diff --git a/workspace-server/internal/handlers/org_import.go b/workspace-server/internal/handlers/org_import.go
index 40a67604..70151e09 100644
--- a/workspace-server/internal/handlers/org_import.go
+++ b/workspace-server/internal/handlers/org_import.go
@@ -51,11 +51,10 @@ func (h *OrgHandler) createWorkspaceTree(ws OrgWorkspace, parentID *string, absX
 		model = defaults.Model
 	}
 	if model == "" {
-		if runtime == "claude-code" {
-			model = "sonnet"
-		} else {
-			model = "anthropic:claude-opus-4-7"
-		}
+		// SSOT: per-runtime defaults live in models/runtime_defaults.go
+		// (see RFC #2873). Consolidated from a duplicate of the same
+		// branch in workspace_provision.go.
+		model = models.DefaultModel(runtime)
 	}
 	tier := ws.Tier
 	if tier == 0 {
diff --git a/workspace-server/internal/handlers/workspace_provision.go b/workspace-server/internal/handlers/workspace_provision.go
index 7734aff0..981ee5da 100644
--- a/workspace-server/internal/handlers/workspace_provision.go
+++ b/workspace-server/internal/handlers/workspace_provision.go
@@ -534,11 +534,10 @@ func (h *WorkspaceHandler) ensureDefaultConfig(workspaceID string, payload model
 	// Generate a minimal config.yaml
 	model := payload.Model
 	if model == "" {
-		if runtime == "claude-code" {
-			model = "sonnet"
-		} else {
-			model = "anthropic:claude-opus-4-7"
-		}
+		// SSOT: per-runtime defaults live in models/runtime_defaults.go
+		// (see RFC #2873). Was previously duplicated here AND in
+		// org_import.go; consolidating prevents silent drift.
+		model = models.DefaultModel(runtime)
 	}
 
 	// Sanitize name/role/model for YAML safety — always double-quote so
diff --git a/workspace-server/internal/models/runtime_defaults.go b/workspace-server/internal/models/runtime_defaults.go
new file mode 100644
index 00000000..320586e8
--- /dev/null
+++ b/workspace-server/internal/models/runtime_defaults.go
@@ -0,0 +1,39 @@
+package models
+
+// runtime_defaults.go — single source of truth for per-runtime defaults
+// the platform applies when the operator/agent didn't supply a value.
+//
+// Why this lives in models/ (not handlers/): default selection is a
+// pure data fact about the runtime, not handler logic. Multiple
+// callers (Create-workspace handler, org-import handler, future
+// auto-provision paths) need the same answer; concentrating the
+// rule here means one edit when a runtime's default changes.
+//
+// Related work (RFC #2873): this is the seed for a future
+// `RuntimeConfig` interface that will also expose `ProvisioningTimeout()`,
+// `CapabilitiesSupported()`, and other per-runtime facts. For now the
+// surface is one helper — extracted from the duplicate branch in
+// workspace_provision.go:537 and org_import.go:54 that diverged silently
+// during refactors before this consolidation.
+
+// DefaultModel returns the model slug to use when a workspace is
+// created without an explicit model and the runtime can't infer one
+// from its own config.
+//
+//   - claude-code: "sonnet" — Anthropic's CLI accepts the short
+//     name and resolves it via the operator's anthropic-oauth or
+//     ANTHROPIC_API_KEY chain.
+//   - everything else (hermes, langgraph, crewai, autogen, deepagents,
+//     codex, openclaw, gemini-cli, external, ""): a fully-qualified
+//     vendor:model slug that the universal MODEL_PROVIDER chain in
+//     molecule-core PR #247 can route via per-vendor required_env.
+//
+// The function never returns an empty string; an unknown runtime
+// gets the universal default rather than failing closed (matches the
+// pre-refactor behavior — both call sites used the same fallback).
+func DefaultModel(runtime string) string {
+	if runtime == "claude-code" {
+		return "sonnet"
+	}
+	return "anthropic:claude-opus-4-7"
+}
diff --git a/workspace-server/internal/models/runtime_defaults_test.go b/workspace-server/internal/models/runtime_defaults_test.go
new file mode 100644
index 00000000..bab673ac
--- /dev/null
+++ b/workspace-server/internal/models/runtime_defaults_test.go
@@ -0,0 +1,62 @@
+package models
+
+import "testing"
+
+// TestDefaultModel pins the contract: known runtimes return their
+// expected default; unknowns and the empty string fall through to the
+// universal default. Add new runtimes here as `case` entries — pre-fix
+// adding a runtime required two source edits + an audit; post-SSOT it
+// requires one entry in DefaultModel + one assertion here.
+func TestDefaultModel(t *testing.T) {
+	cases := []struct {
+		runtime string
+		want    string
+	}{
+		// Known runtimes.
+		{"claude-code", "sonnet"},
+
+		// Universal fallback for everything else. Each runtime is named
+		// explicitly so a future drift (e.g., adding a hermes-specific
+		// branch) shows up as a failure on the runtime that drifted, not
+		// as a generic "unknown" failure.
+		{"hermes", "anthropic:claude-opus-4-7"},
+		{"langgraph", "anthropic:claude-opus-4-7"},
+		{"crewai", "anthropic:claude-opus-4-7"},
+		{"autogen", "anthropic:claude-opus-4-7"},
+		{"deepagents", "anthropic:claude-opus-4-7"},
+		{"codex", "anthropic:claude-opus-4-7"},
+		{"openclaw", "anthropic:claude-opus-4-7"},
+		{"gemini-cli", "anthropic:claude-opus-4-7"},
+		{"external", "anthropic:claude-opus-4-7"},
+
+		// Unknown / empty — fall through to universal default rather
+		// than failing closed. Pre-refactor both call sites also fell
+		// through; pinning the existing behavior, not changing it.
+		{"", "anthropic:claude-opus-4-7"},
+		{"some-future-runtime", "anthropic:claude-opus-4-7"},
+		{"CLAUDE-CODE", "anthropic:claude-opus-4-7"}, // case-sensitive — matches prior behavior
+	}
+	for _, tc := range cases {
+		t.Run(tc.runtime, func(t *testing.T) {
+			got := DefaultModel(tc.runtime)
+			if got != tc.want {
+				t.Errorf("DefaultModel(%q) = %q, want %q", tc.runtime, got, tc.want)
+			}
+		})
+	}
+}
+
+// TestDefaultModel_NeverEmpty — invariant: no input produces an empty
+// string. The handlers that consume this would write empty into
+// config.yaml, which the runtime then can't dispatch — pinning the
+// non-empty contract here protects against a future "return early on
+// unknown runtime" change that would silently break workspace creation.
+func TestDefaultModel_NeverEmpty(t *testing.T) {
+	for _, runtime := range []string{
+		"", "claude-code", "hermes", "unknown-runtime",
+	} {
+		if got := DefaultModel(runtime); got == "" {
+			t.Errorf("DefaultModel(%q) returned empty string", runtime)
+		}
+	}
+}