diff --git a/packages/diagram/generation/src/lib/cloudflare-google-ai-studio.ts b/packages/diagram/generation/src/lib/cloudflare-google-ai-studio.ts
index 319eefe8..635db551 100644
--- a/packages/diagram/generation/src/lib/cloudflare-google-ai-studio.ts
+++ b/packages/diagram/generation/src/lib/cloudflare-google-ai-studio.ts
@@ -15,6 +15,7 @@ import {
 } from "./errors.js";
 import {
   buildGeminiGenerateContentBody,
+  extractGeminiFinishReason,
   extractGeminiText,
   extractGeminiUsage,
   stripGoogleModelPrefix,
@@ -183,8 +184,15 @@ const runGatewayAttempt = Effect.fn(
       }),
   });
   const usage = extractGeminiUsage(raw);
+  const finishReason = extractGeminiFinishReason(raw);
 
   return candidateFromText({
+    diagnostics:
+      finishReason === "MAX_TOKENS"
+        ? [
+            "output_truncated: Gemini stopped at the maximum output-token budget; regenerate the complete diagram.",
+          ]
+        : [],
     model,
     provider: "cloudflare-google-ai-studio",
     raw,
diff --git a/packages/diagram/generation/src/lib/gemini.ts b/packages/diagram/generation/src/lib/gemini.ts
index 20df09a2..10904a51 100644
--- a/packages/diagram/generation/src/lib/gemini.ts
+++ b/packages/diagram/generation/src/lib/gemini.ts
@@ -36,7 +36,7 @@ export class GeminiGenerateContentBody extends Schema.Class<GeminiGenerateConten
   system_instruction: GeminiSystemInstruction,
 }) {}
 
-const DEFAULT_MAX_OUTPUT_TOKENS = 2_048;
+const DEFAULT_MAX_OUTPUT_TOKENS = 16_384;
 const DEFAULT_TEMPERATURE = 0.1;
 
 function messageContent(
@@ -141,3 +141,12 @@ export function extractGeminiUsage(
 
   return Object.keys(usage).length > 0 ? usage : undefined;
 }
+
+export function extractGeminiFinishReason(
+  response: unknown,
+): string | undefined {
+  const candidates = objectValue(response, "candidates");
+  const firstCandidate = Array.isArray(candidates) ? candidates[0] : undefined;
+  const finishReason = objectValue(firstCandidate, "finishReason");
+  return typeof finishReason === "string" ? finishReason : undefined;
+}
diff --git a/packages/diagram/generation/src/lib/messages.ts b/packages/diagram/generation/src/lib/messages.ts
index ac46cc41..cc048461 100644
--- a/packages/diagram/generation/src/lib/messages.ts
+++ b/packages/diagram/generation/src/lib/messages.ts
@@ -36,7 +36,7 @@ export class DiagramGenerationMessages extends Schema.Class<DiagramGenerationMes
 }) {}
 
 const FLOWCHART_IR_INSTRUCTIONS = [
-  "Return only JSON. Do not wrap the JSON in markdown.",
+  "Return only compact, minified JSON on one line. Do not use markdown.",
   'Use type "flowchart".',
   'Every node must have id, label, and kind: "start", "process", "decision", or "end".',
   "Use exactly one start node and at least one end node.",
@@ -49,7 +49,7 @@ const FLOWCHART_IR_INSTRUCTIONS = [
 ];
 
 const MINDMAP_IR_INSTRUCTIONS = [
-  "Return only JSON. Do not wrap the JSON in markdown.",
+  "Return only compact, minified JSON on one line. Do not use markdown.",
   'Use type "mindmap".',
   'Every node must have id, label, kind ("root" or "topic"), and metadata with depth and siblingIndex.',
   "Use exactly one root node at depth 0 with siblingIndex 0.",
@@ -62,62 +62,54 @@ const MINDMAP_IR_INSTRUCTIONS = [
 
 function expectedJsonShape(prompt: DiagramGenerationPrompt): string {
   if (prompt.type === "mindmap") {
-    return JSON.stringify(
-      {
-        id: "short-kebab-case-id",
-        title: prompt.title,
-        type: "mindmap",
-        nodes: [
-          {
-            id: "topic-0",
-            label: "Root topic",
-            kind: "root",
-            metadata: { depth: 0, siblingIndex: 0 },
-          },
-          {
-            id: "topic-0-0",
-            label: "Child topic",
-            kind: "topic",
-            metadata: { depth: 1, siblingIndex: 0 },
-          },
-        ],
-        edges: [
-          {
-            id: "branch-0-0",
-            source: "topic-0",
-            target: "topic-0-0",
-            metadata: { depth: 1, siblingIndex: 0 },
-          },
-        ],
-        layout: { direction: "LR", edgeRouting: "curved" },
-      },
-      null,
-      2,
-    );
-  }
-
-  return JSON.stringify(
-    {
+    return JSON.stringify({
       id: "short-kebab-case-id",
       title: prompt.title,
-      type: "flowchart",
+      type: "mindmap",
       nodes: [
-        { id: "start-id", label: "Human label", kind: "start" },
-        { id: "decision-id", label: "Question?", kind: "decision" },
+        {
+          id: "topic-0",
+          label: "Root topic",
+          kind: "root",
+          metadata: { depth: 0, siblingIndex: 0 },
+        },
+        {
+          id: "topic-0-0",
+          label: "Child topic",
+          kind: "topic",
+          metadata: { depth: 1, siblingIndex: 0 },
+        },
       ],
       edges: [
         {
-          id: "edge-id",
-          source: "decision-id",
-          target: "target-id",
-          label: "yes",
+          id: "branch-0-0",
+          source: "topic-0",
+          target: "topic-0-0",
+          metadata: { depth: 1, siblingIndex: 0 },
         },
       ],
-      layout: { direction: "TB", edgeRouting: "orthogonal" },
-    },
-    null,
-    2,
-  );
+      layout: { direction: "LR", edgeRouting: "curved" },
+    });
+  }
+
+  return JSON.stringify({
+    id: "short-kebab-case-id",
+    title: prompt.title,
+    type: "flowchart",
+    nodes: [
+      { id: "start-id", label: "Human label", kind: "start" },
+      { id: "decision-id", label: "Question?", kind: "decision" },
+    ],
+    edges: [
+      {
+        id: "edge-id",
+        source: "decision-id",
+        target: "target-id",
+        label: "yes",
+      },
+    ],
+    layout: { direction: "TB", edgeRouting: "orthogonal" },
+  });
 }
 
 function requiredList(title: string, values: readonly string[]): string[] {
diff --git a/packages/diagram/generation/src/lib/public-api.test.ts b/packages/diagram/generation/src/lib/public-api.test.ts
index 503b4982..7ab6c397 100644
--- a/packages/diagram/generation/src/lib/public-api.test.ts
+++ b/packages/diagram/generation/src/lib/public-api.test.ts
@@ -73,7 +73,7 @@ const expectedSystem = [
   "You are creating a Sketchi typed intermediate diagram.",
   "",
   "Flowchart IR rules:",
-  "- Return only JSON. Do not wrap the JSON in markdown.",
+  "- Return only compact, minified JSON on one line. Do not use markdown.",
   '- Use type "flowchart".',
   '- Every node must have id, label, and kind: "start", "process", "decision", or "end".',
   "- Use exactly one start node and at least one end node.",
@@ -97,28 +97,24 @@ const expectedUser = [
   "Use these required labels exactly unless the scenario explicitly asks for a clearer synonym.",
   "",
   "Expected JSON shape:",
-  JSON.stringify(
-    {
-      id: "short-kebab-case-id",
-      title: "Pharma batch disposition",
-      type: "flowchart",
-      nodes: [
-        { id: "start-id", label: "Human label", kind: "start" },
-        { id: "decision-id", label: "Question?", kind: "decision" },
-      ],
-      edges: [
-        {
-          id: "edge-id",
-          source: "decision-id",
-          target: "target-id",
-          label: "yes",
-        },
-      ],
-      layout: { direction: "TB", edgeRouting: "orthogonal" },
-    },
-    null,
-    2,
-  ),
+  JSON.stringify({
+    id: "short-kebab-case-id",
+    title: "Pharma batch disposition",
+    type: "flowchart",
+    nodes: [
+      { id: "start-id", label: "Human label", kind: "start" },
+      { id: "decision-id", label: "Question?", kind: "decision" },
+    ],
+    edges: [
+      {
+        id: "edge-id",
+        source: "decision-id",
+        target: "target-id",
+        label: "yes",
+      },
+    ],
+    layout: { direction: "TB", edgeRouting: "orthogonal" },
+  }),
 ].join("\n");
 const expectedGeminiBody = {
   contents: [{ role: "user", parts: [{ text: expectedUser }] }],
@@ -202,7 +198,7 @@ describe("diagram generation prompt mapping", () => {
 
     expect(messages.system).toContain('Use type "mindmap".');
     expect(messages.system).toContain("exactly one root node");
-    expect(messages.user).toContain('"type": "mindmap"');
+    expect(messages.user).toContain('"type":"mindmap"');
   });
 
   it("keeps the Gemini REST body byte-for-byte stable", () => {
@@ -216,6 +212,15 @@ describe("diagram generation prompt mapping", () => {
     ).toEqual(expectedGeminiBody);
   });
 
+  it("uses the expanded output budget unless the request overrides it", () => {
+    expect(
+      buildGeminiGenerateContentBody({
+        model: "google/gemini-3.1-flash-lite",
+        prompt,
+      }).generationConfig.maxOutputTokens,
+    ).toBe(16_384);
+  });
+
   it("normalizes Cloudflare Google model ids for provider-native calls", () => {
     expect(stripGoogleModelPrefix("google/gemini-3.1-flash-lite")).toBe(
       "gemini-3.1-flash-lite",
@@ -774,6 +779,45 @@ layer(malformedClientLayer)("malformed model responses", (it) => {
   );
 });
 
+const truncatedRun = vi.fn(async () =>
+  jsonResponse({
+    candidates: [
+      {
+        content: { role: "model", parts: [{ text: '{"type":"flowchart"' }] },
+        finishReason: "MAX_TOKENS",
+      },
+    ],
+  }),
+);
+const truncatedClientLayer = CloudflareGoogleAiStudioClientLive.pipe(
+  Layer.provide(
+    Layer.mergeAll(
+      Layer.succeed(CloudflareAiGatewayBinding, {
+        gateway: () => ({ getUrl: vi.fn(), run: truncatedRun }),
+      }),
+      configLayer,
+      retryPolicyLayer,
+    ),
+  ),
+);
+
+layer(truncatedClientLayer)("token-budget exhaustion", (it) => {
+  it.effect("returns an explicit output-truncated diagnostic", () =>
+    Effect.gen(function* () {
+      const client = yield* DiagramGenerationClient;
+      const candidate = yield* client.generate({
+        model: "google/gemini-3.1-flash-lite",
+        prompt,
+      });
+
+      assert.isUndefined(candidate.diagram);
+      expect(candidate.diagnostics).toContain(
+        "output_truncated: Gemini stopped at the maximum output-token budget; regenerate the complete diagram.",
+      );
+    }),
+  );
+});
+
 const substitutedClientLayer = Layer.succeed(DiagramGenerationClient, {
   provider: "fixture",
   generate: Effect.fn("diagramGeneration.test.generate")(function* () {
diff --git a/packages/diagram/scenarios/src/lib/prompt.test.ts b/packages/diagram/scenarios/src/lib/prompt.test.ts
index 79fc2704..040a0721 100644
--- a/packages/diagram/scenarios/src/lib/prompt.test.ts
+++ b/packages/diagram/scenarios/src/lib/prompt.test.ts
@@ -45,7 +45,7 @@ describe("scenario prompts", () => {
     expect(prompt.user).toContain("- QA Manager final review");
     expect(prompt.user).toContain("Required decision branch labels:");
     expect(prompt.user).toContain("- retest");
-    expect(prompt.user).toContain('"title": "Pharma batch disposition"');
+    expect(prompt.user).toContain('"title":"Pharma batch disposition"');
   });
 
   it("keeps a flattened prompt for stdin-based generator commands", () => {
@@ -53,7 +53,9 @@ describe("scenario prompts", () => {
 
     expect(flattened).toContain("System message:");
     expect(flattened).toContain("User message:");
-    expect(flattened).toContain("Return only JSON.");
+    expect(flattened).toContain(
+      "Return only compact, minified JSON on one line.",
+    );
     expect(flattened).toContain(scenario.prompt);
   });
 });