diff --git a/server/src/__tests__/environment-selection-route-guards.test.ts b/server/src/__tests__/environment-selection-route-guards.test.ts
index 0f36d4e512..7a9fb2c39e 100644
--- a/server/src/__tests__/environment-selection-route-guards.test.ts
+++ b/server/src/__tests__/environment-selection-route-guards.test.ts
@@ -117,6 +117,7 @@ function buildApp(routerFactory: (app: express.Express) => void) {
     (req as any).actor = {
       type: "board",
       userId: "user-1",
+      companyIds: ["company-1"],
       source: "local_implicit",
     };
     next();
@@ -170,6 +171,10 @@ describe.sequential("execution environment route guards", () => {
     mockProjectService.createWorkspace.mockReset();
     mockProjectService.remove.mockReset();
     mockProjectService.resolveByReference.mockReset();
+    mockProjectService.resolveByReference.mockResolvedValue({
+      ambiguous: false,
+      project: { id: "project-1" },
+    });
     mockProjectService.listWorkspaces.mockReset();
     mockIssueService.create.mockReset();
     mockIssueService.getById.mockReset();
diff --git a/server/src/__tests__/project-routes-env.test.ts b/server/src/__tests__/project-routes-env.test.ts
index d68576a73f..4cb93dc2e8 100644
--- a/server/src/__tests__/project-routes-env.test.ts
+++ b/server/src/__tests__/project-routes-env.test.ts
@@ -158,7 +158,7 @@ describe("project env routes", () => {
       explanation: "Allowed by test mock.",
     });
     mockGetTelemetryClient.mockReturnValue({ track: vi.fn() });
-    mockProjectService.resolveByReference.mockResolvedValue({ ambiguous: false, project: null });
+    mockProjectService.resolveByReference.mockResolvedValue({ ambiguous: false, project: buildProject() });
     mockProjectService.createWorkspace.mockResolvedValue(null);
     mockProjectService.listWorkspaces.mockResolvedValue([]);
     mockEnvironmentService.getById.mockReset();
diff --git a/server/src/__tests__/project-workspace-managed-sandbox-routes.test.ts b/server/src/__tests__/project-workspace-managed-sandbox-routes.test.ts
index 5623fc6b07..b6324d4e26 100644
--- a/server/src/__tests__/project-workspace-managed-sandbox-routes.test.ts
+++ b/server/src/__tests__/project-workspace-managed-sandbox-routes.test.ts
@@ -203,7 +203,7 @@ describe("project workspace host-path floor", () => {
       explanation: "Allowed by test mock.",
     });
     mockGetTelemetryClient.mockReturnValue({ track: vi.fn() });
-    mockProjectService.resolveByReference.mockResolvedValue({ ambiguous: false, project: null });
+    mockProjectService.resolveByReference.mockResolvedValue({ ambiguous: false, project: buildProject() });
     mockProjectService.getById.mockResolvedValue(buildProject());
     mockProjectService.create.mockResolvedValue(buildProject());
     mockProjectService.createWorkspace.mockResolvedValue(buildWorkspace());
diff --git a/server/src/routes/projects.ts b/server/src/routes/projects.ts
index 9d20e4d882..7a5d814935 100644
--- a/server/src/routes/projects.ts
+++ b/server/src/routes/projects.ts
@@ -116,7 +116,11 @@ export function projectRoutes(db: Db) {
     if (req.actor.type === "agent" && req.actor.companyId) {
       return req.actor.companyId;
     }
-    return null;
+    // A single-company actor (the common self-hosted case) has an unambiguous
+    // company context without a `?companyId=` query — shortnames only resolve
+    // inside one company anyway, so require exactly one.
+    const actorCompanyIds = req.actor.companyIds ?? [];
+    return actorCompanyIds.length === 1 ? actorCompanyIds[0] : null;
   }
 
   async function normalizeProjectReference(req: Request, rawId: string) {