feat(agents): rebuild claude task_guidance block from prompting docs

Rebuild the Claude XML residual end to end from Anthropic's prompting docs while keeping CL-7775's measured shape: rationale first, numbered approach, named output contract, one wrapper, leaf-only tail placement. Every line is positively framed and scope-explicit for Sonnet's literal instruction-following; per-line provenance in the PR body.

Author: TheGreatAxios

src/agent/model-family-policy.test.ts

@@ -124,10 +124,14 @@ describe("resolveModelFamilyPolicy", () => {
         orchestrator: true,
       });
       expect(orchestrator.promptResidual).toBeUndefined();
+      // Default-family probe: anthropic/claude-sonnet-4 would hit the claude
+      // row now, and the gpt row has NOT landed yet (#1135), so openai/gpt-4.1
+      // is the probe that still resolves to the default family.
       const base = resolveModelFamilyPolicy({
-        providerName: "anthropic",
-        model: "claude-sonnet-4",
+        providerName: "openai",
+        model: "gpt-4.1",
       });
+      expect(base.family).toBe("default");
       expect(base.promptResidual).toBeUndefined();
     });
   });

src/agent/model-family-policy.ts

@@ -157,9 +157,10 @@ export const GROK_PROMPT_RESIDUAL = [
 // verbatim so the prompt carries exactly one copy.
 export const CLAUDE_TASK_GUIDANCE_NOTE = [
   "<task_guidance>",
-  "- Follow the dispatch brief exactly; its Success criteria are the done-definition.",
-  "- When the done-definition is met, stop calling tools and write the structured report envelope.",
-  "- Batch independent tool calls into a single turn; never re-read a file you already read this session.",
+  "Autonomous coding turn: finish the work in this turn on your best judgment.",
+  "1. Follow the dispatch brief exactly; its Success criteria are the done-definition.",
+  "2. Batch independent tool calls into a single turn; work from files already read this session.",
+  "3. Finish the task when the done-definition is met: prefer the structured report envelope over another tool call.",
   "</task_guidance>",
 ].join("\n");
 

src/agent/prompt-sizes.ts

@@ -18,7 +18,6 @@ import {
   MAX_AGENTS_MD_BYTES,
 } from "./context-extensions.js";
 import { shouldApplyGrokAntiThrash } from "../subagent/provider-family.js";
-import { resolveModelFamilyPolicy } from "./model-family-policy.js";
 import { isCodexProviderName } from "../config/codex-providers.js";
 import { shellCollectDefinition } from "./background-shell-tool.js";
 import {

src/agent/prompts.test.ts

@@ -431,4 +431,24 @@ describe("claude XML task_guidance residual (provider residual, not a prompt for
     expect(countOccurrences(note, "</task_guidance>")).toBe(1);
     expect(note).not.toMatch(/<system_prompt>|<prompt>|<identity>/);
   });
+
+  it("keeps the measured CL-7775 shape: rationale first, numbered approach, named output contract, positively framed", () => {
+    const lines = buildClaudeTaskGuidanceNote().split("\n");
+    // Rationale first: the lead line frames the turn before any directive.
+    expect(lines[1]).toMatch(/^Autonomous coding turn:/);
+    // Numbered approach, not bullets.
+    expect(lines.slice(2, 5).map((l) => l.split(".")[0])).toEqual([
+      "1",
+      "2",
+      "3",
+    ]);
+    // Named output contract.
+    expect(buildClaudeTaskGuidanceNote()).toContain(
+      "structured report envelope",
+    );
+    // Positive framing: no negative imperatives.
+    expect(buildClaudeTaskGuidanceNote()).not.toMatch(
+      /\b(do not|don't|never|stop calling)\b/i,
+    );
+  });
 });

src/agent/prompts.ts

@@ -526,6 +526,9 @@ export function buildGrokLeafAntiThrashNote(): string {
 // Single XML residual for Claude-family workers: a prose residual did
 // nothing, but one <task_guidance> block cut Sonnet tokens. One block only —
 // never a full-prompt XML renderer, never applied outside the claude family.
+// Rebuilt end to end from Anthropic's prompting docs (CL-8309): rationale
+// first, numbered approach, named output contract; every line is positively
+// framed and scope-explicit for Sonnet's literal instruction-following.
 // Single source of truth is the CLAUDE_TASK_GUIDANCE_NOTE block in
 // model-family-policy.ts (policy owns data); this returns that block verbatim
 // so the prompt carries one claude residual with no line twice.
@@ -578,4 +581,3 @@ export function buildSubAgentSystemPrompt(
   }
   return joinSections(sections);
 }
-}