Token exchange for MCP servers

Verified

Exchange the caller’s credential for a backend-scoped token before the gateway forwards a request to an MCP server.

Exchange the incoming token for a backend-scoped token before the gateway forwards a request to an MCP server.

About

MCP servers are a common target for token exchange. The client that calls the gateway authenticates as a user or an agent, but the MCP server behind the gateway expects a token that is scoped to itself, issued by an authorization server that the server trusts. Token exchange lets the gateway make that swap, so the MCP server never sees the incoming token, and the caller never holds a credential for the MCP server.

The configuration is the same oauthTokenExchange backend authentication method that the standard token exchange guide covers. What differs is the target: the policy attaches to an MCP AgentgatewayBackend rather than to a plain Service.

This guide uses an echo MCP server. Its echo tool returns the input that you send it, and with includeHttpHeaders=true it also returns the HTTP headers that it received, which makes the exchanged token directly observable in the tool response.

Before you begin

Follow the Get started guide to install agentgateway.
Follow the Sample app guide to create a gateway proxy with an HTTP listener and deploy the httpbin sample app.
Get the external address of the gateway and save it in an environment variable.
Tip
Kind cluster? Kind does not support LoadBalancer services by default. To use this option with a Kind cluster, install and run cloud-provider-kind.
export INGRESS_GW_ADDRESS=$(kubectl get svc -n agentgateway-system agentgateway-proxy -o jsonpath="{.status.loadBalancer.ingress[0]['hostname','ip']}") echo $INGRESS_GW_ADDRESS
kubectl port-forward deployment/agentgateway-proxy -n agentgateway-system 8080:80

Deploy Keycloak

Deploy a Keycloak authorization server into your cluster to act as the token endpoint. This example imports two realms so that you can exercise both grants:

backend-oauth: The resource realm that performs the exchange. It has an initial-client (mints the user’s inbound token for the RFC 8693 grant), a confidential requester-client (the gateway’s client, with token exchange enabled), a target-client audience, and testuser / testpass user credentials.
idp: A separate identity provider realm that issues the assertion for the RFC 7523 JWT bearer grant. The backend-oauth realm trusts it through a JWT Authorization Grant identity provider.

Steps to deploy Keycloak:

Download the realm definitions and load them into a ConfigMap in the httpbin namespace, alongside the sample app. The sed command rewrites the issuer host in the import (which is pinned to localhost:7080 for local Docker use) to the in-cluster Keycloak address, so that the realms trust each other when Keycloak runs in the cluster.

BASE=https://agentgateway.dev/examples/traffic-token-exchange/jwt-authz-grant/jwtbearer-import
for realm in backend-oauth-realm idp-realm; do
  curl -sL "$BASE/$realm.json" \
    | sed 's#http://localhost:7080#http://keycloak.httpbin.svc.cluster.local:8080#g' \
    > "$realm.json"
done

kubectl create configmap backend-oauth-realm -n httpbin \
  --from-file=backend-oauth-realm.json \
  --from-file=idp-realm.json

Deploy Keycloak and its Service into the httpbin namespace. The --features=preview flag enables Keycloak’s JWT Authorization Grant, which the RFC 7523 JWT bearer grant requires. The KC_HOSTNAME variable pins the token issuer to the in-cluster DNS name, so that tokens minted through a port-forward and the gateway’s token-exchange call agree on the issuer (iss). Without this, Keycloak rejects the token with an issuer mismatch.

kubectl apply -f- <<EOF
apiVersion: apps/v1
kind: Deployment
metadata:
  name: keycloak
  namespace: httpbin
spec:
  replicas: 1
  selector:
    matchLabels:
      app: keycloak
  template:
    metadata:
      labels:
        app: keycloak
    spec:
      containers:
      - name: keycloak
        image: quay.io/keycloak/keycloak:26.7.1
        args: ["start-dev", "--import-realm", "--http-port=8080", "--features=preview"]
        env:
        - name: KC_BOOTSTRAP_ADMIN_USERNAME
          value: admin
        - name: KC_BOOTSTRAP_ADMIN_PASSWORD
          value: admin
        - name: KC_HOSTNAME
          value: "http://keycloak.httpbin.svc.cluster.local:8080"
        - name: KC_HOSTNAME_STRICT
          value: "false"
        - name: KC_HOSTNAME_BACKCHANNEL_DYNAMIC
          value: "false"
        ports:
        - containerPort: 8080
        volumeMounts:
        - name: realm
          mountPath: /opt/keycloak/data/import
          readOnly: true
      volumes:
      - name: realm
        configMap:
          name: backend-oauth-realm
---
apiVersion: v1
kind: Service
metadata:
  name: keycloak
  namespace: httpbin
spec:
  selector:
    app: keycloak
  ports:
  - name: http
    port: 8080
    targetPort: 8080
EOF

Wait for Keycloak to be ready.

kubectl rollout status deployment/keycloak -n httpbin --timeout=180s

Deploy the MCP server

Deploy a sample echo MCP server and expose it through the gateway.

The MCP server goes in the same httpbin namespace as the Keycloak deployment from the previous section, so that the token endpoint backend and the exchange policy can reference each other without a ReferenceGrant.

Deploy the echo MCP server.

kubectl apply -f- <<EOF
apiVersion: apps/v1
kind: Deployment
metadata:
  name: echo
  namespace: httpbin
  labels:
    app: echo
spec:
  replicas: 1
  selector:
    matchLabels:
      app: echo
  template:
    metadata:
      labels:
        app: echo
    spec:
      containers:
      - name: echo
        image: gcr.io/product-excellence-424719/mcp-echo:1.0
        imagePullPolicy: IfNotPresent
        args: ["--oauth-enabled", "false"]
        ports:
        - containerPort: 3002
        readinessProbe:
          httpGet: { path: /healthz, port: 3002 }
          initialDelaySeconds: 10
---
apiVersion: v1
kind: Service
metadata:
  name: echo
  namespace: httpbin
  labels:
    app: echo
spec:
  selector:
    app: echo
  ports:
  - port: 3002
    targetPort: 3002
    appProtocol: agentgateway.dev/mcp
EOF

Create an AgentgatewayBackend that targets the echo server. This backend sets no backend-level authentication, so the policy that you apply later is the only place that token exchange happens.

kubectl apply -f- <<EOF
apiVersion: agentgateway.dev/v1alpha1
kind: AgentgatewayBackend
metadata:
  name: mcp-backend-echo
  namespace: httpbin
spec:
  mcp:
    targets:
    - name: echo-target
      selector:
        services:
          matchLabels:
            app: echo
EOF

Create an HTTPRoute that exposes the MCP backend on the /mcp path of your gateway.

kubectl apply -f- <<EOF
apiVersion: gateway.networking.k8s.io/v1
kind: HTTPRoute
metadata:
  name: mcp-echo
  namespace: httpbin
spec:
  parentRefs:
  - name: agentgateway-proxy
    namespace: agentgateway-system
  rules:
  - matches:
    - path:
        type: PathPrefix
        value: /mcp
    backendRefs:
    - name: mcp-backend-echo
      group: agentgateway.dev
      kind: AgentgatewayBackend
EOF

Verify that the route is accepted.

kubectl -n httpbin get httproute mcp-echo -o jsonpath='{.status.parents[*].conditions[*].type}={.status.parents[*].conditions[*].status}{"\n"}'

Example output:

Accepted ResolvedRefs=True True

Configure token exchange

Configure agentgateway to exchange the incoming token before it reaches the MCP server.

Create an AgentgatewayBackend for the token endpoint, pointing at the in-cluster Keycloak Service.

kubectl apply -f- <<EOF
apiVersion: agentgateway.dev/v1alpha1
kind: AgentgatewayBackend
metadata:
  name: keycloak-token-endpoint
  namespace: httpbin
spec:
  static:
    host: keycloak.httpbin.svc.cluster.local
    port: 8080
EOF

Create a Kubernetes Secret with the gateway client’s secret. This matches the requester-client secret from the imported realm.

kubectl apply -f- <<EOF
apiVersion: v1
kind: Secret
metadata:
  name: oauth-client
  namespace: httpbin
type: Opaque
stringData:
  clientSecret: requester-secret
EOF

Create an AgentgatewayPolicy that attaches the oauthTokenExchange method to the MCP AgentgatewayBackend. Unlike the Service-targeted policies in the other guides, targetRefs names the backend.

kubectl apply -f- <<EOF
apiVersion: agentgateway.dev/v1alpha1
kind: AgentgatewayPolicy
metadata:
  name: mcp-token-exchange
  namespace: httpbin
spec:
  targetRefs:
  - group: agentgateway.dev
    kind: AgentgatewayBackend
    name: mcp-backend-echo
  backend:
    auth:
      oauthTokenExchange:
        backendRef:
          group: agentgateway.dev
          kind: AgentgatewayBackend
          name: keycloak-token-endpoint
        path: /realms/backend-oauth/protocol/openid-connect/token
        grantType: TokenExchange
        audiences:
        - target-client
        clientAuth:
          clientId: requester-client
          method: ClientSecretBasic
          secretRef:
            name: oauth-client
EOF

Review the following table to understand this configuration. For more information, see the API docs.

Field	Description
`backendRef`	Reference to the AgentgatewayBackend for the token endpoint. Mutually exclusive with `url`. Set exactly one of the two.
`url`	The full address of the token endpoint, including the path. Use this field instead of `backendRef` to point at the authorization server directly, without creating an intermediate Kubernetes object. Mutually exclusive with `backendRef`. Do not set `path` when you use `url`.
`path`	Path of the token endpoint on the backend. Must start with `/`. Defaults to `/`.
`grantType`	`TokenExchange` (default, RFC 8693) or `JwtBearer` (RFC 7523).
`clientAuth`	Client authentication for the token endpoint. `method` is `ClientSecretBasic` (default), `ClientSecretPost`, or `PrivateKeyJwt`. Use `secretRef` to read the client secret from a Kubernetes Secret.
`audiences`, `scopes`, `resources`	The `audience`, `scope`, and `resource` parameters sent to the token endpoint. `resources` are RFC 8707 resource indicators.
`subjectToken.source`	Where the gateway reads the incoming token from. Set exactly one of `header`, `queryParameter`, `cookie`, or `expression`, where `expression` is a CEL expression that reads the token from the request, such as a claim of a validated JWT. Defaults to the `Authorization` header with the `Bearer` prefix.
`subjectToken.tokenType`	The type that the gateway reports for that token. Use a built-in name such as `AccessToken` (the default), `Jwt`, or `IdToken`, or a custom absolute URI. See Token types.
`actorToken`	Optional RFC 8693 delegation actor token (`TokenExchange` grant only). Takes the same `tokenType` values as `subjectToken`.
`requestedTokenType`	Optional token type to request, limited to `AccessToken`, `Jwt`, or `IdToken`, and valid only with the `TokenExchange` grant type. The response must return the type that you request. See Request a token type.
`location`	Where to place the exchanged token in the backend request. Defaults to the `Authorization` header.
`additionalParams`	Extra form parameters appended to the token request. Values are CEL expressions.
`cache`	In-memory token cache. Defaults to 8192 entries. Set `inMemory.maxEntries: 0` to disable.

Verify the exchange

Call the echo tool through the gateway and confirm that the Authorization header the MCP server received carries the exchanged token, not the one you sent.

Port-forward the Keycloak Service and the gateway proxy.

kubectl port-forward -n httpbin svc/keycloak 8080:8080 &
kubectl port-forward -n agentgateway-system svc/agentgateway-proxy 8888:80 &

Mint the incoming token as initial-client. The gateway sends this as the subject_token. Tokens expire, so re-mint if you come back later.

export INBOUND_TOKEN="$(curl -s http://localhost:8080/realms/backend-oauth/protocol/openid-connect/token \
  -u initial-client:initial-secret -d grant_type=password \
  -d username=testuser -d password=testpass | jq -r .access_token)"
echo $INBOUND_TOKEN

Call the echo tool with includeHttpHeaders=true, so that the tool returns the HTTP headers that the MCP server received.

npx @modelcontextprotocol/[email protected] \
  --cli http://localhost:8888/mcp \
  --transport http \
  --method tools/call \
  --tool-name echo \
  --tool-arg input=test \
  --tool-arg includeHttpHeaders=true \
  --header "Authorization: Bearer $INBOUND_TOKEN"

The second content item of the response is the request that reached the MCP server. Note that its authorization header carries a different token than the one you sent.

{
  "method": "POST",
  "url": "/mcp",
  "headers": {
    "mcp-session-id": "5cbdbd08-7f27-4adc-a51e-ef0d987f1166",
    "authorization": "Bearer eyJhbGciOiJSUzI1NiIsInR5cCIgOiAiSldUI..."
  }
}

Copy the exchanged token from that authorization header, and save it to an environment variable.
```
export FORWARDED_TOKEN=eyJhbGciOiJSUzI1NiIsInR5cCIgOiAiSldUI...
```

Decode both tokens to confirm the exchange.

for t in "$INBOUND_TOKEN" "$FORWARDED_TOKEN"; do
  echo "$t" | cut -d. -f2 | jq -R 'gsub("-";"+") | gsub("_";"/") | . + ("=" * ((4 - (length % 4)) % 4)) | @base64d | fromjson | {iss, aud, azp}'
done

The inbound token was issued to initial-client for the requester-client audience. The exchanged token was issued for the target-client audience, with the gateway’s own client (requester-client) as the authorized party.

{
  "iss": "http://keycloak.httpbin.svc.cluster.local:8080/realms/backend-oauth",
  "aud": "requester-client",
  "azp": "initial-client"
}
{
  "iss": "http://keycloak.httpbin.svc.cluster.local:8080/realms/backend-oauth",
  "aud": "target-client",
  "azp": "requester-client"
}

Next steps

Validate the incoming token at the edge. The exchange forwards the incoming token to the authorization server as received, without validating it first. Pair the policy with a route-level JWT authentication or MCP authentication policy so that invalid tokens are rejected before any call to the token endpoint. Set preserveToken: true on it, or the exchange finds no subject_token; for a worked example, see Validate the incoming token at the edge.
Scope the exchanged token per MCP server. Attach a separate policy to each MCP AgentgatewayBackend, each with its own audiences, so every server receives a token that is valid only for itself.
Restrict which tools each caller may reach. Token exchange decides which token the gateway sends, not who is allowed through. Add an MCP authorization policy alongside it.

Cleanup

Stop the port-forwards that you started in Verify the exchange.

kill %1 %2

Then delete the resources.

kubectl delete AgentgatewayPolicy mcp-token-exchange -n httpbin
kubectl delete AgentgatewayBackend mcp-backend-echo keycloak-token-endpoint -n httpbin
kubectl delete httproute mcp-echo -n httpbin
kubectl delete secret oauth-client -n httpbin
kubectl delete deployment echo keycloak -n httpbin
kubectl delete service echo keycloak -n httpbin
kubectl delete configmap backend-oauth-realm -n httpbin

JWT bearer grant (RFC 7523)Cross App Access (ID-JAG)

Was this page helpful?