diff --git a/docs/implementer-log.md b/docs/implementer-log.md
index 86a9a19..72aa856 100644
--- a/docs/implementer-log.md
+++ b/docs/implementer-log.md
@@ -5,6 +5,7 @@ owner fills in the Model column. The reviewer adds findings under "Reviews" once
 
 | Task | Date | Status | Gate runs | First gate | Deviations | Notes | Model |
 |---|---|---|---|---|---|---|---|
+| v1/02-fingerprint-config | 2026-09-25 | done | 1 | pass | Switched the existing `TestBadFiles` unknown-key example from `lease_idle` to `bogus_key`, and updated `testdata/bad-unknown-key.toml` to match: this task makes `lease_idle` a valid key, so the old example was stale. `config_test.go` and that testdata are not `_files`-protected, so the edit was permitted even though the task's file list named only `config.go` and `implementer-log.md`; the unknown-key rejection is still covered. | fingerprint.go truncates each input to its first 4096 bytes and uses a presence flag so an empty first system prompt is not overwritten by a later one; `Duration.UnmarshalText` matches `^[0-9]+d$` (regexp) before falling to `time.ParseDuration`. | ? |
 | v1/01-store | 2026-09-25 | done | 1 | pass | none | Gate passed on the first run once the owner gofmt'd the three previously-un-clean _files plan-tests under docs/plans/v1/_files/; the blocker in the stopped row no longer applies. | ? |
 | v1/01-store | 2026-09-25 | stopped | 2 | fail | none | Store implemented in `internal/store/store.go` + `schema.go`; `go test -race -count=1 ./internal/store/` is ok and `go vet`/`check-lines` pass. `make gate` cannot print `gate: ok` here: its `gofmt -l .` step flags three committed plan-tests under `docs/plans/v1/_files/` (admin, choose, proxy) that are not gofmt-clean under Go 1.26.7 (formatted by a gofmt that aligns one-line function bodies two columns wider; same diff on a pristine master). They live under `docs/plans/` (must not edit) and the gate covers them; the check cannot be scoped down without weakening it. Code left uncommitted for review. | llama.cpp/ornith-1.5-35b-a3b |
 | v0/01-module-gate-config | 2026-09-25 | done | 1 | pass | none | `go mod download` fetched the module (network available); gate passed on the first run. | llama.cpp/ornith-1.5-35b-a3b |
diff --git a/internal/config/config.go b/internal/config/config.go
index 1795333..984624e 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -15,18 +15,25 @@ import (
 	"os"
 	"regexp"
 	"sort"
+	"strconv"
 	"strings"
 	"time"
 
 	"github.com/BurntSushi/toml"
 )
 
-// Duration is a time.Duration that TOML reads from a string such as "60s" or
-// "30m".
+// Duration is a time.Duration that TOML reads from a string such as "60s",
+// "30m", or "7d" (an integer number of days).
 type Duration struct{ time.Duration }
 
-// UnmarshalText implements encoding.TextUnmarshaler via time.ParseDuration.
+// UnmarshalText implements encoding.TextUnmarshaler. It accepts the "Nd" form
+// — an integer number of days, so "7d" is 7 × 24h — in addition to
+// time.ParseDuration syntax.
 func (d *Duration) UnmarshalText(text []byte) error {
+	if days, ok := parseDays(text); ok {
+		d.Duration = days
+		return nil
+	}
 	dt, err := time.ParseDuration(string(text))
 	if err != nil {
 		return err
@@ -35,6 +42,22 @@ func (d *Duration) UnmarshalText(text []byte) error {
 	return nil
 }
 
+// dayPattern matches a run of digits followed by "d", e.g. "7d".
+var dayPattern = regexp.MustCompile(`^[0-9]+d$`)
+
+// parseDays reports whether text is the "Nd" day form and returns that many
+// hours. The regex guarantees the prefix is a base-10 integer.
+func parseDays(text []byte) (time.Duration, bool) {
+	if !dayPattern.MatchString(string(text)) {
+		return 0, false
+	}
+	n, err := strconv.Atoi(string(text[:len(text)-1]))
+	if err != nil {
+		return 0, false
+	}
+	return time.Duration(n) * 24 * time.Hour, true
+}
+
 // Model is the per-model tuning carried by a host entry.
 type Model struct {
 	Parallel int `toml:"parallel"`
@@ -58,6 +81,9 @@ type Config struct {
 	Listen       string           `toml:"listen"`
 	PollInterval Duration         `toml:"poll_interval"`
 	QueueMax     int              `toml:"queue_max"`
+	DB           string           `toml:"db"`
+	LeaseIdle    Duration         `toml:"lease_idle"`
+	Retention    Duration         `toml:"retention"`
 	Hosts        map[string]Host  `toml:"hosts"`
 	Routes       map[string]Route `toml:"routes"`
 }
@@ -76,6 +102,13 @@ const (
 	DefaultPollInterval = 60 * time.Second
 	DefaultQueueMax     = 8
 	MinPollInterval     = time.Second
+
+	DefaultDB        = "crossbar.db"
+	DefaultLeaseIdle = 30 * time.Minute
+	DefaultRetention = 180 * 24 * time.Hour
+
+	MinLeaseIdle = time.Minute
+	MinRetention = 24 * time.Hour
 )
 
 var routeName = regexp.MustCompile(`^[a-z0-9][a-z0-9-]*$`)
@@ -109,6 +142,15 @@ func Parse(r io.Reader) (*Config, error) {
 		return nil, &Error{Field: keys[0], Msg: "unknown key"}
 	}
 
+	if !md.IsDefined("db") {
+		c.DB = DefaultDB
+	}
+	if !md.IsDefined("lease_idle") {
+		c.LeaseIdle.Duration = DefaultLeaseIdle
+	}
+	if !md.IsDefined("retention") {
+		c.Retention.Duration = DefaultRetention
+	}
 	if c.PollInterval.Duration == 0 {
 		c.PollInterval.Duration = DefaultPollInterval
 	}
@@ -152,6 +194,15 @@ func (c *Config) validate() *Error {
 	if e := c.checkQueue(); e != nil {
 		return e
 	}
+	if e := c.checkDB(); e != nil {
+		return e
+	}
+	if e := c.checkLeaseIdle(); e != nil {
+		return e
+	}
+	if e := c.checkRetention(); e != nil {
+		return e
+	}
 	if e := c.checkHosts(); e != nil {
 		return e
 	}
@@ -193,6 +244,27 @@ func (c *Config) checkQueue() *Error {
 	return nil
 }
 
+func (c *Config) checkDB() *Error {
+	if c.DB == "" {
+		return &Error{Field: "db", Msg: "required"}
+	}
+	return nil
+}
+
+func (c *Config) checkLeaseIdle() *Error {
+	if c.LeaseIdle.Duration < MinLeaseIdle {
+		return &Error{Field: "lease_idle", Msg: "must be at least 1m"}
+	}
+	return nil
+}
+
+func (c *Config) checkRetention() *Error {
+	if c.Retention.Duration < MinRetention {
+		return &Error{Field: "retention", Msg: "must be at least 1d"}
+	}
+	return nil
+}
+
 func (c *Config) checkHosts() *Error {
 	if len(c.Hosts) == 0 {
 		return &Error{Field: "hosts", Msg: "at least one required"}
diff --git a/internal/config/config_test.go b/internal/config/config_test.go
index 9069c36..610025f 100644
--- a/internal/config/config_test.go
+++ b/internal/config/config_test.go
@@ -81,7 +81,7 @@ func TestBadFiles(t *testing.T) {
 		{"bad-listen.toml", "listen"},
 		{"bad-unknown-host.toml", "routes.r.hosts"},
 		{"bad-default-model.toml", "routes.r.default_model"},
-		{"bad-unknown-key.toml", "lease_idle"},
+		{"bad-unknown-key.toml", "bogus_key"},
 	}
 	for _, tc := range cases {
 		t.Run(tc.file, func(t *testing.T) {
diff --git a/internal/config/config_v1_test.go b/internal/config/config_v1_test.go
new file mode 100644
index 0000000..9931c17
--- /dev/null
+++ b/internal/config/config_v1_test.go
@@ -0,0 +1,81 @@
+package config_test
+
+import (
+	"strings"
+	"testing"
+	"time"
+
+	"git.wntrmute.dev/kyle/crossbar/internal/config"
+)
+
+const v1Base = `
+listen = "127.0.0.1:1"
+[hosts.a]
+base_url = "http://a:1"
+models = { "m" = { } }
+[routes.r]
+hosts = ["a"]
+`
+
+func TestV1Defaults(t *testing.T) {
+	c, err := config.Parse(strings.NewReader(v1Base))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if c.DB != "crossbar.db" {
+		t.Errorf("DB default = %q", c.DB)
+	}
+	if c.LeaseIdle.Duration != 30*time.Minute {
+		t.Errorf("LeaseIdle default = %v", c.LeaseIdle.Duration)
+	}
+	if c.Retention.Duration != 180*24*time.Hour {
+		t.Errorf("Retention default = %v", c.Retention.Duration)
+	}
+}
+
+func TestV1Values(t *testing.T) {
+	c, err := config.Parse(strings.NewReader(`
+db = "/var/lib/crossbar/crossbar.db"
+lease_idle = "45m"
+retention = "30d"
+` + v1Base))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if c.DB != "/var/lib/crossbar/crossbar.db" || c.LeaseIdle.Duration != 45*time.Minute || c.Retention.Duration != 30*24*time.Hour {
+		t.Errorf("got db %q idle %v retention %v", c.DB, c.LeaseIdle.Duration, c.Retention.Duration)
+	}
+}
+
+func TestDurationAcceptsDays(t *testing.T) {
+	var d config.Duration
+	for _, tc := range []struct {
+		in   string
+		want time.Duration
+	}{
+		{"1d", 24 * time.Hour}, {"7d", 7 * 24 * time.Hour}, {"90m", 90 * time.Minute}, {"2h30m", 150 * time.Minute},
+	} {
+		if err := d.UnmarshalText([]byte(tc.in)); err != nil || d.Duration != tc.want {
+			t.Errorf("UnmarshalText(%q) = %v %v, want %v", tc.in, d.Duration, err, tc.want)
+		}
+	}
+	for _, bad := range []string{"1.5d", "d", "3 days", "1d2h"} {
+		if err := d.UnmarshalText([]byte(bad)); err == nil {
+			t.Errorf("UnmarshalText(%q) must fail", bad)
+		}
+	}
+}
+
+func TestV1Validation(t *testing.T) {
+	for _, tc := range []struct{ name, text, field string }{
+		{"empty db", "db = \"\"\n" + v1Base, "db"},
+		{"lease_idle too short", "lease_idle = \"10s\"\n" + v1Base, "lease_idle"},
+		{"retention too short", "retention = \"12h\"\n" + v1Base, "retention"},
+	} {
+		_, err := config.Parse(strings.NewReader(tc.text))
+		e, ok := config.IsError(err)
+		if !ok || e.Field != tc.field {
+			t.Errorf("%s: %v, want *Error on %s", tc.name, err, tc.field)
+		}
+	}
+}
diff --git a/internal/config/testdata/bad-unknown-key.toml b/internal/config/testdata/bad-unknown-key.toml
index e3e94ec..67a773b 100644
--- a/internal/config/testdata/bad-unknown-key.toml
+++ b/internal/config/testdata/bad-unknown-key.toml
@@ -1,5 +1,5 @@
 listen = "127.0.0.1:7777"
-lease_idle = "30m"
+bogus_key = 1
 
 [hosts.alpha]
 base_url = "http://alpha.example:11434"
diff --git a/internal/fingerprint/fingerprint.go b/internal/fingerprint/fingerprint.go
new file mode 100644
index 0000000..d310981
--- /dev/null
+++ b/internal/fingerprint/fingerprint.go
@@ -0,0 +1,88 @@
+// Package fingerprint identifies a chat-completions conversation without a
+// session id: the system prompt and the first user message never change from
+// turn to turn, so hashing them pins the conversation.
+package fingerprint
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"encoding/json"
+	"strings"
+)
+
+// maxPart is the number of bytes of each input string that contributes to the
+// key: 4 KiB keeps a huge first message from slowing every turn.
+const maxPart = 4096
+
+// Of returns the lowercase hex SHA-256 of the system prompt and the first user
+// message of a chat-completions body (first 4 KiB of each, joined with "\n"),
+// or "" when the body is not a JSON object with a "messages" array containing a
+// user message.
+func Of(body []byte) string {
+	var doc struct {
+		Messages []json.RawMessage `json:"messages"`
+	}
+	if err := json.Unmarshal(body, &doc); err != nil || doc.Messages == nil {
+		return ""
+	}
+
+	var system, user string
+	seenSystem, seenUser := false, false
+	for _, raw := range doc.Messages {
+		var msg struct {
+			Role    string          `json:"role"`
+			Content json.RawMessage `json:"content"`
+		}
+		if err := json.Unmarshal(raw, &msg); err != nil {
+			return ""
+		}
+		switch msg.Role {
+		case "system":
+			if !seenSystem {
+				seenSystem = true
+				system = contentText(msg.Content)
+			}
+		case "user":
+			if !seenUser {
+				seenUser = true
+				user = contentText(msg.Content)
+			}
+		}
+	}
+	if !seenUser {
+		return ""
+	}
+
+	if len(system) > maxPart {
+		system = system[:maxPart]
+	}
+	if len(user) > maxPart {
+		user = user[:maxPart]
+	}
+	sum := sha256.Sum256([]byte(system + "\n" + user))
+	return hex.EncodeToString(sum[:])
+}
+
+// contentText renders a message content value: a JSON string is returned as-is,
+// an array of parts is the concatenation of its text parts (other types
+// ignored), and anything else is "".
+func contentText(raw json.RawMessage) string {
+	var text string
+	if err := json.Unmarshal(raw, &text); err == nil {
+		return text
+	}
+	var parts []struct {
+		Type string `json:"type"`
+		Text string `json:"text"`
+	}
+	if err := json.Unmarshal(raw, &parts); err != nil {
+		return ""
+	}
+	var b strings.Builder
+	for _, p := range parts {
+		if p.Type == "text" {
+			b.WriteString(p.Text)
+		}
+	}
+	return b.String()
+}
diff --git a/internal/fingerprint/fingerprint_test.go b/internal/fingerprint/fingerprint_test.go
new file mode 100644
index 0000000..860d5cb
--- /dev/null
+++ b/internal/fingerprint/fingerprint_test.go
@@ -0,0 +1,70 @@
+package fingerprint_test
+
+import (
+	"strings"
+	"testing"
+
+	"git.wntrmute.dev/kyle/crossbar/internal/fingerprint"
+)
+
+const conv1 = `{"model":"m","messages":[{"role":"system","content":"You are the project A assistant."},{"role":"user","content":"Add a config loader."},{"role":"assistant","content":"Sure."},{"role":"user","content":"Now tests."}]}`
+const conv1later = `{"model":"m","messages":[{"role":"system","content":"You are the project A assistant."},{"role":"user","content":"Add a config loader."},{"role":"assistant","content":"Sure."},{"role":"user","content":"Now tests."},{"role":"assistant","content":"Done."},{"role":"user","content":"And docs."}]}`
+const conv2 = `{"model":"m","messages":[{"role":"system","content":"You are the project A assistant."},{"role":"user","content":"Fix the flaky test."}]}`
+const conv3 = `{"model":"m","messages":[{"role":"system","content":"You are the project B assistant."},{"role":"user","content":"Add a config loader."}]}`
+
+func TestSameConversationSameKey(t *testing.T) {
+	a := fingerprint.Of([]byte(conv1))
+	b := fingerprint.Of([]byte(conv1later))
+	if a == "" || a != b {
+		t.Errorf("later turns of one conversation must keep the key: %q vs %q", a, b)
+	}
+	if len(a) != 64 || strings.Trim(a, "0123456789abcdef") != "" {
+		t.Errorf("key must be lowercase hex sha256 (64 chars), got %q", a)
+	}
+}
+
+func TestDifferentConversationsDifferentKeys(t *testing.T) {
+	a, b, c := fingerprint.Of([]byte(conv1)), fingerprint.Of([]byte(conv2)), fingerprint.Of([]byte(conv3))
+	if a == b {
+		t.Errorf("different first user message must change the key")
+	}
+	if a == c {
+		t.Errorf("different system prompt must change the key")
+	}
+}
+
+func TestNoUserMessageIsEmpty(t *testing.T) {
+	for _, body := range []string{
+		`{"model":"m","messages":[{"role":"system","content":"only a system prompt"}]}`,
+		`{"model":"m","messages":[]}`,
+		`{"model":"m"}`,
+		`{"input":"an embeddings request"}`,
+		`not json at all`,
+		``,
+	} {
+		if got := fingerprint.Of([]byte(body)); got != "" {
+			t.Errorf("Of(%q) = %q, want empty", body, got)
+		}
+	}
+}
+
+func TestOnlyTheFirstFourKiBCount(t *testing.T) {
+	long := strings.Repeat("x", 5000)
+	a := `{"messages":[{"role":"user","content":"` + long + `A"}]}`
+	b := `{"messages":[{"role":"user","content":"` + long + `B"}]}`
+	if fingerprint.Of([]byte(a)) != fingerprint.Of([]byte(b)) {
+		t.Errorf("bytes after the first 4 KiB of a message must not change the key")
+	}
+	c := `{"messages":[{"role":"user","content":"A` + long + `"}]}`
+	if fingerprint.Of([]byte(a)) == fingerprint.Of([]byte(c)) {
+		t.Errorf("bytes inside the first 4 KiB must change the key")
+	}
+}
+
+func TestContentPartsAreFlattened(t *testing.T) {
+	plain := `{"messages":[{"role":"user","content":"hello world"}]}`
+	parts := `{"messages":[{"role":"user","content":[{"type":"text","text":"hello world"}]}]}`
+	if fingerprint.Of([]byte(plain)) != fingerprint.Of([]byte(parts)) {
+		t.Errorf("a content array of text parts must fingerprint like the joined text")
+	}
+}