fix(prompt-hook): cap injection under Claude Code's 10k inline limit (#1694) (#1769)

Claude Code persists hook stdout over 10,000 characters to a file and shows
the model a 2 KB preview. The prompt-hook MAX of 16,000 always hit that path
once explore filled the budget. Cap at 9,000 (exported + unit-tested) so the
payload lands inline, with headroom for the wrapper and projectPath nudges.

Lands the approach from #1695 with a testable helper.

Co-authored-by: Colby McHenry <colbymchenry@users.noreply.github.com>

src/bin/codegraph.ts

@@ -41,7 +41,7 @@ try {
 import { Command } from 'commander';
 import * as path from 'path';
 import * as fs from 'fs';
-import { getCodeGraphDir, isInitialized, unsafeIndexRootReason, findNearestCodeGraphRoot, planFrontload, hasStructuralKeyword, extractCodeTokens } from '../directory';
+import { getCodeGraphDir, isInitialized, unsafeIndexRootReason, findNearestCodeGraphRoot, planFrontload, hasStructuralKeyword, extractCodeTokens, capPromptHookInjection } from '../directory';
 import { extractProseCandidates } from '../search/identifier-segments';
 import { detectWorktreeIndexMismatch, worktreeMismatchWarning } from '../sync/worktree';
 import { createShimmerProgress } from '../ui/shimmer-progress';
@@ -1412,8 +1412,11 @@ program
             const text = result.content[0]?.text ?? '';
             if (!result.isError && text.trim()) {
               // Cap the injection so a large-repo explore can't flood the prompt.
-              const MAX = 16000;
-              const body = text.length > MAX ? `${text.slice(0, MAX)}\n…(truncated; call codegraph_explore for the rest)` : text;
+              // Claude Code shows hook stdout inline only up to 10,000 characters;
+              // above that it persists the output to a file and the model sees a
+              // 2 KB preview (#1694). PROMPT_HOOK_INJECTION_MAX (9,000) leaves
+              // room for the wrapper and the projectPath nudge lines below.
+              const body = capPromptHookInjection(text);
               // For a front-loaded SUB-project, a follow-up explore needs its path.
               const more = plan.viaSubScan
                 ? `call codegraph_explore with projectPath: "${plan.exploreRoot}" for more`

src/directory.ts

@@ -568,6 +568,33 @@ export function isStructuralPrompt(prompt: string): boolean {
   return hasStructuralKeyword(prompt) || extractCodeTokens(prompt).length > 0;
 }
 
+/**
+ * Claude Code persists `UserPromptSubmit` hook stdout above this many
+ * characters to a file and shows the model a ~2 KB preview instead (#1694).
+ * Measured on Claude Code 2.1.261; documented in the hooks reference as a
+ * 10,000-character cap on hook output strings.
+ */
+export const CLAUDE_CODE_INLINE_HOOK_OUTPUT_LIMIT = 10_000;
+
+/**
+ * Max characters of explore text injected by `codegraph prompt-hook` before
+ * truncation. Must stay under {@link CLAUDE_CODE_INLINE_HOOK_OUTPUT_LIMIT} so
+ * the host delivers the payload inline. 9,000 leaves ~1k for the
+ * `<codegraph_context>` wrapper and the `projectPath` nudge lines appended
+ * after the cap is applied.
+ */
+export const PROMPT_HOOK_INJECTION_MAX = 9_000;
+
+/**
+ * Cap explore text for the prompt-hook injection, preserving the existing
+ * "call codegraph_explore for the rest" notice when truncated.
+ */
+export function capPromptHookInjection(text: string, max = PROMPT_HOOK_INJECTION_MAX): string {
+  return text.length > max
+    ? `${text.slice(0, max)}\n…(truncated; call codegraph_explore for the rest)`
+    : text;
+}
+
 /**
  * What the front-load hook should do for a prompt issued from a directory.
  */