diff --git a/.agents/prompts/project-grill.md b/.agents/prompts/project-grill.md
index 5644324..2bf160f 100644
--- a/.agents/prompts/project-grill.md
+++ b/.agents/prompts/project-grill.md
@@ -6,6 +6,11 @@ Read `AGENTS.md`, `README.md`, `docs/repository-setup.md`, any existing
 `docs/PROJECT_REQUIREMENTS.md`, accepted ADRs, and the actual repository before
 asking questions.
 
+If `docs/PROJECT_DESCRIPTION.md` exists, read it first. It is the project
+description supplied by the human. Take every decision it already makes as
+given, ask only about what it leaves unresolved or contradictory, and do not
+modify it.
+
 ## Goal
 
 Discover only unresolved decisions that materially affect the product,
diff --git a/.agents/prompts/project-planner.md b/.agents/prompts/project-planner.md
index 1cf3741..e94e4bd 100644
--- a/.agents/prompts/project-planner.md
+++ b/.agents/prompts/project-planner.md
@@ -6,6 +6,9 @@ Read `AGENTS.md`, the approved `docs/PROJECT_REQUIREMENTS.md`,
 `docs/repository-setup.md`, the current architecture, accepted ADRs, and the
 actual repository before changing planning artifacts.
 
+`docs/PROJECT_DESCRIPTION.md`, when present, is background supplied by the
+human. The approved requirements take precedence over it. Do not modify it.
+
 ## Preconditions
 
 Do not proceed unless `docs/PROJECT_REQUIREMENTS.md` contains:
diff --git a/README.md b/README.md
index 531a4b8..d95191b 100644
--- a/README.md
+++ b/README.md
@@ -17,6 +17,8 @@ A lightweight, model-agnostic repository template for agentic software engineeri
 
    ```bash
    ./scripts/start-planning.sh
+   # or, with an existing project description:
+   ./scripts/start-planning.sh --description path/to/description.md
    ```
 
    The script creates `planning/project-bootstrap` in a sibling worktree. The
diff --git a/docs/development.md b/docs/development.md
index 8125d6b..af20ec7 100644
--- a/docs/development.md
+++ b/docs/development.md
@@ -97,9 +97,24 @@ idea and completing `docs/repository-setup.md`:
 The full interface is:
 
 ```text
-./scripts/start-planning.sh [name] [--agent <agent>] [--model <model>]
+./scripts/start-planning.sh [name] [--description <file>] [--agent <agent>] [--model <model>]
 ```
 
+If you already have a project description, pass it with `--description`:
+
+```bash
+./scripts/start-planning.sh --description ~/notes/project-idea.md
+```
+
+The file may be anywhere, including outside the repository. An uncommitted
+description inside the repository is the one change the clean-checkout check
+allows.
+The script copies it to `docs/PROJECT_DESCRIPTION.md` in the planning worktree
+before the first session. Project Grill reads it first and asks only about
+what it leaves unresolved. Neither planning phase may modify it, and it is
+committed with the planning branch as the recorded input. A missing, empty, or
+non-regular file fails before a branch or worktree is created.
+
 Project Grill uses role `project-grill` and the planning session uses role
 `project-planner`; `--agent` and `--model` override both. The optional name
 defaults to `project-bootstrap`. A custom planning cycle such as:
@@ -137,6 +152,7 @@ manually:
 ```bash
 cd ../<repository>-planning-project-bootstrap
 ./scripts/verify.sh
+git add docs/PROJECT_DESCRIPTION.md  # only when --description was used
 git add docs/PROJECT_REQUIREMENTS.md docs/architecture.md docs/roadmap.md docs/decisions
 git commit -m "Plan project bootstrap"
 git push -u origin planning/project-bootstrap
diff --git a/scripts/start-planning.sh b/scripts/start-planning.sh
index cb7b372..1a16788 100755
--- a/scripts/start-planning.sh
+++ b/scripts/start-planning.sh
@@ -6,14 +6,18 @@ source "$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd -P)/lib/agent.sh"
 
 usage() {
   cat <<EOF
-Usage: $0 [name] [--agent <agent>] [--model <model>]
+Usage: $0 [name] [--description <file>] [--agent <agent>] [--model <model>]
 
 The agent and model of each phase come from .agents/agents.conf (roles
 project-grill and project-planner). --agent and --model override both phases.
 
+--description copies an existing project description into the planning
+worktree as docs/PROJECT_DESCRIPTION.md. Project Grill reads it first.
+
 Examples:
   $0
   $0 architecture-refresh
+  $0 --description ~/notes/project-idea.md
   $0 --agent claude --model fable
 EOF
 }
@@ -24,12 +28,41 @@ fail() {
 }
 
 agent_parse_args "$@"
-if [[ "${#AGENT_POSITIONAL[@]}" -gt 1 ]]; then
+
+description_source=""
+names=()
+set -- ${AGENT_POSITIONAL[@]+"${AGENT_POSITIONAL[@]}"}
+while [[ $# -gt 0 ]]; do
+  case "$1" in
+    --description)
+      [[ $# -ge 2 && -n "$2" ]] || fail "--description requires a file."
+      description_source="$2"
+      shift 2
+      ;;
+    --*)
+      fail "unknown option: $1"
+      ;;
+    *)
+      names+=("$1")
+      shift
+      ;;
+  esac
+done
+
+if [[ "${#names[@]}" -gt 1 ]]; then
   usage
   exit 1
 fi
 
-name="${AGENT_POSITIONAL[0]:-project-bootstrap}"
+name="${names[0]:-project-bootstrap}"
+
+if [[ -n "$description_source" ]]; then
+  [[ -f "$description_source" ]] ||
+    fail "project description is not a regular file: $description_source"
+  [[ -r "$description_source" && -s "$description_source" ]] ||
+    fail "project description is empty or unreadable: $description_source"
+  description_source="$(cd "$(dirname "$description_source")" && pwd -P)/$(basename "$description_source")"
+fi
 
 [[ "$name" =~ ^[a-z0-9][a-z0-9-]*$ ]] ||
   fail "planning name must match [a-z0-9][a-z0-9-]*: $name"
@@ -54,7 +87,13 @@ planner_model="$AGENT_MODEL"
 [[ -f "$grill_prompt" ]] || fail "missing Project Grill prompt: $grill_prompt"
 [[ -f "$planner_prompt" ]] || fail "missing project-planner prompt: $planner_prompt"
 
-if [[ -n "$(git -C "$repo_root" status --porcelain)" ]]; then
+# An uncommitted description inside the repository is the only change that
+# does not count as a dirty checkout.
+clean_paths=(.)
+if [[ -n "$description_source" && "$description_source" == "$repo_root"/* ]]; then
+  clean_paths+=(":(exclude,literal)${description_source#"$repo_root"/}")
+fi
+if [[ -n "$(git -C "$repo_root" status --porcelain -- "${clean_paths[@]}")" ]]; then
   fail "current worktree is not clean. Commit or stash changes first."
 fi
 
@@ -71,6 +110,9 @@ echo "  Branch:   $branch"
 echo "  Worktree: $worktree"
 echo "  Grill:    $grill_agent ($grill_model)"
 echo "  Planner:  $planner_agent ($planner_model)"
+if [[ -n "$description_source" ]]; then
+  echo "  Description: $description_source"
+fi
 echo
 
 git -C "$repo_root" fetch origin main ||
@@ -228,6 +270,14 @@ $worktree
 Do not commit, push, open or merge a pull request, create GitHub Issues,
 implement application features, or deploy.
 EOF
+
+  if [[ -n "$description_source" ]]; then
+    cat <<EOF
+
+The user supplied a project description as $description_relative.
+Read it before anything else. Do not modify it.
+EOF
+  fi
 }
 
 run_agent() {
@@ -376,6 +426,20 @@ approve_requirements() {
   approval_tmp=""
 }
 
+# Copy the description before the scope baselines are taken, so it is part of
+# the protected state of both phases instead of an out-of-scope change.
+description_relative="docs/PROJECT_DESCRIPTION.md"
+if [[ -n "$description_source" ]]; then
+  description_target="$worktree/$description_relative"
+  [[ ! -L "$description_target" && (! -e "$description_target" || -f "$description_target") ]] ||
+    post_creation_fail "$description_relative exists in the worktree and is not a regular file."
+  mkdir -p "$worktree/docs" && cp "$description_source" "$description_target" ||
+    post_creation_fail "could not copy the project description into the planning worktree."
+  chmod 644 "$description_target" ||
+    post_creation_fail "could not set permissions on $description_relative."
+  echo "Copied project description to: $description_relative"
+fi
+
 grill_baseline="$state_dir/grill-baseline.tsv"
 planner_baseline="$state_dir/planner-baseline.tsv"
 snapshot_forbidden_paths "grill" "$grill_baseline"
@@ -468,6 +532,9 @@ echo
 echo "Review the planning artifacts, then run:"
 echo "  cd \"$worktree\""
 echo "  ./scripts/verify.sh"
+if [[ -n "$description_source" ]]; then
+  echo "  git add $description_relative"
+fi
 echo "  git add docs/PROJECT_REQUIREMENTS.md docs/architecture.md docs/roadmap.md docs/decisions"
 echo "  git commit -m \"Plan project bootstrap\""
 echo "  git push -u origin \"$branch\""
diff --git a/tests/start-planning-test.sh b/tests/start-planning-test.sh
index af50b3f..921556b 100755
--- a/tests/start-planning-test.sh
+++ b/tests/start-planning-test.sh
@@ -119,6 +119,8 @@ REQUIREMENTS
     printf 'Out-of-scope Grill change.\n' >README.md
   elif [[ "${MOCK_GRILL_MODE:-success}" == "ignored" ]]; then
     printf 'IGNORED_SECRET=test\n' >.env
+  elif [[ "${MOCK_GRILL_MODE:-success}" == "edit-description" ]]; then
+    printf 'Changed by Project Grill.\n' >>docs/PROJECT_DESCRIPTION.md
   elif [[ "${MOCK_GRILL_MODE:-success}" == "weird-paths" ]]; then
     printf 'Tab path.\n' >$'docs/PROJECT_REQUIREMENTS.md\textra'
     printf 'Newline path.\n' >$'unexpected\nfile'
@@ -794,4 +796,97 @@ fi
 [[ ! -e "$tmp/dirty-repo-planning-project-bootstrap" ]] ||
   fail "dirty repository created a worktree"
 
+# A project description from outside the repository is copied into the
+# planning worktree before the phases start and is announced to Project Grill.
+description="$tmp/project idea.txt"
+printf 'A local-first recipe organizer.\n' >"$description"
+description_repo="$(setup_repo description)"
+description_log="$tmp/description.log"
+(
+  cd "$description_repo"
+  printf 'y\n' |
+    PATH="$tmp/bin:/usr/bin:/bin" \
+    MOCK_AGENT_LOG="$description_log" \
+    ./scripts/start-planning.sh --description "$description" --agent claude --model fable \
+    >"$tmp/description.out" 2>&1
+) || {
+  cat "$tmp/description.out" >&2
+  fail "planning with a project description failed"
+}
+description_worktree="$tmp/description-repo-planning-project-bootstrap"
+cmp -s "$description" "$description_worktree/docs/PROJECT_DESCRIPTION.md" ||
+  fail "project description was not copied into the planning worktree"
+grep -Fq "docs/PROJECT_DESCRIPTION.md" "$description_log" ||
+  fail "project description was not announced to the agent"
+[[ -z "$(git -C "$description_repo" status --porcelain)" ]] ||
+  fail "project description changed the primary checkout"
+
+# An uncommitted description inside the repository is accepted, but it does
+# not excuse other uncommitted changes.
+inside_repo="$(setup_repo description-inside)"
+printf 'An untracked description in the checkout.\n' >"$inside_repo/idea.md"
+printf '\nDirty.\n' >>"$inside_repo/AGENTS.md"
+if (
+  cd "$inside_repo"
+  PATH="$tmp/bin:/usr/bin:/bin" \
+    MOCK_AGENT_LOG="$tmp/description-inside.log" \
+    ./scripts/start-planning.sh --description idea.md --agent codex --model astra >/dev/null 2>&1
+); then
+  fail "unrelated uncommitted changes were accepted together with a description"
+fi
+[[ ! -e "$tmp/description-inside-repo-planning-project-bootstrap" ]] ||
+  fail "a dirty checkout created a worktree"
+git -C "$inside_repo" checkout -q -- AGENTS.md
+(
+  cd "$inside_repo"
+  printf 'y\n' |
+    PATH="$tmp/bin:/usr/bin:/bin" \
+    MOCK_AGENT_LOG="$tmp/description-inside.log" \
+    ./scripts/start-planning.sh --description idea.md --agent codex --model astra \
+    >"$tmp/description-inside.out" 2>&1
+) || {
+  cat "$tmp/description-inside.out" >&2
+  fail "an uncommitted description inside the repository was rejected"
+}
+cmp -s "$inside_repo/idea.md" \
+  "$tmp/description-inside-repo-planning-project-bootstrap/docs/PROJECT_DESCRIPTION.md" ||
+  fail "in-repository description was not copied into the planning worktree"
+
+# A phase that modifies the supplied description exceeds its scope.
+description_edit_repo="$(setup_repo description-edit)"
+if (
+  cd "$description_edit_repo"
+  printf 'y\n' |
+    PATH="$tmp/bin:/usr/bin:/bin" \
+    MOCK_AGENT_LOG="$tmp/description-edit.log" \
+    MOCK_GRILL_MODE=edit-description \
+    ./scripts/start-planning.sh --description "$description" --agent codex --model astra \
+    >/dev/null 2>&1
+); then
+  fail "a modified project description returned success"
+fi
+[[ "$(grep -c '^PHASE=' "$tmp/description-edit.log")" -eq 1 ]] ||
+  fail "planner ran after the project description was modified"
+
+# An unusable description fails before any branch or worktree is created.
+: >"$tmp/empty-description.txt"
+description_invalid_repo="$(setup_repo description-invalid)"
+for invalid_description in "$tmp/no-such-description.txt" "$tmp/empty-description.txt" "$tmp/bin" ""; do
+  if (
+    cd "$description_invalid_repo"
+    PATH="$tmp/bin:/usr/bin:/bin" \
+      MOCK_AGENT_LOG="$tmp/description-invalid.log" \
+      ./scripts/start-planning.sh --agent codex --model astra --description "$invalid_description" \
+      >/dev/null 2>&1
+  ); then
+    fail "unusable project description returned success: '$invalid_description'"
+  fi
+done
+[[ ! -e "$tmp/description-invalid-repo-planning-project-bootstrap" ]] ||
+  fail "unusable project description created a worktree"
+if git -C "$description_invalid_repo" show-ref --verify --quiet refs/heads/planning/project-bootstrap; then
+  fail "unusable project description created a branch"
+fi
+[[ ! -e "$tmp/description-invalid.log" ]] || fail "an agent started despite an unusable project description"
+
 echo "start-planning tests passed"