionId }
-        ),
+        actions: decision.actions.map((a) => {
+          if (a.type === "close") return { type: "close", paneId: a.paneId }
+          if (a.type === "replace") return { type: "replace", paneId: a.paneId, newSessionId: a.newSessionId }
+          return { type: "spawn", sessionId: a.sessionId }
+        }),
       })
 
       if (!decision.canSpawn) {
@@ -190,6 +190,13 @@ export class TmuxSessionManager {
             sessionId: action.sessionId,
           })
         }
+        if (action.type === "replace" && actionResult.success) {
+          this.sessions.delete(action.oldSessionId)
+          log("[tmux-session-manager] removed replaced session from cache", {
+            oldSessionId: action.oldSessionId,
+            newSessionId: action.newSessionId,
+          })
+        }
       }
 
       if (result.success && result.spawnedPaneId) {

src/features/tmux-subagent/types.ts

@@ -31,6 +31,7 @@ export type SplitDirection = "-h" | "-v"
 export type PaneAction =
   | { type: "close"; paneId: string; sessionId: string }
   | { type: "spawn"; sessionId: string; description: string; targetPaneId: string; splitDirection: SplitDirection }
+  | { type: "replace"; paneId: string; oldSessionId: string; newSessionId: string; description: string }
 
 export interface SpawnDecision {
   canSpawn: boolean

src/shared/tmux/tmux-utils.ts

@@ -179,6 +179,53 @@ export async function closeTmuxPane(paneId: string): Promise<boolean> {
   return exitCode === 0
 }
 
+export async function replaceTmuxPane(
+  paneId: string,
+  sessionId: string,
+  description: string,
+  config: TmuxConfig,
+  serverUrl: string
+): Promise<SpawnPaneResult> {
+  const { log } = await import("../logger")
+  
+  log("[replaceTmuxPane] called", { paneId, sessionId, description })
+  
+  if (!config.enabled) {
+    return { success: false }
+  }
+  if (!isInsideTmux()) {
+    return { success: false }
+  }
+
+  const tmux = await getTmuxPath()
+  if (!tmux) {
+    return { success: false }
+  }
+
+  const opencodeCmd = `opencode attach ${serverUrl} --session ${sessionId}`
+
+  const proc = spawn([tmux, "respawn-pane", "-k", "-t", paneId, opencodeCmd], {
+    stdout: "pipe",
+    stderr: "pipe",
+  })
+  const exitCode = await proc.exited
+
+  if (exitCode !== 0) {
+    const stderr = await new Response(proc.stderr).text()
+    log("[replaceTmuxPane] FAILED", { paneId, exitCode, stderr: stderr.trim() })
+    return { success: false }
+  }
+
+  const title = `omo-subagent-${description.slice(0, 20)}`
+  spawn([tmux, "select-pane", "-t", paneId, "-T", title], {
+    stdout: "ignore",
+    stderr: "ignore",
+  })
+
+  log("[replaceTmuxPane] SUCCESS", { paneId, sessionId })
+  return { success: true, paneId }
+}
+
 export async function applyLayout(
   tmux: string,
   layout: TmuxLayout,