Azure Functions App host is not able to start

Question

Azure Functions App host is not able to start

Jakob Nielsen from Make Digital Solutions ApS 5

Since yesterday, the Azure Functions host has not been able to start. No changes have been made to the functions.

I am getting the following output in the trace log:

6/13/2025 13:59 Job host stopped

6/13/2025 13:59 Stopping JobHost

6/13/2025 13:59 0 functions loaded

6/13/2025 13:59 0 functions found (Custom)

6/13/2025 13:59 Reading functions metadata (Custom)

6/13/2025 13:59 Loading functions metadata

6/13/2025 13:59 "ConcurrencyOptions

{

""DynamicConcurrencyEnabled"": false,

""MaximumFunctionConcurrency"": 500,

""CPUThreshold"": 0.8,

""SnapshotPersistenceEnabled"": true

}"

6/13/2025 13:59 "HttpOptions

{

""DynamicThrottlesEnabled"": true,

""EnableChunkedRequestBinding"": false,

""MaxConcurrentRequests"": 100,

""MaxOutstandingRequests"": 200,

""RoutePrefix"": ""api""

}"

6/13/2025 13:59 0 functions loaded

6/13/2025 13:59 0 functions found (Custom)

6/13/2025 13:59 Reading functions metadata (Custom)

6/13/2025 13:59 Loading functions metadata

6/13/2025 13:59 Job host started

6/13/2025 13:59 Host started (10343ms)

6/13/2025 13:59 Host initialized (10330ms)

6/13/2025 13:59 "HttpOptions

{

""DynamicThrottlesEnabled"": true,

""EnableChunkedRequestBinding"": false,

""MaxConcurrentRequests"": 100,

""MaxOutstandingRequests"": 200,

""RoutePrefix"": ""api""

}"

6/13/2025 13:59 "Initializing function HTTP routes

No HTTP routes mapped"

6/13/2025 13:59 No job functions found. Try making your job classes and methods public. If you're using binding extensions (e.g. Azure Storage, ServiceBus, Timers, etc.) make sure you've called the registration method for the extension(s) in your startup code (e.g. builder.AddAzureStorage(), builder.AddServiceBus(), builder.AddTimers(), etc.).

6/13/2025 13:59 Generating 0 job function(s)

6/13/2025 13:59 0 functions loaded

6/13/2025 13:59 0 functions found (Custom)

6/13/2025 13:59 Exceeded language worker restart retry count for runtime:node. Shutting down and proactively recycling the Functions Host to recover

6/13/2025 13:59 Reading functions metadata (Custom)

6/13/2025 13:59 Failed to start language worker process for runtime: node. workerId:b95428fb-2f61-4b79-a39a-11fc6e4dda0c

6/13/2025 13:59 Initializing worker process failed

6/13/2025 13:59 Host lock lease acquired by instance ID '88f52b640d06944933df50d1f5e3a10a'.

6/13/2025 13:59 Loading functions metadata

6/13/2025 13:59 Starting Host (HostId=project-vejle-api, InstanceId=446b2ee0-95b3-4ba7-9e96-3390f9cedb98, Version=4.1040.200.25278, ProcessId=5324, AppDomainId=1, InDebugMode=True, InDiagnosticMode=False, FunctionsExtensionVersion=~4)

6/13/2025 13:59 Starting JobHost

Khadeer Ali 5,990 Reputation points Microsoft External Staff Moderator

2025-06-13T14:53:14.2133333+00:00
@Jakob Nielsen from Make Digital Solutions ApS ,

Thanks for reaching out.

Based on the logs you've shared, the root cause appears to be that the Azure Functions host isn't detecting any valid functions to load. The message "0 functions loaded" and "No job functions found" confirms that no executable functions were discovered during host startup.

Additionally, the log line "Failed to start language worker process for runtime: node" suggests that the Node.js language worker is failing to initialize, which is critical for Node-based Azure Functions to run.

From my understanding, this typically happens when:

The function.json or index.js files are missing or incorrectly structured.

There's a mismatch or issue with the configured Node.js version.

The directory structure doesn't follow the expected pattern.

There’s an unhandled error during function initialization.

Since you mentioned no recent changes were made, I’d recommend verifying the following just to rule out any silent failures from previous deployments:

Ensure that each function directory contains a valid function.json and entry script (like index.js).

Confirm that your host.json is syntactically correct and doesn’t have any runtime-breaking configurations.

Check your application settings for the correct WEBSITE_NODE_DEFAULT_VERSION.

Try running the function app locally using func start to see if any initialization errors are surfaced more clearly.
Jakob Nielsen from Make Digital Solutions ApS 5 Reputation points

2025-06-14T10:22:58.4733333+00:00
@Khadeer Ali thank you for your reply and suggestions. Much appreciated.

I did a func start in the local repo and it started without any errors or warnings. The functions are recognized, and I was able to execute functions locally.

I think this indicates that functions.json and index.js are OK, that the directory structure is also OK.

Node version is set to Node.js 20 LTS in the Function App. WEBSITE_NODE_DEFAULT_VERSION is set to ~20.

This is the host.json:

{ "version": "2.0", "logging": { "applicationInsights": { "samplingSettings": { "isEnabled": true, "excludedTypes": "Request" } } }, "extensionBundle": { "id": "Microsoft.Azure.Functions.ExtensionBundle", "version": "[4.*, 5.0.0)" }, "functionTimeout": "00:10:00" }
Khadeer Ali 5,990 Reputation points Microsoft External Staff Moderator

2025-06-16T12:23:49.0866667+00:00

@Jakob Nielsen from Make Digital Solutions ApS

Please respond over private message to investigate the issue.
Jakob Nielsen from Make Digital Solutions ApS 5 Reputation points

2025-06-16T13:48:51.89+00:00

How do I send you a private message @Khadeer Ali

1 answer

Your answer

Khadeer Ali 5,990 Reputation points Microsoft External Staff Moderator

2025-06-13T14:53:14.2133333+00:00

@Jakob Nielsen from Make Digital Solutions ApS ,

Thanks for reaching out.

Based on the logs you've shared, the root cause appears to be that the Azure Functions host isn't detecting any valid functions to load. The message "0 functions loaded" and "No job functions found" confirms that no executable functions were discovered during host startup.

Additionally, the log line "Failed to start language worker process for runtime: node" suggests that the Node.js language worker is failing to initialize, which is critical for Node-based Azure Functions to run.

From my understanding, this typically happens when:

The function.json or index.js files are missing or incorrectly structured.

There's a mismatch or issue with the configured Node.js version.

The directory structure doesn't follow the expected pattern.

There’s an unhandled error during function initialization.

Since you mentioned no recent changes were made, I’d recommend verifying the following just to rule out any silent failures from previous deployments:

Ensure that each function directory contains a valid function.json and entry script (like index.js).

Confirm that your host.json is syntactically correct and doesn’t have any runtime-breaking configurations.

Check your application settings for the correct WEBSITE_NODE_DEFAULT_VERSION.

Try running the function app locally using func start to see if any initialization errors are surfaced more clearly.
Jakob Nielsen from Make Digital Solutions ApS 5 Reputation points

2025-06-14T10:22:58.4733333+00:00

@Khadeer Ali thank you for your reply and suggestions. Much appreciated.

I did a func start in the local repo and it started without any errors or warnings. The functions are recognized, and I was able to execute functions locally.

I think this indicates that functions.json and index.js are OK, that the directory structure is also OK.

Node version is set to Node.js 20 LTS in the Function App. WEBSITE_NODE_DEFAULT_VERSION is set to ~20.

This is the host.json:

{ "version": "2.0", "logging": { "applicationInsights": { "samplingSettings": { "isEnabled": true, "excludedTypes": "Request" } } }, "extensionBundle": { "id": "Microsoft.Azure.Functions.ExtensionBundle", "version": "[4.*, 5.0.0)" }, "functionTimeout": "00:10:00" }
Khadeer Ali 5,990 Reputation points Microsoft External Staff Moderator

2025-06-16T12:23:49.0866667+00:00

@Jakob Nielsen from Make Digital Solutions ApS

Please respond over private message to investigate the issue.
Jakob Nielsen from Make Digital Solutions ApS 5 Reputation points

2025-06-16T13:48:51.89+00:00

How do I send you a private message @Khadeer Ali

Answer 1

@Jakob Nielsen from Make Digital Solutions ApS ,

Thanks for your patience!
To summarize our discussion so far:

The functions are working correctly locally, indicating the function.json, index.js, and directory structure are all valid.

You correctly checked the Kudu (SCM) site under site/wwwroot for an app_offline.htm file — which is known to sometimes cause the host to remain in an offline state post-deployment — but confirmed that the file was not present.

You also verified the Node.js version is set to LTS (~20) and reviewed the host.json file, which all looked fine.

As a next step, you pinned the runtime version to 4.1039.500.25252, which was the version in use when the app was working last week. After doing this, the Functions Host was successfully able to start and load the functions.

This indicates that the issue could be related to a recent runtime update, and reverting to the known stable version resolved the problem.

Please feel free to reach out if you'd like to explore this further or need help keeping the runtime fixed until a more stable update is available.

If this answers your query, do click Accept Answer and Yes for "Was this answer helpful." And if you have any further questions, let us know.

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Azure Functions App host is not able to start

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