diff --git a/.changeset/silly-jaguars-sing.md b/.changeset/silly-jaguars-sing.md
new file mode 100644
index 000000000..ce83abf1c
--- /dev/null
+++ b/.changeset/silly-jaguars-sing.md
@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 3436
+---
+**`/gsd-plan-phase` now documents phase-plan-index-compatible plan conventions** — planner guidance uses canonical `depends_on` and SUMMARY forms, and native phase indexing warns when it ignores noncanonical plan filenames.
diff --git a/agents/gsd-planner.md b/agents/gsd-planner.md
index 37e39c7bb..b23a1d3c8 100644
--- a/agents/gsd-planner.md
+++ b/agents/gsd-planner.md
@@ -426,7 +426,7 @@ phase: XX-name
 plan: NN
 type: execute
 wave: N                     # Execution wave (1, 2, 3...)
-depends_on: []              # Plan IDs this plan requires
+depends_on: []              # Use `01-01`/`01-01-auth-hardening`
 files_modified: []          # Files this plan touches
 autonomous: true            # false if plan has checkpoints
 requirements: []            # REQUIRED — Requirement IDs from ROADMAP this plan addresses. MUST NOT be empty.
@@ -496,7 +496,7 @@ Output: [Artifacts created]
 </success_criteria>
 
 <output>
-After completion, create `.planning/phases/XX-name/{phase}-{plan}-SUMMARY.md`
+Create `.planning/phases/XX-name/{padded_phase}-{plan}-SUMMARY.md` when done
 </output>
 ```
 
diff --git a/sdk/src/query/phase.test.ts b/sdk/src/query/phase.test.ts
index 6b6477120..2be0e4a59 100644
--- a/sdk/src/query/phase.test.ts
+++ b/sdk/src/query/phase.test.ts
@@ -505,4 +505,27 @@ describe('phasePlanIndex', () => {
     expect(planA!.depends_on).toEqual([]);
     expect(planB!.depends_on).toEqual(['15-01']);
   });
+
+  it('#3430: native phase-plan-index warns about noncanonical plan-shaped files it cannot index', async () => {
+    const phase16 = join(tmpDir, '.planning', 'phases', '16-warning');
+    await mkdir(phase16, { recursive: true });
+    await writeFile(join(phase16, '16-PLAN-01-eval-harness.md'), [
+      '---',
+      'phase: 16-warning',
+      'plan: 01',
+      'wave: 1',
+      'autonomous: true',
+      'depends_on: []',
+      '---',
+      '<objective>Noncanonical plan filename.</objective>',
+    ].join('\n'));
+
+    const result = await phasePlanIndex(['16'], tmpDir);
+    const data = result.data as Record<string, unknown>;
+
+    expect(data.plans).toEqual([]);
+    expect(data.warnings).toEqual([
+      'Ignored noncanonical plan files: 16-PLAN-01-eval-harness.md',
+    ]);
+  });
 });
diff --git a/sdk/src/query/phase.ts b/sdk/src/query/phase.ts
index 4fbd249a7..f42272f83 100644
--- a/sdk/src/query/phase.ts
+++ b/sdk/src/query/phase.ts
@@ -278,6 +278,11 @@ export const phasePlanIndex: QueryHandler = async (args, projectDir, workstream)
   const phaseFiles = await readdir(phaseDir);
   const planFiles = phaseFiles.filter(f => f.endsWith('-PLAN.md') || f === 'PLAN.md').sort();
   const summaryFiles = phaseFiles.filter(f => f.endsWith('-SUMMARY.md') || f === 'SUMMARY.md');
+  const nonCanonicalPlanFiles = phaseFiles.filter((f) => (
+    f.toLowerCase().endsWith('.md')
+    && /(^|-)plan(-|\.)/i.test(f)
+    && !(f.endsWith('-PLAN.md') || f === 'PLAN.md')
+  )).sort();
 
   // Build set of plan IDs with summaries — match the planId derivation logic
   const completedPlanIds = new Set(
@@ -433,6 +438,10 @@ export const phasePlanIndex: QueryHandler = async (args, projectDir, workstream)
   let hasCheckpoints = false;
   const warnings: string[] = [];
 
+  if (nonCanonicalPlanFiles.length > 0) {
+    warnings.push(`Ignored noncanonical plan files: ${nonCanonicalPlanFiles.join(', ')}`);
+  }
+
   for (const raw of rawPlans) {
     if (!raw.autonomous) {
       hasCheckpoints = true;
diff --git a/tests/bug-3430-planner-phase-contract.test.cjs b/tests/bug-3430-planner-phase-contract.test.cjs
new file mode 100644
index 000000000..fadca144c
--- /dev/null
+++ b/tests/bug-3430-planner-phase-contract.test.cjs
@@ -0,0 +1,44 @@
+// allow-test-rule: source-text-is-the-product
+// Planner markdown is the deployed planning contract; these checks lock the
+// exact canonical forms that downstream phase-plan-index accepts.
+
+'use strict';
+
+const { test } = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const path = require('node:path');
+
+const PLANNER_PATH = path.join(__dirname, '..', 'agents', 'gsd-planner.md');
+
+function readPlanner() {
+  return fs.readFileSync(PLANNER_PATH, 'utf8');
+}
+
+test('#3430: planner SUMMARY instruction uses canonical padded phase/plan form', () => {
+  const content = readPlanner();
+  assert.match(
+    content,
+    /Create `\.planning\/phases\/XX-name\/\{padded_phase\}-\{plan\}-SUMMARY\.md` when done/,
+    'planner must instruct executors to write SUMMARY files in canonical padded-phase form'
+  );
+  assert.doesNotMatch(
+    content,
+    /After completion, create `\.planning\/phases\/XX-name\/\{phase\}-\{plan\}-SUMMARY\.md`/,
+    'planner must not instruct the broken {phase}-{plan}-SUMMARY.md form'
+  );
+});
+
+test('#3430: planner depends_on docs show canonical in-phase plan ids', () => {
+  const content = readPlanner();
+  assert.match(
+    content,
+    /depends_on:[^\n]*Use `01-01`\/`01-01-auth-hardening`/,
+    'planner must document canonical depends_on examples that phase-plan-index resolves'
+  );
+  assert.doesNotMatch(
+    content,
+    /depends_on:[^\n]*01-trust\/01/,
+    'planner must not document phase-slug/plan-number depends_on examples as canonical'
+  );
+});