Carry evidence through built-in generation retries

docs/roadmap/implementation.md

@@ -465,6 +465,11 @@ attempt, retry-success, exhausted retries, cancellation during backoff, and
 canceled reacquisition. Check evidence retention and timing/accounting; existing
 retry tests own unchanged retry policy. Run observation tests after extraction.
 
+**Completed: 2026-10-13.** Added built-in generation attempt evidence and shared
+presence-aware usage accumulation. Validation passed: `go test -race
+./internal/generation ./internal/observation`, `go vet ./...`, `go test ./...`,
+and `git diff --check`.
+
 ## Stage 10: Retain Injected-Client Evidence Without HTTP Claims
 
 Scope: the injected branch of `internal/generation`.

internal/generation/coordinator.go

@@ -72,14 +72,41 @@ func (e *ExhaustedError) Attempts() uint64 {
 // Generate executes either one opaque client call or a frozen built-in HTTP
 // sequence with one permit per actual dispatch.
 func (c *Coordinator) Generate(ctx context.Context, req domain.GenerateRequest) (*domain.GenerateResponse, error) {
+	response, err, _ := c.generate(ctx, req, false)
+	return response, err
+}
+
+// GenerateWithEvidence runs the built-in retry path and returns its factual
+// attempt evidence alongside any response retained before cancellation.
+func (c *Coordinator) GenerateWithEvidence(
+	ctx context.Context,
+	req domain.GenerateRequest,
+) (*domain.GenerateResponse, error, GenerationEvidence) {
+	return c.generate(ctx, req, true)
+}
+
+func (c *Coordinator) generate(
+	ctx context.Context,
+	req domain.GenerateRequest,
+	captureEvidence bool,
+) (*domain.GenerateResponse, error, GenerationEvidence) {
 	if c.injected != nil {
-		return c.generateInjected(ctx, req)
+		response, err := c.generateInjected(ctx, req)
+		return response, err, GenerationEvidence{}
 	}
 	prepared, err := c.builtIn.PrepareGeneration(req)
 	if err != nil {
-		return nil, err
+		return nil, err, GenerationEvidence{}
 	}
 
+	evidence := GenerationEvidence{}
+	var usage observation.UsageAccumulator
+	finish := func(response *domain.GenerateResponse, err error) (*domain.GenerateResponse, error, GenerationEvidence) {
+		if captureEvidence {
+			evidence.Usage = usage.Observed()
+		}
+		return response, err, evidence
+	}
 	var attempts, retriesUsed uint64
 	recorder := observation.FromContext(ctx)
 	generation := req.Generation
@@ -88,50 +115,82 @@ func (c *Coordinator) Generate(ctx context.Context, req domain.GenerateRequest)
 	}
 	for {
 		if err := ctx.Err(); err != nil {
-			return nil, err
+			return finish(nil, err)
 		}
 		ordinal := attempts + 1
-		outcome, dispatched, capacityWait, generationDuration := c.execute(ctx, req.Target.BackendID, prepared)
+		outcome, dispatched, capacityWait, generationDuration := c.execute(ctx, req.Target.BackendID, prepared, captureEvidence)
 		if dispatched {
 			attempts++
 		}
+		if captureEvidence {
+			attempt := AttemptEvidence{
+				Dispatched:          dispatched,
+				HTTP:                cloneHTTPEvidence(outcome.Evidence),
+				CapacityWait:        capacityWait,
+				RequestDuration:     generationDuration,
+				Failure:             ClassifyFailure(outcome.Err),
+				ResponseDecoded:     outcome.ResponseDecoded,
+				GenerationCompleted: outcome.GenerationCompleted,
+			}
+			if outcome.StructuredOutputRequested != nil {
+				requested := *outcome.StructuredOutputRequested
+				attempt.StructuredOutputRequested = &requested
+			}
+			if dispatched {
+				usage.Add(outcome.Usage, outcome.UsagePresence)
+				var attemptUsage observation.UsageAccumulator
+				attemptUsage.Add(outcome.Usage, outcome.UsagePresence)
+				attempt.Usage = attemptUsage.Observed()
+			}
+			evidence.Attempts = append(evidence.Attempts, attempt)
+		}
 		if recorder != nil {
 			recorder.Attempt(observation.AttemptData{Generation: generation, Attempt: ordinal,
 				Dispatched: dispatched, CapacityWait: capacityWait, GenerationDuration: generationDuration,
-				Outcome: attemptOutcome(outcome.Err), Failure: attemptFailure(outcome.Err),
+				Outcome: attemptOutcome(outcome.Err), Failure: ClassifyFailure(outcome.Err),
 				StatusCode: outcome.StatusCode, Usage: outcome.Usage, UsagePresence: outcome.UsagePresence})
 		}
 		if outcome.Err == nil {
 			if err := ctx.Err(); err != nil {
-				return nil, err
+				if captureEvidence {
+					return finish(outcome.Response, err)
+				}
+				return finish(nil, err)
 			}
-			return outcome.Response, nil
+			return finish(outcome.Response, nil)
 		}
 		if !dispatched || !eligibleStatus(outcome) {
-			return nil, outcome.Err
+			if captureEvidence {
+				return finish(outcome.Response, outcome.Err)
+			}
+			return finish(nil, outcome.Err)
 		}
 		if retriesUsed == req.TransientRetries {
 			if retriesUsed == 0 {
-				return nil, outcome.Err
+				return finish(nil, outcome.Err)
 			}
-			return nil, &ExhaustedError{attempts: attempts, cause: outcome.Err}
+			return finish(nil, &ExhaustedError{attempts: attempts, cause: outcome.Err})
 		}
 		if err := ctx.Err(); err != nil {
-			return nil, err
+			return finish(nil, err)
 		}
 		retryOrdinal := retriesUsed + 1
 		delay := retryDelay(c.backoff(retryOrdinal), outcome.RetryNotBefore, c.now())
 		waitStart := c.now()
 		waitErr := c.wait(ctx, delay)
+		waitDuration := elapsed(waitStart, c.now())
+		if captureEvidence {
+			evidence.RetryDelays = append(evidence.RetryDelays, waitDuration)
+		}
 		if recorder != nil {
 			recorder.Delay(observation.DelayData{Generation: generation, Attempt: attempts,
-				Elapsed: elapsed(waitStart, c.now()), Failure: attemptFailure(waitErr)})
+				Elapsed: waitDuration, Failure: ClassifyFailure(waitErr)})
 		}
 		if waitErr != nil {
-			return nil, waitErr
+			return finish(nil, waitErr)
 		}
 		if err := ctx.Err(); err != nil {
-			return nil, err
+			return finish(nil, err)
 		}
 		retriesUsed++
 	}
@@ -148,7 +207,7 @@ func (c *Coordinator) generateInjected(ctx context.Context, req domain.GenerateR
 	capacityWait := c.capacityWait(req.Target.BackendID, start)
 	if err != nil {
 		if recorder != nil {
-			recorder.Attempt(observation.AttemptData{Generation: generation, Attempt: 1, CapacityWait: capacityWait, Outcome: domain.ExecutionOutcomeError, Failure: attemptFailure(err)})
+			recorder.Attempt(observation.AttemptData{Generation: generation, Attempt: 1, CapacityWait: capacityWait, Outcome: domain.ExecutionOutcomeError, Failure: ClassifyFailure(err)})
 		}
 		return nil, err
 	}
@@ -156,7 +215,7 @@ func (c *Coordinator) generateInjected(ctx context.Context, req domain.GenerateR
 	if err := ctx.Err(); err != nil {
 		release()
 		if recorder != nil {
-			recorder.Attempt(observation.AttemptData{Generation: generation, Attempt: 1, CapacityWait: capacityWait, Outcome: domain.ExecutionOutcomeError, Failure: attemptFailure(err)})
+			recorder.Attempt(observation.AttemptData{Generation: generation, Attempt: 1, CapacityWait: capacityWait, Outcome: domain.ExecutionOutcomeError, Failure: ClassifyFailure(err)})
 		}
 		return nil, err
 	}
@@ -169,7 +228,7 @@ func (c *Coordinator) generateInjected(ctx context.Context, req domain.GenerateR
 	release()
 	if recorder != nil {
 		data := observation.AttemptData{Generation: generation, Attempt: 1, Dispatched: true, CapacityWait: capacityWait, GenerationDuration: duration,
-			Outcome: attemptOutcome(err), Failure: attemptFailure(err)}
+			Outcome: attemptOutcome(err), Failure: ClassifyFailure(err)}
 		if err == nil {
 			data.Usage = response.Usage
 			data.UsagePresence = response.UsagePresence
@@ -185,7 +244,7 @@ func (c *Coordinator) generateInjected(ctx context.Context, req domain.GenerateR
 	return response, nil
 }
 
-func (c *Coordinator) execute(ctx context.Context, backendID string, prepared *llm.PreparedGeneration) (outcome llm.AttemptOutcome, dispatched bool, capacityWait, generationDuration time.Duration) {
+func (c *Coordinator) execute(ctx context.Context, backendID string, prepared *llm.PreparedGeneration, captureEvidence bool) (outcome llm.AttemptOutcome, dispatched bool, capacityWait, generationDuration time.Duration) {
 	start := c.now()
 	release, err := c.manager.Acquire(ctx, backendID)
 	capacityWait = c.capacityWait(backendID, start)
@@ -197,8 +256,14 @@ func (c *Coordinator) execute(ctx context.Context, backendID string, prepared *l
 		return llm.AttemptOutcome{Err: err}, false, capacityWait, 0
 	}
 	dispatchStart := c.now()
-	outcome = prepared.Execute(ctx)
+	if captureEvidence {
-	return outcome, true, capacityWait, elapsed(dispatchStart, c.now())
+		outcome = prepared.ExecuteWithEvidence(ctx)
+		dispatched = outcome.Evidence.DispatchInvoked
+	} else {
+		outcome = prepared.Execute(ctx)
+		dispatched = true
+	}
+	return outcome, dispatched, capacityWait, elapsed(dispatchStart, c.now())
 }
 
 func (c *Coordinator) capacityWait(backendID string, start time.Time) time.Duration {
@@ -222,7 +287,8 @@ func attemptOutcome(err error) domain.ExecutionOutcome {
 	return domain.ExecutionOutcomeError
 }
 
-func attemptFailure(err error) domain.ExecutionFailureKind {
+// ClassifyFailure maps an execution error to the shared content-free category.
+func ClassifyFailure(err error) domain.ExecutionFailureKind {
 	if err == nil {
 		return domain.ExecutionFailureNone
 	}

internal/generation/evidence.go

@@ -0,0 +1,38 @@
+package generation
+
+import (
+	"time"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+	"gitea.maximumdirect.net/eric/promptkit/internal/llm"
+)
+
+// GenerationEvidence contains invocation-local request evidence without raw
+// provider bodies, headers, credentials, or generated content.
+type GenerationEvidence struct {
+	Attempts    []AttemptEvidence
+	RetryDelays []time.Duration
+	Usage       domain.ObservedTokenUsage
+}
+
+// AttemptEvidence describes one capacity acquisition and any dispatch it
+// reached. Generated content remains on the separate response return value.
+type AttemptEvidence struct {
+	Dispatched                bool
+	HTTP                      llm.HTTPExecutionEvidence
+	CapacityWait              time.Duration
+	RequestDuration           time.Duration
+	Failure                   domain.ExecutionFailureKind
+	Usage                     domain.ObservedTokenUsage
+	ResponseDecoded           bool
+	GenerationCompleted       bool
+	StructuredOutputRequested *bool
+}
+
+func cloneHTTPEvidence(evidence llm.HTTPExecutionEvidence) llm.HTTPExecutionEvidence {
+	if evidence.HTTPStatus != nil {
+		status := *evidence.HTTPStatus
+		evidence.HTTPStatus = &status
+	}
+	return evidence
+}

internal/generation/evidence_test.go

@@ -0,0 +1,188 @@
+package generation
+
+import (
+	"context"
+	"errors"
+	"net/http"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+)
+
+func TestGenerateWithEvidenceRetainsAttemptsAndUsage(t *testing.T) {
+	var now = time.Date(2026, 10, 13, 12, 0, 0, 0, time.UTC)
+	calls := 0
+	coordinator := testCoordinator(t, func(*http.Request) (*http.Response, error) {
+		calls++
+		if calls == 1 {
+			now = now.Add(4 * time.Millisecond)
+			return response(http.StatusServiceUnavailable, `{"error":{}}`), nil
+		}
+		now = now.Add(7 * time.Millisecond)
+		return response(http.StatusOK, `{"choices":[{"message":{"content":"ok"}}],"usage":{"prompt_tokens":3}}`), nil
+	})
+	coordinator.now = func() time.Time { return now }
+	coordinator.wait = func(context.Context, time.Duration) error {
+		now = now.Add(19 * time.Millisecond)
+		return nil
+	}
+
+	result, err, evidence := coordinator.GenerateWithEvidence(context.Background(), testRequest(1))
+	if err != nil || result == nil || result.Content != "ok" || calls != 2 {
+		t.Fatalf("result=%+v error=%v calls=%d", result, err, calls)
+	}
+	if len(evidence.Attempts) != 2 || len(evidence.RetryDelays) != 1 || evidence.RetryDelays[0] != 19*time.Millisecond {
+		t.Fatalf("attempts/delays = %#v / %#v", evidence.Attempts, evidence.RetryDelays)
+	}
+	first, second := evidence.Attempts[0], evidence.Attempts[1]
+	if !first.Dispatched || !first.HTTP.RequestObserved || !first.HTTP.ResponseReceived || first.HTTP.HTTPStatus == nil || *first.HTTP.HTTPStatus != http.StatusServiceUnavailable {
+		t.Fatalf("first attempt evidence = %+v", first)
+	}
+	if first.Failure != domain.ExecutionFailureProviderStatus || first.RequestDuration != 4*time.Millisecond {
+		t.Fatalf("first attempt failure/timing = %v / %v", first.Failure, first.RequestDuration)
+	}
+	if !second.Dispatched || !second.GenerationCompleted || !second.ResponseDecoded || second.HTTP.HTTPStatus == nil || *second.HTTP.HTTPStatus != http.StatusOK {
+		t.Fatalf("second attempt evidence = %+v", second)
+	}
+	if second.Failure != domain.ExecutionFailureNone || second.RequestDuration != 7*time.Millisecond {
+		t.Fatalf("second attempt failure/timing = %v / %v", second.Failure, second.RequestDuration)
+	}
+	if !second.Usage.PromptTokens.Known || !second.Usage.PromptTokens.Complete || second.Usage.PromptTokens.Value != 3 {
+		t.Fatalf("per-attempt usage = %+v", second.Usage.PromptTokens)
+	}
+	if !evidence.Usage.PromptTokens.Known || evidence.Usage.PromptTokens.Complete || evidence.Usage.PromptTokens.Value != 3 {
+		t.Fatalf("aggregate usage = %+v", evidence.Usage.PromptTokens)
+	}
+}
+
+func TestGenerateWithEvidenceRetainsExhaustedAttempts(t *testing.T) {
+	calls := 0
+	coordinator := testCoordinator(t, func(*http.Request) (*http.Response, error) {
+		calls++
+		return response(http.StatusServiceUnavailable, `{"error":{}}`), nil
+	})
+	coordinator.wait = func(context.Context, time.Duration) error { return nil }
+
+	result, err, evidence := coordinator.GenerateWithEvidence(context.Background(), testRequest(1))
+	var exhausted *ExhaustedError
+	if result != nil || !errors.As(err, &exhausted) || calls != 2 || exhausted.Attempts() != 2 {
+		t.Fatalf("result=%+v error=%v calls=%d", result, err, calls)
+	}
+	if len(evidence.Attempts) != 2 || len(evidence.RetryDelays) != 1 {
+		t.Fatalf("attempt/delay evidence = %#v / %#v", evidence.Attempts, evidence.RetryDelays)
+	}
+	for _, attempt := range evidence.Attempts {
+		if !attempt.Dispatched || attempt.Failure != domain.ExecutionFailureProviderStatus || attempt.HTTP.HTTPStatus == nil || *attempt.HTTP.HTTPStatus != http.StatusServiceUnavailable {
+			t.Fatalf("exhausted attempt evidence = %+v", attempt)
+		}
+	}
+	if evidence.Usage.PromptTokens.Known || evidence.Usage.PromptTokens.Complete {
+		t.Fatalf("missing provider usage was claimed: %+v", evidence.Usage.PromptTokens)
+	}
+}
+
+func TestGenerateWithEvidenceRetainsInterruptedBackoffWithoutAttempt(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	var now = time.Date(2026, 10, 13, 12, 0, 0, 0, time.UTC)
+	calls := 0
+	coordinator := testCoordinator(t, func(*http.Request) (*http.Response, error) {
+		calls++
+		now = now.Add(3 * time.Millisecond)
+		return response(http.StatusServiceUnavailable, `{"error":{}}`), nil
+	})
+	coordinator.now = func() time.Time { return now }
+	coordinator.wait = func(ctx context.Context, _ time.Duration) error {
+		now = now.Add(11 * time.Millisecond)
+		cancel()
+		return ctx.Err()
+	}
+
+	result, err, evidence := coordinator.GenerateWithEvidence(ctx, testRequest(2))
+	if result != nil || !errors.Is(err, context.Canceled) || calls != 1 {
+		t.Fatalf("result=%+v error=%v calls=%d", result, err, calls)
+	}
+	if len(evidence.Attempts) != 1 || !evidence.Attempts[0].Dispatched || len(evidence.RetryDelays) != 1 || evidence.RetryDelays[0] != 11*time.Millisecond {
+		t.Fatalf("interrupted evidence = %#v", evidence)
+	}
+	if evidence.Attempts[0].HTTP.HTTPStatus == nil || *evidence.Attempts[0].HTTP.HTTPStatus != http.StatusServiceUnavailable {
+		t.Fatalf("last dispatched response was lost: %+v", evidence.Attempts[0])
+	}
+}
+
+func TestGenerateWithEvidenceRecordsCanceledReacquisition(t *testing.T) {
+	var now = time.Date(2026, 10, 13, 12, 0, 0, 0, time.UTC)
+	calls := 0
+	coordinator := testCoordinator(t, func(*http.Request) (*http.Response, error) {
+		calls++
+		now = now.Add(2 * time.Millisecond)
+		return response(http.StatusServiceUnavailable, `{"error":{}}`), nil
+	})
+	coordinator.now = func() time.Time { return now }
+	baseContext, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	contextWithCancelOnWait := &cancelOnDoneContext{Context: baseContext, cancel: cancel}
+	var releaseHeld func()
+	coordinator.wait = func(context.Context, time.Duration) error {
+		release, err := coordinator.manager.Acquire(context.Background(), "limited")
+		if err != nil {
+			t.Fatalf("hold permit: %v", err)
+		}
+		releaseHeld = release
+		contextWithCancelOnWait.armed.Store(true)
+		return nil
+	}
+
+	// Signal cancellation only when the second acquisition has queued behind
+	// the held permit, avoiding scheduler- or sleep-based synchronization.
+	result, err, evidence := coordinator.GenerateWithEvidence(contextWithCancelOnWait, testRequest(2))
+	if releaseHeld != nil {
+		releaseHeld()
+	}
+	if result != nil || !errors.Is(err, context.Canceled) || calls != 1 {
+		t.Fatalf("result=%+v error=%v calls=%d", result, err, calls)
+	}
+	if len(evidence.Attempts) != 2 || !evidence.Attempts[0].Dispatched || evidence.Attempts[1].Dispatched {
+		t.Fatalf("acquisition evidence = %#v", evidence.Attempts)
+	}
+	if evidence.Attempts[1].Failure != domain.ExecutionFailureCanceled {
+		t.Fatalf("canceled acquisition failure = %v", evidence.Attempts[1].Failure)
+	}
+	if evidence.Attempts[0].HTTP.HTTPStatus == nil || *evidence.Attempts[0].HTTP.HTTPStatus != http.StatusServiceUnavailable {
+		t.Fatalf("last dispatched evidence was lost: %+v", evidence.Attempts)
+	}
+}
+
+func TestGenerateWithEvidenceNoDispatchHasCompleteZeroUsage(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+	coordinator := testCoordinator(t, func(*http.Request) (*http.Response, error) {
+		t.Fatal("canceled generation dispatched")
+		return nil, nil
+	})
+
+	result, err, evidence := coordinator.GenerateWithEvidence(ctx, testRequest(0))
+	if result != nil || !errors.Is(err, context.Canceled) || len(evidence.Attempts) != 0 {
+		t.Fatalf("result=%+v error=%v evidence=%+v", result, err, evidence)
+	}
+	if !evidence.Usage.PromptTokens.Known || !evidence.Usage.PromptTokens.Complete || evidence.Usage.PromptTokens.Value != 0 {
+		t.Fatalf("no-dispatch usage = %+v", evidence.Usage.PromptTokens)
+	}
+}
+
+type cancelOnDoneContext struct {
+	context.Context
+	cancel context.CancelFunc
+	once   sync.Once
+	armed  atomic.Bool
+}
+
+func (c *cancelOnDoneContext) Done() <-chan struct{} {
+	if c.armed.Load() {
+		c.once.Do(c.cancel)
+	}
+	return c.Context.Done()
+}

internal/observation/recorder.go

@@ -28,7 +28,7 @@ type Recorder struct {
 	capacityWait         time.Duration
 	generationDuration   time.Duration
 	retryDelay           time.Duration
-	usage                usageTotals
+	usage                UsageAccumulator
 }
 
 // Selection contains resolved identities only. Unresolved fields stay empty.
@@ -79,8 +79,7 @@ func New(observer domain.ExecutionObserver) (*Recorder, error) {
 }
 
 func newRecorder(runID string, now func() time.Time, observer domain.ExecutionObserver) *Recorder {
-	return &Recorder{runID: runID, startedAt: now(), now: now, observer: observer,
+	return &Recorder{runID: runID, startedAt: now(), now: now, observer: observer}
-		usage: newUsageTotals()}
 }
 
 // NewRunID creates the identity shared by observations and successful results.
@@ -119,7 +118,7 @@ func (r *Recorder) Attempt(data AttemptData) {
 			r.lastRepairGeneration = data.Generation
 		}
 		r.generationDuration = addDuration(r.generationDuration, data.GenerationDuration)
-		r.usage.add(data.Usage, data.UsagePresence)
+		r.usage.Add(data.Usage, data.UsagePresence)
 	}
 	event := r.event(domain.ExecutionAttempt)
 	event.Generation = data.Generation
@@ -171,7 +170,7 @@ func (r *Recorder) Finish(data FinishData) {
 	if event.Duration < 0 {
 		event.Duration = 0
 	}
-	event.Usage = r.usage.snapshot(r.attempts)
+	event.Usage = r.usage.Observed()
 	r.deliver(event)
 }
 
@@ -243,7 +242,29 @@ type usageTotals struct {
 	prompt, completion, total, cached, cacheWrite usageField
 }
 
-func newUsageTotals() usageTotals {
+// UsageAccumulator combines presence-aware usage across dispatched attempts.
+// Its zero value represents no dispatched attempts.
+type UsageAccumulator struct {
+	totals   usageTotals
+	attempts uint64
+}
+
+// Add includes the usage reported by one dispatched attempt.
+func (u *UsageAccumulator) Add(usage domain.TokenUsage, presence *domain.TokenUsagePresence) {
+	if u.attempts == 0 {
+		u.totals = completeUsageTotals()
+	}
+	u.attempts++
+	u.totals.add(usage, presence)
+}
+
+// Observed returns aggregate counts, marking every field known and complete
+// when no attempt has been added.
+func (u UsageAccumulator) Observed() domain.ObservedTokenUsage {
+	return u.totals.snapshot(u.attempts)
+}
+
+func completeUsageTotals() usageTotals {
 	return usageTotals{prompt: usageField{complete: true}, completion: usageField{complete: true},
 		total: usageField{complete: true}, cached: usageField{complete: true}, cacheWrite: usageField{complete: true}}
 }

internal/observation/usage_accumulator_test.go

@@ -0,0 +1,27 @@
+package observation
+
+import (
+	"testing"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+)
+
+func TestUsageAccumulatorPreservesPresenceAndNoAttemptSemantics(t *testing.T) {
+	var empty UsageAccumulator
+	zero := empty.Observed()
+	if !zero.PromptTokens.Known || !zero.PromptTokens.Complete || zero.PromptTokens.Value != 0 ||
+		!zero.CacheWriteTokens.Known || !zero.CacheWriteTokens.Complete || zero.CacheWriteTokens.Value != 0 {
+		t.Fatalf("zero-attempt usage = %+v", zero)
+	}
+
+	var usage UsageAccumulator
+	usage.Add(domain.TokenUsage{PromptTokens: 0}, &domain.TokenUsagePresence{PromptTokens: true})
+	usage.Add(domain.TokenUsage{PromptTokens: 3}, &domain.TokenUsagePresence{PromptTokens: true})
+	observed := usage.Observed()
+	if observed.PromptTokens.Value != 3 || !observed.PromptTokens.Known || !observed.PromptTokens.Complete {
+		t.Fatalf("reported prompt usage = %+v", observed.PromptTokens)
+	}
+	if observed.CompletionTokens.Known || observed.CompletionTokens.Complete {
+		t.Fatalf("unreported completion usage = %+v", observed.CompletionTokens)
+	}
+}