diff --git a/PRODUCT.md b/PRODUCT.md
index 0e0d8ba..71cb25a 100644
--- a/PRODUCT.md
+++ b/PRODUCT.md
@@ -29,7 +29,7 @@ that compound.
 - **Bookkeeping compounds across sessions** — Every session makes the next one smarter. Not a wiki nobody reads — extracted learnings become reusable context.
 - **Validation becomes an operating rule** — The doctrine makes separate review, ratcheting, and evidence-backed closeout part of the normal flow.
 - **Primitives and flows scale from one developer to many** — Start with one dev + one agent. The same principles extend to multi-agent orchestration.
-- **Vendor-neutral doctrine** — Works with Claude Code, Cursor, Codex, Copilot, or custom setups. Not locked to any toolchain.
+- **Vendor-neutral doctrine, single-runtime default** — Every factor applies whether you run Claude Code, Cursor, Codex, Copilot, or custom setups. The reference implementation defaults to a single runtime so install-to-first-value stays cheap. Cross-vendor coordination is kept as a falsifiable capability — preserved because in real `/council --mixed` runs the second vendor has changed load-bearing verdicts, not because it sits in the install matrix. The [standing proof](https://12factoragentops.com/sovereignty-proof) is auditable.
 
 ## Competitive Landscape
 
diff --git a/README.md b/README.md
index c158753..203b30c 100644
--- a/README.md
+++ b/README.md
@@ -52,6 +52,18 @@ builds on everything that came before.
 **The hook:** Compounding work is the one thing no amount of model improvement
 replaces. Better models with no bookkeeping still repeat your mistakes.
 
+### Single-runtime by default. Cross-vendor where it pays.
+
+The doctrine is vendor-neutral by design — every factor applies whether you run
+Claude Code, Codex CLI, Cursor, or OpenCode. The reference implementation
+defaults to a single runtime so install-to-first-value stays cheap. Cross-vendor
+coordination (mixed-model councils, runtime-asymmetric planning) is kept because
+in real runs Codex judges have surfaced load-bearing findings that all-Claude
+councils missed. That capability earns its place by changing verdicts, not by
+sitting in the install matrix.
+
+**[Read the sovereignty proof →](https://12factoragentops.com/sovereignty-proof)** — three real `/council --mixed` runs with file:line citations, the standing exhibit for the cross-vendor claim.
+
 ---
 
 ## Quickstart: 5 Minutes, Zero Infrastructure