diff --git a/.mise/config.coverage.toml b/.mise/config.coverage.toml
index c3c16aa..0af0867 100644
--- a/.mise/config.coverage.toml
+++ b/.mise/config.coverage.toml
@@ -52,24 +52,25 @@ eval "$(cargo llvm-cov show-env --sh)"
 # run/drive/worker paths reported 0% while its in-process initialize() was covered. Continuous mode (%c) memory-maps
 # the counter file so increments persist live and survive the kill. %c is incompatible with the merge-pool (%Nm)
 # specifier show-env emits, so use a per-process (%p) continuous file in the same profile dir -- `cargo llvm-cov
-# report` globs every *.profraw there regardless of name. On Linux continuous mode additionally needs relocatable
-# counters (`-Cllvm-args=-runtime-counter-relocation`). The wrapper flags env is CARGO_ENCODED_RUSTFLAGS format
-# (ASCII unit separator 0x1f between flags), not space-separated -- a space append glues the llvm-arg onto the
-# last existing flag and rustc rejects `--cfg=coverage_nightly -Cllvm-args=...` as one invalid --cfg value.
+# report` globs every *.profraw there regardless of name. Branch coverage: `show-env` has no --branch flag (only
+# the top-level `cargo llvm-cov` does), so the nightly `-Zcoverage-options=branch` is appended to the wrapper
+# RUSTFLAGS directly; `report`'s lcov export emits the BRDA records on its own once the covmap carries branch
+# regions. On Linux continuous mode additionally needs relocatable counters
+# (`-Cllvm-args=-runtime-counter-relocation`). The wrapper flags env is CARGO_ENCODED_RUSTFLAGS format (ASCII
+# unit separator 0x1f between flags), not space-separated -- a space append glues the llvm-arg onto the last
+# existing flag and rustc rejects `--cfg=coverage_nightly -Cllvm-args=...` as one invalid --cfg value. goawk
+# generates the separator byte at run time: bash dollar-quote hex is not a valid escape in this TOML basic
+# string, and a TOML unicode escape renders as an invisible byte in the body, so it is constructed visibly.
 cov_dir="$(coreutils dirname "$LLVM_PROFILE_FILE")"
 export LLVM_PROFILE_FILE="$cov_dir/core-cont-%p%c.profraw"
+us="$(goawk 'BEGIN { printf "%c", 31 }')"
+flags="${__CARGO_LLVM_COV_RUSTC_WRAPPER_RUSTFLAGS:-}"
+append_flag() { flags="${flags:+${flags}${us}}$1"; }
+append_flag "-Zcoverage-options=branch"
 if [ "$(coreutils uname -s)" = "Linux" ]; then
-  reloc="-Cllvm-args=-runtime-counter-relocation"
-  flags="${__CARGO_LLVM_COV_RUSTC_WRAPPER_RUSTFLAGS:-}"
-  # Append the reloc flag with a TOML-injected unit separator (ASCII 31).
-  # Bash dollar-quote hex is not a valid TOML escape, so the separator has to be introduced by the
-  # TOML parser into the task body rather than written as a shell escape.
-  if [ -n "$flags" ]; then
-    export __CARGO_LLVM_COV_RUSTC_WRAPPER_RUSTFLAGS="${flags}\u001f${reloc}"
-  else
-    export __CARGO_LLVM_COV_RUSTC_WRAPPER_RUSTFLAGS="${reloc}"
-  fi
+  append_flag "-Cllvm-args=-runtime-counter-relocation"
 fi
+export __CARGO_LLVM_COV_RUSTC_WRAPPER_RUSTFLAGS="$flags"
 # The --features flags compile each runner's coverage code, off by default so non-coverage builds omit it.
 # et-ws-web-runner/coverage adds the browser-module capture (shims + the __et_capture_coverage PUT);
 # et-ws-wasi-runner/coverage adds the guest `/cov` preopen. ET_TEST_COVERAGE (set by the coverage job) still
@@ -206,9 +207,9 @@ cargo clean -p et-ws-wasm-agent --target wasm32-unknown-unknown
 driver="${CHROMEDRIVER:-$(mise which chromedriver)}"
 
 # Build + run cov-server itself instrumented so the launcher is not reported as untested.
-# It uses native -Cinstrument-coverage and flushes its counters to this .profraw on the clean exit that removing
-# the marker below triggers.
-RUSTFLAGS="-Cinstrument-coverage" cargo build -q -p et-ws-test-server --bin cov-server
+# It uses native -Cinstrument-coverage (with nightly branch coverage, matching the rest of the pipeline) and
+# flushes its counters to this .profraw on the clean exit that removing the marker below triggers.
+RUSTFLAGS="-Cinstrument-coverage -Zcoverage-options=branch" cargo build -q -p et-ws-test-server --bin cov-server
 LLVM_PROFILE_FILE="{{ config_root }}/$covdir/cov-server-%p.profraw" ./target/debug/cov-server "$covdir/server-ready" &
 server_pid=$!
 trap 'kill "$server_pid" 2>/dev/null || true' EXIT
diff --git a/.mise/config.toml b/.mise/config.toml
index b0bf7ae..c820daa 100644
--- a/.mise/config.toml
+++ b/.mise/config.toml
@@ -360,9 +360,13 @@ rpath_flag = "-C link-arg=-Wl,-rpath,{{ vars.conda_openssl }}/lib"
 # `python` pin), since the minor-version alias dir isn't created everywhere.
 pylib_flag = "-C link-arg=-Wl,-rpath,{{ vars.a_home }}/.local/share/mise/installs/python/3.13.14/lib"
 wasm_rustflags = "-C target-cpu=mvp -C target-feature=+mutable-globals,+sign-ext,+nontrapping-fptoint,+reference-types"
+# Instrumentation flag pair (leaf var) shared by the wasm coverage builds.
+# The nightly -Z flag adds branch regions to the covmap so the lcov exports carry BRDA records. `a_` prefix so
+# mise's alphabetical [vars] render defines it before wasm_cov_rustflags uses it.
+a_wasm_cov_instr = "-Cinstrument-coverage -Zcoverage-options=branch"
 # RUSTFLAGS for the wasm coverage builds: instrument + emit LLVM IR, no profiler runtime (minicov provides it).
 # Single codegen unit + LTO off so each crate emits one .ll the wasm-cov task can gut and feed to llc/llvm-cov.
-wasm_cov_rustflags = "--emit=llvm-ir -Cinstrument-coverage -Ccodegen-units=1 -Clto=off -Zno-profiler-runtime"
+wasm_cov_rustflags = "--emit=llvm-ir {{ vars.a_wasm_cov_instr }} -Ccodegen-units=1 -Clto=off -Zno-profiler-runtime"
 # Coverage fragments the wasm module build commands append unconditionally (empty unless ET_TEST_COVERAGE is set).
 # wasm_cov is an inline RUSTFLAGS env prefix; the nightly toolchain + conda clang come from the coverage [env].
 # The *_feat vars turn on minicov: cargo `--features` for the WASI cdylibs, wasm-pack `-- --features` for browsers
diff --git a/config/coverage.toml b/config/coverage.toml
index f6c4b31..31118aa 100644
--- a/config/coverage.toml
+++ b/config/coverage.toml
@@ -7,6 +7,11 @@
 # must be a real on-disk path so `coverage xml` can read source.
 
 [tool.coverage.run]
+# Branch coverage must be enabled identically on every data source `coverage combine` merges.
+# This rcfile covers the pytest runs (via COVERAGE_RCFILE); the Pyodide runs set branch=True on their Coverage()
+# constructor directly because no COVERAGE_RCFILE env reaches the in-browser interpreter. A mismatch makes
+# `coverage combine` refuse to merge arc data with line data for the same source files.
+branch = true
 relative_files = true
 
 [tool.coverage.paths]
diff --git a/services/ws-web-runner/src/shims/pycov.js b/services/ws-web-runner/src/shims/pycov.js
index 7e9ee5d..0e3d873 100644
--- a/services/ws-web-runner/src/shims/pycov.js
+++ b/services/ws-web-runner/src/shims/pycov.js
@@ -15,7 +15,7 @@ if (globalThis.__ET_TEST_COVERAGE) {
       await pyodide.loadPackage("coverage");
       pyodide.runPython(`
 import coverage as _et_cov_mod
-_et_cov = _et_cov_mod.Coverage(data_file="/tmp/${pkg}.coverage", source=["${pkg}"])
+_et_cov = _et_cov_mod.Coverage(data_file="/tmp/${pkg}.coverage", source=["${pkg}"], branch=True)
 _et_cov.start()
 `);
     },
diff --git a/utilities/wasm-cov-wrapper/src/main.rs b/utilities/wasm-cov-wrapper/src/main.rs
index 963e390..6a9bc8c 100644
--- a/utilities/wasm-cov-wrapper/src/main.rs
+++ b/utilities/wasm-cov-wrapper/src/main.rs
@@ -40,6 +40,7 @@ fn main() -> ExitCode {
             [
                 "--emit=llvm-ir",
                 "-Cinstrument-coverage",
+                "-Zcoverage-options=branch",
                 "-Ccodegen-units=1",
                 "-Clto=off",
                 "-Zno-profiler-runtime",