diff --git a/server/src/__tests__/environment-runtime-driver-contract.test.ts b/server/src/__tests__/environment-runtime-driver-contract.test.ts
index 373eec43e3b..f2616aae722 100644
--- a/server/src/__tests__/environment-runtime-driver-contract.test.ts
+++ b/server/src/__tests__/environment-runtime-driver-contract.test.ts
@@ -87,6 +87,7 @@ describeEmbeddedPostgres("environment runtime driver contract", () => {
   }) {
     const companyId = randomUUID();
     const agentId = randomUUID();
+    const environmentId = randomUUID();
     const runId = randomUUID();
     const now = new Date();
     let config = input.config;
@@ -140,10 +141,10 @@ describeEmbeddedPostgres("environment runtime driver contract", () => {
             where: (environment, { eq }) => eq(environment.driver, "local"),
           })
         : null;
-    const environmentId = existingLocal?.id ?? randomUUID();
+    const resolvedEnvironmentId = existingLocal?.id ?? environmentId;
     if (!existingLocal) {
       await db.insert(environments).values({
-        id: environmentId,
+        id: resolvedEnvironmentId,
         name: `${input.driver} contract`,
         driver: input.driver,
         status: "active",
@@ -169,7 +170,7 @@ describeEmbeddedPostgres("environment runtime driver contract", () => {
       issueId: null,
       runId,
       environment: {
-        id: environmentId,
+        id: resolvedEnvironmentId,
         companyId,
         name: `${input.driver} contract`,
         description: null,
diff --git a/server/src/__tests__/environment-service.test.ts b/server/src/__tests__/environment-service.test.ts
index dd15546750d..c03dac9e41d 100644
--- a/server/src/__tests__/environment-service.test.ts
+++ b/server/src/__tests__/environment-service.test.ts
@@ -303,6 +303,66 @@ describeEmbeddedPostgres("environmentService leases", () => {
     expect((rows[0]?.metadata as Record<string, unknown>)?.managedKubernetesSandbox).toBe(true);
   });
 
+  it("returns a conflict when creating a second environment with the same name", async () => {
+    await seedEnvironment();
+
+    await svc.create({
+      name: "Shared Fixture",
+      driver: "ssh",
+      status: "active",
+      config: {
+        host: "fixture.example.test",
+        port: 22,
+        username: "fixture",
+        remoteWorkspacePath: "/srv/paperclip",
+      },
+    });
+
+    await expect(svc.create({
+      name: "Shared Fixture",
+      driver: "sandbox",
+      status: "active",
+      config: {
+        provider: "fake-plugin",
+        image: "fake:test",
+        reuseLease: false,
+      },
+    })).rejects.toMatchObject({
+      status: 409,
+      message: 'An environment named "Shared Fixture" already exists for this instance.',
+    });
+  });
+
+  it("returns a conflict when renaming an environment to an existing name", async () => {
+    const { environmentId } = await seedEnvironment();
+    const otherEnvironmentId = randomUUID();
+    const now = new Date();
+
+    await db.insert(environments).values({
+      id: otherEnvironmentId,
+      name: "Other Fixture",
+      driver: "sandbox",
+      status: "active",
+      config: {
+        provider: "fake-plugin",
+        image: "fake:test",
+        reuseLease: false,
+      },
+      createdAt: now,
+      updatedAt: now,
+    });
+
+    await expect(svc.update(otherEnvironmentId, {
+      name: "Lease Fixture",
+    })).rejects.toMatchObject({
+      status: 409,
+      message: 'An environment named "Lease Fixture" already exists for this instance.',
+    });
+
+    const original = await svc.getById(environmentId);
+    expect(original?.name).toBe("Lease Fixture");
+  });
+
   it("rejects a second managed-sandbox row for the same company at the DB level", async () => {
     const companyId = randomUUID();
     await db.insert(companies).values({
diff --git a/server/src/services/environments.ts b/server/src/services/environments.ts
index 4b05f72d225..c746389a6c2 100644
--- a/server/src/services/environments.ts
+++ b/server/src/services/environments.ts
@@ -15,6 +15,7 @@ import {
   type EnvironmentLeaseStatus,
   type UpdateEnvironment,
 } from "@paperclipai/shared";
+import { conflict } from "../errors.js";
 
 type EnvironmentRow = typeof environments.$inferSelect;
 type EnvironmentLeaseRow = typeof environmentLeases.$inferSelect;
@@ -70,6 +71,18 @@ function readEnum<T extends string>(value: string | null, allowed: readonly T[],
   throw new Error(`Unexpected ${fieldName} value: ${value}`);
 }
 
+function hasConstraintName(error: unknown, constraintName: string): boolean {
+  if (typeof error !== "object" || error === null) return false;
+  const candidate = error as {
+    constraint?: unknown;
+    constraint_name?: unknown;
+    cause?: unknown;
+  };
+  return candidate.constraint === constraintName
+    || candidate.constraint_name === constraintName
+    || hasConstraintName(candidate.cause, constraintName);
+}
+
 function toEnvironment(row: EnvironmentRow): Environment {
   return {
     id: row.id,
@@ -82,7 +95,7 @@ function toEnvironment(row: EnvironmentRow): Environment {
     metadata: cloneRecord(row.metadata),
     createdAt: row.createdAt,
     updatedAt: row.updatedAt,
-  } as unknown as Environment;
+  } as Environment;
 }
 
 type EnvironmentListFilters = {
@@ -294,7 +307,16 @@ export function environmentService(db: Db) {
             sql`${environments.driver} = 'sandbox' AND (${environments.metadata} ->> 'managedByPaperclip')::boolean = true`,
         })
         .returning()
-        .then((rows) => rows[0] ?? null);
+        .then((rows) => rows[0] ?? null)
+        .catch((error) => {
+          if (
+            hasConstraintName(error, "environments_name_idx")
+            || hasConstraintName(error, "environments_managed_sandbox_idx")
+          ) {
+            return null;
+          }
+          throw error;
+        });
       if (inserted) return toEnvironment(inserted);
 
       const winner = await db
@@ -359,7 +381,16 @@ export function environmentService(db: Db) {
           updatedAt: now,
         })
         .returning()
-        .then((rows) => rows[0] ?? null);
+        .then((rows) => rows[0] ?? null)
+        .catch((error) => {
+          if (hasConstraintName(error, "environments_name_idx")) {
+            throw conflict(`An environment named "${input.name}" already exists for this instance.`);
+          }
+          if (hasConstraintName(error, "environments_local_driver_idx")) {
+            throw conflict("A local environment already exists for this instance.");
+          }
+          throw error;
+        });
       if (!row) {
         throw new Error("Failed to create environment");
       }
@@ -385,7 +416,16 @@ export function environmentService(db: Db) {
         .set(values)
         .where(eq(environments.id, id))
         .returning()
-        .then((rows) => rows[0] ?? null);
+        .then((rows) => rows[0] ?? null)
+        .catch((error) => {
+          if (hasConstraintName(error, "environments_name_idx")) {
+            throw conflict(`An environment named "${patch.name}" already exists for this instance.`);
+          }
+          if (hasConstraintName(error, "environments_local_driver_idx")) {
+            throw conflict("A local environment already exists for this instance.");
+          }
+          throw error;
+        });
       return row ? toEnvironment(row) : null;
     },
 
diff --git a/server/src/services/heartbeat.ts b/server/src/services/heartbeat.ts
index fb0470c0d99..91dab04264a 100644
--- a/server/src/services/heartbeat.ts
+++ b/server/src/services/heartbeat.ts
@@ -8461,8 +8461,7 @@ export function heartbeatService(db: Db, options: HeartbeatServiceOptions = {})
     const resolvedInstanceSettings = await instanceSettings.get();
     const environmentResolution = resolveExecutionWorkspaceEnvironmentId({
       agentDefaultEnvironmentId: agent.defaultEnvironmentId,
-      instanceDefaultEnvironmentId:
-        (resolvedInstanceSettings as { defaultEnvironmentId?: string | null }).defaultEnvironmentId ?? null,
+      instanceDefaultEnvironmentId: resolvedInstanceSettings.defaultEnvironmentId ?? null,
       localDefaultEnvironmentId: localEnvironment.id,
     });
     const shouldReuseExisting = requestedShouldReuseExisting;