feat(phase2): complete openai api surfaces 2.3-2.7

- 2.3: unknown model errors (400 + RFC 9457 problem+json with valid_models)
- 2.5: GET /v1/models endpoint (derived from config, not hardcoded)
- 2.6: POST /v1/embeddings passthrough (body-based dispatch, no rewrite)
- 2.7: POST /v1/rerank with path rewrite (/v1/rerank → /rerank)
- wire proxy.Handler in main.go (was using dummy handler)
- 140+ tests passing, race detector clean
- all requests: client → nginx → gateway → upstreams
- ready for config deployment to go live

cmd/gateway/main.go

@@ -10,6 +10,7 @@ import (
 	"syscall"
 
 	"github.com/Riotpiaole/homelab-frontend/internal/config"
+	"github.com/Riotpiaole/homelab-frontend/internal/proxy"
 	"github.com/Riotpiaole/homelab-frontend/internal/server"
 )
 
@@ -30,11 +31,8 @@ func main() {
 		}
 	}
 
-	// Create a basic handler (will be replaced with real routing later)
+	// Create the reverse proxy handler that routes requests based on configuration
-	upstreamHandler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+	upstreamHandler := proxy.New(cfg)
-		w.WriteHeader(http.StatusNotFound)
-		fmt.Fprintf(w, "not found")
-	})
 
 	// Create server with health checker
 	srv := server.New(cfg.ListenAddr, cfg.ShutdownTimeout, nil)

internal/proxy/bodydispatch_test.go

@@ -223,7 +223,7 @@ func TestStreamingUnbuffered(t *testing.T) {
 	}
 }
 
-// TestUnknownModelReject verifies that unknown models are rejected.
+// TestUnknownModelReject verifies that unknown models are rejected with 400 and problem+json.
 func TestUnknownModelReject(t *testing.T) {
 	upstreamServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.WriteHeader(http.StatusOK)
@@ -250,14 +250,31 @@ func TestUnknownModelReject(t *testing.T) {
 
 	requestBody := `{"model":"unknown-model","messages":[]}`
 	resp, _ := http.Post(server.URL+"/v1/chat/completions", "application/json", strings.NewReader(requestBody))
-	resp.Body.Close()
+	defer resp.Body.Close()
 
-	if resp.StatusCode != http.StatusNotFound {
+	if resp.StatusCode != http.StatusBadRequest {
-		t.Errorf("expected 404 for unknown model, got %d", resp.StatusCode)
+		t.Errorf("expected 400 for unknown model, got %d", resp.StatusCode)
+	}
+
+	// Verify problem+json content type
+	ct := resp.Header.Get("Content-Type")
+	if !strings.Contains(ct, "application/problem+json") {
+		t.Errorf("expected content-type application/problem+json, got %s", ct)
+	}
+
+	// Verify response is valid JSON
+	var prob map[string]interface{}
+	if err := json.NewDecoder(resp.Body).Decode(&prob); err != nil {
+		t.Errorf("response is not valid JSON: %v", err)
+	}
+
+	// Verify valid_models is included
+	if prob["valid_models"] == nil {
+		t.Errorf("expected valid_models in problem detail")
 	}
 }
 
-// TestMissingModelField verifies that missing model field is rejected.
+// TestMissingModelField verifies that missing model field is rejected with 400 and problem+json.
 func TestMissingModelField(t *testing.T) {
 	upstreamServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.WriteHeader(http.StatusOK)
@@ -284,10 +301,22 @@ func TestMissingModelField(t *testing.T) {
 
 	requestBody := `{"messages":[]}`
 	resp, _ := http.Post(server.URL+"/v1/chat/completions", "application/json", strings.NewReader(requestBody))
-	resp.Body.Close()
+	defer resp.Body.Close()
 
-	if resp.StatusCode != http.StatusNotFound {
+	if resp.StatusCode != http.StatusBadRequest {
-		t.Errorf("expected 404 for missing model, got %d", resp.StatusCode)
+		t.Errorf("expected 400 for missing model, got %d", resp.StatusCode)
+	}
+
+	// Verify problem+json content type
+	ct := resp.Header.Get("Content-Type")
+	if !strings.Contains(ct, "application/problem+json") {
+		t.Errorf("expected content-type application/problem+json, got %s", ct)
+	}
+
+	// Verify response is valid JSON
+	var prob map[string]interface{}
+	if err := json.NewDecoder(resp.Body).Decode(&prob); err != nil {
+		t.Errorf("response is not valid JSON: %v", err)
 	}
 }
 

internal/proxy/embeddings_and_rerank_test.go

@@ -0,0 +1,393 @@
+package proxy
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/Riotpiaole/homelab-frontend/internal/config"
+)
+
+// TestEmbeddingsPassthroughNoRewrite verifies /v1/embeddings is not rewritten
+func TestEmbeddingsPassthroughNoRewrite(t *testing.T) {
+	embeddingsCalled := false
+	upstreamPath := ""
+
+	upstreamServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		embeddingsCalled = true
+		upstreamPath = r.URL.Path
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusOK)
+		fmt.Fprint(w, `{"object":"list","data":[{"object":"embedding","index":0,"embedding":[0.1,0.2]}]}`)
+	}))
+	defer upstreamServer.Close()
+
+	upstreamAddr := strings.TrimPrefix(upstreamServer.URL, "http://")
+
+	cfg := &config.Config{
+		Models: map[string]*config.ModelUpstream{
+			"nomic-ai/nomic-embed-text-v2-moe": {
+				Name:    "nomic-ai/nomic-embed-text-v2-moe",
+				Address: upstreamAddr,
+			},
+		},
+		Routes: make(map[string]*config.Route),
+	}
+
+	handler := New(cfg)
+	defer handler.Close()
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	resp, _ := http.Post(
+		server.URL+"/v1/embeddings",
+		"application/json",
+		bytes.NewReader([]byte(`{"model":"nomic-ai/nomic-embed-text-v2-moe","input":"hello"}`)),
+	)
+	defer resp.Body.Close()
+
+	if !embeddingsCalled {
+		t.Errorf("upstream embeddings service was not called")
+	}
+
+	// Verify path is NOT rewritten (should stay /v1/embeddings)
+	if upstreamPath != "/v1/embeddings" {
+		t.Errorf("expected upstream path /v1/embeddings, got %s", upstreamPath)
+	}
+}
+
+// TestEmbeddingsResponsePassthrough verifies response body is unmodified
+func TestEmbeddingsResponsePassthrough(t *testing.T) {
+	expectedResponse := `{"object":"list","data":[{"object":"embedding","index":0,"embedding":[0.1,0.2,0.3]}]}`
+
+	upstreamServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusOK)
+		fmt.Fprint(w, expectedResponse)
+	}))
+	defer upstreamServer.Close()
+
+	upstreamAddr := strings.TrimPrefix(upstreamServer.URL, "http://")
+
+	cfg := &config.Config{
+		Models: map[string]*config.ModelUpstream{
+			"nomic-ai/nomic-embed-text-v2-moe": {
+				Name:    "nomic-ai/nomic-embed-text-v2-moe",
+				Address: upstreamAddr,
+			},
+		},
+		Routes: make(map[string]*config.Route),
+	}
+
+	handler := New(cfg)
+	defer handler.Close()
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	resp, _ := http.Post(
+		server.URL+"/v1/embeddings",
+		"application/json",
+		bytes.NewReader([]byte(`{"model":"nomic-ai/nomic-embed-text-v2-moe","input":"test"}`)),
+	)
+	defer resp.Body.Close()
+
+	body, _ := io.ReadAll(resp.Body)
+
+	if string(body) != expectedResponse {
+		t.Errorf("response was modified. Expected:\n%s\n\nGot:\n%s", expectedResponse, string(body))
+	}
+}
+
+// TestRerankPathRewrite verifies /v1/rerank is rewritten to /rerank
+func TestRerankPathRewrite(t *testing.T) {
+	upstreamPath := ""
+
+	upstreamServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		upstreamPath = r.URL.Path
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusOK)
+		fmt.Fprint(w, `{"results":[{"index":0,"score":0.9}]}`)
+	}))
+	defer upstreamServer.Close()
+
+	upstreamAddr := strings.TrimPrefix(upstreamServer.URL, "http://")
+
+	cfg := &config.Config{
+		Models: map[string]*config.ModelUpstream{
+			"BAAI/bge-reranker-base": {
+				Name:    "BAAI/bge-reranker-base",
+				Address: upstreamAddr,
+			},
+		},
+		Routes: make(map[string]*config.Route),
+	}
+
+	handler := New(cfg)
+	defer handler.Close()
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	resp, _ := http.Post(
+		server.URL+"/v1/rerank",
+		"application/json",
+		bytes.NewReader([]byte(`{"model":"BAAI/bge-reranker-base","query":"test","texts":["a","b"]}`)),
+	)
+	defer resp.Body.Close()
+
+	// Verify path IS rewritten to /rerank
+	if upstreamPath != "/rerank" {
+		t.Errorf("expected upstream path /rerank, got %s", upstreamPath)
+	}
+}
+
+// TestRerankResponsePassthrough verifies response is unmodified
+func TestRerankResponsePassthrough(t *testing.T) {
+	expectedResponse := `{"results":[{"index":0,"score":0.95},{"index":1,"score":0.85}]}`
+
+	upstreamServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusOK)
+		fmt.Fprint(w, expectedResponse)
+	}))
+	defer upstreamServer.Close()
+
+	upstreamAddr := strings.TrimPrefix(upstreamServer.URL, "http://")
+
+	cfg := &config.Config{
+		Models: map[string]*config.ModelUpstream{
+			"BAAI/bge-reranker-base": {
+				Name:    "BAAI/bge-reranker-base",
+				Address: upstreamAddr,
+			},
+		},
+		Routes: make(map[string]*config.Route),
+	}
+
+	handler := New(cfg)
+	defer handler.Close()
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	resp, _ := http.Post(
+		server.URL+"/v1/rerank",
+		"application/json",
+		bytes.NewReader([]byte(`{"model":"BAAI/bge-reranker-base","query":"q","texts":["a"]}`)),
+	)
+	defer resp.Body.Close()
+
+	body, _ := io.ReadAll(resp.Body)
+
+	if string(body) != expectedResponse {
+		t.Errorf("response was modified")
+	}
+}
+
+// TestEmbeddingsUnknownModel returns error for unknown model
+func TestEmbeddingsUnknownModel(t *testing.T) {
+	cfg := &config.Config{
+		Models: map[string]*config.ModelUpstream{
+			"nomic-ai/nomic-embed-text-v2-moe": {
+				Name:    "nomic-ai/nomic-embed-text-v2-moe",
+				Address: "localhost:9000",
+			},
+		},
+		Routes: make(map[string]*config.Route),
+	}
+
+	handler := New(cfg)
+	defer handler.Close()
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	resp, _ := http.Post(
+		server.URL+"/v1/embeddings",
+		"application/json",
+		bytes.NewReader([]byte(`{"model":"unknown-embeddings","input":"test"}`)),
+	)
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusBadRequest {
+		t.Errorf("expected 400 for unknown embeddings model, got %d", resp.StatusCode)
+	}
+
+	ct := resp.Header.Get("Content-Type")
+	if !strings.Contains(ct, "application/problem+json") {
+		t.Errorf("expected problem+json for unknown model")
+	}
+}
+
+// TestRerankerUnknownModel returns error for unknown model
+func TestRerankerUnknownModel(t *testing.T) {
+	cfg := &config.Config{
+		Models: map[string]*config.ModelUpstream{
+			"BAAI/bge-reranker-base": {
+				Name:    "BAAI/bge-reranker-base",
+				Address: "localhost:9000",
+			},
+		},
+		Routes: make(map[string]*config.Route),
+	}
+
+	handler := New(cfg)
+	defer handler.Close()
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	resp, _ := http.Post(
+		server.URL+"/v1/rerank",
+		"application/json",
+		bytes.NewReader([]byte(`{"model":"unknown-reranker","query":"q","texts":["a"]}`)),
+	)
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusBadRequest {
+		t.Errorf("expected 400 for unknown reranker model, got %d", resp.StatusCode)
+	}
+}
+
+// TestEmbeddingsBodyForwarded verifies body is byte-identical to upstream
+func TestEmbeddingsBodyForwarded(t *testing.T) {
+	receivedBody := ""
+
+	upstreamServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		body, _ := io.ReadAll(r.Body)
+		receivedBody = string(body)
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusOK)
+		fmt.Fprint(w, `{"object":"list","data":[]}`)
+	}))
+	defer upstreamServer.Close()
+
+	upstreamAddr := strings.TrimPrefix(upstreamServer.URL, "http://")
+
+	cfg := &config.Config{
+		Models: map[string]*config.ModelUpstream{
+			"nomic-ai/nomic-embed-text-v2-moe": {
+				Name:    "nomic-ai/nomic-embed-text-v2-moe",
+				Address: upstreamAddr,
+			},
+		},
+		Routes: make(map[string]*config.Route),
+	}
+
+	handler := New(cfg)
+	defer handler.Close()
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	originalBody := `{"model":"nomic-ai/nomic-embed-text-v2-moe","input":"test data with special chars: \u0001"}`
+	resp, _ := http.Post(
+		server.URL+"/v1/embeddings",
+		"application/json",
+		bytes.NewReader([]byte(originalBody)),
+	)
+	resp.Body.Close()
+
+	// The received body should match the original (though may have different formatting)
+	var orig, received map[string]interface{}
+	json.Unmarshal([]byte(originalBody), &orig)
+	json.Unmarshal([]byte(receivedBody), &received)
+
+	if orig["model"] != received["model"] || orig["input"] != received["input"] {
+		t.Errorf("body was not forwarded correctly")
+	}
+}
+
+// TestRerankerBodyForwarded verifies body is byte-identical to upstream
+func TestRerankerBodyForwarded(t *testing.T) {
+	receivedBody := ""
+
+	upstreamServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		body, _ := io.ReadAll(r.Body)
+		receivedBody = string(body)
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusOK)
+		fmt.Fprint(w, `{"results":[]}`)
+	}))
+	defer upstreamServer.Close()
+
+	upstreamAddr := strings.TrimPrefix(upstreamServer.URL, "http://")
+
+	cfg := &config.Config{
+		Models: map[string]*config.ModelUpstream{
+			"BAAI/bge-reranker-base": {
+				Name:    "BAAI/bge-reranker-base",
+				Address: upstreamAddr,
+			},
+		},
+		Routes: make(map[string]*config.Route),
+	}
+
+	handler := New(cfg)
+	defer handler.Close()
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	originalBody := `{"model":"BAAI/bge-reranker-base","query":"test","texts":["a","b","c"]}`
+	resp, _ := http.Post(
+		server.URL+"/v1/rerank",
+		"application/json",
+		bytes.NewReader([]byte(originalBody)),
+	)
+	resp.Body.Close()
+
+	var orig, received map[string]interface{}
+	json.Unmarshal([]byte(originalBody), &orig)
+	json.Unmarshal([]byte(receivedBody), &received)
+
+	if orig["model"] != received["model"] || orig["query"] != received["query"] {
+		t.Errorf("body was not forwarded correctly")
+	}
+}
+
+// TestUpstreamErrorStatusSurfaced verifies upstream errors are returned as-is
+func TestUpstreamErrorStatusSurfaced(t *testing.T) {
+	upstreamServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusInternalServerError)
+		fmt.Fprint(w, `{"error":"upstream failure"}`)
+	}))
+	defer upstreamServer.Close()
+
+	upstreamAddr := strings.TrimPrefix(upstreamServer.URL, "http://")
+
+	cfg := &config.Config{
+		Models: map[string]*config.ModelUpstream{
+			"nomic-ai/nomic-embed-text-v2-moe": {
+				Name:    "nomic-ai/nomic-embed-text-v2-moe",
+				Address: upstreamAddr,
+			},
+		},
+		Routes: make(map[string]*config.Route),
+	}
+
+	handler := New(cfg)
+	defer handler.Close()
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	resp, _ := http.Post(
+		server.URL+"/v1/embeddings",
+		"application/json",
+		bytes.NewReader([]byte(`{"model":"nomic-ai/nomic-embed-text-v2-moe","input":"test"}`)),
+	)
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusInternalServerError {
+		t.Errorf("expected upstream error status 500, got %d", resp.StatusCode)
+	}
+}

internal/proxy/models_endpoint_test.go

@@ -0,0 +1,443 @@
+package proxy
+
+import (
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"sort"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/Riotpiaole/homelab-frontend/internal/config"
+)
+
+// ModelListResponse represents the response shape for GET /v1/models
+type ModelListResponse struct {
+	Object string       `json:"object"`
+	Data   []ModelEntry `json:"data"`
+}
+
+// ModelEntry represents a single model in the list
+type ModelEntry struct {
+	ID      string `json:"id"`
+	Object  string `json:"object"`
+	OwnedBy string `json:"owned_by"`
+	Created int64  `json:"created"`
+}
+
+// TestModelsEndpointReturns200 verifies GET /v1/models returns 200
+func TestModelsEndpointReturns200(t *testing.T) {
+	cfg := &config.Config{
+		Models: map[string]*config.ModelUpstream{
+			"reasoning": {
+				Name:    "reasoning",
+				Address: "localhost:9000",
+			},
+		},
+		Routes: make(map[string]*config.Route),
+	}
+
+	handler := New(cfg)
+	defer handler.Close()
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	resp, err := http.Get(server.URL + "/v1/models")
+	if err != nil {
+		t.Fatalf("request failed: %v", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		t.Errorf("expected 200, got %d", resp.StatusCode)
+	}
+}
+
+// TestModelsEndpointContentType verifies correct content type
+func TestModelsEndpointContentType(t *testing.T) {
+	cfg := &config.Config{
+		Models: map[string]*config.ModelUpstream{
+			"reasoning": {
+				Name:    "reasoning",
+				Address: "localhost:9000",
+			},
+		},
+		Routes: make(map[string]*config.Route),
+	}
+
+	handler := New(cfg)
+	defer handler.Close()
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	resp, _ := http.Get(server.URL + "/v1/models")
+	defer resp.Body.Close()
+
+	ct := resp.Header.Get("Content-Type")
+	if !strings.Contains(ct, "application/json") {
+		t.Errorf("expected content-type application/json, got %s", ct)
+	}
+}
+
+// TestModelsEndpointResponseShape verifies correct JSON structure
+func TestModelsEndpointResponseShape(t *testing.T) {
+	cfg := &config.Config{
+		Models: map[string]*config.ModelUpstream{
+			"reasoning": {
+				Name:    "reasoning",
+				Address: "localhost:9000",
+			},
+		},
+		Routes: make(map[string]*config.Route),
+	}
+
+	handler := New(cfg)
+	defer handler.Close()
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	resp, _ := http.Get(server.URL + "/v1/models")
+	defer resp.Body.Close()
+
+	var result ModelListResponse
+	if err := json.NewDecoder(resp.Body).Decode(&result); err != nil {
+		t.Fatalf("failed to decode response: %v", err)
+	}
+
+	if result.Object != "list" {
+		t.Errorf("expected object='list', got %q", result.Object)
+	}
+
+	if len(result.Data) != 1 {
+		t.Errorf("expected 1 model, got %d", len(result.Data))
+	}
+
+	model := result.Data[0]
+	if model.ID != "reasoning" {
+		t.Errorf("expected id='reasoning', got %q", model.ID)
+	}
+
+	if model.Object != "model" {
+		t.Errorf("expected object='model', got %q", model.Object)
+	}
+}
+
+// TestModelsEndpointEnumeratesAllModels verifies all models are listed
+func TestModelsEndpointEnumeratesAllModels(t *testing.T) {
+	cfg := &config.Config{
+		Models: map[string]*config.ModelUpstream{
+			"reasoning": {
+				Name:    "reasoning",
+				Address: "localhost:9000",
+			},
+			"ornith:35b": {
+				Name:    "ornith:35b",
+				Address: "localhost:9000",
+			},
+			"qwen2.5:3b-instruct": {
+				Name:    "qwen2.5:3b-instruct",
+				Address: "localhost:9000",
+			},
+			"nomic-ai/nomic-embed-text-v2-moe": {
+				Name:    "nomic-ai/nomic-embed-text-v2-moe",
+				Address: "localhost:9000",
+			},
+			"BAAI/bge-reranker-base": {
+				Name:    "BAAI/bge-reranker-base",
+				Address: "localhost:9000",
+			},
+		},
+		Routes: make(map[string]*config.Route),
+	}
+
+	handler := New(cfg)
+	defer handler.Close()
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	resp, _ := http.Get(server.URL + "/v1/models")
+	defer resp.Body.Close()
+
+	var result ModelListResponse
+	json.NewDecoder(resp.Body).Decode(&result)
+
+	if len(result.Data) != 5 {
+		t.Errorf("expected 5 models, got %d", len(result.Data))
+	}
+
+	// Collect actual model IDs
+	modelIDs := make(map[string]bool)
+	for _, model := range result.Data {
+		modelIDs[model.ID] = true
+	}
+
+	// Verify all expected models are present
+	expectedModels := []string{
+		"reasoning",
+		"ornith:35b",
+		"qwen2.5:3b-instruct",
+		"nomic-ai/nomic-embed-text-v2-moe",
+		"BAAI/bge-reranker-base",
+	}
+
+	for _, expected := range expectedModels {
+		if !modelIDs[expected] {
+			t.Errorf("expected model %q in response", expected)
+		}
+	}
+}
+
+// TestModelsEndpointHasRequiredFields verifies all required fields are present
+func TestModelsEndpointHasRequiredFields(t *testing.T) {
+	cfg := &config.Config{
+		Models: map[string]*config.ModelUpstream{
+			"reasoning": {
+				Name:    "reasoning",
+				Address: "localhost:9000",
+			},
+		},
+		Routes: make(map[string]*config.Route),
+	}
+
+	handler := New(cfg)
+	defer handler.Close()
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	resp, _ := http.Get(server.URL + "/v1/models")
+	defer resp.Body.Close()
+
+	var result ModelListResponse
+	json.NewDecoder(resp.Body).Decode(&result)
+
+	model := result.Data[0]
+	if model.ID == "" {
+		t.Errorf("expected id field")
+	}
+
+	if model.Object == "" {
+		t.Errorf("expected object field")
+	}
+
+	if model.OwnedBy == "" {
+		t.Errorf("expected owned_by field")
+	}
+
+	if model.Created == 0 {
+		t.Errorf("expected created field (unix timestamp)")
+	}
+}
+
+// TestModelsEndpointNoUpstreamContact verifies endpoint doesn't contact upstream
+func TestModelsEndpointNoUpstreamContact(t *testing.T) {
+	upstreamCalled := false
+	upstreamServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		upstreamCalled = true
+		w.WriteHeader(http.StatusOK)
+	}))
+	defer upstreamServer.Close()
+
+	upstreamAddr := strings.TrimPrefix(upstreamServer.URL, "http://")
+
+	cfg := &config.Config{
+		Models: map[string]*config.ModelUpstream{
+			"reasoning": {
+				Name:    "reasoning",
+				Address: upstreamAddr,
+			},
+		},
+		Routes: make(map[string]*config.Route),
+	}
+
+	handler := New(cfg)
+	defer handler.Close()
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	_, _ = http.Get(server.URL + "/v1/models")
+
+	if upstreamCalled {
+		t.Errorf("upstream should not be called for /v1/models endpoint")
+	}
+}
+
+// TestModelsEndpointDerivedFromConfig verifies models come from config, not hardcoded
+func TestModelsEndpointDerivedFromConfig(t *testing.T) {
+	// Create config with specific models
+	cfg := &config.Config{
+		Models: map[string]*config.ModelUpstream{
+			"custom-model-1": {
+				Name:    "custom-model-1",
+				Address: "localhost:9000",
+			},
+			"custom-model-2": {
+				Name:    "custom-model-2",
+				Address: "localhost:9000",
+			},
+		},
+		Routes: make(map[string]*config.Route),
+	}
+
+	handler := New(cfg)
+	defer handler.Close()
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	resp, _ := http.Get(server.URL + "/v1/models")
+	defer resp.Body.Close()
+
+	var result ModelListResponse
+	json.NewDecoder(resp.Body).Decode(&result)
+
+	// Verify only the configured models are returned
+	if len(result.Data) != 2 {
+		t.Errorf("expected 2 models from config, got %d", len(result.Data))
+	}
+
+	modelIDs := make([]string, len(result.Data))
+	for i, model := range result.Data {
+		modelIDs[i] = model.ID
+	}
+	sort.Strings(modelIDs)
+
+	expected := []string{"custom-model-1", "custom-model-2"}
+	if !equal(modelIDs, expected) {
+		t.Errorf("expected models %v, got %v", expected, modelIDs)
+	}
+}
+
+// TestModelsEndpointConsistentWithDispatch verifies advertised models can dispatch
+func TestModelsEndpointConsistentWithDispatch(t *testing.T) {
+	upstreamServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte(`{"choices":[{"message":{"content":"ok"}}]}`))
+	}))
+	defer upstreamServer.Close()
+
+	upstreamAddr := strings.TrimPrefix(upstreamServer.URL, "http://")
+
+	cfg := &config.Config{
+		Models: map[string]*config.ModelUpstream{
+			"reasoning": {
+				Name:    "reasoning",
+				Address: upstreamAddr,
+			},
+			"ornith:35b": {
+				Name:    "ornith:35b",
+				Address: upstreamAddr,
+			},
+		},
+		Routes: make(map[string]*config.Route),
+	}
+
+	handler := New(cfg)
+	defer handler.Close()
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	// Get list of models
+	resp, _ := http.Get(server.URL + "/v1/models")
+	var result ModelListResponse
+	json.NewDecoder(resp.Body).Decode(&result)
+	resp.Body.Close()
+
+	// Try to dispatch to each advertised model
+	for _, model := range result.Data {
+		dispatchResp, _ := http.Post(
+			server.URL+"/v1/chat/completions",
+			"application/json",
+			strings.NewReader(`{"model":"`+model.ID+`","messages":[]}`),
+		)
+		defer dispatchResp.Body.Close()
+
+		// Should not return 400 (unknown model error)
+		if dispatchResp.StatusCode == http.StatusBadRequest {
+			body, _ := io.ReadAll(dispatchResp.Body)
+			if strings.Contains(string(body), "unknown model") {
+				t.Errorf("model %q advertised in /v1/models but not accepted for dispatch", model.ID)
+			}
+		}
+	}
+}
+
+// TestModelsEndpointResponseIsConsistent verifies response is consistent across calls
+func TestModelsEndpointResponseIsConsistent(t *testing.T) {
+	cfg := &config.Config{
+		Models: map[string]*config.ModelUpstream{
+			"reasoning": {
+				Name:    "reasoning",
+				Address: "localhost:9000",
+			},
+			"ornith:35b": {
+				Name:    "ornith:35b",
+				Address: "localhost:9000",
+			},
+		},
+		Routes: make(map[string]*config.Route),
+	}
+
+	handler := New(cfg)
+	defer handler.Close()
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	// Call endpoint twice
+	resp1, _ := http.Get(server.URL + "/v1/models")
+	var result1 ModelListResponse
+	json.NewDecoder(resp1.Body).Decode(&result1)
+	resp1.Body.Close()
+
+	time.Sleep(10 * time.Millisecond)
+
+	resp2, _ := http.Get(server.URL + "/v1/models")
+	var result2 ModelListResponse
+	json.NewDecoder(resp2.Body).Decode(&result2)
+	resp2.Body.Close()
+
+	// Verify both responses have same models
+	if len(result1.Data) != len(result2.Data) {
+		t.Errorf("response length inconsistent: %d vs %d", len(result1.Data), len(result2.Data))
+	}
+
+	ids1 := make([]string, len(result1.Data))
+	ids2 := make([]string, len(result2.Data))
+
+	for i, m := range result1.Data {
+		ids1[i] = m.ID
+	}
+	for i, m := range result2.Data {
+		ids2[i] = m.ID
+	}
+
+	sort.Strings(ids1)
+	sort.Strings(ids2)
+
+	if !equal(ids1, ids2) {
+		t.Errorf("responses differ: %v vs %v", ids1, ids2)
+	}
+}
+
+// Helper function to compare string slices
+func equal(a, b []string) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for i := range a {
+		if a[i] != b[i] {
+			return false
+		}
+	}
+	return true
+}

internal/proxy/proxy.go

@@ -2,12 +2,14 @@
 package proxy
 
 import (
+	"encoding/json"
 	"fmt"
 	"io"
 	"net"
 	"net/http"
 	"net/http/httputil"
 	"net/url"
+	"sort"
 	"strings"
 	"time"
 
@@ -37,6 +39,26 @@ type Route struct {
 	Transport *http.Transport
 }
 
+// Error types for model validation
+type modelValidationError struct {
+	Kind    string // "invalid_json", "missing_model", "unknown_model"
+	Message string
+	Model   string // only for unknown_model
+}
+
+func (e *modelValidationError) Error() string {
+	return e.Message
+}
+
+// RFC 9457 Problem Details
+type problemDetail struct {
+	Type       string        `json:"type"`
+	Title      string        `json:"title"`
+	Status     int           `json:"status"`
+	Detail     string        `json:"detail"`
+	ValidModels []string     `json:"valid_models,omitempty"`
+}
+
 // New creates a new reverse proxy handler from configuration.
 // It sets up connection pooling and rewriting rules for each route.
 func New(cfg *config.Config) *Handler {
@@ -174,10 +196,79 @@ func getPeerIP(remoteAddr string) string {
 	return remoteAddr
 }
 
+// writeProblemDetail writes an RFC 9457 problem detail response.
+func writeProblemDetail(w http.ResponseWriter, status int, problemType, title, detail string, validModels []string) {
+	w.Header().Set("Content-Type", "application/problem+json")
+	w.WriteHeader(status)
+
+	problem := problemDetail{
+		Type:        problemType,
+		Title:       title,
+		Status:      status,
+		Detail:      detail,
+		ValidModels: validModels,
+	}
+
+	json.NewEncoder(w).Encode(problem)
+}
+
 // ServeHTTP implements http.Handler.
 func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	// Handle /v1/models endpoint (no routing needed, derived from config)
+	if r.URL.Path == "/v1/models" && r.Method == "GET" {
+		h.handleModelsEndpoint(w, r)
+		return
+	}
+
 	// Try to find a matching route (including body-based dispatch for /v1/chat/completions)
 	route, err := h.RouteRequest(r)
+
+	// Check if this is a model validation error (from body-based dispatch)
+	if validationErr, ok := err.(*modelValidationError); ok {
+		// This is a client error, not a routing error
+		var status int
+		var problemType string
+		var title string
+		var detail string
+
+		switch validationErr.Kind {
+		case "invalid_json":
+			status = http.StatusBadRequest
+			problemType = "https://api.example.com/problems/invalid-request-body"
+			title = "Invalid Request Body"
+			detail = validationErr.Message
+		case "missing_model", "empty_model", "null_model":
+			status = http.StatusBadRequest
+			problemType = "https://api.example.com/problems/missing-model"
+			title = "Missing Model"
+			detail = "The 'model' field is required and must be a non-empty string"
+		case "unknown_model":
+			status = http.StatusBadRequest
+			problemType = "https://api.example.com/problems/unknown-model"
+			title = "Unknown Model"
+			detail = fmt.Sprintf("Model %q is not available. See valid_models for available options.", validationErr.Model)
+		default:
+			status = http.StatusBadRequest
+			problemType = "https://api.example.com/problems/invalid-request"
+			title = "Invalid Request"
+			detail = validationErr.Message
+		}
+
+		// Get list of valid models (only for model-related errors)
+		var validModels []string
+		if validationErr.Kind == "unknown_model" || validationErr.Kind == "missing_model" || validationErr.Kind == "empty_model" || validationErr.Kind == "null_model" {
+			validModels = h.getValidModels()
+		}
+
+		writeProblemDetail(w, status, problemType, title, detail, validModels)
+		logging.Errorf("client error", validationErr, map[string]string{
+			"path":   r.URL.Path,
+			"method": r.Method,
+			"reason": validationErr.Kind,
+		})
+		return
+	}
+
 	if err != nil || route == nil {
 		// Route not found or error determining route
 		w.WriteHeader(http.StatusNotFound)
@@ -252,6 +343,57 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 
 
 
+// getValidModels returns a sorted list of all configured model names.
+func (h *Handler) getValidModels() []string {
+	var models []string
+	for name := range h.config.Models {
+		models = append(models, name)
+	}
+	sort.Strings(models)
+	return models
+}
+
+// modelsListResponse represents the response for GET /v1/models
+type modelsListResponse struct {
+	Object string              `json:"object"`
+	Data   []modelsListEntry   `json:"data"`
+}
+
+// modelsListEntry represents a single model in the list
+type modelsListEntry struct {
+	ID      string `json:"id"`
+	Object  string `json:"object"`
+	OwnedBy string `json:"owned_by"`
+	Created int64  `json:"created"`
+}
+
+// handleModelsEndpoint serves GET /v1/models
+// Returns a list of all configured models, derived from config not hardcoded
+func (h *Handler) handleModelsEndpoint(w http.ResponseWriter, r *http.Request) {
+	// Get all model names from config
+	modelNames := h.getValidModels()
+
+	// Build the response
+	data := make([]modelsListEntry, len(modelNames))
+	for i, name := range modelNames {
+		data[i] = modelsListEntry{
+			ID:      name,
+			Object:  "model",
+			OwnedBy: "api.riotpiao.com",
+			Created: 1700000000, // Fixed timestamp; can be made configurable if needed
+		}
+	}
+
+	response := modelsListResponse{
+		Object: "list",
+		Data:   data,
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(http.StatusOK)
+	json.NewEncoder(w).Encode(response)
+}
+
 // Close closes all underlying transports, releasing their connection pools.
 func (h *Handler) Close() error {
 	for _, transport := range h.transports {

internal/proxy/router.go

@@ -13,12 +13,12 @@ import (
 )
 
 // RouteRequest determines which upstream should handle the request.
-// For /v1/chat/completions, it uses body-based dispatch (reads JSON to find "model" field).
+// For /v1/chat/completions, /v1/embeddings, and /v1/rerank, it uses body-based dispatch.
-// For other routes, it looks up by path prefix.
+// For other routes, it looks up by path in the configured routes.
 func (h *Handler) RouteRequest(r *http.Request) (*Route, error) {
-	// For /v1/chat/completions, use body-based dispatch
+	// For /v1/chat/completions, /v1/embeddings, /v1/rerank use body-based dispatch
-	if r.URL.Path == "/v1/chat/completions" && r.Method == "POST" {
+	if r.Method == "POST" && (r.URL.Path == "/v1/chat/completions" || r.URL.Path == "/v1/embeddings" || r.URL.Path == "/v1/rerank") {
-		return h.routeByModel(r)
+		return h.routeByModel(r, r.URL.Path)
 	}
 
 	// For other paths, try to find a matching route by path
@@ -45,17 +45,25 @@ func (h *Handler) RouteRequest(r *http.Request) (*Route, error) {
 
 // routeByModel reads the request body to find the "model" field and routes accordingly.
 // The body is preserved for forwarding to the upstream.
-func (h *Handler) routeByModel(r *http.Request) (*Route, error) {
+// Returns a modelValidationError for client errors (invalid JSON, missing/unknown model).
+// The path parameter indicates which endpoint is being called (/v1/chat/completions, /v1/embeddings, /v1/rerank)
+func (h *Handler) routeByModel(r *http.Request, path string) (*Route, error) {
 	// If there's no body, we can't determine the model
 	if r.Body == nil {
-		return nil, fmt.Errorf("request body required")
+		return nil, &modelValidationError{
+			Kind:    "missing_model",
+			Message: "request body required",
+		}
 	}
 
 	// Read the body to extract the model name
 	// We need to be careful to preserve the body for the upstream
 	bodyBytes, err := io.ReadAll(r.Body)
 	if err != nil {
-		return nil, fmt.Errorf("failed to read request body: %w", err)
+		return nil, &modelValidationError{
+			Kind:    "invalid_request",
+			Message: fmt.Sprintf("failed to read request body: %v", err),
+		}
 	}
 
 	// Restore the body so it can be read again by the upstream
@@ -64,26 +72,68 @@ func (h *Handler) routeByModel(r *http.Request) (*Route, error) {
 	// Parse the JSON to find the model field
 	var payload map[string]interface{}
 	if err := json.Unmarshal(bodyBytes, &payload); err != nil {
-		return nil, fmt.Errorf("invalid JSON in request body: %w", err)
+		return nil, &modelValidationError{
+			Kind:    "invalid_json",
+			Message: "request body is not valid JSON",
+		}
 	}
 
 	// Extract the model name
-	modelName, ok := payload["model"].(string)
+	modelVal, hasModel := payload["model"]
+	if !hasModel {
+		return nil, &modelValidationError{
+			Kind:    "missing_model",
+			Message: "'model' field is missing",
+		}
+	}
+
+	// Handle null model
+	if modelVal == nil {
+		return nil, &modelValidationError{
+			Kind:    "null_model",
+			Message: "'model' field is null",
+		}
+	}
+
+	// Extract as string
+	modelName, ok := modelVal.(string)
 	if !ok {
-		return nil, fmt.Errorf("model field missing or not a string")
+		return nil, &modelValidationError{
+			Kind:    "missing_model",
+			Message: "'model' field must be a string",
+		}
+	}
+
+	// Handle empty string
+	if modelName == "" {
+		return nil, &modelValidationError{
+			Kind:    "empty_model",
+			Message: "'model' field cannot be empty",
+		}
 	}
 
 	// Look up the model in the registry
 	modelUpstream := h.config.LookupModel(modelName)
 	if modelUpstream == nil {
-		return nil, fmt.Errorf("unknown model: %q", modelName)
+		return nil, &modelValidationError{
+			Kind:    "unknown_model",
+			Message: fmt.Sprintf("unknown model: %q", modelName),
+			Model:   modelName,
+		}
+	}
+
+	// Determine the upstream path based on the request path
+	upstreamPath := path
+	if path == "/v1/rerank" {
+		// Rerank endpoint uses /rerank path on upstream
+		upstreamPath = "/rerank"
 	}
 
 	// Create a route for this model with appropriate timeouts
 	// These are sensible defaults for LLM models
 	upstreamCfg := &config.Upstream{
 		Address:        modelUpstream.Address,
-		PathRewrite:    "/v1/chat/completions",
+		PathRewrite:    upstreamPath,
 		ConnectTimeout: h.defaultConnectTimeout,
 		ReadTimeout:    h.defaultReadTimeout,
 		WriteTimeout:   h.defaultWriteTimeout,

internal/proxy/unknown_model_test.go

@@ -0,0 +1,489 @@
+package proxy
+
+import (
+	"bytes"
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/Riotpiaole/homelab-frontend/internal/config"
+)
+
+// RFC 9457 Problem Details
+type ProblemDetail struct {
+	Type     string                 `json:"type"`
+	Title    string                 `json:"title"`
+	Status   int                    `json:"status"`
+	Detail   string                 `json:"detail"`
+	Instance string                 `json:"instance,omitempty"`
+	Extra    map[string]interface{} `json:"-"`
+}
+
+// UnmarshalJSON allows capturing extra fields
+func (p *ProblemDetail) UnmarshalJSON(data []byte) error {
+	type Alias ProblemDetail
+	aux := &struct {
+		*Alias
+	}{
+		Alias: (*Alias)(p),
+	}
+	if err := json.Unmarshal(data, &aux); err != nil {
+		return err
+	}
+
+	// Capture extra fields
+	var raw map[string]interface{}
+	json.Unmarshal(data, &raw)
+	extra := make(map[string]interface{})
+	for k, v := range raw {
+		if k != "type" && k != "title" && k != "status" && k != "detail" && k != "instance" {
+			extra[k] = v
+		}
+	}
+	p.Extra = extra
+	return nil
+}
+
+// TestUnknownModelReturns4xx verifies unknown model returns client error
+func TestUnknownModelReturns4xx(t *testing.T) {
+	cfg := &config.Config{
+		Models: map[string]*config.ModelUpstream{
+			"reasoning": {
+				Name:    "reasoning",
+				Address: "reasoning-predictor:80",
+				Path:    "/v1/chat/completions",
+			},
+			"ornith:35b": {
+				Name:    "ornith:35b",
+				Address: "ornith-predictor:80",
+				Path:    "/v1/chat/completions",
+			},
+		},
+		Routes: make(map[string]*config.Route),
+	}
+
+	handler := New(cfg)
+	defer handler.Close()
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	resp, err := http.Post(
+		server.URL+"/v1/chat/completions",
+		"application/json",
+		bytes.NewReader([]byte(`{"model":"gpt-4","messages":[]}`)),
+	)
+	if err != nil {
+		t.Fatalf("request failed: %v", err)
+	}
+	defer resp.Body.Close()
+
+	// Verify 4xx status
+	if resp.StatusCode < 400 || resp.StatusCode >= 500 {
+		t.Errorf("expected 4xx status for unknown model, got %d", resp.StatusCode)
+	}
+
+	// Verify RFC 9457 content type
+	ct := resp.Header.Get("Content-Type")
+	if !strings.Contains(ct, "application/problem+json") {
+		t.Errorf("expected content-type application/problem+json, got %s", ct)
+	}
+
+	// Verify response is valid problem detail
+	var prob ProblemDetail
+	body, _ := io.ReadAll(resp.Body)
+	if err := json.Unmarshal(body, &prob); err != nil {
+		t.Errorf("response is not valid JSON: %v", err)
+	}
+
+	if prob.Status == 0 {
+		t.Errorf("expected status in problem detail")
+	}
+
+	if prob.Title == "" {
+		t.Errorf("expected title in problem detail")
+	}
+}
+
+// TestUnknownModelEnumeratesValidModels verifies all models are listed
+func TestUnknownModelEnumeratesValidModels(t *testing.T) {
+	cfg := &config.Config{
+		Models: map[string]*config.ModelUpstream{
+			"reasoning": {
+				Name:    "reasoning",
+				Address: "reasoning-predictor:80",
+			},
+			"ornith:35b": {
+				Name:    "ornith:35b",
+				Address: "ornith-predictor:80",
+			},
+			"qwen2.5:3b-instruct": {
+				Name:    "qwen2.5:3b-instruct",
+				Address: "ornith-predictor:80",
+			},
+		},
+		Routes: make(map[string]*config.Route),
+	}
+
+	handler := New(cfg)
+	defer handler.Close()
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	resp, _ := http.Post(
+		server.URL+"/v1/chat/completions",
+		"application/json",
+		bytes.NewReader([]byte(`{"model":"unknown","messages":[]}`)),
+	)
+	defer resp.Body.Close()
+
+	var prob map[string]interface{}
+	json.NewDecoder(resp.Body).Decode(&prob)
+
+	// Check for valid_models field (as an extra field beyond RFC 9457)
+	validModels, hasModels := prob["valid_models"]
+	if !hasModels {
+		t.Errorf("expected valid_models field in problem detail")
+		return
+	}
+
+	models := validModels.([]interface{})
+	if len(models) != 3 {
+		t.Errorf("expected 3 models in valid_models, got %d", len(models))
+	}
+
+	modelNames := make(map[string]bool)
+	for _, m := range models {
+		modelNames[m.(string)] = true
+	}
+
+	expectedModels := []string{"reasoning", "ornith:35b", "qwen2.5:3b-instruct"}
+	for _, expected := range expectedModels {
+		if !modelNames[expected] {
+			t.Errorf("expected model %s in valid_models", expected)
+		}
+	}
+}
+
+// TestMissingModelFieldReturns4xx verifies missing model field is client error
+func TestMissingModelFieldReturns4xx(t *testing.T) {
+	cfg := &config.Config{
+		Models: map[string]*config.ModelUpstream{
+			"reasoning": {
+				Name:    "reasoning",
+				Address: "reasoning-predictor:80",
+			},
+		},
+		Routes: make(map[string]*config.Route),
+	}
+
+	handler := New(cfg)
+	defer handler.Close()
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	// Request with no model field
+	resp, _ := http.Post(
+		server.URL+"/v1/chat/completions",
+		"application/json",
+		bytes.NewReader([]byte(`{"messages":[]}`)),
+	)
+	defer resp.Body.Close()
+
+	if resp.StatusCode < 400 || resp.StatusCode >= 500 {
+		t.Errorf("expected 4xx for missing model, got %d", resp.StatusCode)
+	}
+
+	ct := resp.Header.Get("Content-Type")
+	if !strings.Contains(ct, "application/problem+json") {
+		t.Errorf("expected problem+json for missing model")
+	}
+
+	// Verify body does not contain the request
+	body, _ := io.ReadAll(resp.Body)
+	if strings.Contains(string(body), "messages") {
+		t.Errorf("response should not echo request body")
+	}
+}
+
+// TestNullModelFieldReturns4xx verifies null model is client error
+func TestNullModelFieldReturns4xx(t *testing.T) {
+	cfg := &config.Config{
+		Models: map[string]*config.ModelUpstream{
+			"reasoning": {
+				Name:    "reasoning",
+				Address: "reasoning-predictor:80",
+			},
+		},
+		Routes: make(map[string]*config.Route),
+	}
+
+	handler := New(cfg)
+	defer handler.Close()
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	// Request with null model
+	resp, _ := http.Post(
+		server.URL+"/v1/chat/completions",
+		"application/json",
+		bytes.NewReader([]byte(`{"model":null,"messages":[]}`)),
+	)
+	defer resp.Body.Close()
+
+	if resp.StatusCode < 400 || resp.StatusCode >= 500 {
+		t.Errorf("expected 4xx for null model, got %d", resp.StatusCode)
+	}
+
+	ct := resp.Header.Get("Content-Type")
+	if !strings.Contains(ct, "application/problem+json") {
+		t.Errorf("expected problem+json for null model")
+	}
+}
+
+// TestEmptyModelFieldReturns4xx verifies empty model string is client error
+func TestEmptyModelFieldReturns4xx(t *testing.T) {
+	cfg := &config.Config{
+		Models: map[string]*config.ModelUpstream{
+			"reasoning": {
+				Name:    "reasoning",
+				Address: "reasoning-predictor:80",
+			},
+		},
+		Routes: make(map[string]*config.Route),
+	}
+
+	handler := New(cfg)
+	defer handler.Close()
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	// Request with empty model string
+	resp, _ := http.Post(
+		server.URL+"/v1/chat/completions",
+		"application/json",
+		bytes.NewReader([]byte(`{"model":"","messages":[]}`)),
+	)
+	defer resp.Body.Close()
+
+	if resp.StatusCode < 400 || resp.StatusCode >= 500 {
+		t.Errorf("expected 4xx for empty model, got %d", resp.StatusCode)
+	}
+}
+
+// TestInvalidJSONIsDistinguishableError verifies invalid JSON is separate from unknown model
+func TestInvalidJSONIsDistinguishableError(t *testing.T) {
+	cfg := &config.Config{
+		Models: map[string]*config.ModelUpstream{
+			"reasoning": {
+				Name:    "reasoning",
+				Address: "reasoning-predictor:80",
+			},
+		},
+		Routes: make(map[string]*config.Route),
+	}
+
+	handler := New(cfg)
+	defer handler.Close()
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	// Request with invalid JSON
+	resp, _ := http.Post(
+		server.URL+"/v1/chat/completions",
+		"application/json",
+		bytes.NewReader([]byte(`not json`)),
+	)
+	defer resp.Body.Close()
+
+	if resp.StatusCode < 400 || resp.StatusCode >= 500 {
+		t.Errorf("expected 4xx for invalid JSON, got %d", resp.StatusCode)
+	}
+
+	var prob map[string]interface{}
+	json.NewDecoder(resp.Body).Decode(&prob)
+
+	// Invalid JSON error should mention JSON parsing, not model
+	detail := prob["detail"].(string)
+	if !strings.Contains(strings.ToLower(detail), "json") {
+		t.Errorf("expected detail to mention JSON for invalid JSON error")
+	}
+}
+
+// TestUnknownModelDoesNotContactUpstream verifies no upstream call is made
+func TestUnknownModelDoesNotContactUpstream(t *testing.T) {
+	upstreamCalled := false
+	upstreamServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		upstreamCalled = true
+		w.WriteHeader(http.StatusOK)
+	}))
+	defer upstreamServer.Close()
+
+	upstreamAddr := strings.TrimPrefix(upstreamServer.URL, "http://")
+
+	cfg := &config.Config{
+		Models: map[string]*config.ModelUpstream{
+			"reasoning": {
+				Name:    "reasoning",
+				Address: upstreamAddr,
+			},
+		},
+		Routes: make(map[string]*config.Route),
+	}
+
+	handler := New(cfg)
+	defer handler.Close()
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	_, _ = http.Post(
+		server.URL+"/v1/chat/completions",
+		"application/json",
+		bytes.NewReader([]byte(`{"model":"unknown","messages":[]}`)),
+	)
+
+	if upstreamCalled {
+		t.Errorf("upstream should not be called for unknown model")
+	}
+}
+
+// TestUnknownModelLogsReason verifies rejection is logged
+func TestUnknownModelLogsReason(t *testing.T) {
+	cfg := &config.Config{
+		Models: map[string]*config.ModelUpstream{
+			"reasoning": {
+				Name:    "reasoning",
+				Address: "reasoning-predictor:80",
+			},
+		},
+		Routes: make(map[string]*config.Route),
+	}
+
+	handler := New(cfg)
+	defer handler.Close()
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	// This test verifies logging behavior by checking the handler's logger output
+	// In a real scenario, you'd capture stderr or use a test logger
+	_, _ = http.Post(
+		server.URL+"/v1/chat/completions",
+		"application/json",
+		bytes.NewReader([]byte(`{"model":"gpt-4","messages":[]}`)),
+	)
+
+	// Logging is verified by checking that no panic occurs
+	// and the request completes successfully
+}
+
+// TestMissingModelAndUnknownModelBothReturn4xx verifies consistent error class
+func TestMissingModelAndUnknownModelBothReturn4xx(t *testing.T) {
+	cfg := &config.Config{
+		Models: map[string]*config.ModelUpstream{
+			"reasoning": {
+				Name:    "reasoning",
+				Address: "reasoning-predictor:80",
+			},
+		},
+		Routes: make(map[string]*config.Route),
+	}
+
+	handler := New(cfg)
+	defer handler.Close()
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	// Test missing model
+	resp1, _ := http.Post(
+		server.URL+"/v1/chat/completions",
+		"application/json",
+		bytes.NewReader([]byte(`{"messages":[]}`)),
+	)
+	resp1.Body.Close()
+
+	// Test unknown model
+	resp2, _ := http.Post(
+		server.URL+"/v1/chat/completions",
+		"application/json",
+		bytes.NewReader([]byte(`{"model":"unknown","messages":[]}`)),
+	)
+	resp2.Body.Close()
+
+	// Both should be in 4xx range
+	if resp1.StatusCode < 400 || resp1.StatusCode >= 500 {
+		t.Errorf("expected 4xx for missing model, got %d", resp1.StatusCode)
+	}
+
+	if resp2.StatusCode < 400 || resp2.StatusCode >= 500 {
+		t.Errorf("expected 4xx for unknown model, got %d", resp2.StatusCode)
+	}
+
+	// Both should be problem+json
+	ct1 := resp1.Header.Get("Content-Type")
+	ct2 := resp2.Header.Get("Content-Type")
+
+	if !strings.Contains(ct1, "application/problem+json") {
+		t.Errorf("expected problem+json for missing model")
+	}
+
+	if !strings.Contains(ct2, "application/problem+json") {
+		t.Errorf("expected problem+json for unknown model")
+	}
+}
+
+// TestProblemDetailHasRequiredFields verifies RFC 9457 compliance
+func TestProblemDetailHasRequiredFields(t *testing.T) {
+	cfg := &config.Config{
+		Models: map[string]*config.ModelUpstream{
+			"reasoning": {
+				Name:    "reasoning",
+				Address: "reasoning-predictor:80",
+			},
+		},
+		Routes: make(map[string]*config.Route),
+	}
+
+	handler := New(cfg)
+	defer handler.Close()
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	resp, _ := http.Post(
+		server.URL+"/v1/chat/completions",
+		"application/json",
+		bytes.NewReader([]byte(`{"model":"unknown","messages":[]}`)),
+	)
+	defer resp.Body.Close()
+
+	var prob map[string]interface{}
+	json.NewDecoder(resp.Body).Decode(&prob)
+
+	// RFC 9457 required fields
+	if prob["type"] == nil {
+		t.Errorf("expected 'type' field in problem detail")
+	}
+
+	if prob["title"] == nil {
+		t.Errorf("expected 'title' field in problem detail")
+	}
+
+	if prob["status"] == nil {
+		t.Errorf("expected 'status' field in problem detail")
+	}
+
+	if prob["detail"] == nil {
+		t.Errorf("expected 'detail' field in problem detail")
+	}
+}

testdata/config/harness-with-models.yaml

@@ -0,0 +1,26 @@
+# Local development harness config with model registry for body-based dispatch.
+# This config is used for testing task 2.2 (body-based dispatch) and task 2.3 (error handling).
+# Every upstream points at the single stub server on loopback 127.0.0.1:9080.
+
+routes: []
+
+models:
+  - name: "reasoning"
+    address: "127.0.0.1:9080"
+    path: "/v1/chat/completions"
+  
+  - name: "ornith:35b"
+    address: "127.0.0.1:9080"
+    path: "/v1/chat/completions"
+  
+  - name: "qwen2.5:3b-instruct"
+    address: "127.0.0.1:9080"
+    path: "/v1/chat/completions"
+  
+  - name: "nomic-ai/nomic-embed-text-v2-moe"
+    address: "127.0.0.1:9080"
+    path: "/v1/embeddings"
+  
+  - name: "BAAI/bge-reranker-base"
+    address: "127.0.0.1:9080"
+    path: "/v1/rerank"