diff --git a/BUILD-LOG.md b/BUILD-LOG.md
index c91754d4..5ff4c6f9 100644
--- a/BUILD-LOG.md
+++ b/BUILD-LOG.md
@@ -106,4 +106,35 @@ No credentials are recorded in this file.
 
 All 11 acceptance criteria demonstrated. The real model request passed.
 
+---
+
+## Phase 5: Configuration-driven Hello World (M1)
+
+### Entry 5.1 — before
+
+- Timestamp: 2026-09-03
+- Intended action: Make the container POC configuration-driven. Baseline committed and tagged `poc-container-hello-v0`. Milestone M1 tracked in Gitea (issues #1-#4): (T1) config module with idempotent bootstrap and strict v1 validation; (T2) wire scripts and compose to config.json with fail-closed behavior; (T3) sandboxed config selftests; (T4) E2E verification and documentation.
+- Reason: Per docs/plans/2026-09-02_atomic-mosaic-foundation.md — config.json must be the sole discovery entry point; updates and runs must never corrupt or invent configuration.
+- Expected result: All M1 acceptance criteria pass; Hello World reproducible from configuration alone.
+
+### Entry 5.2 — after
+
+- Timestamp: 2026-09-03
+- Commands run: `scripts/test-config.sh` (20 cases); fail-closed checks (compose without launcher env, verify/reset with missing config); `scripts/bootstrap.sh`; config-driven `scripts/hello.sh`, `scripts/verify.sh`, negative marker test, sandboxed reset symlink refusal (canary survived), real reset + bootstrap + build + verify; config checksum comparison across the entire flow.
+- Observed result:
+  - Config selftests: 20 passed, 0 failed.
+  - Fail-closed confirmed: compose exits 1 without launcher env; verify/reset exit 1 on missing config before any mutation.
+  - Bootstrap created `~/.config/mosaic-dev/config.json` exclusively; second run validated without rewriting (content + mtime unchanged).
+  - Config-driven hello/verify returned exactly `MOSAIC_HELLO_OK`; verify exit 0; negative marker test exit 1.
+  - Reset refused symlinked dataRoot; canary file survived; real reset removed only the configured data root.
+  - config.json checksum unchanged across hello/verify/reset/bootstrap/build/verify.
+- Failure or correction:
+  1. Selftest harness bug: `cfg` helper invoked without a body for the symlink case (`$2: unbound variable`). Fixed in the harness; product code unaffected.
+  2. E2E rerun-after-reset failure: `verify.sh` did not ensure the configured data root existed before the container mount. With the data root absent, Docker auto-created the host path as root:root, and the container's uid-1000 user could not write the generated system prompt. Fixed by calling `bootstrap_runtime_dir` in `verify.sh`; also hardened it to fail with a clear message when the data root exists but is not writable (root-owned leftover). Clean-slate E2E rerun: all steps green.
+- Credential check: no credential material in config, scripts, logs, or test output.
+
+## Result (M1)
+
+Configuration-driven Hello World verified. `main` merged with M1 and tagged `config-hello-v1`.
+
 
diff --git a/README.md b/README.md
index a79332db..3c062c92 100644
--- a/README.md
+++ b/README.md
@@ -9,24 +9,65 @@ to return exactly `MOSAIC_HELLO_OK`.
 ## Layout
 
 ```text
-BRIEF.md            requirements for this experiment
+BRIEF.md            requirements for the original container proof
 BUILD-LOG.md        append-only build/verification log
 LAYERS.md           implemented layer (L0) and deferred layers (L1-L6)
 Containerfile       image definition (node:24-bookworm-slim, non-root, pinned Pi)
-compose.yaml        one service: mosaic-agent (one-shot)
+compose.yaml        one service: mosaic-agent (one-shot; configured via env)
 package.json        pins @earendil-works/pi-coding-agent at exactly 0.84.4
 package-lock.json   resolved lockfile used by npm ci in the image
-.env.example        non-secret settings only (provider, model)
+.env.example        non-secret settings only (credential-file path, env-var auth)
 contracts/          CONSTITUTION.md, STANDARDS.md, SOUL.md, USER.md (immutable fixtures)
-scripts/            build.sh, hello.sh, verify.sh, reset.sh (+ shared common.sh)
+scripts/            bootstrap/build/hello/verify/reset + config tooling
 src/                load-contracts.sh, run-agent.sh (run inside the container)
+docs/plans/         architecture and milestone plans
 ```
 
+## Configuration
+
+The sole discovery entry point is:
+
+```text
+~/.config/mosaic-dev/config.json
+```
+
+Created only by the explicit, idempotent bootstrap:
+
+```bash
+scripts/bootstrap.sh   # create-if-absent; validates existing config, never rewrites
+```
+
+Minimal shape (`configVersion` 1):
+
+```json
+{
+  "configVersion": 1,
+  "environment": "development",
+  "dataRoot": "/home/jwoltje/.mosaic-dev",
+  "execution": {
+    "backend": "docker",
+    "provider": "zai",
+    "model": "glm-5.3-flash"
+  }
+}
+```
+
+Rules enforced by `scripts/mosaic-config.mjs`:
+
+- Unknown keys, unsupported versions/backends, and malformed JSON exit nonzero; nothing is modified.
+- `dataRoot` must be absolute, canonical, and must not be or contain the home or configuration directory.
+- Validation failures never touch config, state, or images.
+- `scripts/test-config.sh` runs the sandboxed config selftests (no Docker required).
+
+Run paths (`build/hello/verify/reset`) fail closed when configuration is missing or invalid; they never invent it.
+
+See `docs/plans/2026-09-02_atomic-mosaic-foundation.md` for the full plan.
+
 Inside the container:
 
 ```text
 /opt/mosaic/contracts     immutable contract files
-/var/lib/mosaic           generated runtime state (mounted from /home/jwoltje/.mosaic-dev)
+/var/lib/mosaic           generated runtime state (mounted from configured dataRoot)
 /workspace                agent workspace
 ```
 
@@ -48,10 +89,12 @@ Inside the container:
 ## Usage
 
 ```bash
-scripts/build.sh      # build the image
-scripts/hello.sh      # one-shot request; prints the model response
-scripts/verify.sh     # full gated test; exit 0 only on exact MOSAIC_HELLO_OK
-scripts/reset.sh      # delete /home/jwoltje/.mosaic-dev (safety-checked)
+scripts/bootstrap.sh   # create config.json if absent (idempotent)
+scripts/build.sh       # build the image
+scripts/hello.sh       # one-shot request; prints the model response
+scripts/verify.sh      # full gated test; exit 0 only on exact MOSAIC_HELLO_OK
+scripts/test-config.sh # fast config-layer selftests (no Docker)
+scripts/reset.sh       # delete the configured data root (safety-checked)
 ```
 
 Prove the failure path (acceptance criterion 9):
diff --git a/compose.yaml b/compose.yaml
index cd067208..1a447020 100644
--- a/compose.yaml
+++ b/compose.yaml
@@ -6,17 +6,18 @@ services:
     image: mosaic-poc-agent:0.84.4
     user: "1000:1000"
     environment:
-      # Non-secret settings (see .env.example)
-      PI_PROVIDER: ${PI_PROVIDER:-zai}
-      PI_MODEL: ${PI_MODEL:-glm-5.3-flash}
+      # Resolved from config.json by scripts/common.sh (load_config).
+      # Required: compose fails fast when the launcher did not supply them.
+      PI_PROVIDER: ${MOSAIC_PROVIDER:?MOSAIC_PROVIDER must be set by scripts/load_config (run via scripts/*.sh)}
+      PI_MODEL: ${MOSAIC_MODEL:?MOSAIC_MODEL must be set by scripts/load_config (run via scripts/*.sh)}
       # Documented container auth alternative: provider API key via
       # runtime environment variable. Empty by default; when empty Pi
       # falls back to the read-only mounted auth.json credential file.
       ZAI_API_KEY: ${ZAI_API_KEY:-}
       ANTHROPIC_API_KEY: ${ANTHROPIC_API_KEY:-}
     volumes:
-      # Generated runtime state (host dir per brief)
-      - /home/jwoltje/.mosaic-dev:/var/lib/mosaic
+      # Configured runtime state root (from config.json dataRoot).
+      - ${MOSAIC_DATA_ROOT:?MOSAIC_DATA_ROOT must be set by scripts/load_config (run via scripts/*.sh)}:/var/lib/mosaic
       # Runtime credential only: pi auth file mounted READ-ONLY.
       # Never copied into the image.
       - ${PI_AUTH_FILE:-/home/jwoltje/.pi/agent/auth.json}:/home/node/.pi/agent/auth.json:ro
diff --git a/scripts/bootstrap.sh b/scripts/bootstrap.sh
new file mode 100755
index 00000000..0312d2ca
--- /dev/null
+++ b/scripts/bootstrap.sh
@@ -0,0 +1,11 @@
+#!/usr/bin/env bash
+# Explicit, idempotent configuration bootstrap.
+#
+# Creates ~/.config/mosaic-dev/config.json (or $MOSAIC_CONFIG) only when
+# absent. An existing configuration is validated, never modified.
+# Normal run paths (build/hello/verify) deliberately do NOT auto-bootstrap:
+# missing configuration is an error there, not something to invent.
+set -euo pipefail
+cd "$(dirname "$0")/.."
+
+exec node scripts/mosaic-config.mjs bootstrap
diff --git a/scripts/build.sh b/scripts/build.sh
index 851cdd4e..e2758a8d 100755
--- a/scripts/build.sh
+++ b/scripts/build.sh
@@ -5,6 +5,8 @@ cd "$(dirname "$0")/.."
 # shellcheck source=common.sh
 source scripts/common.sh
 
+load_config
+
 bootstrap_runtime_dir
 
 docker compose build
diff --git a/scripts/common.sh b/scripts/common.sh
index f5618473..e3e21abd 100755
--- a/scripts/common.sh
+++ b/scripts/common.sh
@@ -1,13 +1,36 @@
-# Shared helpers for the POC host scripts. Not a documented entry point.
+# Shared helpers for the Mosaic host scripts. Not a documented entry point.
 
-MOSAIC_DEV_DIR="/home/jwoltje/.mosaic-dev"
 POC_ROOT_MARKER=".mosaic-poc-root"
 
-# Ensure the runtime state directory exists and carries this project's
-# ownership marker. The marker is what scripts/reset.sh requires before
-# it will delete anything.
+# Load and validate the Mosaic configuration (config.json), exporting
+# MOSAIC_DATA_ROOT, MOSAIC_PROVIDER, and MOSAIC_MODEL.
+#
+# Fails closed: a missing or invalid configuration aborts the calling
+# script before any container or filesystem mutation. Run paths never
+# auto-bootstrap; use scripts/bootstrap.sh explicitly.
+load_config() {
+    local config_env
+    if ! config_env="$(node scripts/mosaic-config.mjs env)"; then
+        echo "common: configuration load failed" >&2
+        exit 1
+    fi
+    eval "$config_env"
+    export MOSAIC_DATA_ROOT MOSAIC_PROVIDER MOSAIC_MODEL
+    MOSAIC_DEV_DIR="$MOSAIC_DATA_ROOT"
+}
+
+# Ensure the configured runtime data directory exists and carries this
+# project's ownership marker. The marker is what scripts/reset.sh requires
+# before it will delete anything.
 bootstrap_runtime_dir() {
-    if [ ! -d "$MOSAIC_DEV_DIR" ]; then
+    if [ -d "$MOSAIC_DEV_DIR" ]; then
+        if [ ! -w "$MOSAIC_DEV_DIR" ] || [ ! -x "$MOSAIC_DEV_DIR" ]; then
+            echo "bootstrap: $MOSAIC_DEV_DIR exists but is not writable by $(id -un)" >&2
+            echo "bootstrap: a root-owned directory here is usually leftover from a" >&2
+            echo "bootstrap: Docker-created mount source; remove it and rerun." >&2
+            exit 1
+        fi
+    else
         mkdir -p "$MOSAIC_DEV_DIR"
         echo "bootstrap: created $MOSAIC_DEV_DIR"
     fi
diff --git a/scripts/gitea-api.sh b/scripts/gitea-api.sh
new file mode 100755
index 00000000..128a39cd
--- /dev/null
+++ b/scripts/gitea-api.sh
@@ -0,0 +1,83 @@
+#!/usr/bin/env bash
+# Minimal Gitea API client for this repository.
+#
+# Usage: scripts/gitea-api.sh METHOD api/path [json-body]
+#   e.g. scripts/gitea-api.sh GET repos/mosaicstack/stack-v2/issues
+#
+# Security:
+#   - Reads credentials from ~/secrets/mosaic.gitea.json (or
+#     MOSAIC_GITEA_CREDENTIAL_FILE); file must be 0600, non-symlink.
+#   - Token is passed to curl via a config stream (never argv, never disk,
+#     never stdout/stderr).
+#   - Prints the response body on stdout and "HTTP <code>" on stderr.
+#     Exits nonzero when the API reports an error.
+set -euo pipefail
+
+METHOD="${1:?usage: gitea-api.sh METHOD api/path [json-body]}"
+API_PATH="${2:?missing api/path}"
+API_PATH="${API_PATH#/}"
+BODY="${3:-}"
+
+command -v curl >/dev/null || { echo "gitea-api: curl not found" >&2; exit 1; }
+command -v node >/dev/null || { echo "gitea-api: node not found" >&2; exit 1; }
+
+export MOSAIC_GITEA_CREDENTIAL_FILE="${MOSAIC_GITEA_CREDENTIAL_FILE:-$HOME/secrets/mosaic.gitea.json}"
+
+# Validate credential file; emit only the non-secret base URL on stdout.
+BASE="$(node -e '
+const fs = require("fs");
+const p = process.env.MOSAIC_GITEA_CREDENTIAL_FILE;
+let s;
+try { s = fs.lstatSync(p); } catch { process.exit(3); }
+if (!s.isFile() || s.isSymbolicLink() || (s.mode & 0o077) !== 0) process.exit(3);
+let e;
+try { e = JSON.parse(fs.readFileSync(p, "utf8")).mosaicstack || {}; } catch { process.exit(3); }
+const base = String(e.url || "").replace(/\/+$/, "");
+if (!/^https:\/\/git\.mosaicstack\.dev$/.test(base)) process.exit(3);
+if (typeof e.api_token !== "string" || e.api_token.length === 0) process.exit(3);
+process.stdout.write(base);
+')"
+
+# Repo path from the configured origin remote (never from credentials).
+REMOTE_URL="$(git remote get-url origin)"
+REPO_PATH="${REMOTE_URL#https://git.mosaicstack.dev/}"
+REPO_PATH="${REPO_PATH%.git}"
+
+# curl config stream: auth header via fd, never argv.
+gen_curl_cfg() {
+  node -e '
+const fs = require("fs");
+const e = JSON.parse(fs.readFileSync(process.env.MOSAIC_GITEA_CREDENTIAL_FILE, "utf8")).mosaicstack || {};
+process.stdout.write("header = \"Authorization: token " + e.api_token + "\"\n");
+process.stdout.write("header = \"Content-Type: application/json\"\n");
+'
+}
+
+BODY_FILE=""
+cleanup() { [ -n "$BODY_FILE" ] && rm -f "$BODY_FILE"; }
+trap cleanup EXIT
+if [ -n "$BODY" ]; then
+  BODY_FILE="$(mktemp)"
+  chmod 600 "$BODY_FILE"
+  printf '%s' "$BODY" > "$BODY_FILE"
+fi
+
+URL="$BASE/api/v1/$API_PATH"
+if [ -n "$BODY_FILE" ]; then
+  HTTP_CODE="$(curl -sS -K <(gen_curl_cfg) -o /tmp/gitea-api-response.$$ \
+    -w '%{http_code}' -X "$METHOD" "$URL" --data-binary @"$BODY_FILE")" || {
+    echo "gitea-api: request failed" >&2; exit 1; }
+else
+  HTTP_CODE="$(curl -sS -K <(gen_curl_cfg) -o /tmp/gitea-api-response.$$ \
+    -w '%{http_code}' -X "$METHOD" "$URL")" || {
+    echo "gitea-api: request failed" >&2; exit 1; }
+fi
+
+cat /tmp/gitea-api-response.$$ 2>/dev/null || true
+rm -f /tmp/gitea-api-response.$$
+echo "HTTP $HTTP_CODE" >&2
+
+case "$HTTP_CODE" in
+  2*) exit 0 ;;
+  *) echo "gitea-api: $METHOD $API_PATH failed (HTTP $HTTP_CODE)" >&2; exit 1 ;;
+esac
diff --git a/scripts/hello.sh b/scripts/hello.sh
index 4a68729b..c2b23598 100755
--- a/scripts/hello.sh
+++ b/scripts/hello.sh
@@ -10,6 +10,8 @@ cd "$(dirname "$0")/.."
 # shellcheck source=common.sh
 source scripts/common.sh
 
+load_config
+
 bootstrap_runtime_dir
 
 # -T: no pseudo-TTY, so stdout is clean model output.
diff --git a/scripts/mosaic-config.mjs b/scripts/mosaic-config.mjs
new file mode 100755
index 00000000..c209e255
--- /dev/null
+++ b/scripts/mosaic-config.mjs
@@ -0,0 +1,233 @@
+#!/usr/bin/env node
+/**
+ * Mosaic development configuration: bootstrap, validate, resolve.
+ *
+ * Operations:
+ *   bootstrap  Create the default config ONLY if absent; otherwise validate
+ *              the existing file without writing (idempotent).
+ *   validate   Load and strictly validate; print resolved config on stdout.
+ *   env        Print shell-safe exports for launcher scripts:
+ *              MOSAIC_DATA_ROOT, MOSAIC_PROVIDER, MOSAIC_MODEL.
+ *
+ * Config location: $MOSAIC_CONFIG or ~/.config/mosaic-dev/config.json
+ *
+ * Exit codes:
+ *   0 success
+ *   2 configuration exists but is invalid (never modified by this tool)
+ *   3 configuration is missing for a read operation (validate/env)
+ *
+ * Invariants (docs/plans/2026-09-02_atomic-mosaic-foundation.md):
+ *   - Existing configuration is never overwritten.
+ *   - Validation failure modifies nothing.
+ *   - Secrets are never stored in configuration.
+ */
+
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+import process from "node:process";
+
+const SUPPORTED_CONFIG_VERSION = 1;
+const SUPPORTED_ENVIRONMENTS = new Set(["development", "production"]);
+const SUPPORTED_BACKENDS = new Set(["docker"]);
+const NAME_PATTERN = /^[A-Za-z0-9][A-Za-z0-9._:/-]{0,199}$/;
+
+function fail(exitCode, message) {
+  process.stderr.write(`mosaic-config: ${message}\n`);
+  process.exit(exitCode);
+}
+
+function configPath() {
+  return process.env.MOSAIC_CONFIG
+    ? path.resolve(process.env.MOSAIC_CONFIG)
+    : path.join(os.homedir(), ".config", "mosaic-dev", "config.json");
+}
+
+function readRaw(file) {
+  let stat;
+  try {
+    stat = fs.lstatSync(file);
+  } catch {
+    return null; // missing
+  }
+  if (!stat.isFile() || stat.isSymbolicLink()) {
+    fail(2, `configuration path must be a regular, non-symbolic-link file: ${file}`);
+  }
+  try {
+    return fs.readFileSync(file, "utf8");
+  } catch {
+    fail(2, `configuration file is not readable: ${file}`);
+  }
+}
+
+function rejectUnknownKeys(object, allowed, where) {
+  for (const key of Object.keys(object)) {
+    if (!allowed.includes(key)) {
+      fail(2, `unsupported ${where} key: "${key}"`);
+    }
+  }
+}
+
+function isPlainObject(value) {
+  return typeof value === "object" && value !== null && !Array.isArray(value);
+}
+
+function validateDataRoot(value, file) {
+  if (typeof value !== "string" || value.length === 0) {
+    fail(2, 'execution "dataRoot" must be a non-empty string');
+  }
+  if (value.includes("\0")) {
+    fail(2, 'execution "dataRoot" contains a NUL byte');
+  }
+  if (!path.isAbsolute(value)) {
+    fail(2, `execution "dataRoot" must be an absolute path (got "${value}")`);
+  }
+  const resolved = path.resolve(value);
+  if (resolved !== value) {
+    fail(2, `execution "dataRoot" must be canonical without ".", "..", trailing slashes, or redundant separators (got "${value}")`);
+  }
+  if (resolved === path.parse(resolved).root) {
+    fail(2, 'execution "dataRoot" must not be the filesystem root');
+  }
+  const home = path.resolve(os.homedir());
+  if (resolved === home || isAncestorOf(resolved, home)) {
+    fail(2, `execution "dataRoot" must not be or contain the home directory (${home})`);
+  }
+  const configDir = path.dirname(path.resolve(file));
+  if (resolved === configDir || isAncestorOf(resolved, configDir)) {
+    fail(2, `execution "dataRoot" must not be or contain the configuration directory (${configDir})`);
+  }
+  return resolved;
+}
+
+function isAncestorOf(ancestor, candidate) {
+  const rel = path.relative(ancestor, candidate);
+  return rel !== "" && !rel.startsWith("..") && !path.isAbsolute(rel);
+}
+
+function validate(document, file) {
+  if (!isPlainObject(document)) {
+    fail(2, "configuration must be a JSON object");
+  }
+  rejectUnknownKeys(
+    document,
+    ["configVersion", "environment", "dataRoot", "execution"],
+    "configuration",
+  );
+
+  if (document.configVersion !== SUPPORTED_CONFIG_VERSION) {
+    fail(2, `unsupported configVersion: ${JSON.stringify(document.configVersion)} (supported: ${SUPPORTED_CONFIG_VERSION})`);
+  }
+  if (!SUPPORTED_ENVIRONMENTS.has(document.environment)) {
+    fail(2, `unsupported environment: ${JSON.stringify(document.environment)} (supported: ${[...SUPPORTED_ENVIRONMENTS].join(", ")})`);
+  }
+
+  const dataRoot = validateDataRoot(document.dataRoot, file);
+
+  if (!isPlainObject(document.execution)) {
+    fail(2, '"execution" must be a JSON object');
+  }
+  rejectUnknownKeys(document.execution, ["backend", "provider", "model"], '"execution"');
+  if (!SUPPORTED_BACKENDS.has(document.execution.backend)) {
+    fail(2, `unsupported execution.backend: ${JSON.stringify(document.execution.backend)} (supported: ${[...SUPPORTED_BACKENDS].join(", ")})`);
+  }
+  for (const key of ["provider", "model"]) {
+    const value = document.execution[key];
+    if (typeof value !== "string" || !NAME_PATTERN.test(value)) {
+      fail(2, `execution.${key} must match ${NAME_PATTERN} (got ${JSON.stringify(value)})`);
+    }
+  }
+
+  return {
+    configVersion: document.configVersion,
+    environment: document.environment,
+    dataRoot,
+    execution: {
+      backend: document.execution.backend,
+      provider: document.execution.provider,
+      model: document.execution.model,
+    },
+  };
+}
+
+function load(file) {
+  const raw = readRaw(file);
+  if (raw === null) {
+    fail(3, `configuration not found: ${file} (run scripts/bootstrap.sh to create it)`);
+  }
+  let document;
+  try {
+    document = JSON.parse(raw);
+  } catch (error) {
+    fail(2, `configuration is not valid JSON (${file}): ${error.message}`);
+  }
+  return validate(document, file);
+}
+
+function shellQuote(value) {
+  return `'${String(value).replaceAll("'", `'\\''`)}'`;
+}
+
+const DEFAULT_CONFIG = {
+  configVersion: 1,
+  environment: "development",
+  dataRoot: path.join(os.homedir(), ".mosaic-dev"),
+  execution: {
+    backend: "docker",
+    provider: "zai",
+    model: "glm-5.3-flash",
+  },
+};
+
+const operation = process.argv[2];
+const file = configPath();
+
+switch (operation) {
+  case "bootstrap": {
+    if (fs.existsSync(file)) {
+      load(file); // validate only; never rewrite
+      process.stderr.write(`mosaic-config: configuration already present, validated without changes: ${file}\n`);
+      process.exit(0);
+    }
+    fs.mkdirSync(path.dirname(file), { recursive: true });
+    let fd;
+    try {
+      // 'wx': creation is exclusive; an existing file is never overwritten.
+      fd = fs.openSync(file, "wx", 0o644);
+      fs.writeFileSync(fd, `${JSON.stringify(DEFAULT_CONFIG, null, 2)}\n`);
+    } catch (error) {
+      if (error.code === "EEXIST") {
+        load(file);
+        process.exit(0);
+      }
+      fail(1, `unable to create configuration: ${error.message}`);
+    } finally {
+      if (fd !== undefined) fs.closeSync(fd);
+    }
+    load(file);
+    process.stderr.write(`mosaic-config: created default configuration: ${file}\n`);
+    process.exit(0);
+  }
+
+  case "validate": {
+    const resolved = load(file);
+    process.stdout.write(`${JSON.stringify(resolved, null, 2)}\n`);
+    process.exit(0);
+  }
+
+  case "env": {
+    const resolved = load(file);
+    process.stdout.write(
+      [
+        `MOSAIC_DATA_ROOT=${shellQuote(resolved.dataRoot)}`,
+        `MOSAIC_PROVIDER=${shellQuote(resolved.execution.provider)}`,
+        `MOSAIC_MODEL=${shellQuote(resolved.execution.model)}`,
+        "",
+      ].join("\n"),
+    );
+    process.exit(0);
+  }
+
+  default:
+    fail(1, `unknown operation: ${JSON.stringify(operation ?? "")} (expected bootstrap | validate | env)`);
+}
diff --git a/scripts/reset.sh b/scripts/reset.sh
index ba78bdb9..c8138b41 100755
--- a/scripts/reset.sh
+++ b/scripts/reset.sh
@@ -7,9 +7,15 @@
 #      created by this project.
 # Any failed check aborts with nothing deleted.
 set -euo pipefail
+cd "$(dirname "$0")/.."
+# shellcheck source=common.sh
+source scripts/common.sh
 
-TARGET="/home/jwoltje/.mosaic-dev"
-MARKER=".mosaic-poc-root"
+# Reset operates on the CONFIGURED data root. Configuration itself is
+# never a reset target; a missing/invalid configuration aborts here.
+load_config
+TARGET="$MOSAIC_DATA_ROOT"
+MARKER="$POC_ROOT_MARKER"
 
 fail() {
     echo "reset: refusing to delete: $*" >&2
diff --git a/scripts/test-config.sh b/scripts/test-config.sh
new file mode 100755
index 00000000..cc5e34b5
--- /dev/null
+++ b/scripts/test-config.sh
@@ -0,0 +1,142 @@
+#!/usr/bin/env bash
+# Fast, sandboxed selftests for the configuration layer.
+#
+# No Docker, no network, no credentials: every case runs against a
+# temporary config via MOSAIC_CONFIG. Suitable for frequent local runs.
+set -uo pipefail
+cd "$(dirname "$0")/.."
+
+SANDBOX="$(mktemp -d)"
+trap 'rm -rf "$SANDBOX"' EXIT
+
+PASS=0
+FAIL=0
+
+# expect_exit NAME EXPECTED_RC -- command...
+expect_exit() {
+    local name="$1" expected="$2"
+    shift 3 # name, expected, "--"
+    local rc
+    "$@" >/dev/null 2>&1
+    rc=$?
+    if [ "$rc" -eq "$expected" ]; then
+        PASS=$((PASS + 1))
+        echo "ok   $name (exit $rc)"
+    else
+        FAIL=$((FAIL + 1))
+        echo "FAIL $name (exit $rc, expected $expected)"
+    fi
+}
+
+CONFIG_OP="node scripts/mosaic-config.mjs"
+
+valid_body() {
+    cat <<EOF
+{"configVersion":1,"environment":"development","dataRoot":"$1","execution":{"backend":"docker","provider":"zai","model":"glm-5.3-flash"}}
+EOF
+}
+
+cfg() { printf '%s' "$2" > "$SANDBOX/$1"; }
+
+DATA_ROOT="$SANDBOX/data"
+
+# --- bootstrap ---
+rm -f "$SANDBOX/config.json"
+expect_exit "bootstrap creates default when absent" 0 -- \
+    env MOSAIC_CONFIG="$SANDBOX/config.json" $CONFIG_OP bootstrap
+[ -f "$SANDBOX/config.json" ] && { PASS=$((PASS+1)); echo "ok   bootstrap wrote config file"; } \
+    || { FAIL=$((FAIL+1)); echo "FAIL bootstrap wrote config file"; }
+
+SUM_BEFORE=$(sha256sum "$SANDBOX/config.json" | cut -d' ' -f1)
+MTIME_BEFORE=$(stat -c %Y "$SANDBOX/config.json")
+sleep 1.1
+expect_exit "bootstrap is idempotent on existing config" 0 -- \
+    env MOSAIC_CONFIG="$SANDBOX/config.json" $CONFIG_OP bootstrap
+SUM_AFTER=$(sha256sum "$SANDBOX/config.json" | cut -d' ' -f1)
+MTIME_AFTER=$(stat -c %Y "$SANDBOX/config.json")
+if [ "$SUM_BEFORE" = "$SUM_AFTER" ] && [ "$MTIME_BEFORE" = "$MTIME_AFTER" ]; then
+    PASS=$((PASS+1)); echo "ok   bootstrap did not rewrite existing config"
+else
+    FAIL=$((FAIL+1)); echo "FAIL bootstrap rewrote existing config"
+fi
+
+# --- validate ---
+expect_exit "validate missing config exits 3" 3 -- \
+    env MOSAIC_CONFIG="$SANDBOX/absent.json" $CONFIG_OP validate
+
+cfg invalid.json '{'
+expect_exit "malformed JSON exits 2" 2 -- \
+    env MOSAIC_CONFIG="$SANDBOX/invalid.json" $CONFIG_OP validate
+
+cfg badversion.json '{"configVersion":2,"environment":"development","dataRoot":"'$DATA_ROOT'","execution":{"backend":"docker","provider":"zai","model":"m"}}'
+expect_exit "unsupported configVersion exits 2" 2 -- \
+    env MOSAIC_CONFIG="$SANDBOX/badversion.json" $CONFIG_OP validate
+
+cfg unknownkey.json '{"configVersion":1,"environment":"development","dataRoot":"'$DATA_ROOT'","extra":true,"execution":{"backend":"docker","provider":"zai","model":"m"}}'
+expect_exit "unknown top-level key exits 2" 2 -- \
+    env MOSAIC_CONFIG="$SANDBOX/unknownkey.json" $CONFIG_OP validate
+
+cfg unknownexec.json '{"configVersion":1,"environment":"development","dataRoot":"'$DATA_ROOT'","execution":{"backend":"docker","provider":"zai","model":"m","extra":1}}'
+expect_exit "unknown execution key exits 2" 2 -- \
+    env MOSAIC_CONFIG="$SANDBOX/unknownexec.json" $CONFIG_OP validate
+
+cfg badbackend.json '{"configVersion":1,"environment":"development","dataRoot":"'$DATA_ROOT'","execution":{"backend":"podman","provider":"zai","model":"m"}}'
+expect_exit "unsupported backend exits 2" 2 -- \
+    env MOSAIC_CONFIG="$SANDBOX/badbackend.json" $CONFIG_OP validate
+
+cfg badenv.json '{"configVersion":1,"environment":"staging","dataRoot":"'$DATA_ROOT'","execution":{"backend":"docker","provider":"zai","model":"m"}}'
+expect_exit "unsupported environment exits 2" 2 -- \
+    env MOSAIC_CONFIG="$SANDBOX/badenv.json" $CONFIG_OP validate
+
+cfg relative.json '{"configVersion":1,"environment":"development","dataRoot":"relative/path","execution":{"backend":"docker","provider":"zai","model":"m"}}'
+expect_exit "relative dataRoot exits 2" 2 -- \
+    env MOSAIC_CONFIG="$SANDBOX/relative.json" $CONFIG_OP validate
+
+cfg traversal.json '{"configVersion":1,"environment":"development","dataRoot":"/tmp/../home/x","execution":{"backend":"docker","provider":"zai","model":"m"}}'
+expect_exit "non-canonical dataRoot exits 2" 2 -- \
+    env MOSAIC_CONFIG="$SANDBOX/traversal.json" $CONFIG_OP validate
+
+cfg root.json '{"configVersion":1,"environment":"development","dataRoot":"/","execution":{"backend":"docker","provider":"zai","model":"m"}}'
+expect_exit "filesystem root dataRoot exits 2" 2 -- \
+    env MOSAIC_CONFIG="$SANDBOX/root.json" $CONFIG_OP validate
+
+cfg home.json '{"configVersion":1,"environment":"development","dataRoot":"'$HOME'","execution":{"backend":"docker","provider":"zai","model":"m"}}'
+expect_exit "home directory dataRoot exits 2" 2 -- \
+    env MOSAIC_CONFIG="$SANDBOX/home.json" $CONFIG_OP validate
+
+cfg cfgdir.json '{"configVersion":1,"environment":"development","dataRoot":"'$SANDBOX'","execution":{"backend":"docker","provider":"zai","model":"m"}}'
+expect_exit "dataRoot containing config dir exits 2" 2 -- \
+    env MOSAIC_CONFIG="$SANDBOX/cfgdir.json" $CONFIG_OP validate
+
+cfg ctrlchar.json '{"configVersion":1,"environment":"development","dataRoot":"'$DATA_ROOT'","execution":{"backend":"docker","provider":"z\nai","model":"m"}}'
+expect_exit "control character in provider exits 2" 2 -- \
+    env MOSAIC_CONFIG="$SANDBOX/ctrlchar.json" $CONFIG_OP validate
+
+cfg symlink.json 'placeholder'
+ln -sf "$SANDBOX/invalid.json" "$SANDBOX/symlink.json"
+expect_exit "symlinked config file exits 2" 2 -- \
+    env MOSAIC_CONFIG="$SANDBOX/symlink.json" $CONFIG_OP validate
+
+# --- env resolution ---
+cfg valid.json "$(valid_body "$DATA_ROOT")"
+EVAL_OUT="$(MOSAIC_CONFIG="$SANDBOX/valid.json" $CONFIG_OP env)" || true
+if eval "$EVAL_OUT" 2>/dev/null && [ "$MOSAIC_DATA_ROOT" = "$DATA_ROOT" ] \
+    && [ "$MOSAIC_PROVIDER" = "zai" ] && [ "$MOSAIC_MODEL" = "glm-5.3-flash" ]; then
+    PASS=$((PASS+1)); echo "ok   env exports resolve correctly"
+else
+    FAIL=$((FAIL+1)); echo "FAIL env exports resolve correctly"
+fi
+
+# --- validation must not modify the file ---
+SUM_INVALID_BEFORE=$(sha256sum "$SANDBOX/invalid.json" | cut -d' ' -f1)
+MOSAIC_CONFIG="$SANDBOX/invalid.json" $CONFIG_OP validate >/dev/null 2>&1
+SUM_INVALID_AFTER=$(sha256sum "$SANDBOX/invalid.json" | cut -d' ' -f1)
+if [ "$SUM_INVALID_BEFORE" = "$SUM_INVALID_AFTER" ]; then
+    PASS=$((PASS+1)); echo "ok   failed validation modified nothing"
+else
+    FAIL=$((FAIL+1)); echo "FAIL failed validation modified the file"
+fi
+
+echo
+echo "selftest: $PASS passed, $FAIL failed"
+[ "$FAIL" -eq 0 ]
diff --git a/scripts/verify.sh b/scripts/verify.sh
index c2ed20b9..9a3384d7 100755
--- a/scripts/verify.sh
+++ b/scripts/verify.sh
@@ -17,6 +17,12 @@ source scripts/common.sh
 IMAGE="mosaic-poc-agent:0.84.4"
 EXPECTED="${EXPECTED_MARKER:-MOSAIC_HELLO_OK}"
 
+load_config
+
+# Ensure the configured data root exists (host-owned) before the mount,
+# otherwise Docker would auto-create a root-owned directory.
+bootstrap_runtime_dir
+
 # 1. Build the image only if it is not already present.
 if ! docker image inspect "$IMAGE" >/dev/null 2>&1; then
     echo "verify: image $IMAGE not found, building..." >&2