feat(brain): stamp server-owned write provenance; surface it in brain_query

brain_write was an ungated, durable prompt-injection path: a session that
just read an untrusted page could write an entry every later session
retrieves as trusted guidance (OWASP MCP06). Decision (#145): tag
provenance rather than stage writes.

Every write and update stamps origin (static | jwt | unknown) and
written_by (the authenticated principal) into frontmatter. Server-owned:
origin/written_by in caller content or an update fragment are discarded,
so a caller cannot claim a more trusted origin. No principal stamps
origin: unknown, never silence. brain_update re-stamps to the last
writer. capture (the claude.ai connector door) stamps too, via a new
server-derived CaptureContext.AuthPath set by both capture doors after
decode. REST POST /write carries no principal and stamps unknown.

brain_query results carry origin/written_by when the note has them;
older notes omit them rather than fabricate. brain_get already returns
frontmatter.

Refs #145

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ingestion/internal/api/frontmatter.go

@@ -95,6 +95,18 @@ func (f *frontmatter) set(key, value string) {
 	f.lines = append(f.lines, fmLine{key: key, value: value})
 }
 
+// del removes every top-level line for key. Continuation lines of a
+// multi-line value are not tracked per key, so del is only for scalars.
+func (f *frontmatter) del(key string) {
+	kept := f.lines[:0]
+	for _, l := range f.lines {
+		if l.key != key {
+			kept = append(kept, l)
+		}
+	}
+	f.lines = kept
+}
+
 // render serialises the frontmatter back into a `---`-fenced block. An
 // empty frontmatter renders to the empty string so bodies without a
 // header stay header-less.

ingestion/internal/api/handler.go

@@ -118,6 +118,20 @@ type WriteNoteOptions struct {
 	Wing       string
 	Hall       string
 	SourceType string // "internal" marks a first-party observation (e.g. claudewatcher) that needs no external citation
+	// Origin/WrittenBy are server-owned provenance (hyperguild#145): any
+	// origin/written_by in Content's own frontmatter is discarded, so a
+	// caller cannot claim a more trusted origin than it authenticated as.
+	Origin    string
+	WrittenBy string
+}
+
+// provenanceOrigin normalises an origin for the frontmatter: absence must
+// never read as trusted, so empty becomes "unknown".
+func provenanceOrigin(o string) string {
+	if o == "" {
+		return "unknown"
+	}
+	return o
 }
 
 // WriteNote writes a markdown note into the brain. Returns the path
@@ -180,6 +194,11 @@ func writeHallNote(brainDir string, opts WriteNoteOptions) (string, error) {
 	fmt.Fprintf(&fm, "hall: %s\n", opts.Hall)
 	fmt.Fprintf(&fm, "created_at: %s\n", time.Now().UTC().Format(time.RFC3339))
 	emitted["wing"], emitted["hall"], emitted["created_at"] = true, true, true
+	fmt.Fprintf(&fm, "origin: %s\n", provenanceOrigin(opts.Origin))
+	emitted["origin"], emitted["written_by"] = true, true
+	if opts.WrittenBy != "" {
+		fmt.Fprintf(&fm, "written_by: %s\n", opts.WrittenBy)
+	}
 
 	// writeField merges one key: opts.Content's own value (if the note already
 	// carries this field in its own frontmatter) always wins over the fallback,
@@ -295,7 +314,16 @@ func (h *Handler) Write(w http.ResponseWriter, r *http.Request) {
 		writeError(w, http.StatusBadRequest, "invalid JSON")
 		return
 	}
-	relPath, err := WriteNote(h.brainDir, WriteNoteOptions(req))
+	// Field by field, not a struct conversion: provenance is server-owned and
+	// this door carries no principal, so it stamps origin: unknown
+	// (hyperguild#145). A JSON body must never be able to set it.
+	relPath, err := WriteNote(h.brainDir, WriteNoteOptions{
+		Content:    req.Content,
+		Filename:   req.Filename,
+		Wing:       req.Wing,
+		Hall:       req.Hall,
+		SourceType: req.SourceType,
+	})
 	if err != nil {
 		h.logger.Error("write failed", "err", err)
 		writeError(w, http.StatusBadRequest, err.Error())

ingestion/internal/api/update.go

@@ -46,6 +46,11 @@ type UpdateNoteOptions struct {
 	Slug    string
 	Content string // new full body (whole-note replace)
 	Reason  string // optional; stamped as supersede_reason
+	// Origin/WrittenBy re-stamp provenance to the updating principal: the
+	// body is replaced wholesale, so the last writer is the author of what
+	// a reader sees (hyperguild#145). Never taken from Content's fragment.
+	Origin    string
+	WrittenBy string
 }
 
 // UpdateNote supersedes an existing note in place. It replaces the body
@@ -122,12 +127,19 @@ func UpdateNote(brainDir string, opts UpdateNoteOptions) (relPath, contentHash,
 		// overwrites below to save the other two keeps the invariant
 		// explicit at the one place it can be silently broken by a future
 		// reordering, not implicit in call order.
-		if l.key == "" || l.key == "updated_at" || l.key == "supersedes" || l.key == "supersede_reason" {
+		if l.key == "" || l.key == "updated_at" || l.key == "supersedes" || l.key == "supersede_reason" ||
+			l.key == "origin" || l.key == "written_by" {
 			continue
 		}
 		fm.set(l.key, l.value)
 	}
 	fm.set("updated_at", time.Now().UTC().Format(time.RFC3339))
+	fm.set("origin", provenanceOrigin(opts.Origin))
+	if opts.WrittenBy != "" {
+		fm.set("written_by", opts.WrittenBy)
+	} else {
+		fm.del("written_by")
+	}
 	fm.set("supersedes", priorHash)
 	if opts.Reason != "" {
 		fm.set("supersede_reason", opts.Reason)

ingestion/internal/brainstore/store.go

@@ -49,10 +49,12 @@ func (s *Store) Write(ctx context.Context, n capture.Note) (capture.Ref, error)
 	n.Content = content
 
 	relPath, err := api.WriteNote(s.brainDir, api.WriteNoteOptions{
-		Content:  n.Content,
-		Filename: n.Filename,
-		Wing:     n.Wing,
-		Hall:     n.Hall,
+		Content:   n.Content,
+		Filename:  n.Filename,
+		Wing:      n.Wing,
+		Hall:      n.Hall,
+		Origin:    n.Origin,
+		WrittenBy: n.WrittenBy,
 	})
 	if err != nil {
 		return capture.Ref{}, err
@@ -74,7 +76,7 @@ func (s *Store) Update(ctx context.Context, slug string, n capture.Note) (captur
 	}
 	n.Content = content
 
-	opts := api.UpdateNoteOptions{Content: n.Content, Reason: n.Reason}
+	opts := api.UpdateNoteOptions{Content: n.Content, Reason: n.Reason, Origin: n.Origin, WrittenBy: n.WrittenBy}
 	if strings.Contains(slug, "/") {
 		opts.Path = slug
 	} else {

ingestion/internal/capture/entities.go

@@ -60,6 +60,7 @@ type CaptureContext struct {
 	Classification string // caller-declared level token ("" = unspecified)
 	Principal      string // server-derived (auth); audit identity
 	Origin         Zone   // server-derived trust zone; the I1 gate input
+	AuthPath       string // server-derived: "static" or "jwt"; stamped as note provenance (#145)
 }
 
 // Insight is one piece of session knowledge bound for the brain. A

ingestion/internal/capture/ports.go

@@ -40,6 +40,11 @@ type Note struct {
 	Wing     string
 	Hall     string
 	Reason   string
+	// Provenance (hyperguild#145) is stamped by the server from the
+	// authenticated principal, never taken from Content's frontmatter.
+	// Origin is "static", "jwt" or "unknown"; empty means "unknown".
+	Origin    string
+	WrittenBy string
 }
 
 // BrainStore is the brain persistence port — the shared implementation of

ingestion/internal/capture/service.go

@@ -142,7 +142,7 @@ func (s *Service) Capture(ctx context.Context, in CaptureInput) (CaptureReceipt,
 	var landed []string
 
 	for i, ins := range in.Insights {
-		res, item, err := s.persistInsight(ctx, ins)
+		res, item, err := s.persistInsight(ctx, ins, in.Context)
 		receipt.Insights = append(receipt.Insights, res)
 		if err != nil {
 			receipt.Errors = append(receipt.Errors, ItemError{Item: fmt.Sprintf("insight[%d]", i), Error: err.Error()})
@@ -278,8 +278,11 @@ func escalationReason(escalators []string) string {
 	return "required by: " + strings.Join(escalators, ", ")
 }
 
-func (s *Service) persistInsight(ctx context.Context, ins Insight) (InsightResult, string, error) {
-	note := Note{Content: ins.Text, Wing: ins.Wing, Hall: ins.Hall, Filename: brain.Sanitise(firstLine(ins.Text))}
+func (s *Service) persistInsight(ctx context.Context, ins Insight, cc CaptureContext) (InsightResult, string, error) {
+	note := Note{
+		Content: ins.Text, Wing: ins.Wing, Hall: ins.Hall, Filename: brain.Sanitise(firstLine(ins.Text)),
+		Origin: cc.AuthPath, WrittenBy: cc.Principal,
+	}
 	var ref Ref
 	var err error
 	if ins.SupersedeSlug != "" {

ingestion/internal/capturehttp/handler.go

@@ -104,6 +104,10 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	// caller may have tried to put in the body.
 	in.Context.Principal = principal
 	in.Context.Origin = h.resolver.Resolve(principal, viaStatic)
+	in.Context.AuthPath = "jwt"
+	if viaStatic {
+		in.Context.AuthPath = "static"
+	}
 
 	rec, err := h.svc.Capture(r.Context(), in)
 	switch {

ingestion/internal/mcp/audit.go

@@ -12,13 +12,7 @@ import (
 // ended. Brain content is never copied into the record — only its size — so
 // the audit trail does not become a second, unscrubbed copy of the brain.
 func logToolCall(ctx context.Context, name string, args json.RawMessage, start time.Time, err error) {
-	principal, origin := "", "unknown"
-	if info, ok := ctx.Value(principalKey).(principalInfo); ok {
-		principal, origin = info.principal, "jwt"
-		if info.viaStatic {
-			origin = "static"
-		}
-	}
+	origin, principal := provenanceFrom(ctx)
 	attrs := []any{
 		"tool", name,
 		"principal", principal,

ingestion/internal/mcp/handlers.go

@@ -255,11 +255,14 @@ func (s *Server) brainWrite(ctx context.Context, args json.RawMessage) (json.Raw
 	// one implementation (capture uses the same store). The read-after-write
 	// handle {id, path, content_hash} comes back from the store; path is kept
 	// for backward compatibility.
+	origin, writtenBy := provenanceFrom(ctx)
 	ref, err := s.store.Write(ctx, capture.Note{
-		Content:  a.Content,
-		Filename: a.Filename,
-		Wing:     a.Wing,
-		Hall:     a.Hall,
+		Content:   a.Content,
+		Filename:  a.Filename,
+		Wing:      a.Wing,
+		Hall:      a.Hall,
+		Origin:    origin,
+		WrittenBy: writtenBy,
 	})
 	if err != nil {
 		return nil, err
@@ -304,11 +307,14 @@ func (s *Server) brainUpdate(ctx context.Context, args json.RawMessage) (json.Ra
 	if a.Path != "" {
 		slug = a.Path
 	}
+	origin, writtenBy := provenanceFrom(ctx)
 	ref, err := s.store.Update(ctx, slug, capture.Note{
-		Content: a.Content,
-		Wing:    a.Wing,
-		Hall:    a.Hall,
-		Reason:  a.Reason,
+		Content:   a.Content,
+		Wing:      a.Wing,
+		Hall:      a.Hall,
+		Reason:    a.Reason,
+		Origin:    origin,
+		WrittenBy: writtenBy,
 	})
 	if err != nil {
 		return nil, err

ingestion/internal/mcp/provenance_test.go

@@ -0,0 +1,124 @@
+package mcp_test
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// resultText decodes the tool result payload of a tools/call response.
+func resultText(t *testing.T, resp map[string]any) map[string]any {
+	t.Helper()
+	require.NotContains(t, resp, "error", "tool call failed: %v", resp["error"])
+	content := resp["result"].(map[string]any)["content"].([]any)
+	var out map[string]any
+	require.NoError(t, json.Unmarshal([]byte(content[0].(map[string]any)["text"].(string)), &out))
+	return out
+}
+
+func readNote(t *testing.T, dir, rel string) string {
+	t.Helper()
+	b, err := os.ReadFile(filepath.Join(dir, filepath.FromSlash(rel)))
+	require.NoError(t, err)
+	return string(b)
+}
+
+func frontmatterOf(note string) string {
+	parts := strings.SplitN(note, "---\n", 3)
+	if len(parts) < 3 {
+		return ""
+	}
+	return parts[1]
+}
+
+// hyperguild#145: a brain entry records which principal wrote it, so a reader
+// can tell a local-CLI note from one a connector session wrote after reading
+// an untrusted page.
+func TestBrainWriteStampsPrincipalProvenance(t *testing.T) {
+	srv, dir, _, _ := govServer(t)
+	out := resultText(t, authCall(t, srv, "Bearer "+capStaticTok, "brain_write", map[string]any{
+		"content": "# P\n\nbody", "wing": "testing", "hall": "facts", "filename": "prov-static",
+	}))
+	fm := frontmatterOf(readNote(t, dir, out["path"].(string)))
+	assert.Contains(t, fm, "origin: static\n")
+	assert.Contains(t, fm, "written_by: local-cli\n")
+}
+
+func TestBrainWriteWithoutPrincipalStampsOriginUnknown(t *testing.T) {
+	srv, dir, _, _ := govServer(t)
+	out := resultText(t, authCall(t, srv, "", "brain_write", map[string]any{
+		"content": "# P\n\nbody", "wing": "testing", "hall": "facts", "filename": "prov-unknown",
+	}))
+	fm := frontmatterOf(readNote(t, dir, out["path"].(string)))
+	assert.Contains(t, fm, "origin: unknown\n")
+	assert.NotContains(t, fm, "written_by:")
+}
+
+// The control: a caller cannot claim a more trusted origin by putting it in
+// its own frontmatter.
+func TestBrainWriteOverwritesCallerSuppliedProvenance(t *testing.T) {
+	srv, dir, _, _ := govServer(t)
+	forged := "---\norigin: static\nwritten_by: local-cli\n---\n# F\n\nbody"
+	out := resultText(t, authCall(t, srv, "", "brain_write", map[string]any{
+		"content": forged, "wing": "testing", "hall": "facts", "filename": "prov-forged",
+	}))
+	fm := frontmatterOf(readNote(t, dir, out["path"].(string)))
+	assert.Contains(t, fm, "origin: unknown\n")
+	assert.NotContains(t, fm, "origin: static")
+	assert.NotContains(t, fm, "written_by: local-cli")
+	assert.Equal(t, 1, strings.Count(fm, "origin:"), "exactly one origin line")
+}
+
+// brain_update replaces the content wholesale, so provenance follows the
+// last writer — and a forged fragment must not survive the merge either.
+func TestBrainUpdateRestampsProvenanceAndIgnoresForgedFragment(t *testing.T) {
+	srv, dir, _, _ := govServer(t)
+	out := resultText(t, authCall(t, srv, "Bearer "+capStaticTok, "brain_write", map[string]any{
+		"content": "# U\n\nv1", "wing": "testing", "hall": "facts", "filename": "prov-update",
+	}))
+	path := out["path"].(string)
+	resultText(t, authCall(t, srv, "", "brain_update", map[string]any{
+		"path": path, "content": "---\norigin: static\nwritten_by: local-cli\n---\n# U\n\nv2",
+	}))
+	fm := frontmatterOf(readNote(t, dir, path))
+	assert.Contains(t, fm, "origin: unknown\n")
+	assert.NotContains(t, fm, "written_by: local-cli")
+	assert.Equal(t, 1, strings.Count(fm, "origin:"))
+}
+
+func TestBrainQueryReturnsProvenance(t *testing.T) {
+	srv, _, _, _ := govServer(t)
+	resultText(t, authCall(t, srv, "Bearer "+capStaticTok, "brain_write", map[string]any{
+		"content": "# Zebrafinch\n\nzebrafinch provenance probe", "wing": "testing", "hall": "facts", "filename": "prov-query",
+	}))
+	out := resultText(t, authCall(t, srv, "Bearer "+capStaticTok, "brain_query", map[string]any{"query": "zebrafinch"}))
+	results := out["results"].([]any)
+	require.NotEmpty(t, results)
+	hit := results[0].(map[string]any)
+	assert.Equal(t, "static", hit["origin"])
+	assert.Equal(t, "local-cli", hit["written_by"])
+}
+
+// capture is the door a claude.ai connector session uses, so it is the one
+// whose provenance matters most: a JWT principal must be stamped as such.
+func TestCaptureStampsJWTProvenance(t *testing.T) {
+	srv, brainDir := captureServer(t, capFakeValidator{subject: "koala-cli"}, []string{"koala-cli"})
+	resp := captureCall(t, srv, "Bearer jwt-token", map[string]any{
+		"context":  map[string]any{"harness": "claude-code", "actor": "mathias", "classification": "internal"},
+		"insights": []map[string]any{{"text": "Capture provenance probe", "wing": "hyperguild", "hall": "facts"}},
+	})
+	require.Nil(t, resp["error"], "got error: %v", resp["error"])
+	matches, err := filepath.Glob(filepath.Join(brainDir, "wiki/hyperguild/facts/*.md"))
+	require.NoError(t, err)
+	require.Len(t, matches, 1)
+	b, err := os.ReadFile(matches[0])
+	require.NoError(t, err)
+	fm := frontmatterOf(string(b))
+	assert.Contains(t, fm, "origin: jwt\n")
+	assert.Contains(t, fm, "written_by: koala-cli\n")
+}

ingestion/internal/mcp/tools_capture.go

@@ -90,6 +90,7 @@ func (s *Server) brainCapture(ctx context.Context, args json.RawMessage) (json.R
 	// Principal and origin are server-derived — never taken from the body.
 	in.Context.Principal = info.principal
 	in.Context.Origin = s.capture.resolver.Resolve(info.principal, info.viaStatic)
+	in.Context.AuthPath, _ = provenanceFrom(ctx)
 
 	rec, err := s.capture.svc.Capture(ctx, in)
 	if err != nil {
@@ -99,3 +100,17 @@ func (s *Server) brainCapture(ctx context.Context, args json.RawMessage) (json.R
 	}
 	return json.Marshal(rec)
 }
+
+// provenanceFrom maps the authenticated principal in ctx to the provenance
+// stamped on a brain write (hyperguild#145). No principal → "unknown", never
+// silence: an unstamped note must not read as trusted.
+func provenanceFrom(ctx context.Context) (origin, writtenBy string) {
+	info, ok := ctx.Value(principalKey).(principalInfo)
+	if !ok {
+		return "unknown", ""
+	}
+	if info.viaStatic {
+		return "static", info.principal
+	}
+	return "jwt", info.principal
+}

ingestion/internal/search/search.go

@@ -48,6 +48,11 @@ type Result struct {
 	// (infra#72). Read from frontmatter when present, otherwise
 	// inferred from the parent directory.
 	Tier string `json:"tier,omitempty"`
+	// Origin/WrittenBy surface write provenance (hyperguild#145) so a
+	// reader can discount an entry a connector session wrote. Omitted for
+	// notes written before provenance existed — never fabricated.
+	Origin    string `json:"origin,omitempty"`
+	WrittenBy string `json:"written_by,omitempty"`
 }
 
 // tierWeight maps the DIKW tier to a score multiplier applied right
@@ -146,6 +151,7 @@ func QueryContext(ctx context.Context, brainDir string, opts QueryOptions) ([]Re
 			rel = filepath.ToSlash(rel)
 			wing, hall := extractWingHall(string(content), rel)
 			tier := extractTier(string(content), rel)
+			origin, writtenBy := extractProvenance(string(content))
 			results = append(results, Result{
 				Path:    rel,
 				Title:   extractTitle(string(content), d.Name()),
@@ -154,6 +160,9 @@ func QueryContext(ctx context.Context, brainDir string, opts QueryOptions) ([]Re
 				Wing:    wing,
 				Hall:    hall,
 				Tier:    tier,
+
+				Origin:    origin,
+				WrittenBy: writtenBy,
 			})
 			return nil
 		})
@@ -272,6 +281,7 @@ func hydrate(brainDir, relPath string) (Result, error) {
 	}
 	wing, hall := extractWingHall(string(content), relPath)
 	tier := extractTier(string(content), relPath)
+	origin, writtenBy := extractProvenance(string(content))
 	return Result{
 		Path:    relPath,
 		Title:   extractTitle(string(content), filepath.Base(relPath)),
@@ -279,9 +289,45 @@ func hydrate(brainDir, relPath string) (Result, error) {
 		Wing:    wing,
 		Hall:    hall,
 		Tier:    tier,
+
+		Origin:    origin,
+		WrittenBy: writtenBy,
 	}, nil
 }
 
+// extractProvenance reads origin/written_by from the leading frontmatter
+// block only — a body line that happens to start with "origin:" is not
+// provenance.
+func extractProvenance(content string) (origin, writtenBy string) {
+	scanner := bufio.NewScanner(strings.NewReader(content))
+	inFrontmatter := false
+	for scanner.Scan() {
+		line := scanner.Text()
+		if strings.TrimSpace(line) == "---" {
+			if !inFrontmatter {
+				inFrontmatter = true
+				continue
+			}
+			break
+		}
+		if !inFrontmatter {
+			return "", ""
+		}
+		key, val, ok := strings.Cut(line, ":")
+		if !ok || strings.HasPrefix(line, " ") || strings.HasPrefix(line, "\t") {
+			continue
+		}
+		v := strings.Trim(strings.TrimSpace(val), `"'`)
+		switch strings.TrimSpace(key) {
+		case "origin":
+			origin = v
+		case "written_by":
+			writtenBy = v
+		}
+	}
+	return origin, writtenBy
+}
+
 // resolveRoots returns the directories to walk for the given wing/hall
 // filters. Validates hall against the closed vocabulary when set.
 func resolveRoots(brainDir, wing, hall string) ([]string, error) {