Describe the bug

Apollo Router v2.5.0 throws an internal error "Unexpectedly missing edge for __typename field" when executing queries that use inline fragments on interface objects. This occurs when querying interface objects from one subgraph and using inline fragments for interfaces defined in another subgraph, despite successful supergraph composition.

To Reproduce

Setup

1. Create two subgraph definitions with the following schemas:

Subgraph A (subgraph_a.graphql):

extend schema
  @link(
    url: "https://specs.apollo.dev/federation/v2.3"
    import: ["@key", "@interfaceObject"]
  )

type IVehicle @key(fields: "id", resolvable: false) @interfaceObject {
  id: ID!
}

type ILandVehicle @key(fields: "id", resolvable: false) @interfaceObject {
  id: ID!
}

type Query {
  myVehicles_A: [IVehicle]!
}

Subgraph B (subgraph_b.graphql):

extend schema
  @link(
    url: "https://specs.apollo.dev/federation/v2.3"
    import: ["@key", "@shareable", "@external", "@interfaceObject"]
  )

interface IVehicle @key(fields: "id") {
  id: ID!
  color: String
}

interface ILandVehicle implements IVehicle @key(fields: "id") {
  id: ID!
  color: String
  numberOfWheels: Int!
}

type Bike implements ILandVehicle & IVehicle @key(fields: "id") {
  id: ID!
  color: String
  numberOfWheels: Int!
}

type Query {
  myVehicles_B: [IVehicle]!
}

2. Compose the supergraph using Rover:

rover supergraph compose --config ./supergraph_config.yaml --elv2-license=accept > ./supergraph.graphql

Supergraph config (supergraph_config.yaml):

subgraphs:
  graphA:
    routing_url: http://localhost:4010
    schema:
      file: ./subgraphs/subgraph_a.graphql
  graphB:
    routing_url: http://localhost:4011
    schema:
      file: ./subgraphs/subgraph_b.graphql

3. Start the Apollo Router:

./router --dev --supergraph supergraph.graphql

4. Submit the following GraphQL query:

query Test {
  myVehicles_A {
    id
    ... on ILandVehicle {
      color
    }
  }
}

Expected behavior

The query should execute successfully since:

• The supergraph composition succeeds without errors
• The generated supergraph contains all necessary type relationships
• ILandVehicle properly implements IVehicle in the federation schema
• The inline fragment ... on ILandVehicle is valid against the IVehicle return type

Output

Error Response:

{
  "errors": [
    {
      "message": "value retrieval failed: Federation error: An internal error has occurred, please report this bug to Apollo.\n\nDetails: Unexpectedly missing edge for __typename field",
      "extensions": {
        "code": "INTERNAL_SERVER_ERROR"
      }
    }
  ]
}

Note: The same query pattern works correctly when querying myVehicles_B instead of myVehicles_A, confirming that the issue is specifically related to interface objects rather than the general federation setup.

Desktop (please complete the following information):

• OS: macOS
• Apollo Router Version: 2.5.0
• Rover Version: 0.35.0 (used for composition)

Additional context

• The issue appears to be in the query planner's handling of __typename field resolution when dealing with interface objects and inline fragments
• This is a valid federation pattern where one subgraph defines interface objects (stubs) and another defines the actual interface implementations
• The supergraph schema correctly shows the relationships between types with proper @join__implements directives
• Adding __typename explicitly to the query does not resolve the issue
• This error suggests an internal bug in the router's query planning logic rather than a schema composition issue

Minimal reproduction repository: The issue can be reproduced with just the two subgraph files and supergraph config shown above. No running subgraph servers are needed as this issue appears to occur in the query validation or exection planning stage.