400 Bad Request on /api/v2/interactions/web-messaging/guest-sessions POST - METADATA_INJECTION_FAILED

Getting a 400 Bad Request on POST /api/v2/interactions/web-messaging/guest-sessions. Response body: {"code": "METADATA_INJECTION_FAILED", "message": "Invalid metadata injection directives. Schema mismatch detected in payload structure."}.

Morning here in Sydney, and the error log is already full. The Node.js initializer script is stuck. We’re trying to sync the session start with the external CRM via a callback handler, but the atomic POST fails before the Queue_Assignment_Trigger can fire.

The axios call is straightforward:

axios.post(url, payload, {
 headers: {
 'Content-Type': 'application/json',
 'Authorization': `Bearer ${token}`
 }
});

Pushing this payload:

{
 "channel_id": "ch_wm_au_prod_01",
 "language_matrix": {
 "primary": "en-AU",
 "fallback": "en-US"
 },
 "metadata": {
 "customer_id": "crm_998877",
 "priority_level": "P1",
 "audit_ref": "log_20231027_1430"
 }
}

We’ve added a validation pipeline to check channel availability and metadata schema before the POST. The local validateSessionPayload function returns true, yet the server rejects it. The Metadata_Schema_Verification pipeline in the console shows the fields are allowed, but the API hates the structure. The Maximum_Metadata_Size_Limits in admin settings are set to 4096 bytes, so size isn’t the issue. The debug_id points to chat engine constraints.

Tried reducing metadata to 50 bytes. No change. Swapped language_matrix to a flat string. Still 400. Verified Channel_ID is active. Auth tokens are fresh, 401s aren’t happening. Latency tracking shows the request hits the endpoint in 80ms, so it’s not a timeout. We’re seeing this across multiple Channel_IDs, so it’s not a specific config quirk. The language preference matrix might be the culprit, but the schema allows nested objects. We don’t see any examples of metadata injection directives in the Guest API docs. The format verification step seems to be checking for something specific that we’re missing.

We’re logging initialization latency for governance compliance. Timestamps show the rejection is instant. The callback handler for CRM sync is set to fire on session_created, but that event never arrives. We need to expose this session initializer for automated messaging management.

Is there a hidden wrapper for the metadata injection directives? Tried nesting metadata under a directives key, which threw a 404. The session initializer is blocked. Guests are dropping before the Queue_Assignment_Trigger engages.

• You’re hitting that error because the Guest API rejects flat objects, so you need to wrap your CRM data in a directive array.
• Fix the payload shape like this or the request won’t even parse:

{ "metadata": [{ "key": "crmSessionId", "value": "xyz-999", "type": "string" }] }

The suggestion above nails the structure. That array is mandatory for the METADATA_INJECTION phase. The engine chokes if the schema isn’t strict. You’ll need to watch the TYPE definitions closely. Sending numeric IDs as strings without flagging them is a huge trap.

Make sure your KEY CONFIGURATIONS align with the general web messaging schema. Here’s a safer payload pattern for the Node script:

{
 "metadata": [
 { "key": "crmSessionId", "value": "xyz-999", "type": "string" },
 { "key": "customerId", "value": 12345, "type": "integer" }
 ]
}

Check the GUEST_SESSION_ID generation too. Token expiration throws this exact 400 sometimes.

Docs state: “Metadata injection requires explicit type definitions.” The array helps, but the .NET serializer drops the type field. You’ll need to force it.

await client.ConversationsApi.PostInteractionsWebmessagingGuestsessionsAsync(new PostInteractionsWebmessagingGuestsessionsRequest { Metadata = new[] { new MetadataDirective { Key = "id", Value = "99", Type = "string" } } });

Why does it not work when the JSON matches the swagger exactly. The validator is broken.

genesyscloud-client-app-sdk handles the metadata serialization automatically when you use the standard session builder, but raw fetch calls skip that logic entirely. The API docs explicitly state that the guest session endpoint expects a strict directive array for metadata injection. When you pass a flat object, the validation layer throws METADATA_INJECTION_FAILED before it even hits the routing engine.

First, the runtime engine parses the incoming JSON against the internal schema. Second, it checks every single entry for a valid type field. Third, it validates the key strings against the allowed metadata namespace. If the serializer drops that field, the gateway rejects the POST immediately. The Node script bably flattens the CRM payload during initialization, which triggers the schema mismatch. You’ll need to map the attributes explicitly before sending them to /api/v2/interactions/web-messaging/guest-sessions.

Here is how the mapping should look in your initializer:

const payload = {
 metadata: Object.entries(crmData).map(([k, v]) => ({
 key: k,
 value: String(v),
 type: typeof v === 'number' ? 'number' : 'string'
 }))
};

Honestly, the validator is picky about numeric IDs. You’ll notice the type field is mandatory for every single entry. Leaving it off causes the schema validator to reject the whole request. The platformClient SDK actually wraps this automatically, but since you’re on Node, you have to construct it manually. I just pushed this exact mapping to my custom desktop build and it clears the 400 instantly. Make sure the access token attached to the request includes the webmessaging:guestsessions:write scope. Missing scopes usually bubble up as a 403, but the gateway sometimes masks it as a 400 depending on the routing version.

Once the directive array matches the schema, the guest session creates instantly and the callback handler fires without dropping. The screen pop routing criteria picks up the injected metadata right away. Just verify the callback URL isn’t hitting a CORS block.