Fix router model selection: filter available_models, remove same-provider override, iterate candidates

[aashna]

Problem

Three issues cause the router's model recommendation to be silently overridden on the client, sending simple queries to expensive models:

1. available_models / knownEndpoints out of sync

knownEndpoints (from GET /models) and available_models (from POST /auto_models) are separate CAPI calls with independent caches (10-min vs 5-min refresh). When a model like claude-haiku-4.5 appears in available_models before the Models API returns it, the router recommends it but the client can't resolve it to an endpoint — causing a silent fallback to the first available (often the most expensive) model.

Observed in logs: Router returns no_reasoning at 96% confidence with candidate_models: [claude-haiku-4.5, gpt-4.1, ...] for "what day is today", but the actual request goes to claude-sonnet-4.6.

2. Same-provider override negates router decisions

When the router re-runs after /compact or background summarization, _findSameProviderModel overrides the router's recommendation with a same-provider model, often switching to a more expensive one.

3. Only candidate_models[0] attempted

The fallback only tried the first candidate model. If that one model had no matching endpoint, all remaining candidates were discarded.

Fix

Filter available_models before routing

Intersect available_models with knownEndpoints before sending to the router. The router now only sees models the client can actually serve, eliminating the sync gap at the source.

const knownModelIds = new Set(knownEndpoints.map(e => e.model));
const routableModels = token.available_models.filter(m => knownModelIds.has(m));

Remove same-provider override from router path

When the router runs, trust its ranked candidate list directly. Same-provider preference is retained in _selectDefaultModel (the non-router fallback path).

Iterate all candidates via _findFirstAvailableModel

Walk the full candidate_models list instead of only trying [0], as defense-in-depth.

// Before:
const selectedModel = (entry?.endpoint && this._findSameProviderModel(...))
    ?? knownEndpoints.find(e => e.model === result.candidate_models[0]);

// After:
const selectedModel = this._findFirstAvailableModel(result.candidate_models, knownEndpoints);

Forward has_image to router instead of early-returning

Image requests no longer bypass the router. Instead, has_image: true is sent so the server can factor it in. Vision fallback (_applyVisionFallback) still handles downstream model selection if the routed model doesn't support vision.

Emit automode.routerModelSelection telemetry (from #310238)

New event reports candidateModel vs actualModel with overrideReason and filteredModelCount so analysts can detect overrides and CAPI sync gaps without fragile telemetry joins.

Add RouterDecisionError with server error code parsing

Non-OK router responses now parse the JSON error body and surface the error code (e.g. no_vision_models) as the fallbackReason instead of a generic routerError.

Warn-level logs for debuggability

All sync mismatches and fallbacks now emit warn-level logs with full context instead of being silently swallowed.

Behavioral changes

• Image requests now go through the router (with has_image: true) instead of being skipped entirely. Vision fallback still applies after routing.
• conversationId passed to router uses sessionResource ?? sessionId ?? 'unknown' (matching cache key) instead of raw chatRequest?.sessionId.

Supersedes #310238.

[Copilot]

Pull request overview

Updates Copilot’s automode router model selection so that when the router returns a ranked candidate_models list, the service selects the first candidate that actually exists in the user’s configured endpoint list (instead of only trying candidate_models[0]).

Changes:

• Iterate through candidate_models to find the first model with a matching known endpoint.
• Keep same-provider preference, but apply it against the candidate list before falling back to the first available candidate endpoint.

Show a summary per file

File	Description
extensions/copilot/src/platform/endpoint/node/automodeService.ts	Adjusts router-result handling to choose the first viable endpoint from the router’s ordered candidates.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 2

[github-actions]

Screenshot Changes

Base: 4ab3ca55 Current: e5dff969

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blocks-ci screenshots changed

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