Add cache efficiency, burn rate, and fix context calculation by himattm · Pull Request #18 · himattm/prism

himattm · 2026-02-12T16:36:13Z

Summary

Fix legacy context calculation: Excluded CacheReadTokens from the context percentage computation — cache reads don't consume additional context window space, so including them inflated the reported usage
Extend SessionContext: Added token data fields (InputTokens, OutputTokens, CacheCreationTokens, CacheReadTokens, ContextWindowSize) to make token-level data available to plugins
Add cache efficiency indicator (⌁XX%): Shows the percentage of input tokens served from cache, giving visibility into cache hit rates (~10x cheaper tokens)
Add session burn rate (~$X.XX/h): Tracks cost velocity using snapshot-based calculation; only appears after 60+ seconds of data and when rate exceeds $0.01/h
New burnrate package: Manages snapshot persistence in temp files for cost velocity tracking across status line renders
Configurable display: Both indicators can be toggled via show_cache and show_burn_rate in the api_billing config section
Cleanup on session end: Burn rate snapshot files are removed when a session ends
Updated README: Documented new display format, configuration options, and visual breakdown

Test Plan

Verify go test ./... passes (all new and existing tests)
Verify legacy context percentage is unchanged when CacheReadTokens is 0
Verify legacy context percentage no longer inflates when CacheReadTokens is large
Verify cache indicator (⌁XX%) appears when cache read tokens are present
Verify cache indicator is hidden when show_cache: false or no cache reads
Verify burn rate appears after 60s of session data with cost > $0.01/h
Verify burn rate snapshot file is cleaned up on session end
Verify config parsing for show_burn_rate and show_cache options

🤖 Generated with Claude Code

…culation - Fix legacy context calculation to exclude CacheReadTokens (they don't consume additional context window space) - Extend SessionContext with token data fields (input, output, cache creation, cache read, context window size) - Add cache efficiency indicator (⌁XX%) showing cache read hit percentage - Add session burn rate (~$X.XX/h) with configurable display - Add burnrate package for snapshot-based cost velocity tracking - Clean up burn rate snapshot files on session end - Update README with new API billing display features and config options Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Copilot

Pull request overview

This PR improves status/cost reporting by fixing legacy context usage math around cache tokens, exposing token-level fields to plugins, and adding two new API-billing indicators (cache efficiency and session burn rate) with supporting snapshot persistence.

Changes:

Fix legacy context % calculation by excluding CacheReadTokens from the denominator.
Extend plugin.SessionContext with token usage fields and context window size.
Add cache efficiency (⌁XX%) and burn rate (~$X.XX/h) display, plus a new internal/burnrate package and session-end cleanup.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 14 comments.

Show a summary per file

File	Description
internal/statusline/statusline.go	Fix legacy context math; add burn rate + cache indicator to built-in `cost`; expose token fields to plugins.
internal/statusline/statusline_test.go	Add tests for legacy context cache exclusion and cache indicator/ratio.
internal/statusline/burnrate_test.go	Add tests for burn rate rendering and snapshot creation behavior.
internal/plugins/usage.go	Add `show_cache` / `show_burn_rate` config and cache indicator/burn rate suffix in API billing cost rendering.
internal/plugins/usage_test.go	Expand config parsing tests and add cache-indicator + cacheRatio tests.
internal/plugin/types.go	Add token-related fields to `SessionContext` JSON payload for plugins.
internal/hooks/hooks.go	Remove burn rate snapshot file on session end.
internal/hooks/hooks_test.go	Add test ensuring burn rate snapshot cleanup on session end.
internal/burnrate/burnrate.go	New package for snapshot persistence + burn rate calculation/formatting.
internal/burnrate/burnrate_test.go	Tests for snapshot lifecycle, rate calculation thresholds, formatting, cleanup.
README.md	Document new usage display format and config toggles.

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Copilot · 2026-02-12T18:18:02Z

internal/hooks/hooks_test.go

+func TestHandleSessionEnd_CleansBurnRateFile(t *testing.T) {
+	sessionID := "test-session-end-burn"
+


The test uses a fixed sessionID (and thus fixed temp file names). This can become flaky when multiple go test runs overlap on the same machine/user (e.g., CI retries) because they share os.TempDir(). Prefer generating a unique session ID per test (timestamp + random suffix) to avoid collisions.

Copilot · 2026-02-12T18:18:03Z

internal/statusline/burnrate_test.go

+	data, _ := json.Marshal(snap)
+	os.WriteFile(path, data, 0644)


Test setup ignores errors from json.Marshal / os.WriteFile. If either fails, the assertions later can produce misleading results. Please handle these errors and fail fast to make the test output actionable.

Suggested change

data, _ := json.Marshal(snap)

os.WriteFile(path, data, 0644)

data, err := json.Marshal(snap)

if err != nil {

t.Fatalf("failed to marshal burn rate snapshot: %v", err)

}

if err := os.WriteFile(path, data, 0644); err != nil {

t.Fatalf("failed to write burn rate snapshot file: %v", err)

}

Copilot · 2026-02-12T18:18:03Z

internal/burnrate/burnrate_test.go

+	data, _ := json.Marshal(original)
+	os.WriteFile(path, data, 0644)


This test writes the snapshot file without checking the json.Marshal / os.WriteFile errors, which can hide test setup problems and cause confusing failures downstream. Please handle and assert these errors.

Suggested change

data, _ := json.Marshal(original)

os.WriteFile(path, data, 0644)

data, err := json.Marshal(original)

if err != nil {

t.Fatalf("failed to marshal original snapshot: %v", err)

}

if err := os.WriteFile(path, data, 0644); err != nil {

t.Fatalf("failed to write snapshot file: %v", err)

}

Copilot · 2026-02-12T18:18:04Z

internal/plugins/usage.go

+	// Append burn rate if enabled and session ID is available
+	if cfg.showBurnRate && input.Prism.SessionID != "" {
+		costStr += p.getBurnRateSuffix(input.Prism.SessionID, cost)
+	}
+
+	// Append cache efficiency indicator if enabled
+	if cfg.showCache {
+		if ratio, ok := cacheRatio(input.Session.InputTokens, input.Session.CacheCreationTokens, input.Session.CacheReadTokens); ok {
+			costStr += fmt.Sprintf(" ⌁%d%%", ratio)
+		}
+	}


Output ordering doesn’t match the README/example format: this appends burn rate before the cache indicator, producing $X.XX ~$Y.YY/h ⌁ZZ% rather than $X.XX ⌁ZZ% ~$Y.YY/h. Either swap the append order in code or adjust the documented format so users get what they expect.

Copilot · 2026-02-12T18:18:04Z

internal/statusline/statusline.go

+	costStr := fmt.Sprintf("$%.2f", cost)
+
+	// Compute burn rate if session ID is available
+	if sl.input.SessionID != "" {
+		snap, existed, err := burnrate.LoadOrCreateSnapshot(sl.input.SessionID, cost)
+		if err == nil && existed {
+			if rate, show := burnrate.CalculateRate(snap, cost, time.Now()); show {
+				costStr += " " + burnrate.FormatRate(rate)
+			}
+		}
+	}
+
+	// Add cache efficiency indicator
+	usage := sl.input.Context.CurrentUsage
+	if ratio, ok := cacheRatio(usage.InputTokens, usage.CacheCreationTokens, usage.CacheReadTokens); ok {
+		costStr += fmt.Sprintf(" ⌁%d%%", ratio)
+	}


The README/example format shows cost, then cache indicator, then burn rate, but this builds the string as cost → burn rate → cache. Align the order (or update docs) so the status line output matches the documented display format.

Copilot · 2026-02-12T18:18:05Z

internal/burnrate/burnrate.go

+	if err := os.WriteFile(path, data, 0644); err != nil {
+		return nil, false, err
+	}
+


LoadOrCreateSnapshotAt writes the snapshot JSON directly to the target file. With concurrent renders/processes, this can lead to torn writes and JSON unmarshal failures. Consider writing to a temp file and os.Rename for atomic replacement (and/or using a per-session lock) to make snapshot persistence robust.

Suggested change

if err := os.WriteFile(path, data, 0644); err != nil {

return nil, false, err

}

// Write to a temporary file and atomically replace the target to avoid torn writes.

dir := filepath.Dir(path)

tmpFile, err := os.CreateTemp(dir, "prism-burn-*.tmp")

if err != nil {

return nil, false, err

}

tmpName := tmpFile.Name()

if _, err := tmpFile.Write(data); err != nil {

tmpFile.Close()

os.Remove(tmpName)

return nil, false, err

}

if err := tmpFile.Sync(); err != nil {

tmpFile.Close()

os.Remove(tmpName)

return nil, false, err

}

if err := tmpFile.Close(); err != nil {

os.Remove(tmpName)

return nil, false, err

}

if err := os.Chmod(tmpName, 0644); err != nil {

os.Remove(tmpName)

return nil, false, err

}

if err := os.Rename(tmpName, path); err != nil {

os.Remove(tmpName)

return nil, false, err

}

Copilot · 2026-02-12T18:18:05Z

internal/hooks/hooks_test.go

+
+	// Also create idle file (normal cleanup)
+	idleFile := filepath.Join(os.TempDir(), fmt.Sprintf("prism-idle-%s", sessionID))
+	os.WriteFile(idleFile, []byte{}, 0644)


This test ignores the error from os.WriteFile when creating the idle file, which can mask failures on restricted/readonly temp directories. Please check and fail the test on write errors (similar to the burn-rate file setup above).

Suggested change

os.WriteFile(idleFile, []byte{}, 0644)

if err := os.WriteFile(idleFile, []byte{}, 0644); err != nil {

t.Fatalf("failed to create idle file: %v", err)

}

Copilot · 2026-02-12T18:18:06Z

internal/statusline/statusline.go

+	if sl.input.SessionID != "" {
+		snap, existed, err := burnrate.LoadOrCreateSnapshot(sl.input.SessionID, cost)
+		if err == nil && existed {
+			if rate, show := burnrate.CalculateRate(snap, cost, time.Now()); show {
+				costStr += " " + burnrate.FormatRate(rate)
+			}
+		}


Status line sections are rendered concurrently (goroutines in renderLine). If a user config includes both cost and usage sections, burn-rate snapshot file reads/writes can happen at the same time for the same session, risking corrupted/partial JSON reads and flickering output. Consider adding synchronization/atomic file writes in the burnrate package or ensuring only one renderer touches the snapshot per session.

Copilot · 2026-02-12T18:18:06Z

internal/statusline/statusline.go

+	// Add cache efficiency indicator
+	usage := sl.input.Context.CurrentUsage
+	if ratio, ok := cacheRatio(usage.InputTokens, usage.CacheCreationTokens, usage.CacheReadTokens); ok {
+		costStr += fmt.Sprintf(" ⌁%d%%", ratio)
+	}


Cache efficiency is appended unconditionally for the built-in cost section, but the PR description/README describe show_cache as a configurable toggle. Either wire show_cache into this code path (similar to the usage plugin), or keep cache efficiency confined to the usage plugin to avoid undocumented behavior differences.

Copilot · 2026-02-12T18:18:06Z

internal/hooks/hooks.go

+		// Clean up burn rate snapshot file
+		burnRateFile := filepath.Join(os.TempDir(), fmt.Sprintf("prism-burn-%s", input.SessionID))
+		os.Remove(burnRateFile)


This duplicates burn-rate path construction logic instead of calling burnrate.Cleanup(sessionID). Using the package helper keeps the cleanup behavior consistent if the burnrate file naming/sanitization changes and reduces the chance of drift.

- Sanitize sessionID in burnrate.FilePath() to prevent path traversal - Use atomic writes (tmp + rename) in LoadOrCreateSnapshotAt() - Fix burn rate never appearing: switch to LoadOrCreateSnapshot in usage plugin - Fix output ordering: cache indicator (⌁XX%) now appears before burn rate (~$X/h) - Wire show_cache and show_burn_rate config in statusline (skip I/O when disabled) - Replace manual burn rate path cleanup in hooks with burnrate.Cleanup() - Add error checks for json.Marshal/os.WriteFile in all test files - Use unique session IDs per test to prevent collisions Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

- Sanitize idle marker file paths to prevent directory traversal (hooks, statusline) - Clamp costDecimals config to 0-10 range for safe fmt.Sprintf formatting - Cache burn rate snapshots in memory via sync.Map to avoid per-render disk I/O - Fix calculateContextPct to derive used% from RemainingPercentage when UsedPercentage is zero - Add corrupt/invalid JSON snapshot recovery with Timestamp.IsZero() validation - Fix unchecked errors in statusline_test.go test setup code - Extract duplicate cacheRatio logic to shared internal/tokens.CacheEfficiency Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

himattm requested a review from Copilot February 12, 2026 18:12

Copilot started reviewing on behalf of himattm February 12, 2026 18:13 View session

Copilot AI reviewed Feb 12, 2026

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Add cache efficiency, burn rate, and fix context calculation#18

Add cache efficiency, burn rate, and fix context calculation#18
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himattm/cost-context-accuracy

himattm commented Feb 12, 2026 •

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		func TestHandleSessionEnd_CleansBurnRateFile(t *testing.T) {
		sessionID := "test-session-end-burn"

-	data, _ := json.Marshal(snap)
-	os.WriteFile(path, data, 0644)
+	data, err := json.Marshal(snap)
+	if err != nil {
+		t.Fatalf("failed to marshal burn rate snapshot: %v", err)
+	}
+	if err := os.WriteFile(path, data, 0644); err != nil {
+		t.Fatalf("failed to write burn rate snapshot file: %v", err)
+	}

		data, _ := json.Marshal(original)
		os.WriteFile(path, data, 0644)

-	if err := os.WriteFile(path, data, 0644); err != nil {
-		return nil, false, err
-	}
+	// Write to a temporary file and atomically replace the target to avoid torn writes.
+	dir := filepath.Dir(path)
+	tmpFile, err := os.CreateTemp(dir, "prism-burn-*.tmp")
+	if err != nil {
+		return nil, false, err
+	}
+	tmpName := tmpFile.Name()
+	if _, err := tmpFile.Write(data); err != nil {
+		tmpFile.Close()
+		os.Remove(tmpName)
+		return nil, false, err
+	}
+	if err := tmpFile.Sync(); err != nil {
+		tmpFile.Close()
+		os.Remove(tmpName)
+		return nil, false, err
+	}
+	if err := tmpFile.Close(); err != nil {
+		os.Remove(tmpName)
+		return nil, false, err
+	}
+	if err := os.Chmod(tmpName, 0644); err != nil {
+		os.Remove(tmpName)
+		return nil, false, err
+	}
+	if err := os.Rename(tmpName, path); err != nil {
+		os.Remove(tmpName)
+		return nil, false, err
+	}

-	os.WriteFile(idleFile, []byte{}, 0644)
+	if err := os.WriteFile(idleFile, []byte{}, 0644); err != nil {
+		t.Fatalf("failed to create idle file: %v", err)
+	}

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himattm commented Feb 12, 2026 •

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