diff --git a/server/src/__tests__/environment-run-orchestrator.test.ts b/server/src/__tests__/environment-run-orchestrator.test.ts
index df72db38439..d8273d84984 100644
--- a/server/src/__tests__/environment-run-orchestrator.test.ts
+++ b/server/src/__tests__/environment-run-orchestrator.test.ts
@@ -749,6 +749,20 @@ describe("environmentRunOrchestrator — realizeForRun", () => {
 
 
 describe("native runner lifecycle changes before lease acquisition", () => {
+  it("fences computer warm admission when the selected model changes", async () => {
+    const environment = { ...makeEnvironment("computer"), config: { provider: "boat" } };
+    mockGetEnvironment.mockResolvedValue(environment);
+    const acquireRunLease = vi.fn(async (input) => ({ environment: input.environment, lease: makeLease(), leaseContext: {} }));
+    const orchestrator = environmentRunOrchestrator({} as never, { environmentRuntime: makeMockRuntime({ acquireRunLease }) });
+    const input = { companyId: "company-1", selectedEnvironmentId: "env-1", localEnvironmentId: "local", adapterType: "paperclip_runner",
+      issueId: "task", heartbeatRunId: "run-1", agentId: "agent-1", persistedExecutionWorkspace: null, executionWorkspaceSettings: null };
+    for (const model of ["model-a", "model-a", "model-b"]) await orchestrator.acquireForRun({ ...input, adapterConfig: { provider: "codex", model } });
+    const keys = acquireRunLease.mock.calls.map(([input]) => input.executionConfigurationKey);
+    expect(keys[0]).toMatch(/^[a-f0-9]{64}$/);
+    expect(keys[1]).toBe(keys[0]);
+    expect(keys[2]).not.toBe(keys[0]);
+  });
+
   it("requests a reusable lease when an existing task switches from per-turn to warm", async () => {
     const environment = { ...makeEnvironment("sandbox"), config: { provider: "daytona", reuseLease: false } };
     mockGetEnvironment.mockResolvedValue(environment);
diff --git a/server/src/services/computer-environment-driver.test.ts b/server/src/services/computer-environment-driver.test.ts
index f3c3dd6cd2f..816c0ce3123 100644
--- a/server/src/services/computer-environment-driver.test.ts
+++ b/server/src/services/computer-environment-driver.test.ts
@@ -4,7 +4,7 @@ import { createComputerEnvironmentDriver } from "./computer-environment-driver.j
 const state = vi.hoisted(() => {
   const binding = { owner: { computerId: "computer", ownerId: "owner", generation: 1 },
     listenerPort: 43127, remoteCwd: "/remote/workspace", placementId: "placement", agentHome: "/remote/home" };
-  return { admit: vi.fn(async () => binding), admitProbe: vi.fn(async () => binding),
+  return { admit: vi.fn(async (_input: { sessionKey: string }) => binding), admitProbe: vi.fn(async () => binding),
     retire: vi.fn(async () => ({ retired: true })), acquireLease: vi.fn(async (input: unknown) => input) };
 });
 vi.mock("../modules/computers/index.js", () => ({ computerService: () => state }));
@@ -12,6 +12,20 @@ vi.mock("./environments.js", () => ({ environmentService: () => state }));
 vi.mock("./instance-settings.js", () => ({ instanceSettingsService: () => ({ getExperimental: async () => ({ enableBoatEnvironments: true }) }) }));
 
 describe("computer environment probe ownership", () => {
+  it("keeps stable configuration on one session key and fences a changed model", async () => {
+    state.admit.mockClear();
+    const db = { update: () => ({ set: () => ({ where: async () => undefined }) }) };
+    const driver = createComputerEnvironmentDriver(db as never);
+    for (const executionConfigurationKey of ["model-a", "model-a", "model-b"]) {
+      await driver.acquireRunLease({ companyId: "company", agentId: "agent", heartbeatRunId: "run",
+        environment: { id: "environment", config: {} }, adapterType: "paperclip_runner", executionConfigurationKey } as never);
+    }
+    const keys = state.admit.mock.calls.map(([input]) => input.sessionKey);
+    expect(keys[0]).toBe(keys[1]);
+    expect(keys[0]).not.toBe(keys[2]);
+    state.admit.mockClear();
+  });
+
   it("admits a bounded probe without fabricating a heartbeat run", async () => {
     const db = { update: () => ({ set: () => ({ where: async () => undefined }) }) };
     const driver = createComputerEnvironmentDriver(db as never);
diff --git a/server/src/services/computer-environment-driver.ts b/server/src/services/computer-environment-driver.ts
index fba7139f1ba..b89e70dffc1 100644
--- a/server/src/services/computer-environment-driver.ts
+++ b/server/src/services/computer-environment-driver.ts
@@ -49,7 +49,7 @@ export function createComputerEnvironmentDriver(db: Db): EnvironmentRuntimeDrive
       const idleTimeoutMs = resolvePaperclipRunnerIdleTimeoutMs(input.environment.config.runnerIdleTimeoutMs);
       const binding = input.heartbeatRunId ? await computers.admit({ companyId: input.companyId, environmentId: input.environment.id,
         agentId: input.agentId, runId: input.heartbeatRunId, idleTimeoutMs,
-        sessionKey: createHash("sha256").update(JSON.stringify([input.agentId, input.executionWorkspaceId ?? input.issueId, input.adapterType])).digest("hex") })
+        sessionKey: createHash("sha256").update(JSON.stringify([input.agentId, input.executionWorkspaceId ?? input.issueId, input.adapterType, input.executionConfigurationKey])).digest("hex") })
         : await computers.admitProbe({ companyId: input.companyId, environmentId: input.environment.id,
           agentId: input.agentId, probeId: randomUUID(), idleTimeoutMs });
       try {
diff --git a/server/src/services/environment-run-orchestrator.ts b/server/src/services/environment-run-orchestrator.ts
index 37ad6e59704..82d7997efc7 100644
--- a/server/src/services/environment-run-orchestrator.ts
+++ b/server/src/services/environment-run-orchestrator.ts
@@ -15,6 +15,7 @@
  * and transport logic.
  */
 
+import { createHash } from "node:crypto";
 import type { Db } from "@paperclipai/db";
 import type {
   Environment,
@@ -203,6 +204,7 @@ export function environmentRunOrchestrator(
    * Wraps the runtime driver's acquire call with standardized error handling.
    */
   async function acquireLease(input: {
+    executionConfigurationKey?: string;
     companyId: string;
     environment: Environment;
     issueId: string | null;
@@ -267,6 +269,7 @@ export function environmentRunOrchestrator(
     localEnvironmentId: string;
     adapterType: string;
     adapterConfig?: Record<string, unknown>;
+    executionConfigurationKey?: string;
     admittedLifecycleMode?: "warm" | "per_turn";
     issueId: string | null;
     heartbeatRunId: string;
@@ -289,6 +292,9 @@ export function environmentRunOrchestrator(
 
     // Step 2: Acquire lease
     const acquisitionInput = {
+      executionConfigurationKey: environment.driver === "computer" ? createHash("sha256").update(JSON.stringify([
+        input.adapterConfig, input.executionConfigurationKey, environment.config, environment.envVars,
+      ])).digest("hex") : undefined,
       companyId: input.companyId,
       environment,
       issueId: input.issueId,
diff --git a/server/src/services/environment-runtime.ts b/server/src/services/environment-runtime.ts
index b4812ff2fdf..94558bb4dd4 100644
--- a/server/src/services/environment-runtime.ts
+++ b/server/src/services/environment-runtime.ts
@@ -433,6 +433,8 @@ function stripSecretRefValuesFromPluginLeaseMetadata(input: {
 }
 
 export interface EnvironmentDriverAcquireInput {
+  /** Stable selected execution configuration; command-backed warm owners fence changes. */
+  executionConfigurationKey?: string;
   companyId: string;
   environment: Environment;
   issueId: string | null;
@@ -3979,6 +3981,7 @@ export function environmentRuntimeService(
     },
 
     async acquireRunLease(input: {
+      executionConfigurationKey?: string;
       companyId: string;
       environment: Environment;
       issueId: string | null;
@@ -4017,6 +4020,7 @@ export function environmentRuntimeService(
       });
       const driver = requireDriver(input.environment);
       const lease = await driver.acquireRunLease({
+        executionConfigurationKey: input.executionConfigurationKey,
         companyId: input.companyId,
         environment: input.environment,
         issueId: input.issueId,
diff --git a/server/src/services/heartbeat.ts b/server/src/services/heartbeat.ts
index 75c9ffda65a..b70e34e6a61 100644
--- a/server/src/services/heartbeat.ts
+++ b/server/src/services/heartbeat.ts
@@ -4443,7 +4443,7 @@ export function heartbeatService(
       }
       await bindIssueToPersistedExecutionWorkspace(persistedExecutionWorkspace);
       const projectRepositoryPaths: string[] = [];
-      if (executionWorkspace.projectId && resolvedWorkspace.source === "project_primary" && !resolvedWorkspace.baseCwdFallback) {
+      if (selectedEnvironmentForConfig?.driver !== "computer" && executionWorkspace.projectId && resolvedWorkspace.source === "project_primary" && !resolvedWorkspace.baseCwdFallback) {
         const repositoryRows = await db.select().from(projectWorkspaces).where(and(
           eq(projectWorkspaces.companyId, agent.companyId),
           eq(projectWorkspaces.projectId, executionWorkspace.projectId),
@@ -4492,6 +4492,9 @@ export function heartbeatService(
           localEnvironmentId: localEnvironment.id,
           adapterType: agent.adapterType,
           adapterConfig: parseObject(agent.adapterConfig),
+          executionConfigurationKey: createHash("sha256").update(JSON.stringify([
+            config, agent.runtimeConfig, agent.permissions, managedAiRuntime?.sessionIdentity,
+          ])).digest("hex"),
           admittedLifecycleMode: persistedNativeExecutionInput?.session.lifecyclePolicy.mode,
           issueId: issueId ?? null,
           heartbeatRunId: run.id,
diff --git a/server/src/services/native-runtime/native-session-executor.test.ts b/server/src/services/native-runtime/native-session-executor.test.ts
index 79c6e7b0efe..5807a868630 100644
--- a/server/src/services/native-runtime/native-session-executor.test.ts
+++ b/server/src/services/native-runtime/native-session-executor.test.ts
@@ -7433,6 +7433,36 @@ describe("native warm session supervision", () => {
     await closeWarmNativeSessionsForRun({ runId: owners[1]!.runId, reason: "fixture cleanup" });
   });
 
+  it("replaces a computer warm runner after model configuration admits a fresh owner", async () => {
+    const name = "computer-model-change";
+    const current = { ...execution, binding: { ...execution.binding, runId: `${name}-one`, agentId: name, executionWorkspaceId: name },
+      session: { ...execution.session, normalizedSessionId: name, lifecyclePolicy: { mode: "warm" as const, idleTimeoutMs: 60_000 } } } as NativeExecutionInputV1;
+    const firstClose = vi.fn(async () => undefined), secondClose = vi.fn(async () => undefined);
+    const firstRetire = vi.fn(async () => true), secondRetire = vi.fn(async () => true);
+    const target = { kind: "remote", transport: "computer", environmentId: name, remoteCwd: `/home/user/${name}`,
+      resourceAuthority: { kind: "computer-owner", computerId: name, ownerId: "prior-owner", generation: 1 },
+      retainWarm: vi.fn(async () => undefined), retire: firstRetire };
+    const result = { result: { summary: "completed" }, terminal: { runTerminalState: "succeeded" }, turnId: name,
+      normalizedSessionId: name, providerSessionId: name, driverKind: "test", driverVersion: "1", nativeEventCount: 1,
+      highestContiguousSourceSeq: 1, usage: null };
+    state.execute.mockReset().mockImplementationOnce(async options => {
+      await options.onSession?.({ close: firstClose }); return result;
+    }).mockImplementationOnce(async options => {
+      expect(options.existingSession).toBeUndefined();
+      expect(secondRetire).not.toHaveBeenCalled();
+      await options.onSession?.({ close: secondClose }); return result;
+    });
+    await executePaperclipNativeSession({ db: leaseDb(current), execution: current, runnerInstanceId: name, runnerExecutionTarget: target as never });
+    const next = { ...current, binding: { ...current.binding, runId: `${name}-two` }, provider: { ...current.provider, model: "changed-model" } } as NativeExecutionInputV1;
+    await executePaperclipNativeSession({ db: leaseDb(next), execution: next, runnerInstanceId: name,
+      runnerExecutionTarget: { ...target, resourceAuthority: { ...target.resourceAuthority, ownerId: "fresh-owner" }, retire: secondRetire } as never });
+    expect(firstRetire).toHaveBeenCalledOnce();
+    expect(firstClose).toHaveBeenCalledOnce();
+    expect(secondRetire).not.toHaveBeenCalled();
+    await closeWarmNativeSessionsForRun({ runId: next.binding.runId, reason: "fixture cleanup" });
+    expect(secondRetire).toHaveBeenCalledOnce();
+  });
+
   it("keeps a computer transport alive when its old idle timer loses to warm admission", async () => {
     const name = "computer-idle-admission-race";
     const current = { ...execution, binding: { ...execution.binding, runId: `${name}-one`, agentId: name, executionWorkspaceId: name },