diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index a642e1a3..f2512cb9 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -272,6 +272,14 @@ jobs:
           find tests/e2e infra/scripts -type f -name '*.sh' -print0 \
             | xargs -0 shellcheck --severity=warning
 
+      - if: needs.changes.outputs.scripts == 'true'
+        name: Lint cleanup-trap hygiene (RFC #2873)
+        # Asserts every shell E2E test that calls `mktemp` also installs
+        # an EXIT trap. Catches the /tmp-leak class — a missing trap
+        # silently leaks scratch into CI runners (~10-100KB per run).
+        # See tests/e2e/lint_cleanup_traps.sh for the rule + fix pattern.
+        run: bash tests/e2e/lint_cleanup_traps.sh
+
       - if: needs.changes.outputs.scripts == 'true'
         name: Run E2E bash unit tests (no live infra)
         # Pure-bash unit tests for E2E helper libs (lib/*.sh). These pin
diff --git a/tests/e2e/lint_cleanup_traps.sh b/tests/e2e/lint_cleanup_traps.sh
new file mode 100755
index 00000000..a20d9a6f
--- /dev/null
+++ b/tests/e2e/lint_cleanup_traps.sh
@@ -0,0 +1,40 @@
+#!/usr/bin/env bash
+# lint_cleanup_traps.sh — regression gate for the OSS-shape program's
+# "all E2E tests must have proper cleanup" bar (RFC #2873).
+#
+# Asserts: every shell file under tests/e2e/ that calls `mktemp` ALSO
+# installs an `EXIT` trap somewhere in the file. The trap is the
+# minimum-viable guarantee that scratch files won't leak when an
+# assertion or curl exits the script non-zero.
+#
+# Why this lints (instead of the test runner enforcing): shell scripts
+# can't easily be wrapped by an outer harness without breaking the
+# `WSID=… ./test_x.sh` invocation contract. Static gate is the cheap
+# defense.
+#
+# Usage:
+#   tests/e2e/lint_cleanup_traps.sh
+#
+# Exits non-zero if any test_*.sh has unmatched mktemp/trap. CI invokes
+# it from the existing Shellcheck (E2E scripts) workflow.
+
+set -euo pipefail
+
+cd "$(dirname "$0")"
+
+violations=0
+for f in test_*.sh; do
+  if grep -qE '\bmktemp\b' "$f"; then
+    if ! grep -qE 'trap[[:space:]]+.*EXIT' "$f"; then
+      echo "::error file=tests/e2e/$f::has 'mktemp' but no 'trap … EXIT' — scratch will leak when test exits non-zero. Pattern: TMPDIR_E2E=\$(mktemp -d -t prefix-XXX); trap 'rm -rf \"\$TMPDIR_E2E\"' EXIT INT TERM"
+      violations=$((violations + 1))
+    fi
+  fi
+done
+
+if [ "$violations" -gt 0 ]; then
+  echo "::error::$violations shell E2E file(s) leak scratch on early exit. See above."
+  exit 1
+fi
+
+echo "✓ all $(grep -lE '\bmktemp\b' test_*.sh | wc -l | tr -d ' ') shell E2E files with mktemp also install an EXIT trap"
diff --git a/tests/e2e/test_chat_attachments_e2e.sh b/tests/e2e/test_chat_attachments_e2e.sh
index 76f46706..3d352f05 100755
--- a/tests/e2e/test_chat_attachments_e2e.sh
+++ b/tests/e2e/test_chat_attachments_e2e.sh
@@ -22,6 +22,13 @@ set -euo pipefail
 WSID="${WSID:?WSID=<workspace-id> required}"
 BASE="${BASE:-http://localhost:8080}"
 
+# Per-run scratch dir collected under one trap so every mktemp leak path
+# (assertion failure, SIGINT, exit non-zero) is plugged. Pre-fix this test
+# created a /tmp/hermes-e2e-XXXXXX.txt and never deleted it — ~10 KB ×
+# every CI run leaked into the runner. RFC #2873 cleanup-hygiene PR.
+TMPDIR_E2E=$(mktemp -d -t chat-attachments-e2e-XXXXXX)
+trap 'rm -rf "$TMPDIR_E2E"' EXIT INT TERM
+
 log() { printf "\n=== %s ===\n" "$*"; }
 
 log "Preflight: workspace online?"
@@ -29,7 +36,9 @@ STATUS=$(curl -s "$BASE/workspaces/$WSID" | python3 -c 'import json,sys;print(js
 [ "$STATUS" = "online" ] || { echo "workspace not online ($STATUS)"; exit 1; }
 
 log "Step 1 — Upload a text file via /chat/uploads"
-TEST_FILE=$(mktemp -t hermes-e2e-XXXXXX.txt)
+# `mktemp <full-template>` is portable across BSD (macOS) + GNU; -p is
+# GNU-only and breaks local dev runs on Mac.
+TEST_FILE=$(mktemp "$TMPDIR_E2E/hermes-e2e-XXXXXX.txt")
 echo "secret code: $(openssl rand -hex 4)-$(openssl rand -hex 4)" > "$TEST_FILE"
 EXPECTED=$(cat "$TEST_FILE" | awk '{print $NF}')
 UPLOAD=$(curl -s -X POST "$BASE/workspaces/$WSID/chat/uploads" -F "files=@$TEST_FILE")
diff --git a/tests/e2e/test_chat_attachments_multiruntime_e2e.sh b/tests/e2e/test_chat_attachments_multiruntime_e2e.sh
index 9601132e..3d93e186 100755
--- a/tests/e2e/test_chat_attachments_multiruntime_e2e.sh
+++ b/tests/e2e/test_chat_attachments_multiruntime_e2e.sh
@@ -24,6 +24,15 @@ set -uo pipefail
 BASE="${BASE:-http://localhost:8080}"
 fails=0
 
+# Per-run scratch dir collected under one trap so every per-runtime
+# round_trip mktemp leak path (assertion failure, SIGINT, exit
+# non-zero, function early-return between mktemp and rm) is plugged.
+# Pre-fix, round_trip's `rm -f "$test_file"` only fired on the success
+# path inside the function — every test_failure path before the rm
+# leaked the scratch into /tmp permanently. RFC #2873 cleanup-hygiene PR.
+TMPDIR_E2E=$(mktemp -d -t mr-attachments-e2e-XXXXXX)
+trap 'rm -rf "$TMPDIR_E2E"' EXIT INT TERM
+
 has_patch_in_container() {
   local container="$1"
   # Signal that platform helpers are available AND wired into the
@@ -74,12 +83,16 @@ print(f"executor: claude-code monkey-patch active ({name})")
 round_trip() {
   local label="$1" wsid="$2"
   local test_file expected upload uri payload reply reply_text
-  test_file=$(mktemp -t e2e-mr-XXXX.txt)
+  # Scratch goes under TMPDIR_E2E; the script-level trap rm -rf's the
+  # whole dir on exit, so per-file rm calls are unnecessary AND make
+  # error paths leak when forgotten.
+  # `mktemp <full-template>` is portable across BSD (macOS) + GNU; -p is GNU-only.
+  test_file=$(mktemp "$TMPDIR_E2E/e2e-mr-${label}-XXXX.txt")
   expected="secret-$(openssl rand -hex 6)"
   echo "$expected" > "$test_file"
   upload=$(curl -s -X POST "$BASE/workspaces/$wsid/chat/uploads" -F "files=@$test_file")
   uri=$(echo "$upload" | python3 -c 'import json,sys;print(json.load(sys.stdin)["files"][0]["uri"])' 2>/dev/null)
-  [ -z "$uri" ] && { echo "FAIL $label: upload returned no URI: $upload"; rm -f "$test_file"; return 1; }
+  [ -z "$uri" ] && { echo "FAIL $label: upload returned no URI: $upload"; return 1; }
   payload=$(URI="$uri" python3 -c '
 import json, os
 uri = os.environ["URI"]
@@ -103,7 +116,8 @@ try:
 except Exception as exc:
     print(f"(parse failed: {exc})")
 ' 2>&1)
-  rm -f "$test_file"
+  # $test_file lives under TMPDIR_E2E; the script-level trap rm -rf's
+  # the dir on exit, covering every return path including SIGINT.
 
   if echo "$reply_text" | grep -qF "$expected"; then
     echo "PASS $label round-trip: agent quoted $expected"
diff --git a/tests/e2e/test_notify_attachments_e2e.sh b/tests/e2e/test_notify_attachments_e2e.sh
index 6957c7e3..2a5f12a7 100755
--- a/tests/e2e/test_notify_attachments_e2e.sh
+++ b/tests/e2e/test_notify_attachments_e2e.sh
@@ -29,11 +29,20 @@ FAIL=0
 WSID=""
 
 cleanup() {
+  # Workspace teardown — best-effort, ignore errors so an unrelated CP
+  # outage doesn't shadow a real test failure.
   if [ -n "$WSID" ]; then
     curl -s -X DELETE "$BASE/workspaces/$WSID?confirm=true" > /dev/null || true
   fi
+  # /tmp scratch — pre-fix only ran on success path (the unconditional
+  # rm at the bottom of the script). Trap-based path lets the file leak
+  # whenever the script exits non-zero before reaching the rm. RFC #2873
+  # cleanup-hygiene PR.
+  if [ -n "${TMPF:-}" ]; then
+    rm -f "$TMPF"
+  fi
 }
-trap cleanup EXIT
+trap cleanup EXIT INT TERM
 
 assert() {
   local label="$1"
@@ -230,7 +239,8 @@ for r in rows:
   assert "stored URI matches uploaded URI" "$STORED_URI" "$URI"
 fi
 
-rm -f "$TMPF"
+# $TMPF cleanup happens via the trap-cleanup function above — covers
+# both the success path and any early exit / SIGINT.
 
 echo ""
 echo "=== Results: $PASS passed, $FAIL failed ==="
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)
+		}
+	}
+}