diff --git a/agents/gsd-pattern-mapper.md b/agents/gsd-pattern-mapper.md
index a007513dc..fb850b29b 100644
--- a/agents/gsd-pattern-mapper.md
+++ b/agents/gsd-pattern-mapper.md
@@ -118,6 +118,12 @@ Grep("router\.(get|post|put|delete)", type: "ts")
 
 ## Step 4: Extract Patterns from Analogs
 
+**Never re-read the same range.** For small files (≤ 2,000 lines), one `Read` call is enough — extract everything in that pass. For large files, multiple non-overlapping targeted reads are fine; what is forbidden is re-reading a range already in context.
+
+**Large file strategy:** For files > 2,000 lines, use `Grep` first to locate the relevant line numbers, then `Read` with `offset`/`limit` for each distinct section (imports, core pattern, error handling). Use non-overlapping ranges. Do not load the whole file.
+
+**Early stopping:** Stop analog search once you have 3–5 strong matches. There is no benefit to finding a 10th analog.
+
 For each analog file, Read it and extract:
 
 | Pattern Category | What to Extract |
@@ -297,6 +303,16 @@ Pattern mapping complete. Planner can now reference analog patterns in PLAN.md f
 
 </structured_returns>
 
+<critical_rules>
+
+- **No re-reads:** Never re-read a range already in context. Small files: one Read call, extract everything. Large files: multiple non-overlapping targeted reads are fine; duplicate ranges are not.
+- **Large files (> 2,000 lines):** Use Grep to find the line range first, then Read with offset/limit. Never load the whole file when a targeted section suffices.
+- **Stop at 3–5 analogs:** Once you have enough strong matches, write PATTERNS.md. Broader search produces diminishing returns and wastes tokens.
+- **No source edits:** PATTERNS.md is the only file you write. All other file access is read-only.
+- **No heredoc writes:** Always use the Write tool, never `Bash(cat << 'EOF')`.
+
+</critical_rules>
+
 <success_criteria>
 
 Pattern mapping is complete when:
diff --git a/tests/pattern-mapper.test.cjs b/tests/pattern-mapper.test.cjs
index 11116119f..435e0337a 100644
--- a/tests/pattern-mapper.test.cjs
+++ b/tests/pattern-mapper.test.cjs
@@ -1,11 +1,12 @@
 /**
- * Tests for Pattern Mapper feature (#1861)
+ * Tests for Pattern Mapper feature (#1861, #2312)
  *
  * Covers:
  * - Config key workflow.pattern_mapper in VALID_CONFIG_KEYS
  * - Default value is true
  * - Config round-trip (set/get)
  * - init plan-phase output includes patterns_path (null when missing, path when present)
+ * - Agent prompt contains no-re-read and early-stop constraints (#2312)
  */
 
 const { describe, test, beforeEach, afterEach } = require('node:test');
@@ -109,3 +110,33 @@ describe('init plan-phase patterns_path', () => {
     assert.ok(data.patterns_path.includes('01-foundation'), `Expected path to include phase dir, got: ${data.patterns_path}`);
   });
 });
+
+describe('gsd-pattern-mapper agent prompt efficiency constraints (#2312)', () => {
+  const agentPath = path.join(__dirname, '..', 'agents', 'gsd-pattern-mapper.md');
+  let agentContent;
+
+  beforeEach(() => {
+    agentContent = fs.readFileSync(agentPath, 'utf-8');
+  });
+
+  test('READS-01: prompt contains no-re-read constraint', () => {
+    assert.ok(
+      /read each.*file.*once/i.test(agentContent) || /never re-read/i.test(agentContent),
+      'Agent prompt must instruct the model to read each analog file only once'
+    );
+  });
+
+  test('READS-02: prompt contains early-stop instruction', () => {
+    assert.ok(
+      /stop.*analog|3.?5.*analog|early.stop/i.test(agentContent),
+      'Agent prompt must instruct the model to stop after finding 3-5 analogs'
+    );
+  });
+
+  test('READS-03: prompt contains large-file strategy', () => {
+    assert.ok(
+      /2[,.]?000.*line|offset.*limit|large file/i.test(agentContent),
+      'Agent prompt must include guidance for reading large files with offset/limit'
+    );
+  });
+});