fix: rewrite cached system prompt model identity to match requested model

src/index.test.ts

@@ -19,6 +19,10 @@ import {
 } from "./claude-tools.js";
 import { extractOAuthErrorDetail } from "./oauth.js";
 import { createSseProcessor, parseSseEvent, buildSseEvent } from "./stream.js";
+import {
+  deriveModelDisplayName,
+  rewriteSystemBlocksForModel,
+} from "./index.js";
 
 // ── Helpers (extracted / reimplemented from index.ts for unit testing) ────────
 
@@ -926,3 +930,60 @@ describe("parseSseEvent / buildSseEvent round trip", () => {
     assert.deepEqual(parsed, { event: null, data: '{"ok":true}' });
   });
 });
+
+describe("deriveModelDisplayName", () => {
+  it("parses claude-opus-4-7 → Opus 4.7", () => {
+    assert.equal(deriveModelDisplayName("claude-opus-4-7"), "Opus 4.7");
+  });
+
+  it("parses claude-sonnet-4-6 → Sonnet 4.6", () => {
+    assert.equal(deriveModelDisplayName("claude-sonnet-4-6"), "Sonnet 4.6");
+  });
+
+  it("ignores a trailing date suffix (claude-haiku-4-5-20251001 → Haiku 4.5)", () => {
+    assert.equal(deriveModelDisplayName("claude-haiku-4-5-20251001"), "Haiku 4.5");
+  });
+
+  it("falls back to the raw id when the convention doesn't match", () => {
+    assert.equal(deriveModelDisplayName("weird-custom-model"), "weird-custom-model");
+    assert.equal(deriveModelDisplayName("claude-3-5-sonnet-20241022"), "claude-3-5-sonnet-20241022");
+  });
+});
+
+describe("rewriteSystemBlocksForModel", () => {
+  const sourceBlock = {
+    type: "text",
+    text:
+      "You are a Claude agent, built on Anthropic's Claude Agent SDK.\n" +
+      " - You are powered by the model named Sonnet 4.6. The exact model ID is claude-sonnet-4-6.\n" +
+      " - Assistant knowledge cutoff is August 2025.",
+  };
+
+  it("rewrites the identity line to match the requested model", () => {
+    const [out] = rewriteSystemBlocksForModel([sourceBlock], "claude-opus-4-7");
+    assert.ok(out.text?.includes("named Opus 4.7"));
+    assert.ok(out.text?.includes("claude-opus-4-7"));
+    assert.ok(!out.text?.includes("Sonnet 4.6"));
+    assert.ok(!out.text?.includes("claude-sonnet-4-6"));
+  });
+
+  it("leaves blocks untouched when no model id is provided", () => {
+    const [out] = rewriteSystemBlocksForModel([sourceBlock], undefined);
+    assert.equal(out, sourceBlock);
+  });
+
+  it("handles a dated variant without re-including the date suffix in the display name", () => {
+    const [out] = rewriteSystemBlocksForModel([sourceBlock], "claude-haiku-4-5-20251001");
+    assert.ok(out.text?.includes("named Haiku 4.5"));
+    assert.ok(out.text?.includes("The exact model ID is claude-haiku-4-5-20251001."));
+  });
+
+  it("preserves non-text blocks and non-matching text", () => {
+    const nonText = { type: "other", text: "unchanged" };
+    const noMatch = { type: "text", text: "no identity line here" };
+    const blocks = [nonText, noMatch];
+    const out = rewriteSystemBlocksForModel(blocks, "claude-opus-4-7");
+    assert.deepEqual(out[0], nonText);
+    assert.equal(out[1].text, "no identity line here");
+  });
+});

src/index.ts

@@ -159,6 +159,52 @@ type PluginClient = {
 const CLAUDE_PREFIX =
   "You are a Claude agent, built on Anthropic's Claude Agent SDK.";
 
+/**
+ * Derive a human-readable display name from a Claude model ID.
+ *
+ * Matches `claude-{family}-{major}-{minor}[-{date}]` (the convention used
+ * for every Claude model family to date) and renders e.g.
+ *   claude-opus-4-7            -> "Opus 4.7"
+ *   claude-haiku-4-5-20251001  -> "Haiku 4.5"
+ *   claude-sonnet-4-6          -> "Sonnet 4.6"
+ *
+ * Falls back to the raw model ID if the convention doesn't match, so future
+ * naming changes are surfaced truthfully instead of silently wrong.
+ */
+export function deriveModelDisplayName(modelId: string): string {
+  const m = modelId.match(/^claude-([a-z]+)-(\d+)-(\d+)(?:-\d+)?$/i);
+  if (!m) return modelId;
+  const [, family, major, minor] = m;
+  const capitalized = family.charAt(0).toUpperCase() + family.slice(1).toLowerCase();
+  return `${capitalized} ${major}.${minor}`;
+}
+
+/**
+ * Rewrite model-identity lines inside the cached Claude Code system prompt
+ * so they reflect the model actually being requested. Without this, every
+ * request (regardless of selected model) gets a system prompt claiming
+ * "You are powered by the model named Sonnet 4.6", which can bias behavior.
+ */
+export function rewriteSystemBlocksForModel(
+  blocks: Array<{ type?: string; text?: string }>,
+  modelId: string | undefined,
+): Array<{ type?: string; text?: string }> {
+  if (!modelId) return blocks;
+  const display = deriveModelDisplayName(modelId);
+
+  return blocks.map((block) => {
+    if (block?.type !== "text" || typeof block.text !== "string") return block;
+    let text = block.text;
+
+    text = text.replace(
+      /You are powered by the model named [^\n]+? The exact model ID is [a-z0-9.-]+\./g,
+      `You are powered by the model named ${display}. The exact model ID is ${modelId}.`,
+    );
+
+    return { ...block, text };
+  });
+}
+
 const oauthProfileCache = new Map<string, Promise<OAuthProfile | null>>();
 const SYSTEM_PROMPT_CACHE_PATH = process.env.ANTHROPIC_SYSTEM_PROMPT_PATH
   || join(process.env.HOME || "", ".cache", "opencode-claude-bridge", "claude-system-prompt.json");
@@ -648,9 +694,13 @@ const OpenCodeClaudeBridge = async ({ client }: { client: PluginClient }) => {
 
                 // If we have a locally captured Claude Code system prompt, prefer it.
                 if (cachedClaudeSystem) {
+                  const rewritten = rewriteSystemBlocksForModel(
+                    cachedClaudeSystem,
+                    typeof parsed.model === "string" ? parsed.model : undefined,
+                  );
                   parsed.system = [
                     { type: "text", text: billingHeader },
-                    ...cachedClaudeSystem,
+                    ...rewritten,
                   ];
                 } else if (parsed.system && Array.isArray(parsed.system)) {
                   // Otherwise keep the existing prompt but shape it closer to Claude Code.