diff --git a/Justfile b/Justfile
index 53d9e896cf032170f8e0f4336b5e1170170c79f7..b1b6eab73c9a32d190e64c5a97f72a2e8b48023f 100644
--- a/Justfile
+++ b/Justfile
@@ -71,16 +71,9 @@ build-doc:
     go build -o {{doc_plugin}} ./cmd/protoc-gen-tarantool-doc
 
 # Build the optional C-acceleration runtime into runtime/pb/c_runtime.{so,dylib}.
-# Source lives under runtime/pb/c/ once bd-ra6 lands; until then this recipe
-# prints a helpful error and exits 1. The runtime is opt-in via PB_ENABLE_C=1
-# — see docs/specs/c_accel_build_packaging.md for the full contract.
+# The runtime is opt-in via PB_ENABLE_C=1 — see docs/specs/c_accel_build_packaging.md
+# for the full contract.
 build-c:
-    @if [ ! -d runtime/pb/c ]; then \
-        echo "build-c: runtime/pb/c/ does not exist yet."; \
-        echo "  The C runtime arrives with bd-ra6 (generic C codec)."; \
-        echo "  See docs/specs/c_accel_build_packaging.md."; \
-        exit 1; \
-    fi
     make -C runtime/pb/c
 
 # Remove built C-runtime artifacts.
@@ -180,6 +173,19 @@ #   just test-one protobuf_test.lua::hello.full.test_packed_repeated_int32
 test-one filter: gen
     LUA_PATH="{{lua_path}}" LUA_CPATH="{{lua_cpath}}" {{luatest}} -v test/{{filter}}
 
+# Same as `test`, but with PB_ENABLE_C=1 so pb.encode / pb.decode dispatch
+# through the C runtime (runtime/pb/c_runtime.{so,dylib}). Builds the C
+# module first. Together with `test`, this is the parity gate — every
+# luatest assertion is against a reference output (golden bytes, txtpb,
+# conformance result), so if Lua passes and C passes, both equal the
+# reference and Lua ≡ C by transitivity. See bd-43t.
+test-c: build-c gen
+    PB_ENABLE_C=1 LUA_PATH="{{lua_path}}" LUA_CPATH="{{lua_cpath}}" {{luatest}} -v test/
+
+# Parity gate: run the suite under both codecs. Use before pushing C-runtime
+# or codec changes.
+test-all: test test-c
+
 # ---------------------------------------------------------------------------
 # Bench
 # ---------------------------------------------------------------------------
@@ -187,6 +193,15 @@
 # Microbench: alloc + throughput per op across 5 payload sizes, both modes.
 bench: gen
     tarantool bench/bench.lua --print
+
+# Same as `bench`, but with PB_ENABLE_C=1 so the runtime-mode results
+# exercise the C codec. The runtime-mode column is relabelled to
+# `c-runtime` in the output. Full-mode results are unchanged (full mode
+# uses inline wire calls, not pb.encode). Allocation baselines (`bench-baseline`
+# / `bench-compare`) remain Lua-only — C alloc shape differs by design
+# and would noise the gate.
+bench-c: build-c gen
+    PB_ENABLE_C=1 tarantool bench/bench.lua --print
 
 # Overwrite bench/baseline.json with current alloc-per-op numbers.
 bench-baseline: gen
diff --git a/README.md b/README.md
index 2a5df5f44adaaae8e42a2574882345f5f0fb10af..5b8073a1850d24cfc045f4b3f83ff9c3dca0f6e4 100644
--- a/README.md
+++ b/README.md
@@ -385,12 +385,33 @@ `ignore_unknown_fields=true` to `pb.json.decode`). The self-test in
 `test/conformance_test.lua` exercises the runner with crafted requests on
 every `just test` run.
 
+### C-runtime parity
+
+The optional C acceleration runtime (`runtime/pb/c_runtime.{so,dylib}`,
+built via `just build-c`, activated by `PB_ENABLE_C=1`) must produce
+byte-identical output to the pure-Lua codec. The parity gate reuses the
+existing test suites — no separate diff harness:
+
+```bash
+just test-all       # luatest under both codecs (752 + 1043 tests)
+just conformance    # Google suite, Lua codecs
+just conformance-c  # Google suite, PB_ENABLE_C=1
+```
+
+Every assertion checks against a reference (golden bytes, txtpb,
+conformance result). If Lua passes and C passes, both equal the
+reference, so Lua ≡ C by transitivity. The interop fixtures under
+`test/interop/fixtures/` are parameterized over both codegen modes;
+under `PB_ENABLE_C=1` the runtime-mode iteration transparently
+exercises the C codec.
+
 [gconf]: https://github.com/protocolbuffers/protobuf/tree/main/conformance
 
 ## Benchmarks
 
 ```bash
 just bench           # print throughput + alloc per op (5 sizes × 2 modes)
+just bench-c         # same with PB_ENABLE_C=1 — runtime column → `c-runtime`
 just bench-baseline  # overwrite bench/baseline.json (run on a quiet machine)
 just bench-compare   # exit 1 if any alloc-per-op regressed >5% vs baseline
 ```
diff --git a/bench/bench.lua b/bench/bench.lua
index bcb14e5927dc81efa66f81ba298a302b48d0ca6a..0a868011abdd96794b89ac0dc6923c5d387df082 100644
--- a/bench/bench.lua
+++ b/bench/bench.lua
@@ -17,6 +17,13 @@ package.path = './runtime/?.lua;./runtime/?/init.lua;'
     .. './examples/expected/?.lua;./examples/expected/?/init.lua;'
     .. package.path
 
+-- Extend cpath so `require('pb.c_runtime')` finds runtime/pb/c_runtime.{so,dylib}
+-- when PB_ENABLE_C=1. `.dylib` first matches the order in the Justfile (see
+-- the comment there for why this matters when both extensions coexist).
+package.cpath = './runtime/?.dylib;./runtime/?.so;'
+    .. './runtime/?/init.dylib;./runtime/?/init.so;'
+    .. package.cpath
+
 local clock = require('clock')
 local json  = require('json')
 local fio   = require('fio')
@@ -436,8 +443,35 @@
 local args = {...}
 local mode_flag = args[1] or '--print'
 
-io.stderr:write(string.format('tarantool-protobuf bench (%s)\n', _TARANTOOL))
+local C_ENABLED = (os.getenv('PB_ENABLE_C') == '1') and (require('pb').c_runtime ~= nil)
+
+if C_ENABLED and (mode_flag == '--baseline' or mode_flag == '--compare') then
+    -- The alloc-per-op baseline is Lua-only by design. The C codec has a
+    -- different allocation shape (C-side scratch + a single Lua-side
+    -- result string) that would noise the gate. Re-run without
+    -- PB_ENABLE_C=1 for baseline/compare ops.
+    io.stderr:write('bench.lua: --baseline and --compare are Lua-only; '
+        .. 'unset PB_ENABLE_C and rerun\n')
+    os.exit(2)
+end
+
+io.stderr:write(string.format('tarantool-protobuf bench (%s)%s\n',
+    _TARANTOOL, C_ENABLED and ', C runtime ENABLED' or ''))
 local doc = run_all()
+
+-- Relabel runtime-mode results to `c-runtime` when the C dispatch is
+-- active. The bench fixture iterates over generated modules from
+-- examples/expected/runtime/, which call pb.encode / pb.decode — those
+-- dispatch through the C codec when desc.c_plan compiles. Full-mode
+-- modules inline wire calls and don't go through pb.encode, so the
+-- full-mode column is unchanged from the Lua run.
+if C_ENABLED then
+    for _, schema in ipairs(doc.schemas) do
+        for _, r in ipairs(schema.results) do
+            if r.mode == 'runtime' then r.mode = 'c-runtime' end
+        end
+    end
+end
 
 if mode_flag == '--print' then
     io.write(render(doc))