diff --git a/framework/modelcatalog/datasheet/costonlyparams_test.go b/framework/modelcatalog/datasheet/costonlyparams_test.go
new file mode 100644
index 00000000000..c9b1bf58d77
--- /dev/null
+++ b/framework/modelcatalog/datasheet/costonlyparams_test.go
@@ -0,0 +1,119 @@
+package datasheet
+
+import (
+	"encoding/json"
+	"slices"
+	"testing"
+
+	"github.com/maximhq/bifrost/core/schemas"
+)
+
+// Regression test for issue #6276: a datasheet row that declares ONLY pricing (no
+// capability flags, no model_parameters) must not produce a supported-parameters
+// allowlist. The default "reasoning_with_tool_calls" marker used to be added even to
+// capability-empty rows, turning them into a one-element allowlist; the compat plugin
+// (should_drop_params) then stripped tools / tool_choice / temperature / everything
+// else from requests to that model.
+func TestCostOnlyRowProducesNoParamAllowlist(t *testing.T) {
+	// Hosted-datasheet shape reported in the issue: pricing keys only, none of
+	// which map to ModelCapabilities fields. The issue's original row
+	// (gemini-3.6-flash) has since been fixed upstream; claude-opus-4-7-20260416
+	// ({"deprecation_date": "2027-04-16"}) still reproduces on the live feed and
+	// parses to the same empty ModelCapabilities.
+	costOnlyRow := json.RawMessage(`{
+		"input_cost_per_token_batches": 0.000000075,
+		"output_cost_per_token_batches": 0.0000003
+	}`)
+
+	t.Run("extractSupportedParams on a capability-empty row", func(t *testing.T) {
+		var caps schemas.ModelCapabilities
+		if err := json.Unmarshal(costOnlyRow, &caps); err != nil {
+			t.Fatalf("unmarshal cost-only row: %v", err)
+		}
+		if !IsEmptyModelCapabilities(&caps) {
+			t.Fatalf("precondition: cost-only row should parse to empty ModelCapabilities")
+		}
+		if got := extractSupportedParams(&caps); len(got) != 0 {
+			t.Errorf("extractSupportedParams(cost-only row) = %v, want empty — a row declaring no request parameters must not synthesize an allowlist", got)
+		}
+	})
+
+	t.Run("applyModelParameters end-to-end", func(t *testing.T) {
+		s := &Store{}
+		// A second, genuinely populated row keeps applied > 0 so the index swap
+		// happens, mirroring the real datasheet where thousands of populated
+		// rows sit alongside the cost-only gemini-3.6-flash row.
+		s.applyModelParameters(map[string]json.RawMessage{
+			"claude-opus-4-7-20260416": costOnlyRow,
+			"gpt-4o":                   json.RawMessage(`{"supports_function_calling":true,"supports_tool_choice":true}`),
+		})
+
+		if got := s.GetSupportedParameters("claude-opus-4-7-20260416"); got != nil {
+			t.Errorf("GetSupportedParameters(claude-opus-4-7-20260416) = %v, want nil (unknown) — compat treats any non-nil list as a complete allowlist and drops tools/tool_choice", got)
+		}
+	})
+
+	t.Run("explicit-false-only row keeps an authoritative allowlist", func(t *testing.T) {
+		// A row saying only {"supports_function_calling": false} IS a statement
+		// about the parameter surface: it must keep a non-nil allowlist (with the
+		// default marker) so compat still drops tools, instead of degrading to
+		// "unknown, do not drop".
+		var caps schemas.ModelCapabilities
+		if err := json.Unmarshal(json.RawMessage(`{"supports_function_calling":false}`), &caps); err != nil {
+			t.Fatalf("unmarshal explicit-false-only row: %v", err)
+		}
+		got := extractSupportedParams(&caps)
+		if len(got) == 0 {
+			t.Fatalf("extractSupportedParams(explicit-false-only row) = empty, want the default reasoning_with_tool_calls marker so the allowlist stays authoritative")
+		}
+		if !slices.Contains(got, "reasoning_with_tool_calls") {
+			t.Errorf("extractSupportedParams(explicit-false-only row) = %v, want reasoning_with_tool_calls present", got)
+		}
+		if slices.Contains(got, "tools") {
+			t.Errorf("extractSupportedParams(explicit-false-only row) = %v, want tools absent", got)
+		}
+	})
+
+	t.Run("populated row keeps the default marker", func(t *testing.T) {
+		// Backward-compat contract from #4630: rows declaring real capabilities but
+		// lacking the reasoning_with_tool_calls flag still get the default marker.
+		var caps schemas.ModelCapabilities
+		if err := json.Unmarshal(json.RawMessage(`{"supports_function_calling":true}`), &caps); err != nil {
+			t.Fatalf("unmarshal populated row: %v", err)
+		}
+		got := extractSupportedParams(&caps)
+		if !slices.Contains(got, "reasoning_with_tool_calls") {
+			t.Errorf("extractSupportedParams(populated row) = %v, want it to include the default reasoning_with_tool_calls marker", got)
+		}
+		if !slices.Contains(got, "tools") {
+			t.Errorf("extractSupportedParams(populated row) = %v, want it to include tools", got)
+		}
+	})
+
+	t.Run("explicit flag is honored regardless of other capabilities", func(t *testing.T) {
+		var explicitTrue schemas.ModelCapabilities
+		if err := json.Unmarshal(json.RawMessage(`{"supports_reasoning_with_tool_calls":true}`), &explicitTrue); err != nil {
+			t.Fatalf("unmarshal explicit-true row: %v", err)
+		}
+		if got := extractSupportedParams(&explicitTrue); !slices.Contains(got, "reasoning_with_tool_calls") {
+			t.Errorf("extractSupportedParams(explicit true) = %v, want reasoning_with_tool_calls present", got)
+		}
+
+		var explicitFalse schemas.ModelCapabilities
+		if err := json.Unmarshal(json.RawMessage(`{"supports_function_calling":true,"supports_reasoning_with_tool_calls":false}`), &explicitFalse); err != nil {
+			t.Fatalf("unmarshal explicit-false row: %v", err)
+		}
+		if got := extractSupportedParams(&explicitFalse); slices.Contains(got, "reasoning_with_tool_calls") {
+			t.Errorf("extractSupportedParams(explicit false) = %v, want reasoning_with_tool_calls absent", got)
+		}
+
+		// Explicit false must also override a marker sourced from model_parameters ids.
+		var falseWithParamID schemas.ModelCapabilities
+		if err := json.Unmarshal(json.RawMessage(`{"model_parameters":[{"id":"reasoning_with_tool_calls"}],"supports_reasoning_with_tool_calls":false}`), &falseWithParamID); err != nil {
+			t.Fatalf("unmarshal explicit-false row with param id: %v", err)
+		}
+		if got := extractSupportedParams(&falseWithParamID); slices.Contains(got, "reasoning_with_tool_calls") {
+			t.Errorf("extractSupportedParams(explicit false + model_parameters id) = %v, want reasoning_with_tool_calls absent", got)
+		}
+	})
+}
diff --git a/framework/modelcatalog/datasheet/types.go b/framework/modelcatalog/datasheet/types.go
index a1aacfc525b..c720f21d101 100644
--- a/framework/modelcatalog/datasheet/types.go
+++ b/framework/modelcatalog/datasheet/types.go
@@ -515,9 +515,6 @@ func extractSupportedParams(parsed *schemas.ModelCapabilities) []string {
 	if parsed.SupportsReasoning != nil && *parsed.SupportsReasoning {
 		addParam("reasoning")
 	}
-	if parsed.SupportsReasoningWithToolCalls == nil || *parsed.SupportsReasoningWithToolCalls {
-		addParam("reasoning_with_tool_calls")
-	}
 	if parsed.SupportsNoneReasoningEffort != nil && *parsed.SupportsNoneReasoningEffort {
 		addParam("supports_none_reasoning_effort")
 	}
@@ -539,6 +536,46 @@ func extractSupportedParams(parsed *schemas.ModelCapabilities) []string {
 		addParam("web_search_options")
 	}
 
+	// declaredParamSurface reports whether the row said anything at all about the
+	// request-parameter surface, including explicit "supports X: false". That is a
+	// different set from "produced a parameter name above": a row carrying only
+	// explicit-false flags produces no names but IS making an authoritative
+	// statement, and must keep a (possibly empty-of-tools) allowlist rather than
+	// degrade to "unknown, do not drop".
+	declaredParamSurface := len(parsed.ModelParameters) > 0 ||
+		parsed.SupportsAssistantPrefill != nil ||
+		parsed.SupportsFunctionCalling != nil ||
+		parsed.SupportsParallelFunctionCalling != nil ||
+		parsed.SupportsToolChoice != nil ||
+		parsed.SupportsReasoning != nil ||
+		parsed.SupportsResponseSchema != nil ||
+		parsed.SupportsNoneReasoningEffort != nil ||
+		parsed.SupportsServiceTier != nil ||
+		parsed.SupportsPromptCaching != nil ||
+		parsed.SupportsWebSearch != nil
+
+	// reasoning_with_tool_calls defaults on for rows that lack the flag, so models
+	// missing it don't get reasoning stripped when tools are present. Restrict the
+	// default to rows that declared a parameter surface: for a row that said
+	// nothing about parameters (cost-only, deprecation-only, mode-only) the marker
+	// alone would otherwise become a one-element allowlist, and compat with
+	// should_drop_params would strip every other parameter (tools, tool_choice,
+	// ...) from requests. This block must stay last in the function: the explicit
+	// false below removes the marker regardless of how it was added above.
+	if parsed.SupportsReasoningWithToolCalls != nil {
+		if *parsed.SupportsReasoningWithToolCalls {
+			addParam("reasoning_with_tool_calls")
+		} else {
+			// Explicit false wins even when a model_parameters id already added
+			// the marker via the default case above.
+			supported = slices.DeleteFunc(supported, func(p string) bool {
+				return p == "reasoning_with_tool_calls"
+			})
+		}
+	} else if declaredParamSurface {
+		addParam("reasoning_with_tool_calls")
+	}
+
 	return supported
 }