diff --git a/AGENTS.md b/AGENTS.md
index 59a0201..1bc24d1 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -310,7 +310,7 @@ sourced as needed.
 | `lib/matrix_listener.sh` | Long-poll Matrix sync daemon. Dispatches thread replies to the correct agent via well-known files (`/tmp/{agent}-escalation-reply`). Handles supervisor, gardener, dev, review, vault, and action reply routing. Run as systemd service. | Standalone daemon |
 | `lib/formula-session.sh` | `acquire_cron_lock()`, `check_memory()`, `load_formula()`, `build_context_block()`, `start_formula_session()`, `formula_phase_callback()`, `build_prompt_footer()`, `run_formula_and_monitor()` — shared helpers for formula-driven cron agents (lock, memory guard, formula loading, prompt assembly, tmux session, monitor loop, crash recovery). | planner-run.sh, predictor-run.sh |
 | `lib/file-action-issue.sh` | `file_action_issue()` — dedup check, label lookup, and issue creation for formula-driven cron wrappers. Sets `FILED_ISSUE_NUM` on success. | gardener-run.sh |
-| `lib/agent-session.sh` | Shared tmux + Claude session helpers: `create_agent_session()`, `inject_formula()`, `agent_wait_for_claude_ready()`, `agent_inject_into_session()`, `agent_kill_session()`, `monitor_phase_loop()`, `read_phase()`. `create_agent_session(session, workdir, [phase_file])` optionally installs a PostToolUse hook (matcher `Bash\|Write`) that detects phase file writes in real-time — when Claude writes to the phase file, the hook writes a marker so `monitor_phase_loop` reacts on the next poll instead of waiting for mtime changes. Also installs a StopFailure hook (matcher `rate_limit\|server_error\|authentication_failed\|billing_error`) that writes `PHASE:failed` with an `api_error` reason to the phase file and touches the phase-changed marker, so the orchestrator discovers API errors within one poll cycle instead of waiting for idle timeout. When `MATRIX_THREAD_ID` is exported, also installs a Stop hook (`on-stop-matrix.sh`) that streams each Claude response to the Matrix thread. `monitor_phase_loop` sets `_MONITOR_LOOP_EXIT` to one of: `done`, `idle_timeout`, `idle_prompt` (Claude returned to `❯` for 3 consecutive polls without writing any phase — callback invoked with `PHASE:failed`, session already dead), `crashed`, or a `PHASE:*` string. **Callers must handle `idle_prompt`** in both their callback and their post-loop exit handler — see [`docs/PHASE-PROTOCOL.md` § idle_prompt](docs/PHASE-PROTOCOL.md#idle_prompt-exit-reason) for the full contract. | dev-agent.sh, gardener-agent.sh, action-agent.sh |
+| `lib/agent-session.sh` | Shared tmux + Claude session helpers: `create_agent_session()`, `inject_formula()`, `agent_wait_for_claude_ready()`, `agent_inject_into_session()`, `agent_kill_session()`, `monitor_phase_loop()`, `read_phase()`, `write_compact_context()`. `create_agent_session(session, workdir, [phase_file])` optionally installs a PostToolUse hook (matcher `Bash\|Write`) that detects phase file writes in real-time — when Claude writes to the phase file, the hook writes a marker so `monitor_phase_loop` reacts on the next poll instead of waiting for mtime changes. Also installs a StopFailure hook (matcher `rate_limit\|server_error\|authentication_failed\|billing_error`) that writes `PHASE:failed` with an `api_error` reason to the phase file and touches the phase-changed marker, so the orchestrator discovers API errors within one poll cycle instead of waiting for idle timeout. Also installs a SessionStart hook (matcher `compact`) that re-injects phase protocol instructions after context compaction — callers write the context file via `write_compact_context(phase_file, content)`, and the hook (`on-compact-reinject.sh`) outputs the file content to stdout so Claude retains critical instructions. When `MATRIX_THREAD_ID` is exported, also installs a Stop hook (`on-stop-matrix.sh`) that streams each Claude response to the Matrix thread. `monitor_phase_loop` sets `_MONITOR_LOOP_EXIT` to one of: `done`, `idle_timeout`, `idle_prompt` (Claude returned to `❯` for 3 consecutive polls without writing any phase — callback invoked with `PHASE:failed`, session already dead), `crashed`, or a `PHASE:*` string. **Callers must handle `idle_prompt`** in both their callback and their post-loop exit handler — see [`docs/PHASE-PROTOCOL.md` § idle_prompt](docs/PHASE-PROTOCOL.md#idle_prompt-exit-reason) for the full contract. | dev-agent.sh, gardener-agent.sh, action-agent.sh |
 
 ---
 
diff --git a/action/action-agent.sh b/action/action-agent.sh
index a1fd470..f0920a4 100644
--- a/action/action-agent.sh
+++ b/action/action-agent.sh
@@ -88,7 +88,7 @@ cleanup() {
   agent_kill_session "$SESSION_NAME"
   # Best-effort docker cleanup for containers started during this action
   (cd "${PROJECT_REPO_ROOT}" 2>/dev/null && docker compose down 2>/dev/null) || true
-  rm -f "$PHASE_FILE" "$IMPL_SUMMARY_FILE" "$PREFLIGHT_RESULT"
+  rm -f "$PHASE_FILE" "${PHASE_FILE%.phase}.context" "$IMPL_SUMMARY_FILE" "$PREFLIGHT_RESULT"
 }
 trap cleanup EXIT
 
@@ -193,6 +193,9 @@ fi
 # Build phase protocol from shared function (Path B covered in Instructions section above)
 PHASE_PROTOCOL_INSTRUCTIONS="$(build_phase_protocol_prompt "$PHASE_FILE" "$IMPL_SUMMARY_FILE" "$BRANCH")"
 
+# Write phase protocol to context file for compaction survival
+write_compact_context "$PHASE_FILE" "$PHASE_PROTOCOL_INSTRUCTIONS"
+
 INITIAL_PROMPT="You are an action agent. Your job is to execute the action formula
 in the issue below.
 
@@ -272,16 +275,16 @@ case "${_MONITOR_LOOP_EXIT:-}" in
     # Escalate to supervisor (idle_prompt already escalated via _on_phase_change callback)
     echo "{\"issue\":${ISSUE},\"pr\":${PR_NUMBER:-0},\"reason\":\"idle_timeout\",\"ts\":\"$(date -u +%Y-%m-%dT%H:%M:%SZ)\"}" \
       >> "${FACTORY_ROOT}/supervisor/escalations-${PROJECT_NAME}.jsonl"
-    rm -f "$PHASE_FILE" "$IMPL_SUMMARY_FILE" "$THREAD_FILE" "$SCRATCH_FILE"
+    rm -f "$PHASE_FILE" "${PHASE_FILE%.phase}.context" "$IMPL_SUMMARY_FILE" "$THREAD_FILE" "$SCRATCH_FILE"
     ;;
   idle_prompt)
     # Notification + escalation already handled by _on_phase_change(PHASE:failed) callback
-    rm -f "$PHASE_FILE" "$IMPL_SUMMARY_FILE" "$THREAD_FILE" "$SCRATCH_FILE"
+    rm -f "$PHASE_FILE" "${PHASE_FILE%.phase}.context" "$IMPL_SUMMARY_FILE" "$THREAD_FILE" "$SCRATCH_FILE"
     ;;
   done)
     # Belt-and-suspenders: callback handles primary cleanup,
     # but ensure sentinel files are removed if callback was interrupted
-    rm -f "$PHASE_FILE" "$IMPL_SUMMARY_FILE" "$THREAD_FILE" "$SCRATCH_FILE"
+    rm -f "$PHASE_FILE" "${PHASE_FILE%.phase}.context" "$IMPL_SUMMARY_FILE" "$THREAD_FILE" "$SCRATCH_FILE"
     ;;
 esac
 
diff --git a/dev/dev-agent.sh b/dev/dev-agent.sh
index 6d3e463..4beaf95 100755
--- a/dev/dev-agent.sh
+++ b/dev/dev-agent.sh
@@ -588,6 +588,9 @@ printf 'PHASE:failed\nReason: refused\n' > \"${PHASE_FILE}\"
 printf 'PHASE:failed\nReason: %s\n' \"describe what failed\" > \"${PHASE_FILE}\"
 \`\`\`"
 
+# Write phase protocol to context file for compaction survival
+write_compact_context "$PHASE_FILE" "$PHASE_PROTOCOL_INSTRUCTIONS"
+
 if [ "$RECOVERY_MODE" = true ]; then
   # Build recovery context
   GIT_DIFF_STAT=$(git -C "$WORKTREE" diff "origin/${PRIMARY_BRANCH}..HEAD" --stat 2>/dev/null | head -20 || echo "(no diff)")
@@ -759,7 +762,8 @@ case "${_MONITOR_LOOP_EXIT:-}" in
     else
       cleanup_worktree
     fi
-    rm -f "$PHASE_FILE" "$IMPL_SUMMARY_FILE" "$THREAD_FILE" "$SCRATCH_FILE" \
+    rm -f "$PHASE_FILE" "${PHASE_FILE%.phase}.context" \
+      "$IMPL_SUMMARY_FILE" "$THREAD_FILE" "$SCRATCH_FILE" \
       "/tmp/ci-result-${PROJECT_NAME}-${ISSUE}.txt"
     [ -n "${PR_NUMBER:-}" ] && rm -f "/tmp/review-injected-${PROJECT_NAME}-${PR_NUMBER}"
     ;;
@@ -769,7 +773,8 @@ case "${_MONITOR_LOOP_EXIT:-}" in
   done)
     # Belt-and-suspenders: callback in phase-handler.sh handles primary cleanup,
     # but ensure sentinel files are removed if callback was interrupted
-    rm -f "$PHASE_FILE" "$IMPL_SUMMARY_FILE" "$THREAD_FILE" "$SCRATCH_FILE" \
+    rm -f "$PHASE_FILE" "${PHASE_FILE%.phase}.context" \
+      "$IMPL_SUMMARY_FILE" "$THREAD_FILE" "$SCRATCH_FILE" \
       "/tmp/ci-result-${PROJECT_NAME}-${ISSUE}.txt"
     [ -n "${PR_NUMBER:-}" ] && rm -f "/tmp/review-injected-${PROJECT_NAME}-${PR_NUMBER}"
     CLAIMED=false
diff --git a/gardener/gardener-agent.sh b/gardener/gardener-agent.sh
index 48e0091..d4cb1ac 100644
--- a/gardener/gardener-agent.sh
+++ b/gardener/gardener-agent.sh
@@ -267,9 +267,16 @@ When all work is done and verify confirms zero tech-debt:
 On unrecoverable error:
   printf 'PHASE:failed\nReason: %s\n' 'describe error' > '${PHASE_FILE}'"
 
+# Write phase protocol to context file for compaction survival
+write_compact_context "$PHASE_FILE" "## Phase protocol (REQUIRED)
+When all work is done and verify confirms zero tech-debt:
+  echo 'PHASE:done' > '${PHASE_FILE}'
+On unrecoverable error:
+  printf 'PHASE:failed\nReason: %s\n' 'describe error' > '${PHASE_FILE}'"
+
 # ── Reset phase + result files ────────────────────────────────────────────
 agent_kill_session "$SESSION_NAME"
-rm -f "$PHASE_FILE" "$RESULT_FILE"
+rm -f "$PHASE_FILE" "${PHASE_FILE%.phase}.context" "$RESULT_FILE"
 touch "$RESULT_FILE"
 
 # ── Create tmux session ───────────────────────────────────────────────────
diff --git a/lib/agent-session.sh b/lib/agent-session.sh
index 81d6aae..501d955 100644
--- a/lib/agent-session.sh
+++ b/lib/agent-session.sh
@@ -218,6 +218,39 @@ create_agent_session() {
   fi
   rm -f "$exit_marker"
 
+  # Install SessionStart hook for context re-injection after compaction:
+  # when Claude Code compacts context during long sessions, the phase protocol
+  # instructions are lost. This hook fires after each compaction and outputs
+  # the content of a context file so Claude retains critical instructions.
+  # The context file is written by callers via write_compact_context().
+  if [ -n "$phase_file" ]; then
+    local compact_hook_script="${FACTORY_ROOT}/lib/hooks/on-compact-reinject.sh"
+    if [ -x "$compact_hook_script" ]; then
+      local context_file="${phase_file%.phase}.context"
+      local compact_hook_cmd="${compact_hook_script} ${context_file}"
+      if [ -f "$settings" ]; then
+        jq --arg cmd "$compact_hook_cmd" '
+          if (.hooks.SessionStart // [] | any(.[]; .hooks[]?.command == $cmd))
+          then .
+          else .hooks.SessionStart = (.hooks.SessionStart // []) + [{
+            matcher: "compact",
+            hooks: [{type: "command", command: $cmd}]
+          }]
+          end
+        ' "$settings" > "${settings}.tmp" && mv "${settings}.tmp" "$settings"
+      else
+        jq -n --arg cmd "$compact_hook_cmd" '{
+          hooks: {
+            SessionStart: [{
+              matcher: "compact",
+              hooks: [{type: "command", command: $cmd}]
+            }]
+          }
+        }' > "$settings"
+      fi
+    fi
+  fi
+
   # Install Stop hook for Matrix streaming: when MATRIX_THREAD_ID is set,
   # each Claude response is posted to the Matrix thread so humans can follow.
   local matrix_hook_script="${FACTORY_ROOT}/lib/hooks/on-stop-matrix.sh"
@@ -392,6 +425,16 @@ monitor_phase_loop() {
   done
 }
 
+# Write context to a file for re-injection after context compaction.
+# The SessionStart compact hook reads this file and outputs it to stdout.
+# Args: phase_file content
+write_compact_context() {
+  local phase_file="$1"
+  local content="$2"
+  local context_file="${phase_file%.phase}.context"
+  printf '%s\n' "$content" > "$context_file"
+}
+
 # Kill a tmux session gracefully (no-op if not found).
 agent_kill_session() {
   local session="${1:-}"
diff --git a/lib/formula-session.sh b/lib/formula-session.sh
index f09eec1..21622e7 100644
--- a/lib/formula-session.sh
+++ b/lib/formula-session.sh
@@ -202,6 +202,11 @@ run_formula_and_monitor() {
     exit 1
   fi
 
+  # Write phase protocol to context file for compaction survival
+  if [ -n "${PROMPT_FOOTER:-}" ]; then
+    write_compact_context "$PHASE_FILE" "$PROMPT_FOOTER"
+  fi
+
   agent_inject_into_session "$SESSION_NAME" "$PROMPT"
   log "Prompt sent to tmux session"
   matrix_send "$agent_name" "${agent_name^} session started for ${CODEBERG_REPO}" 2>/dev/null || true
diff --git a/lib/hooks/on-compact-reinject.sh b/lib/hooks/on-compact-reinject.sh
new file mode 100755
index 0000000..320f2a1
--- /dev/null
+++ b/lib/hooks/on-compact-reinject.sh
@@ -0,0 +1,15 @@
+#!/bin/bash
+# on-compact-reinject.sh — SessionStart (compact) hook for dark-factory agent sessions.
+#
+# Called by Claude Code after context compaction. Reads a context file and
+# outputs its content to stdout, which Claude Code injects as system context.
+# No-op if the context file doesn't exist.
+#
+# Usage (in .claude/settings.json):
+#   {"type": "command", "command": "this-script /tmp/dev-session-PROJECT-ISSUE.context"}
+#
+# Args: $1 = context file path
+
+cat > /dev/null  # consume hook JSON from stdin
+[ -n "${1:-}" ] && [ -f "$1" ] && cat "$1"
+exit 0