diff --git a/packages/mosaic/framework/tools/qa/qa-hook-stdin.sh b/packages/mosaic/framework/tools/qa/qa-hook-stdin.sh
index 8c3ce162..ad5fccec 100755
--- a/packages/mosaic/framework/tools/qa/qa-hook-stdin.sh
+++ b/packages/mosaic/framework/tools/qa/qa-hook-stdin.sh
@@ -50,6 +50,21 @@ if ! [[ "$FILE_PATH" =~ \.(ts|tsx|js|jsx|mjs|cjs)$ ]]; then
     exit 0
 fi
 
+# Deps preflight (#856): this hook is the common gate-entry seam the delivery
+# cycle invokes on every Edit/Write/MultiEdit — it fires before any pnpm-based
+# gate (test/lint/typecheck/format:check) runs against the edited file. In a
+# freshly created git worktree (pnpm workspaces do NOT share node_modules
+# across worktrees), node_modules/.bin is empty until `pnpm install` has run,
+# so gate binaries (tsc/eslint/prettier/vitest) fail with a raw, illegible
+# `sh: 1: <tool>: not found` that is indistinguishable from a real failure.
+# Fail legibly here instead, before that raw error has a chance to surface.
+BIN_DIR="$PROJECT_ROOT/node_modules/.bin"
+if [ ! -d "$BIN_DIR" ] || [ -z "$(ls -A "$BIN_DIR" 2>/dev/null)" ]; then
+    echo "deps not installed — run pnpm install" >&2
+    echo "[$(date '+%Y-%m-%d %H:%M:%S')] [ERROR] deps not installed — run pnpm install ($BIN_DIR is missing or empty)" >> "$LOG_FILE"
+    exit 1
+fi
+
 # Call the main QA handler with extracted parameters
 if [ -f ~/.config/mosaic/tools/qa/qa-hook-handler.sh ]; then
     echo "[$(date '+%Y-%m-%d %H:%M:%S')] Calling QA handler for $FILE_PATH" >> "$LOG_FILE"
diff --git a/packages/mosaic/framework/tools/qa/test-deps-preflight.sh b/packages/mosaic/framework/tools/qa/test-deps-preflight.sh
new file mode 100755
index 00000000..acdbf61d
--- /dev/null
+++ b/packages/mosaic/framework/tools/qa/test-deps-preflight.sh
@@ -0,0 +1,116 @@
+#!/usr/bin/env bash
+# Regression harness for #856: worker git-worktrees under a fresh `git worktree
+# add` have no node_modules until `pnpm install` runs (pnpm workspaces do NOT
+# share node_modules across worktrees). Before the fix, the gate-entry seam
+# (qa-hook-stdin.sh, registered as the PostToolUse hook for every Edit/Write/
+# MultiEdit in runtime/claude/settings.json) silently let a raw
+# `sh: 1: <tool>: not found` surface from any downstream gate invocation —
+# indistinguishable from a real test/lint failure (false-red).
+#
+# Asserts:
+#   1. RED (documented): a completely fresh worktree with no node_modules/.bin
+#      at all produces the raw "not found" for a gate binary — this is the
+#      defect the fix prevents from reaching the operator un-annotated.
+#   2. With node_modules/.bin missing entirely, the seam exits nonzero with
+#      the legible sentinel "deps not installed — run pnpm install" instead
+#      of silently proceeding (exit 0) into a would-be raw not-found.
+#   3. With node_modules/.bin present but empty, same legible-sentinel
+#      behavior (covers `git worktree add` immediately followed by an
+#      as-yet-incomplete/interrupted install).
+#   4. Once node_modules/.bin is populated (post `pnpm install`), the seam
+#      proceeds normally (exit 0) — the preflight does not false-positive.
+#   5. Non-JS/TS files are unaffected (existing skip behavior preserved).
+
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+HOOK="$SCRIPT_DIR/qa-hook-stdin.sh"
+
+TMP_DIR=$(mktemp -d)
+trap 'rm -rf "$TMP_DIR"' EXIT
+
+fail=0
+
+fail_msg() {
+    echo "FAIL: $*" >&2
+    fail=1
+}
+
+run_hook() {
+    local file_path="$1"
+    printf '{"tool_name":"Edit","tool_input":{"file_path":"%s"}}' "$file_path" | "$HOOK"
+}
+
+make_fixture_repo() {
+    local dir="$1"
+    mkdir -p "$dir"
+    git -C "$dir" init -q .
+    git -C "$dir" -c user.email=fixture@test -c user.name=fixture commit -q --allow-empty -m init
+}
+
+# --- Scenario 1: RED — document the pre-fix raw not-found a gate hits when
+#     node_modules/.bin is entirely absent (this is what the preflight now
+#     intercepts before any gate command runs).
+RED_DIR="$TMP_DIR/red-fixture"
+make_fixture_repo "$RED_DIR"
+RED_OUTPUT=$(PATH="/usr/bin:/bin" sh -c 'tsc --noEmit' 2>&1) && RED_STATUS=0 || RED_STATUS=$?
+case "$RED_OUTPUT" in
+    *"not found"*) ;;
+    *) fail_msg "expected the raw un-preflighted invocation to demonstrate 'not found'; got: $RED_OUTPUT" ;;
+esac
+[[ "$RED_STATUS" -ne 0 ]] || fail_msg "expected raw invocation without deps installed to fail"
+
+# --- Scenario 2: node_modules/.bin missing entirely -> legible sentinel, nonzero.
+MISSING_DIR="$TMP_DIR/missing-bin"
+make_fixture_repo "$MISSING_DIR"
+echo "console.log(1)" > "$MISSING_DIR/x.ts"
+OUTPUT=$(cd "$MISSING_DIR" && run_hook "$MISSING_DIR/x.ts" 2>&1) && STATUS=0 || STATUS=$?
+[[ "$STATUS" -ne 0 ]] || fail_msg "missing node_modules/.bin: expected nonzero exit, got 0"
+case "$OUTPUT" in
+    *"deps not installed"*"pnpm install"*) ;;
+    *) fail_msg "missing node_modules/.bin: expected legible sentinel, got: $OUTPUT" ;;
+esac
+
+# --- Scenario 3: node_modules/.bin present but empty -> legible sentinel, nonzero.
+EMPTY_DIR="$TMP_DIR/empty-bin"
+make_fixture_repo "$EMPTY_DIR"
+mkdir -p "$EMPTY_DIR/node_modules/.bin"
+echo "console.log(1)" > "$EMPTY_DIR/x.ts"
+OUTPUT=$(cd "$EMPTY_DIR" && run_hook "$EMPTY_DIR/x.ts" 2>&1) && STATUS=0 || STATUS=$?
+[[ "$STATUS" -ne 0 ]] || fail_msg "empty node_modules/.bin: expected nonzero exit, got 0"
+case "$OUTPUT" in
+    *"deps not installed"*"pnpm install"*) ;;
+    *) fail_msg "empty node_modules/.bin: expected legible sentinel, got: $OUTPUT" ;;
+esac
+
+# --- Scenario 4: node_modules/.bin populated (post `pnpm install`) -> proceeds normally.
+OK_DIR="$TMP_DIR/installed-bin"
+make_fixture_repo "$OK_DIR"
+mkdir -p "$OK_DIR/node_modules/.bin"
+printf '#!/bin/sh\necho ok\n' > "$OK_DIR/node_modules/.bin/tsc"
+chmod +x "$OK_DIR/node_modules/.bin/tsc"
+echo "console.log(1)" > "$OK_DIR/x.ts"
+OUTPUT=$(cd "$OK_DIR" && run_hook "$OK_DIR/x.ts" 2>&1) && STATUS=0 || STATUS=$?
+[[ "$STATUS" -eq 0 ]] || fail_msg "populated node_modules/.bin: expected exit 0, got $STATUS ($OUTPUT)"
+case "$OUTPUT" in
+    *"deps not installed"*) fail_msg "populated node_modules/.bin: unexpected sentinel fired: $OUTPUT" ;;
+    *) ;;
+esac
+
+# --- Scenario 5: non-JS/TS files are unaffected by the preflight (still
+#     skipped before the deps check, regardless of node_modules state).
+NONJS_DIR="$TMP_DIR/nonjs"
+make_fixture_repo "$NONJS_DIR"
+echo "# doc" > "$NONJS_DIR/README.md"
+OUTPUT=$(cd "$NONJS_DIR" && run_hook "$NONJS_DIR/README.md" 2>&1) && STATUS=0 || STATUS=$?
+[[ "$STATUS" -eq 0 ]] || fail_msg "non-JS/TS file: expected exit 0 (skip), got $STATUS ($OUTPUT)"
+case "$OUTPUT" in
+    *"deps not installed"*) fail_msg "non-JS/TS file: preflight incorrectly fired: $OUTPUT" ;;
+    *) ;;
+esac
+
+if [[ "$fail" -eq 0 ]]; then
+    echo "deps-preflight regression passed (5/5 scenarios)"
+fi
+
+exit "$fail"
diff --git a/packages/mosaic/package.json b/packages/mosaic/package.json
index b6a37282..55942bc7 100644
--- a/packages/mosaic/package.json
+++ b/packages/mosaic/package.json
@@ -25,7 +25,7 @@
     "lint": "eslint src",
     "typecheck": "tsc --noEmit",
     "test": "vitest run --passWithNoTests && pnpm run test:framework-shell",
-    "test:framework-shell": "python3 src/lease-broker/daemon_deadline_unittest.py && python3 src/lease-broker/normative_fragments_unittest.py && python3 src/lease-broker/receipt_challenge_unittest.py && python3 src/lease-broker/context_recovery_unittest.py && python3 src/lease-broker/recovery_runtime_unittest.py && python3 src/lease-broker/recovery_b1_adversarial_unittest.py && python3 src/lease-broker/framework_skill_portability_unittest.py && python3 src/mutator-gate/runtime_tools_unittest.py && python3 src/mutator-gate/runtime_launch_guard_unittest.py && python3 framework/tools/lease-broker/check-runtime-launches.py --root ../.. && bash framework/tools/codex/test-pr-diff-context.sh"
+    "test:framework-shell": "python3 src/lease-broker/daemon_deadline_unittest.py && python3 src/lease-broker/normative_fragments_unittest.py && python3 src/lease-broker/receipt_challenge_unittest.py && python3 src/lease-broker/context_recovery_unittest.py && python3 src/lease-broker/recovery_runtime_unittest.py && python3 src/lease-broker/recovery_b1_adversarial_unittest.py && python3 src/lease-broker/framework_skill_portability_unittest.py && python3 src/mutator-gate/runtime_tools_unittest.py && python3 src/mutator-gate/runtime_launch_guard_unittest.py && python3 framework/tools/lease-broker/check-runtime-launches.py --root ../.. && bash framework/tools/codex/test-pr-diff-context.sh && bash framework/tools/qa/test-deps-preflight.sh"
   },
   "dependencies": {
     "@mosaicstack/brain": "workspace:*",
diff --git a/plugins/mosaic-framework/src/index.ts b/plugins/mosaic-framework/src/index.ts
index 1fb9400a..551bccc9 100644
--- a/plugins/mosaic-framework/src/index.ts
+++ b/plugins/mosaic-framework/src/index.ts
@@ -217,12 +217,22 @@ git fetch origin
 mkdir -p ~/src/${projectName}-worktrees
 git worktree add ~/src/${projectName}-worktrees/<task-slug> -b <branch-name> origin/main
 cd ~/src/${projectName}-worktrees/<task-slug>
+pnpm install --frozen-lockfile --prefer-offline
 # ... all work happens here ...
 git push origin <branch-name>
 cd ~/src/${projectName} && git worktree remove ~/src/${projectName}-worktrees/<task-slug>
 \`\`\`
 
-Worktrees path: \`~/src/<repo>-worktrees/<task-slug>\` — NEVER use /tmp.`);
+Worktrees path: \`~/src/<repo>-worktrees/<task-slug>\` — NEVER use /tmp.
+
+\`pnpm install --frozen-lockfile --prefer-offline\` MUST run immediately after
+\`git worktree add\`/\`cd\`, BEFORE any gate (\`pnpm test\`/\`lint\`/\`typecheck\`/\`format:check\`)
+is invoked. pnpm workspaces do NOT share \`node_modules\` across separate git
+worktrees — a fresh worktree has an empty \`node_modules/.bin\`, so every gate
+binary (\`tsc\`/\`eslint\`/\`prettier\`/\`vitest\`) fails \`sh: 1: <tool>: not found\`
+until deps are installed. That failure is indistinguishable from a real
+test/lint failure — a false-red gate. Never skip this step and never reorder
+it after the first gate invocation.`);
 
   // 6. Completion gates
   sections.push(`# Completion Gates — ENFORCED