Revamp code coverage tooling

.bazelrc

@@ -283,16 +283,10 @@ build:stamp-full --stamp --workspace_status_command='./bazel/stamp_vars.sh full'
 # Coverage
 # =================================
 coverage --config=coverage
-build:coverage --action_env=BAZEL_USE_LLVM_NATIVE_COVERAGE=1
-build:coverage --action_env=GCOV=llvm-profdata
-build:coverage --copt=-DNDEBUG
-build:coverage --define=dynamic_link_tests=true
+build:coverage --repo_env=BAZEL_USE_LLVM_NATIVE_COVERAGE=1
 build:coverage --combined_report=lcov
 build:coverage --experimental_use_llvm_covmap
 build:coverage --experimental_generate_llvm_lcov
-build:coverage --experimental_split_coverage_postprocessing
-build:coverage --experimental_fetch_all_coverage_outputs
-build:coverage --collect_code_coverage
 build:coverage --instrumentation_filter="^//src/v[/:]"
 
 # sanitizer is a deprecated alias for debug

.claude/skills/improve-coverage/SKILL.md

@@ -0,0 +1,106 @@
+---
+name: improve-coverage
+description: Improve code coverage for a Bazel C++ test target or source file
+user-invocable: true
+---
+
+# Improve Code Coverage
+
+Improve test coverage for a Bazel C++ test target or source file path.
+
+## Input Resolution
+
+1. **Bazel test target** (starts with `//`, e.g. `//src/v/bytes/tests:iobuf_test`):
+   use it directly.
+
+2. **Source file path** (e.g. `src/v/bytes/iobuf.cc`): find the test target:
+   ```bash
+   bazel query "kind('.*_test', rdeps(//src/v/..., //$(dirname $FILE):$(basename $FILE .cc), 1))" 2>/dev/null
+   ```
+   If multiple targets match, pick the one in the closest `tests/` directory.
+   If none match, tell the user and stop.
+
+## Workflow
+
+### Step 1: Baseline Coverage
+
+Run coverage and read the LLM report:
+
+```bash
+tools/run-cov <target> -r llm
+```
+
+Note the **total line coverage percentage** — this is the baseline to beat.
+
+Read the report and identify:
+- **Uncovered functions** — highest impact. One test can cover many lines.
+- **Partially covered functions** — uncovered branches indicate missing
+  error-path or edge-case tests.
+- **Zero-coverage files in scope** — may need new test cases entirely.
+
+### Step 2: Understand the Code
+
+Read the source files named in the report. Focus on:
+- Uncovered functions: what they do, inputs, preconditions.
+- The existing test file: patterns, fixtures, helpers.
+
+Key codebase patterns:
+- `redpanda_cc_gtest` targets use GTest (`TEST()`, `TEST_F()`).
+  `redpanda_cc_btest` targets use Seastar Boost test
+  (`SEASTAR_THREAD_TEST_CASE`). Match whichever the test file uses.
+- Many functions are `ss::future<>` coroutines — test with
+  `SEASTAR_THREAD_TEST_CASE` or the Seastar GTest runner.
+- Use `ss` namespace prefix for Seastar types.
+- Use `EXPECT_*` for most checks, `ASSERT_*` only when continuing would crash.
+
+### Step 3: Write Tests
+
+Prioritize by impact:
+1. **Uncovered functions** — call each one. A 20-line function = 20 lines covered.
+2. **Uncovered error paths** — trigger the uncovered branches in partially
+   covered functions.
+3. **Zero-coverage files** — add smoke tests if practical.
+
+Rules:
+- Add tests to the **existing test file**. Don't create new files or BUILD
+  targets unless necessary.
+- Follow existing naming conventions in the file.
+- One logical behavior per test case.
+- Prefer deterministic tests — construct error conditions directly.
+- Do NOT add comments that restate the code.
+
+### Step 4: Verify Build
+
+```bash
+bazel build <target>
+```
+
+Fix compilation errors before proceeding.
+
+### Step 5: Verify Coverage
+
+Re-run coverage:
+
+```bash
+tools/run-cov <target> -r llm
+```
+
+Compare to the baseline. Report:
+
+```
+Coverage: X.Y% → A.B% (+N.N%)
+New lines covered: <count>
+```
+
+If coverage did not improve, investigate:
+- Build errors preventing the new tests from running?
+- Code compiled out by preprocessor guards?
+- Template instantiations not triggered by test types?
+
+### Step 6: Summary
+
+Report:
+- Which functions/paths are now covered
+- What test cases were added
+- Before/after coverage numbers
+- Remaining significant gaps and suggestions

.gitignore

@@ -138,6 +138,7 @@ coverage.xml
 .hypothesis/
 .pytest_cache/
 genhtml/
+coverage-out/
 
 # Translations
 *.mo

tools/BUILD

@@ -3,6 +3,8 @@ load("@rules_pkg//pkg:tar.bzl", "pkg_tar")
 load("@rules_python//python:defs.bzl", "py_binary")
 load("@rules_shell//shell:sh_binary.bzl", "sh_binary")
 
+exports_files(["run-cov"])
+
 buildifier(
     name = "buildifier.check",
     exclude_patterns = [