mcpServerSettings is silently dropped for CustomContainer session pools — code-interpreter-custom sample cannot work

Summary

The code-interpreter-custom sample (and the Custom code interpreter tool for agents doc that walks through it) deploys a Microsoft.App/sessionPools resource with containerType: 'CustomContainer' and mcpServerSettings: { isMcpServerEnabled: true }, then reads sessionPool.properties.mcpServerSettings.mcpServerEndpoint and calls fetchMCPServerCredentials.

In practice, the ARM RP accepts and silently drops mcpServerSettings for CustomContainer pools: the deployment succeeds, but a GET on the pool returns mcpServerSettings: null, mcpServerEndpoint is never issued, and fetchMCPServerCredentials fails with:

ERROR: Bad Request({"error":{"code":"SessionMCPServerNotEnabled","message":"Session pool '<pool>' does not have MCP Server enabled.","traceId":"fc0fd0d9859c929e74c91f49dbe563c0"}})

The identical mcpServerSettings block on a containerType: 'Shell' pool works immediately (MCP endpoint issued), so this is specific to the CustomContainer + MCP combination — the exact combination this sample is built on.

What we tested (2026-07-17)

SessionPoolsSupportMCP AFEC feature Registered, Microsoft.App provider re-registered (state Registered) well before all tests. All pools reached provisioningState: Succeeded.

Also ruled out:

• Property casing: both isMcpServerEnabled (as in the sample's infra.bicep) and isMCPServerEnabled (as in the Container Apps MCP tutorials) behave identically — dropped on CustomContainer, honored on Shell.
• Update vs. create: PATCH/PUT with mcpServerSettings on an existing CustomContainer pool returns success (provisioningState: Succeeded) but the property remains null. Freshly created pools behave the same, so it is not an update-only limitation.
• Region: reproduced in both eastus2 and westus2 (the region used in the tutorials).
• Other feature flags: az feature list --namespace Microsoft.App shows no other MCP-related gate; SessionPoolsSupportMCP is the only one and it is Registered.

The CustomContainer test pools used the same image and configuration as the sample (mcr.microsoft.com/k8se/services/codeinterpreter:0.9.18-python3.12, the four SYS_RUNTIME_SANDBOX/AZURE_CODE_EXEC_ENV/AZURECONTAINERAPPS_SESSIONS_SANDBOX_VERSION/JUPYTER_TOKEN env vars, ingress targetPort: 6000).

Minimal repro

# Feature registered, provider re-registered beforehand:
az feature show --namespace Microsoft.App --name SessionPoolsSupportMCP --query properties.state   # Registered
az provider show -n Microsoft.App --query registrationState                                        # Registered

# Shell pool — MCP works:
az rest --method put \
  --url "/subscriptions/<sub>/resourceGroups/<rg>/providers/Microsoft.App/sessionPools/mcpshell?api-version=2025-10-02-preview" \
  --body '{"location":"eastus2","properties":{"poolManagementType":"Dynamic","containerType":"Shell","scaleConfiguration":{"maxConcurrentSessions":5},"sessionNetworkConfiguration":{"status":"EgressEnabled"},"dynamicPoolConfiguration":{"lifecycleConfiguration":{"lifecycleType":"Timed","cooldownPeriodInSeconds":300}},"mcpServerSettings":{"isMCPServerEnabled":true}}}'
# GET afterwards -> "mcpServerSettings": { "isMCPServerEnabled": true, "mcpServerEndpoint": "https://eastus2.dynamicsessions.io/..." }

# CustomContainer pool (sample-equivalent) — mcpServerSettings silently dropped:
az rest --method put \
  --url "/subscriptions/<sub>/resourceGroups/<rg>/providers/Microsoft.App/sessionPools/mcpcustom?api-version=2025-10-02-preview" \
  --body '{"location":"eastus2","properties":{"environmentId":"<managed-environment-id>","poolManagementType":"Dynamic","containerType":"CustomContainer","scaleConfiguration":{"maxConcurrentSessions":5,"readySessionInstances":1},"sessionNetworkConfiguration":{"status":"EgressEnabled"},"dynamicPoolConfiguration":{"lifecycleConfiguration":{"lifecycleType":"Timed","cooldownPeriodInSeconds":300}},"customContainerTemplate":{"containers":[{"name":"codeinterpreter","image":"mcr.microsoft.com/k8se/services/codeinterpreter:0.9.18-python3.12","env":[{"name":"SYS_RUNTIME_SANDBOX","value":"AzureContainerApps-DynamicSessions"},{"name":"AZURE_CODE_EXEC_ENV","value":"AzureContainerApps-DynamicSessions-Py3.12"},{"name":"AZURECONTAINERAPPS_SESSIONS_SANDBOX_VERSION","value":"7758"},{"name":"JUPYTER_TOKEN","value":"AzureContainerApps-DynamicSessions"}],"resources":{"cpu":1,"memory":"2Gi"}}],"ingress":{"targetPort":6000}},"mcpServerSettings":{"isMCPServerEnabled":true}}}'
# GET afterwards -> "mcpServerSettings": null, provisioningState Succeeded

Impact on the sample

Because mcpServerSettings never persists on the CustomContainer pool:

• sessionPool.properties.mcpServerSettings.mcpServerEndpoint in infra.bicep evaluates to null and the Foundry connection target is empty/invalid.
• The deployment script's az rest --method post --url "$SESSION_POOL_ID/fetchMCPServerCredentials?..." fails with SessionMCPServerNotEnabled, so the whole az deployment group create run fails.

Documentation cross-references

• Custom code interpreter tool for agents (preview) — declares this scenario ("create an agent that uses a custom code interpreter MCP server"), requires only the SessionPoolsSupportMCP feature registration, and points at this sample's infra.bicep.
• Azure Container Apps custom container sessions — does not mention the platform-managed MCP server at all.
• The platform-managed MCP tutorials only cover Shell and PythonLTS pool types.

Questions

1. Is the platform-managed MCP server actually supported for containerType: 'CustomContainer' session pools today? If yes, what is missing beyond SessionPoolsSupportMCP registration (extra gate, allow-list, specific regions)?
2. If it is not yet rolled out, can the sample and the custom-code-interpreter doc be updated to say so — and ideally can the RP reject mcpServerSettings on unsupported pool types instead of silently dropping it? The silent drop turns a configuration error into a late, confusing SessionMCPServerNotEnabled failure.

Environment

• Date tested: 2026-07-17
• Regions: eastus2, westus2
• API versions: 2025-02-02-preview, 2025-10-02-preview
• SessionPoolsSupportMCP: Registered; Microsoft.App provider: Registered
• Example failure traceId: fc0fd0d9859c929e74c91f49dbe563c0 (eastus2, fetchMcpServerCredentials → SessionMCPServerNotEnabled)