diff --git a/packages/core/src/__tests__/canvas-log.test.ts b/packages/core/src/__tests__/canvas-log.test.ts
index 96797de58..b0b2aed2c 100644
--- a/packages/core/src/__tests__/canvas-log.test.ts
+++ b/packages/core/src/__tests__/canvas-log.test.ts
@@ -134,6 +134,44 @@ describe("CanvasArtifactSchema", () => {
     const result = CanvasArtifactSchema.safeParse({ ...validMarkdown, id: "BAD ID" });
     expect(result.success).toBe(false);
   });
+
+  it("rejects diff canvases with too many hunk lines", () => {
+    // 1001 lines exceeds DIFF_MAX_LINES_PER_HUNK (1000). This is the cap that
+    // prevents agent-emitted diffs from rendering hundreds of thousands of DOM
+    // rows on every 5s poll.
+    const tooManyLines = Array.from({ length: 1001 }, (_, i) => ({
+      kind: "context" as const,
+      text: `line ${i}`,
+    }));
+    const result = CanvasArtifactSchema.safeParse({
+      version: 1,
+      id: "huge",
+      type: "diff",
+      title: "Too many lines",
+      createdAt: "2026-05-06T00:00:00Z",
+      updatedAt: "2026-05-06T00:00:00Z",
+      payload: { files: [{ path: "a.ts", status: "modified", hunks: [{ header: "@@", lines: tooManyLines }] }] },
+    });
+    expect(result.success).toBe(false);
+  });
+
+  it("rejects diff canvases with too many files", () => {
+    const tooManyFiles = Array.from({ length: 201 }, (_, i) => ({
+      path: `f${i}.ts`,
+      status: "modified" as const,
+      hunks: [],
+    }));
+    const result = CanvasArtifactSchema.safeParse({
+      version: 1,
+      id: "wide",
+      type: "diff",
+      title: "Too many files",
+      createdAt: "2026-05-06T00:00:00Z",
+      updatedAt: "2026-05-06T00:00:00Z",
+      payload: { files: tooManyFiles },
+    });
+    expect(result.success).toBe(false);
+  });
 });
 
 describe("parseUnifiedDiff", () => {
diff --git a/packages/core/src/canvas-schema.ts b/packages/core/src/canvas-schema.ts
index c9c443cee..541cb8a83 100644
--- a/packages/core/src/canvas-schema.ts
+++ b/packages/core/src/canvas-schema.ts
@@ -24,10 +24,19 @@ const diffLine = z
   })
   .strict();
 
+// Bounds matching the synthesized git-diff path's caps in canvas-log.ts.
+// Without these, an agent-emitted diff canvas could pack tens of thousands of
+// hunk lines into a 256 KB payload and stall the session detail page on every
+// 5s poll. Numbers chosen to be generous for legitimate review/refactor diffs
+// while preventing DOM blowup. See canvas-log.ts: DIFF_MAX_FILES, DIFF_MAX_LINES.
+const DIFF_MAX_LINES_PER_HUNK = 1000;
+const DIFF_MAX_HUNKS_PER_FILE = 50;
+const DIFF_MAX_FILES_PER_CANVAS = 200;
+
 const diffHunk = z
   .object({
     header: z.string(),
-    lines: z.array(diffLine),
+    lines: z.array(diffLine).max(DIFF_MAX_LINES_PER_HUNK),
   })
   .strict();
 
@@ -36,13 +45,13 @@ const diffFile = z
     path: z.string().min(1),
     oldPath: z.string().optional(),
     status: z.enum(["added", "modified", "deleted", "renamed"]),
-    hunks: z.array(diffHunk),
+    hunks: z.array(diffHunk).max(DIFF_MAX_HUNKS_PER_FILE),
   })
   .strict();
 
 const diffPayload = z
   .object({
-    files: z.array(diffFile),
+    files: z.array(diffFile).max(DIFF_MAX_FILES_PER_CANVAS),
   })
   .strict();