llama: preserve field order in user-defined JSON schemas (ollama#8002)

Previously we decoded and re-encoded JSON schemas during validation,
which served no purpose since json.RawMessage already validates JSON
syntax. Worse, the re-encoding lost field ordering from the original
schema, which affects inference quality during step-by-step reasoning.

While fixing this ordering issue by using json.RawMessage directly,
testing revealed that schema_to_grammar (from llama.cpp) also fails to
preserve field order during grammar generation. This appears to be the
root cause of inference degradation.

This change prevents us from mangling the user's original schema order,
but we still need to address the ordering issue in schema_to_grammar.
That will be a separate change.

Updates ollama#7978

llama/grammar_test.go

@@ -0,0 +1,80 @@
+package llama
+
+import (
+	"bufio"
+	"bytes"
+	"strings"
+	"testing"
+)
+
+// https://github.com/ollama/ollama/issues/7978
+const issue7978JSONSchema = `{
+  "type": "object",
+  "properties": {
+    "steps": {
+      "type": "array",
+      "items": {
+	"type": "object",
+	"properties": {
+	  "explanation": { "type": "string" },
+	  "output": { "type": "string" }
+	},
+	"required": ["explanation", "output"],
+	"additionalProperties": false
+      }
+    },
+    "final_answer": { "type": "string" }
+  },
+  "required": ["steps", "final_answer"],
+  "additionalProperties": false
+}`
+
+func TestIssue7978(t *testing.T) {
+	t.Skip("schema_to_grammar is broken; skipping until fixed")
+
+	g := SchemaToGrammar([]byte(issue7978JSONSchema))
+	if g == nil {
+		t.Fatal("failed to convert JSON schema to grammar")
+	}
+
+	t.Logf("grammar:\n%s", g)
+	t.Log()
+
+	var sawSteps bool
+	s := bufio.NewScanner(bytes.NewReader(g))
+	for s.Scan() {
+		line := s.Text()
+		if strings.Contains(line, "steps") {
+			sawSteps = true
+		}
+		if strings.Contains(line, "final-answer") && !sawSteps {
+			t.Error("expected 'steps' before 'final-answer'")
+		}
+	}
+}
+
+func TestSchemaToGrammer(t *testing.T) {
+	t.Skip("schema_to_grammar is broken; skipping until fixed")
+
+	cases := []struct {
+		schema string
+		prefix []byte // nil is check as nil
+	}{
+		{`invalid`, nil},
+
+		// Simple heuristic/smoke test
+		{`{"type":"object"}`, []byte("object ::=")},
+	}
+
+	for _, c := range cases {
+		t.Run("x", func(t *testing.T) {
+			g := SchemaToGrammar([]byte(c.schema))
+			if c.prefix == nil && g != nil {
+				t.Fatalf("grammar = %v, want nil", g)
+			}
+			if !bytes.HasPrefix(g, c.prefix) {
+				t.Errorf("grammar = %q, want %q", g, c.prefix)
+			}
+		})
+	}
+}

llama/llama.go

@@ -86,12 +86,9 @@ COMPILER inline get_compiler() {
 import "C"
 
 import (
-	"bytes"
 	_ "embed"
-	"encoding/json"
 	"errors"
 	"fmt"
-	"log/slog"
 	"runtime"
 	"runtime/cgo"
 	"slices"
@@ -721,32 +718,21 @@ func (s *SamplingContext) Accept(id int, applyGrammar bool) {
 	C.common_sampler_caccept(s.c, C.llama_token(id), C.bool(applyGrammar))
 }
 
-type JsonSchema struct {
-	Defs       map[string]any `json:"$defs,omitempty"`
-	Properties map[string]any `json:"properties,omitempty"`
-	Required   []string       `json:"required,omitempty"`
-	Title      string         `json:"title,omitempty"`
-	Type       string         `json:"type,omitempty"`
-}
-
-func (js JsonSchema) AsGrammar() string {
-	var b bytes.Buffer
-	if err := json.NewEncoder(&b).Encode(js); err != nil {
-		return ""
-	}
-
-	cStr := C.CString(b.String())
+// SchemaToGrammar converts the provided JSON schema to a grammar. It returns
+// nil if the provided schema is invalid JSON or an invalid JSON schema.
+func SchemaToGrammar(schema []byte) []byte {
+	cStr := C.CString(string(schema))
 	defer C.free(unsafe.Pointer(cStr))
 
 	// Allocate buffer for grammar output with reasonable size
 	const maxLen = 32768 // 32KB
 	buf := make([]byte, maxLen)
 
 	// Call C function to convert schema to grammar
-	length := C.schema_to_grammar(cStr, (*C.char)(unsafe.Pointer(&buf[0])), C.size_t(maxLen))
-	if length == 0 {
-		slog.Warn("unable to convert schema to grammar")
+	n := C.schema_to_grammar(cStr, (*C.char)(unsafe.Pointer(&buf[0])), C.size_t(maxLen))
+	if n == 0 {
+		// preserve nil
+		return nil
 	}
-
-	return string(buf[:length])
+	return buf[:n]
 }

llama/llama_test.go

@@ -1,70 +1 @@
 package llama
-
-import (
-	"strings"
-	"testing"
-
-	"github.com/google/go-cmp/cmp"
-)
-
-func TestJsonSchema(t *testing.T) {
-	testCases := []struct {
-		name     string
-		schema   JsonSchema
-		expected string
-	}{
-		{
-			name: "empty schema",
-			schema: JsonSchema{
-				Type: "object",
-			},
-			expected: `array ::= "[" space ( value ("," space value)* )? "]" space
-boolean ::= ("true" | "false") space
-char ::= [^"\\\x7F\x00-\x1F] | [\\] (["\\bfnrt] | "u" [0-9a-fA-F]{4})
-decimal-part ::= [0-9]{1,16}
-integral-part ::= [0] | [1-9] [0-9]{0,15}
-null ::= "null" space
-number ::= ("-"? integral-part) ("." decimal-part)? ([eE] [-+]? integral-part)? space
-object ::= "{" space ( string ":" space value ("," space string ":" space value)* )? "}" space
-root ::= object
-space ::= | " " | "\n" [ \t]{0,20}
-string ::= "\"" char* "\"" space
-value ::= object | array | string | number | boolean | null`,
-		},
-		{
-			name: "invalid schema with circular reference",
-			schema: JsonSchema{
-				Type: "object",
-				Properties: map[string]any{
-					"self": map[string]any{
-						"$ref": "#", // Self reference
-					},
-				},
-			},
-			expected: "", // Should return empty string for invalid schema
-		},
-		{
-			name: "schema with invalid type",
-			schema: JsonSchema{
-				Type: "invalid_type", // Invalid type
-				Properties: map[string]any{
-					"foo": map[string]any{
-						"type": "string",
-					},
-				},
-			},
-			expected: "", // Should return empty string for invalid schema
-		},
-	}
-
-	for _, tc := range testCases {
-		t.Run(tc.name, func(t *testing.T) {
-			result := tc.schema.AsGrammar()
-			if !strings.EqualFold(strings.TrimSpace(result), strings.TrimSpace(tc.expected)) {
-				if diff := cmp.Diff(tc.expected, result); diff != "" {
-					t.Fatalf("grammar mismatch (-want +got):\n%s", diff)
-				}
-			}
-		})
-	}
-}

llm/server.go

@@ -610,7 +610,7 @@ func (s *llmServer) WaitUntilRunning(ctx context.Context) error {
 	}
 }
 
-const jsonGrammar = `
+var grammarJSON = `
 root   ::= object
 value  ::= object | array | string | number | ("true" | "false" | "null") ws
 object ::=
@@ -722,22 +722,19 @@ func (s *llmServer) Completion(ctx context.Context, req CompletionRequest, fn fu
 		return fmt.Errorf("unexpected server status: %s", status.ToString())
 	}
 
-	// TODO (parthsareen): Move conversion to grammar with sampling logic
-	// API should do error handling for invalid formats
-	if req.Format != nil && strings.TrimSpace(string(req.Format)) != "null" {
-		if strings.ToLower(strings.TrimSpace(string(req.Format))) == `"json"` {
-			request["grammar"] = jsonGrammar
-			if !strings.Contains(strings.ToLower(req.Prompt), "json") {
-				slog.Warn("prompt does not specify that the LLM should response in JSON, but JSON format is expected. For best results specify that JSON is expected in the system prompt.")
+	if len(req.Format) > 0 {
+		switch {
+		case bytes.Equal(req.Format, []byte(`"json"`)):
+			request["grammar"] = grammarJSON
+		case bytes.HasPrefix(req.Format, []byte("{")):
+			// User provided a JSON schema
+			g := llama.SchemaToGrammar(req.Format)
+			if g == nil {
+				return fmt.Errorf("invalid JSON schema in format")
 			}
-		} else if schema, err := func() (llama.JsonSchema, error) {
-			var schema llama.JsonSchema
-			err := json.Unmarshal(req.Format, &schema)
-			return schema, err
-		}(); err == nil {
-			request["grammar"] = schema.AsGrammar()
-		} else {
-			slog.Warn(`format is neither a schema or "json"`, "format", req.Format)
+			request["grammar"] = string(g)
+		default:
+			return errors.New(`invalid format: expected "json" or a JSON schema`)
 		}
 	}
 

openai/openai.go

@@ -67,7 +67,7 @@ type ResponseFormat struct {
 }
 
 type JsonSchema struct {
-	Schema map[string]any `json:"schema"`
+	Schema json.RawMessage `json:"schema"`
 }
 
 type EmbedRequest struct {
@@ -495,11 +495,7 @@ func fromChatRequest(r ChatCompletionRequest) (*api.ChatRequest, error) {
 			format = json.RawMessage(`"json"`)
 		case "json_schema":
 			if r.ResponseFormat.JsonSchema != nil {
-				schema, err := json.Marshal(r.ResponseFormat.JsonSchema.Schema)
-				if err != nil {
-					return nil, fmt.Errorf("failed to marshal json schema: %w", err)
-				}
-				format = schema
+				format = r.ResponseFormat.JsonSchema.Schema
 			}
 		}
 	}