diff --git a/.github/workflows/dry-run.yml b/.github/workflows/dry-run.yml
index 906ba3041..4d880f25f 100644
--- a/.github/workflows/dry-run.yml
+++ b/.github/workflows/dry-run.yml
@@ -15,6 +15,11 @@ on:
         description: 'Skip build+smoke steps (faster iteration on lint/tests)'
         type: boolean
         default: false
+      soak_level:
+        description: 'Soak test level: full (quick+asan+nightly), quick (10min only), none'
+        type: choice
+        options: ['full', 'quick', 'none']
+        default: 'quick'
 
 permissions:
   contents: read
@@ -503,3 +508,88 @@ jobs:
             exit 1
           }
           Write-Host "=== Windows Defender: clean ==="
+
+  # ── Step 6: Soak tests (after smoke, per-platform) ──────────
+  soak-quick:
+    if: ${{ inputs.soak_level != 'none' && !inputs.skip_builds }}
+    needs: [smoke-unix]
+    runs-on: ${{ matrix.os }}
+    timeout-minutes: 30
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - os: ubuntu-latest
+            goos: linux
+            goarch: amd64
+            cc: gcc
+            cxx: g++
+          - os: ubuntu-24.04-arm
+            goos: linux
+            goarch: arm64
+            cc: gcc
+            cxx: g++
+          - os: macos-14
+            goos: darwin
+            goarch: arm64
+            cc: cc
+            cxx: c++
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4
+
+      - name: Install deps (Linux)
+        if: startsWith(matrix.os, 'ubuntu')
+        run: sudo apt-get update && sudo apt-get install -y zlib1g-dev python3 git
+
+      - name: Build (release mode)
+        run: scripts/build.sh CC=${{ matrix.cc }} CXX=${{ matrix.cxx }}
+
+      - name: Quick soak (10 min)
+        run: scripts/soak-test.sh build/c/codebase-memory-mcp 10
+
+      - name: Upload soak metrics
+        if: always()
+        uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4
+        with:
+          name: soak-quick-${{ matrix.goos }}-${{ matrix.goarch }}
+          path: soak-results/
+          retention-days: 14
+
+  soak-asan:
+    if: ${{ inputs.soak_level == 'full' && !inputs.skip_builds }}
+    needs: [smoke-unix]
+    runs-on: ${{ matrix.os }}
+    timeout-minutes: 45
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - os: ubuntu-latest
+            goarch: amd64
+          - os: ubuntu-24.04-arm
+            goarch: arm64
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4
+
+      - name: Install deps
+        run: sudo apt-get update && sudo apt-get install -y zlib1g-dev python3 git
+
+      - name: Build (ASan + LeakSanitizer)
+        run: |
+          scripts/test.sh CC=gcc CXX=g++
+          # test.sh builds with ASan; the test-runner binary has ASan linked
+          cp build/c/test-runner build/c/codebase-memory-mcp-asan || true
+          scripts/build.sh CC=gcc CXX=g++
+
+      - name: ASan soak (15 min)
+        env:
+          ASAN_OPTIONS: "detect_leaks=1:halt_on_error=0"
+        run: scripts/soak-test.sh build/c/codebase-memory-mcp 15
+
+      - name: Upload ASan soak metrics
+        if: always()
+        uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4
+        with:
+          name: soak-asan-linux-${{ matrix.goarch }}
+          path: soak-results/
+          retention-days: 14
diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index faffd8b76..d00089bca 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -16,6 +16,11 @@ on:
         required: false
         type: boolean
         default: false
+      soak_level:
+        description: 'Soak test level: full (quick+asan), quick (10min only), none (skip)'
+        type: choice
+        options: ['full', 'quick', 'none']
+        default: 'quick'
 
 permissions:
   contents: write
@@ -505,9 +510,84 @@ jobs:
           }
           Write-Host "=== Windows Defender: clean ==="
 
-  # ── Step 5: Create DRAFT release (not public yet) ─────────────
+  # ── Step 5a: Quick soak (parallel with smoke, per-platform) ────
+  soak-quick:
+    if: ${{ inputs.soak_level != 'none' }}
+    needs: [build-unix]
+    runs-on: ${{ matrix.os }}
+    timeout-minutes: 30
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - os: ubuntu-latest
+            goos: linux
+            goarch: amd64
+            cc: gcc
+            cxx: g++
+          - os: ubuntu-24.04-arm
+            goos: linux
+            goarch: arm64
+            cc: gcc
+            cxx: g++
+          - os: macos-14
+            goos: darwin
+            goarch: arm64
+            cc: cc
+            cxx: c++
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4
+      - name: Install deps (Linux)
+        if: startsWith(matrix.os, 'ubuntu')
+        run: sudo apt-get update && sudo apt-get install -y zlib1g-dev python3 git
+      - name: Build (release mode)
+        run: scripts/build.sh --version ${{ inputs.version }} CC=${{ matrix.cc }} CXX=${{ matrix.cxx }}
+      - name: Quick soak (10 min)
+        run: scripts/soak-test.sh build/c/codebase-memory-mcp 10
+      - name: Upload soak metrics
+        if: always()
+        uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4
+        with:
+          name: soak-quick-${{ matrix.goos }}-${{ matrix.goarch }}
+          path: soak-results/
+          retention-days: 14
+
+  # ── Step 5b: ASan soak (Linux only, parallel) ─────────────────
+  soak-asan:
+    if: ${{ inputs.soak_level == 'full' }}
+    needs: [build-unix]
+    runs-on: ${{ matrix.os }}
+    timeout-minutes: 45
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - os: ubuntu-latest
+            goarch: amd64
+          - os: ubuntu-24.04-arm
+            goarch: arm64
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4
+      - name: Install deps
+        run: sudo apt-get update && sudo apt-get install -y zlib1g-dev python3 git
+      - name: Build (release mode for soak)
+        run: scripts/build.sh CC=gcc CXX=g++
+      - name: ASan soak (15 min)
+        env:
+          ASAN_OPTIONS: "detect_leaks=1:halt_on_error=0"
+        run: scripts/soak-test.sh build/c/codebase-memory-mcp 15
+      - name: Upload ASan soak metrics
+        if: always()
+        uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4
+        with:
+          name: soak-asan-linux-${{ matrix.goarch }}
+          path: soak-results/
+          retention-days: 14
+
+  # ── Step 6: Create DRAFT release (not public yet) ─────────────
   release-draft:
-    needs: [smoke-unix, smoke-windows, security-static, codeql-gate]
+    needs: [smoke-unix, smoke-windows, security-static, codeql-gate, soak-quick]
+    if: ${{ !cancelled() && !failure() }}
     runs-on: ubuntu-latest
     permissions:
       contents: write
diff --git a/Makefile.cbm b/Makefile.cbm
index a5dc2120d..b3bb4a8c1 100644
--- a/Makefile.cbm
+++ b/Makefile.cbm
@@ -101,7 +101,8 @@ FOUNDATION_SRCS = \
     src/foundation/compat_thread.c \
     src/foundation/compat_fs.c \
     src/foundation/compat_regex.c \
-    src/foundation/mem.c
+    src/foundation/mem.c \
+    src/foundation/diagnostics.c
 
 # Existing extraction C code (compiled from current location)
 EXTRACTION_SRCS = \
diff --git a/scripts/soak-test.sh b/scripts/soak-test.sh
new file mode 100755
index 000000000..f7a5c33fb
--- /dev/null
+++ b/scripts/soak-test.sh
@@ -0,0 +1,320 @@
+#!/bin/bash
+# soak-test.sh — Endurance test for codebase-memory-mcp.
+#
+# Runs compressed workload cycles: queries, file mutations, reindexes, idle periods.
+# Reads diagnostics from /tmp/cbm-diagnostics-<pid>.json (requires CBM_DIAGNOSTICS=1).
+# Outputs metrics to soak-results/ and exits 0 (pass) or 1 (fail).
+#
+# Usage:
+#   scripts/soak-test.sh <binary> <duration_minutes> [--skip-crash-test]
+#
+# Tiers:
+#   10 min  = quick soak (CI gate)
+#   15 min  = ASan soak (leak detection)
+#   240 min = nightly (compressed 4h = ~5 days real usage)
+
+set -euo pipefail
+
+BINARY="${1:?Usage: soak-test.sh <binary> <duration_minutes>}"
+DURATION_MIN="${2:?Usage: soak-test.sh <binary> <duration_minutes>}"
+SKIP_CRASH="${3:-}"
+BINARY=$(cd "$(dirname "$BINARY")" && pwd)/$(basename "$BINARY")
+
+RESULTS_DIR="soak-results"
+mkdir -p "$RESULTS_DIR"
+
+METRICS_CSV="$RESULTS_DIR/metrics.csv"
+LATENCY_CSV="$RESULTS_DIR/latency.csv"
+SUMMARY="$RESULTS_DIR/summary.txt"
+
+echo "timestamp,uptime_s,rss_bytes,heap_committed,fd_count,query_count,query_max_us" > "$METRICS_CSV"
+echo "timestamp,tool,duration_ms,exit_code" > "$LATENCY_CSV"
+> "$SUMMARY"
+
+DURATION_S=$((DURATION_MIN * 60))
+
+echo "=== soak-test: binary=$BINARY duration=${DURATION_MIN}m ==="
+
+# ── Helper: create test project ──────────────────────────────────
+
+SOAK_PROJECT=$(mktemp -d)
+mkdir -p "$SOAK_PROJECT/src" "$SOAK_PROJECT/lib"
+
+# Create a realistic small project with multiple languages
+cat > "$SOAK_PROJECT/src/main.py" << 'PYEOF'
+from lib.utils import compute, validate
+
+def process(data):
+    if validate(data):
+        return compute(data)
+    return None
+
+def main():
+    result = process({"key": "value"})
+    print(result)
+
+if __name__ == "__main__":
+    main()
+PYEOF
+
+cat > "$SOAK_PROJECT/lib/utils.py" << 'PYEOF'
+def compute(data):
+    total = sum(len(str(v)) for v in data.values())
+    return total
+
+def validate(data):
+    return isinstance(data, dict) and len(data) > 0
+
+def helper():
+    return 42
+PYEOF
+
+cat > "$SOAK_PROJECT/src/server.go" << 'GOEOF'
+package main
+
+import "fmt"
+
+func StartServer(port int) error {
+    fmt.Printf("listening on %d\n", port)
+    return nil
+}
+
+func HandleRequest(path string) string {
+    return "ok: " + path
+}
+GOEOF
+
+cat > "$SOAK_PROJECT/config.yaml" << 'YAMLEOF'
+database:
+  host: localhost
+  port: 5432
+server:
+  workers: 4
+YAMLEOF
+
+# Init git repo (required for watcher)
+git -C "$SOAK_PROJECT" init -q 2>/dev/null
+git -C "$SOAK_PROJECT" add -A 2>/dev/null
+git -C "$SOAK_PROJECT" -c user.email=test@test -c user.name=test commit -q -m "init" 2>/dev/null
+
+# ── Helper: run CLI tool call and record latency ─────────────────
+
+cli_call() {
+    local tool="$1"
+    local args="$2"
+    local t0
+    t0=$(python3 -c "import time; print(int(time.time()*1000))")
+    "$BINARY" cli "$tool" "$args" > /dev/null 2>&1
+    local rc=$?
+    local t1
+    t1=$(python3 -c "import time; print(int(time.time()*1000))")
+    local dur=$((t1 - t0))
+    echo "$(date +%s),$tool,$dur,$rc" >> "$LATENCY_CSV"
+}
+
+# ── Helper: collect diagnostics snapshot ─────────────────────────
+
+collect_snapshot() {
+    local diag_file="/tmp/cbm-diagnostics-${SERVER_PID}.json"
+    if [ -f "$diag_file" ]; then
+        local ts=$(date +%s)
+        local uptime=$(python3 -c "import json; d=json.load(open('$diag_file')); print(d.get('uptime_s',0))" 2>/dev/null || echo "0")
+        local rss=$(python3 -c "import json; d=json.load(open('$diag_file')); print(d.get('rss_bytes',0))" 2>/dev/null || echo "0")
+        local commit=$(python3 -c "import json; d=json.load(open('$diag_file')); print(d.get('heap_committed_bytes',0))" 2>/dev/null || echo "0")
+        local fds=$(python3 -c "import json; d=json.load(open('$diag_file')); print(d.get('fd_count',0))" 2>/dev/null || echo "0")
+        local qcount=$(python3 -c "import json; d=json.load(open('$diag_file')); print(d.get('query_count',0))" 2>/dev/null || echo "0")
+        local qmax=$(python3 -c "import json; d=json.load(open('$diag_file')); print(d.get('query_max_us',0))" 2>/dev/null || echo "0")
+        echo "$ts,$uptime,$rss,$commit,$fds,$qcount,$qmax" >> "$METRICS_CSV"
+    fi
+}
+
+# ── Phase 1: Start MCP server with diagnostics ──────────────────
+
+echo "--- Phase 1: start server ---"
+# Keep stdin open with a sleeping process — kill it to send EOF
+sleep 999999 | CBM_DIAGNOSTICS=1 "$BINARY" > /dev/null 2>"$RESULTS_DIR/server-stderr.log" &
+SERVER_PID=$!
+SLEEP_PID=$(jobs -p | head -1)
+sleep 3
+
+if ! kill -0 "$SERVER_PID" 2>/dev/null; then
+    echo "FAIL: server did not start"
+    kill "$SLEEP_PID" 2>/dev/null || true
+    exit 1
+fi
+echo "OK: server running (pid=$SERVER_PID)"
+
+# ── Phase 2: Initial index ───────────────────────────────────────
+
+echo "--- Phase 2: initial index ---"
+cli_call index_repository "{\"repo_path\":\"$SOAK_PROJECT\"}"
+sleep 6  # wait for diagnostics write
+collect_snapshot
+
+# Derive project name (same logic as cbm_project_name_from_path)
+PROJ_NAME=$(echo "$SOAK_PROJECT" | sed 's|^/||; s|/|-|g')
+
+BASELINE_RSS=$(python3 -c "import json; d=json.load(open('/tmp/cbm-diagnostics-${SERVER_PID}.json')); print(d.get('rss_bytes',0))" 2>/dev/null || echo "0")
+BASELINE_FDS=$(python3 -c "import json; d=json.load(open('/tmp/cbm-diagnostics-${SERVER_PID}.json')); print(d.get('fd_count',0))" 2>/dev/null || echo "0")
+echo "OK: baseline RSS=${BASELINE_RSS} FDs=${BASELINE_FDS}"
+
+# ── Phase 3: Compressed workload loop ────────────────────────────
+
+echo "--- Phase 3: workload loop (${DURATION_MIN}m) ---"
+START_TIME=$(date +%s)
+END_TIME=$((START_TIME + DURATION_S))
+CYCLE=0
+LAST_MUTATE=0
+LAST_REINDEX=0
+
+while [ "$(date +%s)" -lt "$END_TIME" ]; do
+    NOW=$(date +%s)
+    CYCLE=$((CYCLE + 1))
+
+    # Queries every 2 seconds
+    cli_call search_graph "{\"project\":\"$PROJ_NAME\",\"name_pattern\":\".*compute.*\"}"
+    cli_call trace_call_path "{\"project\":\"$PROJ_NAME\",\"function_name\":\"compute\",\"direction\":\"both\"}"
+
+    # File mutation every 2 minutes
+    if [ $((NOW - LAST_MUTATE)) -ge 120 ]; then
+        echo "# mutation at cycle $CYCLE $(date)" >> "$SOAK_PROJECT/src/main.py"
+        git -C "$SOAK_PROJECT" add -A 2>/dev/null
+        git -C "$SOAK_PROJECT" -c user.email=test@test -c user.name=test commit -q -m "cycle $CYCLE" 2>/dev/null || true
+        LAST_MUTATE=$NOW
+    fi
+
+    # Full reindex every 5 minutes
+    if [ $((NOW - LAST_REINDEX)) -ge 300 ]; then
+        cli_call index_repository "{\"repo_path\":\"$SOAK_PROJECT\"}"
+        LAST_REINDEX=$NOW
+    fi
+
+    # Collect diagnostics every 30 seconds
+    if [ $((CYCLE % 15)) -eq 0 ]; then
+        collect_snapshot
+    fi
+
+    sleep 2
+done
+
+# ── Phase 4: Idle period + final snapshot ────────────────────────
+
+echo "--- Phase 4: idle (30s) ---"
+sleep 30
+collect_snapshot
+
+# Check idle CPU
+IDLE_CPU=$(ps -o %cpu= -p "$SERVER_PID" 2>/dev/null | tr -d ' ' || echo "0")
+echo "OK: idle CPU=${IDLE_CPU}%"
+
+# ── Phase 5: Crash recovery test ────────────────────────────────
+
+if [ "$SKIP_CRASH" != "--skip-crash-test" ]; then
+    echo "--- Phase 5: crash recovery ---"
+
+    # Start a reindex then kill -9
+    cli_call index_repository "{\"repo_path\":\"$SOAK_PROJECT\"}" &
+    INDEX_PID=$!
+    sleep 1
+    kill -9 "$INDEX_PID" 2>/dev/null || true
+    wait "$INDEX_PID" 2>/dev/null || true
+
+    # Verify the server is still alive (kill was on CLI, not server)
+    if kill -0 "$SERVER_PID" 2>/dev/null; then
+        echo "OK: server survived crash test"
+
+        # Verify clean re-index after crash
+        cli_call index_repository "{\"repo_path\":\"$SOAK_PROJECT\"}"
+        echo "OK: clean re-index after crash"
+    else
+        echo "FAIL: server died during crash test"
+    fi
+fi
+
+# ── Phase 6: Shutdown + analysis ─────────────────────────────────
+
+echo "--- Phase 6: shutdown + analysis ---"
+kill "$SLEEP_PID" 2>/dev/null || true  # close stdin → EOF → server exits
+sleep 2
+kill "$SERVER_PID" 2>/dev/null || true
+wait "$SERVER_PID" 2>/dev/null || true
+
+# Final diagnostics (written by thread before exit)
+FINAL_DIAG="/tmp/cbm-diagnostics-${SERVER_PID}.json"
+
+# ── Analysis ─────────────────────────────────────────────────────
+
+PASS=true
+
+# Check 1: RSS growth
+# For short runs (<10min): check absolute ceiling (warmup noise dominates slope)
+# For long runs (>=10min): use linear regression on post-warmup samples (skip first 20%)
+TOTAL_SAMPLES=$(awk -F, 'NR>1 && $3>0 { n++ } END { print n+0 }' "$METRICS_CSV")
+MAX_RSS=$(awk -F, 'NR>1 && $3>0 { if ($3>max) max=$3 } END { printf "%.0f", max/1024/1024 }' "$METRICS_CSV")
+echo "RSS max: ${MAX_RSS}MB (${TOTAL_SAMPLES} samples)" | tee -a "$SUMMARY"
+
+if [ "$DURATION_MIN" -lt 10 ]; then
+    # Short run: absolute ceiling check
+    if [ "${MAX_RSS:-0}" -gt 100 ] 2>/dev/null; then
+        echo "FAIL: RSS ${MAX_RSS}MB > 100MB ceiling" | tee -a "$SUMMARY"
+        PASS=false
+    fi
+else
+    # Long run: slope after skipping warmup (first 20% of samples)
+    RSS_SLOPE=$(awk -F, -v skip="$((TOTAL_SAMPLES / 5))" '
+    NR>1 && $3>0 {
+        row++
+        if (row <= skip) next
+        n++; x=$1; y=$3; sx+=x; sy+=y; sxx+=x*x; sxy+=x*y
+    }
+    END {
+        if (n<5) { print 0; exit }
+        slope = (n*sxy - sx*sy) / (n*sxx - sx*sx)
+        printf "%.0f", slope * 3600 / 1024
+    }' "$METRICS_CSV")
+    echo "RSS slope (post-warmup): ${RSS_SLOPE} KB/hr" | tee -a "$SUMMARY"
+    if [ "${RSS_SLOPE:-0}" -gt 500 ] 2>/dev/null; then
+        echo "FAIL: RSS slope ${RSS_SLOPE} KB/hr > 500 KB/hr threshold" | tee -a "$SUMMARY"
+        PASS=false
+    fi
+fi
+
+# Check 2: FD drift
+FD_DRIFT=$(awk -F, 'NR>1 && $5>0 { if (!first) first=$5; last=$5 } END { print last-first }' "$METRICS_CSV")
+echo "FD drift: ${FD_DRIFT:-0}" | tee -a "$SUMMARY"
+if [ "${FD_DRIFT:-0}" -gt 20 ] 2>/dev/null; then
+    echo "FAIL: FD drift ${FD_DRIFT} > 20" | tee -a "$SUMMARY"
+    PASS=false
+fi
+
+# Check 3: Idle CPU
+IDLE_INT=$(echo "$IDLE_CPU" | cut -d. -f1)
+echo "Idle CPU: ${IDLE_CPU}%" | tee -a "$SUMMARY"
+if [ "${IDLE_INT:-0}" -gt 5 ] 2>/dev/null; then
+    echo "FAIL: idle CPU ${IDLE_CPU}% > 5%" | tee -a "$SUMMARY"
+    PASS=false
+fi
+
+# Check 4: Max query latency
+MAX_LATENCY=$(awk -F, 'NR>1 { if ($3>max) max=$3 } END { print max+0 }' "$LATENCY_CSV")
+echo "Max query latency: ${MAX_LATENCY}ms" | tee -a "$SUMMARY"
+if [ "${MAX_LATENCY:-0}" -gt 10000 ] 2>/dev/null; then
+    echo "FAIL: max latency ${MAX_LATENCY}ms > 10s" | tee -a "$SUMMARY"
+    PASS=false
+fi
+
+# Check 5: Query count (sanity — should have many)
+TOTAL_QUERIES=$(awk -F, 'NR>1 { n++ } END { print n+0 }' "$LATENCY_CSV")
+echo "Total queries: $TOTAL_QUERIES" | tee -a "$SUMMARY"
+
+# ── Cleanup ──────────────────────────────────────────────────────
+
+rm -rf "$SOAK_PROJECT"
+
+echo ""
+if $PASS; then
+    echo "=== soak-test: PASSED ===" | tee -a "$SUMMARY"
+else
+    echo "=== soak-test: FAILED ===" | tee -a "$SUMMARY"
+    exit 1
+fi
diff --git a/src/foundation/diagnostics.c b/src/foundation/diagnostics.c
new file mode 100644
index 000000000..ff6abac17
--- /dev/null
+++ b/src/foundation/diagnostics.c
@@ -0,0 +1,199 @@
+/*
+ * diagnostics.c — Periodic diagnostics file writer.
+ *
+ * Writes JSON to /tmp/cbm-diagnostics-<pid>.json every 5 seconds.
+ * Atomic: writes .tmp then renames to avoid partial reads.
+ */
+#include "foundation/diagnostics.h"
+#include "foundation/mem.h"
+#include "foundation/compat.h"
+#include "foundation/compat_thread.h"
+#include "foundation/compat_fs.h"
+#include "foundation/platform.h"
+
+#include <mimalloc.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <time.h>
+
+#ifdef _WIN32
+#include <process.h>
+#define getpid _getpid
+#else
+#include <dirent.h>
+#include <unistd.h>
+#endif
+
+/* ── Globals ─────────────────────────────────────────────────────── */
+
+cbm_query_stats_t g_query_stats = {0};
+static atomic_int g_diag_stop = 0;
+static cbm_thread_t g_diag_thread;
+static bool g_diag_started = false;
+static time_t g_start_time = 0;
+static char g_diag_path[256] = "";
+
+/* ── Query stats ─────────────────────────────────────────────────── */
+
+void cbm_diag_record_query(long long duration_us, bool is_error) {
+    atomic_fetch_add(&g_query_stats.count, 1);
+    atomic_fetch_add(&g_query_stats.time_us, duration_us);
+    if (is_error) {
+        atomic_fetch_add(&g_query_stats.errors, 1);
+    }
+    /* Update max (lock-free CAS loop) */
+    long long old_max = atomic_load(&g_query_stats.max_us);
+    while (duration_us > old_max) {
+        if (atomic_compare_exchange_weak(&g_query_stats.max_us, &old_max, duration_us)) {
+            break;
+        }
+    }
+}
+
+/* ── FD count (platform-specific) ────────────────────────────────── */
+
+static int count_open_fds(void) {
+#ifdef __linux__
+    int count = 0;
+    DIR *d = opendir("/proc/self/fd");
+    if (d) {
+        while (readdir(d)) {
+            count++;
+        }
+        closedir(d);
+        count -= 2; /* . and .. */
+    }
+    return count;
+#elif defined(__APPLE__)
+    /* Use proc_pidinfo on macOS — but for simplicity, count via /dev/fd */
+    int count = 0;
+    DIR *d = opendir("/dev/fd");
+    if (d) {
+        while (readdir(d)) {
+            count++;
+        }
+        closedir(d);
+        count -= 2; /* . and .. */
+    }
+    return count;
+#else
+    return -1; /* Not available on Windows */
+#endif
+}
+
+/* ── Writer ──────────────────────────────────────────────────────── */
+
+#define DIAG_INTERVAL_S 5
+#define DIAG_PATH_EXTRA 24 /* ".tmp" + safety margin */
+
+static void write_diagnostics(void) {
+    /* Collect mimalloc stats */
+    size_t elapsed_ms = 0;
+    size_t user_ms = 0;
+    size_t sys_ms = 0;
+    size_t current_rss = 0;
+    size_t peak_rss = 0;
+    size_t current_commit = 0;
+    size_t peak_commit = 0;
+    size_t page_faults = 0;
+    mi_process_info(&elapsed_ms, &user_ms, &sys_ms, &current_rss, &peak_rss, &current_commit,
+                    &peak_commit, &page_faults);
+
+    /* Fallback RSS for ASan builds */
+    if (current_rss == 0) {
+        current_rss = cbm_mem_rss();
+    }
+
+    int fds = count_open_fds();
+    time_t now = time(NULL);
+    long uptime = (long)(now - g_start_time);
+
+    int qcount = atomic_load(&g_query_stats.count);
+    int qerrors = atomic_load(&g_query_stats.errors);
+    long long qtime = atomic_load(&g_query_stats.time_us);
+    long long qmax = atomic_load(&g_query_stats.max_us);
+    long long qavg = qcount > 0 ? qtime / qcount : 0;
+
+    /* Write to .tmp then rename (atomic) */
+    char tmp_path[sizeof(g_diag_path) + DIAG_PATH_EXTRA];
+    snprintf(tmp_path, sizeof(tmp_path), "%s.tmp", g_diag_path);
+
+    FILE *f = fopen(tmp_path, "w");
+    if (!f) {
+        return;
+    }
+
+    fprintf(f,
+            "{\n"
+            "  \"uptime_s\": %ld,\n"
+            "  \"rss_bytes\": %zu,\n"
+            "  \"peak_rss_bytes\": %zu,\n"
+            "  \"heap_committed_bytes\": %zu,\n"
+            "  \"peak_committed_bytes\": %zu,\n"
+            "  \"page_faults\": %zu,\n"
+            "  \"fd_count\": %d,\n"
+            "  \"query_count\": %d,\n"
+            "  \"query_errors\": %d,\n"
+            "  \"query_total_us\": %lld,\n"
+            "  \"query_avg_us\": %lld,\n"
+            "  \"query_max_us\": %lld,\n"
+            "  \"pid\": %d\n"
+            "}\n",
+            uptime, current_rss, peak_rss, current_commit, peak_commit, page_faults, fds, qcount,
+            qerrors, qtime, qavg, qmax, (int)getpid());
+    fclose(f);
+    rename(tmp_path, g_diag_path);
+}
+
+static void *diag_thread_fn(void *arg) {
+    (void)arg;
+    while (!atomic_load(&g_diag_stop)) {
+        write_diagnostics();
+        struct timespec ts = {DIAG_INTERVAL_S, 0};
+        cbm_nanosleep(&ts, NULL);
+    }
+    /* Final write before exit */
+    write_diagnostics();
+    return NULL;
+}
+
+/* ── Public API ──────────────────────────────────────────────────── */
+
+bool cbm_diag_start(void) {
+    // NOLINTNEXTLINE(concurrency-mt-unsafe)
+    const char *env = getenv("CBM_DIAGNOSTICS");
+    if (!env || (strcmp(env, "1") != 0 && strcmp(env, "true") != 0)) {
+        return false;
+    }
+
+    g_start_time = time(NULL);
+    atomic_store(&g_diag_stop, 0);
+
+    snprintf(g_diag_path, sizeof(g_diag_path), "%s/cbm-diagnostics-%d.json", cbm_tmpdir(),
+             (int)getpid());
+
+    if (cbm_thread_create(&g_diag_thread, 0, diag_thread_fn, NULL) != 0) {
+        return false;
+    }
+
+    g_diag_started = true;
+    fprintf(stderr, "level=info msg=diagnostics.start path=%s interval=%ds\n", g_diag_path,
+            DIAG_INTERVAL_S);
+    return true;
+}
+
+void cbm_diag_stop(void) {
+    if (!g_diag_started) {
+        return;
+    }
+    atomic_store(&g_diag_stop, 1);
+    cbm_thread_join(&g_diag_thread);
+    g_diag_started = false;
+
+    /* Clean up file */
+    cbm_unlink(g_diag_path);
+    char tmp_path[sizeof(g_diag_path) + DIAG_PATH_EXTRA];
+    snprintf(tmp_path, sizeof(tmp_path), "%s.tmp", g_diag_path);
+    cbm_unlink(tmp_path);
+}
diff --git a/src/foundation/diagnostics.h b/src/foundation/diagnostics.h
new file mode 100644
index 000000000..b34511927
--- /dev/null
+++ b/src/foundation/diagnostics.h
@@ -0,0 +1,35 @@
+/*
+ * diagnostics.h — Periodic diagnostics file writer.
+ *
+ * When CBM_DIAGNOSTICS=1, writes /tmp/cbm-diagnostics-<pid>.json every 5s.
+ * Soak tests read this file to track memory, FDs, query stats over time.
+ */
+#ifndef CBM_DIAGNOSTICS_H
+#define CBM_DIAGNOSTICS_H
+
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdatomic.h>
+
+/* Global query stats — updated by the MCP server on each tool call. */
+typedef struct {
+    atomic_int count;     /* total tool calls */
+    atomic_int errors;    /* tool calls that returned isError=true */
+    atomic_llong time_us; /* cumulative wall-clock time (microseconds) */
+    atomic_llong max_us;  /* max single call time (microseconds) */
+} cbm_query_stats_t;
+
+/* Singleton query stats — MCP server increments these. */
+extern cbm_query_stats_t g_query_stats;
+
+/* Record a completed tool call. */
+void cbm_diag_record_query(long long duration_us, bool is_error);
+
+/* Start the diagnostics writer thread (if CBM_DIAGNOSTICS env is set).
+ * Call once from main(). Returns true if started. */
+bool cbm_diag_start(void);
+
+/* Stop the writer thread and delete the diagnostics file. */
+void cbm_diag_stop(void);
+
+#endif /* CBM_DIAGNOSTICS_H */
diff --git a/src/main.c b/src/main.c
index d21c761f9..70eadcdcd 100644
--- a/src/main.c
+++ b/src/main.c
@@ -19,6 +19,7 @@
 #include "store/store.h"
 #include "cli/cli.h"
 #include "foundation/log.h"
+#include "foundation/diagnostics.h"
 #include "foundation/platform.h"
 #include "foundation/compat_thread.h"
 #include "foundation/mem.h"
@@ -192,6 +193,7 @@ int main(int argc, char **argv) {
     cbm_http_server_set_binary_path(argv[0]);
     cbm_log_set_sink(cbm_ui_log_append);
     cbm_log_info("server.start", "version", CBM_VERSION);
+    cbm_diag_start(); /* starts if CBM_DIAGNOSTICS=1 */
 
     /* Parse --ui and --port flags (persisted config) */
     cbm_ui_config_t ui_cfg;
@@ -302,6 +304,7 @@ int main(int argc, char **argv) {
 
     g_watcher = NULL;
     g_server = NULL;
+    cbm_diag_stop();
 
     return rc;
 }
diff --git a/src/mcp/mcp.c b/src/mcp/mcp.c
index 545b7c4a7..4542584b8 100644
--- a/src/mcp/mcp.c
+++ b/src/mcp/mcp.c
@@ -14,6 +14,7 @@
 #include "cli/cli.h"
 #include "watcher/watcher.h"
 #include "foundation/mem.h"
+#include "foundation/diagnostics.h"
 #include "foundation/platform.h"
 #include "foundation/compat.h"
 #include "foundation/compat_fs.h"
@@ -2721,7 +2722,6 @@ char *cbm_mcp_handle_tool(cbm_mcp_server_t *srv, const char *tool_name, const ch
     if (strcmp(tool_name, "ingest_traces") == 0) {
         return handle_ingest_traces(srv, args_json);
     }
-
     char msg[256];
     snprintf(msg, sizeof(msg), "unknown tool: %s", tool_name);
     return cbm_mcp_text_result(msg, true);
@@ -2997,7 +2997,17 @@ char *cbm_mcp_server_handle(cbm_mcp_server_t *srv, const char *line) {
         char *tool_args =
             req.params_raw ? cbm_mcp_get_arguments(req.params_raw) : heap_strdup("{}");
 
+        struct timespec t0;
+        cbm_clock_gettime(CLOCK_MONOTONIC, &t0);
         result_json = cbm_mcp_handle_tool(srv, tool_name, tool_args);
+        struct timespec t1;
+        cbm_clock_gettime(CLOCK_MONOTONIC, &t1);
+        long long dur_us = ((long long)(t1.tv_sec - t0.tv_sec) * 1000000LL) +
+                           ((long long)(t1.tv_nsec - t0.tv_nsec) / 1000LL);
+        // NOLINTNEXTLINE(readability-implicit-bool-conversion)
+        bool is_err = (result_json != NULL) && (strstr(result_json, "\"isError\":true") != NULL);
+        cbm_diag_record_query(dur_us, is_err);
+
         result_json = inject_update_notice(srv, result_json);
         free(tool_name);
         free(tool_args);