feat(models): emit the context window under three client dialects

OpenAI's /v1/models schema carries no context field, so every client that
wants one reads a vendor dialect, and a client whose dialect is absent does
not ask — it substitutes its own default. GET /v1/models now emits the same
resolved integer under "context_window", "context_length" (the OpenRouter
spelling) and "max_model_len" (the vLLM spelling). All three are omitted
together when the window is unknown, exactly as the single field was.

Measured case: omp 18.1.15's openai-models-list discovery resolves
`max_model_len ?? context_length ?? <bundled catalog> ?? 128000` and never
reads context_window, so a 1048576-context local model reached the client as
128000, which also clamped its maxTokens.

The metadata fetcher already parses all of these dialects on the inbound
side (provider_openai.go), so this makes the outbound direction speak what
the inbound one already understands.

Claude-Session: https://claude.ai/code/session_0168ukk3uwp8PgiuRAzeLPXp

README.md

@@ -666,8 +666,8 @@ identical file with nothing to review.
 
 ### `GET /v1/models` extension fields
 
-Two OpenAI-compat-safe extra fields on each model object, present only
-when known:
+OpenAI-compat-safe extra fields on each model object, present only when
+known:
 
 ```json
 {
@@ -675,11 +675,19 @@ when known:
   "object": "model",
   "owned_by": "openai",
   "context_window": 400000,
+  "context_length": 400000,
+  "max_model_len": 400000,
   "pricing": {"input_per_mtok_usd": 1.25, "output_per_mtok_usd": 10}
 }
 ```
 
-- `context_window` — an integer token count, omitted when unknown.
+- `context_window`, `context_length`, `max_model_len` — the same integer
+  token count under all three dialect spellings, omitted together when
+  unknown. OpenAI's own schema has no context field, so each client reads
+  whichever vendor dialect it was built against: `context_length` is the
+  OpenRouter spelling, `max_model_len` the vLLM one. A client whose
+  dialect is missing does not ask — it silently substitutes its own
+  default (128k is a common one), so all three are emitted.
 - `pricing` — `{input_per_mtok_usd, output_per_mtok_usd}`, both floats
   in USD per 1,000,000 tokens. Omitted when unknown; present with `0`
   values for an explicitly free model — a known zero, not an absent

registry.go

@@ -1563,17 +1563,29 @@ func (m *modelRegistry) aliasEntriesFor(grp *group) map[string]map[string]any {
 }
 
 // modelObject builds one OpenAI-compatible model list entry, extended
-// with two OpenAI-compat-safe extension fields (feature v0.23):
-// "context_window" (int) and "pricing" ({"input_per_mtok_usd",
-// "output_per_mtok_usd"} floats, USD per million tokens) — both omitted
-// entirely when meta reports them unknown, never emitted as a
-// misleading zero. "pricing" IS emitted with zeros for an explicitly
-// free model (meta.CostKnown true, both cost fields 0) — that is a
-// known, meaningful zero, not an absent one.
+// with OpenAI-compat-safe extension fields (feature v0.23): the context
+// window and "pricing" ({"input_per_mtok_usd", "output_per_mtok_usd"}
+// floats, USD per million tokens) — both omitted entirely when meta
+// reports them unknown, never emitted as a misleading zero. "pricing" IS
+// emitted with zeros for an explicitly free model (meta.CostKnown true,
+// both cost fields 0) — that is a known, meaningful zero, not an absent
+// one.
+//
+// The context window is emitted under THREE keys carrying the identical
+// integer: "context_window", "context_length" (the OpenRouter spelling)
+// and "max_model_len" (the vLLM spelling). OpenAI's own /v1/models
+// schema has no context field at all, so every client that wants one
+// reads some vendor dialect, and a client whose dialect is missing
+// silently substitutes its own default instead of asking. Emitting all
+// three costs two map entries and removes that whole failure class —
+// the same reason the metadata FETCHER already parses all of these
+// dialects on the inbound side (provider_openai.go's modelMetadata).
 func modelObject(id, ownedBy string, meta resolvedModelMeta) map[string]any {
 	obj := map[string]any{"id": id, "object": "model", "owned_by": ownedBy}
 	if meta.ContextKnown {
 		obj["context_window"] = meta.ContextTokens
+		obj["context_length"] = meta.ContextTokens
+		obj["max_model_len"] = meta.ContextTokens
 	}
 	if meta.CostKnown {
 		obj["pricing"] = map[string]any{

registry_modelmeta_test.go

@@ -375,3 +375,36 @@ func TestModelRegistry_RefreshProvider_MetadataPanicDoesNotFailDiscovery(t *test
 		t.Error("want at least one log line recording the metadata-capture panic")
 	}
 }
+
+// TestModelObject_ContextDialectMirrors proves every model object carries its
+// context window under all three dialect keys OpenAI-compatible clients
+// actually read — "context_window", "context_length" and "max_model_len" —
+// always the identical integer, and that all three vanish together when the
+// window is unknown.
+//
+// Client-side reason this exists (2026-09-10): omp's openai-models-list
+// discovery reads `max_model_len ?? context_length ?? <bundled catalog> ??
+// 128000` and never looks at context_window, so a 1M-context local model was
+// advertised to it as the 128000 default. Other clients read the OpenRouter
+// spelling (context_length). One resolved value, three spellings, no
+// per-client configuration.
+//
+// Mutation that must make this test fail: drop either mirror key from
+// modelObject, or let one carry a different value from context_window.
+func TestModelObject_ContextDialectMirrors(t *testing.T) {
+	t.Parallel()
+
+	known := modelObject("m", "prov", resolvedModelMeta{ContextTokens: 1048576, ContextKnown: true})
+	for _, key := range []string{"context_window", "context_length", "max_model_len"} {
+		if known[key] != 1048576 {
+			t.Errorf("known[%q] = %v, want 1048576", key, known[key])
+		}
+	}
+
+	unknown := modelObject("m", "prov", resolvedModelMeta{})
+	for _, key := range []string{"context_window", "context_length", "max_model_len"} {
+		if v, present := unknown[key]; present {
+			t.Errorf("unknown[%q] present (%v), want omitted entirely", key, v)
+		}
+	}
+}