diff --git a/server/src/__tests__/execution-workspace-policy.test.ts b/server/src/__tests__/execution-workspace-policy.test.ts
index 4ed5de9b4fe..507db5551c3 100644
--- a/server/src/__tests__/execution-workspace-policy.test.ts
+++ b/server/src/__tests__/execution-workspace-policy.test.ts
@@ -148,16 +148,93 @@ describe("execution workspace policy helpers", () => {
     });
   });
 
-  it("prefers persisted environment selection over issue and project defaults", () => {
+  it("reuses persisted workspace environment when it agrees with the assignee's identity", () => {
+    expect(
+      resolveExecutionWorkspaceEnvironmentId({
+        projectPolicy: { enabled: true, environmentId: "agent-env" },
+        issueSettings: { environmentId: "agent-env" },
+        workspaceConfig: { environmentId: "agent-env" },
+        agentDefaultEnvironmentId: "agent-env",
+        defaultEnvironmentId: "default-env",
+      }),
+    ).toEqual({
+      environmentId: "agent-env",
+      source: "workspace",
+      conflict: null,
+    });
+  });
+
+  it("refuses silent reuse when the persisted workspace env disagrees with the assignee (PAPA-380: sandbox agent on local workspace)", () => {
+    // Claude E2B was assigned to a child issue whose parent had already
+    // realized a `Local` workspace. The persisted workspace env must not
+    // shadow the agent's intended sandbox env.
+    expect(
+      resolveExecutionWorkspaceEnvironmentId({
+        projectPolicy: { enabled: true, environmentId: null },
+        issueSettings: { environmentId: "sandbox-env", mode: "shared_workspace" },
+        workspaceConfig: { environmentId: "local-env" },
+        agentDefaultEnvironmentId: "sandbox-env",
+        defaultEnvironmentId: "local-env",
+      }),
+    ).toEqual({
+      environmentId: "sandbox-env",
+      source: "issue",
+      conflict: {
+        reason: "reused_workspace_environment_mismatch",
+        workspaceEnvironmentId: "local-env",
+        assigneeIntendedEnvironmentId: "sandbox-env",
+        assigneeIntendedSource: "issue",
+      },
+    });
+  });
+
+  it("refuses silent reuse when a null-default (local) agent inherits a non-local workspace env (PAPA-431: Manual QA on engineer SSH workspace)", () => {
+    // Manual QA agent has defaultEnvironmentId: null. When a sibling issue's
+    // SSH workspace is inherited via inheritExecutionWorkspaceFromIssueId,
+    // the persisted SSH env must NOT shadow the agent's deliberate local
+    // identity. The inherited issueSettings.environmentId is treated as a
+    // promoted artifact, not an explicit operator choice.
+    expect(
+      resolveExecutionWorkspaceEnvironmentId({
+        projectPolicy: { enabled: true, environmentId: null },
+        issueSettings: { environmentId: "ssh-env", mode: "isolated_workspace" },
+        workspaceConfig: { environmentId: "ssh-env" },
+        agentDefaultEnvironmentId: null,
+        defaultEnvironmentId: "local-env",
+      }),
+    ).toEqual({
+      environmentId: "local-env",
+      source: "default",
+      conflict: {
+        reason: "reused_workspace_environment_mismatch",
+        workspaceEnvironmentId: "ssh-env",
+        assigneeIntendedEnvironmentId: "local-env",
+        assigneeIntendedSource: "default",
+      },
+    });
+  });
+
+  it("ignores issue/project env promotion for null-default agents even when no workspace is being reused", () => {
+    // A null-default (local-only) agent must not silently inherit an env that
+    // could have been auto-promoted from a different assignee. With no
+    // reused workspace there is no conflict to surface, but the resolved env
+    // is still the local default rather than the issue setting.
     expect(
       resolveExecutionWorkspaceEnvironmentId({
         projectPolicy: { enabled: true, environmentId: "project-env" },
         issueSettings: { environmentId: "issue-env" },
-        workspaceConfig: { environmentId: "workspace-env" },
-        agentDefaultEnvironmentId: "agent-env",
-        defaultEnvironmentId: "default-env",
+        workspaceConfig: null,
+        agentDefaultEnvironmentId: null,
+        defaultEnvironmentId: "local-env",
       }),
-    ).toBe("workspace-env");
+    ).toEqual({
+      environmentId: "local-env",
+      source: "default",
+      conflict: null,
+    });
+  });
+
+  it("prefers the explicit issue environment over project and agent defaults when no workspace is reused", () => {
     expect(
       resolveExecutionWorkspaceEnvironmentId({
         projectPolicy: { enabled: true, environmentId: "project-env" },
@@ -166,7 +243,11 @@ describe("execution workspace policy helpers", () => {
         agentDefaultEnvironmentId: "agent-env",
         defaultEnvironmentId: "default-env",
       }),
-    ).toBe("issue-env");
+    ).toEqual({
+      environmentId: "issue-env",
+      source: "issue",
+      conflict: null,
+    });
     expect(
       resolveExecutionWorkspaceEnvironmentId({
         projectPolicy: { enabled: true, environmentId: "project-env" },
@@ -175,7 +256,11 @@ describe("execution workspace policy helpers", () => {
         agentDefaultEnvironmentId: "agent-env",
         defaultEnvironmentId: "default-env",
       }),
-    ).toBe("project-env");
+    ).toEqual({
+      environmentId: "project-env",
+      source: "project",
+      conflict: null,
+    });
   });
 
   it("falls back to the agent default environment before the company default", () => {
@@ -187,7 +272,11 @@ describe("execution workspace policy helpers", () => {
         agentDefaultEnvironmentId: "agent-env",
         defaultEnvironmentId: "default-env",
       }),
-    ).toBe("agent-env");
+    ).toEqual({
+      environmentId: "agent-env",
+      source: "agent",
+      conflict: null,
+    });
     expect(
       resolveExecutionWorkspaceEnvironmentId({
         projectPolicy: { enabled: true, environmentId: null },
@@ -196,7 +285,11 @@ describe("execution workspace policy helpers", () => {
         agentDefaultEnvironmentId: "agent-env",
         defaultEnvironmentId: "default-env",
       }),
-    ).toBe("default-env");
+    ).toEqual({
+      environmentId: "default-env",
+      source: "project",
+      conflict: null,
+    });
     expect(
       resolveExecutionWorkspaceEnvironmentId({
         projectPolicy: null,
@@ -205,7 +298,11 @@ describe("execution workspace policy helpers", () => {
         agentDefaultEnvironmentId: null,
         defaultEnvironmentId: "default-env",
       }),
-    ).toBe("default-env");
+    ).toEqual({
+      environmentId: "default-env",
+      source: "default",
+      conflict: null,
+    });
     expect(
       resolveExecutionWorkspaceEnvironmentId({
         projectPolicy: { enabled: true, environmentId: null },
@@ -214,7 +311,11 @@ describe("execution workspace policy helpers", () => {
         agentDefaultEnvironmentId: null,
         defaultEnvironmentId: "default-env",
       }),
-    ).toBe("default-env");
+    ).toEqual({
+      environmentId: "default-env",
+      source: "default",
+      conflict: null,
+    });
   });
 
   it("maps persisted execution workspace modes back to issue settings", () => {
diff --git a/server/src/services/execution-workspace-policy.ts b/server/src/services/execution-workspace-policy.ts
index ea89d4414f1..de97fe9a893 100644
--- a/server/src/services/execution-workspace-policy.ts
+++ b/server/src/services/execution-workspace-policy.ts
@@ -119,26 +119,93 @@ export function parseIssueExecutionWorkspaceSettings(raw: unknown): IssueExecuti
   };
 }
 
+export type ExecutionWorkspaceEnvironmentSource =
+  | "workspace"
+  | "issue"
+  | "project"
+  | "agent"
+  | "default";
+
+export type ExecutionWorkspaceEnvironmentConflict = {
+  reason: "reused_workspace_environment_mismatch";
+  workspaceEnvironmentId: string;
+  assigneeIntendedEnvironmentId: string;
+  assigneeIntendedSource: Exclude<ExecutionWorkspaceEnvironmentSource, "workspace">;
+};
+
+export type ExecutionWorkspaceEnvironmentResolution = {
+  environmentId: string;
+  source: ExecutionWorkspaceEnvironmentSource;
+  conflict: ExecutionWorkspaceEnvironmentConflict | null;
+};
+
+function resolveAssigneeIntendedExecutionWorkspaceEnvironment(input: {
+  projectPolicy: ProjectExecutionWorkspacePolicy | null;
+  issueSettings: IssueExecutionWorkspaceSettings | null;
+  agentDefaultEnvironmentId: string | null;
+  defaultEnvironmentId: string;
+}): {
+  environmentId: string;
+  source: Exclude<ExecutionWorkspaceEnvironmentSource, "workspace">;
+} {
+  // Companion bug to PAPA-380 / PAPA-431: when the assignee has no explicit
+  // defaultEnvironmentId, the agent is deliberately scoped to the local
+  // default. Such agents (e.g. Manual QA today) must NOT silently inherit an
+  // issue/project environment that may have been promoted from a different
+  // agent's identity via inheritExecutionWorkspaceFromIssueId or sibling
+  // assignment. Treat null as a positive "local default" signal rather than
+  // "inherit anything."
+  if (input.agentDefaultEnvironmentId === null) {
+    return { environmentId: input.defaultEnvironmentId, source: "default" };
+  }
+  if (input.issueSettings?.environmentId !== undefined) {
+    return {
+      environmentId: input.issueSettings.environmentId ?? input.defaultEnvironmentId,
+      source: "issue",
+    };
+  }
+  if (input.projectPolicy?.environmentId !== undefined) {
+    return {
+      environmentId: input.projectPolicy.environmentId ?? input.defaultEnvironmentId,
+      source: "project",
+    };
+  }
+  return { environmentId: input.agentDefaultEnvironmentId, source: "agent" };
+}
+
 export function resolveExecutionWorkspaceEnvironmentId(input: {
   projectPolicy: ProjectExecutionWorkspacePolicy | null;
   issueSettings: IssueExecutionWorkspaceSettings | null;
   workspaceConfig: { environmentId?: string | null } | null;
   agentDefaultEnvironmentId: string | null;
   defaultEnvironmentId: string;
-}) {
+}): ExecutionWorkspaceEnvironmentResolution {
+  const assigneeIntended = resolveAssigneeIntendedExecutionWorkspaceEnvironment(input);
+
   if (input.workspaceConfig?.environmentId !== undefined) {
-    return input.workspaceConfig.environmentId ?? input.defaultEnvironmentId;
+    const workspaceEnvironmentId =
+      input.workspaceConfig.environmentId ?? input.defaultEnvironmentId;
+    // PAPA-380 / PAPA-431: a reused workspace's persisted environmentId must
+    // never silently shadow the current assignee's environment identity.
+    // When they disagree, refuse the silent reuse: return the assignee's
+    // intended env and surface a conflict signal so the caller forces a fresh
+    // workspace realization (or otherwise alerts the operator) instead of
+    // running the agent on someone else's environment.
+    if (workspaceEnvironmentId !== assigneeIntended.environmentId) {
+      return {
+        environmentId: assigneeIntended.environmentId,
+        source: assigneeIntended.source,
+        conflict: {
+          reason: "reused_workspace_environment_mismatch",
+          workspaceEnvironmentId,
+          assigneeIntendedEnvironmentId: assigneeIntended.environmentId,
+          assigneeIntendedSource: assigneeIntended.source,
+        },
+      };
+    }
+    return { environmentId: workspaceEnvironmentId, source: "workspace", conflict: null };
   }
-  if (input.issueSettings?.environmentId !== undefined) {
-    return input.issueSettings.environmentId ?? input.defaultEnvironmentId;
-  }
-  if (input.projectPolicy?.environmentId !== undefined) {
-    return input.projectPolicy.environmentId ?? input.defaultEnvironmentId;
-  }
-  if (input.agentDefaultEnvironmentId !== null) {
-    return input.agentDefaultEnvironmentId;
-  }
-  return input.defaultEnvironmentId;
+  return { environmentId: assigneeIntended.environmentId, source: assigneeIntended.source, conflict: null };
 }
 
 export function defaultIssueExecutionWorkspaceSettingsForProject(
diff --git a/server/src/services/heartbeat.ts b/server/src/services/heartbeat.ts
index 8c34e99233a..bf59a2d84fc 100644
--- a/server/src/services/heartbeat.ts
+++ b/server/src/services/heartbeat.ts
@@ -7106,13 +7106,47 @@ export function heartbeatService(db: Db, options: HeartbeatServiceOptions = {})
     }
     const existingExecutionWorkspace =
       issueRef?.executionWorkspaceId ? await executionWorkspacesSvc.getById(issueRef.executionWorkspaceId) : null;
-    const shouldReuseExisting =
+    const requestedShouldReuseExisting =
       issueRef?.executionWorkspacePreference === "reuse_existing" &&
       existingExecutionWorkspace !== null &&
       existingExecutionWorkspace.status !== "archived";
-    const reusableExecutionWorkspaceConfig = shouldReuseExisting
+    const requestedReusableExecutionWorkspaceConfig = requestedShouldReuseExisting
       ? existingExecutionWorkspace?.config ?? null
       : null;
+    const defaultEnvironment = await environmentsSvc.ensureLocalEnvironment(agent.companyId);
+    const environmentResolution = resolveExecutionWorkspaceEnvironmentId({
+      projectPolicy: projectExecutionWorkspacePolicy,
+      issueSettings: issueExecutionWorkspaceSettings,
+      workspaceConfig: requestedReusableExecutionWorkspaceConfig,
+      agentDefaultEnvironmentId: agent.defaultEnvironmentId,
+      defaultEnvironmentId: defaultEnvironment.id,
+    });
+    // PAPA-380 / PAPA-431: when the resolver refuses silent reuse of the
+    // persisted workspace environment, also force a fresh workspace
+    // realization on the assignee's intended env. Reusing the on-disk
+    // workspace while swapping the env underneath it would mismatch the cwd's
+    // runtime expectations (e.g. an SSH-targeted worktree running on the
+    // local default driver).
+    if (environmentResolution.conflict) {
+      logger.warn(
+        {
+          runId: run.id,
+          issueId,
+          agentId: agent.id,
+          adapterType: agent.adapterType,
+          existingExecutionWorkspaceId: existingExecutionWorkspace?.id ?? null,
+          workspaceEnvironmentId: environmentResolution.conflict.workspaceEnvironmentId,
+          assigneeIntendedEnvironmentId:
+            environmentResolution.conflict.assigneeIntendedEnvironmentId,
+          assigneeIntendedSource: environmentResolution.conflict.assigneeIntendedSource,
+        },
+        "Refusing silent reuse of execution workspace whose environment does not match the assignee's intended environment; forcing fresh realization",
+      );
+    }
+    const shouldReuseExisting = requestedShouldReuseExisting && !environmentResolution.conflict;
+    const reusableExecutionWorkspaceConfig = shouldReuseExisting
+      ? requestedReusableExecutionWorkspaceConfig
+      : null;
     const persistedExecutionWorkspaceMode = shouldReuseExisting && existingExecutionWorkspace
       ? issueExecutionWorkspaceModeForPersistedWorkspace(existingExecutionWorkspace.mode)
       : null;
@@ -7122,14 +7156,7 @@ export function heartbeatService(db: Db, options: HeartbeatServiceOptions = {})
       persistedExecutionWorkspaceMode === "agent_default"
         ? persistedExecutionWorkspaceMode
         : requestedExecutionWorkspaceMode;
-    const defaultEnvironment = await environmentsSvc.ensureLocalEnvironment(agent.companyId);
-    const selectedEnvironmentId = resolveExecutionWorkspaceEnvironmentId({
-      projectPolicy: projectExecutionWorkspacePolicy,
-      issueSettings: issueExecutionWorkspaceSettings,
-      workspaceConfig: reusableExecutionWorkspaceConfig,
-      agentDefaultEnvironmentId: agent.defaultEnvironmentId,
-      defaultEnvironmentId: defaultEnvironment.id,
-    });
+    const selectedEnvironmentId = environmentResolution.environmentId;
     const workspaceManagedConfig = shouldReuseExisting
       ? { ...config }
       : buildExecutionWorkspaceAdapterConfig({