feat(perf): emit prompt-prefix hash fingerprint per turn

internal/agent/loop.go

@@ -213,6 +213,26 @@
 		exposed, _ := partitionToolsCached(registry, permissionMode, options, loaded, toolDefCache)
 		toolPartitionSpan.End()
 
+		// Prompt-prefix fingerprint: hash the seven cacheable sub-components
+		// of this turn's request (system prompt sections, project context,
+		// skills, partitioned tool list, tool schemas) and emit one trace
+		// event. Stable across turns => cacheable. Drift in any sub-hash
+		// names the sub-component that broke the cache for this turn. The
+		// computation is pure and cheap (seven SHA-256s of small strings)
+		// and is a no-op when tracing is off.
+		if options.Trace != nil {
+			fp := ComputePrefixFingerprint(options, exposed)
+			options.Trace.EmitPrefixHash(trace.PrefixHash{
+				BaseInstructionsHash:   fp.BaseInstructionsHash,
+				ConfirmationPolicyHash: fp.ConfirmationPolicyHash,
+				ProjectContextHash:     fp.ProjectContextHash,
+				SkillsHash:             fp.SkillsHash,
+				ToolsHash:              fp.ToolsHash,
+				SchemaHash:             fp.SchemaHash,
+				CompletePrefixHash:     fp.CompletePrefixHash,
+			})
+		}
+
 		// PROACTIVE compaction: if the history is approaching the model's
 		// context window, summarize the oldest middle before building the
 		// request. A no-op when ContextWindow == 0 (compaction disabled).
@@ -2691,7 +2711,7 @@
 // through tool_search. Non-deferred tools (including tool_search) are always
 // exposed. The exposed slice is alpha-sorted by name, matching the legacy order
 // so the inactive path is stable.
 func partitionTools(registry *tools.Registry, permissionMode PermissionMode, options Options, loaded map[string]bool) ([]zeroruntime.ToolDefinition, string) {
 	return partitionToolsCached(registry, permissionMode, options, loaded, nil)
 }
 

internal/agent/prompt_fingerprint.go

@@ -0,0 +1,300 @@
+package agent
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"encoding/json"
+	"fmt"
+	"sort"
+	"strings"
+
+	"github.com/Gitlawb/zero/internal/zeroruntime"
+)
+
+// promptSubstrings are the seven cacheable sub-components of the prompt a run
+// sends to a model. Each is the literal content the sub-component would emit
+// into the system prompt or tool list, hashed so a downstream observer can
+// detect drift turn-over-turn. The substrings are produced by
+// buildPromptSubstrings and consumed by ComputePrefixFingerprint.
+type promptSubstrings struct {
+	baseInstructions   string
+	confirmationPolicy string
+	projectContext     string
+	skills             string
+	tools              string
+	schema             string
+}
+
+// buildPromptSubstrings assembles the seven cacheable sub-components of the
+// prompt without joining them. The system prompt sections are produced by the
+// same private builders that buildSystemPrompt uses, so the substrings are
+// byte-identical to what would have appeared in the joined prompt. The tools
+// and schema substrings are derived from the partitioned tool list (caller-
+// provided) so this helper does not need to re-run the partition.
+//
+// This is the seam ComputePrefixFingerprint reads from. It exists as a
+// separate function (rather than returning both prompt and substrings from
+// buildSystemPrompt) so existing callers of buildSystemPrompt are unaffected
+// and the substrings helper is independently testable.
+func buildPromptSubstrings(options Options, exposed []zeroruntime.ToolDefinition) promptSubstrings {
+	core := strings.TrimSpace(options.SystemPrompt)
+	if core == "" {
+		core = strings.TrimSpace(coreSystemPrompt)
+	}
+	if core == "" {
+		core = fallbackSystemPrompt
+	}
+
+	// The confirmation policy is appended unconditionally in buildSystemPrompt;
+	// the substring here is the same constant.
+	policy := strings.TrimSpace(confirmationPolicy)
+
+	// projectContext is the joined output of workspaceContext, which itself
+	// walks the AGENTS.md / ZERO.md / .zero/AGENTS.md chain. Hashing the
+	// joined string captures every guideline file's contribution and the
+	// ordering between them.
+	project := workspaceContext(options.Cwd)
+
+	// skills is the joined output of skillsContext, with each skill on its own
+	// line. The substring is the literal text the model would see.
+	skills := skillsContext(options)
+
+	// tools and schema are derived from the partitioned tool list. The list
+	// passed in is already in stable order (partitionToolsCached is the
+	// canonical stable partitioner) so concatenating in slice order is
+	// sufficient — we sort names again defensively in case a future caller
+	// hands us a list from a different partitioner.
+	toolsSubstr, schemaSubstr := toolSubstrings(exposed)
+
+	return promptSubstrings{
+		baseInstructions:   core,
+		confirmationPolicy: policy,
+		projectContext:     project,
+		skills:             skills,
+		tools:              toolsSubstr,
+		schema:             schemaSubstr,
+	}
+}
+
+// toolSubstrings extracts a stable, name-sorted digest of the partitioned tool
+// list. The tools substring is "name1\nname2\n..." (sorted) so a reordering of
+// the partition produces a different hash. The schema substring is the
+// concatenation of each tool's JSON schema in name order, so a schema change
+// is detected independently of the tool ordering.
+func toolSubstrings(exposed []zeroruntime.ToolDefinition) (toolsSubstr, schemaSubstr string) {
+	if len(exposed) == 0 {
+		return "", ""
+	}
+	names := make([]string, 0, len(exposed))
+	byName := make(map[string]zeroruntime.ToolDefinition, len(exposed))
+	for _, def := range exposed {
+		names = append(names, def.Name)
+		byName[def.Name] = def
+	}
+	sort.Strings(names)
+	var toolsSB, schemaSB strings.Builder
+	for i, name := range names {
+		if i > 0 {
+			toolsSB.WriteByte('\n')
+		}
+		toolsSB.WriteString(name)
+		def := byName[name]
+		// Parameters is rendered to a canonical JSON string per tool. The
+		// schema-render cache in internal/agent (used by partitionToolsCached)
+		// guarantees the same render is byte-identical across turns for the
+		// same tool, so this substring is a stable hash input.
+		schemaSB.WriteString(name)
+		schemaSB.WriteByte('\n')
+		schemaSB.WriteString(canonicalSchemaString(def.Parameters))
+		schemaSB.WriteByte('\n')
+	}
+	return toolsSB.String(), schemaSB.String()
+}
+
+// canonicalSchemaString renders a tool's parameter schema to a stable string.
+// tools.ToolDef.Parameters is a map[string]any in the common case; Go's
+// fmt.Sprintf("%v", m) iterates a map in random order, which would produce
+// a different hash for the same Parameters value across calls and defeat
+// the fingerprint. encoding/json marshals maps with keys sorted
+// alphabetically, so json.Marshal is the primary stable render. The
+// fallback for the rare non-JSON-compatible value (functions, channels,
+// NaN/Inf floats, cyclic references) is a stable key-sorted stringifier
+// that walks the value with sorted keys for maps and a leading
+// "__non_json:" prefix so a future schema change is visible in the trace
+// (a SchemaHash collision from the same value) rather than a silent hash
+// drift.
+//
+// Note: SchemaHash is a stability signal, not a wire-identity signal. The
+// bytes json.Marshal produces for a Go map[string]any may differ from
+// the bytes the provider actually sends on the wire (provider encoders
+// use their own JSON conventions; an Anthropic schema may serialize with
+// different whitespace, key order, or number formatting). A consumer
+// must NOT assume SchemaHash matches the provider's wire schema — only
+// that two turns with the same Go-side Parameters produce the same
+// SchemaHash. This is sufficient for the trace's purpose (drift
+// detection) but not for a content-based equality check.
+func canonicalSchemaString(params map[string]any) string {
+	if len(params) == 0 {
+		return ""
+	}
+	data, err := json.Marshal(params)
+	if err != nil {
+		return "__non_json:" + stableStringify(params)
+	}
+	return string(data)
+}
+
+// stableStringify renders v to a deterministic string. Maps have their keys
+// sorted alphabetically; slices are walked in order; primitives use their
+// natural Go format. The function is recursive but bounded by the size of
+// the input, so a cyclic reference is not reachable here (the json.Marshal
+// fallback is hit when json.Marshal itself returns an error, which for the
+// common case is a non-JSON-compatible value, not a cycle — cycles are
+// rare in tool schemas and acceptable to mis-render in the fallback path
+// since the trace's contract is "the hash is stable for the same input,"
+// not "the fallback is lossless"). Used only when json.Marshal fails.
+func stableStringify(v any) string {
+	var sb strings.Builder
+	writeStable(&sb, v)
+	return sb.String()
+}
+
+func writeStable(sb *strings.Builder, v any) {
+	switch x := v.(type) {
+	case nil:
+		sb.WriteString("null")
+	case bool:
+		if x {
+			sb.WriteString("true")
+		} else {
+			sb.WriteString("false")
+		}
+	case string:
+		sb.WriteByte('"')
+		sb.WriteString(x)
+		sb.WriteByte('"')
+	case float64:
+		// Use %g for compact, deterministic float rendering. NaN and Inf are
+		// not JSON-encodable (which is why we are in the fallback path) and
+		// render as "NaN" / "+Inf" / "-Inf" — distinct, stable strings.
+		fmt.Fprintf(sb, "%g", x)
+	case float32:
+		fmt.Fprintf(sb, "%g", float64(x))
+	case int:
+		fmt.Fprintf(sb, "%d", x)
+	case int64:
+		fmt.Fprintf(sb, "%d", x)
+	case int32:
+		fmt.Fprintf(sb, "%d", x)
+	case uint:
+		fmt.Fprintf(sb, "%d", x)
+	case uint64:
+		fmt.Fprintf(sb, "%d", x)
+	case uint32:
+		fmt.Fprintf(sb, "%d", x)
+	case map[string]any:
+		sb.WriteByte('{')
+		keys := make([]string, 0, len(x))
+		for k := range x {
+			keys = append(keys, k)
+		}
+		sort.Strings(keys)
+		for i, k := range keys {
+			if i > 0 {
+				sb.WriteByte(',')
+			}
+			sb.WriteByte('"')
+			sb.WriteString(k)
+			sb.WriteString(`":`)
+			writeStable(sb, x[k])
+		}
+		sb.WriteByte('}')
+	case []any:
+		sb.WriteByte('[')
+		for i, item := range x {
+			if i > 0 {
+				sb.WriteByte(',')
+			}
+			writeStable(sb, item)
+		}
+		sb.WriteByte(']')
+	default:
+		// Last-resort: include the type so distinct values produce distinct
+		// strings even if their default format collides. Without the type
+		// tag, fmt.Sprintf("%v", x) for two different types could produce
+		// the same bytes (rare, but the type prefix makes it impossible).
+		fmt.Fprintf(sb, "<%T:%v>", x, x)
+	}
+}
+
+// ComputePrefixFingerprint returns a trace.PrefixHash (a 7-field fingerprint
+// of the prompt prefix) for one turn of a run. The seven sub-hashes are
+// independent SHA-256s of the corresponding sub-component; the complete-prefix
+// hash is a SHA-256 of the canonical concatenation of the other six, so any
+// sub-component drift is observable both individually and in aggregate.
+//
+// I/O side effects: buildPromptSubstrings calls workspaceContext, which runs
+// git (gitBranchForPrompt, FindProjectGitRoot) and reads the AGENTS.md /
+// ZERO.md / .zero/AGENTS.md chain plus the repo map. The same workspace-
+// context work is performed separately by buildSystemPrompt a few lines
+// later in the request-build path, so on every traced turn the file and
+// git reads happen twice. The duplication is intentional for this PR (a
+// follow-up will pass substrings through) and is a known perf cost only on
+// the opt-in trace path.
+//
+// A stable CompletePrefixHash across turns means the four captured
+// sub-components (baseInstructions, confirmationPolicy, projectContext,
+// skills) are byte-identical. It does NOT rule out drift in the seven
+// uncaptured sections of buildSystemPrompt (modelPromptAddendum,
+// sessionRuntimeContext, approvedCommandPrefixContext, workspaceSeedContext,
+// userGuidelines, specialistDelegationContext, responseStyleContext). For
+// default Options the four captured substrings are the full prompt; for
+// non-default Options the seven uncaptured sections may contribute, and a
+// consumer correlating CompletePrefix stability with cached_input_tokens
+// must cross-check the model_switches counter (modelPromptAddendum changes
+// on a model switch) to disambiguate "no drift" from "drift in an
+// uncaptured section."
+
+func ComputePrefixFingerprint(options Options, exposed []zeroruntime.ToolDefinition) prefixFingerprint {
+	subs := buildPromptSubstrings(options, exposed)
+	base := sha256hex(subs.baseInstructions)
+	policy := sha256hex(subs.confirmationPolicy)
+	project := sha256hex(subs.projectContext)
+	skills := sha256hex(subs.skills)
+	toolsH := sha256hex(subs.tools)
+	schema := sha256hex(subs.schema)
+	complete := sha256hex(strings.Join([]string{
+		base, policy, project, skills, toolsH, schema,
+	}, "|"))
+	return prefixFingerprint{
+		BaseInstructionsHash:   base,
+		ConfirmationPolicyHash: policy,
+		ProjectContextHash:     project,
+		SkillsHash:             skills,
+		ToolsHash:              toolsH,
+		SchemaHash:             schema,
+		CompletePrefixHash:     complete,
+	}
+}
+
+// prefixFingerprint is the agent-side shape of a prompt-prefix fingerprint. It
+// is converted to a trace.PrefixHash at the loop boundary (see EmitPrefixHash
+// in loop.go) so the trace package does not need to import this one. The field
+// names match the trace.PrefixHash JSON tags 1:1.
+type prefixFingerprint struct {
+	BaseInstructionsHash   string
+	ConfirmationPolicyHash string
+	ProjectContextHash     string
+	SkillsHash             string
+	ToolsHash              string
+	SchemaHash             string
+	CompletePrefixHash     string
+}
+
+// sha256hex returns the hex-encoded SHA-256 of s. Empty input produces the
+// hash of the empty string, which is a constant; callers that want to
+// distinguish "absent" from "empty" should check s before calling.
+func sha256hex(s string) string {
+	sum := sha256.Sum256([]byte(s))
+	return hex.EncodeToString(sum[:])
+}