claude-code

src/tools/FileReadTool/limits.ts

@@ -0,0 +1,93 @@
+/**
+ * Read tool output limits.  Two caps apply to text reads:
+ *
+ *   | limit         | default | checks                    | cost          | on overflow     |
+ *   |---------------|---------|---------------------------|---------------|-----------------|
+ *   | maxSizeBytes  | 256 KB  | TOTAL FILE SIZE (not out) | 1 stat        | throws pre-read |
+ *   | maxTokens     | 25000   | actual output tokens      | API roundtrip | throws post-read|
+ *
+ * Known mismatch: maxSizeBytes gates on total file size, not the slice.
+ * Tested truncating instead of throwing for explicit-limit reads that
+ * exceed the byte cap (#21841, Mar 2026).  Reverted: tool error rate
+ * dropped but mean tokens rose — the throw path yields a ~100-byte error
+ * tool-result while truncation yields ~25K tokens of content at the cap.
+ */
+import memoize from 'lodash-es/memoize.js'
+import { getFeatureValue_CACHED_MAY_BE_STALE } from 'src/services/analytics/growthbook.js'
+import { MAX_OUTPUT_SIZE } from 'src/utils/file.js'
+export const DEFAULT_MAX_OUTPUT_TOKENS = 25000
+
+/**
+ * Env var override for max output tokens. Returns undefined when unset/invalid
+ * so the caller can fall through to the next precedence tier.
+ */
+function getEnvMaxTokens(): number | undefined {
+  const override = process.env.CLAUDE_CODE_FILE_READ_MAX_OUTPUT_TOKENS
+  if (override) {
+    const parsed = parseInt(override, 10)
+    if (!isNaN(parsed) && parsed > 0) {
+      return parsed
+    }
+  }
+  return undefined
+}
+
+export type FileReadingLimits = {
+  maxTokens: number
+  maxSizeBytes: number
+  includeMaxSizeInPrompt?: boolean
+  targetedRangeNudge?: boolean
+}
+
+/**
+ * Default limits for Read tool when the ToolUseContext doesn't supply an
+ * override. Memoized so the GrowthBook value is fixed at first call — avoids
+ * the cap changing mid-session as the flag refreshes in the background.
+ *
+ * Precedence for maxTokens: env var > GrowthBook > DEFAULT_MAX_OUTPUT_TOKENS.
+ * (Env var is a user-set override, should beat experiment infrastructure.)
+ *
+ * Defensive: each field is individually validated; invalid values fall
+ * through to the hardcoded defaults (no route to cap=0).
+ */
+export const getDefaultFileReadingLimits = memoize((): FileReadingLimits => {
+  const override =
+    getFeatureValue_CACHED_MAY_BE_STALE<Partial<FileReadingLimits> | null>(
+      'tengu_amber_wren',
+      {},
+    )
+
+  const maxSizeBytes =
+    typeof override?.maxSizeBytes === 'number' &&
+    Number.isFinite(override.maxSizeBytes) &&
+    override.maxSizeBytes > 0
+      ? override.maxSizeBytes
+      : MAX_OUTPUT_SIZE
+
+  const envMaxTokens = getEnvMaxTokens()
+  const maxTokens =
+    envMaxTokens ??
+    (typeof override?.maxTokens === 'number' &&
+    Number.isFinite(override.maxTokens) &&
+    override.maxTokens > 0
+      ? override.maxTokens
+      : DEFAULT_MAX_OUTPUT_TOKENS)
+
+  const includeMaxSizeInPrompt =
+    typeof override?.includeMaxSizeInPrompt === 'boolean'
+      ? override.includeMaxSizeInPrompt
+      : undefined
+
+  const targetedRangeNudge =
+    typeof override?.targetedRangeNudge === 'boolean'
+      ? override.targetedRangeNudge
+      : undefined
+
+  return {
+    maxSizeBytes,
+    maxTokens,
+    includeMaxSizeInPrompt,
+    targetedRangeNudge,
+  }
+})
+