diff --git a/Taskfile.yml b/Taskfile.yml
index b5e9007..1263462 100644
--- a/Taskfile.yml
+++ b/Taskfile.yml
@@ -110,7 +110,11 @@ tasks:
         #    harness instead of erroring.
         sh '{{.TASKFILE_DIR}}/bin/check-surfaces' '{{.TASKFILE_DIR}}' || fail=1
 
-        # 5. Plugin manifest version must match the latest tag, or installers
+        # 5. A documented session_log call must be one the tool would accept:
+        #    session_id is its only required field, and nine skills omitted it.
+        sh '{{.TASKFILE_DIR}}/bin/check-session-log' '{{.TASKFILE_DIR}}' || fail=1
+
+        # 6. Plugin manifest version must match the latest tag, or installers
         #    never see the update (Claude Code gates updates on this field).
         if [ -f '{{.PLUGIN_MANIFEST}}' ]; then
           manifest_v=$(sed -n 's/.*"version"[[:space:]]*:[[:space:]]*"\([^"]*\)".*/\1/p' '{{.PLUGIN_MANIFEST}}' | head -1)
diff --git a/bin/check-session-log b/bin/check-session-log
new file mode 100755
index 0000000..cb8770a
--- /dev/null
+++ b/bin/check-session-log
@@ -0,0 +1,39 @@
+#!/bin/sh
+# check-session-log — a skill that documents a session_log call must document a
+# call the tool would actually accept.
+#
+# Nine skills documented the payload and none named `session_id`, which is the
+# tool's only REQUIRED field. An agent following any of them either failed the
+# call or invented an id — and invented ids do not group, so the pass-rate the
+# endpoint computes was being assembled from entries that could not be joined
+# (#36, found by a pressure test against `debug`).
+#
+# Also rejected: `duration_ms: <wall-clock>`. An agent does not observe a clock.
+# A documented field nobody can honestly populate gets fabricated or silently
+# dropped, and fabricated numbers in a metrics stream are worse than no numbers.
+#
+# Usage: check-session-log [checkout-root]
+
+set -u
+
+ROOT=${1:-$(CDPATH= cd -- "$(dirname -- "$0")/.." && pwd)}
+fail=0
+
+for dir in "$ROOT"/skills/*/; do
+  [ -f "$dir/SKILL.md" ] || continue
+  name=$(basename "$dir")
+
+  grep -q 'session_log' "$dir/SKILL.md" || continue
+
+  if ! grep -q 'session_id' "$dir/SKILL.md"; then
+    echo "ERROR: skills/$name documents session_log without session_id — the tool's only required field" >&2
+    fail=1
+  fi
+
+  if grep -q 'duration_ms:[[:space:]]*<' "$dir/SKILL.md"; then
+    echo "ERROR: skills/$name documents duration_ms as a placeholder an agent cannot measure — omit it or pass a real measurement" >&2
+    fail=1
+  fi
+done
+
+exit $fail
diff --git a/skills/cad-babysit/SKILL.md b/skills/cad-babysit/SKILL.md
index 75f3ab4..7369423 100644
--- a/skills/cad-babysit/SKILL.md
+++ b/skills/cad-babysit/SKILL.md
@@ -234,8 +234,8 @@ wrong" — it means "nothing on the known list is wrong."
 Call `session_log` once at the end of every phase to record the outcome.
 
 **At end of each phase:**
-- `session_log` with `{skill: "cad-babysit", phase: "<phase-name>", final_status: "pass" |
-  "fail" | "skip", message: "<one-line summary>", duration_ms: <wall-clock>, project_root:
+- `session_log` with `{session_id: "<this session's id>", skill: "cad-babysit", phase: "<phase-name>", final_status: "pass" |
+  "fail" | "skip", message: "<one-line summary>", project_root:
   "<absolute path>"}`
 
 **Phases for this skill:** query, read-comments, read-actions-logs, classify-fault-vs-outcome,
diff --git a/skills/code-review/SKILL.md b/skills/code-review/SKILL.md
index 7f05048..9ef66d3 100644
--- a/skills/code-review/SKILL.md
+++ b/skills/code-review/SKILL.md
@@ -253,7 +253,7 @@ Pass-rate is computed downstream by the `/pass-rate` HTTP endpoint, which
 treats `pass` as success, `fail` as failure, `skip` as neither.
 
 **At end of each phase:**
-- `session_log` with `{skill: "code-review", phase: "<phase-name>", final_status: "pass" | "fail" | "skip", message: "<one-line summary>", duration_ms: <wall-clock>, project_root: "<absolute path>"}`
+- `session_log` with `{session_id: "<this session's id>", skill: "code-review", phase: "<phase-name>", final_status: "pass" | "fail" | "skip", message: "<one-line summary>", project_root: "<absolute path>"}`
 
 **Phases for this skill:** phase-0-iron-laws, phase-1-context, phase-2-architectural-analysis, phase-3-go-checklist, phase-4-code-smell-detection, phase-5-test-quality
 
diff --git a/skills/debug/SKILL.md b/skills/debug/SKILL.md
index 5052289..290da99 100644
--- a/skills/debug/SKILL.md
+++ b/skills/debug/SKILL.md
@@ -154,7 +154,7 @@ Pass-rate is computed downstream by the `/pass-rate` HTTP endpoint, which
 treats `pass` as success, `fail` as failure, `skip` as neither.
 
 **At end of each phase:**
-- `session_log` with `{skill: "debug", phase: "<phase-name>", final_status: "pass" | "fail" | "skip", message: "<one-line summary>", duration_ms: <wall-clock>, project_root: "<absolute path>"}`
+- `session_log` with `{session_id: "<this session's id>", skill: "debug", phase: "<phase-name>", final_status: "pass" | "fail" | "skip", message: "<one-line summary>", project_root: "<absolute path>"}`
 
 **Phases for this skill:** read-failure, generate-hypotheses, output
 
diff --git a/skills/experiment-spec/SKILL.md b/skills/experiment-spec/SKILL.md
index f4782f4..1654cda 100644
--- a/skills/experiment-spec/SKILL.md
+++ b/skills/experiment-spec/SKILL.md
@@ -185,7 +185,7 @@ If silhouette ≤ 0.20:
 
 **After experiment concludes:**
 - `brain_write` to `jepa-fx/failures/` with any new failure modes discovered
-- `session_log` with outcome
+- `session_log` with `{session_id: "<this session's id>", skill: "experiment-spec", phase: "conclude", final_status: "pass" | "fail" | "skip", message: "<one-line outcome>"}`
 
 ## Cross-References
 
diff --git a/skills/feature-spec/SKILL.md b/skills/feature-spec/SKILL.md
index 29c5628..572af74 100644
--- a/skills/feature-spec/SKILL.md
+++ b/skills/feature-spec/SKILL.md
@@ -245,7 +245,7 @@ Pass-rate is computed downstream by the `/pass-rate` HTTP endpoint, which
 treats `pass` as success, `fail` as failure, `skip` as neither.
 
 **At end of each phase:**
-- `session_log` with `{skill: "feature-spec", phase: "<phase-name>", final_status: "pass" | "fail" | "skip", message: "<one-line summary>", duration_ms: <wall-clock>, project_root: "<absolute path>"}`
+- `session_log` with `{session_id: "<this session's id>", skill: "feature-spec", phase: "<phase-name>", final_status: "pass" | "fail" | "skip", message: "<one-line summary>", project_root: "<absolute path>"}`
 
 **Phases for this skill:** draft
 
diff --git a/skills/session-retrospective/SKILL.md b/skills/session-retrospective/SKILL.md
index bbb7f77..1c090d9 100644
--- a/skills/session-retrospective/SKILL.md
+++ b/skills/session-retrospective/SKILL.md
@@ -116,7 +116,7 @@ Pass-rate is computed downstream by the `/pass-rate` HTTP endpoint, which
 treats `pass` as success, `fail` as failure, `skip` as neither.
 
 **At end of each phase:**
-- `session_log` with `{skill: "session-retrospective", phase: "<phase-name>", final_status: "pass" | "fail" | "skip", message: "<one-line summary>", duration_ms: <wall-clock>, project_root: "<absolute path>"}`
+- `session_log` with `{session_id: "<this session's id>", skill: "session-retrospective", phase: "<phase-name>", final_status: "pass" | "fail" | "skip", message: "<one-line summary>", project_root: "<absolute path>"}`
 
 **Phases for this skill:** surfaced
 
diff --git a/skills/spec-driven-dev/SKILL.md b/skills/spec-driven-dev/SKILL.md
index 39e9dba..9634193 100644
--- a/skills/spec-driven-dev/SKILL.md
+++ b/skills/spec-driven-dev/SKILL.md
@@ -332,7 +332,7 @@ Pass-rate is computed downstream by the `/pass-rate` HTTP endpoint, which
 treats `pass` as success, `fail` as failure, `skip` as neither.
 
 **At end of each phase:**
-- `session_log` with `{skill: "spec-driven-dev", phase: "<phase-name>", final_status: "pass" | "fail" | "skip", message: "<one-line summary>", duration_ms: <wall-clock>, project_root: "<absolute path>"}`
+- `session_log` with `{session_id: "<this session's id>", skill: "spec-driven-dev", phase: "<phase-name>", final_status: "pass" | "fail" | "skip", message: "<one-line summary>", project_root: "<absolute path>"}`
 
 **Phases for this skill:** specify, plan, tasks, implement
 
diff --git a/skills/tdd/SKILL.md b/skills/tdd/SKILL.md
index 3b7f09c..14840f0 100644
--- a/skills/tdd/SKILL.md
+++ b/skills/tdd/SKILL.md
@@ -406,13 +406,13 @@ Pass-rate is computed downstream by the `/pass-rate` HTTP endpoint, which
 treats `pass` as success, `fail` as failure, `skip` as neither.
 
 **At end of `red` phase:**
-- `session_log` with `{skill: "tdd", phase: "red", final_status: "pass" | "fail" | "skip", message: "<one-line summary>", duration_ms: <wall-clock>, project_root: "<absolute path>"}`
+- `session_log` with `{session_id: "<this session's id>", skill: "tdd", phase: "red", final_status: "pass" | "fail" | "skip", message: "<one-line summary>", project_root: "<absolute path>"}`
 
 **At end of `green` phase:**
-- `session_log` with `{skill: "tdd", phase: "green", final_status: "pass" | "fail" | "skip", message: "<one-line summary>", duration_ms: <wall-clock>, project_root: "<absolute path>"}`
+- `session_log` with `{session_id: "<this session's id>", skill: "tdd", phase: "green", final_status: "pass" | "fail" | "skip", message: "<one-line summary>", project_root: "<absolute path>"}`
 
 **At end of `refactor` phase:**
-- `session_log` with `{skill: "tdd", phase: "refactor", final_status: "pass" | "fail" | "skip", message: "<one-line summary>", duration_ms: <wall-clock>, project_root: "<absolute path>"}`
+- `session_log` with `{session_id: "<this session's id>", skill: "tdd", phase: "refactor", final_status: "pass" | "fail" | "skip", message: "<one-line summary>", project_root: "<absolute path>"}`
 
 **Status semantics:**
 - `pass` — the phase's intended outcome was reached (red: test fails as expected; green: test passes; refactor: tests still pass after refactor).
diff --git a/skills/trainer/SKILL.md b/skills/trainer/SKILL.md
index 8d8db67..b2e6bdc 100644
--- a/skills/trainer/SKILL.md
+++ b/skills/trainer/SKILL.md
@@ -231,10 +231,10 @@ Pass-rate is computed downstream by the `/pass-rate` HTTP endpoint, which
 treats `pass` as success, `fail` as failure, `skip` as neither.
 
 **At end of `read` phase:**
-- `session_log` with `{skill: "trainer", phase: "read", final_status: "pass" | "fail" | "skip", message: "<one-line summary>", duration_ms: <wall-clock>, project_root: "<absolute path>"}`
+- `session_log` with `{session_id: "<this session's id>", skill: "trainer", phase: "read", final_status: "pass" | "fail" | "skip", message: "<one-line summary>", project_root: "<absolute path>"}`
 
 **At end of `write` phase:**
-- `session_log` with `{skill: "trainer", phase: "write", final_status: "pass" | "fail" | "skip", message: "<one-line summary>", duration_ms: <wall-clock>, project_root: "<absolute path>"}`
+- `session_log` with `{session_id: "<this session's id>", skill: "trainer", phase: "write", final_status: "pass" | "fail" | "skip", message: "<one-line summary>", project_root: "<absolute path>"}`
 
 **Phases for this skill:** read, write
 
diff --git a/tests/check_session_log_test.sh b/tests/check_session_log_test.sh
new file mode 100644
index 0000000..c29a214
--- /dev/null
+++ b/tests/check_session_log_test.sh
@@ -0,0 +1,67 @@
+#!/bin/sh
+# Tests for bin/check-session-log.
+#
+# The regression: nine skills documented a session_log payload and not one of
+# them named `session_id` — the tool's ONLY required field. Every documented
+# call was incomplete, so an agent following any of them either failed the call
+# or invented an id, and invented ids do not group. Found by a pressure test
+# against `debug`, then found to be fleet-wide (#36).
+#
+# Also gated: `duration_ms: <wall-clock>`, which an agent cannot measure. A
+# documented field nobody can honestly populate gets fabricated or silently
+# dropped; both are worse than omitting it.
+
+set -u
+
+ROOT=$(CDPATH= cd -- "$(dirname -- "$0")/.." && pwd)
+SUT="$ROOT/bin/check-session-log"
+
+pass=0
+fail=0
+WORK=${TMPDIR:-/tmp}/check_session_log_test.$$
+
+if [ ! -f "$SUT" ]; then
+  printf 'FAIL: %s does not exist\n' "$SUT"
+  exit 1
+fi
+
+cleanup() { rm -rf "$WORK"; }
+trap cleanup EXIT INT TERM
+
+# fixture <name> <body> — a checkout with one skill whose SKILL.md is <body>.
+fixture() {
+  _f="$WORK/$1"
+  mkdir -p "$_f/skills/alpha"
+  printf -- '---\nname: alpha\ndescription: a\n---\n\n%s\n' "$2" > "$_f/skills/alpha/SKILL.md"
+  printf '%s' "$_f"
+}
+
+# expect <rc> <label> <fixture> [needle]
+expect() {
+  _want=$1; _label=$2; _fx=$3; _needle=${4:-}
+  _out=$(sh "$SUT" "$_fx" 2>&1); _rc=$?
+  if [ "$_rc" -ne "$_want" ]; then
+    fail=$((fail + 1)); printf '  FAIL %s (want rc=%s got %s)\n%s\n' "$_label" "$_want" "$_rc" "$_out"; return
+  fi
+  if [ -n "$_needle" ] && ! printf '%s' "$_out" | grep -q "$_needle"; then
+    fail=$((fail + 1)); printf '  FAIL %s (rc ok, report never says %s)\n%s\n' "$_label" "$_needle" "$_out"; return
+  fi
+  pass=$((pass + 1)); printf '  ok   %s\n' "$_label"
+}
+
+expect 0 'a skill that never mentions session_log is fine' \
+  "$(fixture quiet 'Just a skill. No logging here.')"
+
+expect 0 'a documented call naming session_id passes' \
+  "$(fixture good 'Call `session_log` with `{session_id: "<id>", skill: "alpha", phase: "red", final_status: "pass"}`')"
+
+expect 1 'a documented call omitting session_id fails' \
+  "$(fixture bad 'Call `session_log` with `{skill: "alpha", phase: "red", final_status: "pass"}`')" \
+  'session_id'
+
+expect 1 'documenting duration_ms as wall-clock fails' \
+  "$(fixture clock 'Call `session_log` with `{session_id: "<id>", skill: "alpha", duration_ms: <wall-clock>}`')" \
+  'duration_ms'
+
+printf '\ncheck_session_log: %d passed, %d failed\n' "$pass" "$fail"
+[ "$fail" -eq 0 ]