TL;DR — Terraform 1.10 ephemeral resources finally let you fetch a Vault-issued AWS token during an apply without it ever touching the state file. That removes the most common reason AI infra leaks credentials.

What you'll set up

A Terraform configuration that stands up: an EKS cluster, an OpenSearch Serverless collection (vector store), an Application Load Balancer with WebSocket support, and IRSA roles for the voice-agent pod — all using ephemeral Vault credentials so no static AWS keys exist in TF state or CI.

Architecture

flowchart TD
  TF[terraform apply] --> VAULT[(Vault)]
  VAULT -->|ephemeral STS| AWS[AWS APIs]
  AWS --> EKS[EKS cluster]
  AWS --> OSS[OpenSearch Serverless]
  AWS --> ALB[ALB WebSocket]
  EKS --> IRSA[Pod IRSA]
  IRSA --> OSS

Step 1 — Configure the Vault provider with ephemeral output

```hcl terraform { required_version = ">= 1.10" required_providers { vault = { source = "hashicorp/vault", version = "> 4.4" } aws = { source = "hashicorp/aws", version = "> 5.70" } } }

provider "vault" { address = "https://vault.example.com" }

ephemeral "vault_aws_access_credentials" "tf_role" { backend = "aws" role = "terraform-deployer" }

provider "aws" { region = "us-east-1" access_key = ephemeral.vault_aws_access_credentials.tf_role.access_key secret_key = ephemeral.vault_aws_access_credentials.tf_role.secret_key token = ephemeral.vault_aws_access_credentials.tf_role.security_token } ```

The ephemeral block is read fresh each phase (plan, apply, refresh). Nothing lands in terraform.tfstate. When the apply finishes, Vault revokes the STS lease.

Step 2 — Stand up EKS with OIDC

```hcl module "eks" { source = "terraform-aws-modules/eks/aws" version = "~> 20.30" cluster_name = "voice-prod" cluster_version = "1.31" vpc_id = aws_vpc.voice.id subnet_ids = aws_subnet.private[*].id enable_irsa = true eks_managed_node_groups = { voice = { instance_types = ["m7g.large"] min_size = 2; max_size = 10; desired_size = 3 } } } ```

ARM Graviton (m7g) cuts our voice-agent compute spend ~30% vs x86, and the OpenAI Realtime client and LiveKit both run cleanly on arm64 in 2026.

Step 3 — OpenSearch Serverless vector store

```hcl resource "aws_opensearchserverless_collection" "vectors" { name = "voice-vectors" type = "VECTORSEARCH" }

resource "aws_opensearchserverless_security_policy" "encr" { name = "voice-vectors-encr" type = "encryption" policy = jsonencode({ Rules = [{ ResourceType = "collection", Resource = ["collection/voice-vectors"] }] AWSOwnedKey = true }) } ```

We use this for tool-result caching and for the per-tenant FAQ embeddings. VECTORSEARCH is the right type — SEARCH will silently bill more.

Step 4 — IRSA for the voice-agent pod

```hcl data "aws_iam_policy_document" "agent_assume" { statement { actions = ["sts:AssumeRoleWithWebIdentity"] principals { type = "Federated"; identifiers = [module.eks.oidc_provider_arn] } condition { test = "StringEquals"; variable = "${replace(module.eks.cluster_oidc_issuer_url,"https://","")}:sub" values = ["system:serviceaccount:voice:voice-agent"] } } }

resource "aws_iam_role" "agent" { name = "voice-agent" assume_role_policy = data.aws_iam_policy_document.agent_assume.json } ```

Now the voice-agent pod assumes voice-agent via service-account annotation eks.amazonaws.com/role-arn — no static IAM keys, ever.

Step 5 — ALB with WebSocket idle timeout tuning

```hcl resource "aws_lb" "voice" { name = "voice-alb" load_balancer_type = "application" subnets = aws_subnet.public[*].id idle_timeout = 3600 enable_http2 = true } ```

idle_timeout = 3600 (1 hour) is critical. Default 60s drops long voice sessions; 3600s plus a TCP keepalive at 30s on the agent keeps WebSockets alive.

Step 6 — Run terraform plan with a generated lockfile in CI

```yaml

.github/workflows/terraform.yml

• run: terraform init -backend-config=bucket=tf-state -backend-config=key=voice/prod
• run: terraform validate
• run: terraform plan -out=tfplan
• run: terraform apply -auto-approve tfplan ```

The state goes to S3 with versioning and SSE-KMS; PRs only get plan permissions, main gets apply.

Step 7 — Drift-detection cron

```yaml on: { schedule: [{ cron: '0 3 * * *' }] } jobs: drift: steps: - run: terraform plan -detailed-exitcode || (gh issue create --title "Drift detected" --body "see logs"; exit 1) ```

-detailed-exitcode returns 2 when there's drift — turns into a Slack alert before someone "fixed it in the console" causes a 3am page.

Pitfalls

• Ephemeral resources in output — outputs cannot reference ephemeral. The provider error message is misleading; just don't output ephemeral values.
• OpenSearch Serverless eventual consistency — collection ARNs aren't ready immediately; add a time_sleep of ~30s before granting policies.
• ALB idle_timeout vs upstream — set server_side_timeout on the agent ≥ ALB idle_timeout, otherwise upstream closes mid-call.
• EKS OIDC issuer URL formatting — the replace(...,"https://","") is required for the sub condition; many tutorials get this wrong.
• Vault role TTL too short — if max_ttl < apply duration, mid-apply refreshes fail. Set 30 min minimum.

How CallSphere does this in production

CallSphere's primary infra is a self-hosted k3s cluster (not EKS) with Postgres at 72.62.162.83 behind Cloudflare Tunnel — but tenant-isolated HIPAA installs use exactly this Terraform pattern on customer AWS accounts. We never store long-lived AWS keys in CI; Vault issues 30-min STS for every apply. 37 agents, 90+ tools, 115+ DB tables, $149/$499/$1499, 14-day trial, 22% affiliate, see pricing.

FAQ

Q: Terraform vs OpenTofu in 2026? OpenTofu is now wire-compatible with TF 1.10's ephemeral feature; pick OpenTofu if license matters, TF if you need HCP integrations.

Q: How do I share state across teams? S3 + DynamoDB lock with SSE-KMS, or HCP Terraform Workspaces with VCS-driven runs.

Q: Why not store AWS keys in CI secrets and skip Vault? You can, but you're now committed to rotating those keys. OIDC + Vault is set-and-forget.

Q: Where do model API keys go? External Secrets Operator pulls them from Vault into the pod at runtime. Never in TF state.

Sources

• Terraform 1.10 improves handling secrets with ephemeral values — HashiCorp
• HashiCorp Terraform 1.10 adds Ephemeral Values — InfoQ
• Terraform Latest Trends 2026 — Clanker Cloud
• Terraform Ephemeral Resources — Infisical
• Terraform + AI: 5 Futuristic Ways — TecAdmin