diff --git a/backend/internal/domain/market/research/assets/researcher_schema_ollama.json b/backend/internal/domain/market/research/assets/researcher_schema_ollama.json
new file mode 100644
index 0000000..6336924
--- /dev/null
+++ b/backend/internal/domain/market/research/assets/researcher_schema_ollama.json
@@ -0,0 +1,79 @@
+{
+  "type": "object",
+  "required": ["markt_name", "recherche_datum", "status", "quellen_gesamt", "felder"],
+  "properties": {
+    "markt_name": {"type": "string"},
+    "recherche_datum": {"type": "string"},
+    "status": {"type": "string", "enum": ["bestaetigt", "unklar", "vorjahr_unbestaetigt", "abgesagt"]},
+    "quellen_gesamt": {"type": "array", "items": {"type": "string"}},
+    "felder": {
+      "type": "object",
+      "required": ["website", "strasse", "plz", "stadt", "bundesland", "land", "veranstalter", "start_datum", "end_datum", "oeffnungszeiten", "eintrittspreise", "bild_url"],
+      "properties": {
+        "website":     {"type": "object", "required": ["wert", "quellen", "extraktion", "hinweis"], "properties": {"wert": {"anyOf": [{"type": "string"}, {"type": "null"}]}, "hinweis": {"anyOf": [{"type": "string"}, {"type": "null"}]}, "quellen": {"type": "array", "items": {"type": "string"}}, "extraktion": {"type": "string", "enum": ["direkt", "kombiniert"]}}},
+        "strasse":     {"type": "object", "required": ["wert", "quellen", "extraktion", "hinweis"], "properties": {"wert": {"anyOf": [{"type": "string"}, {"type": "null"}]}, "hinweis": {"anyOf": [{"type": "string"}, {"type": "null"}]}, "quellen": {"type": "array", "items": {"type": "string"}}, "extraktion": {"type": "string", "enum": ["direkt", "kombiniert"]}}},
+        "plz":         {"type": "object", "required": ["wert", "quellen", "extraktion", "hinweis"], "properties": {"wert": {"anyOf": [{"type": "string"}, {"type": "null"}]}, "hinweis": {"anyOf": [{"type": "string"}, {"type": "null"}]}, "quellen": {"type": "array", "items": {"type": "string"}}, "extraktion": {"type": "string", "enum": ["direkt", "kombiniert"]}}},
+        "stadt":       {"type": "object", "required": ["wert", "quellen", "extraktion", "hinweis"], "properties": {"wert": {"anyOf": [{"type": "string"}, {"type": "null"}]}, "hinweis": {"anyOf": [{"type": "string"}, {"type": "null"}]}, "quellen": {"type": "array", "items": {"type": "string"}}, "extraktion": {"type": "string", "enum": ["direkt", "kombiniert"]}}},
+        "bundesland":  {"type": "object", "required": ["wert", "quellen", "extraktion", "hinweis"], "properties": {"wert": {"anyOf": [{"type": "string"}, {"type": "null"}]}, "hinweis": {"anyOf": [{"type": "string"}, {"type": "null"}]}, "quellen": {"type": "array", "items": {"type": "string"}}, "extraktion": {"type": "string", "enum": ["direkt", "kombiniert"]}}},
+        "land":        {"type": "object", "required": ["wert", "quellen", "extraktion", "hinweis"], "properties": {"wert": {"anyOf": [{"type": "string"}, {"type": "null"}]}, "hinweis": {"anyOf": [{"type": "string"}, {"type": "null"}]}, "quellen": {"type": "array", "items": {"type": "string"}}, "extraktion": {"type": "string", "enum": ["direkt", "kombiniert"]}}},
+        "veranstalter":{"type": "object", "required": ["wert", "quellen", "extraktion", "hinweis"], "properties": {"wert": {"anyOf": [{"type": "string"}, {"type": "null"}]}, "hinweis": {"anyOf": [{"type": "string"}, {"type": "null"}]}, "quellen": {"type": "array", "items": {"type": "string"}}, "extraktion": {"type": "string", "enum": ["direkt", "kombiniert"]}}},
+        "bild_url":    {"type": "object", "required": ["wert", "quellen", "extraktion", "hinweis"], "properties": {"wert": {"anyOf": [{"type": "string"}, {"type": "null"}]}, "hinweis": {"anyOf": [{"type": "string"}, {"type": "null"}]}, "quellen": {"type": "array", "items": {"type": "string"}}, "extraktion": {"type": "string", "enum": ["direkt", "kombiniert"]}}},
+        "start_datum": {"type": "object", "required": ["wert", "quellen", "extraktion", "hinweis"], "properties": {"wert": {"anyOf": [{"type": "string"}, {"type": "null"}]}, "hinweis": {"anyOf": [{"type": "string"}, {"type": "null"}]}, "quellen": {"type": "array", "items": {"type": "string"}}, "extraktion": {"type": "string", "enum": ["direkt", "kombiniert"]}}},
+        "end_datum":   {"type": "object", "required": ["wert", "quellen", "extraktion", "hinweis"], "properties": {"wert": {"anyOf": [{"type": "string"}, {"type": "null"}]}, "hinweis": {"anyOf": [{"type": "string"}, {"type": "null"}]}, "quellen": {"type": "array", "items": {"type": "string"}}, "extraktion": {"type": "string", "enum": ["direkt", "kombiniert"]}}},
+        "oeffnungszeiten": {
+          "type": "object",
+          "required": ["wert", "quellen", "extraktion", "hinweis"],
+          "properties": {
+            "wert": {
+              "anyOf": [
+                {
+                  "type": "array",
+                  "items": {
+                    "type": "object",
+                    "required": ["datum_von", "datum_bis", "von", "bis"],
+                    "properties": {
+                      "datum_von": {"anyOf": [{"type": "string"}, {"type": "null"}]},
+                      "datum_bis": {"anyOf": [{"type": "string"}, {"type": "null"}]},
+                      "von":       {"anyOf": [{"type": "string"}, {"type": "null"}]},
+                      "bis":       {"anyOf": [{"type": "string"}, {"type": "null"}]}
+                    }
+                  }
+                },
+                {"type": "null"}
+              ]
+            },
+            "hinweis":    {"anyOf": [{"type": "string"}, {"type": "null"}]},
+            "quellen":    {"type": "array", "items": {"type": "string"}},
+            "extraktion": {"type": "string", "enum": ["direkt", "kombiniert"]}
+          }
+        },
+        "eintrittspreise": {
+          "type": "object",
+          "required": ["wert", "quellen", "extraktion", "hinweis"],
+          "properties": {
+            "wert": {
+              "anyOf": [
+                {
+                  "type": "array",
+                  "items": {
+                    "type": "object",
+                    "required": ["name", "betrag", "waehrung"],
+                    "properties": {
+                      "name":     {"type": "string"},
+                      "betrag":   {"type": "number"},
+                      "waehrung": {"type": "string", "enum": ["EUR", "CHF"]}
+                    }
+                  }
+                },
+                {"type": "null"}
+              ]
+            },
+            "hinweis":    {"anyOf": [{"type": "string"}, {"type": "null"}]},
+            "quellen":    {"type": "array", "items": {"type": "string"}},
+            "extraktion": {"type": "string", "enum": ["direkt", "kombiniert"]}
+          }
+        }
+      }
+    }
+  }
+}
diff --git a/backend/internal/domain/market/research/orchestrator.go b/backend/internal/domain/market/research/orchestrator.go
index 5237625..3bfa970 100644
--- a/backend/internal/domain/market/research/orchestrator.go
+++ b/backend/internal/domain/market/research/orchestrator.go
@@ -6,6 +6,7 @@ import (
 	"errors"
 	"fmt"
 	"net/url"
+	"strings"
 	"time"
 
 	"marktvogt.de/backend/internal/pkg/ai"
@@ -75,19 +76,37 @@ func (o *Orchestrator) Run(ctx context.Context, in Input) (Output, error) {
 	}
 
 	// 4. LLM call with one retry on schema violation
-	resp, err := callLLM(ctx, o.AI, userPrompt, SchemaJSON)
+	// Providers with constrained decoding (Ollama) use a simplified schema
+	// without $defs, union types, or patterns — and are validated against that
+	// same simplified schema. Providers that embed the schema in the prompt
+	// (Mistral) get the full schema for both generation and validation.
+	constraintSchema := SchemaJSON
+	validationSchema := SchemaJSON
+	if o.AI.SupportsJSONSchema() {
+		constraintSchema = ConstraintSchemaJSON
+		validationSchema = ConstraintSchemaJSON
+	}
+	validate := func(content string) error {
+		normalized := normalizeNullStrings(content)
+		return ai.ValidateSchema(validationSchema, []byte(normalized))
+	}
+	resp, err := callLLM(ctx, o.AI, userPrompt, constraintSchema)
 	if err == nil {
-		if verr := ai.ValidateSchema(SchemaJSON, []byte(resp.Content)); verr != nil {
+		if verr := validate(resp.Content); verr != nil {
 			err = &ai.ProviderError{Code: ai.ErrSchemaViolation, Retryable: true, RawOutput: resp.Content, Inner: verr}
+		} else {
+			resp.Content = normalizeNullStrings(resp.Content)
 		}
 	}
 	if err != nil {
 		var pe *ai.ProviderError
 		if errors.As(err, &pe) && pe.Code == ai.ErrSchemaViolation {
-			resp, err = callLLM(ctx, o.AI, userPrompt+"\n\nYour previous response failed schema validation. Re-emit the JSON strictly matching the schema.", SchemaJSON)
+			resp, err = callLLM(ctx, o.AI, userPrompt+"\n\nYour previous response failed schema validation. Re-emit the JSON strictly matching the schema.", constraintSchema)
 			if err == nil {
-				if verr := ai.ValidateSchema(SchemaJSON, []byte(resp.Content)); verr != nil {
+				if verr := validate(resp.Content); verr != nil {
 					err = &ai.ProviderError{Code: ai.ErrSchemaViolation, Retryable: false, RawOutput: resp.Content, Inner: verr}
+				} else {
+					resp.Content = normalizeNullStrings(resp.Content)
 				}
 			}
 		}
@@ -145,6 +164,61 @@ func buildUserPrompt(in Input, pages []Page) (string, error) {
 	return string(buf), nil
 }
 
+// normalizeNullStrings replaces the string literal "null" with JSON null
+// throughout any JSON value. Constrained-decoding providers that use a
+// simplified schema (type: string, no union types) emit "null" as a string
+// when they intend the absence of a value. The full schema expects JSON null
+// for nullable fields, so we normalise before validation.
+// It also strips markdown code fences (```json ... ```) that some models
+// emit even when constrained decoding is requested.
+func normalizeNullStrings(content string) string {
+	content = stripMarkdownFences(content)
+	var v any
+	if err := json.Unmarshal([]byte(content), &v); err != nil {
+		return content
+	}
+	normalized := replaceNullStrings(v)
+	b, err := json.Marshal(normalized)
+	if err != nil {
+		return content
+	}
+	return string(b)
+}
+
+func replaceNullStrings(v any) any {
+	switch val := v.(type) {
+	case string:
+		if val == "null" {
+			return nil
+		}
+		return val
+	case map[string]any:
+		for k, elem := range val {
+			val[k] = replaceNullStrings(elem)
+		}
+		return val
+	case []any:
+		for i, elem := range val {
+			val[i] = replaceNullStrings(elem)
+		}
+		return val
+	default:
+		return v
+	}
+}
+
+func stripMarkdownFences(s string) string {
+	s = strings.TrimSpace(s)
+	if strings.HasPrefix(s, "```") {
+		if i := strings.Index(s, "\n"); i != -1 {
+			s = s[i+1:]
+		}
+		s = strings.TrimSuffix(s, "```")
+		s = strings.TrimSpace(s)
+	}
+	return s
+}
+
 func countDomains(urls []string) int {
 	seen := map[string]struct{}{}
 	for _, raw := range urls {
diff --git a/backend/internal/domain/market/research/schema.go b/backend/internal/domain/market/research/schema.go
index cf0ccd5..dffd4ae 100644
--- a/backend/internal/domain/market/research/schema.go
+++ b/backend/internal/domain/market/research/schema.go
@@ -4,3 +4,11 @@ import _ "embed"
 
 //go:embed assets/researcher_schema.json
 var SchemaJSON []byte
+
+// ConstraintSchemaJSON is a simplified, flat JSON Schema for providers that
+// support constrained decoding but cannot handle $defs, union types, or
+// pattern constraints (e.g. Ollama with llama.cpp grammar generation).
+// Post-hoc validation always uses SchemaJSON regardless.
+//
+//go:embed assets/researcher_schema_ollama.json
+var ConstraintSchemaJSON []byte